A Review of Inverse Gas Chromatography and its Development ...
Inverse Gas Chromatography infinite dilution for the ...The inverse gas chromatography (IGC) is an...
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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
240 Mohamed Hassen V Baouab et al., J. Tun. Chem. Soc., 2017, 19, 237-249
(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 .
Mohamed Hassen V Baouab et al., J. Tun. Chem. Soc., 2017, 19, 237-249 241
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
242 Mohamed Hassen V Baouab et al., J. Tun. Chem. Soc., 2017, 19, 237-249
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
Mohamed Hassen V Baouab et al., J. Tun. Chem. Soc., 2017, 19, 237-249 243
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
244 Mohamed Hassen V Baouab et al., J. Tun. Chem. Soc., 2017, 19, 237-249
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
Mohamed Hassen V Baouab et al., J. Tun. Chem. Soc., 2017, 19, 237-249 245
%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.
246 Mohamed Hassen V Baouab et al., J. Tun. Chem. Soc., 2017, 19, 237-249
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.
Mohamed Hassen V Baouab et al., J. Tun. Chem. Soc., 2017, 19, 237-249 247
%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
248 Mohamed Hassen V Baouab et al., J. Tun. Chem. Soc., 2017, 19, 237-249
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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