Preparation of Different Cationic Gemini Surfactants and their Applications as Corrosion Inhibitors and Oil Dispersants

Review Article

By

Maher Ibrahim Nessim

Associate Professor

Analysis and Evaluation Department,

Egyptian Petroleum Research Institute

Associate Professor

E-mail:maherni@yahoo.com

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Contents

Item PageCover 1Contents 2I. Introduction 3II. Chemical Structures of Gemini Surfactants 5III. State Of Gemini Surfactants in the Premicellar Range of Concentration 10IV. Behavior at Interfaces 11V. Efficiency of gemini surfactants as corrosion inhibitors for carbon steel in acid media 17VI. Micelle Formation And Solubilization 19 a. Aqueous Dimeric Surfactant Solutions At Concentrations Below The Critical Micellization Concentration

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b. dimeric Surfactants At Interfaces 24 b.1. Air–Solution Interface 24 b.2. Solid-Solution Interface 27 c. Solubility In Water, CMC, And Thermodynamics Of Micellization Of Dimeric and Oligomeric Surfactants

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d. Critical Micellization Concentration 29 e. Thermodynamics of Micellization 30VII. Correlation between the Efficiency of the Cationic Gemini Surfactants as Corrosion Inhibitors and their Quantum Chemical Calculation Studies

33

1- Introduction 332- Quantum chemical parameters 332.1. Atomic charges 34

2.2. Molecular orbital energies 34 2.3. Dipole moment 35 2.4. Energy 35 3. Quantum Chemical Calculation Methods 35

3.1. Ab-initio and MP2 methods 363.2. Semiempirical methods 36

3.2.1.MNDO Method (modified neglect of differential overlap) 36 3.2.2.AM1 Method 36Optimized structure of some cationic gemini surfactants of the type N2,N3-dialkyl-N2,N2,N3,N3-tetramethylbutane diamminium bromide:

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The frontier molecule orbital density distributions of an example of the prepared cationic gemini surfactants

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VIII: Different examples of Prepared Cationic Gemini Surfactants in Our work 38References 39

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I. Introduction

Gemini or dimeric surfactants (Fig. 1) are new types of amphiphilic molecules that

have attracted attention from various industrial and academic research groups. These

surfactants are made up of two amphiphilic moieties connected at the level of the head

groups, or very close to the head groups, by a spacer group, which can be hydrophobic or

hydrophilic, flexible or rigid [1–3]. The gemini surfactants behave mainly as normal

surfactants, with similar sequences of phases and aggregates structures, but they also show

some interesting differences. They exhibit some unusual physicochemical properties that

might prove useful for technical applications in the future. The current interest in such

surfactants arises from three essential properties: (1) gemini surfactants are characterized by

critical micelle concentrations CMC that are one to two orders of magnitude lower than for

the corresponding conventional (monomeric) surfactants [1,4]; (2) gemini surfactants are

much more efficient than the corresponding conventional (monomeric) surfactants at

decreasing the surface tension of water [5–8]; (3) aqueous solutions of some gemini

surfactants with short spacers can have remarkable rheological properties (viscoelasticity,

gelification and shear thickening) at relatively low surfactant concentration whereas the

solution of the corresponding monomer remains low viscous [9]. In addition to those

properties, gemini surfactants appear to have better solubilizing, wetting, foaming, and lime

soap dispersing properties than conventional surfactants [10–13].

Owing to these unique properties, gemini surfactants have been widely used in

industrial detergency, gelation of organic solvents, template synthesis of various materials,

etc. Besides, the Krafft temperatures of gemini surfactants with hydrophilic spacers are

generally very low [10–12,14,15], giving these surfactants the capacity to be used in cold

water. These properties are commonly used to evaluate surfactant performance and are

important for applications such as cleaning and stabilization of dispersed systems. The first

reports on gemini surfactants concerned bis quaternary ammonium halide surfactants.

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FIG. 1 Schematic representation of a gemini surfactant.

The biological activity of gemini surfactants, in aqueous solution, was studied [13, 16,

17], and micellar solutions of these surfactants were used to catalyze chemical reactions [18].

Most studies, however, reported on the surface tension of the aqueous solutions of gemini

surfactants for CMC determinations and an assessment of their capacity in reducing the

surface tension of water [3, 5–8]. These studies did not raise much interest among surfactant

scientists in spite of the much lower CMC and stronger biological activity found for gemini

surfactants compared with the corresponding monomeric conventional surfactants. It was

only in the early 1990s, following the synthesis of gemini surfactants in a great variety of

chemical structures, that more systematic studies revealed that such surfactants possess

properties that make them superior to conventional surfactants [14]. Thus, their values of C 20,

the surfactant concentration where the surface tension is decreased by 20 mN/m, are much

lower for equal or lower values of CMC (surface tension at the CMC) [14]. The idea

underlying the study of gemini surfactants is that linking surfactants two by two may provide

a new way to control the shape of their assemblies and thus some of their properties [19].

The vast majority of work on gemini surfactants has been made with symmetrical

gemini surfactants, i.e., containing identical polar head groups and identical hydrophobic

tails. These are true gemini surfactants and they can also be regarded as a kind of dimeric

surfactants. Several reviews on gemini surfactants have recently been published [14, 20–25].

It is well known from research on structure–property relationships that surfactants with an

asymmetrical geometry may give interesting characteristics in terms of self-assembly into

aggregates and packing at interfaces. For instance, fine tuning of the internal curvature of

micro emulsions in order to obtain systems with very large solubilization capacity can be

made by designing surfactants with different lengths of the two hydrophobic tails or with

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asymmetrical branching of one tail [26–29]. Diglycerides and phospholipids are typical

examples [30]. Studies on asymmetrical gemini surfactants are still scarce. Recent studies

have been made on gemini surfactants in which the polar head groups are chemically

different.

Gemini surfactants with non-identical ‘‘halves’’ have been referred to as hetero-

gemini [31,32]. By using a surfactant with such a chemical structure, one may expect to get

the best out of two attractive concepts: (1) gemini surfactants and (2) a 1:1 molar

combination of two different surfactants built into the same molecule. This novel geometry in

the molecule may allow a new way to control the adsorption and shape of surfactant

assemblies and may provide properties that are often obtained by mixed surfactants at

equimolar ratio. The alkanediyl-N-bis(alkyl dimethyl ammonium bromide) or bisquaternary

ammonium bromides have been by far the most investigated gemini surfactants because their

synthesis and purification are relatively easy. These surfactants are designated by the

abbreviation m-s-m, 2Br-, s and m being the carbon numbers of the alkanediyl group (spacer)

and of the alkyl chain of the amphiphilic moieties. These surfactants are formally the dimers

of the quaternary dimethyl ammonium bromide surfactants with two unequal alkyl chains of

carbon numbers m and s/2. The symbolism used above for symmetrical dimeric surfactants

can be easily extended to asymmetrical dimeric surfactants (m-s-mV, 2Br-) and to surfactant

oligomers (m-s-m-s-m, 3Br- for a trimeric surfactant, for instance).

This review refers to dimeric gemini surfactants. Throughout the review,

‘‘conventional surfactants’’ and ‘‘monomeric surfactants’’ are given the same meaning. The

next section discusses chemical structures, of gemini surfactants. Section III reviews the

behavior of gemini surfactants in solutions below the CMC. Section IV deals with their

behavior at interfaces whereas section V emphasizes on their efficiency as corrosion

inhibitors for mild steel. The sixth section reviews micelle formation and solubilization. The

last section predicts the correlation between the efficiency of the cationic gemini surfactants

as corrosion inhibitors and their quantum chemical calculation studies.

II. Chemical Structures of Gemini Surfactants

Gemini surfactants with a great variety of chemical structures have been obtained by

acting on the nature of the head group and spacer group, as illustrated in Fig. 1. The head

group can be anionic, cationic, nonionic, or zwitterionic while the spacer group is hydrophilic

or hydrophobic, rigid or flexible [1–3, 5–13,15–18,33–53]. Gemini surfactants with non-

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identical head groups have been recently synthesized [31,54–60]. The hydrophobic moieties

are generally normal alkyl chains, CmH2m+1. However, gemini surfactants with mixed

fluorinated-hydrogenated alkyl chains, C8F17C4H8 for instance, have been synthesized [61].

Examples of chemical structures for gemini surfactants are given in Fig. 2. Surfactants [A]–

[D] are cationic, [E]–[I] and [L] are anionic, [J] can be made zwitter ionic, [K] is a nonionic

sugar-based gemini surfactant, and [N] is a surfactant with one positive head group and one

negative head group. [A], [B], [D], [G–J] and [L] have a flexible spacer, whereas [C], [E] and

[F] have a rigid spacer, Fig.2.

G Form [45] H Form [46] I Form [47]

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J Form [48] K Form [51] L Form [52]

M Form [53] N Form [54] O Form [61]

P Form [59] Q Form [31]FIG. 2 Examples of gemini surfactants

In [A]–[C], [E], [F], [L]–[O] the spacer is hydrophobic whereas it is hydrophilic in

[D] and [G]–[J] where Y=O or O(CH2CH2O)x. [M] and [N] are functional gemini surfactants:

the electrical charge of [M] can be acted upon electrochemically and [N] is cleavable. [O] is a

gemini surfactant with mixed hydrocarbon/fluorocarbon chains. [P] is a gemini surfactant

with one anionic and one cationic head group separated by two methylene groups, the two

alkyl chains may be symmetrical or asymmetrical. These surfactants will be denoted m, mV,

where m and mV being the carbon numbers of the chains bound to the phosphate and

trimethylammonium head groups, respectively.[Q] is a nonionic gemini surfactant when X =

OH and an anionic gemini when X=SO4Na.

Due to lack of space, only three examples of synthesis of gemini surfactants are given.

Surfactant [A] and its homologs (R=C12H25 and Y=(CH2)s-) were obtained via the single-step

reaction of dodecyldimethylamine with the corresponding -dibromoalkane (molar ratio

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2.1/1) in dry ethanol under reflux for 48 h [1]. Other solvents can be used, such as ethyl

acetate, acetone, methyl cyanide, etc., depending on the surfactant synthesized [1–3,5–

8,13,16– 18,33–35,37–42].

The phosphate-quaternary ammonium hetero-gemini surfactant [P] of Fig. 2 was

synthesized by a surprisingly simple two-step, one-pot reaction, shown in Scheme 1 [58]. A

fatty alcohol was reacted with the cyclic ethylene chlorophosphate to yield the intermediate 1,

which was subsequently ring opened by an alkyldimethylamine to form the desired gemini

surfactant 2.

Scheme 1 Synthesis of surfactants [P] [59].

A large series of surfactants of this type has been prepared with hydrocarbon chains

containing 8–18 carbon atoms. The synthesis is attractive and versatile in that the two main

building blocks, the fatty alcohol and the alkyldimethylamine, are readily available. The

method therefore allows easy access to all types of surfactants with identical or different

hydrocarbon tails. The cyclic ethylene chlorophosphate is somewhat expensive, but the use of

this starting material may be circumvented by preparing the intermediate 1 by reaction the

fatty alcohol with phosphoryl chloride followed by addition of ethylene glycol and a tertiary

amine, as is also shown in Scheme 1.

Contrary to the gemini surfactant [A] of Fig. 2, compound [P] is counter ion free, i.e.,

the zwitterionic surfactant is in the form of an inner salt. Compound [Q] of Fig. 2 is a gemini

surfactant with different polar head groups, one hydroxyl group or sulfate and one methyl-

capped poly (ethylene glycol). The published synthesis is outlined in Scheme 2 [31]. A fatty

nitrile derived from oleic acid was used as starting material. Epoxidation with hydrogen

peroxide over a tungstic acid catalyst yielded the epoxide 1, which was ring-opened by

8

methyl-capped poly (ethylene glycol) to yield the nonionic gemini surfactant 2. Compound

[Q] with the corresponding sulfate 3, was obtained by treatment of 2 with chlorosulfonic acid

[60]. The synthesis strategy lends itself to production in large scale (in which case

chlorosulfonic acid would probably be replaced by SO3). Analogous products with other

anionic groups than sulfate can probably be synthesized from the intermediate 2 by

employing the synthesis routes discussed above.

The synthesis strategy shown in Scheme 2 can in principle be used on all unsaturated

lipophilic starting materials. In practice there may not be many alternatives to oleic acid

derivatives for large-scale preparation. An analog to the nonionic surfactant 2 has recently

been synthesized from methyl oleate [62]. The real difficulty when dealing with gemini

surfactants lies in the purification of the raw surfactants. The purification of the crude gemini

surfactants is essential, particularly in studies of adsorption and behavior at interfaces. The

purification procedures are somewhat easier for the quaternary ammonium gemini

surfactants.

Sophisticated procedures must often be used. Indeed, one or more reaction steps that

lead to gemini surfactants usually involves the two ends of some intermediate compound.This

reaction rarely reaches full completion. It results in the formation of a mixture of mono- and

di-functionalized compounds. Their separation is usually achieved through chromatography.

Scheme 2, Synthesis of Surfactants Q [31]

III. State Of Gemini Surfactants in the Premicellar Range of Concentration

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Many papers discussed the state of ionic gemini surfactants at concentration below the

CMC. Several situations were considered. First, the gemini may be dissociated, and give rise

to one gemini ion and two counter ions, or not completely dissociated, with partial binding of

one counter ion to the gemini ion. The binding equilibrium obeys the mass action law and the

binding is expected to increase with the surfactant concentration. Second, depending on the

conformation of the spacer group of the surfactant gemini, its two alkyl chains may or may

not interact. Third, premicellar aggregation may take place, giving rise to small aggregates of

gemini ions of low aggregation number. Some surface tension data have been interpreted on

the basis that in the premicellar range, one bromide ion of gemini surfactants [A] binds to the

surfactant ion [7,8,13,43], thereby reducing its charge. This effect would be similar to what

has been assumed for the closely related bolaform surfactants or bolaamphiphiles

(amphiphilic molecules that have hydrophilic groups at both ends of a sufficiently long chain

hydrophobic hydrocarbon chain), alkanediyl-,-bis(trimethylammonium bromide) [63,64].

Compared to single-headed amphiphiles, the introduction of a second head group generally

induces a higher solubility in water, an increase in the critical micelle concentration CMC,

and a decrease in aggregation number. The aggregate morphologies of bolaamphiphiles

include spheres, cylinders, disks, and vesicles. Bolaamphiphiles are also known to form

helical structures that can form micro tubular self-assemblies [67].

However, potentiometric studies using surfactant-specific electrodes for 12-s-12

surfactants [A] did not reveal any ion pairing [1]. Indeed, for these surfactants the linear

variation of the electromotive force with lnC (C = surfactant concentration) below CMC

yielded an e.m.f. change close to 30 mV for a concentration change by a factor 10, a value

close to that expected for divalent-univalent (2:1) electrolytes. However, conductivity

measurements suggested that ion pairing takes place in submicellar solutions of 8-s-8, 2Br-

and 10-s-10, 2Br- surfactants which are characterized by high CMC values which favor ion

pairing [35,65].

Another issue concerns a possible premicellar association of gemini surfactants into

dimers and larger oligomers for surfactants [A] with CH2CHOHCH2 or (CH2CHOH)2 spacers

[37,38] and surfactants [C], [E], and [F] with a hydrophobic rigid spacer [3], at m z 14. The

decrease of the surface tension lowering effect of these surfactants and the increase of their

CMC with increasing m, where m ≥ 14–16, were explained on this basis[2, 3, 37, 38]. Besides

some surfactants [A] with R = CmH2m+1 [66] or CmH2m+1OC(O)CH2 [7, 42] and with a

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polymethylene or other spacer groups did not show this abnormal behavior up to m = 16.

Thus, premicellization appears to depend on the gemini surfactant nature. It is also favored by

long polyethylene spacer as shown in a recent study [65].

IV. Behavior at Interfaces

Extensive surface tension measurements have been performed on aqueous solutions of

gemini surfactants with the purpose of investigating their behavior at the air–solution

interface (measurement of surface area a occupied by one surfactant molecule at the

interface) and determining CMCs. The surface areas (a) were obtained from the slope of the

variation of the surface tension with lnC (C = surfactant concentration) using the Gibbs

expression of the surface excess concentration :

(1)

Where, R is the gas constant and T the absolute temperature. The constant n takes the values

2 for univalent-univalent monomeric ionic surfactants and 3 for divalent-univalent ionic

gemini surfactant, in the absence of a swamping electrolyte. The surface area occupied by

one surfactant at the interface, (a) is then obtained as (NA)-1, NA being Avogadro’s number.

The value n = 2 was used for ionic gemini surfactants in several studies [6–8, 43], on the

assumption that one of the two charged head groups is neutralized by a bound counter ion.

Several other studies used the values n = 3 [3, 68–72]. At any rate, the value used for n does

not affect the qualitative conclusions inferred from the (a) values for a series of homologous

surfactants.

The effectiveness of gemini surfactants in lowering the surface tension of water is

close to that of the corresponding monomeric surfactant. Indeed, the values of the surface

tension at the CMC, CMC, are close for monomeric gemini surfactants, as illustrated by the

results shown in Fig. 3 for 12-3-12, 2Br- [4] and its corresponding monomeric surfactant,

dodecyltrimethylammonium bromide (DTAB) [73].

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FIG.3 Surface tension vs. concentration of the gemini surfactant 12-3-12, 2Br- (●) and of dodecyltrimethylammonium bromide (■) (Adapted from data in Refs.4 and 73).

However, the former are always more efficient surface-active agents than the latter

because their CMCs are much lower. Thus, the values of the surfactant concentration C20 for

which is lowered by 20 mN/m are much lower for gemini than for monomeric surfactants.

This result is very important for the utilization of gemini surfactants. The behavior of gemini

surfactants at the air-solution interface has been extensively investigated and some important

results are summarized as follows:

1. Surface activity is favored by flexible spacers such as polymethylene or polyoxy-

ethylene chains. Bulky and/or rigid aromatic spacers result in larger values of CMC [3,

37]. Bulky and/or rigid aromatic groups in the hydrocarbon tails near the spacer also

have an unfavorable effect on surface tension lowering [74]. An effect of the aging of

the solution on the measured surface tension has been reported for surfactants with

rigid spacers [2, 3]. This effect is probably in relation with the high Krafft temperature

of these surfactants [75].

2. The alkyl chain carbon number m of the gemini surfactant has generally a small effect

on (a) as long as, say, m < 12–14 [6, 8, 76]. However, for some cationic gemini

surfactants an effect sets in at higher values of m and results in larger values of CMC

and a [2,3,7,37,38,77]. Premicellar association, self-coiling of the alkyl chains, and a

peculiar configuration of the surfactant with its two alkyl chains lying more or less

flat on the interface have been proposed to explain this behavior [2,3,37,38].

Premicellar association appears to be at the origin of the observed behavior [65].

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3. For flexible spacers, of the -(CH2)iY(CH2)i- type, the value of (a) depends on the

nature of the chemical group Y. Thus, the value of a (in nm2, in parentheses) increases

in the order: -S- (0.84)< -N(CH3)- (1.08)< -CH2- (1.14)< -O- (1.28), for the gemini

surfactants [C12H25(CH3)2N+, Br-]2[(CH2)2Y(CH2)2] [7, 8].

4. Figure 4 shows the variation of (a) with the spacer carbon number s for the

[C12H25(CH3)2N+, Br-]2(CH2)s series (hydrophobic polymethylene spacer [4]) and with

the total number nT of oxygen and carbon atoms separating the two head groups in

anionic gemini which have a hydrophilic polyoxyethylene spacer (Y is O (nT = 9),

OCH2CH2O- (nT = 12), -O(CH2CH2O)2 (nT = 15), and -O(CH2CH2O)3 (nT = 18)) [45].

Focusing only on the qualitative features, (a) is observed to go through a maximum at

s = 10–12 for the hydrophobic spacer series but not for the hydrophilic spacer series.

A similar maximum in a at about the same value of s appears to occur for the

bolaform surfactants [(CH3)3N+, Br- ]2(CH2)s [78]. For the bolaform and the 12-s-12

surfactants this maximum was explained in terms of a change of location of the

polymethylene chain as s increased. At s < 10, the chain has little flexibility and lies

flat with a fairly linear conformation in the air–solution interface. This is supported by

X-ray scattering studies of the lamellar and hexagonal phases in the water/12-s-12,

2Br- surfactant mixtures [79] and by the rapid initial increase of a with s in Fig. 4. At

s > 10 the chain is too hydrophobic to remain in contact with water and moves to the

air side of the interface, adopting a wicket-like or looped conformation in doing so

[4,64,78], which results in an overall decrease of a. This effect may be enhanced by a

change of orientation of the alkyl chains with respect to the interface as s increases, at

large s values. The absence of a maximum for gemini surfactants having a hydrophilic

spacer supports this explanation. The maximum observed for gemini surfactants with

a hydrophobic spacer has been accounted for theoretically [80, 81]. The spacer

conformational entropy and the attractive and repulsive interactions between

surfactant molecules appear to be the dominant factors in determining the variation of

a with s.

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FIG. 4 Variation of the surface area per gemini surfactant at the air–water interface for the 12-s-12 series(●, from Ref.4) and for[C10H21OCH2CH(OCH2CO2

-, Na+)(CH2)]2O(CH2CH2O) x series (o, from Ref. 45) vs. the spacer carbon number s or the total number of atoms nT

between charged groups at 25oC.

5- The values of the surface area a per surfactant [Q] with X = OH and a number x of

ethylene oxide units equal to 7, 12, and 16, have been found to be 43, 50, and 55 Å 2,

respectively, from surface tension data [31]. Note that for these surfactants n = 1 in

Eq. (1). These values are lower than for a nonionic surfactant with a single

hydrophobic chain and a polyoxyethylene head group even though surfactants [Q]

contain two hydrophobic chains[81].For instance, the values of(a) for C12E8 and C10E8

are 63 Å2 and 70 Å2, respectively. This suggests that the monolayer formed by gemini

surfactants [Q] is relatively closely packed. For surfactants [Q] with X = SO4Na, the

surface tension at the CMC (CMC) increases with the polyoxyethylene chain length, a

phenomenon that is also observed with CmEx surfactants [83]. The value of (a)

calculated from the surface tension data with n = 2 increases with the number of

oxyethylene units, as expected. The surfactant with x = 7 appears to be the most

effective one, with the lowest values of the CMC and CMC. The variation of CMC with

the number of oxyethylene units x is often attributed to steric crowding of the

nonionic head groups, which naturally increases with x.

6- The Gibbs equation was used with n = 1 for the analysis of the surface tension data for

surfactants [P]. Indeed, these surfactants are counter ion free and therefore considered

as neutral molecules. The surface areas per molecule have been obtained to be around

30 Å2 [59]. These values are much lower than one would expect for a surface

monolayer. They are also significantly lower than the values of a measured for

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equimolecular mixtures of two oppositely charged surfactants. The unrealistically low

values of a obtained for gemini surfactants [P] may be due to some kind of surface

aggregation [59], which remains to be clarified. Dynamic surface tension (DST)

studies of the cationic surfactants [A] with Y = CHOH showed that their adsorption at

the air–solution interface is controlled by diffusion [84]. On the contrary, for gemini

surfactants [Q] (Fig. 5), the results suggest that at the beginning (short times) the

adsorption is essentially diffusion controlled. However, close to equilibrium (long

times) the DST decays are not consistent with a diffusion-controlled adsorption

mechanism [31,60].

FIG. 5 Dynamic surface tension t vs. time for gemini [Q] with X = SO4Na and x = 12 (from Ref. 60). From top to bottom: 0.032, 0.064, 0.12, 0.207, 0.252, and 0.393 wt. %.

The adsorption of gemini cationic surfactants [A], 12-s-12, 2Br- on solid surfaces has

been investigated. The surfactant 12-2-12, 2Br- was less adsorbed than its corresponding

monomer, DTAB, on silica [85] and titanium dioxide [86], when the adsorption was

expressed in moles of adsorbed dodecyl chain per gram of solid. A similar conclusion was

reached for the adsorption of DTAB and 12-2-12, 2Br- on laponite clay [87]. No explanation

was provided for these results. The maximum amount of 12-s-12, 2Br- gemini surfactant

adsorbed on silica was shown to decrease very much as the spacer carbon number was

increased from 2 to 10 [88].

Adsorption of surfactants [Q] with X = OH and various values of x at silica surfaces

have been investigated by optical reflectometry [31] (Fig. 6). The adsorbed amount of each

surfactant on hydrophilic silica was about twice that on hydrophobic silica. The adsorbed

15

amount depends little on x in the case of hydrophobic silica but decreases upon increasing x

for hydrophilic silica, showing the effect of steric hindrance from the relatively larger head

group. It was concluded that surfactants [Q] are better packed at solid–liquid interfaces than

conventional surfactants. Surfactants [Q] with X = SO4Na show a significant decrease of the

adsorbed amount on hydrophilic silica with increasing number of oxyethylene units [60] (Fig.

6). On both hydrophilic and hydrophobic silica, a increases with the polyoxyethylene chain

length. This result indicates that packing at the solid–liquid interface is improved when the

poly (oxyethylene) molecular weight decreases.

FIG. 6 Amount of adsorbed gemini surfactant [Q], on the hydrophilic silica surface (from Refs. 31 and 60). From top to bottom surfactant (X=OH, x=12), (X=OH, x = 16), (X = SO4Na, x = 8), (X = SO4Na, x = 12), (X = SO4Na, x = 16), respectively.

This is a well-known phenomenon for monodisperse poly(ethylene glycol) monoalkyl

ethers such as CmEx. Thus, the anionic character of the surfactants, introduced via the sulfate

group, is of minor importance for adsorption. It should be pointed out that surfactants [Q]

seem to pack better with X=OH than with X = SO4-Na+ at the air–water and solid–water

interfaces. The sulfate group seems to decrease the ability of the surfactant to align tightly in

a monolayer. The adsorbed amount of surfactants [P] is higher at the surface of hydrophilic

silica than on hydrophobic silica. The adsorbed amount for the pair 10,12 and 12,10 is higher

than for 14,8 and 8,14 on both hydrophilic and hydrophobic silica. This might be due to the

effect of the relatively larger difference between the lengths of the two alkyl chains of the

latter surfactant pair. This asymmetry may be unfavorable for efficient packing at planar

surfaces [59].

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V. Efficiency of gemini surfactants as corrosion inhibitors for carbon steel in acid media

Low carbon steel is being used extensively under different conditions in industries

because of its low cost and excellent mechanical properties. However, some corrosion

problems take place due to the effect of acid solutions in cooling systems, storage reservoirs,

and water transport pipelines for injection systems [89]. So the study of corrosion of steel in

acid solutions is industrially important field of research [90-92]. The use of inhibitors is one

of the most practical methods for protection against corrosion especially in acidic media [93-

95]. In order to reduce the corrosion of metals, several techniques have been applied; where

among that utilization of organic compounds are gaining high space as corrosion inhibitors.

Among efficient corrosion inhibitors, there are heterocyclic organic compounds consisting of

-system and/ or O, N, P, or S heteroatoms [96].

It is well known that the presence of hydrophilic and hydrophobic groups in the

inhibitor favors the adsorption process at the electriferous surface [97]. Thus, the application

of conventional surfactants made up of one hydrophilic head group and one hydrophobic

chain as corrosion inhibitors has been widely studied. It was found that these amphiphilic

compounds could adsorb on metal surface to form a protective layer and have a marked

inhibiting efficiency near their critical micellar concentrations [98-105].

As a new generation of surfactants, gemini surfactants have attracted great interest in

recent years. This new generation of surfactants has been investigated as corrosion inhibitors

for acid solutions [106]. However, there are few reports on the use of gemini surfactants as

inhibitors of metal corrosion [107, 108].

It has been demonstrated that gemini surfactants are more efficient to form micelles

than conventional surfactants containing one hydrophilic group and one hydrophobic group,

and show lower critical micelle concentration (CMC), better solubilization and greater

efficiency in lowering the surface tension of water. Given that, gemini surfactants show many

unique properties in comparison with single chain conventional surfactants, it is reasonable to

study their effects on corrosion inhibition of metals [109, 110]. It describes corrosion

inhibition of carbon steel in 1M HCl by cationic gemini surfactants of formula Cm-x-Cm. In

general, cationic surfactants and particularly gemini surfactant possessing effective inhibitory

effect. They accumulate in special order at the interfaces and modify the interfaces and thus,

control, reduce, or prevent reactions between a substrate and its surroundings when added to

the medium in small quantities [111]. In order to evaluate compounds as corrosion inhibitors

17

and to design novel inhibitors, much more research works were concentrated on the studies of

the relationship between structural characteristics of the organic compounds and their

inhibiting effects [112].

Gemini surfactants control corrosion, acting over the anodic or the cathodic surface or

both. We and some other researchers have demonstrated that quaternary ammonium gemini

surfactants are excellent candidates for iron and steel in acidic medium [113-119]. The

adsorption behavior of these gemini surfactants on metal surface in acidic medium was found

to be affected remarkably not only by length of hydrophobic chains [120], but also by the

spacer length of the gemini surfactants [121-124]. It was found that adsorption mechanism of gemini surfactants on metal surface is different from that of conventional single-chained surfactants, and the spacer length of the gemini surfactants was found to determine the adsorption mechanism of these gemini surfactants onto metal surface. Also the efficiency of the used gemini

surfactants on the corrosion inhibition is affected by the spacer length between the two heads

[125]. Moreover, the adsorption mechanism of gemini surfactants on metal surface in acid

medium has been rarely investigated till now.

We noticed that many gemini surfactants have been synthesized. The structure of the

gemini surfactants can be tailored either by introducing different types of spacers such as

polymethylene, polyoxyethylene, and aromatic rings in the molecule [129, 131]. Some

research works revealed that the inhibition efficiency of Schiff bases is much greater than that

of corresponding amines and aldehydes due to the presence of a –CH=N- group in the

molecules [132]. The polar unit is regarded as the reaction center for the adsorption process.

Thus, polar organic compounds are adsorbed on the metal surface, forming a charge transfer

complex bond between their polar atoms and the metal. The size, orientation, shape and

electric charge on the molecule determine the degree of adsorption and hence the

effectiveness of the inhibitor [123-132]. Studies have been carried out on the effect of

variation of the spacer polarity and chain length on the physicochemical properties of gemini

surfactants [128–130, 132]. However, the effect of variation in the head group polarity on the

properties of cationic gemini surfactants, which can offer interesting physicochemical

properties, has not been studied earlier. Hence an attempt has been made by researchers,

synthesize and study the physicochemical properties of the cationic gemini surfactants with

variation in their head group polarity.

18

VI. Micelle Formation And Solubilization

Introduction:

It has been known for many years that amphiphilic molecules, which consist of a

hydrophilic head and a hydrophobic tail, can form a wide variety of aggregates including

spherical micelles, worm-like micelles, bilayers, and reverse micelles with properties

different from those of the unassembled molecules. The current interest in such surfactants arises from three essential properties.

1. Dimeric surfactants are characterized by CMC that are one to two orders of magnitude lower than those for the corresponding conventional (monomeric) surfactants [133, 134]. For example, the CMC of the dimeric surfactant 12-2-12 (dimethylene-1,2-bis(dodecyldimethylammonium bromide)) is about 0.055 wt. %, whereas that of the corresponding monomeric surfactant DTAB (dodecyltrimethylammonium bromide) is 0.50 wt. %.

2. Dimeric surfactants are much more efficient than the corresponding monomeric surfactants at decreasing the surface tension of water. For instance, the concentration required for lowering the surface tension of water by 0.02 N/m is 0.21 wt. % for DTAB only 0.0083 wt. % for 12-2-12 [133, 134].

3. Aqueous solutions of dimeric surfactants with short spacers can have very high

viscosities at relatively low concentrations whereas solutions of the corresponding

monomeric surfactants have low viscosities. For instance the viscosity of aqueous

solutions of DTAB is only marginally larger than that of pure water up to a surfactant

concentration of at least 10 wt. % whereas a 5 wt. % solution of 12-2-12 has a

viscosity of several hundred Pa/s and is viscoelastic [133, 134]. Solutions of 12-2-12

can also display shear thickening at fairly low concentrations [135]. These properties

reflect the ability of dimeric surfactants with a short spacer, such as 12-2-12, to give

rise to worm-like micelles at fairly low surfactant concentrations, even in the absence

of added salt [136].

The fact that the properties of dimeric surfactants can differ greatly from those of

conventional surfactants has been related to the distribution of distances between head groups

19

in micelles formed by these two types of surfactants [137]. For conventional surfactants, this

distribution goes through a maximum at a thermodynamic equilibrium distance dT ≈ 0.7–0.9

nm. For dimeric surfactants the distribution is bimodal, with a first maximum at the

thermodynamic distance dT and another more narrow maximum at a distance ds that

corresponds to the length of the spacer. This length is determined by the bond lengths and

bond angles between the atoms making up the spacer group. The bimodal distribution of head

group distances and the effect of the chemical link between head groups on the packing of

surfactant alkyl chains in the micelle core are expected to strongly affect the curvature of

surfactant layers and thus the micelle shape and the properties of the solution. The distance ds

can be adjusted to be smaller than, equal to, or larger than dT by modifying the structure of the spacer. Those different situations are expected to give rise to a rich variety of behaviors.Dimeric surfactants have been also reported to have better solubilizing, wetting, foaming, and lime-soap-dispersing properties than conventional surfactants [14]. These properties are commonly used to evaluate surfactant performances. Besides, the Krafft temperatures of dimeric surfactants with hydrophilic spacers (see below) are generally very low [14], giving these surfactants the capacity to be used in cold water. Finally, cationic dimeric surfactants have been shown to possess a strong biological activity [138]. The interesting properties displayed by dimeric surfactants has led to the synthesis of longer homologs that are subsequently referred to as oligomeric surfactants [139]. The schematic representation of a trimeric surfactant is given in Fig. 1B. Results for oligomeric surfactants are reviewed below whenever appropriate. At the outset it is pointed out that most of the gain in properties is achieved by going from a monomeric to a dimeric surfactant.

Several reviews on dimeric surfactants have been recently published [133, 134, 14, 25, 140, 141]. The present review specifically focuses on the effect of the spacer group. It does not attempt to be thorough. Only the most significant results that refer to the effect of the nature and length of the spacer group on solution properties of dimeric surfactants are presented. Indeed, and this will become clearer below, the

20

nature and length of the spacer group are the most important parameters in determining the properties of dimeric surfactants.

Most of the reviewed results concern cationic dimeric surfactants of the bisquaternary

ammonium type. Table 1 shows some of these surfactants [1,142–151]. The abbreviations

used to refer to the surfactants in Table 1 are retained throughout this review. The surfactants

m-s-m (A1) and m-EOz -m (A3) are the ones that are referred to the most.

TABLE 1

Examples of Bisquaternary Ammonium Bromide Dimeric Surfactants

_________________________________________________________________________A1: R1 = R2 = Cm H2m+1; Y = CH2 ; x + y + 1 = s; m-s-m surfactants [1].A2: R1 = R2 = Cm H2m+1 ; Y = CH2 , O, S, N(CH3 ), x = y = 2 [142, 143].A2l : R1 = R2 = Cm H2m+1 ; Y = CHOH, (CHOH)2 ; x = y = 1 [143].A3: R1 = R2 = Cm H2m+1 ; Y = (OCH2 CH2 )z , x = 2; y = 0; m-EOz -m surfactants [144].A4: R1 = R2 = Cm H2m+1 ; Y = C ≡ C; x = y = 1 [145].A5a : R1 = R2 = Cm H2m+1 ; Y = x = y = 1 [146].A6: R1 = R2 = Cm H2m+1 OC(O)CH2 ; no Y ; x = y = 1; counter ion = chloride [147, 148].A7: R1 = R2 = Cm F2m C4 H8 ; no Y ; x = y = 1 [149].A8: R1 = Cm H2m+1 ; R2 = Cm

/ H2m/+1 ; no Y ; x = y = 1; s; m-2-m/ surfactants [150, 151].

__________________________________________________________________________a represents a phenylene group.

a) Aqueous Dimeric Surfactant Solutions At Concentrations Below The CriticalMicellization Concentration

Many papers discussed the state of ionic dimeric surfactants at concentrations below

the cmc. Several situations were considered. First, the dimer may be completely dissociated,

giving rise to one dimeric ion and two counter ions, or not completely dissociated, with

partial binding of one counter ion to the dimeric ion. Second, depending on the conformation

of the spacer group of the dimeric surfactant, its two alkyl chains may or may not interact.

Third, premicellar aggregation may take place, giving rise to small aggregates of dimeric ions

21

of low aggregation number. The possibility of a partial binding of a counter ion by a dimeric

surfactant ion at concentrations C < CMC arose in the analysis of the plots of the surface

tension γ vs C for dimeric surfactant solutions. These plots permit the determination of the

surface excess and a = 1/NA (NA = Avogadro’s number), surface area occupied by one

surfactant at the air/water interface, on the basis of the Gibbs equation:

= (2)

In Eq. (2) R is the gas constant and T the absolute temperature. The constant n takes

the value 2 for an ionic surfactant where the surfactant ion and the counter ion are univalent

and the value 3 for a dimeric surfactant made up of a divalent surfactant ion and two

univalent counter ions, in the absence of a swamping electrolyte. The reported a values were

calculated on the basis of n = 2 [142, 152–154], on the assumption that one of the two

charged groups is neutralized by a bound counter ion, or n = 3, on the assumption of a full

dissociation of the dimeric surfactants [145, 146, 155–158]. Some studies reported two sets of

values of a based on n = 2 and 3 [144, 4]. The same problem arises for ionic trimeric

surfactants for which the value n = 4 should be used if the surfactant is fully ionized at C <

CMC. The knowledge of the surface area occupied by a surfactant at an interface is very

important in surfactant science.

The problem of the value of n in the case of oligomeric surfactants therefore needed

to be solved. An attempt in this direction was performed by means of neutron reflectivity, a

technique that permits a direct determination of the surface excess [159]. The comparison

of the value of from neutron reflectivity to that of (dγ/d lnC)/RT from surface tension data

made it possible to determine the value of n at any surfactant concentrations. The comparison

yielded n = 2 for the dimeric surfactants 12-2-12, 12-3-12, and 12-12-12 and n = 3 for the

surfactant A5 with m = 12 [159]. For 12-6-12 the value of n was found to decrease from 3 at

low C to 2 at higher C but still below the CMC. The authors concluded that one dimeric

surfactant ion binds one bromide ion (ion pairing) in submicellar solutions of 12-s-12

surfactants. However it has been shown that other effects may explain differences between

neutron reflectivity and surface tension data [160].

Besides, recent electrical conductivity studies did not reveal any ion pairing in

submicellar solutions of 12-s-12 surfactants [161]. However, ion pairing was evidenced in

submicellar solutions of 8-s-8 and 10-s-10 surfactants, owing to the larger CMC values of

these surfactants with respect to 12-s-12 surfactants. Ion pairing is thus a real possibility for

dimeric surfactants with high CMC values, i.e., those with an alkyl chain containing 8 or 10

22

carbon atoms but apparently not those with an alkyl chain containing 12 carbon atoms.

Additional studies are required to explain the different conclusions reached in neutron

reflectivity and electrical conductivity studies of 12-s-12 surfactants. Some consideration

should be given to a possible explanation of the neutron reflectivity results in terms of

premicellar aggregation.

Thermodynamic studies suggested that the two alkyl chains in the surfactant A6 might

be partly associated in the molecularly dispersed state [148]. This suggestion was not

supported by the values of the free energy change associated with the transfer of one alkyl

(dodecyl) chain from the aqueous phase to the micelle, G◦(C12), for two series of oligomeric

surfactants [162]. These values were calculated with the available CMC and ionization data,

using the appropriate equation [163]. Self-coiling of the alkyl chains was mentioned as a

possible explanation for the aging effect observed in measurements of surface tension of

solutions of dimeric surfactants with long alkyl chains, below the CMC [146]. However such

effects are not observed with conventional surfactants having the same long alkyl chains. A 13C and 1H-NMR investigation of the conformation of dimeric surfactant A5 with m = 8

below the CMC did not reveal interactions between octyl chains [36]. Premicellar

aggregation occurs in solutions of conventional surfactants that are sufficiently hydrophobic.

The situation is somewhat similar with dimeric surfactants. Menger and Littau [146]

provided the first evidence for premicellar aggregation in solutions of several series of

dimeric surfactants, in particular for surfactant A5. The variation of the log cmc with the

alkyl chain carbon number m showed a strong upward curvature or a minimum for m ≥ 16,

instead of the usual linear variation. Premicellar aggregation was later reported for other

series of dimeric surfactants [143, 145, 165, 166]. Electrical conductivity measurements are

particularly well suited for obtaining evidence of premicellar aggregation [161, 166]. This is

illustrated in Fig. 7A and 7B. The variations of the electrical conductivity K with the

surfactant concentrations C and of the molar conductivity with C1/2 are represented for the

surfactants 12-8-12 and 16-8-16 [161]. The plots for the 12-8-12 surfactant show a normal

behavior.

For the 16-8-16 surfactant the K vs C plot shows a small upward curvature and the

vs C1/2 plot shows a pronounced maximum in the submicellar concentrations range. The vs

C1/2 plot for the dimers 14-8-14, 16-paraxylylene-16, and 18-8-18 also show a maximum, the

amplitude of which increases rapidly with m [161]. This maximum is the signature of

premicellar association in surfactant solutions. It arises because the equivalent conductivity of

23

a small aggregate of surfactant ions (whether monomeric or dimeric) is larger than the sum of

the equivalent conductivities of the ions constituting it [161, 166]. This effect shows at

different values of m for different dimeric surfactants [143, 145, 165, 38] but always at m ≥

14–16.

However, it is already detected at m = 11 for more complex dimeric surfactants

derived from arginine where the peptide segment separating the two charged groups may help

in stabilizing small premicellar aggregates [166]. For the dimeric surfactants A1 the spacer

carbon number has a strong effect on the occurrence of premicellization. Thus this effect does

not occur for the 12-s-12 dimers up to s = 12 and for 16-3-16 and 16-4-16. However the vs

C1/2 plots for 12-14-12, 12-16-12, 12-20-12, 16-6-16, and 16-8-16 show a maximum [161].

A study of two series of anionic trimeric surfactants suggests that premicellization occurs at a

value of m much lower than that for dimeric surfactants [167, 168].

FIG.7 Variations of the electrical conductivity K with the surfactant concentration and of the molar conductivity with the square root of the surfactant concentration for the dimeric surfactants 12-8-12 (A) and 16-8-16 (B). The arrow in B (top) indicates the CMC as obtained from the K vs C plot in B (bottom). From Ref. (161).

b) dimeric Surfactants At Interfaces

24

1. Air–Solution Interface The studies of the adsorption of dimeric surfactant at the air/solution interface

aimed to assess the efficiency and effectiveness of these surfactants in reducing the

surface tension of water. These measurements also aimed to measure the CMC and

surface area a occupied by one dimeric surfactant at the air/water interface on the

basis of the Gibbs equation (Eq. (2)). The efficiency and effectiveness are

characterized by the value of the surfactant concentration C20 at which the surface

tension of water is reduced by 0.02 N/m and by the value of the surface tension at the

CMC, γCMC, respectively. Figure 8 shows the variation of a with the spacer carbon

number for 12-s-12 surfactants [155, 4] and with the number nT = 3z + 2 of oxygen

and carbon atoms in the spacer group in the case of 12-EOz-12 surfactants [144]. For

the 12-s-12 surfactants (hydrophobic spacer) a is a maximum at a value of s around

10–12. A similar maximum in a at about the same value of s appears to occur for the

bolaform surfactants [N+(CH3)3, Br−]2(CH2)s (42).A maximum in a was observed for

the dimeric surfactants derived from arginine and which also contain a hydrophobic

polymethylene spacer [170]. However only a rather slow increase of a with nT is seen

for the 12-EOz-12 surfactants. A series of anionic surfactants having a poly (ethylene-

oxide) spacer showed the same behavior as the 12-EOz-12 surfactants [171]. Such a

behavior is expected upon increasing the volume and, thus, the surface of the poly

(ethyleneoxide) spacer, which remains located on the water side of the interface.

FIG.8 Surface area a occupied by one dimeric surfactant at the air–water interface. Variation with the spacer carbon number s at 25◦C for 12-s-12 surfactants(●, data from Refs. 155 and 4) and with nT for 12-EOz-12 surfactants (□, from Ref. 144).

25

For the 12-s-12 surfactants the maximum of a was explained in terms of a change in location of the polymethylene spacer upon increasing s. At s < 10, the spacer group is rather rigid and lies flat with a fairly linear conformation at the air–solution interface. This is suggested by the rapid initial increase of a with s in Fig. 8 and is supported by X-ray scattering studies of the lamellar and hexagonal phases in water/12-s-12 surfactant mixtures [172]. At s > 10 the spacer becomes too hydrophobic to remain in contact with water and moves to the air side of the interface, where it adopts a looped (wicket-like) conformation [4, 169]. This results in an overall decrease of a. This effect may be enhanced by a change of orientation of the alkyl chains with respect to the interface as s increases.

The absence of a maximum for dimeric surfactants with a hydrophilic spacer [144, 171] also supports this explanation. The maximum of a observed for the 12-s-12 surfactants was accounted for theoretically by statistical-mechanical calculations [173]. The main factors that determine the variation of a with s were shown to be the spacer conformational entropy and the attractive and repulsive interactions between surfactant molecules. At s > 12 the calculated value of a decreases upon increasing s more slowly than experimentally observed [173]. This discrepancy may be due to premicellar aggregation. Indeed, this effect occurs at s > 12 for 12-s-12 surfactants [161].

It was not taken into account in the calculations [46] and was not considered in the experimental study (30). Other studies do show that premicellar aggregation can cause a significant decrease in the apparent value of a [143, 165]. Monte Carlo simulations were also performed to understand the behavior of dimeric surfactants at the air–solution interface [174]. The results in Fig. 9 are of the utmost importance for explaining the properties of m-s-m surfactants in aqueous solution that are described below as well as the differences in behavior between the two closely related series of

26

dimeric surfactants m-s-m and m-EOz -m. Indeed the large changes of a with s for the 12-s-12 surfactants indicate correspondingly large changes of the surfactant packing parameter that results in changes of micelle shape.

On the contrary the relatively slow change of a with nT = 3z + 2 seen for 12-EOz-12 surfactants suggests that the micelle structure should vary slowly with the number of ethylene oxide groups in the spacer. For cationic oligomeric surfactants the surface area per amphiphilic moiety has been shown to decrease in going from the monomer (DTAB) to the dimer (12-3-12) and the trimer (12-3-12-3-12). It then levels off when going to the tetramer (12-3-12- 4-12-3-12) [162].

The kinetics of adsorption of dimeric surfactants at the air–water interface strongly depends on the nature of the surfactant. Two reports on anionic [180] and cationic [176] dimeric surfactants, the latter of type A2´ and A5, indicated a diffusion controlled adsorption of the investigated surfactants. On the contrary, a large barrier to adsorption was reported to exist for cationic surfactant dimers derived from disulfur betaine [156]. Dimeric surfactants with a flexible spacer lowered the surface tension of water faster than dimers with a rigid spacer.

FIG.9 Spacer carbon number dependence of the maximum amount of adsorbed 12-s-12 surfactant on raw (●) and HCl-treated(o)silica (from the results in Refs. 178 and 179).

27

b.2 Solid-Solution Interface:The adsorption isotherms of dimeric 12-s-12 [182-184],

trimeric 12-s-12-s-12 [162, 180], and tetrameric 12-3-12-4-12-3-12 cationic surfactants on macro porous amorphous silica showed that the adsorption involves two steps, as for conventional surfactants [162]. The first step occurs at very low concentration and corresponds to a binding of individual dimeric surfactants to charged sites on the silica surface by an ion exchange mechanism. The second step occurs at concentrations slightly below the CMC and corresponds to the formation of surface aggregates. Figure 4 shows that in the case of the adsorption of 12-s-12 surfactants on both raw and HCl-treated silica the maximum amount of adsorbed surfactant, max, decreases as s increases [177-179]. This variation is in relation with the structure of the surface aggregates. An atomic force microscopy study [181] showed that 12-2-12 adsorbs as a flat bilayer, whereas 12-4-12 and 12-6-12 adsorb as parallel cylinders. A surfactant with still lower packing parameters was shown to adsorb under the form of spherical surface aggregates [181]. Obviously the maximum amount of adsorbed surfactant decreases as the structure goes from a flat bilayer to parallel cylinders and to spheres. Since mica and silica surfaces differ mostly by their charge density, it is likely that differences in structure of the surface aggregates similar to those seen on mica occur on silica, thereby explaining the change of max.

c. Solubility In Water, CMC, And Thermodynamics Of Micellization Of Dimeric and Oligomeric Surfactants

Solubility in water, Krafft temperature, and melting temperature of dimeric surfactants:

28

Ionic dimeric surfactants with m ≤ 12 are generally highly soluble in water particularly those with a hydrophilic spacer. The reported Krafft temperatures, TK, of several series of anionic dimeric surfactants with hydrophobic or hydrophilic spacers are below 0oC [134, 14, 25, 171]. Such low values of TK permit the use of dimeric surfactants in cold water. The variation of TK with the spacer carbon number s has been determined for solutions of the cationic dimeric surfactants 12-s-12 and 16-s-16 [24, 183] and compared to the variation of the melting temperature, TM, of the solid surfactants. The results are shown in Fig. 10. There appears to be no correlation between the variations of TK and TM with s. For homologous series of conventional surfactants it is usually observed that the variations of TK and TM are correlated. For instance both TK and TM increase with the surfactant chain length. The maximum of TM at s = 5 for the two series of surfactants in Fig. 6 is noteworthy. The minimum of TM at s = 10–12 for the 12-s-12 surfactants was mentioned in Ref. [172]. Figure 10A also shows the variation of the melting temperature of 12-EOz-12 surfactants with nT = 3z + 2, total number of oxygen and carbon atoms in the spacer [184]. The plot apparently goes through a minimum at nT = 11, a value close to that of s for which TM is a minimum for the 12-s-12 series. The few instances where the Krafft temperatures of a dimeric surfactant and of the corresponding monomeric surfactant could be directly compared revealed no systematic trend. Thus, the values of TK for 16-2-16 and CTAB (cetyltrimethylammonium bromide) are, respectively, 45 and 25o C [182, 24]. Those for 12-2-12 and DTAB are 15◦C and below O◦C, respectively [182, 24]. On the contrary, dimeric surfactants derived from arginine have TK values lower than those of the corresponding monomeric surfactants [185].

d. Critical Micellization Concentration

29

One of the main reasons for the current interest in dimeric surfactants is that their CMC are much lower than those of the corresponding monomeric surfactants, by about one order of magnitude and more. For instance, the CMC of 12-2-12 and DTAB are 0.055 and 0.50 wt%, respectively [1]. The low CMC values of dimeric surfactants with respect to the corresponding conventional surfactants arise mainly because two alkyl chains, rather than one, are transferred at a time from water to the micelle pseudo phase [163, 186]. The results concerning the effects of various parameters on the CMC of dimeric surfactants are briefly summarized.

(i) The CMC of surfactant A2 with the flexible hydrophobic spacer (CH2)2Y(CH2)2 depends little on the chemical nature of Y . Thus, the CMC of surfactant A2 with m =12 were found to be [139, 145] 1.2, 1.1, 1.0 and 0.84 mM for Y≡N(CH3), O, CH2, and s, respectively.

(ii) The CMC of anionic or cationic dimeric surfactants with a poly (ethylene oxide) spacer increase with the number z of ethylene oxide groups [144, 171, 187] whereas the CMC of conventional surfactants decreases upon intercalation of ethylene oxide groups between the alkyl chain and the charged group.

(iii) The CMC of surfactants m-s-m with a hydrophobic polymethylene spacer is a maximum at s = 5–6, irrespective of the value of m (see Fig. 11) [1, 188, 189]. This value of s is also that for which the melting temperature of the m-s-m surfactants is a maximum. A similar maximum of CMC was observed for dimeric surfactants derived from arginine that also includes a hydrophobic spacer [43]. On the contrary, for both anionic and cationic surfactants with a hydrophilic poly(ethylene oxide) spacer the CMC increased progressively with nT = 3z + 2, total number of oxygen and carbon atoms in the spacer, as seen in Fig. 11 for 12-EOz-12 surfactants [144, 171]. The increase of CMC with s observed for s ≤ 6 for m-s-m surfactants is probably due to

30

a conformational change in the surfactant molecule. The two alkyl chains would be in a gauche or trans position at low s values and in a cis position at higher s. Monte Carlo simulations of dimeric surfactant solutions attempted to account for the behavior of the CMC of the surfactant [174]. The calculations correctly predicted the presence of a maximum in the CMC vs s plot for surfactants with a hydrophobic spacer. However, the calculated overall variations of the CMC with s were much larger than experimentally observed. Also, the calculations predicted a decrease of CMC with increasing nT for hydrophilic spacers [174], whereas the experimental results show the opposite behavior.

(iv) The CMC of quaternary ammonium oligomeric surfactants with m = 12 decreased in a somewhat hyperbolic manner in going from the monomer to the dimer, trimer, and tetramer [162]. The largest part of the decrease occurred in going from the monomer to the dimer.

e. Thermodynamics of Micellization Several recent papers have reported on the thermodynamics of

micellization of 12-s-12 surfactants [190-193].Figure 8 shows the variation of the enthalpy of micellization, HoM, of 12-s-12 surfactants with the spacer carbon number, as obtained from direct calorimetric measurements [190-192]. The two sets of data represented show important differences in numerical values that may be due to the calibration of the measuring devices. Nevertheless the trends are similar, with -HoM going through a rather shallow minimum at around s = 5–6, i.e., at about the same s value as some of the properties reviewed above. A very large decrease of -HoM is observed in going from 12-2-12 to 12-4-12; -HoM depends only weakly on s at s > 4. The large decrease of -HoM at low s has been attributed to the conformational change discussed in the preceding section [192]. The value of the free energy of micellization of a dimeric surfactant, GoM, can be

31

obtained from Eq. (3), where α is the micelle degree of ionization [163]:

GoM = 2RT(1.5 − α) ln cmc (3)A recently reported set of α values (see Ref. [191] and below)

has been used together with available cmc data [1] to calculate the values of GoM and of the entropy of micellization SoM of 12-s-12 surfactants. The data show that the free energy of micellization is nearly independent of s and that most of the variation of SoM and GoM with s occurs in going from s = 2 to s = 4. The volume change upon micellization VoM of m-s-m surfactants has been determined [193]. For 12-s-12 surfactants VoM goes through a shallow minimum at s around 5–6 (see Fig. 12). The free energy of micellization per dodecyl chain Go (C12) for DTAB, 12-3-12, 12-3-12-3-12, and 12-3-12-4-12-3-12 was calculated by inserting the available CMC and α values into Eq. 4, valid for oligomeric surfactants [163]:

GoM (C12) = RT(1 + 1/x − α) lnCMC. (4) In Eq. (4), x is the number of dodecyl chains in the oligomeric

surfactant. GoM (C12) was found to be equal to −18.3, −20.8, −21.5, and −22.8 kJ/mol, respectively [162]. This variation is rather small, within the experimental error on α. A similar result was found for the three surfactants 12-3 (monomer), 12-6-12, and 12-6-12-6-12 [162]. Such results do not support the existence of an intramolecular association of the alkyl chains of the oligomeric surfactant below the CMC postulated in Ref. [151].

32

FIG.10 Spacer carbon number dependence of the Krafft temperature TK (o) and of the melting temperature TM (●,□). (A) 12-s-12 (●) and 12-EOz-12 (□) surfactants; (B) 16-s-16 surfactants (from Refs. 56 and 58). The lines are guides to the eye. The data points for the Krafft tempe- rature of surfactants 12- 6-12, 12-8-12, and 12-10-12 have been set at 0o

C, while in fact the TK values for these surfactants could not be measured (TK <0o C).

FIG.11 Dependence of the CMC on the spacer carbon number s for the surfactants 10-s-10 (□, from Ref. 62), 12-s-12 (∇, from Ref. 12), and 16-s-16 (o, from Ref. 12; ●, from Ref. 189) and on nT = 3z + 2 for the 12-EOz-12 surfactants (▲, from Ref. 144).

33

FIG.12 Variation of the enthalpy of micellization _H◦M (●, from Refs. 52 and 66; ■, from Ref. 191) and of the volume change upon micellization ∆VoM (o, from Ref. 193) for the 12-s-12 surfactants at 25◦C.

VII. Correlation between the Efficiency of the Cationic Gemini Surfactants as Corrosion

Inhibitors and their Quantum Chemical Calculation Studies

1. Introduction:

Quantum chemical methods have already proven to be very useful in determining the

molecular structure as well as elucidating the electronic structure and reactivity [194]. Thus,

it has become a common practice to carry out quantum chemical calculations in corrosion

inhibition studies. The concept of assessing the efficiency of a corrosion inhibitor with the

help of computational chemistry is to search for compounds with desired properties using

chemical intuition and experience into a mathematically quantified and computerized form.

Once a correlation between the structure and activity or property is found, any number of

compounds, including those not yet synthesized, can be readily screened employing

34

computational methodology [195] and a set of mathematical equations which are capable of

representing accurately the chemical phenomenon under study [196, 197].

The study of corrosion processes and their inhibition by organic inhibitors is a very

active field of research [198]. Many researchers report that the inhibition effect mainly

depends on some physicochemical and electronic properties of the organic inhibitor which

relate to its functional groups, steric effects, electronic density of donor atoms, and orbital

character of donating electrons, and so on [199, 200]. The inhibiting mechanism is generally

explained by the formation of a physically and/or chemically adsorbed film on the metal

surface [201, 202]. It is well known that organic compounds which act as inhibitors are rich

in heteroatoms, such as sulfur, oxygen, and nitrogen [203, 204]. These compounds and their

derivatives are excellent corrosion inhibitors in a wide range of media and are selected

essentially from empirical knowledge based on their macroscopic physicochemical

properties. Recently, theoretical prediction of the efficiency of corrosion inhibitors has

become very popular in parallel with the progress in computational hardware and the

development of efficient algorithms which assisted the routine development of molecular

quantum mechanical calculations [205]. Due to the enormous complexity of this type of

studies which need to consider the metallic surface, inhibitor and solvent molecules,

theoretical calculations of the corrosion inhibition processes cannot be achieved in a rigorous

way from the viewpoint of quantum chemistry.

2. Quantum chemical parameters:

Quantum chemical methods and molecular modeling techniques enable the definition

of a large number of molecular quantities characterizing the reactivity, shape, and binding

properties of a complete molecule as well as of molecular fragments and substituents. The

use of theoretical parameters presents two main advantages: firstly, the compounds and their

various fragments and substituents can be directly characterized on the basis of their

molecular structure only; and secondly, the proposed mechanism of action can be directly

accounted for in terms of the chemical reactivity of the compounds under study [206].

Quantum chemically derived parameters are fundamentally different from experimentally

measured quantities, although there is some natural overlap. Unlike experimental measure-

ments there is no statistical error in quantum chemical calculations. There is inherent error

however, associated with the assumptions required to facilitate the calculations. In most cases

the direction but not the magnitude of the error is known [206]. In using quantum chemistry

based parameters with a series of related compounds, the computational error is considered to

35

be approximately constant throughout the series. The prominent quantum chemical

parameters can be subdivided as follows:

2.1. Atomic charges

All chemical interactions are either electrostatic (polar) or orbital (covalent). Electric

charges in the molecule are obviously responsible for electrostatic interactions. The local

electron densities or charges are important in many chemical reactions and for physico-

chemical properties of compounds. Thus, charge-based parameters have been widely

employed as chemical reactivity indices or as measures of weak intermolecular interactions.

Despite its usefulness, the concept of a partial atomic charge is somewhat arbitrary, because it

depends on the method used to delimit between one atom and the next. As a consequence,

there are many methods for estimating the partial charges. Mulliken population analysis [207]

is mostly used for the calculation of the charge distribution in a molecule. These numerical

quantities are easy to obtain and they provide at least a qualitative understanding of the

structure and reactivity of molecules [208]. Furthermore, atomic charges are used for the

description of the molecular polarity of molecules

2.2. Molecular orbital energies

Highest occupied molecular orbital energy (EHOMO) and lowest unoccupied molecular

orbital energy (ELUMO) are very popular quantum chemical parameters. These orbitals, also

called the frontier orbitals, determine the way the molecule interacts with other species. The

HOMO is the orbital that could act as an electron donor, since it is the outermost (highest

energy) orbital containing electrons. The LUMO is the orbital that could act as the electron

acceptor, since it is the innermost (lowest energy) orbital that has room to accept electrons.

According to the frontier molecular orbital theory, the formation of a transition state is due to

an interaction between the frontier orbitals (HOMO and LUMO) of reactants [209]. The

energy of the HOMO is directly related to the ionization potential and the energy of the

LUMO is directly related to the electron affinity. The HOMO–LUMO gap, i.e. the difference

in energy between the HOMO and LUMO, is an important stability index [210]. A large

HOMO–LUMO gap implies high stability for the molecule in chemical reactions [211]. The

concept of ‘‘activation hardness” has been also defined on the basis of the HOMO–LUMO

energy gap. The qualitative definition of hardness is closely related to the polarizability, since

a decrease of the energy gap usually leads to easier polarization of the molecule [212].

36

2.3. Dipole moment

The most widely used quantity to describe the polarity is the dipole moment of the

molecule [213]. Dipole moment is the measure of polarity of a polar covalent bond. It is

defined as the product of charge on the atoms and the distance between the two bonded

atoms. The total dipole moment, however, reflects only the global polarity of a molecule. For

a complete molecule the total molecular dipole moment may be approximated as the vector

sum of individual bond dipole moments.

2.4. Energy

The total energy calculated by quantum chemical methods is also a beneficial parameter.

The total energy of a system is composed of the internal, potential, and kinetic energy.

Hohenberg and Kohn [214] proved that the total energy of a system including that of the

many body effects of electrons (exchange and correlation) in the presence of static external

potential (for example, the atomic nuclei) is a unique functional of the charge density. The

minimum value of the total energy functional is the ground state energy of the system. The

electronic charge density which yields this minimum is then the exact single particle ground

state energy.

3. Quantum Chemical Calculation Methods:

3.1. Ab-initio and MP2 methods

Semiempirical methods serve as efficient computational tools which can yield fast

quantitative estimates for a number of properties. This may be particularly useful for

correlating large sets of experimental and theoretical data, for establishing trends in classes of

related molecules, and for scanning a computational problem before proceeding with higher

level treatments. There remains the need to improve semiempirical methods with regard to

their accuracy and range of applicability, without compromising their computational

efficiency. In addition, there is a need to develop new algorithms in order to exploit modern

computer architectures and extend semiempirical calculations to ever larger molecules.

3.2. Semiempirical methods

3.2.1 MNDO Method (modified neglect of differential overlap)

It is based on the NDDO (neglect of diatomic differential overlap) approximation and in

turn NDDO is an improved version of INDO (intermediate neglect of differential overlap)

method. INDO itself is an improvement over the CNDO (complete neglect of differential

overlap) approximation. There are several such semiempirical LCAO MO methods,

developed for specific purposes.

3.2.2 AM1 Method

37

AM1 is a semiempirical method based on the neglect of differential diatomic overlap

integral approximation. Specifically, it is a generalization of the modified neglect of diatomic

differential overlap approximation. AM1 was developed by Michael Dewar and coworkers

reported in 1985 [215]. AM1 is an attempt to improve the MNDO model by reducing the

repulsion of atoms at close separation distances. The atomic core–atomic core terms in the

MNDO equations were modified through the addition of off-center attractive and repulsive

Gaussian functions. The complexity of the parameterization problem increased in AM1 as the

number of parameters per atom increased from seven in MNDO to 13–16 per atom in AM1.

By using these methods, the experimentally obtained inhibition efficiency was

correlated with energy gap which is theoretically calculated by quantum chemical

calculations. The difference in energy between the EHOMO and ELUMO decreased with

increasing the inhibition efficiency. The energy gap (ΔE) is an important stability index

[210]. A large HOMO–LUMO gap implies high stability for the molecule in chemical

reactions [211]. The values of (ΔE) indicate remarkably that the smaller energy gap results in

a high corrosion inhibition, reflecting the stronger interaction between the inhibitors and

metal surface. The interactions are probably physical adsorption [216-218].

Optimized structure of some cationic gemini surfactants of the type N2,N3-dialkyl-N2,N2,N3,N3-tetramethylbutane diamminium bromide:

38

HOMO LUMO

The frontier molecule orbital density distributions of an example of the prepared cationic gemini surfactants

VIII: Different examples of Prepared Cationic Gemini Surfactants in Our work

Where R = C12H25, C14H29 and C16H33 4,4'-((1E,1'E)-(1,4-phenylenebis(azanylylidene))bis(methanylylidene))bis(N-alkyl-N,N-dimethyl-benzenaminium) bromide

39

Where R = C12H25, C14H29 and C16H33

N,N'-(1,4-phenylenebis(methylene))bis(N,N-dimethylalkan-1-aminium) bromide

Where R = C12H25, C14H29 and C16H33

N2,N3-dialkyl-N2,N2,N3,N3-tetramethylbutane-2,3-diaminium bromide

Where R = C12H25, C14H29 and C16H33

N1,N2-dialkyl-N1,N1,N2,N2-tetramethylethane-1,2-diaminium bromide

Where R = C10H21 and C12H25

N1,N5-dialkyl-N1,N1,N5,N5-tetramethylpentane-1,5-diaminium bromide

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