Characterization of Adsorption of Quaternary Ammonium Cationic

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Langmuir 1996, 12, 2130-2135

Articles Characterization of Adsorption of Quaternary Ammonium Cationic Surfactants and Their Adsolubilization Behaviors on Silica Kunio Esumi,* Mika Matoba, and Yoko Yamanaka Department of Applied Chemistry and Institute of Colloid and Interface Science, Science University of Tokyo, Kagurazaka, Shinjuku-ku, Tokyo 162, Japan Received October 31, 1995X. In Final Form: January 29, 1996 Adsorption of quaternary ammonium cationic surfactants with one, two, or three alkyl chains on silica has been studied by measuring adsorption density, ζ potential, and dispersion stability. The adsorbed amounts at saturation decrease with increasing chain number of the surfactants. Silica suspensions exhibit a process of dispersion-flocculation-redispersion with the surfactant concentration for three surfactants which can be well correlated with the change in ζ potentials. Fluorescence and ESR measurements using probes show that the microproperties in the adsorbed layers are considerably affected by their chain numbers of the surfactants. Under a constant feed concentration of 2-naphthol, the adsolubilized amounts of 2-naphthol increase, reach a maximum, and then decrease with single-chain or double-chain surfactant concentration, whereas only a slight decrease in the adsolubilized amount of 2-naphthol is observed for the triple-chain surfactant. The ratios of amount of 2-naphthol adsolubilized to the adsorbed amount of surfactant on silica for the double-chain and triple-chain surfactants are not so different and are quite large compared to that for the single-chain surfactant. In addition, from a two-step adsorption-adsolubilization procedure, it is found that the double-chain or triple-chain surfactant adsorbs strongly on the silica surface, keeping 2-naphthol molecules in the adsorbed layer in comparison with the single-chain surfactant.

1. Introduction Adsorption of surfactant at the solid/aqueous solution interface has been extensively studied1-3 to understand interactions between the surfactant and the solid surface. The information obtained is very useful in improving the efficiency of the industrial applications such as detergency, dispersion stability of solids, flotation, etc. It has been found4 that the adsorption of surfactant is strongly affected by surfactant structure, pH, ionic strength, and temperature. In particular, it is very important to study the effect of surfactant structure5-9 on the adsorption. However, there is no systematic work on the effect of surfactants with different chain numbers on adsorption from aqueous solution. It is also interesting to characterize the microproperties of a surfactant adsorbed layer on solid * To whom correspondence should be addressed. Fax: 03-32352214. E-mail: [email protected]. X Abstract published in Advance ACS Abstracts, April 15, 1996. (1) Clunie, J. S.; Ingram, B. T. In Adsorption from Solution at the Solid/Liquid Interface; Parfitt, G. D., Rochester, C. H., Eds.; Academic Press: New York, 1983; p 105. (2) Schwuger, M. J. In Anionic Surfactants; Lucassen-Reynders, E. H., Ed.; Surfactant Science Series; Marcel Dekker: New York, 1981; Vol. 11. (3) Dobias, B. In Coagulation and Flocculation; Dobias, B., Ed.; Marcel Dekker: New York, 1993; Vol. 47, Chapter 12. (4) Clint, J. H. Surfactant aggregation; Chapman and Hall: New York, 1992; Chapter 9. (5) Kumagai, S.; Fukushima, S. J. Colloid Interface Sci. 1973, 42, 539. (6) Sivakumav, A.; Somasundaran, P.; Thach, S. J. Colloid Interface Sci. 1993, 159, 481. (7) Wangnerud, P.; Berling, D.; Olofsson, G. J. Colloid Interface Sci. 1995, 169, 365. (8) Esumi, K.; Yamanaka, Y. J. Colloid Interface Sci. 1995, 172, 116. (9) Yao, J.; Strauss, G. Langmuir 1991, 7, 2353.

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particles. For this purpose, spectroscopic tools including fluorescence,10-12 ESR,13,14 and NMR15,16 have been used. The surfactant adsorbed layer formed at the solid/ aqueous solution interface provides hydrophobic circumstances which can incorporate water-insoluble compounds. This incorporation is referrred to as “adsolubilization”.1 Some groups18-21 reported that the adsolubilized amounts of water-insoluble compounds increase, reach maxima, and then decrease with increasing surfactant concentration, where this decrease occurs probably due to the solubilization of the water-insoluble compounds into micelles in bulk. However, most of the studies for adsolubilization have been limited to the use of singlechain surfactants. On the other hand, we have performed the adsolubilization experiments using a double-chain surfactant22 such (10) Chandar, P.; Somasundaran, P.; Turro, N. J. J. Colloid Interface Sci. 1987, 117, 31. (11) Levitz, P.; Van Damme, H.; Keravis, D. J. Phys. Chem. 1984, 88, 2228. (12) Esumi, K.; Sakamoto, Y.; Meguro, K. Colloid Polym. Sci. 1989, 267, 525. (13) Waterman, K. C.; Turro, N. J.; Chandar, P.; Somasundaran, P. J. Phys. Chem. 1986, 90, 6830. (14) Esumi, K.; Otsuka, H.; Meguro, K. J. Colloid Interface Sci. 1990, 136, 224. (15) Soderlind, E.; Stibs, P. Langmuir 1993, 9, 1678. (16) Quist, P.-Q.; Soderlind, E. J. Colloid Interface Sci. 1995, 172, 510. (17) Wu, J.; Harwell, J. H.; O’Rear, E. A. Langmuir 1987, 3, 531. (18) Lee, C.; Yeskie, M. A.; Harwell, J. H.; O’Rear, E. A. Langmuir 1990, 6, 1758. (19) Zhu, B.-Y.; Zhao, X.; Gu, T. J. Chem. Soc., Faraday Trans. 1 1988, 84, 3951. (20) Nagahama, T.; Esumi, K.; Meguro, K. Shikizai Kyokaishi 1991, 64, 366. (21) Monticone, V.; Mannebach, M. H.; Treiner, C. Langmuir 1994, 10, 2395. (22) Esumi, K.; Sugimura, A.; Yamada, T. Shikizai Kyokaishi 1993, 66, 142.

© 1996 American Chemical Society

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as dioctadecyldimethylammonium chloride (DODAC), which is well-known to provide unilamellar vesicles23,24 in aqueous solution. It was found that the maximum ratio of Yellow OB adsolubilized to the DODAC adsorbed layer was remarkably greater than that of Yellow OB to a singlechain surfactant. This difference can be understood, taking into consideration that the volume of hydrocarbon chains in the DODAC-adsorbed layer is greater than that in the single-chain surfactant layer. Consequently, the adsolubilization behavior of multichain surfactants is expected to be different from that of single-chain surfactants. This adsolubilization process can also be applicable for wastewater treatment,25,26 in particular of toxic hydrophobic organic compounds such as 2-naphthol. The objective of this study is to characterize adsorption of quaternary ammonium cationic surfactants and their adsorbed layers on silica, which is a representative mineral oxide. The adsolubilization behavior of 2-naphthol into the quaternary ammonium surfactant adsorbed layer formed on silica has also been studied. The quaternary ammonium cationic surfactants used are single-, double-, or triple-chain ones. 2. Experimental Section 2.1. Materials. Dodecyltrimethylammonium chloride (DTAC), didodecyldimethylammonium bromide (DDAB), and tridodecylmethylammonium chloride (TMAC) were obtained from Tokyo Kasei Industries, Co. DTAC was used after several recrystallizations from acetone, while the others were used as received. Pyrene-1-carboxaldehyde (PCA) and pyrene were obtained from Aldrich Chemical Co. and Tokyo Kasei Industries, Co., respectively. PCA was used as received, and pyrene was purified four times by recrystallization from ethanol. The spin probe methyl 12-doxyl stearate (12-DOXYL ME) from Sigma Chemical Co. was used as received. 2-Naphthol was of reagent grade and used as received. The water used in this study was purified by passing it through a Milli-Q System until the specific conductivity fell below 0.1 µS cm-1. Silic was kindly supplied by Nihon Shokubai Co. The specific surface area and average particle size were 16.7 m2 g-1 and 0.3 µm, respectively. 2.2. Methods. Adsorption of cationic surfactants and adsolubilization of 2-naphthol were carried out as follows. A series of surfactants with desired concentrations in aqueous solutions was prepared in the absence and presence of 2-naphthol containing 10 mmol dm-3 of NaCl or NaBr. For DTAC and TMAC NaCl was used, and for DDAB NaBr was used. The final feed concentration of 2-naphthol was taken at 0.4 mmol dm-3. Then, 20 cm3 of the solutions was added to silica (0.3 g) in a glass vial with a cap. The glass vial was equilibrated by shaking at 25 °C for 24 h in a water bath. After equilibration, the solids were separated by centrifugation and the supernatant was analyzed for 2-naphthol using a UV spectrophotometer (HP 8452A) and for the surfactants using a dye method.27 The pH of the solutions was not adjusted but was around 6. The suspension prepared above without 2-naphthol was added into a flask in which pyrene-1-carboxaldeyde (PCA) or pyrene (1 × 10-6 mol dm-3) dissolved in ethanol had first been introduced, and the solvent was then allowed to evaporate for fluorescence measurements. A similar procedure was carried out for ESR measurements using 12-DOXYL ME, 1 × 10-5 mol dm-3. Another experiment was done. At first, the surfactant only was adsorbed on silica from its aqueous solution for 24 h, and then, the supernatant was discarded. Then the aqueous solutions containing 2-naphthol (0.4 mmol dm-3) were added to the (23) Kunitake, T.; Okahata Y. J. Am. Chem. Soc. 1977, 99, 3860. (24) Deguchi, K.; Mino, J. J. Colloid Interface Sci. 1978, 65, 155. (25) Laha, S.; Liu, Z.; Edwards, D. A.; Luthy, R. G. In Aquatic Chemistry; Huang, C. P., Melia, C. R. O., Morgan, J. J., Eds.; Advanced Chemistry Series No. 244; American Chemical Society: Washington, DC, 1995; Chapter 17. (26) Nayyar, S. P.; Sabatini, D. A.; Harwell, J. H. Environ. Sci. Technol. 1994, 28, 1874. (27) Scott, G. V. Anal. Chem. 1968, 40, 769.

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Figure 1. Adsorption isotherms of cationic surfactants on silica. surfactant adsorbed silica and the suspensions were again equilibrated by shaking at 25 °C for 24 h. 2.3. Measurements. ζ potential and dispersion stability measurements were performed using the same samples used to measure adsorption. The ζ potential measurements were made with an electrophoretic apparatus (Pen Kem 500). The stability of the silica dispersion was evaluated by measuring the absorbance at 600 nm of the top portion of the aqueous suspension in a sedimentation tube kept for one day after the adsorption. A high absorbance indicates a high dispersion stability, whereas a low one indicates a flocculated or settled state. The steady-state emission spectra of PCA or pyrene in the suspension and supernatant were obtained by using a Hitachi 650-10S fluorescence spectrophotometer. The excitation wavelengths of PCA and pyrene were 356 and 335 nm, respectively. The ESR spectra were recorded on a JEOL JES FE-3-X spectrometer utilizing a 100-kHz field modulation and X-band microwaves. The slurries for the ESR measurements were obtained by centrifugation of the adsorption samples. All measurements were carried out at 25 °C.

3. Results and Discussion Figure 1 shows the adsorption isotherms of the cationic surfactants on silica. It is seen that the adsorbed amount increases with the surfactant concentration and levels off for three surfactants. In particular, for DDAB and TMAC, the adsorbed amount increased markedly at lower concentrations. The adsorbed amounts at saturation were about 8 × 10-5, 6 × 10-5, and 5 × 10-5 mol g-1 for DTAC, DDAB, and TMAC, respectively. The limiting surface areas per surfactant molecule were 35 Å2 for DTAC, 46 Å2, for DDAB, and 55 Å2 for TMAC, respectively, as calculated from the adsorbed amount at saturation and the specific surface area of the silica. Since the molecular size of these cationic surfactants depends on their chain numbers, a small difference in the adsorbed amounts at saturation may derive from different molecular organized structures; DDAB or TMAC molecules adsorbed are much more packed than DTAC. It is known28 that when ionic single-chain surfactants adsorb on oppositely charged particles, a bilayer of their surfactants is formed with increasing surfactant concentrations. In the cases of double-chain and triple-chain surfactants,29 there is a great possibility of formation of vesicle-like structure on particles. Figure 2 shows the ζ potential of silica suspensions by adsorption of the cationic surfactants. For three surfactants, the ζ potential increased from negative to zero and (28) Meguro, K.; Kondo, T. Nippon Kagaku Zasshi, 1955, 76, 642. (29) Kunitake, T.; Kimizuka, N.; Higashi, N.; Nakashima, N. J. Am. Chem. Soc. 1984, 106, 1978.

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Figure 2. Change in ζ potential of silica suspensions by adsorption of cationic surfactants.

Figure 3. Change in absorbance of silica suspensions by adsorption of cationic surfactants.

converted to positive with increasing surfactant concentration. The features in the change of the ζ potential are very similar to those of adsorption. The ζ potential change with the surfactant concentration can be interpreted as follows. The cationic surfactants adsorb on negatively charged silica, orienting the hydrophilic groups of the surfactants to the negatively charged sites on silica. As a result, the surface of silica becomes hydrophobic. With increasing surfactant concentration, the formation of a bilayer by DTAC or of a vesicle-like layer by DDAB or TMAC increases the ζ potential positively. Such ζ potential changes considerably influenced their stabilities of the silica dispersions. Figure 3 shows the changes in the absorbance of the silica suspensions by adsorption of the cationic surfactants. It should be mentioned that the absorbances of the aqueous solutions of DDAB or TMAC alone are negligibly small compared with those of the suspensions. The absorbances of the silica suspensions decreased, reached a minimum, and then quickly increased with increasing surfactant concentration for three cationic surfactants. These changes in the absorbance with the surfactant concentration can be well correlated with a process of dispersion-flocculation-redispersion.30 Furthermore, these dispersion stabilities of silica are dominantly controlled by the electrostatic forces between silica particles: the dispersion stability is roughly proportional to the absolute value of the ζ potentials of silica suspensions. (30) Ogihara, K.; Tomioka, S.; Esumi, K.; Meguro, K. Shikizai Kyokaishi 1982, 55, 546.

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Figure 4. Maximum wavelength of PCA for surfactants-silica systems vs equilibrium surfactant concentration.

In order to elucidate the microproperties of the cationic surfactant adsorbed layer on silica, fluorescence and ESR techniques have been applied. It is known31 that the maximum fluorescence wavelength (λmax) of PCA is very sensitive to solvent polarity; λmax decreases with decreasing polarity of the solvents. Figure 4 shows the λmax of PCA in supernatant solutions and at the silica/liquid interface. It is seen that the λmax of PCA in the suspension and supernatant for DTAC decreases slightly at low concentrations but abruptly falls down at around 452 nm with increasing DTAC concentration. Over a wide DTAC concentration region, the λmax of PCA in the suspension was much lower than that in the supernatant, indicating that the polarity sensed by PCA in the DTAC adsorbed layer is lower than that in the supernatant. A very similar change of the λmax for the hexadecyltrimethylammonium bromide-silica system has been observed.32 On the other hand, for DDAB and TMAC, the λmax of PCA in the suspension was only slightly lower than that in the supernatant. Thus, the micropolarities sensed by PCA were clearly different among the three surfactants. These differences can be understood by the view that since PCA molecules are not penetrated into highly compacted adsorbed layers of DDAB or TMAC, they are placed near the surface of the adsorbed layers so that a higher polarity in the DDAB or TMAC adsorbed layer is obtained compared with that in the DTAC adsorbed layer. The same idea is also applied to the polarity of the supernatants. A similar experiment using pyrene, which has a much lower polarity than PCA, was performed. The pyrene fluorescence fine structure has been found to be remarkably dependent on the solvent: the intensity ratio of I1/I3 (I1 first vibronic band and I3 third vibronic band, at 373 and 383 nm, respectively) is sensitive to the solvent polarity.33 Figure 5 shows the changes in I1/I3 in the supernatants and at the solid/liquid interfaces for the three surfactants. In the DTAC-silica system, I1/I3 in the supernatant gradually decreased and showed a break point at around the cmc of DTAC, while that at the solid/ liquid interface decreased and remained almost constant with increasing DTAC concentration. This indicates that the polarity of the DTAC adsorbed layer sensed by pyrene is considerably lower than that in the supernatant. A similar behavior was observed for the DDAB- and (31) Kalyanasundaram, K.; Thomas, J. K. J. Phys. Chem. 1977, 81, 2176. (32) Esumi, K.; Nagahama, T.; Meguro, K. Colloid Polym. Sci. 1991, 269, 1274. (33) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99, 2039.

Quaternary Ammonium Cationic Surfactants

Figure 5. I1/I3 of pyrene for surfactant-silica systems vs equilibrium surfactant concentration.

Figure 6. ESR spectra of 12-DOXYL ME by adsorption of surfactants on silica: (a) without surfactant; (b) DTAC, 27 mmol dm-3; (c) DDAB, 1.2 mmol dm-3; (d) TMAC, 0.6 mmol dm-3 of feed concentration.

TMAC-silica systems. In addition, the polarity of the DDAB adsorbed layer was much lower compared with those of the DTAC and TMAC layers. Figure 6 shows ESR spectra of 12-DOXYL ME recorded for the silica/aqueous solution interface with adsorption of three surfactants. In the absence of surfactant, the spectrum of 12-DOXYL ME was characterized by hyperfine splittings. By adsorption of the surfactant, the line broadening of the ESR spectra occurred, indicating that the mobility of 12-DOXYL ME in the surfactant adsorbed layer decreases. In order to obtain more informative knowledge, the order parameter (S) of 12-DOXYL ME was calculated from maximum hyperfine splitting and the known hyperfine crystal tensors Axx, Ayy, and Azz for DOXYL.34 The order parameters systematically increased with increasing surfactant concentration and remained constant (Figure 7). Among three surfactants, the order parameters by adsorption of DTAC were lower than those of DDAB or TMAC. The order parameters by adsorption of TMAC were also slightly greater than those of DDAB. Since the order parameters represent the mobility of the probe in the adsorbed layer which is correlated with the microviscosity in the adsorbed layer, it is suggested that (34) Griffith, O. H.; Jost, P. C. In Spin LabelingsTheory and Applications; Berliner, L. J., Ed.; Academic Press: London, 1976; Chapter 12.

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Figure 7. Order parameters vs equilibrium surfactant concentration.

the microviscosity of the DDAB or TMAC adsorbed layer is fairly large compared to that of DTAC, which is probably derived from the highly packed surfactant layer. Figure 8a shows the change in the adsolubilized amount of 2-naphthol by adsorption of DTAC on silica. In this study, “adsolubilization” means an incorporation of hydrophobic compounds in surfactant adsorbed layers on particles including surfactant coverage above zero. It is important to note that adsorption of 2-naphthol on silica is hardly observed in the absence of surfactant. The adsolubilized amount of 2-naphthol increased with increasing adsorbed amount of DTAC but steeply decreased at above 15 mmol dm-3 of DTAC. The adsorbed amounts of DTAC in the presence of 2-naphthol were greater than those in the absence of 2-naphthol. Since a monolayer of DTAC at lower DTAC concentration and a bilayer of DTAC at higher concentration are formed on silica, the adsolubilized amount of 2-naphthol increases with DTAC concentration. However, since the formation of a DTAC micelle occurs in the bulk solution at above 15 mmol dm-3 of DTAC, 2-naphthol becomes partitioned between the adsorbed layer and the micelle in the bulk solution. As a result, the reduction in the adsolubilization occurs at about 15 mmol dm-3 of DTAC. The ratio of the adsolubilized amount of 2-naphthol to the adsorbed amount of DTAC decreased from 0.2 at 3 mmol dm-3 of DTAC to 0.08 at 15 mmol dm-3 of DTAC. A similar adsolubilization behavior for single-chain cationic surfactants and the silica system has been reported by Monticone et al.21 Figure 8b also shows the amount of 2-naphthol adsolubilized into the DDAB adsorbed layer on silica. The adsorbed amount of DDAB increased sharply and then reached a plateau, while the adsolubilized amount of 2-naphthol also increased steeply and reached a maximum and then decreased. It is interesting to note that DDAB is known to form vesicles in aqueous solution similar to DDAC.23,24 In this study, since the formation of a vesicle was not observed below 0.1 mmol dm-3 of DDAB in aqueous solution, it is reasonable to assume that the DDAB molecule is adsorbed on silica, orienting its hydrophobic chain to the water phase, and that, at the same time, a vesicle-like bilayer of DDAB is possibly formed on silica below 0.1 mmol dm-3 of DDAB. At above 0.1 mmol dm-3 of DDAB, the vesicles are formed in the bulk. Therefore the decrease in the adsolubilized amount of 2-naphthol is derived from the incorporation of 2-naphthol into the vesicles in the bulk. The ratio of the adsolubilized amount of 2-naphthol to the adsorbed amount of DDAB decreased from 0.53 at the feed concentration of DDAB ) 0.4 mmol

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Figure 9. Change in adsolubilized amount of 2-naphthol and surfactant adsorption by the procedure of adsorption-adsolubilization.

Figure 8. Change in adsolubilized amount of 2-naphthol (closed marks) and in adsorbed amount of surfactant (open marks) by adsorption of cationic surfactants.

dm-3 to 0.36 at the feed concentration of DDAB ) 0.8 mmol dm-3. In Figure 8c, the amount of 2-naphthol adsolubilized into the TMAC adsorbed layer on silica linearly increased and reached a maximum and then slightly decreased. This decrease is due to a large aggregate formation from an extremely low concentration of TMAC in aqueous solution.29 The ratio of the adsolubilized amount of 2-naphthol to the adsorbed amount of TMAC decreased from 0.8 at the feed concentration of TMAC ) 0.4 mmol dm-3 to 0.4 at the feed concentration of TMAC ) 0.8 mmol dm-3. From the comparison of the adsolubilization behavior obtained by these three surfactants, it is found that the maximum adsolubilized amount of 2-naphthol is in the order TMAC > DDAB > DTAC, although the corresponding amounts of surfactant adsorbed are different at the

maximum conditions. In addition, a more direct evaluation of the adsolubilization by the three surfactants is to calculate the ratio of the amount of 2-naphthol adsolubilized to the adsorbed amount of surfactant on silica, which may be called the “adsolubilized efficiency by surfactant”. The efficiency at the maximum adsolubilized amount is as follows: DTAC ) 0.08; DDAB ) 0.26; TMAC ) 0.33. The efficiencies of DDAB and TMAC are not so different and are appreciably larger than those of DTAC. This difference in the efficiencies can be interpreted by the fact that the hydrophobic volume in which 2-naphthol can be incorporated is roughly proportional to the hydrocarbon chain number of the surfactant and is also controlled by the surfactant chain packing. However, since the efficiency of DDAB is slightly smaller than that of TMAC, it seems likely that the chains in the TMAC molecules adsorbed are more tightly packed than those in DDAB so that the 2-naphthol molecule would not be incorporated easily into the TMAC adsorbed layer, unlike into the DDAB adsorbed layer. Further, the interaction of three cationic surfactants with the silica surface for 2-naphthol adsolubilization was studied by a two-step process of adsorptionadsolubilization: after the adsorption of the cationic surfactants alone on silica, the supernatant obtained by centrifugation of the suspensions was removed and then an aqueous solution containing 2-naphthol was added to the cationic adsorbed silica. Then the amount of surfactant remaining on silica and the adsolubilized amount of 2-naphthol were measured. The results are shown in Figure 9. In the case of DTAC, the adsolubilized amount of 2-naphthol extremely decreased accompanying the desorption of DTAC. This result suggests that a second layer of DTAC at the bilayer would desorb due to a weak interaction between the hydrophobic hydrocarbon chains of DTAC. On the other hand, in the cases of DDAB and TMAC, the adsorbed amounts of the surfactants as well as the adsolubilized amounts of 2-naphthol were hardly altered by the above procedure, indicating that a doublechain surfactant, DDAB, and a triple-chain surfactant, TMAC, adsorb strongly on the silica surface, incorporating 2-naphthol firmly. A similar study of the dioctadecyldimethylammonium chloride/silica system shows that the double-chain surfactant is hardly desorbed from the silica surface after several washings and that a water-insoluble dye, Yellow OB, adsolubilized is almost completely retained.21 The above results suggest that the adsorption of the cationic surfactants and the adsolubilization behavior of

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2-naphthol using single-, double-, or triple-chain cationic surfactants on a silica surface are significantly affected by surfactant structure. 4. Conclusion Although the adsorbed amount of the double-chain or triple-chain surfactant at saturation is smaller than that of the single-chain surfactant, the structure of the doublechain or triple-chain surfactant in the adsorbed layer on silica is different from that of the single-chain surfactant: the molecular packing of the multichain surfactants is higher, and the desorption of these surfactants is hardly observed compared with those of the single-chain surfactant. These differences affect strongly the adsolubilization behavior of 2-naphthol into the surfactant adsorbed

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layer. The efficiencies of the adsolubilization are not so different for the double-chain and triple-chain surfactants, which are markedly higher than that for the single-chain surfactant. In addition, from the two-step adsorptionadsolubilization experiment, it is inferred that the doublechain or triple-chain surfactant adsorbs strongly on the silica surface and that their adsorbed layers retain 2-naphthol molecules firmly compared with the singlechain surfactant. Thus, it is found that the adsorption characteristics and adsolubilization by the multichain surfactants are very different from those of the single-chain surfactant. LA950545K