Langmuir 1997, 13, 2585-2587
2585
Adsorption and Adsolubilization by Cationic Surfactants on Laponite Clay Kunio Esumi,*,† Yasuko Takeda,† Masaya Goino,† Kenji Ishiduki,† and Yoshifumi Koide‡ Department of Applied Chemistry and Institute of Colloid and Interface Science, Science University of Tokyo, Kagurazaka, Shinjuku-ku, Tokyo 162, Japan, and Department of Applied Chemistry and Biochemistry, Faculty of Engineering, Kumamoto University, Kurokami, Kumamoto 860, Japan Received December 19, 1996. In Final Form: February 18, 1997
Introduction Water-insoluble substances are often incorporated in surfactant adsorbed layers on solids which is called “adsolubilization”.1 For such studies, many solids such as alumina,2-6 titania,6 silica,5-10 and latex11 have been used so far. One of the interesting applications of adsolubilization is to study removal of toxic substances in wastewater. Indeed, many workers12,13 have reported removal of toxic substances from soils using surfactants, although the mechanisms for removing toxic substances are not fully understood due to a very complicated phenomenon. From a fundamental point of view, it is very important to understand interactions among soils, toxic substances, and surfactants. In this study, adsorption and adsolubilization by cationic surfactants with different structures on clay are reported in which laponite is used as clay because physicochemical properties of laponite are well-known.14-16 As an adsolubilizate, 2-naphthol is employed, which is one of the representative toxic substances. Experimental Section Materials. Laponite kindly supplied by Nippon Silica Co. was used as an adsorbent. The cationic exchange capacity and the BET surface area determined by nitrogen adsorption were 0.72 mequiv/g and 224.9 m2/g, respectively. † ‡
Science University of Tokyo. Kumamoto University.
(1) Wu, J.; Harwell, J. H.; O’Rear, E. A. Langmuir 1987, 3, 531. (2) Lee, C.; Yeskie, M. A.; Harwell, J. H.; O’Rear, E. A. Langmuir 1990, 6, 1758. (3) Nayyar, S. P.; Sabatini, D.; Harwell, J. H. Environ. Sci. Technol. 1994, 28, 1874. (4) Esumi, K.; Yamanaka, Y. J. Colloid Interface Sci. 1995, 172, 116. (5) Schieder, D.; Dobias, B.; Klumpp, E.; Schwuger, M. J. Colloids Surf. 1994, 88, 103. (6) Favoriti, P.; Monticone, V.; Treiner, C. J. Colloid Interface Sci. 1996, 179, 173. (7) Gao, Y.; Du, J.; Gu, T. J. Chem. Soc., Faraday Trans. 1 1986, 83, 2671. (8) Esumi, K.; Nagahama, T.; Meguro, K. Colloid Polym. Sci. 1991, 269, 1274. (9) Monticone, V.; Treiner, C. J. Colloid Interface Sci. 1994, 166, 394. (10) Esumi, K.; Matoba, M.; Yamanaka, Y. Langmuir 1996, 12, 2130. (11) Jansen, J.; Treiner, C.; Vaution, C. J. Colloid Interface Sci. 1996, 179, 578. (12) Laha, S.; Liu, Z.; Edwards, D. A.; Luthy, R. G. In Aquatic Chemistry: Interfacial and Interspecies Processes; Huang, C. P., O’Melia, C. R., Morgan, J. J., Eds.; Adv. Chem. Ser. 244; American Chemical Society: Washington, DC, 1995; Chapter 17. (13) Sabatini, D. A., Knox, R. C., Harwell, J. H., Eds. SurfactantEnhanced Subsurface Remediation; ACS Symp. Ser. 594; American Chemical Society: Washington, DC, 1995. (14) Perkins, R.; Brace, R.; Matijevic, E. J. Colloid Interface Sci. 1974, 48, 417. (15) Fripiat, J.; Cases, J.; Francois, M.; Letellier, M. J. Colloid Interface Sci. 1982, 89, 378. (16) Ramsay, J. D. F. J. Colloid Interface Sci. 1986, 109, 441.
S0743-7463(96)02120-8 CCC: $14.00
Figure 1. Chemical structures of cationic surfactants used in this study. As surfactants, dodecyltrimethylammonium bromide (1RQ), 1,2-bis(dodecyldimethylammonio)ethane dibromide (2RenQ), and methyldodecylbis[2-(dimethyldodecylammonio)ethyl]ammonium tribromide (3RdienQ) were used. 1RQ was obtained from Tokyo Kasei Co. and was used after crystallization from acetone. 2RenQ and 3RdienQ were synthesized and purified.17 Their surfactant structures are given in Figure 1. The water used in this study was purified through a Milli-Q system. 2-Naphthol was of reagent grade and was used as received. Methods and Measurements. The adsorption of surfactant and adsolubilization of 2-naphthol on laponite in the presence of 10 mmol dm-3 NaBr were performed as follows. A series of aqueous solutions of the desired surfactant concentrations was prepared with 10 mmol dm-3 NaBr added. The final feed concentrations used of 2-naphthol were 0.4 and 2.0 mmol dm-3. Then the solution was added to laponite suspensions which had been previously prepared in a capped vial. The laponite concentration in the suspensions was 10 wt %. The glass vial was equilibrated by shaking in a water bath at 25 °C for 24 h. After equilibration, the solids were separated by filtration and the supernatant was analyzed for surfactants using a dye method and for 2-naphthol using a UV spectrophotometer (HP 8452A). The pH of the solutions was not adjusted but was about 6. The ζ potential of the suspensions was measured with an electrophoretic apparatus (Pen Kem 500). The suspensions containing pyrene were prepared as follows. A known volume of the pyrene/ ethanol solution was placed into a glass vial and the solvent was removed by pumping; the suspensions were added in the vial. Then the suspensions were shaken at 25 °C for 24 h. The concentration of pyrene was from 1 × 10-6 to 1 × 10-7 mol dm-3. The steady-state emission spectra of pyrene in the suspensions were measured by using a fluorescence spectrophotometer (65010S, Hitachi Co.). The excitation wavelength for pyrene was 335 nm. The critical micelle concentrations (cmc’s) of the surfactants containing 10 mmol dm-3 NaBr were 12.0 for 1RQ, 3.8 × 10-2 for 2RenQ, and 1.7 × 10-2 mmol dm-3 for 3RdienQ, respectively.19 X-ray diffraction analyses were carried out using a Mac Science X-ray diffractometer (M03X-HF, Model 1031). The measurements covered an angular range of 2θ ) 2-15° using Cu KR radiation, Ni filter.
Results and Discussion Figure 2 shows the adsorption isotherms of 1RQ, 2RenQ, and 3RdienQ on laponite. It is apparent that the adsorption of 1RQ and 2RenQ increases sharply at low (17) Esumi, K.; Taguma, K.; Koide, Y. Langmuir 1996, 12, 4039. (18) Kalynasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99, 2039. (19) Esumi, K.; Goino, M.; Koide, Y. J. Colloid Interface Sci. 1996, 183, 539.
© 1997 American Chemical Society
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Figure 2. Adsorption isotherms of 1RQ, 2RenQ, and 3RdienQ on laponite.
Figure 3. Change in ζ potential of laponite with the adsorption of surfactants.
concentration and then gradually increases. At low 1RQ and 2RenQ concentrations, the amounts adsorbed of 1RQ and 2RenQ were more than that of CEC (cationic exchange capacity). On the other hand, the amount adsorbed on 3RdienQ was considerably smaller than those of 1RQ and 2RenQ. Compared with the amount adsorbed of surfactant at each cmc, the amount adsorbed was increased in the following order: 1RQ > 2RenQ > 3RdienQ. These adsorption behaviors would affect the surface charge of laponite particles. The ζ potential of laponite suspensions with the adsorption of 1RQ and 2RenQ was measured (Figure 3). The ζ potential of laponite particles with a negative value increased with the surfactant concentration and reached zero, and with a further increasing surfactant concentration the ζ potential showed a positive value. These changes in the ζ potential occur by the adsorption of cationic
Notes
Figure 4. Change in I1/I3 ratios with the adsorption of surfactants.
surfactant on the outer surface as well as the exchange of cation by surfactant ions. At positive ζ potential region, a surfactant bilayer would be formed on the outer surface. In the case of 3RdienQ, large flocs were formed so that the ζ potential could not be measured in some 3RdienQ concentration region. The adsorbed state of the surfactants on laponite was estimated by using fluorescence spectra of pyrene. Pyrene probe exhibits five fluorescence absorption bands, where the ratio of the first and the third vibronic bands (I1/I3) is very sensitive to the polarity of the environment where the pyrene is located.18 As shown in Figure 4, the ratios of I1/I3 for 1RQ-, 2RenQ-, and 3RdienQ-laponite systems decreased with the surfactant equilibrium concentration, indicating that the micropolarity in the surfactantadsorbed layer is much lower compared with that of laponite alone. The ratios of I1/I3 for 2RenQ were greater than those for 1RQ and 3RdienQ, although the reason is not known at the present time. Thus, it is found that the microenvironment in the surfactant-adsorbed layer becomes hydrophobic with increasing surfactant adsorption. It is interesting to elucidate adsolubilization behavior of 2-naphthol by the cationic adsorbed layer on laponite. Figure 5 shows the amount adsolubilized of 2-naphthol with the adsorption of 1RQ, 2RenQ, and 3RdienQ. It is noteworthy that the amounts adsorbed of the surfactants in the absence of 2-naphthol are almost the same as those in the presence of 2-naphthol. Interestingly, all the naphthol molecules added as 0.4 mmol dm-3 feed concentration were incorporated in the adsorbed layer of the three surfactants even at their low equilibrium concentration. Since laponite itself has no capacity for incorporation of 2-naphthol, this large incorporation of 2-naphthol is predominantly controlled by the surfactantadsorbed layer, which shows hydrophobic. The ratio of incorporated naphthol concentration to amount adsorbed of the surfactant is appreciably dependent on the surfactant equilibrium concentration: The ratio is very large at low surfactant concentration but decreases rapidly with increasing surfactant concentration. A similar change in the ratio with surfactant adsorption has been reported for the 1-pentanol-sodium dodecyl sulfate-alumina system.2 In the case of the feed concentration of 2-naphthol
Notes
Langmuir, Vol. 13, No. 9, 1997 2587
Figure 5. Adsolubilization of 2-naphthol in the surfactantadsorbed layers.
Figure 6. Basal spaces of laponite with the adsorption of cationic surfactants.
of 2 mmol dm-3, all the naphthol molecules were also incorporated for the three surfactant systems except at lower and higher 1RQ concentrations. Compared the ratio by laponite with the other particles such as silica10,19 and alumina,4 the ratio by laponite was very low because the adsorbed amount of surfactants on laponite was extremely large. This adsorption characteristic of laponite can be correlated with the layered structure of laponite. In the case of 1RQ, the amount adsolubilized of 2-naphthol was decreased at higher 1RQ concentrations (above cmc). This result suggests that 2-naphthol is distributed into the surfactant-adsorbed layer and micelles in bulk. Figure 6 shows the results of basal spacings of laponite with the adsorption of cationic surfactants. It is noteworthy that the basal spacings of laponite with surfactant are almost the same as those by 2-naphthol adsolubilization. According to the model concept of Lagaly,20 layer broadenings of 0.4 and 0.8 nm for low-charged smectite correspond to a monolayer and a double layer incorporation by monoalkyl chains arranged parallel to the silicate layers. Layer broadening was obtained from a comparison of the layer spacing after incorporation of the surfactants with the layer spacing of untreated laponite. In addition, layer broadenings of 3.4-3.5 nm and 4.9-5.0 nm point a
bilayer incorporation by double-chain dodecyl and triplechain dodecyl surfactants arranged perpendicular to the silicate layers.21 In this study, the layer broadening for laponite changed with the amounts adsorbed of the surfactants: in the case of 1RQ the layer broadening of 0.8 nm was observed in a wide adsorbed amount region, while those by 2RenQ and 3RdienQ were 0.9-1.1 nm and around 0.9 nm, respectively. These results may imply that 1RQ adsorbs as a double layer of flat lying, while 2RenQ and 3RdienQ adsorb as a monolayer of flat-lying at low adsorption and perpendicular to the silicate layer at high adsorption. In the interlayer volume of laponite, most of the adsorbed surfactants in this study are incorporated into the interlayer. It is concluded from the above results that adsorption of cationic surfactants is increased from 3RdienQ to 1RQ in the layer of laponite, resulting in the layer broadening, where the adsorption by 2RenQ and 3RdienQ makes the layer much larger than that by 1RQ. Most of 2-naphthol molecules as the feed concentrations (0.4 and 2 mmol dm-3) are adsolubilized in the surfactant-adsorbed layer.
(20) Lagaly, G.; Weiss, A. Kolloid Z. Z. Polym. 1970, 243, 48.
LA962120J (21) Okahata, Y.; Shimizu, A. Langmuir 1989, 5, 954.
