Journal of Molecular Liquids 221 (2016) 224–234

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Journal of Molecular Liquids

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Synthesis and characterization of newly cationic surfactants based on2-(2-(dimethylamino)ethoxy)ethanol: physiochemical, thermodynamicand evaluation as biocide

Samy M. Shaban a,⁎, A.S. Fouda b, S.M. Rashwan c, Hoyeda E. Ibrahim c, M.F. El-Bhrawy d

a Egyptian Petroleum Research Institute (EPRI), Nasr City, Cairo, Egyptb Department of Chemistry, Faculty of Science, El-Mansoura University, Egyptc Department of Chemistry, Faculty of Science, Ismailia University, Egyptd Dara Petroleum Company, Egypt

⁎ Corresponding author.E-mail address: dr.samyshaban@yahoo.com (S.M. Sha

http://dx.doi.org/10.1016/j.molliq.2016.05.0880167-7322/© 2016 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 23 February 2016Accepted 29 May 2016Available online 1 June 2016

Three newly cationic surfactants based on 2-(2-(dimethyl amino) ethoxy) ethanol were prepared through directquaternization with different alkyl bromide. The chemical structures of the synthesized surfactants were con-firmed by proton nuclear magnetic resonance and Fourier transform infrared spectroscopy. The critical micelleconcentrations have been estimated using two different techniques surface tension and conductometric mea-surements at three different temperatures 25, 45 and 65 °C. The surface parameters were found hydrophobicand temperature dependent. The tail elongation and raising the solution temperature has a reverse effect onthe criticalmicelle concentration. The calculated thermodynamic parameters proved that the synthesized surfac-tant favor the adsorption at the interfaces than aggregation in micelles. The synthesized surfactant HEDDB hasthe maximum antibiotic effect against the tested gram positive and negative bacteria and even higher than theused drug. The surfactant HEDHB has higher antifungal activity than the reference drug. The simply synthesizedsurfactant showed a high effect against sulfate reducing bacteria at low concentration.

© 2016 Elsevier B.V. All rights reserved.

Keywords:Surface tensionConductivityMicellization & adsorption thermodynamicsBiological activitySulfate reducing bacteria

1. Introduction

The surfactants have attracted great interest in both technical appli-cations and fundamental researches. Surfactants are organic com-pounds have the ability to lower the surface or interfacial tension ofthe system inwhich they be dissolved. They comprised of two opposingparts attached together in the same molecule. The tail of surfactant hasthe ability to dissolve in non-polar media while the head dissolve inpolar media. Therefore, the amazing structure of surfactant force themto migrate to the interface or aggregate in micelles in the bulk solution.The mention behavior of surfactant lowers the free energy of the sys-tem. The adsorption and the micellization phenomena are responsiblefor thewide application of surfactants in various fields and fundamentalresearch. In petroleum sectors, the surfactants participate in many ap-plications serving the production of the crude oil like corrosion inhibi-tors, demulsifier, pour point depressant, biocide and drilling mud [1–8]. Surfactants used in detergent, paints, coating, drug industry [9–13].The surfactants participate in nanotechnology research as cappingagents for controlling size [14–17]. The surfactant can be used as

ban).

capping agent and reducing agent at the same time as reported by T.Mishra et al. [18]. The similarity in the chemical structure between thesurfactants and cell membrane of bacteria facilitates the destructive ac-tion of surfactant. The production of crude oil sometimes accompaniedby certain types of bacteria called sulfate-reducing bacteria (SRB). Thegrowth of sulfate reducing bacteria cause severe corrosion problemsin transfer pipelines and storage tanks and with high growth, it maylead to the souring of the oil well. Therefore, it is necessary to mitigatethe sulfate reducing bacteria. The cationic surfactant showed high effi-ciency in killing the sulfate reducing bacteria, at the same time it partic-ipates in the oil production [19–22]. The cationic surfactant is a categoryof surfactant where the head group carries positive charge [23–25].Many reports exploited the biological activity of cationic surfactant[26–28]. The presence of some functional groups like NH, OH, SH andhalogen in the chemical structure of the surfactant enhance the biolog-ical activity and the corrosion inhibition [29,30]. The work aimed tostudy the physicochemical and thermodynamic parameters of new sim-ply synthesized surfactant in order to manage them is specific applica-tion. The adsorption and micellization propensity are the central ofvarious application. The chemical structures of the surfactants wereconfirmed using FTIR and 1H NMR spectroscopy. The activity againstbacteria and fungi were determined. The foaming and emulsification

225S.M. Shaban et al. / Journal of Molecular Liquids 221 (2016) 224–234

power have been determined to assess whether they can facilities theoil production or not when they be used as biocide against sulfate re-ducing bacteria.

2. Materials & experimental

2.1. Materials

2-(2-(dimethylamino)ethoxy)ethanol was used as purchased fromSigma Aldrich chemical company. The octyl bromide, dodecyl bromideand hexadecyl bromide alkylating agents were purchased from aMerck company and used without any purification. All the used organicsolvents (ethanol and diethyl ether) were purchased from theAlgomhoria Chemical Company. All themention chemicals and solventswere used without any more purification.

2.2. Synthesis of cationic surfactants

The cationic surfactants were prepared through direct low energysynthetic pathway. The 2-(2-(dimethylamino)ethoxy)ethanol0.01 mol (1.33 g) was refluxed with 0.01 mol from the alkylatingagent separately (octyl bromide (1.93 g), dodecyl bromide (2.49) andhexadecyl bromide (3.05 g) in 100 mL ethyl alcohol as solvent for15 h). After evaporating the absolute alcohol, the residual was purifiedwith diethyl ether [31]. The short name for the obtained product hence-forward are HEDOB, HEDDB and HEDHB for the synthesized cationicsurfactant with octyl, dodecyl and hexadecyl alkyl chain respectively.The synthetic pathway is shown in Scheme 1.

Scheme 1. Synthetic route of n

2.3. Structure confirmation

The synthetic route of newly cationic surfactantswas investigated byFourier transform infrared (FTIR) and proton nuclear magnetic reso-nance spectroscopy (1H NMR). The FTIR analysis was carried out inEgyptian Petroleum Research Institute using ATI Mattsonm Infinity Se-ries™, Bench top 961 controlled by Win First ™ V2.01 Software. The1H NMR was done in National Research Institute using GEMINI 200(1H 200 MHz) in DMSO-d6.

2.4. Measurements

2.4.1. Surface tension measurements and interfacial tensionThe surface tension of the synthesized cationic surfactant aqueous

solutions was measured by a platinum ring detachment method usinga K6 Krüss (Hamburg, Germany) tensiometer at three different temper-atures 25, 45 and 65 ± 0.1 °C. The accuracy of the measurements was±0.5 mN m−1. The solutions were poured into a clean Teflon cupwith a mean diameter of 28 mm. The platinum ring was cleaned beforeeach measurement with diluted chromic acid mixture solution thenwashed with double distilled water. Each concentration was measuredthree times and the average was recorded and usedwithout correction.The critical micelle concentration (CMC) was determined from thebreak point in surface tension (γ) versus [log c] plots [31].

The interfacial tension of the synthesized cationic surfactants wasobtained between 0.1% aqueous solutions of surfactants by weight andlight paraffin oil at 25 °C using the same procedures of the surface ten-sion measurements [32].

ewly cationic surfactants.

Fig. 1. IR spectrum of N-(2-(2-hydroxyethoxy)ethyl)-N,N-dimethylhexadecan-1-aminium bromide (HEDHB).

226 S.M. Shaban et al. / Journal of Molecular Liquids 221 (2016) 224–234

2.4.2. Foaming powerFoam power was estimated after shaking 100mL of 0.1% the surfac-

tant solution vigorously in a stoppered graduated 250 mL cylinder atroom temperature 25 °C. The foamheightmeasured by the initially pro-duced foam height (in mL). The Foam stability was measured by thetime required by the produced foam be diminished to the halve of ini-tially produced [33].

2.4.3. Emulsion stabilityEmulsion stability was estimated by vigorously stirring a mixture of

10mL (0.1%) of the synthesized cationic surfactant solutions and 10 mLof paraffin oil at 25 °C. The emulsion stability of the synthesized cationicsurfactants was expressed as the time required for separation of 9mL ofpure surfactant solution [34].

2.4.4. Conductivity measurementsThe CMCs of the synthesized cationic surfactants have been estimat-

ed by conductivity using a digital conductivity meter Cond 3210 SET 1,

Fig. 2. 1H NMR spectrum of N-(2-(2-hydroxyethoxy)ethy

Probe tetra corn 325 (Wissenschaftlich Technische Werkstattern), hav-ing a sensitivity of 1 μS·cm−1. Each surfactant aqueous solution wasmeasured trice and the average was taken. The deionized double dis-tilled water used in the surfactant dilution has a conductivity equal to1.9 μS·cm−1 [35].

2.4.5. The biological activity evaluationThe antibiotic effect of the synthesized surfactants against pathogen-

ic bacteria (Gram-positive and Gram negative) and fungi has beenassessed using filter paper disc agar method [36,37]. The source ofmicro-organism isMicroanalytical Center, Cairo University. The bacteriaunder evaluation was Escherichia coli and Staphylococcus aureus. TheCandida albicans and Aspergillus flavus were used as an example forfungi.

The procedures for the assessment were as follows:

1. Themelted agar flask was inoculated with the organism to be tested,and then pour them into a Petri dish.

l)-N,N-dimethyloctan-1-aminium bromide (HEDOB).

Table1

Thesurfaceprop

erties

ofsynthe

sizedcation

icsu

rfactant

atva

riou

stempe

ratures.

Comp.

Temp.

°CCM

Ca

(mM·L

−1)

CMCb

(mM·L

−1)

αC 2

0∗10

−5

(mol·L

−1)

π CMC

(mN·m

−1)

Г max

∗10

−10

(mol·cm

−2)

Amin

A2

CMC/C 2

0

HED

OB

253.98

13.98

40.41

6.31

35.93

0.80

207.69

63.10

452.58

12.58

30.43

4.44

34.59

0.67

246.09

58.17

651.42

21.43

30.44

2.29

32.35

0.54

306.75

62.01

HED

DB

251.71

91.74

10.39

2.62

36.83

0.82

202.01

65.57

451.05

61.11

60.41

1.21

34.88

0.68

244.88

87.53

650.85

00.87

00.42

0.87

33.25

0.56

298.91

97.33

HED

HB

250.82

70.82

80.38

1.21

37.79

0.85

195.27

68.59

450.68

40.69

80.40

0.54

36.35

0.69

241.64

127.61

650.53

70.55

40.41

0.42

34.04

0.58

288.26

128.82

aTh

eva

lues

obtained

from

surfacetens

ionmeasu

remen

ts.

bTh

eva

lues

obtained

from

cond

uctometry

measu

remen

ts.

227S.M. Shaban et al. / Journal of Molecular Liquids 221 (2016) 224–234

2. After the solidification, a multilobed disc impregnated with the syn-thesized surfactants laid on top of agar.

3. Each surfactant in each lope of disc diffuses into the prepared gar. Ifthe organisms under test are sensitive to the synthesized surfactants,no growth occurs.

4. The diameters of inhibition zones were measured after 24–48 h. at35–37 °C (for bacteria) and 3–4 days at 25–27 °C (for fungi).

5. The zone was used to compare between the synthesized surfactantsto determine the most effective one.

The antibiotic effect of the synthesized surfactants on the sulfate re-ducing bacteria (SRB) was assessed using the serial dilution method ac-cording to ASTMD4412-84 [38]. SRB-contaminatedwater was suppliedfromPetro Dara PetroleumCo. (EasternDesert, Egypt). The contaminat-ed water was subject to growth of 107 bacteria cell/mL. The preparedcationic surfactants were tested as biocide against the SRB by doses of2.5 × 10−3, 1 × 10−3, 5 × 10−4 and 1 × 10−4 M. The system was incu-bated to contact time of 3.0 h. Each system was cultured in SRB specificmedia for 21 days at 37–40 °C.

3. Results and discussion

3.1. Structure confirmation

3.1.1. FTIR spectraThe three synthesized surfactants showed nearly the same bands in

infrared spectra. Fig. 1 shows the FTIR spectra of the surfactant HEDHBas an example of the synthesized surfactant. Fig. 1 confirms the pres-ence of a stretching vibration band of –C–H aliphatic symmetric andasymmetric at 2852 and 2920 cm−1 respectively in addition–CH2 bend-ing at 1398 cm−1, –CH3 bending at 1471 cm−1. The hydroxyl group ap-peared at 3376 cm−1 while the ether bond and C–N bond at 1131 cm−1

and 1081 cm−1 respectively. The aromatic proton appeared at3008 cm−1.

3.1.2. 1H NMR spectraThe number and distribution of the prepared cationic surfactant pro-

tons were estimated by 1H NMR spectra. Fig. 2 shows the 1H NMR spec-tra of N-(2-(2-hydroxy ethoxy) ethyl)-N,N-dimethyloctan-1-aminiumbromide (HEDOB) with signals at: δ = 0.79 (t,3H, CH3 alkyl chain);δ = 1.18 (m,10H, N⊕CH2CH2(CH2)5CH3); δ = 1.59 (m,2H, N⊕-CH2CH2(CH2)5CH3); δ = 2.73 (m,2H, N⊕CH2CH2(CH2)5CH3); δ = 3.06(m,6H, (CH3)2N⊕CH2CH2(CH2)5CH3); δ = 3.33 (t,2H, OCH2CH2N⊕-CH2CH2(CH2)5CH3); δ = 3.44 (t,2H, OCH2CH2N⊕CH2CH2(CH2)5CH3);δ = 3.51 (t,2H, HOCH2CH2OCH2CH2N⊕); δ = 3.77 (t,2H, t,2H,HOCH2CH2OCH2CH2N⊕) and δ = 4.69 (s,1H, t,2H,HOCH2CH2OCH2CH2N⊕).

3.2. Specific conductivity study

Altering the hydrophobic chain length of the synthesized cationicsurfactant containing free hydroxyl group and the raising solution tem-perature have a great effect on the specific conductivity. The recordedvalues of the degree of counter ion dissociation (α) in Table 1, were ob-tained through Frahm's method and equal to the ration between thepostmicellar to premicellar region slops. Data in Table 1 and Fig. 3showed that the specific conductivity is inversely proportional to thehydrophobicity. Increasing the hydrophobic chain length of the synthe-sized cationic surfactants is accompanied by decreasing the solutionspecific conductivity. This behavior ascribed to increasing both thetotal number of counter ions and the bond strength between the surfac-tant head and the counter ion. Increasing the hydrophobicity is followedby decreasing the hydration, hence the charge density around the mi-celle enhanced so the bond between the micelle and the counter ion isstrengthened (values of counter ion dissociation decrease). By increas-ing the hydrophobic chain length, the molecular weight increase, so

Fig. 4. The plots of specific conductivity against concentrations of the prepared cationicsurfactants (HEDHB) in distilled water at 25, 45 and 65 °C.

Fig. 3. The plots of specific conductivity against concentrations of the prepared cationicsurfactants in distalled water at 45 °C.

228 S.M. Shaban et al. / Journal of Molecular Liquids 221 (2016) 224–234

the number of counter ions decreases [39–43]. The degree of counterion dissociation values of the synthesized cationic surfactants HEDOB,HEDDB and HEDHB was 0.43, 0.41 and 0.40 at 45 °C respectively.

By inspection data in Table 1, and Fig. 4, it was found that the raisingthe solution temperature is directly proportional to the specific conduc-tivity of the synthesized cationic surfactant. For example, the degree ofcounter ion dissociation of the synthesized surfactant HEDHB at tem-perature 25, 45 and 65 °C was 0.38, 0.40 and 0.41 respectively. This be-havior ascribed to increasing the dissociation of the counter ion fromthe head of synthesized surfactant monomer or their micelle with rais-ing the temperature and this effect is dominant than the columbic at-traction force between the head and its counter ion [44–47].

3.3. Critical micelle concentration (CMC) and surface activity

The characteristic CMC of the synthesized cationic surfactant aque-ous solution at 25, 45 and 65 °C, was estimated by two differentmethods; surface tension and conductivity. The CMC corresponds tothe intersection point in the specific conductivity-concentration curveobtained from conductivity measurements as exemplified in Figs. 3and 4.

The abrupt change in the surface tension curves of the synthesizedcationic surfactant against their bulk concentration as exemplified inFigs. 5–7, correspond to the CMC. The measured CMC from the conduc-tivity method is slightly higher than that obtained from the surface ten-sion as depicted in Table 1, due to the premicellar region [48–50].

Increasing the hydrophobicity has a decreasing effect on the CMC asobvious in the recorded data (Table 1 & Fig. 8). Increasing the chainlength of the synthesized cationic surfactants, increase their affinity toaggregate in clusters, hence the micelle be formed at lower concentra-tions. The measured CMCs at 25 °C were 3.98, 1.72 and 0.83 mM/L forthe synthesized surfactants HEDOB, HEDDB and HEDHB respectively.

Increasing the solution hydrophobicity by increasing the length ofthe synthesized cationic surfactants destroys the water structure,hence the free energy of surfactant solution increase. The surfactantunimers aggregate together in the micelle form or migrate to the inter-face to avoid contactingwith the polarmedia, hence the free energy, de-crease so the affinity to form the micelle increase (CMC decrease) [51–54].

During the micellization process, the hydration of the hydrophilichead group increase compared to the surfactant unimers. This behaviorcan be distinguished in Figs. 3 and 4 through increasing the specific con-ductivity as discussed previously. When micelle starts to be formed, wenotice steady in the values of surface tension as shown in Figs. 5–7, dueto the micelle be formed in the bulk not surface.

Inspection data in Table 1, and Fig. 8, we reveal that, raising the so-lution temperature has a decreasing effect on the CMC; for example,the CMC of the synthesized surfactant HEDDB are 1.72, 1.06 and0.85 mM/L at 25, 45 and 65 °C respectively. This behavior is attributedto the decreasing the hydration around the hydrophilic head and dis-ruption the water structure around the hydrophobic tail. The magni-tude of the two opposing behaviors determines whether the CMCincrease or decrease over a particular range. As depicted in Table 1, itclarified that the predominant effect is decreasing the hydration aroundthe hydrophilic head group, hence CMC decreased [55–57].

The effectiveness (πCMC) is the difference in the surface tensionvalues of blank water (γo) and at critical micelle concentration (γCMC)of the synthesized cationic surfactants (Table 1). The effectiveness wascalculated using surface tension measurements through the followingequation:

πCMC ¼ γo−γCMC ð1Þ

On inspection the effectiveness values of the synthesized cationicsurfactant in Table 1, we reveal that the synthesized surfactant with hy-drophobic tail contains sixteen carbon atoms (HEDHB) is the most

effective surfactant at the three tested temperatures. The πCMC equal to35.9, 36.8 and 37.8 mN m−1 for the synthesized cationic surfactantsHEDOB, HEDDB andHEDHB at 25 °C respectively. The higher value of ef-fectiveness is indicative of the condensed nature of synthesized cationicsurfactants unimers at the aqueousmedium/air interface and the lowervalue refer to that the formed monolayer from the monomers is moreexpanded [58–59]. The surfactant HEDHB is the most effective onedue to it has the higher CMC/C20 as indicated in Table 1. The CMC/C20values of the synthesized surfactants HEDHB were 68.6, 127.6 and128.8 at 25, 45 and 65 °C, respectively.

The efficiency (C20) values were calculated from surface tensionmeasurements and recorded in Table 1. The C20 represents the concen-tration of the surfactant required to suppress the surface tension by20 dyne/cm. Inspecting the data in Table 1, we note that by elevatingthe solution temperature and increasing the hydrophobicity, the effi-ciency increase. Raising the aqueous solution temperature from 25 to65 °C, the hydration around the hydrophilic and hydrophobic decrease.From the behavior of the temperature on the CMC,we can conclude thatthe effect of decreasing the hydration around thehydrophilic head is theprevailing one, so the rate of migration of the surfactant unimers to sur-face increase so more reduction in surface tension. The efficiency in-crease by increasing the hydrophobic chain length as a result ofincreasing the hydrophobicity, therefore the adsorption at surfaceincrease.

Fig. 7. The relation between the surface tension and log concentration of prepared cationicsurfactant (HEDHB) at various temperatures.

Fig. 5. The relation between surface tension and log concentration of prepared cationicsurfactant (HEDOB) at various temperatures.

229S.M. Shaban et al. / Journal of Molecular Liquids 221 (2016) 224–234

The surface excess (Γmax) of the prepared cationic surfactantsexpressed as the concentration of the surfactant unimers at interfaceper unit area. The (Γmax) calculated from surface tension measurementsusing Gibb's adsorption equation (Eq. (2)) [60]:

Γ max ¼ 12:303 nRT

� �δ γ

δ logc

� �T

ð2Þ

Where R is the gas constant, n is the number of active species (nequal 2 for cationic surfactant with monovalent counter ion), (δ γ/δlog c) is the surface pressure (slope at the premicellar region of surfacetension-concentration curve) and T is the absolute temperature.

Maximum surface excess describes the accumulation of preparedcationic surfactants at the interface. Hence increasing the surface pres-sure gives an indication on the high accumulation of the surfactantmonomers at the interface.

Minimum average surface area is the average area (in square ang-strom) occupied by each cationic unimer adsorbed at the system inter-face. Amin values give information about the orientation of the preparedsurfactant at the interface [61]. The minimum surface area (Amin) calcu-lated from Gibb's adsorption equation (Eq. (3)):

Amin ¼ 1016

Γ maxNð3Þ

WhereN is Avogadro's number. Themathematically calculatedmax-imum surface excess and minimum surface area of the prepared

Fig. 6. The relation between surface tension and log concentration of prepared cationicsurfactant (HEDDB) at various temperatures.

cationic surfactant at 25, 45 and 65 °C based on surface tension mea-surements were recorded in Table 1.

By inspection the Γmax and Amin values in Table 1, we concluded thatthey are hydrophobic and temperature depended. Raising the solutiontemperature from 25 to 65 °C, the Γmax decreases. This means that theaccumulation of the surfactant unimers decreases at the air-water inter-face by raising the solution temperature. The Γmax of the synthesizedsurfactant HEDOB were 0.8 × 10−10, 0.67 × 10−10 and0.54 × 10−10 mol/cm2 at temperature 25, 45 and 65 °C respectively. In-creasing the hydrophobic tail of the synthesized surfactant, the accumu-lation at the air-water interface increase. The Γmax were found to equalto 0.8 × 10−10, 0.82 × 10−10 and 0.85 × 10−10 mol/cm2 at 25 °C forthe prepared HEDOB, HEDDB and HEDHB cationic surfactantrespectively.

The cluster formation arises from two opposing forces. An attractiveforce arises from the hydrophobic effect of the hydrophobic surfactanttail. The electrostatic repulsive force arises from the interaction betweenthe positively charged head groups. The hydrophobic effect favors theaggregation in micelle and closer packing by breaking down the highlydynamic hydrogen bond betweenwatermolecules; thus, the longer hy-drophobic tails will induce stronger hydrophobic attraction, whichforces the surfactants unimers to aggregates in larger micelle withhigh accumulation at the interfaces [62–64].

Fig. 8. Temperatures & hydrophobic chain length effect of synthesized cationic surfactanton critical micelle concentration values.

Table2

Micelliz

ationan

dad

sorption

thermod

ynam

icpa

rametersof

theprep

ared

cation

icsurfactants.

Comp.

Temp.

°CΔGomic

kJ/m

olΔHmic

kJ·m

ol−

1ΔS m

ic

kJ·m

ol−

1·K

−1

ΔGoad

s

kJ·m

ol−

1ΔHad

s

kJ·m

ol−

1ΔS a

ds

kJ·m

ol−

1·K

−1

HED

OB

25−

37.58

––

−42

.07

––

45−

41.47

20.49

0.19

5−

46.60

25.42

0.22

665

−46

.42

37.22

0.24

7−

52.40

45.63

0.29

0HED

DB

25−

41.39

––

−45

.87

––

45−

45.81

24.56

0.22

1−

50.96

29.97

0.25

465

−49

.22

8.38

0.17

−55

.20

16.62

0.21

2HED

HB

25−

44.60

––

−49

.04

––

45−

47.91

4.72

0.16

5−

53.20

12.88

0.20

865

−51

.63

11.41

0.18

6−

57.54

15.96

0.21

7

230 S.M. Shaban et al. / Journal of Molecular Liquids 221 (2016) 224–234

Theminimum surface area is related to the accumulation of the sur-factants unimers at the interfaces. As depicted in Table 1, the accumula-tion at the interface increase by increasing the hydrophobic effect andwith decreasing the solution temperature. The high accumulationforce the surfactant unimers to be dense packing and the surfactants or-dered vertically to the interface; hence the Amin decreases [42,63].

3.4. Thermodynamic aspects

The propensity of the synthesized cationic surfactant to adsorb at in-terfaces or aggregate inmicelles in bulk solution is a central to their var-ious useful properties. Hence, their behavior at three differenttemperatures 25, 45 and 65 °C, was studied thermodynamically usingthe phase separation model proposed by Zana [65]. The standard freeenergy of micellization (ΔGo

mic), the standard free energy of adsorption(ΔGo

ads), micellization entropy (ΔSmic), adsorption entropy (ΔSads), mi-cellization enthalpy (ΔHmic) and adsorption enthalpy (ΔHads) were cal-culated by following Eqs. (4)–(9):

ΔGomic ¼ 2−αð ÞRT In XCMCð Þ ð4Þ

ΔGoads ¼ ΔGo

mic−6:023� 10−2 πCMC=Γ max

� �ð5Þ

ΔSmic ¼ −d ΔGomic=ΔT

� � ð6Þ

ΔSads ¼ −d ΔGoads=ΔT

� � ð7Þ

ΔHmic ¼ ΔGomic þ TΔSmic ð8Þ

ΔHads ¼ ΔGoads þ TΔSads ð9Þ

Where α is degree of counter ion dissociation for the synthesizedcationic surfactants (conductivity measurements) and XCMC is the criti-cal micelle concentration in molar fraction (surface tensionmeasurements).

The calculated thermodynamic parameterswere depicted in Table 2.The data reveal that both the adsorption andmicellization processes arespontaneous as indicated by the negative values of change in free ener-gy of adsorption andmicellization. TheΔGo

mic of the synthesized cation-ic surfactants HEDOB, HEDDB and HEDHB was −37.58, −41.39 and−44.6 kJ/mol respectively at 25 °C. The ΔGo

ads of the HEDOB, HEDDBand HEDHB was −42.07, −45.87 and −49.04 kJ/mol respectively at25 °C. On Comparing between the both values of ΔGo

ads and ΔGomic for

the synthesized cationic surfactants, we conclude that ΔGoads is more

negative than that of micellizationΔGomic; which show the high adsorp-

tive propensity of the synthesized cationic surfactants at the air-waterinterface than aggregation in micelles (adsorption is energetically fa-vored than micellization). According to the previous results, we canconclude that the synthesized cationic surfactants tend to adsorb atair-water interface first until surface saturation then it aggregates inmi-celles in the solution bulk [55–57].

Inspection data in Table 2,we reveal that the change in free energy ofadsorption and micelliztion of synthesized cationic surfactants increasein the negative direction upon raising the solution temperature from 25to 65 °C. This indicates the higher stability of the adsorbed and aggregat-ed surfactant molecules than the dispersed surfactants unimers in thebulk solution. The ΔGo

mic of the synthesized surfactants HEDOB in-creased in the negative direction from −37.58 to −46.42 kJ/mol uponraising the solution temperature from 25 to 65 °C, while the ΔGo

ads in-creased in the negative direction from −42.07 to −52.4 kJ/mol.

The same trend has been observed upon increasing the hydrophobiccharacter of the synthesized surfactants through increasing the lengthof the surfactants tails from eight to sixteen carbon atoms. The ΔGo

mic

of the synthesized surfactants increased in the negative direction from−37.58 to −44.60 kJ/mol upon increasing the hydrophobic chain

231S.M. Shaban et al. / Journal of Molecular Liquids 221 (2016) 224–234

length from eight to sixteen carbon atoms at solution temperature 25 °C, while the ΔGo

ads increased in the negative direction from −42.07 to−49.04 kJ/mol at 25 °C.

Elevating the solution temperature causes a decrease of hydrationaround the hydrophilic group, so the hydrophobicity of the system in-crease and accompanied by increasing the energy of the system, somol-ecules of surfactant tend to adsorb and form micelle to decrease theenergy of the system [52–54]. Increasing the chain length of preparedcationic surfactants containing free hydroxyl group were accompaniedby increasing the hydrophobicity of the aqueous system; which leadto the destroying the water structure thus increasing the free energyof the system. Thus, the surfactant monomers migrate to surface or ag-gregate in clusters. Themigration to surface or aggregation in cluster de-creases the energy of the system, so the change in the free energy of theprepared surfactant-solvent system will be decreased and increased inthe negative direction. The chemical structure of synthesized cationicsurfactants containing free hydroxyl group is the main factor influenc-ing their thermodynamic aspects.

The recorded ΔSmic and ΔSads values in Table 2, are positive whichrefer to the ordering of the surfactant molecules when they aggregateinmicelles and/or adsorb at the air-water interface. The ordering of sur-factant molecules is evident to their compactness whether theyadsorbed at air-water interface or aggregates in micelle where the hy-drophilic tail coiled interior the micelle and the head faced to water.This ordering decreases the repulsion in the surfactant-aqueous phasesystem and leads to more stabilization for the formed micelles and/oradsorbed unimers at the air-water interface. The adsorption entropychange (ΔSads) is more positive than micellization entropy change(ΔSmic); which reflect the greater freedom of hydrophobic part throughmotion to the interface than micelle formation [58–59].

3.5. Foaming power of synthesized surfactants

In some applications like fire extinguishing and metal flotation sys-tems, the foam formation is very important. In other, the foam forma-tion is a very serious matter, which should be prevented like in jetsystem and high-pressure fluid. The foam can increase the pressure ofthose systems, causing an exploitation. Inspection data set in Table 3,we reveal that the prepared cationic surfactants with the free hydroxylgroup have a low foamheightwith very low stability except the synthe-sized surfactant HEDHB, which has foam stability, reach to 240 min.Hence, the synthesized surfactant with eight and twelve carbon atomscan be used in the various applications, which not require the foam for-mation affinity such as washing machine laundry and additives inoilfield applications as corrosion inhibitors and as a biocide for sulfatereducing bacteria [66].

3.6. Emulsification power

The emulsification affinity is an indicator of the synthesized cationicsurfactants ability of to locate at the boundaries between the differentphases. The emulsification efficiency was measured by the time re-quired for the separation of 9 mL of pure water from the emulsion sys-tem. Increasing the separation time, the stability of the formed emulsionincrease and vice versa.

Table 3Emulsion stability, interfacial tension, foam high and foam stability of the synthesized cationic

Surfactant Emulsion stability (min) Interfacial tension (

HEDOB 3 9HEDDB 4 7HEDHB 180 6

Inspection data in Table 3, it can be seen that increasing the emulsionstability depend on the chemical structure of the synthesized cationicsurfactants containing free hydroxyl group, where by increasing the hy-drophobic character upon increasing the chain length of the preparedsurfactant, the emulsion stability increase. These results matched withthe obtained Γmax in Table 1. By increasing the length of the hydrophobicchain length, the accumulation of surfactant unimers at the interfacesincrease, hence the interfacial tension decrease and the emulsion stabil-ity increase. The synthesized surfactants HEDOBandHEDDBexhibit lowemulsification power range from 3–4 min. According to this low emul-sification power, these two surfactants can be used in applications asdemulsifier, corrosion inhibitors and biocide for sulfate reducing bacte-ria in oilfield system in which any chemical used should have ademulsifier ability in order to facilities the oil production processes. Ac-cording the dataset in Table 3, the synthesized surfactant with chainlength equal to sixteen carbon atoms (HEDHB) has very high emulsionstability (180min.). Hence, the surfactants HEDHB can be used in sever-al applications as emulsifier including emulsion paints, textile industryand shampoos [67].

3.7. Interfacial tension

The measured interfacial tension of the synthesized cationic surfac-tants containing free hydroxyl group with light paraffin oil at 25 °Cwas depicted in Table 3. The dataset reveals that the synthesized cation-ic surfactant exhibit low interfacial tension values as an indication forthe high accumulation of the surfactants unimers at the water-paraffinoil interface. It was found that increasing the hydrophobic chain lengthof the synthesized surfactants followed by decreasing the interfacialtension value of water-paraffin oil system at 25 °C. These results are ingood agreements with the trend of maximum surface excess depictedin Table 1. The interfacial tension values of the synthesized surfactantHEDOB, HEDDB andHEDHBwere 9, 7 and 6mN/m at 25 °C respectively.The lower values of the interfacial tension reflect the ability of these sur-factants to be used as a biocides and corrosion inhibitors [67].

3.8. Antimicrobial activity

The antibiotic and antifungal effect of the synthesized cationic sur-factants containing free hydroxyl group were screened against patho-genic Gram-negative bacteria (Escherichia coli), Gram-positive(Staphylococcus aureus) bacteria and fungi (Aspergillus flavus and Candi-da albicans). The obtained datasets were depicted in Table 4. The syn-thesized three cationic surfactants with different hydrophobic tailshowed high potency action against the tested bacteria and fungi. In-spection data in Table 4, we reveal that the synthesized surfactantshave antimicrobial activity higher than the used drug references. In-spection dataset in Table 4, it is clear that the antimicrobial activity ofthe prepared cationic surfactants (HEDOB, HEDDB and HEDHB) de-pends on their chemical structures. The synthesized biocide character-ized by its amphipathic structure owing a unique behavior. It canadsorb on the cell wall membrane and aggregate in clusters. The killingaction of the biocide depends on the adsorption on the cell wall mem-brane. The synthesized biocide (surfactants) have different surface pa-rameters depending on their chemical structure; like CMC, maximumsurface excess, minimum surface area, the change in free energy of

surfactant at 25 °C.

mN/m) Foaming height (mL) Foam stability (min)

10 2.520 560 240

Table4

Thean

tibioticeffect

ofsynthe

sizedcation

icsurfactantsag

ains

tpa

thog

enicba

cteria

andfung

i.

Microorga

nism

Gram

reaction

Inhibition

zone

diam

eter

(mm/m

gsample)

Usedstan

dard

referenc

eRe

f.inhibition

zone

diam

eter

(mm/m

gsample)

HED

OB

HED

DB

HED

HB

Esch

erichiacoli

G−

1622

20Ampicillin

15Stap

hylococcus

aureus

G+

1522

17Ampicillin

13Aspergillu

sfla

vus

Fung

us13

1417

Amph

otericin

B14

Cand

idaalbicans

Fung

us13

1415

Amph

otericin

B16

232 S.M. Shaban et al. / Journal of Molecular Liquids 221 (2016) 224–234

micellization and change in free energy of adsorption; which discussedin details. Hence themagnitude of these parameters responsible for theactivity as antimicrobial.

Inspection the dataset in Table 4, we reveal that the antibiotic andantifungal effect against bacteria and fungi are hydrophobic dependent.In case of the tested bacteria, the antibiotic effect of the synthesized cat-ionic surfactants gradually increases with increasing the hydrophobicchain length of the surfactants until maximum effect at chain length oftwelve carbon atoms (HEDDB) then the activity decrease again. Thistrend is known by the cut-off effect [68–70]. The antibiotic effect ofthe synthesized biocides HEDOB, HEDDB andHEDHBagainst Escherichiacoli bacteria was 16, 22 and 20 mm/mg respectively.

Regarding to the tested fungi, the synthesized biocide showedprom-ising high antifungal activity which increases upon increasing the hy-drophobic chain of the biocide surfactant. The antifungal activity ofthe synthesized biocides HEDOB, HEDDB and HEDHB against Aspergillusflavus fungus was 13, 14 and 17 mm/mg respectively.

The cut-off effect depends on the surface parameters of the synthe-sized surfactants, these parameters are CMC, change in free energy ofadsorption, accumulation of the surfactants at the cell wall and thesize of diffused surfactant unimers [71–74].

Elongation the tail of the synthesized surfactant accompanied by adecreasing the CMC as depicted in Table 1, hence the biocide surfactantconcentration at the cell membrane became lower with higher chain.According to this hypothesis, the antimicrobial activity should be or-dered as follows HEDOB ˃HEDDB ˃HEDHB. The change in the free ener-gy of adsorption increase with increasing the hydrophobic chain lengthas indicated in Table 2, hence the rate of migration to the cell wall inter-face increase. According this parameter, the activity can be ordered asfollows HEDHB ˃ HEDDB ˃ HEDOB. Another hypothesis related to a de-crease in the perturbation of the membrane at higher alkyl tail, assum-ing that the longer the chain, the better mimic molecule in the lipidlayer, leading to disruption in the membrane. As depicted in Table 1,the maximum accumulation of the synthesized biocide surfactants in-crease with increasing the hydrophobic chain length, hence the activitypredicted to increase. Therefore, the magnitude of all the previous de-termines the behavior of the antibiotic. From the obtained dataset inTable 4, we can conclude that the synthesized surfactants HEDDB havethe optimum antibiotic effect, while the synthesized biocide HEDHBhas the maximum antifungal effect against the tested microorganism.

Sulfate Reducing Bacteria (SRB) is one of the problemswhich obsta-cle the petroleum industry. The SRB bacteria produce H2S, which causesouring in the petroleum well and corrosion for the equipment besideits hazard effect on the environment. Therefore, it is necessary to miti-gate the SRB bacteria to control the microbial induced corrosion(MIC). The synthesized cationic surfactants were evaluated against Sul-fate reducing bacteria using serial dilution method and the obtained re-sults were depicted in Table 5. The contaminated water was suppliedfrom Petro Dara Petroleum Co. (Eastern Desert, Egypt), and subject togrowth 107 cells/mL. The synthesized cationic surfactants were evaluat-ed at concentration 2.5 × 10−3, 1 × 10−3, 5 × 10−4 and 1 × 10−4 M asan antibiotic against sulfate reducing bacteria. The prepared biocideshowed high potent action against SRB. The synthesized biocideHEDDB has themaximumantibiotic effect against SRB. At concentration5 × 10−4 M from the tested biocide HEDOB, HEDDB and HEDHB, thegrowth of SRB were 104, nil and 102 cells/mL respectively.

The bacterial cell membrane comprised from thick wall containingmany layers of peptidoglycan and teichoic acids as exemplified in Fig.9 [75,76]. The positively charged surfactant adsorbed on lipoteichonicacid layer of the Gram-positive bacteria and on the lipid layer of theGram-negative bacteria. Therefore, the expectedmechanism of the syn-thesized cationic surfactants depends on their affinity to adsorb on thecell wall membrane through the interaction between the positivelyhead group and lipoteichonic acid layer and lipid layer in Gram-positiveand Gram-negative bacteria respectively. The adsorption on the cellwall membrane, facilitate the penetration of the hydrophobic tail and

Table 5Biocidal effect of the prepared compounds against sulfate reducing bacteria, SRB.

Biocide

Conc.

2.5 ∗ 10−3

(M)1 ∗ 10−3

(M)5 ∗ 10−4

(M)1 ∗ 10−4

(M)

Blank 107

HEDOB 102 103 104 105

HEDDB Nil Nil Nil 103

HEDHB Nil 102 102 103

233S.M. Shaban et al. / Journal of Molecular Liquids 221 (2016) 224–234

the counter ion and hence disturbing the selective permeability of themembrane leading to the cell death [77–81].

3.8.1. Surface activity — antimicrobial activity relationshipFrom the obtained dataset in both Tables 3 & 4, it is clear that the

chemical structures of the synthesized cationic surfactants containingfree hydroxyl group are responsible for the antibiotic and antifungal ac-tivity. By increasing the hydrophobic chain length, the antimicrobial ac-tivity increase as cleared in Tables 3 & 4. The tested biocides are cationicsurfactants, which characterized by their amphipathic structure con-taining hydrophilic head and a hydrophobic tail. This unique structureresponsible for the surfactants surface parameters including the adsorp-tion andmicellization. The antimicrobial activity of the biocide dependson their adsorption on cell wall membrane then penetration inside theliving cell and disturbing the selective permeability of the membrane.As depicted in Tables 1 & 2, increasing the hydrophobic chain length isaccompanied by increasing the adsorption at the interface (ΔGo

ads)and the accumulation at the interface increase (Γmax), these two

Fig. 9. The structure of the bac

parameters enhance the population of the surfactants unimers at thecell wall membrane and adsorption through the positively charged sur-factant head group. The surfactant adsorption at cell wall permit thepenetration of the surfactant tail inside the cell beside the counter ionand the free hydroxyl group which bind to ribosomal protein. Hence,the selective permeability of the cell wall membrane destroyed, causingcell death [82].

4. Conclusion

Three newly cationic surfactant containing free hydroxyl groupwereprepared by a simple and direct method. The chemical structures wereconfirmed using proton nuclear magnetic resonance and Fourier trans-form infrared spectroscopy. The surface parameters of the synthesizedcationic surfactants are chemical structure dependent. The CMC obtain-ed from both surface tension and conductivity measurements were ingood agreement. The thermodynamic aspect showed the propensityof the prepared surfactants to adsorb at the air-water interface then

terial cell wall membrane.

234 S.M. Shaban et al. / Journal of Molecular Liquids 221 (2016) 224–234

aggregation in micelle in bulk solution. The synthesized surfactantsHEDOB and HEDDB showed low foaming stability, low interfacial ten-sion and low emulsion stability. Therefore can be used as additives (cor-rosion inhibitors and biocides) in the oilfield system. While thesurfactant HEDHB showed high emulsification power, therefore it maybe used in emulsion paints, textile industry and shampoos. The synthe-sized surfactants showed antibiotic and antifungal activity compared tothe used references drugs. The synthesized HEDDB surfactant showedhighly biocidal activity against the sulfate reducing bacteria present inthe petroleum oilfield.

Acknowledgements

This work was financially supported by Egyptian Petroleum Re-search Institute.

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