Langmuir 1997, 13, 4505-4511
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Articles Polysaccharide-Surfactant Interaction. 1. Adsorption of Cationic Surfactants at the Cellulose-Water Interface S. C. Biswas and D. K. Chattoraj* Department of Food Technology and Biochemical Engineering, Jadavpur University, Calcutta-700 032, India Received September 17, 1996. In Final Form: April 14, 1997X The extent of adsorption (Γ21) of cetyltrimethylammonium bromide (CTAB), myristyltrimethylammonium bromide (MTAB), and dodecyltrimethylammonium bromide (DTAB) from aqueous solution onto a cellulosewater interface has been measured analytically in a wide range of surfactant concentrations below and above the critical micelle concentration (cmc) at different physicochemical conditions and in the presence of different electrolytes and urea. Γ21 is found to increase with increase of bulk surfactant concentration C2 until it reaches a maximum value Γ2m when C2 reaches a critical value, C2m. With further increase of C2 beyond C2m, Γ21 decreases from Γ2m and becomes zero with attainment of surface azeotropic state at a surfactant concentration C2azeo. For C2 > C2azeo, values of Γ21 are negative due to the excess hydration of cellulose fibril and desorption of surfactant micelles from the surface to the bulk phase. The value of Γ2m depends upon the different physicochemical conditions and presence of different electrolytes and urea. Values of C2m lie considerably below the cmc in most cases. Γ2m decreases with decrease of hydrocarbon chain length of surfactant molecules, and in the case of DTAB all values of Γ21 are negative. The results also predict involvement of hydrophobic interaction in the adsorption process. The standard free energy change ∆G° for the transfer of surfactant molecules to 1 kg of cellulose at the state of surface saturation has been calculated using an integrated form of the Gibbs adsorption equation. The values of ∆G° follow the same order as those of Γ2m. The average slope of the linear plot of ∆G° vs Γ2m is equal to -34.3 ( 0.1 kJ/mol. This corresponds to the standard free energy change (∆GB°) for the transfer of 1 mol of surfactant from the bulk solution to the cellulose surface when bulk mole fraction of surfactant is altered from zero to unity. The values of ∆Ghi° for different systems at high surfactant concentration (>C2azeo) have been also calculated using a linear extrapolation method, and they are found to be positive in all cases due to excess positive hydration of cellulose.
Introduction Recently, extensive studies have been made on the interaction of cationic and anionic surfactants with natural and synthetic polymers.1-5 Such studies are found to be of importance from fundamental and technological standpoints. In the last two decades, thermodynamic and kinetic aspects of binding of cationic and anionic surfactants to proteins5-8 and nucleic acids9 have been extensively investigated using various experimental techniques. Compared to this, the study of interaction of ionic surfactants with different types of polysaccharides has been undertaken in depth only in recent years with many interesting results. The physicochemical principles involved in cellulosesurfactant interactions are known for a long time to be X
Abstract published in Advance ACS Abstracts, July 1, 1997.
(1) Robb, I. D. Polymer/Surfactant Interaction. In Anionic Surfactants-Physical Chemistry of Surfactant Action; Reynders, E. L., Ed.; Surfactant Science Series; Marcel Dekker: New York, 1981; Vol. 11, Chapter 3. (2) Hayakwa, K.; Kwak, J. C. T. J. Phys. Chem. 1982, 86, 3866. (3) Kresheck, G. C.; Hargraves, W. A. J. Colloid Interface Sci. 1981, 83, 1. (4) Almgren, M.; Hansson, P.; Mukhtar, E.; Stam, J. V. Langmuir 1992, 8, 2405. (5) Goddard, E. D., Anathapadmanabhan, K. P., Eds. Interactions of Surfactants with polymer and proteins; CRC Press: Boca Raton, FL, 1993. (6) Das, M.; Chattoraj, D. K. Colloids Surf. 1991, 61, 1. (7) Sadhukhan, B. K.; Chattoraj, D. K. In Surfactants in Solutions; Mittal, K. L., Lindman, B., Eds.; Plenum Press: New York, 1984; Vol. 3, p 1249. (8) Chattoraj, D. K.; Birdi, K. S. Adsorption at interfaces and Gibbs Surface Excess; Plenum Press: New York, 1984. (9) Chatterjee, R.; Chattoraj, D. K. Biopolymers 1979, 18, 147.
S0743-7463(96)00905-5 CCC: $14.00
closely associated with detergent action and laundry cleaning of oiled fabrics.10-12 By use of equilibrium dialysis and other techniques, the extent of binding of surfactants to soluble cellulose derivatives has been investigated under different physicochemical conditions.13-16 Goddard et al.17,18 in their reviews have extensively covered the techniques and principles used for the study of interaction of surfactants with cellulose derivatives. Lindman and co-workers19-22 and others23 have studied the interaction of ionic surfactants with hydrophobically modified cellulose and other derivatives using different physicochemical conditions. Interactions of surfactants with cellulose (10) Cutter, W. G.; Kissa, E. Detergency Theory and Technology; Marcel Dekker: New York, 1987. (11) Schwartz, A. M. In Surface and Colloid Science; Matiieuer, Ed.; Wiley: New York, 1972; Vol. 5, p 195. (12) Lim, J.; Miller, C. A. In Surfactants in Solutions; Mittal, K. L., Shah, D. O., Eds.; Plenum Press: New York and London, 1991; Vol. 12, p 491. (13) Jones, M. N. J. Colloid Interface Sci. 1967, 23, 36. (14) Shirahama, K. J. Colloid Polym. Sci. 1974, 252, 978. (15) Hayakawa, K.; Kwak, J. C. T. J. Phys. Chem. 1983, 87, 506. (16) Obhu, K.; Hiraishi, O.; Kashiwa, J. J. Am. Oil Chem. Soc. 1982, 59, 108. (17) Goddard, E. D. Colloids Surfs. 1986, 19, 255. (18) Goddard, E. D. In Surfactants in Solution; Mittal, K. L., Shah, D. O., Eds.; Plenum Press: New York and London; 1990; Vol. 11. (19) Thuresso, K.; Nystrom, B.; Wang, G.; Lindman, B. Langmuir 1995, 11, 3730. (20) Nystrom, B.; Lindman, B. Macromolecules 1995, 28, 967. (21) Zhang, K.; Jonstromen, M.; Lindman, B. J. Phys. Chem. 1994, 98, 2459. (22) Piculell, L.; Lindman, B. Adv. Colloid Interface Sci. 1992, 41, 149. (23) Bloor, D. M.; Mwakibete, H. K. O.; Wyn-Jones, E. J. Colloid Interface Sci. 1996, 178, 334.
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have been studied by few workers recently.24-26 Giles and Arshid27,28 from the adsorption of a series of chemically related solutes onto cellulose and chitin have reported that cellulose is hydrophobic in nature. In our present work, we have studied the adsorption of cationic surfactants of varied chain lengths onto cellulose at different physicochemical conditions and in the presence of different neutral electrolytes and urea. The role of hydration over the binding interaction of surfactants has been critically examined. The changes of free energy due to the binding of surfactants to cellulose under different physicochemical conditions have been evaluated using an integrated form of the Gibbs adsorption equation. The nature of surfactant-cellulose interaction has also been analyzed from thermodynamic considerations. Experimental Section Materials. Highly pure cellulose (TLC-Grade) was obtained from CSIR Centre for Biochemicals, New Delhi. The cationic surfactants cetyltrimethylammonium bromide (CTAB), myristyltrimethylammonium bromide (MTAB), and dodecyltrimethylammonium bromide (DTAB) were obtained from Nakarai Chemicals Ltd. (Guaranteed Reagent), Japan, and TCI (GR), Japan, respectively. The purity and critical micelle concentration (cmc) of the surfactants have been determined by measurement of surface tension of solutions as a function of surfactant concentration. The values of cmc for CTAB, MTAB, and DTAB were found to be 0.89, 3.4, and 13.4 mM, respectively, which agreed with those reported earlier.7,9,29 Other common electrolytes, acids, and urea used were of analytical grade. Urea was recrystallized from warm alcohol before use. Double distilled water was used all throughout the experimental work. Before use, cellulose was dehydrated completely in a vacuum desiccator containing concentrated sulfuric acid for 7 days, and then the dried cellulose was kept in a desiccator containing anhydrous CaCl2. Stock quantities of phosphate buffer solution of desired pH 6.0 and 8.0 and acetate buffer of pH 4.0 were used in these experiments. The ionic strength of the solution was maintained by direct addition of calculated amount of NaCl. Adsorption Experiment. In the adsorption experiments, a fixed amount, W (equal to 2 × 10-4 kg), of dried cellulose was taken in different standard joint stoppered round bottom conical flasks (capacity 100 mL). Then a fixed volume, V (equal to 20 mL), of solutions of different molar concentrations (C2t) was taken into each flask. The flasks were then sealed and shaken gently on a horizontal shaker for 24 h at constant temperature. The flasks were taken away from the shaker and kept undisturbed for another 24 h. The temperature of the systems was kept constant by an air thermostat ((0.1 °C accuracy). After attainment of equilibrium, the supernatant in the bottle was found by spectroscopic examination to be free from suspended particles after centrifugation at 7000 rpm. The concentration of the surfactant (C2) in the supernate was estimated by the dye partition technique.30,31 Here 1 mL of surfactant solution (after proper dilution) was taken in a set of standard joint stoppered test tubes (capacity 60 mL) and to it 1 mL of dye solution (disulphine blue of concentration 200 mg/L in 0.02 N H2SO4) was added. After 10 min, 10 mL of chloroform was added into each test tube. The test tubes were then stoppered tightly and shaken (24) Martin, K; Helsten; E.; Klingborg, A. W. J. Am. Oil Chem. Soc. 1989, 166, 1381. (25) Jukiewicz, K.; Janust, W.; Spraycha, R.; Stczypa, J. In Surfactants in Solution; Mittal, K. L., Ed.; Plenum Press: New York and London, 1989; Vol. 9, p 371. (26) Sobisch, C. Tenside, Surfactants, Deterg. 1992, 29 (3), 199. (27) Arshid, F. M.; Giles, C. H.; Melure, E. C.; Ogilicic, A.; Rose, T. J. J. Chem. Soc. 1953, 67. (28) Arshid, F. M.; Giles, C. H.; Jain, S. K. J. Chem. Soc. 1956, 859. (29) Samanta, A.; Chattoraj, D. K. In Properties of Surfactants in Solution; Mittal, K. L., Bothorel, J., Eds.; Plenum Press: New York, 1986. (30) Mukherjee, P. Anal. Chem. 1956, 28, 870. (31) Biswas, H. K.; Mondal, B. M. Anal. Chem. 1972, 44, 1636.
Biswas and Chattoraj vigorously for 10 min and then kept undisturbed for another 45 min at 30 °C in a thermostatic chamber. The organic layer was withdrawn by a syringe and absorbance was read in a spectrophotometer (Hitachi U 2000) at 625 nm. The concentration of surfactant (C2) was calculated using a standard curve obtained from the measurement of the absorbance of the surfactant solutions of known concentrations. The moles of surfactant adsorbed per kilogram of cellulose (Γ21) at a bulk concentration C2 can be calculated using relation (1)
Γ21 )
Vt (C t - C2) 1000 2
(1)
where Vt stands for volume of the surfactant solution in mL per kg of cellulose and is equal to V/W. The limit of standard deviation in the measurement of Γ21 was found to be less than 6% calculated for ten sets, and the errors are shown in Figure 1.
Results and Discussion Cellulose is a polysaccharide made up of monomeric glucose residues forming a linear polymer chain. It is fibrous, tough, and water insoluble unbranched homopolysaccharide of 10 000 or more glucose units connected by 1-4glycosidic bonds. These 1-4 bonds are in β configuration as a result of which the D-glucose chain assumes an extended conformation and undergoes side by side aggregation into insoluble fibrils. Linear chains of cellulose in the fibrils are held together by cross links of a large number of hydrogen bonds.32,33 Each glucose residue of the linear polymer may involve interchain hydrogen bond formation, but few hydroxyl groups may combine with external water molecules as a result of polymer hydration. The surface of cellulose powder is significantly hydrophilic in nature. Recently Banerjee and Chattoraj34 have shown from the isopiestic vapor pressure experiments that two and three molecules of water are adsorbed per glucose residue of the polysaccharide chain of cellulose at 30 and 37 °C, respectively, at unit water activity. In Figures 1 and 2, the extent of adsorption of CTAB from the aqueous solution to the surface of 1 kg of cellulose (Γ21) has been plotted against equilibrium concentration (C2) of surfactant in the bulk liquid medium. All isotherms are type IV in shape according to the classification given by Schay.35 An interesting feature of all these adsorption isotherms is that Γ21 increases with increase of C2 from zero until the adsorption reaches maximum positive value Γ2m at a critical concentration C2m. In most cases, C2m is less than bulk cmc of CTAB. The value of Γ21 in all cases sharply decreases from Γ2m until it becomes zero. On further increase of C2, Γ21 becomes negative. According to eq 1, Γ21 depends on the difference between C2t and C2 both estimated for bulk solution before and after adsorption. The difference C2t - C2 may result from preferential accumulation of surfactant at the cellulose interface or from its desorption from the surface depending on the value of C2. The value of C2t - C2 may also depend on the relative state of preferential hydration or dehydration of dry cellulose in the presence of bulk surfactant solution which may result on the swelling or deswelling of the polyscccharide depending on the value of C2. As a (32) Lehninger, A. L. Principles of Biochemistry, 2nd ed.; CBS: Delhi, 1987; p 277. (33) Dey, P. M.; Brinson, K. In Advances in Carbohydrate Chemistry and Biochemistry; Tipson, R. S.; Horton, D., Eds.; Academic Press, Inc.: New York, London, 1984; Vol. 42, p 294. (34) Banerjee, P.; Chattoraj, D. K. J. Indian Chem. Soc. 1993, 70, 1. (35) Schay, G. Surface and Colloid Science; Matiyeoic, E., Ed.; Wiley Interscience: New York, 1969; Vol. 2, p 155.
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Γ21 ) n2t - n1t
n2 n1
(2)
Here n1t and n2t are total moles of solvent and solute components in the whole system per kilogram of dry cellulose powder and n1 and n2 are their values in free bulk solution at adsorption equilibrium. M1 stands for the molecular weight of water. From eq 2, one can derive eq 3 for the moles of water (Γ12) adsorbed in excess per kilogram of polysaccharide.36
Γ12 ) n1t - n2t ) -Γ21
Figure 1. Plot of Γ21 vs C2 for adsorption of CTAB at cellulosewater interface at pH ) 6.0, µ ) 0.15: A, 24 °C; B, 30 °C; C, 37 °C.
n1 n2
n1 n2
(3)
We thus find that Γ21 and Γ12 are not independent of each other but they are relative excesses which are earlier defined as the Gibbs excess quantities.37 Let us now imagine that ∆n1 and ∆n2 moles of solvent and solute respectively are present per kilogram of dry cellulose at the inhomogenous interfacial phase which are in contact with the homogenous bulk phase containing n1 and n2 moles of solvent and solute, respectively, we can then write
n1t ) n1 + ∆n1 ) n1′ + (n1′′ + ∆n1)
(4)
and
n2t ) n2 + ∆n2 ) n2′ + (n2′′ + ∆n2)
(5)
Here n1′′ and n2′′ are moles of solvent and solute components belonging to the bulk phase adjacent to the interfacial inhomogenous region and n1′ and n2′ are the moles of these components present far away from the interface. Combining eqs 2-5
(
Γ21 ) ∆n2 - ∆n1
) (
)
n2 n2 + n2′′ - n1′′ + n1 n1
(
n2′ - n1′
Figure 2. Plot of Γ21 vs C2 for adsorption of CTAB at the cellulose-water interface in the presence of neutral salts and urea at pH ) 6.0, T ) 30 °C: A, 2 M LiCl; B, 2 M NaCl; C, 2 M KCl; D, 0.667 M Na2SO4; E, 6.0 M urea.
result of combined effects of all these, C2t may be greater or less than C2 so that Γ21 may become positive or negative at a given value of C2. Since the surfactant solution in contact with surface is dilute, one can assume that its molarity (C2) and molality (m2) in the bulk solution are same. Also for such a solution Wt is equal to Vt, where Wt is the weight of the solvent in grams per kilogram of cellulose. Replacing C2t, C2, and Wt terms by 1000n2t/M1n1t, 1000n2/M1n1, and n1tM1, respectively, one finds from eq 18
) ∆n2 - ∆n1
n2 n1
)
n2 n1
(6)
since throughout the bulk phase solute concentration is uniform8,37 so that
n2 n2′ n2′′ X2 C2 ) = ) ) n1 n1′ n1′′ X1 55.5
(7)
Here X1 and X2 are mole fractions of solvent and solute components, respectively. (36) Nag, A.; Sadhukhan, B.; Chattoraj, D. K.Colloids Surf. 1987, 116, 168. (37) Chattoraj, D. K. Indian J. Chem. 1981, 20A, 941.
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Table 1. Values of C2azeo, ∆n1, and ∆n2 for Adsorption of Cationic Surfactants at the Cellulose-Water Interface
surfactants
temp (°C)
pH
CTAB CTAB CTAB CTAB CTAB CTAB CTAB MTAB DTAB CTAB 2 M LiCl CTAB 2 M NaCl CTAB 2 M KCl CTAB 0.667 M Na2SO4 CTAB 6 M urea
24 30 37 30 30 24 24 30 30 37 37 37 37 37
6.0 6.0 6.0 6.0 6.0 4.0 8.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0
ionic strength (µ) 0.15 0.15 0.15 0.05 0.10 0.15 0.15 0.15 0.15 2.05 2.05 2.05 2.05
103C2azeo (M)
10-2∆n1 (mol of H2O/kg of cellulose)
∆n1 (mol of H2O/mol of glucose residue)
102 ∆n2 (mol of surfactant/kg of cellulose)
102 ∆n2 (mol of surfactant/mol of glucose residue)
5.47 4.14 8.08 5.40 3.09 3.68 4.98 7.92 6.29 17.4 4.52 9.15 13.1
88.6 67.1 131 87.5 50.0 59.6 80.7 128 102 282 73.2 148 212
1.42 1.67 1.42 2.91 1.50 1.37 1.19 0.70 0.000 4.07 1.27 2.41 1.18
0.230 0.285 0.230 0.471 0.243 0.222 0.193 0.113 0.000 0.659 0.206 0.390 0.191
1.45 2.25 1.00 3.05 2.65 2.05 1.25 0.50 1.53 1.53 1.43 0.425
On combination of eqs 6 and 7, relation 8 will be obtained
Γ21 ) ∆n2 - ∆n1
= ∆n2 - ∆n1
X2 X1 C2 55.5
(8)
Similarly it can be shown that
Γ12 ) ∆n1 - ∆n2 = ∆n1 - ∆n2
X1 X2 55.5 C2
(9)
Equations 8 and 9 indicate that values of Γ21 and Γ12 have contributions of both ∆n1 and ∆n2 moles of solvent and solute components present in the inhomogeneous surface phase. We like to mention here that Priggogine and Defay,38 using a mass balance approach, derived eq 2 for the Gibbs surface excess for the adsorption of solute at liquid-gas and liquid-liquid interfaces. Equation 8 has been derived by Chattoraj8,37 for adsorption at liquid interfaces, and it has been extensively used39 for the calculation of ∆n1 and ∆n2 for electrolyte solutions in contact with air and oil. It thus appears from eq 8 that for positive values of Γ21, ∆n2 > ∆n1X2/X1 but ∆n2 will be less than ∆n1X2/X1 when Γ21 becomes negative at higher values of surfactant concentrations. When ∆n2 becomes equal to ∆n1X2/X1, the inhomogeneous composition ∆n2/∆n1 of the surface phase becomes equal to the composition of the bulk phase whereby a surface azeotropic state is reached. Values of C2azeo are included in Table 1. When C2 exceeds C2azeo, Γ21 becomes negative as a result of relative transfer of a significant amount of water from bulk to the inhomogeneous interfacial phase. ∆n1 will increase and simultaneously ∆n2 may decrease when such relative transfer occurs above the cmc. One also notes from Figures 1 and 2 (dashed portion of the curve) that Γ21 beyond C2m varies linearly with C2 for every system in a certain limited range of concentration of CTAB. From the slope and intercept of the linear plot, the values of ∆n1 and ∆n2 have been calculated using eq (38) Defay, R.; Priggogine, I.; Bellemans, A. Surface Tension and Adsorption (Translated by Everett, D. H.); Longmans: London, 1966. (39) Ghosh, L. N.; Das, K. P.; Chattoraj, D. K. J. Colloid Interface Sci. 1988, 121, 278.
8. These values of ∆n1 and ∆n2 (which are constant in this range of concentration) have been included in Table 1. One finds from Table 1 that the ratio ∆n1/∆n2 is of the order 10 000. At this stage, water (moles) bound per glucose residue is of the order of 100 whereas that of CTAB is 0.001. In the linear region of the plot of Γ21-C2, the average concentration of CTAB equal to 55.5 ∆n2/∆n1 and its magnitude equal to C2azeo is higher than the bulk cmc of CTAB. We thus notice that as C2 exceeds C2m, CTAB molecules present in the surface bound phase surrounding cellulose tend to form micellar aggregrates at the interface. This observation may be of some importance for interpretation of the mechanism of detergent action. In Figure 1, the isotherms for adsorption of CTAB by cellulose at different temperatures have been compared with each other at pH 6.0 and ionic strength 0.15. One finds that with increase of temperature, from 24 to 30 °C values of Γ2m (as well as ∆n2) increase, whereas its value decreases when temperature increases from 30 to 37 °C. From Table 1, one also notes that ∆n1 increases significantly and monotonously with increase of temperature. Thus, the effect of interaction of water to cellulose is mixed up with temperature dependent hydrophobic interaction of CTAB to cellulose so that the results of Γ2m as function of temperatue is not regular in nature. Nonionic polysaccharide cellulose on binding CTAB becomes positively charged. The effect of ionic strength on the values of Γ2m (vide Table 2) indicates that electrostatic effect for the CTAB-cellulose binding process is insignificant. We also note (vide Table 2) that at pH 4.0, 6.0, and 8.0, values of Γ2m are 0.0112, 0.0076, and 0.0093 mol of CTAB per kg of cellulose at 24 °C and at ionic strength 0.15. One can relate this change all at high ionic strength to the change in hydration and alteration of conformation of cellulose with pH. Moles of CTAB bound per mole of glucose remain in the range 0.001-0.008 whereas in the absence of CTAB, 2-10 molecules of water34 can attach per glucose residue. It thus appears that only a small fraction of glucose residues can be accessible for binding of CTAB. It has already been shown from isopiestic experiments34 that in the presence of excess salts (of nearly 2.0 ionic strength) such as Na2SO4, KCl, NaCl, and LiCl, water molecules bound to glucose residue are 21, 14, 16, and 12, respectively. One finds from Table 2 and also from Figure 2 that Γ2m values in the presence of these excess salts are relatively high and their values stand in the order KCl > NaCl > LiCl > Na2SO4. It appears that Γ2m has a close relation with hydrated states of cellulose in the presence of excess salts. Urea is known to behave as a water
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Table 2. Parameters for Adsorption of Cationic Surfactants at the Cellulose-Water Interface
surfactants
temp (°C)
pH
CTAB CTAB CTAB CTAB CTAB CTAB CTAB MTAB DTAB CTAB 2 M LiCl CTAB 2 M NaCl CTAB 2 M KCl CTAB 0.667 M Na2SO4 CTAB 6 M urea
24 30 37 30 30 24 24 30 30 37 37 37 37 37
6.0 6.0 6.0 6.0 6.0 4.0 8.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0
ionic strength (µ) 0.15 0.15 0.15 0.05 0.10 0.15 0.15 0.15 0.15 2.05 2.05 2.05 2.05
103 C2m (mol/L)
103 Γ2m (mol/kg)
(Γ2m)-1 (mol of glucose residue/mol of surfactant)
-∆GB° (kJ/mol)
-∆G° × 102 (kJ/kg)
+∆Ghi° × 102 (kJ/kg)
0.507 0.625 0.416 1.51 1.14 0.525 0.516 0.258
0.928 1.12 0.845 1.04 0.953 0.762 0.841 0.425
665 551 731 594 648 810 734 1452
32.3 32.4 32.2 32.5 31.2 32.8 31.3 34.7
30.0 36.3 27.2 33.8 29.7 25.0 26.3 14.7
1.11 0.054 0.205 0.098 2.97
1.09 1.26 1.60 0.816 2.29
566 490 386 756 270
35.0 35.9 34.4 35.9 33.6
38.2 45.2 55.0 29.3 76.9
104 159 188 134 31.0 60.8 120 133 62.7 473 165 248 214
of CTAB with the hydrophobic region existing predominently in the hydrophilic surface of cellulose. We also observe with interest that value of ∆n2 is almost zero for DTAB but its positive value increases as one passes from MTAB to CTAB. From this, one can again conclude that there exists some hydrophobic spots or small islands in the cellulose fiber surrounded by large areas covered by bound water. CTAB and MTAB possessing long hydrophobic groups may occupy fractions of these hydrophobic spots by overcoming the effect due to the water binding to cellulose. In the case of DTAB, the binding does not occur in excess since surrounding water at the interface is sufficient to resist such binding interaction by hydrophobic effects. In the case of adsorption of Γ21 mol of surfactant from solution to unit area of the contact surface, it has been shown from the integration of the Gibbs equation6,8,37
∆G° )
Figure 3. Plot of Γ21 vs C2 for adsorption of cationic surfactants of varied chain length at the cellulose-water interface at pH ) 6.0, µ ) 0.15, T ) 30 °C: A, CTAB; B, MTAB; C, DTAB.
∫0a ) 1dγ ) -∫0a ) 1Γ21 dµ2 2
2
(10)
Here γ stands for the surface free energies per unit area in the presence of the surfactant in the bulk medium of activity a2 and chemical potential µ2 of the surfactant respectively. ∆G° is the standard free energy change for surfactant-cellulose interaction when a2 is altered from zero to unity in the rational scale. It is expressed in kilojoules per kilogram of solid if Γ21 is expressed in moles of surfactant adsorbed per kilogram of solid. In the case of many adsorption isotherms in solid-liquid systems, Γ21 increases with increase of C2 (equal to 55.5X2 for dilute solution) until it reaches maximum value Γ2m and then it becomes independent of C2. For such a system, eq 10 may be written in the form6,8,37,41
structure breaker,40 and hydrophobicity of cellulose surface is enhanced in the presence of 6 M urea, as a result of which Γ2m becomes 0.03 mol of CTAB per kg cellulose (vide Table 2). With further increase of C2 from C2m, Γ2m remains apparently constant and positive. It is wellknown29 that in the presence of 6 M urea, cmc of CTAB has increased to a large extent so that value of C2 does not attain the value of C2azeo in the range of concentration considered by us. In Figure 3, the isotherms for adsorption of CTAB, MTAB, and DTAB to cellulose have been compared under identical physicochemical conditions. The most interesting obsevation is that Γ21 for DTAB is always negative and there is no maximum in the Γ21-C2 plot. Hydrocarbon chain length for this surfactant is insufficient for excess positive adsorption of DTAB to the hydrophilic surface of cellulose. When the -CH2- group in the hydrocarbon chain is increased from 12 to 14 for MTAB, Γ2m becomes positive but its magnitude is small. However, its value becomes maximum for CTAB bearing 16 -CH2- groups. This is indicating involvement of hydrophobic interaction
This involves the assumption that Γ2m remains constant when X2 is altered from X2m to unity. From the graphical integration of the integrated part of eq 11 based on the plot of Γ21/X2 against X2 using a computer, values of ∆G° have been calculated when X2 is altered from zero to unity. These values for different systems are presented in Table 2. In an alternative method, apparent free energy change (∆Gap°) can be calculated from eq 11 by arbitrarily putting X2 for X2m and Γ21 for Γ2m. These values of ∆Gap° will refer to the different unsaturated states for surfactantbiopolymer complexes of fractional surface coverage Γ21/ Γ2m. In Figure 4, ∆Gap° plotted against Γ21/Γ2m in the range equal to zero to unity are found to be linear. Further, the
(40) Franks, F. In Water A Comprehensive Treatise; Franks, F., Ed.; Plenum Press: New York and London, 1975; Vol. 4, Chapter 1, p 1.
(41) Chattoraj, D. K.; Mahapatra, P. K.; Roy, A. M. Biophys. Chem. 1996, 63, 37.
∆G° ) -RT
∫0Γ
m
2
Γ21 d ln X2 + RTΓ2m ln X2m
(11)
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Figure 4. Plot of ∆Gap° vs Γ21/Γ2m for adsorption of cationic surfactants at the cellulose-water interface: A, MTAB, pH ) 6.0, µ ) 0.15, 30 °C; B, CTAB, pH ) 8.0, µ ) 0.15, 24 °C; C, CTAB, pH ) 6.0, µ ) 0.05, 30 °C; D, CTAB, pH ) 6.0, 2 M NaCl, 37 °C.
Figure 5. Plot of ∆G° vs Γ2m for adsorption of cationic surfactants at the cellulose-water interface.
slope of the plot is observed to agree with the value of ∆G° obtained when Γ21 is equal to Γ2m so that6,8,37
∆Gap° ) ∆G°
Γ21 Γ2m
(12)
This also means that values of ∆G° for saturated and unsaturated complexes remain unchanged. From Table 2, it is noted that -∆G° increases with increase of Γ2m for different systems so that both these quantities may represent the relative affinity of the surfactant to cellulose under various physicochemical conditions. In Figure 5, values of ∆G° for different systems are found to vary linearly with Γ2m, and the slope of this curve ∆G°/Γ2m equal to ∆GB° in kilojoules per mole of surfactant becomes equal to -34.3 ( 0.1. ∆GB° stands for the standard free energy change for the transfer of 1 mol of surfactant from the bulk to the surface when X2 is altered from zero to unity. From eq 12, one can show that6,8,37
∆G° ∆Gap° ) ) ∆GB° Γ2m Γ21 or
∆G° ) Γ2m ∆GB°
(13)
∆G° thus is the product of ∆GB° and maximum adsorption value Γ2m. From Table 2, ∆G° for CTAB and MTAB at 30 °C and at ionic strength 0.15 are respectively found to be -363 and -147 J per kg of cellulose. From this, one can find
Figure 6. Plot of (∆Gap°)hi vs 1/xX2 for adsorption of cationic surfactants at the cellulose-water interface at pH ) 6.0, T ) 30 °C: A, CTAB, µ ) 0.10; B, CTAB, µ ) 0.15; C, MTAB, µ ) 0.15; D, DTAB, µ ) 0.15.
that the standard free energy change per CH2 group (∆GCH2°) due to its interaction with hydrophobic surface of cellulose is -108 J per kg of polysaccharide. Chattoraj et al.8,42 have shown that in the case of a micellar solution in the presence of excess neutral salt, the expression for the Gibbs adsorption equation remains the same as that shown in the right hand side of eqs 10 and 11. Values of (∆Gap°)hi for different systems have been calculated using eq 11 when C2 exceeds C2m and C2azeo, respectively, such that it represents apparent standard free energy change for water-surfactantcellulose interaction when X2 is altered from zero to unity. In the region of high values of C2 exceeding C2m, a negative value of ∆Gap° is observed to decrease from ∆G° until it becomes positive beyond C2azeo. Positive values of apparent free energy change(∆Gap°)hi have been plotted against 1/xX2, and the linear region of the plot has been extrapolated (vide Figure 6) to X2 equal to unity for evaluation of standard free energy change ∆Ghi° which in all cases (vide Table 2) are positive due to the excess hydration of the cellulose in a hypothetically standard state of X2 equal to unity. Free energy change ∆Gh° due to the excess hydration of cellulose in the presence of surfactant can be calculated from the integration of the Gibbs adsorption equation (10) written in an alternative form34,37
∆Gh° )
∫aa ))01dγ ) -∫aa ))01Γ12 dµ1 1
1
1
1
∫aa ))10Γ21 dµ2 ) +∫aa ))01Γ21 dµ2 ) -∆G° 2
2
)-
2
2
(14)
Using eq 14, we find that the free energy change (∆Gh°) due to the excess hydration of cellulose-surfactant mixture as a result of change of the bulk water activity hypothetically from zero to unity can be obtained multiplying ∆G° in Table 2 by -1. It will be of interest to note that for the negative values of Γ21, ∆G° is positive, which indicates that hydration rather than surfactant binding in excess is spontaneous in this region. The reverse is the case when Γ21 is positive. We shall also conclude that ∆G° is constant and negative when Γ21 varies from zero to Γ2m. When C2 > C2m, negative values of standard free energy decrease until its value becomes positive when X2 is close to unity. (42) Chattoraj, D. K. J. Phys. Chem. 1967, 71, 455.
Polysaccharide-Surfactant Interaction
Langmuir, Vol. 13, No. 17, 1997 4511
Table 3. Thermodynamic Parameters for Adsorption of CTAB at Cellulose-Water Interface at pH ) 6.0, µ ) 0.15 CTAB concn range low concn
temp, T (K)
∆G° (kJ/kg)
Tav (K)
297
∆Havo (kJ/kg)
∆Savo (kJ kg-1 K-1)
Tav∆Savo (kJ/kg)
-0.332
2.82
0.011
3.30
-0.318
-4.30
-0.013
-3.98
-0.300 300
303
-0.363 306.5
high concn
∆Gavo (kJ/kg)
310 297
-0.272 1.04 300
303
1.32
-26.2
-0.091
-27.5
1.74
-10.9
-0.041
-12.7
1.58 306.5
310
1.88
From integration of the Gibbs-Helmholtz equation at two different temperatures, it can be shown that
[
]
∆G2° ∆G1° 1 1 ) ∆Hav° T2 T1 T2 T1
(15)
Here T1 and T2 are a pair of two temperatures close to each other and ∆G1° and ∆G2° are corresponding free energies of adsorption. With these equations, values of ∆Hav° and ∆Gav° (equal to 0.5 (∆G1° + ∆G2°)) can be evaluated. From ∆Hav° - ∆Gav° equal to Tav∆Sav°, values of ∆Sav° for the systems studied have been calculated for
various systems (vide Table 3). It appears from Table 3 that in all these cases Tav∆Sav° ≈ ∆Hav°, which means that the binding process is controlled mainly by enthalpy and entropy effects. Acknowledgment. We are grateful to the Council of Scientific and Industrial Research, New Delhi, for the award of a Senior Research Fellowship to S.C.B. Thanks are also due to the Indian National Science Academy, New Delhi, for financial assistance. LA960905J
