Inverse Gas Chromatography infinite dilution for the ...The inverse gas chromatography (IGC) is an analysis technique of the solids surface. It is based on the physical adsorption

Journal of the Tunisian Chemical Society, 2017, 19, 237-249 237

* Corresponding author: Institut Préparatoire aux Etudes d’Ingénieurs, Avenue de l’environnement, 5000 Monastir - Tunisia

e-mail address : [email protected] Tel.:+216.98.22.82.10 Fax: +216.73.50.05.12

Inverse Gas Chromatography infinite dilution

for the determination of the dispersive surface free energy

and acid–base interactions of cationized cotton fibers

Mohamed Hassen V Baouab*, Aroussi Chaabane

Laboratoire de microélectronique & instrumentation, Faculté des Sciences de Monastir,

Université de Monastir, Tunisia

(Received: 07 November 2016, accepted: 11 September 2017)

Abstract: Inverse gas chromatography (IGC) was used to characterize Epoxypropyltrimethylammonium chloride

grafted cotton fibers (EPTMAC-Cotton) for ions exchanger biomaterial applications. Using IGC at infinite dilution,

we have investigated the influence of %N (0-2) content and temperature on the dispersive component of surface

energy ( ) and the variation of the cationized cotton acid-base behavior was determined. The treatment by

Epoxypropyltrimethylammonium chloride was fund to reduce values in 0 to 0.5 %N range and beyond 1% N, the

values varies very little and approaches the polypropylene (PP) one. As generally observed, values decrease when

the temperature increases in the (29-60)°C range. The free energy of adsorption, the enthalpy of adsorption and the entropy of adsorption of CHCl3 as acid probe and THF as basic probe are evaluated. Ka and Kb numbers describing the

acid-base behavior of the stationary phase were calculated. At low %N, EPTMAC treatment enhances the acidic

behavior of cotton fibers. Moreover, a particular higher basicity is fund at %N=0.5 value. At high %N, (-CH3) groups

of EPTMAC decreases the acidity of cotton by steric hindrance and approves neutral behavior to cationzed fibers.

Key words: Cotton; inverse gas chromatography; dispersive component of surface energy, acid-base behavior.

S

D

DS

S

D

INTRODUCTION The inverse gas chromatography (IGC) is an

analysis technique of the solids surface. It is based

on the physical adsorption of well known probes

by the samples solid surface [1,2]. This method is qualified as a reverse because in the case of IGC,

we try to analyze the stationary phase, Contrary to

conventional chromatographic techniques the IGC is qualified as a reverse technique since the column

is typically packed with the solid sample under

investigation and a single gas vapor is injected into the column [3]. Thus, the fundamental physical

phenomenon on which the IGC repose is the

reversible adsorption of gas molecules on the

surface of the solid phase. The thermodynamic phenomenon accompanied reverse adsorption can

be exploited in various ways to acquire a lot of

information on the material of the stationary phase [3-7]. This method is simple, fast, results are

reproducible and reliable and well suited to the finely divided powders such as cellulosic fibers

[8-15]. The physicochemical surface properties of

cellulose and lingnocellulosic materials are of

major importance in the context of the production of composites in textile area, waste water treatment

and, particularly, in the production of composites

with polymeric matrices [16-28]. These properties can be evaluated by using IGC. At infinite dilution

conditions of appropriate gas probes, IGC may

provide important parameters including the dispersive component of the surface energy of the

material under analysis, thermodynamic data on

the adsorption of polar probes and acid-base

interaction parameters among the matrix and the stationary phase [28]. This article outlines some of

the important basic concepts and applications for

IGC infinite dilution technique in the determination of the dispersive surface free energy

238 Mohamed Hassen V Baouab et al., J. Tun. Chem. Soc., 2017, 19, 237-249

and acid-base interactions of Epoxypropyltri-methylammonium chloride grafted cotton fibers

(EPTMAC-Cotton) by the use of adequate probes.

MATERIALS AND METHODS 1. Theoretical background

Adsorbed molecule on a solid surface is subjected

to various intermolecular forces, electrostatic in nature. They are classified usually into two

categories: i) Dispersion forces or no polar,

existing between any two molecules and not including permanent dipole: they are the result of

the instantaneous polarization induced by dipole 1

onto dipole 2. ii) The non-dispersive or polar

forces regrouping orientation effects (Keesom effect) between two permanent dipoles and the

induction effects (Debye effect) resulting from the

induction of a dipole by a permanent dipole such as the hydrogen bonds and acid-base interactions.

It is therefore convenient to express the surface

tension of a solid as the algebraic sum of two components: first one attributed to dispersive

forces and the second one to polar forces. Thus:

(2)

D and P indexes referring to the dispersive and

polar interactions respectively.

Conder and Young [29] defined retention time (tR) as the basic quantities in IGC. tR, probe gas is the

difference time between the injection of the

gaseous mixture at the inlet of the column and the

detection output of the solute marked at the top of the column. A fraction of this time named retention

time (tM) is required for the simple crossing of a

gaseous probe not retained in the column and assumed not establish any interaction with the

stationary phase. Methane (CH4) probe is generally

used to evaluate tM. In practice, we reflect in terms of retention volume

which corresponds to the volume of carrier gas

required to elute each of the constituents:

VR = DC.tR and VM = DC.tM (3)

Where DC the carrier gas flow and VM is the empty volume or dead volume of the column. Therefore,

the real retention volume of a gas molecule is

given by:

V = VR – VM = DC.(tR – tM) (4)

P

S

D

SS

On the other hand, maintaining a constant flow rate of carrier gas can’t be done only if exist pressure

drop between the inlet and the outlet of the

column: it follows that the gas volume flow is not

constant over the crossing of the column. j, factor introduced by James and Martin [30] to take

account of this phenomenon and the net retention

volume (VN) is defined by:

VN = j.V = j.DC.(tR – tM) (5)

Where j is compressibility factor of the carrier gas

defined as:

(6)

Where Pe pressure at the inlet of the column and Ps

pressure at the outlet of the column. Typically the carrier gas flow is measured by a bubble flow

meter at room temperature; DC the carrier gas flow

is defined as:

(7)

Where TC column temperature, Ta ambient

temperature, and are viscosities of gas at

TC and Ta respectively. DC and Da carrier gas flow at TC and Ta respectively. It is generally assumed

that , which gives:

(8)

We finally define a net volume of specific

retention ( ), normalized to the temperature and

expressed per unit mass of the stationary phase:

=

= (9)

1

1

2

33

2

Ps

Pe

P

Pe

jS

c

a

T

T

a

C

aCT

TDD

..

CT aT

aC TT

a

C

aCT

TDD .

0

gV

Vg

o

m

V

T

N

c

.273

Vg

o).(

..

273MR

a

a

ttm

Dj

T

Mohamed Hassen V Baouab et al., J. Tun. Chem. Soc., 2017, 19, 237-249 239

Where: m mass of the stationary phase. It remains to connect this parameter retention to thermo-

dynamic quantities of interest.

The work at infinite dilution of solute (in the mobile

phase and the stationary phase) allows to neglect: i) interactions between solute molecules to the

(adsorbent/adsorbate) interactions, ii) solute diffusion

phenomena in the solid phase, it is assumed depends only of adsorption phenomena on the surface of the

stationary phase. Partition coefficient (KS) of the

solute between the phases is equal to the ratio of the concentrations of the solute in the stationary phase

and the mobile phase [31, 32].

(10)

Where q the solute concentration in the stationary phase and c concentration of solute in the mobile

phase. is the fraction of time spent by a solute molecule in the mobile phase; F can then be

expressed as:

(11)

In addition, F is equal at equilibrium to the fraction

of the total number of solute molecules in the

mobile phase, Vs being the volume of the stationary phase, hence:

(12)

By adding the time spent by solute molecules in

each phase along the entire length of the column,

can be considered: i) when solute molecule is in

the gas phase, a total volume equal to ( ) of

carrier gas must pass through the column to elute.

ii) When solute molecule is in the mobile phase and that it interacts with the stationary phase, the

volume of gas required for elution is equal to

( ) , where:

(13)

Combining (12) and (13), one obtains:

Kq

cS

F

sM

M

VqVc

VcF

..

.0

0

ssM

M

VKV

VF

.0

0

0

MV

0

RV

o

R

o

M

V

VF

(14)

VS represents in IGC infinite dilution the entire

(gas/solid) interface:

(15)

With Sspé the surface area of the stationary phase

and m the mass of the stationary phase hence

(16)

A number of thermodynamic quantities can be

deducted in calculating (Ks) and its dependence on temperature. The standard free energy change in

the adsorption isotherm of one mole of adsorbate

from a gas standard state to the standard state solid

surface is given by De Boer [33]:

(17)

With Psg vapor pressure of the adsorbate to the reference gas state, Pss Vapor pressure at the

equilibrium in the standard adsorption state, R

ideal gas constant and T temperature

(corresponding to the temperature of the column). We can join this expression to Ks at the

equilibrium, if we expressed Ks as:

(18)

With: Γ is the surface of the solute concentration in

the solid phase and c concentration of solute in the

gas phase [31, 34-36]. Considering the ideal gas at low pressures:

or (19)

With Π is the bi-dimensional spreading factor,

considering the adsorbed gas as a thin film. If the

adsorbed gas is in the standard state, while Π = Πs

hence .

By substituting this equation in the (17) equation, we obtain:

SSNSS

o

M

o

R VKVVKVV ..

mSV sps .

sp

N

SSm

VK

.

sg

sso

P

PTRG ln..

Kcs

P

TRK

TR

Pc s

.

. RT

KPs

s

ss


(20)

Either by taking (16)

(21)

The second term is a constant

for a given column at a given temperature. Hence the fundamental equation of IGC:

(22)

Two standard states have been defined: i) One by De Boer [33] which considers that the adsorbed

molecules are far apart from each other by a

distance equal on average to that between the molecules of an ideal gas in the reference state

defined by .

ii) The other defined by Ries and Kimball [37]

who considers that a gaseous molecule adsorbed on the surface (A) of the solid has a volume equal

to (A.e) to the reference state defined by P° = 1

atm at the temperature T. e is the thickness of the layer of solute (taken equal to 6 Å).

1.1. Case of nonpolar probes (n-alkanes)

Nonpolar probe develop only dispersive

interactions with the stationary phase. Gibbs free energy in the isotherm adsorption process of a

mole of gas molecules is connected to the adhesion

energy (Wa) between the solid and the probe [34]

ΔG° = -a.N.Wa (23)

Where Wa represents the work that must be done per unit area to separate the contact linking gas

probe and the stationary solid, a area of the probe

and N Avogadro's number. From Fowkes [38] if one of the compounds at least is nonpolar,

interactions will only dispersive nature and the

adhesion energy is reduced to:

s

ssso PKLnTRG

...

s

P

mS

VRTLnG

sg

spé

No .

sps

sgg

S

PLnTRLnVTR

.....

sps

sg

S

PLnTR

...

te

g

o CLnVTRG ..

atmP

CT

1

0

0

. (24)

Where: the subscripts s and L refer to the

stationary phase (the studied fibers in this case) and to the probe molecule, respectively. By

combining (22), (23) and (24) expression is

obtained [39]

(25)

Knowing a and γL to a series of alkenes, draw of

runs to obtain a straight

line of slope , thereby allowing the

calculation of . Note that the values used γL are

those for probes in the liquid state.

1.2. Case of polar probes

When injected polar probes, specific interactions are added to the dispersive ones. Schultz and

Lavielle [39] shows that Gibbs free energy

associated to the adsorption of gas molecules may be considered as the sum of the dispersive ΔGdisp

and specific ΔGsp components:

ΔG = ΔGdisp + ΔGsp (26)

By performing the plot of ,

the representative point of a probe having a polar

character acid and/or base will be located above the reference line that is the same representation

for n-alkanes probe having the same γL. Thus:

(27)

For polar probes ΔGsp = ΔG A-B , where ΔG A-B is the free energy of Gibbs change due to the acid-

base interaction. To determine the acid-base

character of a solid with IGC technique, we

generally use the concept of Guttmann [40], which allows taking into account the amphoteric

character of the solid. The free energy of

adsorption ΔGsp corresponding to the specific acid-base interactions is related to the enthalpy of

adsorption ΔH sp by:

(28)

D

L

D

s

D

aa WW 2

te

Lsg CNaLnVTR ....2..

).(.. D

Lg afLnVTR

D

SN ..2

S

D

D

Lg afLnVTR ..

réf

g

gréf

gg

sp

V

VRTLnRTLnVRTLnVG

spspsp STHG .


Where ΔSsp is the entropy of adsorption corresponding to the specific acid-base

interactions. A plot of ΔGsp versus T (temperature)

should yield straight line with intercept equal to

ΔHsp. The enthalpy of adsorption corresponding to the specific acid-base interaction is related to the

acceptor and donor parameters, Ka and Kb of the

fibers. According to Saint-Flour and Papirer [4],

(29)

Where DN and AN are the donor and acceptor

numbers, respectively, of the acid-base probes as defined by Guttmann [40]. A plot of –ΔHsp / AN

versus DN / AN should yield a straight line with

slop Ka and intercept Kb.

2.2. Materials All reagents [dimethylformamide (DMF), tri-n-

propylamine (TPA), EPTMAC and nitric acid] and used probes [n-hexane (C6), n-heptane (C7),

n-octane (C8), n-nonane (C9), n-decane (C10)

diethyl ether (Et2O), tetrahydrofurane (THF),

chloroforme (CHCl3), dichloromethane (CH2Cl2), ethyl acetate (AcEthyl) and acetone] were supplied

by Aldrich (Sigma-Aldrich Chimie Sarl, Saint-

Quentin Fallavier, France) in analyses grade and used without further purification. Table I provides

the data necessary to the different probes used for

subsequent calculations. Tunisian Cotton fibers

ANKDNKH ba

sp ..

(99 % cellulose) were supplied by SITEX (Société Internationale de Textile, Monastir, Tunisia) and

purified by a mild alkaline scouring (2% NaOH

and 0.2% of a wetting agent) for 5 h at 110°C in an

Ahiba Nuance® laboratory machine using a liquor ratio of 1:10 (w/v), followed by washing with

distilled water and drying in air at room

temperature. Four samples of EPTMAC–Cotton (I–IV), with differing ammonium group content

(%N) depending on the reaction time are prepared,

analyzed and characterized as previously described [16]. Chemical structure of EPTMAC–Cotton is

presented in Fig.1. A blank (%N=0) is always

performed with the same chemical treatment as the

other samples except exposure to EPTMAC. The nitrogen content analysis of the final products is

shown in Table II.

2. IGC infinite dilution operating conditions

Samples of Cotton fibers are dried in an oven with

air circulation at 60 °C for 48 hours, and then they are reduced to fine fibrils with a coffee mill type

(MOULINEX®). Cotton fibrils (1.5 g) are

subsequently packed into stainless-steel columns

of 1m in length and 4.7mm internal diameter, previously cleaned with an aqueous solution of

nitric acid 15% and rinsed with acetone. Once

filled, the columns are conditioned overnight at 120 °C under helium flow (flow rates 25-30

cm3.min-1) to desorb any trace of water or solvent

and volatile impurities. The apparatus used was a

DELSI Di700® instrument equipped with a flame-ionization detector and integrating recorder DELSI

ENICA 21®. The temperature of the column was

controlled within ±0.1°C. A series of n-alkanes were injected at infinite dilution (0.1μL) to

determine at 29, 40 and 60°C. Polar probes

were also injected in order to have access to acidic and basic character of the studied samples. A small

amount of methane is injected with probe. This

light n-alkane hardly interacts with the stationary phase and get out of the column almost instantly.

The detection of methane gives the beginning of

time and corrects the offsets due to the dead volumes.

RESULTS AND DISCUSSION

1. Dispersive component of the surface energy Fig. 2 shows as an example the plot of RTLnVg

versus for studied stationary phases at 40°C. Calculation of the dispersion component of surface

energy requires the retention time of three (or

S

D

D

La

Fig.1. Chemical structure of EPTMAC–Cotton


Probe Area (Å2) (mJ/m2) d

L DN (kcal/mol) AN (kcal/mol) Character

n-hexane 51.5 18.4 - - Neutral

n-heptane 57 20.3 - - Neutral

n-octane 62.8 21.3 - - Neutral

n-nonane 68.9 22.7 - - Neutral

n-decane 75 23.4 - - Neutral

CHCl3 44 25.9 - 23.1 Acidic

AcEthyl 48 16.5 17.1 9.3 Amphoteric

Et2O 47 15 19.2 3.9 Basic

CH2Cl2 31.5 27.6 - 20.4 Acidic

THF 45 22.5 20.1 8 Basic

Acetone 42.5 16.5 17 12.5 Amphoteric

Table I: Physicochemical proprieties of the IGC probes [39,40]

above) n-alkane probes [14]. During the IGC experiments on I and II-EPTMAC-cotton, the

retention times of four probes (n-hexane,

n-heptane, n-octane and n-nonane) and (n-heptane,

n-octane, n-nonane and n-decane) were respec-tively obtained at 29, 40 and 60°C and three probes

(n-heptane, n-octane and n-nonane) were obtained

at 29, 40 and 60°C for blank, III and IV-EPTMAC-cotton. It was hypothesized that the peaks for the

other n-alkane probes took too long to elute at

these measurement temperatures (over 60 min which is maximum value provided by the

equipment). The linear relationship vs. n-alkanes

chain length illustrate that this technique works

well in case of cotton fibers and we can notice that

we had rather good results to calculate

because the retention times were reproducible and

the chromatographic peak shape of each probe had to be as symmetrical as possible.

Table II summarize values at the studied range

of temperatures and for different %N of EPTMAC-cotton obtained in this work. For blank (%N = 0),

values (51.4 mJ/m2 at 29°C, 44.45 mJ/m2 at

40°C and 37 mJ/m2 at 60°C) were different to that found in the literature (50.4 mJ/m2 at 20°C, 36.8

mJ/m2 at 40°C and 20.6 mJ/m2 at 60°C) [13], it is

difficult to make comparisons since the fiber source is not the same. Indeed, although cotton

cellulose has a well-defined molecular structure,

D

s

D

s

D

s

the fact this material is difficult to obtain in pure chemical form (i.e., without surface contaminants)

should greatly account for some variety of the

reported IGC results. The difference in values can be also attributed to the chemically treatment,

it has been proposed that besides the chemical

composition of the cellulose surface, during the

chemically treatment other factors such as modifications of crystallinity, diffusion of solvents

into the bulk volume and final surface morphology

D

s

Fig.2. Pot of

for studied stationary phases at 40°C. )()(..

DsafgVLnTR


also play an important role in the interaction between the probes and the cellulose and thus

influence the IGC data [9, 41, 42]. Generally and

as described in the literature, the dispersive

component of the surface energy ( ) the

majority of the reported values fall within the

range of 40-50 mJ.m-2, although other values have

been published. For instance at 40°C, cotton

cellulose was reported to have a values of 50

mJ.m-2 [34], purified hardwood -cellulose showed

a value of 47.4 mJ.m-2 [5, 8], and cellulose

powder presented a of 48 mJ.m-2 [43]. If %N

maintained constant, of studied stationary

phases showed a negative temperature coefficient

over this entire range due to chemical rearrangements. The temperature coefficients of

dispersion component of surface energy

( ) for blank and (I to IV)-EPTMAC-cotton are summarized in Table II.

The dispersive component of surface energy of

studied systems are rather sensitive to temperature

with ( ) of (-0.4527 to -0.5037) mJ.m-2. °C-1,

which is in the usual range of lignocellulosic material [(-0.07 to -0.5) mJ.m-2.°C-1] [6, 9, 13]. At

constant temperature, and as shown in fig 3,

values decrease when %N increase in (0-1) range,

beyond 1 %N, the values varies slightly and

at 40°C approaches the polypropylene (PP) values

(31.7 mJ/m2 at 25°C and 32.8 mJ/m2 at 60°C) as measured by IGC [10,24]. These results indicate

that graft of EPTMAC onto cotton take place on

the surface of the fibers and at high %N, EPTMAC

-cotton has a behavior toward PP which is comparable to a hydrophobic polyolefin [10] and

D

s

D

s

D

sD

sD

s

dTd D

s /

dTd D

s /

D

s

D

s

Table II: Characteristics of samples, dispersive component of surface free energy ( ) and the temperature

dependence of surface energy ( ) of the studied materials.

d

s

dTd d

s

Materials (mJ/m2) d

s

( )

(mJ.m-2.°C-1)

dTd d

s

R2 Sample Reaction

time (min)

%N

29°C

40°C

60°C

Blank 0 0 51.4 44.45 37 -0.4533 0.9988

I-EPTMAC-Cotton 30 0.5 46.26 40 32 -0.4527 0.9909

II-EPTMAC-Cotton 60 1 38 32.5 22.75 -0.4914 1

III-EPTMAC-Cotton 90 1.5 38.46 33.2 23 -0.5001 0.9997

IV-EPTMAC-Cotton 120 2 38.3 32.85 22.7 -0.5037 0.9997

this suggests that other mechanisms such as acid-base interactions may be responsible for the

observed improvement [40]. This subject is

discussed in more details in the following section.

2. The surface free energy

Using Schultz’s methode [39], Fig. 4 shows as an

example the plot of RTLnVg vs. for

I-EPTMAC-Cotton at 29°C for used polar probes, which enable the detrermination of –ΔGsp. The

results for the other studied systems are presented

in Table III. We can notice that in some cases, the

polar probes are too much retained by stationary phases and can’t be detected output of the column,

a sign of strong interactions with this type of

chemically modified fiber at the studied temperature range. Regarding the acid-base

properties of cellulose, it has been found by IGC

D

La

Fig.3. Plot of for studied

stationary phases at different temperatures.

)(%NfD

s


that the cellulose surface has an amphoteric behavior [43-46]. In addition, it is predominantly

acidic rather than basic. These conclusions have

been drawn from the determination of –ΔGsp with

several polar probes, being the greatest vales obtained with amphoteric probes, such as acetone.

Moreover, when comparing the –ΔGsp values

obtained with THF basic probe and CHCl3 acid probe, these are higher with the basic probes,

indicating a more acidic than basic behavior of the

cellulose surfaces. Fig. 5 shows for I-EPTMAC-cotton, that the basic

character, as indicated by the –ΔGsp value of

CHCl3, increase slightly when some OH of

Fig.4. Determination of –ΔGsp by Schultz’s method

for I-EPTMAC-Cotton at 29°C. Fig. 5. Plot of –ΔGsp = f(%N) for CHCl3 and THF

at different temperatures.

Fig.6. Proposal schem of adsorption of acid and basic probes onto: 6a. Blank Cotton (%N = 0);

6b. I and II-EPTMAC-Cotton (%N = 0.5 and 1); 6c. III and IV-EPTMAC-Cotton (%N = 1.5 and 2).

cellulose of cotton are substituted. At this low %N, grafted EPTMAC moieties are sufficiently distant

and the disposition of the OH groups creates basic

cavities that attract strongly acidic probes (Fig.

6.a). For II, III and IV-EPTMAC-cotton, and at constant temperature, basicities of chemically

modified fibers are nearly regular. This effect, due

to the interaction with the electron pairs oxygen atoms, is maximum with the lower %N of

stationary phase. Higher %N seems to disfavor this

interaction by steric hindrance (Fig. 6.b). The acidic character, measured by the –ΔGsp values for

THF, highly decreases among 0 and 1 %N. At low

%N (0 - 0.5) range, access of acid probe through


%N

Polar probe

–ΔGsp (J/mol)

29°C 40°C 60°C

0 CH2Cl2 1317 1467 1707

CHCl3 1256 1126 810

Et2O 5513.5 3954 1780

Acetone 8643 7771 5910

THF 6204 6012 5703

AcEthyl - - -

0.5 CH2Cl2 1760 2138 2690

CHCl3 1560 2015 2700

Et2O 4420 3774 2590

Acetone 7830 8249 8832

THF 5620 5074 4229

AcEthyl 5999 5243 4309

1 CH2Cl2 1200 1050 830

CHCl3 1493 1323 1069

Et2O - - -

Acetone 4780 - -

THF 2371 1955 1337

AcEthyl 3251 2107 230

1.5 CH2Cl2 1120 970 500

CHCl3 1380 1100 636

Et2O - - -

Acetone - - -

THF 2200 2005 1670

AcEthyl - - -

2 CH2Cl2 1305 - -

CHCl3 1400 1045 500

Et2O - - -

Acetone 3892 1764 -

THF 2636 2020 1080

AcEthyl 2812 1833 800

Table III: Free Energy of adsorption, ΔGsp, of the polar

probes at different studied temperatures. grafted ammonium groups was stericaly permitted and maximum interactions are performed.

Contrary, for II, III and IV-EPTMAC-cotton,

highly grafted (–CH3) groups act as a mask for the

numerous ammonium groups present at the fiber surface (Fig. 6c). This result explains the obtained

values in this %N range.

Table IV summarizes the calculated ratio ΔGsp CHCl3 / ΔGsp THF for the different studied

systems. The evolution of the ratio from blank to

I-EPTMAC-cotton, corresponds to an increases in surface basicity (due to the interaction with the

electron pairs of oxygen atoms of hydroxyl groups)

and simultaneously, to a small decrease in surface acidity (due to grafted ammonium groups). For II,

III and IV-EPTMAC-cotton, and at constant

temperature, the studied ratio was regular due to

steric hindrance of (–CH3) groups that prevents acidic and basic probes to interact with OH of

stationary phases. 3. Thermodynamic and acid-base evaluation By Plotting the values of –ΔGsp / T against 1/T, the

adsorption enthalpy, –ΔHsp, and the adsorption entropy, –ΔSsp, of blank and EPTMAC-cotton

samples were determined for polar probes and the

results were given in Table V. Fig. 7 shows plots of –ΔHsp vs. %N for CHCl3 and THF, the two

studied polar probes provides endothermic heat of

adsorption onto blank cotton with –ΔHsp (THF) <

–ΔHsp (CHCl3) indicating a prevalence of acidic character. The particular decrease in the heat of

adsorption of CHCl3 and simultaneously the

increase of the heat of adsorption of THF onto I-EPTMAC-cotton confirm the amplification of

basic character and the reduction of acid character of

cotton in the %N (0 - 0.5) range. In the %N (1 - 2) range endothermic heat adsorption of acidic and

basic probes are quite similar indicating a modest

Lewis acid-base character of (II, III and IV-

EPTMAC-cotton) samples. In Fig. 8 showing a

D

s

%N (meq/g)

T(°C) 0 0.5 1 1.5 2

)(

)( 3

THFG

CHClGsp

sp

29 0.20 0.28 0.63 0.63 0.55

40 0.19 0.40 0.68 0.55 0.52

60 0.14 0.64 0.80 0.38 0.46

Table IV: [ΔGsp (CHCl3) / ΔGsp (THF)] vs %N for the studied materials at different temperatures.


plots of –ΔSsp vs. %N, negative value of entropy of adsorption of CHCl3 indicating an attract of the

acid probe and positive value for THF a repulse for

the basic probe and confirming the basic scale of I-

EPTMAC-cotton. For II, III and IV-EPTMAC-cotton, as for enthalpy, the entropy of adsorption

varies humble with %N. The acidic-basic

characters Ka and Kb were determined (Table V) using the AN (electron acceptor) (without

dimension) and DN (donor numbers) (kJ/mol)

numbers, and the plot of –ΔHsp / AN vs. DN / AN. Typically, the Ka values are higher than the Kb

ones. This may confirm a prevalence of the acidic

behavior of cellulose [39]. Ka value of blank cotton

(Ka= 0.3013) was comparable to the Ka value of cellulose (Ka= 0.31) [40] indicating a surface

initially rich in hydroxyl groups and Kb value of

blank cotton (0.073) was 1/3 lower than cellulose ones (Kb= 0.24) due to the chemically treatment.

Indeed, a great number of studies concerned the

chemically treatments of cellulose and lingnocellulosics and its effect on the surface

properties of these materials has been realized [47].

The surface modification of cellulose with reagent

has been carried out and the most important finding obtained by IGC analyses was the great

modifications of the Lewis acid-base character of

the cellulose surface after these surface treatments [3, 48]. Fig. 9 which is a plots of Ka and Kb vs %N,

reveal that: i) partial cationization of cotton surface

increases the basic behavior and decreases the acidic behavior of I-EPTMAC-cotton. ii) for II, III

and IV-EPTMAC-cotton, Ka values are among 1/3

and 1/2 of blank one and Kb values are bit more

than the blank one. The decrease of Ka values and low Kb values of II-III and IV-EPTMAC-cotton

indicates that hydroxyl groups still accessible to

the polar probes are much less numerous and high cationized cotton surface gradually enriched by

(–CH3) groups that decrease the cotton acidity by

steric hindrance and transforms cotton from amphoteric or slightly acidic (blank) to neutral

behavior (II, III and IV-EPTMAC-cotton). Indeed,

Ka and Kb values of IV-EPTMAC-cotton are

comparable to the neutral PP ones (Ka = 0.01 and Kb = 0) [47]. The Ka/Kb ratios (Table V) indicate

that for blank cotton there is a larger number of

highest-energy acidic sites relative to that of highest-energy basic sites. This ratios decrease

gradually from 4.28 to 1.1 when the %N increases

from 0 for blank to 2 for IV-EPTMAC-cotton. This

was interpreted as being due to a decrease of the accessibility on the fibers surface of the OH

functional groups responsible of the Lewis acid-

base interactions by (-CH3) steric hindrance. Grafted [N(CH3)3

+,Cl-] groups, act as neutral

Fig.7. Polts of ΔH = f(%N) for CHCl3 and THF onto

studied stationnairy phases.

Fig.8. Polts of ΔS = f(%N) for CHCl3 and THF onto

studied stationnary phases. Fig.9. Plots of Ka and Kb vs %N for studied

stationnary phases.


%N Polar probe

).( 1

molkJ

H sp

)..( 11

molKJ

S sp

aK

bK

ba KK /

0 CH2Cl2 25.665 -14.56

0.30

0.07

4.28

CHCl3 24.56 12.51

Et2O 41.356 -119.01

Acetone 35.488 -88.755

THF 11.053 -16.075

AcEthyl - -

0.5 CH2Cl2 -7.192 29.71

0.25

0.23

1.08

CHCl3 -9.432 36.46

Et2O 22.255 -59.05

Acetone -1.788 31.94

THF 19.053 -44.55

AcEthyl 22.111 -53.56

1 CH2Cl2 4.20 -19.5

0.13

0.08

1.63

CHCl3 5.58 -13.56

Et2O - -

Acetone - -

THF 12.334 -33.06

AcEthyl 32.519 97.01

1.5 CH2Cl2 10.20 -13.76

0.12

0.10

1.2

CHCl3 8.592 -23.90

Et2O - -

Acetone - -

THF 7.347 -17.05

AcEthyl - -

2 CH2Cl2 - -

0.1

0.09

1.1

CHCl3 7.347 -28.8

Et2O - -

Acetone - -

THF 7.651 -19.8

AcEthyl 21.816 -63.29

Table V: Enthalpy, Entropy, Ka and Kb values of studied materials.

moieties. Indeed, the increases of the acidic ammonium groups don’t affect the Lewis acid-base

character of the cationized cotton surface.

CONCLUSION Inverse gas chromatography (IGC) at infinite

dilution has proven to be a versatile, powerful,

sensitive and relatively fast technique for measurement of surface energy and acid-base

characteristics of cationized cotton. At constant

temperature, grafting EPTMAC onto cotton fibers

is accompanied by a decrease in the value of .

This reduction is highly dependent on %N and is

very significant among 0 and 0.5 %N. Temperature increasing is followed by a significant decrease in

the value at low %N and at high %N,

approaches the PP value at 40°C and EPTMAC-cotton behavior toward polyolefin. According to

values of –ΔGsp, ΔHsp, ΔSsp , Ka and Kb, (–CH3)

groups acts as a mask and decrease the acidity of

D

s

D

sD

s


blank cotton (%N = 0) by steric hindrance. Low cationized cotton (%N = 0.5) approves a basic

behavior and high cationized cotton approves

neutral behavior (%N = 1; 1.5; 2).

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Inverse Gas Chromatography infinite dilution for the ...The inverse gas chromatography (IGC) is an analysis technique of the solids surface. It is based on the physical adsorption