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Adsorption and diffusion of sodium p-dodecylbenzene sulfonate of tetramer type in cellulose
Adsorption and Diffusion of Sodium p-Dodecylbenzene Sulfonate of Tetramer Type in Cellulose ZENZO MORITA ~ AND TOSHIRO IIJIMA Department of Polymer Science, Faculty of Engineering, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152, Japan Received January 23, 1980; accepted November 18, 1980 The adsorption and diffusion of sodium p-dodecylbenzene sulfonate of a tetramer type (DBS) in cellulose from the wide concentration range of DBS solution were investigated by the method of the cylindrical film role at temperature between 10 and 50°C. The surface concentration of DBS in cellulose at first increased gradually with increase in the concentration of DBS in solution, then increased abruptly in the vicinity of the critical micelle concentration (CMC), reached a maximum, and finally decreased gradually. The diffusion profile of DBS in cellulose near and above the CMC showed the existence of two kinds of adsorbed species, the monomer and dimer of DBS. The adsorption of dimer corresponded to the adsorption maximum in the adsorption isotherm. The adsorption of monomer DBS ion on cellulose showed a Langmuir-type adsorption isotherm. The diffusion behavior of DBS in cellulose also supported these findings. With an increase in the DBS concentration above the CMC, larger micelles of DBS, which were not adsorbed by cellulose, were also formed and the adsorption of dimer gradually decreased due to the fact that the concentration of dimer in solution decreased as the micelle formation proceeded. Thus, a so-called adsorption maximum was observed in the adsorption isotherm. The adsorption of monomer DBS ions on cellulose was enhanced by the addition of sodium chloride. The diffusion coefficient of DBS in cellulose increased with increase in the concentration of DBS and/or NaC1 added to the DBS solution. INTRODUCTION
Adsorption of ionic surfactants on solid substrates such as textile fibers, carbon black, clays, and glasses from aqueous solution has been intensively studied (1-3). Since Aickin found an adsorption maximum when measuring the adsorption of anionic surfactants from aqueous solution on cotton and wool (4), a number of workers have reported results similar to that of Aickin on the adsorption of anionic surfactants on cellulose and/or wool (5-8), of anionic surfactants on carbon black or graphite (8-10), of cationic surfactants on cellulose (11), and of anionic and cationic surfactants on porous glass (12). 1 Present address: Department of Textiles and Polymer Science, Faculty of Engineering, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184, Japan.
The adsorption maxima observed in the isotherms have been interpreted in terms of the activity change of single surfactant ions, the micelle adsorption of surfactants (5, 6, 8, 10-13), and the multilayer adsorption or the swelling of cellulose (7). On the other hand, some workers have reported that no adsorption maxima of ionic surfactants were observed upon purification of the adsorbents and have proposed that the adsorption maxima were due to impurities in either the surfactants or the adsorbents (14-18). Thereafter, Mukerjee has proposed the exclusion effect in adsorption as the reason of the adsorption maximum on porous glass (12). Trogus, Schechter, and Wade, on the other hand, have reported a new interpretation of adsorption maxima and minima based on the micellar-monomer equilibria in
155
0021-9797/81/070155-07502.00/0 Journal of Colloid and Interface Science, Vol. 82, No. 1, July 1981
Copyright © 1981 by Academic Press, Inc. All rights of reproduction in any form reserved.
156
MORITA
mixed surfactant systems (19). Although their theory has described the various types of apparent behavior of surfactant adsorption, it has to be verified experimentally. There still remain, therefore, some problems concerning the mechanism and theory of surfactant adsorption. In the present paper, the adsorption of sodium p-dodecylbenzene sulfonate of a tetramer type (DB S) is studied by the method of the cylindrical film roll and, at the same time the diffusion of DBS in cellulose is examined. The diffusion profiles of DBS in cellulose obtained by this method give new information to elucidate the adsorption mechanism of surfactants and various adsorption theories proposed so far can be reexamined from a new point of view. EXPERIMENTAL
Materials. The purified sample of sodium p-dodecylbenzene sulfonate of a tetramer type (DBS; extra pure chemicals of Tokyo Kasei Kogyo Co. Ltd.) was obtained by extracting the commercial product by a Soxhlet extractor with absolute alcohol and by removing the insoluble impurities. The product was crystallized from fresh alcohol and dried in vacuo. The infrared spectra of purified sample were measured by a Hitachi EPIG3 spectrophotometer and were compared with those of Hummel (20). The DBS used was identified as tetramer type from the absorption at 1367 and 1380 cm -1 and the fact that only the para isomer was present was inferred from the absorption at 840 cm -1 and from the absence of absorption at 800 cm -1. The concentrations of the aqueous surfactant solutions were determined by a modification of Epton's method (21). The purity of the purified sample was 98% (wt) using cetylpyridinium chloride (reagent grade chemical of E. Merck A.G., dried at l l0°C under vacuum) as the reference of concentration. The other chemicals used were of reagent grade. The critical micelle concentration (CMC) Journal of Colloid and Interface Science, Vol. 82, No. 1, July 1981
AND IIJIMA
of DBS was measured by the solubilization method (22). In glass-stoppered flasks, various concentrations of surfactant solution were prepared by diluting the stock solution and amounts of dimethylaminoazobenzene in excess of the solubility limit were added. The solutions were allowed to stand for about 2 weeks at 60.0°C with occasional stirring. After equilibrium, a known amount of solution was pipetted out through a glassfiber filter and was diluted with absolute alcohol to 50% (by vol). The solution was diluted again by 50% alcohol, if necessary, and the optical density was mesured at a wavelength of 419 nm. From the results of solubilization, the CMC of DBS used was found to be 1.3 mmole/dmz at 60°C. This value is similar to the results reported earlier (23, 24). Cellophane film roll. Cellophane sheets (#350, Tokyo Cellophane Sheet Co. Ltd.) were cut to 4 or 6 cm width and 55 or 100 cm length, scoured by boiling water three times and stored in distilled water for 2 or 3 days. A film roll was made of the film by winding it carefully on a glass tubing (gb = 1 cm) so that no bubbles of air were trapped between the layers (25, 26). The free end of the film was held by a slender glass rod. This corresponds to the boundary between layers after unwinding. Adsorption and diffusion. A film roll was inserted into the surfactant solution of a given concentration and diffusion was allowed to proceed for three different times at each concentration of the solution. At the end of the diffusion, the surface of the film roll was immediately washed with water. Both sides of the film roll were cut a little at the point of the holding glass rod to distinguish between layers. After drying, the optical densities of the respective layers were measured at the wavelength of 222.5 nm by a Shimadzu SV-50A recording spectrometer. The amount of adsorption was calculated using a calibration curve which was made by extracting the DBS on cellophane by water at 90°C and determining the con-
ADSORPTION AND DIFFUSION OF DBS centration of the solution. It was confirmed that B e e r ' s law held in all the concentration ranges examined on the film.
o =
157
2
RESULTS C o n c e n t r a t i o n Profiles in Cellulose
A typical e x a m p l e of profiles for the diffusion of DBS in cellulose is shown in Fig. 1. A curve of the concentration distribution was obtained by B o l t z m a n n ' s transformation (in terms of x / 2 t 1/2) of the profiles for three diffusion times, t. It was confirmed that in all cases the surface concentration of DBS was constant during diffusion and that the diffusion coefficient did not depend on the diffusion time. The film roll m a y r e a s o n a b l y be taken as a semiinfinite substrate in the direction of the radius for short diffusion times. The diffusion equation along the radial direction of the film roll can be described by F i c k ' s law, OC Ot
02C -
D
- -
[1]
Ox 2
where D (cm2/min) is the diffusion coefficient, C (mole/kg) is the concentration of
x %(3
I 3 e
o
8 Oi 0
2
4
6
8
x/2t/t- x tO4crn rnin -1/2
FIG. 2. Concentration distribution of DBS in cellulose at 30°C. DBS (mole/dm3): (1) 2.66 × 10-4, (2) 5.31 × 10-4, (3) 1.59 × 10-3;diffusiontime (min): (1) [~, 400; [], 600; [], 900; (2) A, 400; ~, 600; A, 900; (3) ©, 364; O, 598; O, 881. surfactant in cellulose, t (min) is the diffusion time, and x (cm) is the distance f r o m the surface along the radial direction of the film roll. When the surface concentration, Co (mole/kg), is kept constant during the diffusion experiment, as shown in Fig. 1, the solution of Eq. [1] is given by
'k
C = Coerfc-
X
2(Dt) 1/2
[2]
where erfc z
% x
o_ 8 0
0
2
4
6
8
10
X/2~" x 104 crn rnin -1/2
FlG. 1. Concentration distribution of DBS in cellulose at 30°C. DBS: 2.81 x 10-3 mole/dmZ; diffusion time: El, 900 rain; /x, 1200 rain; (3, 1500 min).
-
77"21/2
e-'2d~.
[3]
By applying these equations to the concentration profiles obtained experimentally, the values of D and Co can be calculated (25, 26). The diffusion experiments were carried out with various concentrations of surfactant solution below and a b o v e the C M C and the results are s h o w n in Figs. 2 and 3. The diffusion below the C M C was o b s e r v e d to be Fickian from the concentration profiles (curves 1 and 2 in Fig. 2) and the values of Co increased with increase in the concentration of surfactant solution. On the other hand, a b o v e the CMC, the values of Co inJournal o f Colloid a n d Interface Science, Vol. 82, N o . 1, July 1981
158
MORITA AND IIJIMA
31
scribed by Eq. [2], i.e., of Fickian type, the values of D and Co for both the profiles were calculated by the method of Sekido and Matsui (25, 26). The results calculated are shown in Figs. 4 - 6 . The values of Co obtained from curve A are shown as isotherm A and those obtained by extrapolation of curve B as isotherm B, which correspond to the total adsorption, in the figures. The values of D calculated from curve A are shown in the upper parts of the figures. They had a slight tendency to increase with increase in the surfactant concentration.
2
% x
8
Salt Effect X/2~/~" X 104 cm rain "1/2
FIG. 3. Concentration distribution of DBS in cellulose at 30°C. DBS (mole/din3): (1) 2.66 × 10-3, (2) 3.32 × 10-8, (3) 4.65 × 10-3, (4) 7.97 x 10-3;diffusion time (min): (1) [~, 368; [], 602; [], 885; (2) A, 604; Ax,893; (3) ©, 371; (I), 607; (4)-~7, 605. creased rapidly with increasing concentration (curve 3 in Fig. 2 and curve 1 in Fig. 3) and the diffusion profiles were found to be composed of the two kinds of distribution. That is to say, the diffusion profile in Fig. 1 was divided into curve A whose diffusion was of Fickian type similar to curves 1 and 2 in Fig. 2 and curve B which was present only near the surface. With increasing solution concentration, Co increased and give a maximum value. B e y o n d this point, Co gradually decreased accompanying the reduction of concentration distribution corresponded to curve B until only the profile of curve A was observed (curve 4 in Fig. 3). The adsorption and diffusion behavior at 10, 20, and 50°C was similar to that at 30°C shown in Figs. 1-3. Adsorption Isotherms and Diffusion Coefficients As the distribution profiles below the CMC and those of curve A above the CMC fitted a theoretical distribution profile deJournal o f Colloid and Interface Science,
Vol. 82, No. 1, July 1981
Though the results mentioned above are those in the absence of salt, the salt effect on the D and Co will give important information for elucidation of the adsorption mechanism of surfactants. The diffusion profiles with salt addition of 1, 3, and 10 g/dm 3 were similar to those without salt shown in Figs. 1-3. The values of D and Co calculated by the method of Sekido and Matsui from them are shown in Table I. Only curve A was observed at higher DBS concentration. The adsorption of DBS clearly increased with in-
:
8
%
4
O
x
P
I
i
i
<_ ~ Io
~" ~ ,B
3
x \ ~
2
~
d
f)'" (3-
m
oG
~
1
Conc. o f D B S in soln.
6
6
{ x l O - 3 m o l / d m a)
F~G.4. Adsorption and diffusion of DBS in cellulose at 30°C. A, total adsorption; ©, monomer adsorption.
159
ADSORPTION AND DIFFUSION OF DBS
creasing salt addition. The values of D gradually increased, passed through a maximum, and finally fell off with increase in salt concentration as in the case of direct dyes (27). DISCUSSION
I
i
i
i
E
%
Although intensive studies on the micelle formation have been undertaken, the theory of micelle formation has not yet been completely established. The micelle size and their distribution in ionic and nonionic surfactants above the CMC have been studied from the theoretical and experimental points of view (28-30). The smallest aggregates and their distribution, which are expected to relate closely to their adsorption, are at present incompletely understood. Many workers, however, have reported the fact that no aggregates of intermediate size were found in measurable amount in the aqueous solution of surfactants above the CMC (3135). Mukerjee has pointed out that there is substantial evidence that a number of surfactants dimerize well below the CMC (36). So far as the monomer concentration of surfactants above the CMC is concerned, there remains much to be learned. Tanford has developed a micelle formation theory by
2
x
o
x t)
2
i Conc. of DBS in soln, ( x l O - 3 m o l / / d m 3)
FIG. 5. Adsorption and diffusion of DBS in cellulose at 50°C. A, total adsorption (B); O, monomer adsorption (A).
x v
u m
1 Conc.
of
i
i
r
I
2
4
6
8
DBSin
soln.
( x l()3mol//drn3~
FIG. 6. Adsorptionand diffusionof DBS in cellulose at 10 and 20°C. Total adsorption: 10°C, A; 20°C, ~3; monomeradsorption: 10°C,O; 20°C, V; diffusioncoefficient: 10°C, O; 20°C, V. presupposing that the monomer concentration above the CMC is constant (28, 37). According to results by Cutler et al., who measured the activity of sodium dodecyl sulfate (SDS) by the use of an ion-selective electrode (38), the activity of SDS anion decreases with increase of the total concentration of surfactant but that of the counterion increases and the mean activity of SDS increases at a lower rate than is the case below the CMC. NMR measurements also showed a decrease of monomer concentration with increasing of total surfactant concentration (39). On the other hand, the adsorption isotherms of surfactant seem in general to be more complex in shape, as Trogus et al. (19) noted, than the distribution of monomer ions in aqueous solution as mentioned above. Thus, many workers have observed maxima of different shapes near the CMC in the adsorption isotherms on various substrates (4-12). The result of the present study using DBS o f a tetramer type also showed a broad maximum in the adsorption isotherms, which was similar to that of Fava and Eyring (6). The present results, on the other hand, showed clearly the existence of two kinds of species of DBS in the adsorption judging Journal of Colloid and Interface Science, Vol. 82, No. 1, July 1981
160
MORITA AND IIJIMA TABLE I
The Salt Effect on the Adsorption and Diffusion of DBS in Cellulose at 30°C DBS conc. (mole/rim 3)
NaCI cone. (g/din ~)
3.0 x 10-4
0 1 3 10
3.0 x 10-3
0
4.21 x 5.38 x 7.01 x 1.04 x
10-3 10-3 10-8 10-2
Diffusion coeff. (from curve A) (cm2/rnin)
1.29 x 5.96 x 5.03 × 3.54 x
10-r 10-7 10-7 10-r
3 10
9.56 x 10-3 1.27 x 10-2 1.25 x 10-2 9.12 x 10-3
4.93 9.15 9.71 7.12
x × x ×
10-~ 10-7 10-r 10-r
0 1 3 i0
1.25 x 10-2 1.42 X 10-2 1.54 x 10-z 1.45 x 10-~
1.03 x 7.31 x 7.01 x 7.43 x
10-6 10-r 10-~ 10-r
1
8.0 x 10-3
DBS adsorption (mole/kg)
from the diffusion profiles near and above the CMC. The species of DBS responsible for the adsorption maximum diffused only into a few layers from the surface of the film roll. The absorption spectra of DBS on cellophane film in which the profile of curve B was contained were found to be the same in shape as those containing only the profile of curve A. Beer's law held over all the wavelengths observed on the cellophane. If the sample of DBS used contained isomers having a small diffusion coefficient corresponding to curve B, the profile of curve B should have been observed in the whole range of surfactant concentration. Thus, the results of the present study cannot be explained by the theory of Trogus et al. (19). If we consider that the dimers of DBS are formed in solution at the concentrations where the distribution profiles of curve B are observed and that they, as well as the monomer, are adsorbed by cellulose, we may well explain the present results. With increasing surfactant concentration, the dimerization of DBS c o m m e n c e s near the CMC, reaches the maximum at a concentration a little higher than the CMC, and Journal of Colloid and Interface Science, Vol. 82, No. 1~ July 1981
then decreases gradually. This behavior is consistent with the view of micelle formation in the surfactant solution mentioned above. As the larger micelles are not adsorbed by cellulose due to the exclusion effect (12), an adsorption maximum may be observed in the a d s o r p t i o n isotherms as a result of m o n o m e r and dimer adsorption. These results were evidenced by the concentration distribution of DBS in cellulose as well as by the diffusion behavior. The diffusion of DBS m o n o m e r corresponding to curve A showed reasonable behavior over all the concentration range discussed. It was confirmed, as a result, that the adsorption isotherm of the DBS m o n o m e r was of Langmuir-type. Diffusion experiments similar to the present study by the use of s o d i u m p - n - d o d e c y l benzene sulfonate showed a sharp adsorption maximum and the existence of an adsorption minimum in the adsorption isotherm (40). The diffusion profiles were similar to those of the present study. The concentration range of the existence of dimer may depend on the kind of surfactant, the purity, and the composition of solution and so on. According to the adsorption theory based on the presence of impurities in cellulose, the decrease in the adsorption above the CMC is attributed to the removal of the impurities by the surfactant (14-18). According to this theory we would expect that the surface concentration of the present study decreases as the diffusion proceeds. Such behavior, however, was not observed as shown in Figs. 1-3. As the addition of sodium chloride to the DBS solution increased the adsorption of DBS on cellulose, it was considered that there was a D o n n a n equilibrium in this adsorption system similar to the adsorption of direct dyes on cellulose (41). The results of salt addition appear to provide no new information to be discussed except an increase in adsorption. Further investigations will be required to
ADSORPTION AND DIFFUSION OF DBS
elucidate the temperature dependence of adsorption shown in Figs. 4 - 6 . SUMMARY
The diffusion of DBS in cellulose was determined by the cylindrical film roll method. From the profiles the equilibrium adsorption and diffusion behavior were discussed in connection with the micelle formation. The adsorption of DBS on cellulose increases gradually with increase in the concentration of DBS in solution and reaches a maximum. The maximum was explained by the adsorption of DBS dimer. The diffusion behavior of DBS in cellulose is consistent with these interpretations. REFERENCES 1. Rosen, M. J., "Surfactants and Interfacial Phenomena," Chap. 2. John Wiley, New York, 1978. 2. White, H. J., Jr., "Cationic Surfactants" (E. Jungermann, Ed.), Chap. 9. Marcel Dekker, New York, 1970. 3. Ginn, M. E., "Cationic Surfactants" (E. Jungermann, Ed.), Chap. 11. Marcel Dekker, New York, 1970. 4. Aickin, R. G., J. Soc. Dyers Colour 60, 60 (1944). 5. Meader, A. L., and Fries, B. A.,Ind. Eng. Chem. 44, 1636 (1952). 6. Fava, A., and Eyring, H., J. Phys. Chem. 60, 890 (1956). 7. Evans, H. C., J. Colloid Sci. 13, 537 (1958). 8. Void, R. D., and Phansalkar, A. K., Rec. Tray. Chim. 74, 41 (1955). 9. Con-in, M. L., Lind, E. L., Roginsky, A., and Harkins, W. D., J. Colloid Sci. 4, 485 (1949). 10. Vold, R. D., and Sivaramakrishnan, N. H.,J. Phys. Chem. 62, 984 (1958). 11. Sexsmith, F. H., and White, H. J., Jr., J. Colloid Sci. 14, 596 (1959). 12. Mukerjee, P., and Anavil, A., "Adsorption at Interfaces" (K. L. Mittal, Ed.), p. 107. Am. Chem. Soc. Symp. Ser. No. 8, Am. Chem. Soc., Washington, D. C., 1975. 13. Sexsmith, F. H., and White, H. J., Jr., J. Colloid Sci. 14, 630 (1959). 14. Kitchener, J. A., J. Phot. Sci. 13, 152 (1965). 15. Saleeb, F. Z., and Kitchener, J. A.,J. Chem. Soc. 1965, 911. 16. Ginn, M. E., Kinney, F. B., and Harris, J. C., J. Amer. Oil Chem. Soc. 38, 138 (1961).
161
17. Schwarz, W. J., Martin, A. R., and Davis, R. C., Text. Res. J. 32, 1 (1962). 18. Schott, H., Text. Res. J. 37, 336 (1967). 19. Trogus, F. J., Schechter, R. S., and Wade, W. H., J. Colloid Interface Sci. 70, 293 (1979). 20. Hummel, D., "Identification and Analysis of Surface Active Agents by Infrared and Chemical Methods." Interscience, New York, 1962. 21. Japanese Industrial Standard, JIS K 3362-1970, "Testing Methods for Synthetic Detergents." 22. Kolthoff, I. M., and Stricks, W,, J. Phys. Colloid Chem. 52, 915 (1948). 23. Paquette, R. G., Lingafelter, E. C., and Tartar, H. V.,J. Amer. Chem. Soc. 65, 686 (1943). 24. Gershman, J. W., J. Phys. Chem. 61, 581 (1957). 25. Sekido, M., and Matsui, K., Sen-i Gakkaishi 20, 778 (1964). 26. Morita, Z., Kobayashi, R., Uchimura, K., and Motomura, H., J. Appl. Polym. Sci. 19, 1095 (1975). 27. McGregor, R., Peters, R. H., and Petropoulos, J. H., Trans. Faraday Soc. 58, 1045 (1962). 28. Tanford, C., "The Hydrophobic Effect," 2nd ed. John Wiley, New York, 1980. 29. Mittal, K. L. (Ed.), "Micellization, Solubilization and Microemulsions." Plenum Press, New York, 1977. 30. Wennerstrrm, H., and Lindman, B., Phys. Rep. 52, 1 (1979). 31. Hoeve, C. A. J., and Benson, G. C., J. Phys. Chem. 61, 1149 (1957). 32. Poland, D. C., and Sheraga, H. A.,J. Phys. Chem. 69, 2431 (1965); J. Colloid Interface Sci, 21,273 (1966). 33. Ruckenstein, E., and Nagarajan, R., "Micellization, Solubilization and Microemulsions" (K. L. Mittal, Ed.), Vol. l, p. 133. Plenum Press, New York, 1977. 34. Aniansson, E. A. G.,Ber. Bunsenges. Phys. Chem. 82, 981 (1978). 35. Mukerjee, P., Bet. Bunsenges. Phys. Chem. 82, 931 (1978). 36. Mukerjee, P., Adv. Colloid Interface Sci. 1, 241 (1967). 37. Tanford, C., J. Phys. Chem. 76, 3020 (1972); 78, 2469 (1974). 38. Cutler, S. G., and Meares, P., J. Chem. Soc. Faraday 1 74, 1758 (1978). 39. Aniansson, E. A. G., et al., J. Phys. Chem. 80, 905 (1976). 40. Duong Xuan Mai, Masters Dissertation, Tokyo University of Agriculture and Technology, 1978. 41. Vickerstaff, T., "The Physical Chemistry of Dyeing," 2nd ed,, Chap. 7. Oliver and Boyd, London, 1954.
Journal of Colloidand Interface Science, Vol.82, No. 1, July 1981
Adsorption of ionic surfactants on solid substrates such as textile fibers, carbon black, clays, and glasses from aqueous solution has been intensively studied (1-3). Since Aickin found an adsorption maximum when measuring the adsorption of anionic surfactants from aqueous solution on cotton and wool (4), a number of workers have reported results similar to that of Aickin on the adsorption of anionic surfactants on cellulose and/or wool (5-8), of anionic surfactants on carbon black or graphite (8-10), of cationic surfactants on cellulose (11), and of anionic and cationic surfactants on porous glass (12). 1 Present address: Department of Textiles and Polymer Science, Faculty of Engineering, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184, Japan.
The adsorption maxima observed in the isotherms have been interpreted in terms of the activity change of single surfactant ions, the micelle adsorption of surfactants (5, 6, 8, 10-13), and the multilayer adsorption or the swelling of cellulose (7). On the other hand, some workers have reported that no adsorption maxima of ionic surfactants were observed upon purification of the adsorbents and have proposed that the adsorption maxima were due to impurities in either the surfactants or the adsorbents (14-18). Thereafter, Mukerjee has proposed the exclusion effect in adsorption as the reason of the adsorption maximum on porous glass (12). Trogus, Schechter, and Wade, on the other hand, have reported a new interpretation of adsorption maxima and minima based on the micellar-monomer equilibria in
155
0021-9797/81/070155-07502.00/0 Journal of Colloid and Interface Science, Vol. 82, No. 1, July 1981
Copyright © 1981 by Academic Press, Inc. All rights of reproduction in any form reserved.
156
MORITA
mixed surfactant systems (19). Although their theory has described the various types of apparent behavior of surfactant adsorption, it has to be verified experimentally. There still remain, therefore, some problems concerning the mechanism and theory of surfactant adsorption. In the present paper, the adsorption of sodium p-dodecylbenzene sulfonate of a tetramer type (DB S) is studied by the method of the cylindrical film roll and, at the same time the diffusion of DBS in cellulose is examined. The diffusion profiles of DBS in cellulose obtained by this method give new information to elucidate the adsorption mechanism of surfactants and various adsorption theories proposed so far can be reexamined from a new point of view. EXPERIMENTAL
Materials. The purified sample of sodium p-dodecylbenzene sulfonate of a tetramer type (DBS; extra pure chemicals of Tokyo Kasei Kogyo Co. Ltd.) was obtained by extracting the commercial product by a Soxhlet extractor with absolute alcohol and by removing the insoluble impurities. The product was crystallized from fresh alcohol and dried in vacuo. The infrared spectra of purified sample were measured by a Hitachi EPIG3 spectrophotometer and were compared with those of Hummel (20). The DBS used was identified as tetramer type from the absorption at 1367 and 1380 cm -1 and the fact that only the para isomer was present was inferred from the absorption at 840 cm -1 and from the absence of absorption at 800 cm -1. The concentrations of the aqueous surfactant solutions were determined by a modification of Epton's method (21). The purity of the purified sample was 98% (wt) using cetylpyridinium chloride (reagent grade chemical of E. Merck A.G., dried at l l0°C under vacuum) as the reference of concentration. The other chemicals used were of reagent grade. The critical micelle concentration (CMC) Journal of Colloid and Interface Science, Vol. 82, No. 1, July 1981
AND IIJIMA
of DBS was measured by the solubilization method (22). In glass-stoppered flasks, various concentrations of surfactant solution were prepared by diluting the stock solution and amounts of dimethylaminoazobenzene in excess of the solubility limit were added. The solutions were allowed to stand for about 2 weeks at 60.0°C with occasional stirring. After equilibrium, a known amount of solution was pipetted out through a glassfiber filter and was diluted with absolute alcohol to 50% (by vol). The solution was diluted again by 50% alcohol, if necessary, and the optical density was mesured at a wavelength of 419 nm. From the results of solubilization, the CMC of DBS used was found to be 1.3 mmole/dmz at 60°C. This value is similar to the results reported earlier (23, 24). Cellophane film roll. Cellophane sheets (#350, Tokyo Cellophane Sheet Co. Ltd.) were cut to 4 or 6 cm width and 55 or 100 cm length, scoured by boiling water three times and stored in distilled water for 2 or 3 days. A film roll was made of the film by winding it carefully on a glass tubing (gb = 1 cm) so that no bubbles of air were trapped between the layers (25, 26). The free end of the film was held by a slender glass rod. This corresponds to the boundary between layers after unwinding. Adsorption and diffusion. A film roll was inserted into the surfactant solution of a given concentration and diffusion was allowed to proceed for three different times at each concentration of the solution. At the end of the diffusion, the surface of the film roll was immediately washed with water. Both sides of the film roll were cut a little at the point of the holding glass rod to distinguish between layers. After drying, the optical densities of the respective layers were measured at the wavelength of 222.5 nm by a Shimadzu SV-50A recording spectrometer. The amount of adsorption was calculated using a calibration curve which was made by extracting the DBS on cellophane by water at 90°C and determining the con-
ADSORPTION AND DIFFUSION OF DBS centration of the solution. It was confirmed that B e e r ' s law held in all the concentration ranges examined on the film.
o =
157
2
RESULTS C o n c e n t r a t i o n Profiles in Cellulose
A typical e x a m p l e of profiles for the diffusion of DBS in cellulose is shown in Fig. 1. A curve of the concentration distribution was obtained by B o l t z m a n n ' s transformation (in terms of x / 2 t 1/2) of the profiles for three diffusion times, t. It was confirmed that in all cases the surface concentration of DBS was constant during diffusion and that the diffusion coefficient did not depend on the diffusion time. The film roll m a y r e a s o n a b l y be taken as a semiinfinite substrate in the direction of the radius for short diffusion times. The diffusion equation along the radial direction of the film roll can be described by F i c k ' s law, OC Ot
02C -
D
- -
[1]
Ox 2
where D (cm2/min) is the diffusion coefficient, C (mole/kg) is the concentration of
x %(3
I 3 e
o
8 Oi 0
2
4
6
8
x/2t/t- x tO4crn rnin -1/2
FIG. 2. Concentration distribution of DBS in cellulose at 30°C. DBS (mole/dm3): (1) 2.66 × 10-4, (2) 5.31 × 10-4, (3) 1.59 × 10-3;diffusiontime (min): (1) [~, 400; [], 600; [], 900; (2) A, 400; ~, 600; A, 900; (3) ©, 364; O, 598; O, 881. surfactant in cellulose, t (min) is the diffusion time, and x (cm) is the distance f r o m the surface along the radial direction of the film roll. When the surface concentration, Co (mole/kg), is kept constant during the diffusion experiment, as shown in Fig. 1, the solution of Eq. [1] is given by
'k
C = Coerfc-
X
2(Dt) 1/2
[2]
where erfc z
% x
o_ 8 0
0
2
4
6
8
10
X/2~" x 104 crn rnin -1/2
FlG. 1. Concentration distribution of DBS in cellulose at 30°C. DBS: 2.81 x 10-3 mole/dmZ; diffusion time: El, 900 rain; /x, 1200 rain; (3, 1500 min).
-
77"21/2
e-'2d~.
[3]
By applying these equations to the concentration profiles obtained experimentally, the values of D and Co can be calculated (25, 26). The diffusion experiments were carried out with various concentrations of surfactant solution below and a b o v e the C M C and the results are s h o w n in Figs. 2 and 3. The diffusion below the C M C was o b s e r v e d to be Fickian from the concentration profiles (curves 1 and 2 in Fig. 2) and the values of Co increased with increase in the concentration of surfactant solution. On the other hand, a b o v e the CMC, the values of Co inJournal o f Colloid a n d Interface Science, Vol. 82, N o . 1, July 1981
158
MORITA AND IIJIMA
31
scribed by Eq. [2], i.e., of Fickian type, the values of D and Co for both the profiles were calculated by the method of Sekido and Matsui (25, 26). The results calculated are shown in Figs. 4 - 6 . The values of Co obtained from curve A are shown as isotherm A and those obtained by extrapolation of curve B as isotherm B, which correspond to the total adsorption, in the figures. The values of D calculated from curve A are shown in the upper parts of the figures. They had a slight tendency to increase with increase in the surfactant concentration.
2
% x
8
Salt Effect X/2~/~" X 104 cm rain "1/2
FIG. 3. Concentration distribution of DBS in cellulose at 30°C. DBS (mole/din3): (1) 2.66 × 10-3, (2) 3.32 × 10-8, (3) 4.65 × 10-3, (4) 7.97 x 10-3;diffusion time (min): (1) [~, 368; [], 602; [], 885; (2) A, 604; Ax,893; (3) ©, 371; (I), 607; (4)-~7, 605. creased rapidly with increasing concentration (curve 3 in Fig. 2 and curve 1 in Fig. 3) and the diffusion profiles were found to be composed of the two kinds of distribution. That is to say, the diffusion profile in Fig. 1 was divided into curve A whose diffusion was of Fickian type similar to curves 1 and 2 in Fig. 2 and curve B which was present only near the surface. With increasing solution concentration, Co increased and give a maximum value. B e y o n d this point, Co gradually decreased accompanying the reduction of concentration distribution corresponded to curve B until only the profile of curve A was observed (curve 4 in Fig. 3). The adsorption and diffusion behavior at 10, 20, and 50°C was similar to that at 30°C shown in Figs. 1-3. Adsorption Isotherms and Diffusion Coefficients As the distribution profiles below the CMC and those of curve A above the CMC fitted a theoretical distribution profile deJournal o f Colloid and Interface Science,
Vol. 82, No. 1, July 1981
Though the results mentioned above are those in the absence of salt, the salt effect on the D and Co will give important information for elucidation of the adsorption mechanism of surfactants. The diffusion profiles with salt addition of 1, 3, and 10 g/dm 3 were similar to those without salt shown in Figs. 1-3. The values of D and Co calculated by the method of Sekido and Matsui from them are shown in Table I. Only curve A was observed at higher DBS concentration. The adsorption of DBS clearly increased with in-
:
8
%
4
O
x
P
I
i
i
<_ ~ Io
~" ~ ,B
3
x \ ~
2
~
d
f)'" (3-
m
oG
~
1
Conc. o f D B S in soln.
6
6
{ x l O - 3 m o l / d m a)
F~G.4. Adsorption and diffusion of DBS in cellulose at 30°C. A, total adsorption; ©, monomer adsorption.
159
ADSORPTION AND DIFFUSION OF DBS
creasing salt addition. The values of D gradually increased, passed through a maximum, and finally fell off with increase in salt concentration as in the case of direct dyes (27). DISCUSSION
I
i
i
i
E
%
Although intensive studies on the micelle formation have been undertaken, the theory of micelle formation has not yet been completely established. The micelle size and their distribution in ionic and nonionic surfactants above the CMC have been studied from the theoretical and experimental points of view (28-30). The smallest aggregates and their distribution, which are expected to relate closely to their adsorption, are at present incompletely understood. Many workers, however, have reported the fact that no aggregates of intermediate size were found in measurable amount in the aqueous solution of surfactants above the CMC (3135). Mukerjee has pointed out that there is substantial evidence that a number of surfactants dimerize well below the CMC (36). So far as the monomer concentration of surfactants above the CMC is concerned, there remains much to be learned. Tanford has developed a micelle formation theory by
2
x
o
x t)
2
i Conc. of DBS in soln, ( x l O - 3 m o l / / d m 3)
FIG. 5. Adsorption and diffusion of DBS in cellulose at 50°C. A, total adsorption (B); O, monomer adsorption (A).
x v
u m
1 Conc.
of
i
i
r
I
2
4
6
8
DBSin
soln.
( x l()3mol//drn3~
FIG. 6. Adsorptionand diffusionof DBS in cellulose at 10 and 20°C. Total adsorption: 10°C, A; 20°C, ~3; monomeradsorption: 10°C,O; 20°C, V; diffusioncoefficient: 10°C, O; 20°C, V. presupposing that the monomer concentration above the CMC is constant (28, 37). According to results by Cutler et al., who measured the activity of sodium dodecyl sulfate (SDS) by the use of an ion-selective electrode (38), the activity of SDS anion decreases with increase of the total concentration of surfactant but that of the counterion increases and the mean activity of SDS increases at a lower rate than is the case below the CMC. NMR measurements also showed a decrease of monomer concentration with increasing of total surfactant concentration (39). On the other hand, the adsorption isotherms of surfactant seem in general to be more complex in shape, as Trogus et al. (19) noted, than the distribution of monomer ions in aqueous solution as mentioned above. Thus, many workers have observed maxima of different shapes near the CMC in the adsorption isotherms on various substrates (4-12). The result of the present study using DBS o f a tetramer type also showed a broad maximum in the adsorption isotherms, which was similar to that of Fava and Eyring (6). The present results, on the other hand, showed clearly the existence of two kinds of species of DBS in the adsorption judging Journal of Colloid and Interface Science, Vol. 82, No. 1, July 1981
160
MORITA AND IIJIMA TABLE I
The Salt Effect on the Adsorption and Diffusion of DBS in Cellulose at 30°C DBS conc. (mole/rim 3)
NaCI cone. (g/din ~)
3.0 x 10-4
0 1 3 10
3.0 x 10-3
0
4.21 x 5.38 x 7.01 x 1.04 x
10-3 10-3 10-8 10-2
Diffusion coeff. (from curve A) (cm2/rnin)
1.29 x 5.96 x 5.03 × 3.54 x
10-r 10-7 10-7 10-r
3 10
9.56 x 10-3 1.27 x 10-2 1.25 x 10-2 9.12 x 10-3
4.93 9.15 9.71 7.12
x × x ×
10-~ 10-7 10-r 10-r
0 1 3 i0
1.25 x 10-2 1.42 X 10-2 1.54 x 10-z 1.45 x 10-~
1.03 x 7.31 x 7.01 x 7.43 x
10-6 10-r 10-~ 10-r
1
8.0 x 10-3
DBS adsorption (mole/kg)
from the diffusion profiles near and above the CMC. The species of DBS responsible for the adsorption maximum diffused only into a few layers from the surface of the film roll. The absorption spectra of DBS on cellophane film in which the profile of curve B was contained were found to be the same in shape as those containing only the profile of curve A. Beer's law held over all the wavelengths observed on the cellophane. If the sample of DBS used contained isomers having a small diffusion coefficient corresponding to curve B, the profile of curve B should have been observed in the whole range of surfactant concentration. Thus, the results of the present study cannot be explained by the theory of Trogus et al. (19). If we consider that the dimers of DBS are formed in solution at the concentrations where the distribution profiles of curve B are observed and that they, as well as the monomer, are adsorbed by cellulose, we may well explain the present results. With increasing surfactant concentration, the dimerization of DBS c o m m e n c e s near the CMC, reaches the maximum at a concentration a little higher than the CMC, and Journal of Colloid and Interface Science, Vol. 82, No. 1~ July 1981
then decreases gradually. This behavior is consistent with the view of micelle formation in the surfactant solution mentioned above. As the larger micelles are not adsorbed by cellulose due to the exclusion effect (12), an adsorption maximum may be observed in the a d s o r p t i o n isotherms as a result of m o n o m e r and dimer adsorption. These results were evidenced by the concentration distribution of DBS in cellulose as well as by the diffusion behavior. The diffusion of DBS m o n o m e r corresponding to curve A showed reasonable behavior over all the concentration range discussed. It was confirmed, as a result, that the adsorption isotherm of the DBS m o n o m e r was of Langmuir-type. Diffusion experiments similar to the present study by the use of s o d i u m p - n - d o d e c y l benzene sulfonate showed a sharp adsorption maximum and the existence of an adsorption minimum in the adsorption isotherm (40). The diffusion profiles were similar to those of the present study. The concentration range of the existence of dimer may depend on the kind of surfactant, the purity, and the composition of solution and so on. According to the adsorption theory based on the presence of impurities in cellulose, the decrease in the adsorption above the CMC is attributed to the removal of the impurities by the surfactant (14-18). According to this theory we would expect that the surface concentration of the present study decreases as the diffusion proceeds. Such behavior, however, was not observed as shown in Figs. 1-3. As the addition of sodium chloride to the DBS solution increased the adsorption of DBS on cellulose, it was considered that there was a D o n n a n equilibrium in this adsorption system similar to the adsorption of direct dyes on cellulose (41). The results of salt addition appear to provide no new information to be discussed except an increase in adsorption. Further investigations will be required to
ADSORPTION AND DIFFUSION OF DBS
elucidate the temperature dependence of adsorption shown in Figs. 4 - 6 . SUMMARY
The diffusion of DBS in cellulose was determined by the cylindrical film roll method. From the profiles the equilibrium adsorption and diffusion behavior were discussed in connection with the micelle formation. The adsorption of DBS on cellulose increases gradually with increase in the concentration of DBS in solution and reaches a maximum. The maximum was explained by the adsorption of DBS dimer. The diffusion behavior of DBS in cellulose is consistent with these interpretations. REFERENCES 1. Rosen, M. J., "Surfactants and Interfacial Phenomena," Chap. 2. John Wiley, New York, 1978. 2. White, H. J., Jr., "Cationic Surfactants" (E. Jungermann, Ed.), Chap. 9. Marcel Dekker, New York, 1970. 3. Ginn, M. E., "Cationic Surfactants" (E. Jungermann, Ed.), Chap. 11. Marcel Dekker, New York, 1970. 4. Aickin, R. G., J. Soc. Dyers Colour 60, 60 (1944). 5. Meader, A. L., and Fries, B. A.,Ind. Eng. Chem. 44, 1636 (1952). 6. Fava, A., and Eyring, H., J. Phys. Chem. 60, 890 (1956). 7. Evans, H. C., J. Colloid Sci. 13, 537 (1958). 8. Void, R. D., and Phansalkar, A. K., Rec. Tray. Chim. 74, 41 (1955). 9. Con-in, M. L., Lind, E. L., Roginsky, A., and Harkins, W. D., J. Colloid Sci. 4, 485 (1949). 10. Vold, R. D., and Sivaramakrishnan, N. H.,J. Phys. Chem. 62, 984 (1958). 11. Sexsmith, F. H., and White, H. J., Jr., J. Colloid Sci. 14, 596 (1959). 12. Mukerjee, P., and Anavil, A., "Adsorption at Interfaces" (K. L. Mittal, Ed.), p. 107. Am. Chem. Soc. Symp. Ser. No. 8, Am. Chem. Soc., Washington, D. C., 1975. 13. Sexsmith, F. H., and White, H. J., Jr., J. Colloid Sci. 14, 630 (1959). 14. Kitchener, J. A., J. Phot. Sci. 13, 152 (1965). 15. Saleeb, F. Z., and Kitchener, J. A.,J. Chem. Soc. 1965, 911. 16. Ginn, M. E., Kinney, F. B., and Harris, J. C., J. Amer. Oil Chem. Soc. 38, 138 (1961).
161
17. Schwarz, W. J., Martin, A. R., and Davis, R. C., Text. Res. J. 32, 1 (1962). 18. Schott, H., Text. Res. J. 37, 336 (1967). 19. Trogus, F. J., Schechter, R. S., and Wade, W. H., J. Colloid Interface Sci. 70, 293 (1979). 20. Hummel, D., "Identification and Analysis of Surface Active Agents by Infrared and Chemical Methods." Interscience, New York, 1962. 21. Japanese Industrial Standard, JIS K 3362-1970, "Testing Methods for Synthetic Detergents." 22. Kolthoff, I. M., and Stricks, W,, J. Phys. Colloid Chem. 52, 915 (1948). 23. Paquette, R. G., Lingafelter, E. C., and Tartar, H. V.,J. Amer. Chem. Soc. 65, 686 (1943). 24. Gershman, J. W., J. Phys. Chem. 61, 581 (1957). 25. Sekido, M., and Matsui, K., Sen-i Gakkaishi 20, 778 (1964). 26. Morita, Z., Kobayashi, R., Uchimura, K., and Motomura, H., J. Appl. Polym. Sci. 19, 1095 (1975). 27. McGregor, R., Peters, R. H., and Petropoulos, J. H., Trans. Faraday Soc. 58, 1045 (1962). 28. Tanford, C., "The Hydrophobic Effect," 2nd ed. John Wiley, New York, 1980. 29. Mittal, K. L. (Ed.), "Micellization, Solubilization and Microemulsions." Plenum Press, New York, 1977. 30. Wennerstrrm, H., and Lindman, B., Phys. Rep. 52, 1 (1979). 31. Hoeve, C. A. J., and Benson, G. C., J. Phys. Chem. 61, 1149 (1957). 32. Poland, D. C., and Sheraga, H. A.,J. Phys. Chem. 69, 2431 (1965); J. Colloid Interface Sci, 21,273 (1966). 33. Ruckenstein, E., and Nagarajan, R., "Micellization, Solubilization and Microemulsions" (K. L. Mittal, Ed.), Vol. l, p. 133. Plenum Press, New York, 1977. 34. Aniansson, E. A. G.,Ber. Bunsenges. Phys. Chem. 82, 981 (1978). 35. Mukerjee, P., Bet. Bunsenges. Phys. Chem. 82, 931 (1978). 36. Mukerjee, P., Adv. Colloid Interface Sci. 1, 241 (1967). 37. Tanford, C., J. Phys. Chem. 76, 3020 (1972); 78, 2469 (1974). 38. Cutler, S. G., and Meares, P., J. Chem. Soc. Faraday 1 74, 1758 (1978). 39. Aniansson, E. A. G., et al., J. Phys. Chem. 80, 905 (1976). 40. Duong Xuan Mai, Masters Dissertation, Tokyo University of Agriculture and Technology, 1978. 41. Vickerstaff, T., "The Physical Chemistry of Dyeing," 2nd ed,, Chap. 7. Oliver and Boyd, London, 1954.
Journal of Colloidand Interface Science, Vol.82, No. 1, July 1981