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Adsorption of Cationic Surfactants on Titanium Dioxide with a Hydrophobic Group Anchor and Their Adsolubilization Behaviors Kunio Esumi,*,† Syuji Uda,† Masaya Goino,† Kenji Ishiduki,† Tsuneo Suhara,‡ Hiroshi Fukui,‡ and Yoshifumi Koide§ Department of Applied Chemistry and Institute of Colloid and Interface Science, Science University of Tokyo, Kagurazaka, Shinjuku-ku, Tokyo 162, Japan; Shiseido Basic Research Laboratories, 1050 Nippa-Cho, Kohoku-ku, Yokohama 223, Japan; and Department of Applied Chemistry and Biochemistry, Faculty of Engineering, Kumamoto University, Kurokami, Kumamoto 860, Japan Received January 3, 1997X Adsorption of cationic surfactants (dodecyltrimethylammonium bromide, 1RQ; 1,2-bis(dodecyldimethylammonio)ethane dibromide, 2RenQ) has been investigated on titanium dioxide with a dodecyl chain anchor. The adsorbed amounts of 1RQ and 2RenQ increase with increasing concentration of the dodecyl chain anchor on titanium dioxide, where the adsorbed amounts of 1RQ at saturation are considerably greater than those of 2RenQ. Changes in the dispersion stabilities and the ζ potentials of the titanium dioxide suspensions are well correlated with the adsorption behaviors of the surfactants. The adsolubilized amounts of 2-naphthol in the surfactant-adsorbed layer also increase with increasing concentration of the dodecyl chain anchor for 1RQ and 2RenQ, whereas those with 2RenQ are greater than those with 1RQ even though there is a smaller amount of adsorbed 2RenQ. These differences in the adsolubilization are attribued to the surfactant compactness in the adsorbed layer estimated by ESR.
Introduction When ionic surfactants are adsorbed on oppositely charged solid particles, a bilayer formation often occurs. Since the microenvironmental properties in such a bilayer show a hydrophobic property, water-insoluble compounds are incorporated in the bilayer, which is called “adsolubilization”.1 Many groups2 have been trying to understand the adsolubilization behaviors, which can be applied to many fields such as cosmetics, paints, waste-water treatment, and drugs, etc. It has been observed3-5 that the amounts adsolubilized of water-insoluble compounds decrease when micelles are present in the bulk because the water-insoluble compounds are distributed between the bilayer and micelles. It has also been reported6 that the amounts of naphthalene derivatives adsolubilized are decreased above the cmc for the adsorption of cationic surfactants on titanium dioxide. Accordingly, in order to enhance the adsolubilization capacity, it is necessary to design the structure of a surfactant-adsorbed layer on particles in the absence of micelles in the bulk. Recently, Suhara et al.7-9 have reported that various oxide surfaces can be modified by chemical vapor deposi†
Science University of Tokyo. Shiseido Basic Research Laboratories. § Kumamoto University. X Abstract published in Advance ACS Abstracts, April 15, 1997. ‡
(1) Wu, J.; Harwell, J. H.; O’Rear, E. A. Langmuir 1987, 3, 531. (2) Haver, J. H.; Harwell, J. H.; Lobban, L. L.; O’Rear, E. A. Solubilization in Surfactant Aggregates; Christian, S. D., Scamehorn, J. F., Eds.; Marcel Dekker: New York, 1995; Chapter 8 and references therein. (3) Monticone, V.; Mannebach, M. H.; Treiner, C. Langmuir 1994, 10, 1395. (4) Klumpp, E.; Heitmann, H.; Lewandowski, H.; Schwuger, M. J. Prog. Colloid Polym. Sci. 1992, 89, 181. (5) Esumi, K.; Yamanaka, Y. J. Colloid Interface Sci. 1995, 172, 116. (6) Favoriti, P.; Monticone, V.; Treiner, C. J. Colloid Interface Sci. 1996, 179, 173. (7) Suhara, T.; Kutsuna, H.; Fukui, H.; Yamaguchi, M. J. Jpn. Soc. Colour Mater. 1992, 65, 264. (8) Suhara, T.; Kutsuna, H.; Fukui, H.; Yamaguchi, M. Colloid Polym. Sci. 1993, 271, 486. (9) Suhara, T.; Fukui, H.; Yamaguchi, M. Colloids Surf. 1995, 101, 29.
S0743-7463(97)00012-7 CCC: $14.00
tion using 1,3,5,7-tetramethylcyclotetrasiloxane, followed by various reactions to obtain hydrophobic modified oxides with long hydrocabon chains. This method may allow us to provide a first adsorption layer of surfactant on particles which is actually chemically bonded with the oxide surface. When surfactants are added to the surface-modified oxides described above, the surfactant molecules would adsorb on the hydrocarbon chains of the modified surface as a “second layer” from well below the cmc’s. Therefore it is expected to form a sufficient quantity of bilayer on the modified oxides in the absence of micelles in aqueous solution. Indeed, Leimbach and Rupprecht10 showed that chemiadsorbed alkyl chains on oxides serve as anchors for small surface aggregates of ionic surfactants. In this study, adsorption behaviors of cationic surfactants on chemically bonded dodecyl chains on titanium dioxide have been reported for two surfactants in which one or two quaternary ammonium species are linked at the level of the head groups by a hydrocarbon spacer. In addition, adsolubilization of 2-naphthol in the surfactantadsorbed layers has also been characterized to elucidate a role of the surface modification. Experimental Section Materials. Titanium dioxide obtained from Ishihara Sangyo Co., Ltd (specific surface area, 53.3 m2 g-1) was contacted with 1,3,5,7-tetramethylcyclotetrasiloxane (TMCTS) vapor (Shin-Etsu chemicals Co., Ltd) at 80 °C for 16 h to obtain various amounts coated with polymerized TMCTS. Then Si-H groups on the titanium dioxide were reacted with 1-dodecene in ethanol in the presence of H2PtCl6 as catalysis at 60 °C for 4 h.9 After reaction, the surface-modified samples were filtered and were washed with ethanol and then were dried in vacuo. The surface-modified samples were analyzed using elemental analysis and infrared spectroscopy. The surface coverages with TMCTS were 25, 76, and 95% from the calculation of the density and molecular area of TMCTS. By a further reaction between 1-dodecene and Si-H groups, the concentrations of 1-dodecene grafted on the surfaces covered with 25, 76, and 95% TMCTS became 11.8, 26.3, and (10) Leimbach, J.; Rupprecht, H. Colloid Polym. Sci. 1993, 271, 307.
© 1997 American Chemical Society
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47.2 µmol g-1, respectively, and these samples will be referred to as D-10, D-30, and D-40 as well as D-0 (bare sample). As surfactants, dodecyltrimethylammonium bromide (1RQ) and 1,2-bis(dodecyldimethylammonio)ethane dibromide (2RenQ) were used. 1RQ was obtained from Tokyo Kasei Co. and was used after crystallization from acetone. 2RenQ was synthesized and was used after purification.11 The spin probe methyl 12doxylstearate (12-DOXYL ME) from Sigma was used as received. The water used in all experiments was purified through a Milli-Q system. The other chemicals were of guaranteed grade. Method. The adsorption of surfactants was obtained by measuring their concentrations in solutions before and after adsorption at 25 °C. Experiments were performed in glass vials with caps. All suspensions (0.15 g of TiO2/15 mL) in the presence of 10 mmol dm-3 NaBr were equilibrated for 24 h in a water shaker bath at 25 °C. The solid content in the suspension was 10 wt %. After equilibration, the solids were separated by centrifugation and the supernatant was analyzed using a dye method.12 The pH of the suspensions was not adjusted but was shown to be about 6.5. Suspensions containing 1 × 10-6 mol dm-3 12-DOXYL ME were prepared for ESR measurements. The ESR spectra were recorded on a JEOL JES FE-3X spectrometer at room temperature, where solid slurries obtained by centrifugation of the suspensions were used. The ζ potential of the suspensions was measured with an electrophoretic apparatus (Pen Kem 500). The stability of titanium dioxide suspensions was evaluated by measuring the absorbance at 600 nm of the top portion of the aqueous suspension in a sedimentation tube kept for 24 h after the adsorption. A high absorbance indicates a well dispersed state. In the adsolubilization of 2-naphthol, a series of surfactant solutions containing 2-naphthol (0.4 mmol dm-3) was prepared. The adsolubilization was carried out by the same procedure described above. The concentration of 2-naphthol in the supernatant was determined using a UV spectrophotometer.13 The critical micelle concentrations (cmc’s) of 1RQ and 2RenQ containing 10 mmol dm-3 NaBr were determined using a KRUSS tensiometer K12. The cmc’s were hardly changed by addition of 0.4 mmol dm-3 2-naphthol.
Results and Discussion Figure 1a shows the adsorption of 1RQ on D-0, D-10, D-30, and D-40 in the presence of 0.4 mmol dm-3 2-naphthol. The adsorption of 1RQ on D-0 and D-10 increased gradually with 1RQ concentration, whereas that of 1RQ on D-30 and D-40 increased rather sharply. This difference is probably derived from the different hydrophobicities of the samples, which increase with increasing dodecyl chain concentration as the anchor. The adsorbed amounts of 1 RQ at the saturation increased with increasing dodecyl chain concentration as anchor. Comparing the occupied area of 1RQ (0.49 nm2/molecule)11 at the air/water interface with that of 1RQ (0.27 nm2/ molecule) at the D-0 surface calculated from the surface area and amount adsorbed suggests that 1RQ adsorbs on the D-0 surface as a bilayer at the saturation level. Assuming that the increased amount of 1RQ adsorbed on the modified samples is proportional to the dodecyl chain concentration of the anchor layer, about 5-8 molecules of 1RQ are obtained for adsorption per one dodecyl chain of the anchor layer at saturation. It has also been reported that about 5-20 surfactant ions are adsorbed per one chemisorbed anchor group on metal oxides.14 Figure 1b shows the adsolubilization of 2-naphthol with adsorption of 1RQ on D-0, D-10, D-30, and D-40. It is seen that the amounts of 2-naphthol adsolubilized increase and reach a maximum and then decrease with 1RQ concentration (11) Esumi, K.; Taguma, K.; Koide, Y. Langmuir 1996, 12, 4039. (12) Scott, G. V. Anal. Chem. 1968, 40, 769. (13) Esumi, K.; Matoba, M.; Yamanaka, Y. Langmuir 1996, 12, 2130. (14) Rupprecht, H.; Gu, T. Colloid Polym. Sci. 1991, 269, 506.
Figure 1. (a) Adsorption isotherms of 1RQ on titanium dioxide with various C12 anchor concentrations at 25 °C. (b) Adsolubilization of 2-naphthol on titanium dioxide with various C12 anchor concentrations with 1RQ adsorption at 25 °C.
for all the samples. The decrease in the adsolubilization of 2-naphthol has been interpreted3,5,13 by distribution of 2-naphthol between the adsorbed layer and micelles in the bulk. Interestingly, the amount adsolubilized was markedly dependent on the dodecyl chain concentration as anchor: the amounts adsolubilized increase in the order D-40 > D-30 > D-10 > D-0. In particular, the maximum amount of 2-naphthol adsolubilized by D-40 was 2.5 times that adsolubilized by D-0. Thus, an enhancement in the adsolubilization of 2-naphthol by introducing the dodecyl chain anchor on the titanium dioxide is clearly demonstrated. In addition, in the absence of 1RQ the incorporation of 2-naphthol on the surface-modified samples increased with increasing dodecyl chain concentration as anchor. This incorporation occurs probably due to hydrophobic interaction between 2-naphthol and the dodecyl chain anchor. The ratio of the number of 2-naphthol molecules incorporated per one dodecyl chain as the anchor
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Figure 2. Change in the ζ potential of titanium dioxide suspensions with 1RQ adsorption at 25 °C.
Figure 3. Change in the absorbance of titanium dioxide suspensions with 1RQ adsorption at 25 °C.
Figure 4. (a) Adsorption isotherms of 2RenQ on titanium dioxide with various C12 anchor concentrations at 25 °C. (b) Adsolubilization of 2-naphthol on titanium dioxide with various C12 anchor concentrations with 2RenQ adsorption at 25 °C.
was about 0.10-0.15 in the absence of surfactant. When 1RQ is added to the surface-modified samples, 1RQ molecules are adsorbed on the surface-modified samples, orienting their hydrophobic chains to the surface through their hydrophobic interactions. As a result, the surfacemodified titanium dioxide particles become dispersed by the electrostatic repulsion force between the 1RQ adsorbed particles accompanying an increase of the adsolubilization capacity. The ζ potentials of the titanium dioxide suspensions are shown in Figure 2. It is seen that the ζ potentials increase with increasing 1RQ concentration for all the samples and level off. In addition, the ζ potentials increase with increasing dodecyl chain concentration as anchor. The dispersion stability of the titanium dioxide suspensions was also increased with the 1RQ concentration: the higher the dodecyl chain concentration as anchor, the higher the dispersion stability (Figure 3). However, a
good dispersion stability of D-0 was not obtained. These results suggest that the dispersion stability can be well correlated with the electrostatic repulsion force. Comparing with the adsolubilization efficiencies expressed by the ratio of the maximum amount of 2-naphthol adsolubilized to the amount of surfactant adsorbed, the adsolubilization efficiency for D-0 (0.048) was almost the same as that for D-10 (0.046) and was smaller than those for D-30 (0.078) and D-40 (0.065). These differences in the adsolubilization efficiency may arise from the different surfactant compactnesses in the adsorbed layer. Figure 4 shows the adsorption of 2RenQ and the adsolubilization of 2-naphthol on D-0, D-10, D-30, and D-40. All isotherms showed a sharp increase at low 2RenQ concentration and then reached a plateau, indicating a strong affinity of 2RenQ for the titanium dioxide surfaces. The amounts of 2RenQ adsorbed increased with increasing dodecyl chain concentration as anchor and were almost
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Figure 5. Change in ζ potential of titanium dioxide suspensions with 2RenQ adsorption at 25 °C. Figure 7. Change in the order parameters of 12 DOXYL ME with surfactant adsorption.
Figure 6. ESR spectra of 12 DOXYL ME at the surfactantadsorbed layer on titanium dioxide: (a) D-0, 0 mmol dm-3 1RQ; (b) D-40, 3 mmol dm-3 1RQ.
the same for D-30 and D-40. The amounts of 2RenQ adsorbed were appreciably smaller than those of 1RQ even taking into consideration their molecular sizes. The amounts of 2-naphthol adsolubilized increased, reached a maximum, and then decreased for all the samples. These behaviors were similar to those of 1RQ. However, the amounts of 2-naphthol adsolubilized were not so different for all the samples. The adsolubilization efficiency at the maximum adsolubilization by 2RenQ ranged between 0.46 and 0.48. Thus, the adsolubilization efficiency by 2RenQ is considerably higher than that by 1RQ, suggesting that the 2RenQ molecules in the adsorbed layer are packed more tightly than the 1RQ molecules. As the 2RenQ molecules adsorbed per dodecyl chain of the anchor layer, 1-2 molecules are obtained by the calculation. The dispersion stabilities of the suspensions with the adsorption of 2RenQ increased with increasing 2RenQ concentration as well as increasing dodecyl chain concentration as anchor, similar to those of 1RQ. The ζ potentials of the titanium dioxide suspensions increased at low 2RenQ concentration and leveled off with a further increasing concentration for all the samples (Figure 5). These features are similar to those of the adsorption isotherms. From these results, it is suggested that 2RenQ is adsorbed on the bare surface as a patchlike bilayer.
To understand a considerable difference in the adsolubilization efficiency by 1RQ and 2RenQ, the microproperties of the surfactant-adsorbed layers on the samples were estimated using an ESR technique. Figure 6 shows ESR spectra of 12-DOXYL ME recorded for the solids (D-0 and D-40)/aqueous solution interface with adsorption of 1RQ. The spectrum of 12-DOXYL ME for D-0 in the absence of 1RQ was characterized by hyperfine splittings, whereas that for D-40 by 1RQ adsorption became broad. This line broadening of the ESR spectra indicates that the mobility of 12-DOXYL ME in the surfactant-adsorbed layer decreases. To obtain more information, the order parameter (S) of 12-DOXYL ME was calculated from the maximum hyperfine splittings (A| and A⊥) and the known hyperfine crystal tensors (Axx, Ayy, and Azz) for DOXYL using the equation15
S ) 2(A| - A⊥)(Axx + Ayy + Azz)/ (2 Azz - Axx - Ayy)(A| + 2A⊥) The order parameters with adsorption of 1RQ and 2RenQ became large with increasing dodecyl chain concentration as anchor (Figure 7); in particular, the order parameters with the adsorption of 2RenQ were greater than those with 1RQ. Since the order parameters represent the mobility of the probe in the adsorbed layer, which can be correlated with the microviscosity and chain packing, it can be said that the chain packing of the 2RenQ-adsorbed layer is higher than that of 1RQ. This large packing in the adsorbed layer of 2RenQ will make the adsolubilization efficiency of 2-naphthol more enhanced compared with that of the adsorbed layer of 1RQ. Conclusions It is found that adsorption of 1RQ and 2RenQ cationic surfactants on titanium dioxide particles increases with increasing dodecyl chain concentration as anchor, where the amounts adsorbed of 1RQ are appreciably greater than (15) Hubbell, W. L.; McConnell, H. M. J. Am. Chem. Soc. 1971, 93, 314.
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those of 2RenQ. By adsorption of these surfactants, the dispersion stabilities of titanium dioxide suspensions are enhanced with increasing dodecyl chain concentration as anchor due to an electrostatic repulsion force. In the adsolubilization study, the amounts of adsolubilized of 2-naphthol by the 1RQ-adsorbed layer increase considerably with increasing dodecyl chain concentration as anchor, but those by the 2RenQ-adsorbed layer are almost
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independent of the dodecyl chain concentration as anchor. These diferences in the adsolubilization behavior can be correlated with differences in the compactness of the surfactant-adsorbed layer, which has been evaluated using a spin probe. LA9700127
