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Adsorption and Adsolubilization Behaviors of Cationic Surfactant and Hydrophobically Modified Polymer Mixtures on Na-Kaolinite Jinben Wang,† Buxing Han,† Haike Yan,*,† Zhixin Li,‡ and R. K. Thomas‡ Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100080, People’s Republic of China, and Physical and Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QZ, U.K. Received February 19, 1999. In Final Form: July 14, 1999 The adsolubilization of 2-naphthol by tetradecyltrimethylammonium bromide (TTAB) and by mixtures of TTAB and hydrophobically modified polyacrylamide (HMPAM) on Na-kaolinite was investigated. At concentrations of TTAB in the region of the CMC, only small differences were found in the amounts of 2-naphthol adsolubilized in the presence and in the absence of HMPAM. However, at much lower TTAB concentrations, adsolubilization of 2-naphthol onto the Na-kaolinite surface was greatly enhanced in the presence of HMPAM. The stability of the clay dispersion was found to be strongly diminished by the addition of HMPAM.
Introduction It is well known that water-insoluble compounds can be incorporated into surfactant layers adsorbed at the solid/aqueous solution interface. This phenomenon of “adsolubilization” has attracted much attention because of its importance in both applications and fundamental research.1 For example, adsolubilization can be a precursor to the formation of thin supported polymer films,2,3 which may then have applications in lubrication or in reversephase chromatographic packings.4 It has also been proposed that adsolubilization might be a basis for new surfactant-based separation processes.5 One particularly promising variation is the use of chiral surfactants for resolving mixtures of optical isomers.6 Other authors have reported applications in wastewater treatment,7,8 removal of toxic substances from soils using surfactants,7,9 and chemical reaction engineering.3,10,11 Further studies have suggested that there is some potential for the use of cationic surfactants in the remediation of contaminated subsoil and aquifers.12-17 In this case, adsorption of cationic * To whom correspondence should be addressed. Fax: +86-01062559373. E-mail:
[email protected]. † Chinese Academy of Sciences. ‡ University of Oxford. (1) Esumi, K.; Takeda, Y.; Goino, M.; Ishiduki, K.; Koide, Y. Langmiur 1997, 13, 2585. (2) Wu, J.; Harwell, H.; O’Rear, E. A. J. Phys. Chem. 1987, 91, 623. (3) Wu, J.; Harwell, J. H.; O’Rear, E. A. Langmuir 1987, 3, 531. (4) Wu, J.; Harwell, J. H.; O’Rear, E. A.; Christian, S. D. AIChE J. 1988, 34, 1511. (5) Barton, J. W.; Fitzgerald, T. P.; Lee, C.; O′Rear, E. A.; Harwell, J. H. Sep. Sci. Technol. 1988, 23, 637. (6) Lee, C.; O’Rear, E. A.; Harwell, J. H.; Sheffield, J. A. J. Colloid Interface Sci. 1990, 137, 296. (7) Laha, S.; Liu, Z.; Edwards, D. A.; Luthy, R. G. In Aquatic Chemistry; Huang, C. P., O’Melia, C. R., Morgan, J. J., Eds.; Advanced Chemistry Series 244; American Chemical Society: Washington, DC, 1995; Chapter 17. (8) Nayar, S. P.; Sabatini, D. A.; Harwell, J. H. Environ. Sci. Technol. 1994, 28, 1847. (9) Sabatini, D. A., Knox, R. C., Harwell J. H., Eds. SurfactantEnhanced Subsurface Remediation; ACS Symposium 594; American Chemical Society: Washington, DC, 1995. (10) Yu, C.; Wong, D. W.; Lobban, L. L. Langmuir 1992, 8, 2582. (11) O’Haver, J. H.; Harwell, J. H.; O’Rear, E. A.; Snodgrass, L. J.; Waddell, W. H. Langmuir 1994, 10, 2588. (12) Boyd S. A.; Lee, J. F.; Mortland, M. M. Nature 1988, 333, 345.
surfactants onto clay results in organo-clays with an enhanced capability for removing hydrophobic organic contaminants from aqueous solution, offering a means to reduce the transport of contaminants through engineered clay barriers, soil profiles, and aquifers.17 From a fundamental point of view, it is important to understand the interactions involved in adsolubilization. Some researchers18-21 have focused on the dependence of the extent of adsolubilization on surfactant concentrations. Thus Esumi et al.22,23 reported the adsorption and adsolubilization by cationic surfactants with one, two, or three alkyl chains using 2-naphthol, a representative toxic substance, as adsolubilizate. Most of the studies of adsolubilization, however, have focused on variation of the adsolubilizate using only a single surfactant. Although some authors24 have studied the adsolubilization behavior using surfactant-polymer mixtures, the adsolubilization behavior of cationic surfactant-HMPAM on clay minerals has not been studied. Hydrophobically modified polymers, in particular, exhibit a variety of unusual properties and display a specific pattern of interactions with cationic surfactants. Studying the factors that govern the adsolubilization behavior of surfactant-hydrophobically modified polymer systems will provide a further possibility of designing rational polymer-surfactant systems for various applications; furthermore, clay mineral is very rich in natural resources, and it is directly related to practical usages, especially in the remediation of contaminated soil (13) Boyd, S. A.; Mortland, M. M.; Chiou, C. T. Soil Sci. Soc. Am. J. 1988, 52, 652. (14) Boyd, S. A.; Sun, S.; Lee, J. F.; Mortland, M. M. Clays Clay Miner. 1988, 36, 125. (15) Jaynes, W. F.; Boyd, S. A. Soil Sci. Soc. Am. J. 1991, 55, 43. (16) Lee, J. F.; Crum, J.; Boyd, S. A. Environ. Sci. Technol. 1989, 23, 1365. (17) Burris, D. R.; Antworth, C. P. J. Contam. Hydrol. 1992, 10, 325. (18) Esumi, K.; Yamanaka, Y. J. Colloid Interface Sci. 1995, 172, 116. (19) Yao, J.; Strauss, G. Langmuir 1991, 7, 2353. (20) Chandar, P.; Somasundaran, P.; Turro, N. J. J. Colloid Interface Sci. 1987, 117, 31. (21) Levitz, P.; Van Damme, H.; Keravis, D. J. Phys. Chem. 1984, 88, 2228. (22) Esumi, K.; Matoba, M.; Yamanaka, Y. Langmuir 1996, 12, 2130. (23) Esumi, K.; Takeda, Y.; Goino, M.; Ishiduki, K.; Koid, Y. Langmuir 1997, 13, 2585. (24) Esumi, K.; Mizuno, K.; Yamanaka, Y. Langmuir 1995, 11, 1571.
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Figure 1. Structure and nomenclature of the hydrophobically modified polymer.
and subsoils and other wastewater treatment applications, as well as in oil recovery. The importance of using it as adsorbent is obvious. In previous studies in our laboratory,25 the interaction between tetradecyl trimethylammonium bromide (TTAB) and hydrophobically modified poly(acrylamide) (HMPAM) in bulk solution was investigated. In the present paper, we studied the adsorption of mixtures of TTAB and HMPAM on Na-kaolinite, followed by adsolubilization of 2-naphthol by the mixtures. Experimental Section Materials. A well-crystallized kaolinite was obtained from The Clay Minerals Society, Source Clay Minerals Repository, University of Missouri, Columbia. The clay was treated with 30% H2O2 for 18 h to remove organic matter. It was then repeatedly washed with 1 M NaCl and then washed with distilled water until no Cl- could be detected. Its surface area was 10.1 m2 g-1 (measured by the Brunauer-Emmett-Teller (BET) (N2) method on an Omnisorp 100 CX). Tetradecyltrimethylammonium bromide (TTAB) (99%) was obtained from Aldrich Chemical Co., Inc. and used as received. The variation of surface tension (γ) with the logarithm of the concentration (ln C) showed no evidence of a minimum, indicating that the surfactant was pure with respect to its surface chemistry. 2-Naphthol was of reagent grade and was used without further purification. Hydrophobically modified poly(acrylamide) was prepared by radical copolymerization of acrylamide and N-alkylacrylamide in tert-butyl alcohol with azobis(isobutyronitrile) as initiator at 60 °C. The synthetic method was as described previously.26,27 The structure and nomenclature of the polymer is shown in Figure 1. The average molecular weight determined by viscometry was approximately 300 000. Twice-distilled water was used in all experiments.
Methods and Measurement Adsorption and Adsolubilization. The adsorption and adsolubilization were studied by adding the desired amounts of solution containing surfactant, polymer, and/or 2-naphthol (the feed concentration of 2-naphthol is 0.4 mmol dm-3) at known concentrations to a series of glass flasks containing defined amounts of Na-kaolinite. The flasks were shaken in a constanttemperature bath (Yamato BT-25, Japan) at a fixed temperature of 20 ( 0.05 °C. The pH values of the suspensions were about 6. Initial experiments showed that shaking for 48 h was sufficient to establish equilibrium. After shaking for this period, the solid particles were separated from the supernatant liquid by centrifugation, and the resultant clear supernatant was removed for analysis of the concentrations of the adsorbates. The concentration of TTAB was determined using the two-phase titration technique,28 with 1,2-dichloroethane as the organic solvent and a hydrophobic dye, tetrabromophenolphthalein ethyl ester (pKa ) 4.2) as the indicator. The concentration of 2-naphthol (25) Wang Y.; Han B.; Yan H.; Kwak, J. C. T. Langmuir 1997, 13, 3119. (26) Effing, J. J.; McLennan, I. J.; Kwak, J. C. T. J. Phys. Chem. 1994, 98, 2499. (27) Effing, J. J.; McLennan, I. J.; Van Os, N. M.; Kwak, J. C. T. J. Phys. Chem. 1994, 98, 12397. (28) Tsubouch, M.; Mitsushio, H.; Yamasaki, N. Anal. Chem. 1981, 53, 1957.
Figure 2. Adsorption of TTAB onto Na-kaolinite. was determined using a UV-vis spectrophotometer (GENERAL TU-1201) at 327 nm. A starch-triiodide colorimetric method29 was used for determining the concentration of HMPAM at a wavelength of 420 nm. Measurements of Zeta Potential. Samples of 5-mg kaolinite were dispersed by ultrasonic homogenization in a 25 mL solution of known concentration of TTAB or TTAB and HMPAM. The zeta potential measurements were made using a microelectrophoresis meter (Type JS94B, Jecheng Instrument Co. Ltd., China). All experimental conditions were kept as similar as possible to those used for the adsorption experiments. Measurements of Dispersion Stability of Na-Kaolinite. Determination of the stability of Na-kaolinite dispersions was performed using the same samples as for the adsorption. After equilibration, the clay was gently mixed and then allowed to stand without disturbance for 2 h before measurement of the light absorption using the UV-vis spectrophotometer. The degree of dispersion of the clay was evaluated by the absorbance at 500 nm of the upper 5 mL of each suspension. A high absorbance indicated a high dispersion stability, whereas a low one indicated a flocculated or settled state.
Results and Discussion Adsorption of TTAB and HMPAM onto Na-Kaolinite. The adsorption isotherm of TTAB on the Nakaolinite surface is shown in Figure 2, in which CTTAB stands for the equilibrium concentration of TTAB in mol dm-3. The isotherm can be approximately divided into three adsorption regions. At very low bulk concentrations, the adsorbed amount increases slowly with increasing surfactant concentration. In the second region, the surfactant concentration is below the critical micelle concentration (CMC) of the surfactant. Here, there is a sharp increase in the adsorbed amount, indicating that there is some cooperativity in the adsorption. This behavior may be attributed to lateral aggregation of the surfactant molecules on the solid surface. The third region is around the CMC of the surfactant. Above the CMC, the concentration of the surfactant monomer remains nearly constant and, in parallel with this, the adsorption remains constant, indicating that there is no adsorption of micelles on the clay surface.30 Figure 3 shows the uptake of polyacrylamide on 1-g of Na-kaolinite. In the absence of surfactant, the adsorption of HMPAM increases gradually and reaches a limiting value of approximately 1.2 × 10-2 g of polymer at a polymer concentration of about 0.4 g dm-3. A suggested mechanism of binding of polyacrylamides onto clay involves the formation of hydrogen bonds between the (29) Scoggins M. W.; Miller J. W. J. Soc. Pet. Eng. 1979, 19, 111. (30) Hough, D. B.; Rendall, H. M. In Adsorption from Solution at the Solid/Liquid Interface; Parafitt, G. D., Rochester, C. H., Eds.; Academic Press: New York, 1983; p 247
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Figure 3. Adsorption of HMPAM onto Na-kaolinite.
Figure 5. Adsolubilization amount of 2-naphthol on the kaolinite surface in the TTAB-adsorbed layers.
Figure 4. Adsorption of TTAB and HMPAM from the mixtures of TTAB + HMPAM onto Na-kaolinite. The initial concentration of HMPAM was 1 g dm-3.
surface hydroxyls and the CdO groups of the polymer.31,32 The adsorption of HMPAM in the presence of TTAB is shown in Figure 4. The amount of adsorbed HMPAM increases slightly in the low TTAB concentration region, but at about 5 × 10-4 mol dm-3 of TTAB, the amount of adsorbed HMPAM decreases steeply. It is interesting that this coincides with the TTAB concentration of 5 × 10-4 mol dm-3, where there is the onset of cooperative adsorption of the TTAB. This phenomenon is consistent with the adsorption being determined mainly by competition between TTAB and HMPAM, although there is a slight increase in the amount of TTAB adsorbed in the presence of HMPAM. In the region of high concentrations of TTAB, the presence of HMPAM causes a marked enhancement in the adsorption of TTAB. This is due to the hydrophobic interaction between the polymer and the surfactant, which would cause an increase in the adsorbed amount of TTAB onto the clay surface. The “necklace model” of polymersurfactant binding describes the polymer-surfactant complex as a series of spherical micelles with their surfaces covered by polymer segments and connected by polymer strands belonging to the same polymer molecule.33 It seems reasonable to postulate that there are some micelles that were adsorbed in the form of a complex with the polymer. Further work on this subject is being undertaken in our lab. Adsolubilization of 2-Naphthol by TTAB onto a Clay Surface. Figure 5 shows the extent of adsolubilization of 2-naphthol by TTAB adsorbed on Na-kaolinite. (31) Stutzmann, T.; Siffert, B. Clays Clay Miner. 1977, 25, 392. (32) Michaels, A. S.; Morelos, S. Ind. Eng. Chem. 1955, 47, 1801. (33) Shirahama, K.; Tsujii, K.; Tsksgi, T. J. Biochem. (Tokyo) 1974, 75, 309.
Figure 6. Adsolubilization amount of 2-naphthol on the kaolinite surface by TTAB and the mixture of TTAB + HMPAM.
There is almost no adsolubilization of 2-naphthol on Nakaolinite in the absence of adsorbed surfactant. In the presence of TTAB, the adsolubilization of 2-naphthol increases with the adsorbed amount of TTAB up to the CMC of TTAB. However, once micelles are formed in the bulk solution, 2-naphthol starts to be partitioned between the adsorbed layer and bulk micelles. As a result, there is a maximum in the adsolubilization of 2-naphthol, and after the maximum, the reduction of the adsolubilization amount occurs. It seems that the reduction begins below the CMC of TTAB. This is attributed to the fact that the concentration for the maximum in the adsolubilization is actually a concentration region and not a precise concentration point. As a matter of fact, some aggregates of TTAB begin to form when the concentration is close to the CMC, which may have a lower aggregation number than that of the micelles; however, they also have the ability to incorporate 2-naphthol molecules, and this may cause the adsolubilization to be lowered before the CMC. The presence of 2-naphthol does not affect the adsorption amount of TTAB significantly. Adsolubilization of 2-Naphthol by the Mixtures of TTAB + HMPAM. Figure 6 compares the adsolubilization of 2-naphthol on Na-kaolinite in the presence of mixtures of TTAB and HMPAM with the adsolubilization in the presence of just TTAB. In the case of adsolubilization by the mixtures, the adsolubilized amount of 2-naphthol
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increases gradually with the concentration of TTAB, reaching a maximum at about 5 × 10-4 mol dm-3 of TTAB before decreasing sharply. This pattern of adsorption is similar to that of the adsolubilization of 2-naphthol by TTAB only, but the presence of HMPAM greatly affects the adsolubilization of 2-naphthol. At the maximum, it is smaller than in the absence of HMPAM, and the concentration of TTAB at the maximum is decreased (Figure 6). This trend coincides with previous results of our own,25 which show that the presence of HMPAM decreases the value of the CMC of TTAB. The result of this lowering of the CMC is that the onset of the partitioning of the 2-naphthol between the adsorbed layer and the micelle occurs at lower TTAB concentrations. However, below about 5 × 10-4 mol dm-3 of TTAB, the adsolubilization of 2-naphthol is greatly enhanced by the addition of HMPAM. The enhancement of the adsolubilization of 2-naphthol is attributed to the fact that HMPAM interacts with TTAB molecules at the clay surface, forming the polymersurfactant complexes. In the lower equilibrium concentration, the complexes have a greater ability to incorporate 2-naphthol molecules than does TTAB itself. In this case, the concentration of TTAB in the bulk solution is not high enough to form complexes with HMPAM. At the clay surface, the TTAB is concentrated and interacts with HMPAM when HMPAM is adsorbed on the surface. At adsorbed layers, the interaction between TTAB and HMPAM involves a surfactant aggregation process similar to micellization. Through hydrophobic interaction, the polymer segments interact with the surfactants already adsorbed on the clay surface, especially in the hydrophobically modified polymer systems. Thus, for TTAB at 0.1 mmol dm-3, the ratio of adsolubilized 2-naphthol to adsorbed TTAB is about 0.8, but this increases to 1.2 in the presence of HMPAM. The efficiency of adsolubilization is obviously increased by the presence of HMPAM. Similar to that of just TTAB, the downturn of the adsolubilization amount also occurs after a maximum. The adsolubilization behavior and the conformation of the adsorbed layers are related to the interaction between TTAB and HMPAM molecules. In our previous work,25 we studied the interaction between HMPAM and TTAB and the conformation of the HMPAM-TTAB complex in the bulk solution. It was found that HMPAM and TTAB can form a complex through hydrophobic interaction between the alkyl chain in the TTAB and HMPAM segments. It was concluded that mixed micelles may be formed with surfactant aggregates and HMPAM segments. In this study, in which the adsorption of TTAB and HMPAM from their binary mixture is carried out on a negatively charged clay surface, TTAB reaches the solid surface much faster than HMPAM due to the greater diffusion coefficient of TTAB. The TTAB molecule is concentrated at the clay surface, orienting its hydrophobic chain to the solution. Then, HMPAM adsorbs on the clay surface through the hydrophobic interaction between HMPAM and TTAB. It is reasonable to postulate that complexes were formed between TTAB and HMPAM at the solid surface. The complex, like a mixed micelle, forms a hydrophobic environment, which has the ability to incorporate 2-naphthol. It was found in our experiment that HMPAM itself has no ability to adsolubilize 2-naphthol. So, the enhancement in the adsolubilization is apparently due to the formation of the HMPAM-TTAB complex on the clay surface. At a low TTAB concentration, there are no TTAB aggregates or polymer-surfactant complexes forming in the bulk solution. So, the reduction of the 2-naphthol concentration in solution can only be attributed to the adsolubilization of 2-naphthol onto the clay surface. But, at the high TTAB concentration region,
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Figure 7. ζ-Potential of Na-kaolinite in TTAB and TTAB + HMPAM solutions.
Figure 8. Degree of dispersion of Na-kaolinite as the TTAB concentration increases.
there are surfactant aggregates or polymer-surfactant complex that exist in the bulk solution. 2-Naphthol can be incorporated in both the TTAB aggregates or polymerTTAB complexes in the solution and the adsorbed layers at the clay surface. The driving force to the distribution of the polar oil between the solution and the clay surface is mainly the hydrophobic interaction. Electrophoretic Mobility and the Degree of Clay Dispersion. Figure 7 shows the ζ potential of kaolinite suspensions following adsorption of TTAB in the presence and absence of HMPAM. The ζ potential is increased from negative values to zero and then becomes positive as the surfactant concentration increases. These changes in ζ potential have a considerable influence on the stability of the clay dispersions. Figure 8 shows the changes in the UV absorbance of Na-kaolinite suspensions after the adsorption of TTAB or TTAB + HMPAM mixtures. Note that the absorbances of the aqueous solutions of TTAB alone and TTAB + HMPAM mixtures are negligibly small compared with the clay suspensions. The absorbance of the suspensions in the presence of TTAB and TTAB + HMPAM decreases to a minimum, then increases with increasing surfactant concentration. These results show a similar trend to those reported previously on surfactantcontaining systems.22,34 The stability of kaolinite dispersions is greatly affected by the electrostatic forces between clay particles and is approximately proportional to the (34) Xu, S.; Boyd, S. A. Langmuir 1995, 11, 2508.
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absolute value of the ζ potential. The degree of dispersion is also significantly affected by the presence of HMPAM and, as indicated by the UV absorbance, is higher in the absence of HMPAM (Figure 8). That the presence of HMPAM induces clay flocculation or settlement is clearly indicated by the decrease of the absorbance of the clay suspension. Conclusions The adsorption of TTAB onto Na-kaolinite in the presence and absence of HMPAM has been determined. The subsequent adsolubilization of 2-naphthol by adsorbed TTAB and by mixtures of TTAB and HMPAM has also been investigated. It was found that the presence of HMPAM strongly affects the amounts of 2-naphthol adsolubilized onto the clay surface. At high TTAB concentrations (in the region of the CMC), the adsolubilization
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efficiency of 2-naphthol by the mixture is not higher than with TTAB alone. At lower TTAB concentrations, however, the amount of 2-naphthol adsolubilized is much higher in the presence of HMPAM than in the absence of HMPAM, at a given concentration of TTAB. Finally, the degree of clay dispersion is greatly influenced by the presence of HMPAM. The stability of the clay dispersion is significantly lower in the presence of HMPAM, as indicated by the UV absorbance of the clay suspensions. Acknowledgment. The authors are grateful to the Royal Society U.K., the Chinese Academy of Sciences, the National Natural Science Foundation of China and the State Science and Technology Commission of China for their financial support. LA9901837
