КОЛЛОИДНЫЙ ЖУРНАЛ, 2008, том 70, № 1, с. 112-115
УДК 541.182
ADSORPTION AND ELECTROCHEMICAL STUDIES ON SURFACTANT MODIFIED CELLULOSE ACETATE MEMBRANES
© 2008 Kehar Singh, A. K. Tiwari, Chandra Shekhar Dwivedi
Department of Chemistry, D.D.U. Gorakhpur University Gorakhpur-273009, India Поступила в редакцию 01.08.2006 г.
Possibility of modification of cellulose acetate membrane upon adsorption of ionic surfactants, cetyl pyridini-um chloride and sodium dodecyl sulphate has been investigated on the basis of membrane potential studies. For the estimation of extent of adsorption, surface tension measurements were carried out in the presence and absence of cellulose acetate membrane using surfactants solutions of variable composition. The results have also been used for the characterization of thermodynamic equilibrium which the membrane surfactant system attains upon surfactant adsorption, in terms of immobilization constant.
INTRODUCTION
Electrochemical modification of membranes is of considerable theoretical and practical interest [1, 2]. Inherent tendency of surfactants to undergo immobilization on membrane surface can be exploited with advantage to manipulate the permselective behaviour of membranes [3, 4]. In this study, possibility of modification of cellulose acetate, a model membrane system, upon equilibration with cetyl pyridinium chloride (CPC) and sodium dodecyl sulphate (SDS) has been investigated on the basis of surface tension and membrane potential measurements. Cellulose acetate is practically uncharged and exhibits slight cation selectivity because of dissociation of its gluconic groups [57]. Upon surfactant adsorption, the membrane matrix may acquire charge and the resulting membrane modification is expected to depend on the nature and concentration of the surfactant solution used. Surface tension measurements have been carried out with and without surfactants to ascertain the extent of surfactant adsorption and its variation with surfactant concentration. These results have been used to obtain the so called "membrane immobilization constant", a characteristic parameter which describes equilibrium of the given surfactant with the membrane. The results have also been used to obtain adsorption and desorption rate constants for the systems under investigation.
Membrane potential measurements using cellulose acetate membrane and sodium chloride solutions of unequal concentration with and without surfactants have also been carried out to obtain transport numbers of ions in the membrane phase for the determination of membrane permselectivity. The results clearly show that adsorption of the surfactants results in electrochemical modification of the cellulose acetate membrane and this varies with surfactant concentration and approaches saturation at surfactant concentrations
comparable to that of the critical micelle concentrations (CMC) of the surfactants.
MATERIALS AND METHODS
SDS and CPC (CDH, India) were used as such without further purification. Cellulose acetate membrane was prepared using procedure described in our earlier studies [3, 4]. Membrane surface area is 4.52 cm2. Maximum pore size available in pore size curve ranges from 45 x 10-6 cm to 49 x 10-6 cm and its pore fraction is found in the curve to be 0.35.
Surface tension measurements were carried out using Tensiomat Model 21 (Fisher Scientific Instruments, USA). Surface tension values are accurate within ±0.1 dyne/cm.
For adsorption studies a known mass of the membrane was equilibrated with the surfactant solution of desired concentration. The surfactant concentration changed due to adsorption and this change was estimated on the basis of change in the measured surface tension.
For membrane potential measurements, a piece of the prepared membrane was fixed in a glass cell and equilibrated with 1 M solution of the desired electrolyte. Thereafter, it was kept in the experimental solutions overnight for equilibration. Solutions were renewed before measurements were started. The set-up used for the membrane potential measurements may be represented as shown in Fig. 1. Membrane potentials developed across the membrane were measured using a digital multimeter (Philips, PM 2518). The maximum limit of the error in membrane potential measurement is ±0.5 mV. The ratio between the sum of electrical resistances of the saturated calomel electrodes and salt bridges, and multimeter input impedance is 6.2 x 10-6.
RESULTS AND DISCUSSION
Surface tension data obtained using 0.05 M NaCl solutions having different concentrations of CPC and SDS with and without cellulose acetate membrane are presented in Table 1. The changes in surface tension in the presence of the membrane arise on account of adsorption of the surfactants by the membrane. The number of moles of the surfactant adsorbed, n, were estimated using the relationship [8]
n =
ACV
m
(1)
or
n = KA(1 - 0)CY,
(2)
where (1 - 0) represents fraction of the membrane surface which remains uncovered and is available for further surfactant immobilization, C denotes surfactant concentration and Y may be called order of adsorption. We call KA the adsorption rate constant which is characteristic of the surfactant-membrane system. It is obviously that
0 =
n
nS
(3)
where ns is the number of adsorbed surfactant molecules at saturation when CMC is reached. From equation (2) it follows that
log
1-0
= log K A+ Y log C.
(4)
Validity of the equation (4) is tested in Fig. 1 for cellulose acetate/SDS and cellulose acetate/CPC systems. The values of KA and Y are included in Table 2. In both cases Y = 1 which conforms to unimolecular adsorption of the surfactants.
SCE
Electrolyte (C2)
Electrolyte (Cj)
Surfactant (C) ^ Surfactant (C)
SCE
SB
M
SB
where AC = C0 - C1, C0 is the molar concentration of the surfactant before adsorption, C1 is the molar concentration of the surfactant at equilibrium, V is the volume of the solution in liter, m is the mass of the membrane.
It may be noted that CMC of the surfactants and their concentrations beyond which further surfactant adsorption does not occur, are comparable, consistent with the observation that surface saturation is expected at CMC of the surfactants [9]. This gives credence to the belief that adsorption occurs through surfactant monomers only. Adsorption of surfactant micelles does not take place.
The number of surfactant molecules adsorbed at any concentration before micellization commences may be written as
^ - 4( 1-0)CY dt dt
Fig. 1. The scheme of the set-up used for the membrane potential measurements. SB, M and SCE represent KCl salt bridge, membrane and saturated calomel electrode respectively.
When the membrane is kept in the surfactant solution, only monomeric surfactant species undergo immobilization to establish equilibrium provided the concentration of the surfactant does not exceed CMC. This equilibrium between the surfactant molecules in solution and those immobilized at the interface as a mono-
Table 1. Surface tension and surfactant adsorption values
SDS-CA System
Surface tension, dyne/cm 0.05 M NaCl + Surfactant concentration, mM n
Without membrane With membrane Without membrane With membrane |imole/g
67.5 73.0 0.30 0.10 110
62.5 71.0 0.40 0.14 192
57.5 68.5 0.82 0.80 260
47.5 58.0 1.00 0.89 317
42.0 47.5 1.50 1.00 332
40.7 42.0 3.00 2.50 340
34.0 36.0 5.50 5.00 352
33.3 35.0 8.00 7.50 352
33.0 34.0 10.50 10.00 352
CPC-CA System
Surface tension, dyne/cm 0.05 M NaCl + Surfactant concentration x 104, mM n x 10 | mole/g
Without membrane With membrane Without membrane With membrane
63.2 65.3 5.0 3.12 1.49
59.3 62.1 25.0 13.29 9.34
53.8 55.4 50.0 34.84 12.09
51.0 52.1 100.0 78.01 17.54
47.5 50.6 200.0 99.34 80.31
47.0 48.5 250.0 150.19 79.63
114
SINGH et al.
Table 2. Thermodynamic parameters for cellulose acetate-SDS and cellulose acetate-CPC systems with 0.05M NaCl
Parameters Sodium dodecyl sulphate Cetyl pyridinium chloride
CMC 7.5 mM 0.03 mM
nmax 34.1 dyne/cm 21.75 dyne/cm
œ 7.038 x 108 cm2/mole 19.84 x 108 cm2/mole
AG0 -14.47 kJ/mole -30.13 kJ/mole
K 3.40 x 102 18.89 x 104
KA 4.0 x 103 7.5
Kd 12 4.0 x 10-3
s nm 357 |imole/g 7.46 |imole/g
Y 1.0 0.958
layer can be characterized using thermodynamic considerations [10, 12]
Chemical potential of free monomeric surfactant may be expressed as
| = |0 + RTlnC, (5)
where T is the absolute temperature, R is the gas constant.
In the case of immobilized monolayer of the surfactant, the chemical potential is
|s = |0s + no), (6)
where | 0 and | 0s represent standard chemical potential in the solution and the membrane surface, respectively. The surface pressure, is defined as
n = Y h2o — Y. (7)
where yh O and y respectively denote surface tension
of water and the surfactant solution. Surface area per mole of the surfactant is given by [11]
RTd ln C
œ = —
dy
(8)
log[n/(1 - 9)]
2 1 0 -1
-2
log C [mM]
Fig. 2. Test of validity of Eq. (4) for different systems: 1
CPC/CA, 0.05 M NaCl, 2 - SDS/CA, 0.05 M NaCl.
At critical micelle concentration of the surfactant,
| = |0 + tf71nCMC (9)
and
Is = |0s + nmaxœ, (10)
where
nmax = ( Y h2o- Y cmc ). (11)
Furthermore, when the surfactant immobilization results in the establishment of equilibrium,
I = Is. (12)
From equations (9), (10) and (12) it follows that
0s 0 max
L-JL + n—° = lnCMC. RT RT
The standard free energy change,
AG0 = |0s - |0.
From equations (13) and (14) it follows that
AG0 = RTlnCMC - nmaxo.
(13)
(14)
(15)
CMC values estimated on the basis of surface tension measurements alongwith nmax and o> values for the cellulose acetate/SDS and cellulose acetate/CPC systems are summarized in Table 2 alongwith AG0 values.
Now,
AG0 = RTln K.
The equilibrium constants K characteristic of the equilibria established upon immobilization of the surfactants estimated using the above equation are also included in Table 2. It may be noted that
K =
-K----A--
-K----
Для дальнейшего прочтения статьи необходимо приобрести полный текст. Статьи высылаются в формате PDF на указанную при оплате почту. Время доставки составляет менее 10 минут. Стоимость одной статьи — 150 рублей.