Langmuir 2000, 16, 9217-9220
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Mixed Surfactant Adsolubilization of 2-Naphthol on Alumina Kunio Esumi,* Noriko Maedomari, and Kanjiro Torigoe Department of Applied Chemistry and Institute of Colloid and Interface Science, Science University of Tokyo, Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan Received April 11, 2000. In Final Form: August 30, 2000 The adsolubilization of 2-naphthol by surfactant mixtures of an anionic surfactant, sodium dodecyl sulfate, and a nonionic surfactant, hexaoxyethylenedodecyl ether on alumina was investigated. In a singlesurfactant system, only the anionic surfactant was found to show an appreciable adsolubilization. In the mixed-surfactant system, the adsolubilization grew with an increase in the total amount of adsorbed surfactant, whereas the adsolubilization became greater with an increase in the anionic surfactant content in the initial mixtures. On the other hand, the ratios of adsolubilized amount to surfactant adsorbed amount became smaller when the anionic surfactant content in the initial mixtures increased. This behavior is attributed to the properties of the mixed-surfactant adsorbed layer at the alumina-water interface.
Introduction Incorporation of water-insoluble compounds into surfactant adsorbed layers on particles is referred to as “Adsolubilization”.1 Until now, adsolubilization has been studied for many systems consisting of different combinations of surfactants and particles.2-8 For example, Nayyar et al.3 have reported the adsolubilization of nonpolar, polar, and ionizable organic compounds using sodium dodecyl sulfate and alumina particles. They have explained the variations in adsolubilization results on the basis of surfactant fundamentals and contaminant properties. Favoriti et al.8 have also studied the adsolubilization of naphthalene derivatives on alumina, titanium dioxide, and silica by adsorption of cetyltrimethylammonium bromide; they found that the partition coefficient for each solute is independent of the solid substrate and is a function of the surfactant adsorption and the neutral solute properties. Thus, as one of the applications of adsolubilization, surfactants have been used to remove pollutants from aqueous media. So far as we know, only one paper has been published on adsolubilization using surfactant mixtures.2 Adsorption from surfactant mixtures onto solids includes the adsorption of binary surfactant mixtures of anionic surfactants, anionic-nonionic surfactants, and cationic-nonionic surfactants.9-13 In particular, it is interesting that in ionic-nonionic surfactants, the ad* To whom correspondence should be addressed. (1) Harwell, J. H.; Hoskins, J. C.; Schechter, R. S.; Wade, W. H. Langmuir 1985, 1, 251. (2) Esumi, K.; Sakamoto, Y.; Nagahama, T.; Meguro, K. Bull. Chem. Soc. Jpn. 1989, 62, 2502. (3) Nayyar, S. P.; Sabatini, D. A.; Harwell, J. H. Environ. Sci. Technol. 1994, 28, 1874. (4) Schieder, D.; Dobias, B.; Klumpp, E.; Schwuger, M. J. Colloids Surf. 1994, 88, 103. (5) Monticone, V.; Treiner, C. J. Colloid Interface Sci. 1994, 166, 394. (6) Esumi, K.; Matoba, M.; Yamanaka, Y. Langmuir 1996, 12, 2130. (7) Esumi, K.; Toyoda, H.; Goino, M.; Suhara, T.; Fukui, H. Langmuir 1998, 14, 199. (8) Favoriti, P.; Monticone, V.; Treiner, C. J. Colloid Interface Sci. 1996, 179, 173. (9) Harwell, J. H.; Roberts, B. L.; Scamehorn, J. F. Colloids Surf. 1988, 32, 1. (10) Somasundaran, P.; Fu. E.; Xu, Q. Langmuir 1992, 8, 1065. (11) Esumi, K.; Sakamoto, Y.; Meguro, K. J. Colloid Interface Sci. 1990, 134, 283.
sorption of one surfactant is often enhanced by addition of a small amount of the other surfactant. Surfactant mixtures provide several advantages over single surfactants, because the adsorption of surfactants on particles can be controlled using appropriate surfactants and solution properties. It would be expected that the removal of pollutants could be enhanced using a minimum amount of surfactant mixtures of ionic and nonionic surfactants. The objective of this study is to investigate the adsolubilization of 2-naphthol using surfactant mixtures of an anionic surfactant, sodium dodecyl sulfate (SDS), and a nonionic surfactant, hexaoxyethylenedodecyl ether (C10E6), on alumina. Experimental Section Materials. SDS was obtained commercially and used after several recrystallizations with ethanol. C10E6 was supplied by Nikko Chemicals Co. and used as received. R-alumina was supplied by Showa Denkou Co.; its surface area and average particle size were 30.2 m2 g-1 and 0.3 µm, respectively. Water used in this study was purified with the use of a Milli-Q Plus system (Millipore). The other chemicals used were of analytical grade. Methods and Measurements. Adsolubilization of 2-naphthol was carried out as follows. A series of solutions were prepared, containing fixed concentrations of 2-naphthol (0.4 mmol dm-3) and NaCl (10 mmol dm-3) in SDS/C10E6 aqueous solutions. The solutions were then added to alumina in glass vials with caps. All suspensions were adjusted to about pH 3.5 with HCl, and the glass vials were equilibrated at 25 °C for 24 h in a shaker-water bath. After equilibration, the solids were separated by centrifugation and the supernatant was analyzed for 2-naphthol (at 328 nm, using a UV detector) and for surfactants (using an RI detector of a high-performance liquid chromatography). The surface tensions of SDS/C10E6 aqueous mixtures in the presence of 10 mmol dm-3 of NaCl were measured using a Kruss K12 tensiometer. The Wilhelmy plate technique was used in the measurements.
Results and Discussion Before studying the adsolubilization of 2-naphthol, it is important to obtain the interaction between SDS and C10E6 in aqueous solution. Surface-tension curves for various (12) Huang, L.; Maltesh, C.; Somasundaran, P. J. Colloid Interface Sci. 1996, 177, 222. (13) Otsuka, H.; Esumi, K. Langmuir 1994, 10, 45.
10.1021/la000543m CCC: $19.00 © 2000 American Chemical Society Published on Web 10/26/2000
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Figure 1. Surface tension vs total surfactant concentration for SDS and C10E6 and their mixtures of different compositions; 25 °C, 10 mmol dm-3 NaCl.
Figure 2. Cmc’s in for binary anionic-nonionic mixtures of SDS/C10E6 in 10 mmol dm-3 NaCl at 25 °C. The plotted points are experimental results; the solid line is the prediction of the regular solution theory with β ) -3.4; and the dotted line is the prediction for ideal mixing, β ) 0.
mixtures of SDS and C10E6 are given in Figure 1. The surface tensions of the mixtures decreased with an increase in the mixture concentration and showed break points that were taken as mixed critical micelle concentrations (cmc’s). The surface tensions of the mixtures at the mixed cmc’s are not so different compared with those of SDS or C10E6 alone. Figure 2 shows the mixed cmc vs molar fraction of SDS in the mixtures. One can see that the mixed cmc’s are lower than that of ideal mixing, indicating a synergism in the mixed micelle formation between SDS and C10E6. In addition, because the interaction parameter, β, of the SDS/C10E6 mixed system calculated using the regular solution theory14 is about -3.4, it is suggested that the mutual phobicity between the hydrocarbon chains, as well as the reduction in Coulombic
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Figure 3. Adsorption of SDS and C10E6 from single-surfactant systems and mixtures at different initial compositions as a function of individual surfactant equilibrium concentration; 25 °C, 10 mmol dm-3 NaCl, pH 3.5.
repulsion between the headgroups, dominates the interactions between SDS and C10E6. The interaction parameter between nonionic and anionic surfactants ranges from -2 to -5,14 and the interaction parameter for the present system is within this range. The adsorption of SDS-C10E6 mixtures was studied at the alumina-water interface from mixtures at several different initial molar fractions of SDS. Figure 3 shows the isotherms for adsorption of SDS and C10E6 on alumina from their respective surfactants and mixtures. Here, the isotherms for the surfactant mixtures were measured at below their mixed cmc’s. The adsorption of C10E6 alone at the alumina-water interface is very low. This observation is similar to the results on the adsorption of polyethoxylated monolaurate on titania15 and poly(ethylene oxide) on alumina.16 It is known15,16 that for adsorption to occur on polar surfaces, the nonionic surfactants must have a moiety that has adsorption force sufficient to overcome the strong interaction between water molecules and the ethylene oxide groups. Accordingly, it is conceivable that in the case of the strongly hydrated alumina surface, C10E6 cannot displace enough water molecules to enable for adsorption. On the other hand, SDS alone adsorbs appreciably on alumina. At pH 3.5 the alumina surface is positively charged (the isoelectric point of alumina is pH 9.0), and hence the electrostatic attraction with the anionic SDS will be dominant. In the region of 0.5 and 1.4 mmol dm-3 SDS, the occupied area of SDS calculated is about 0.2 nm2, which corresponds to the formation of an SDS monolayer on alumina. At the individual surfactant adsorption isotherms for the mixtures, it is seen that C10E6 adsorption is enhanced significantly when SDS content in the mixtures is increased, and the adsorption isotherms are shifted to lower concentration ranges. It is suggested that the initial (14) Holland, P. M. Mixed Surfactant Systems; Holland, P. M., Rubingh, D. N., Eds.; ACS Symposium Series 501; American Chemical Society: Washington, DC, 1992; Chapter 2. (15) Fukushima, S.; Kumagai, S. J. Colloid Interface Sci. 1973, 42, 539. (16) Koksal, E.; Ramachandran, R.; Somasundaran, P.; Maltesh, C. Powder Technol. 1990, 62, 253.
Adsolubilization of 2-Naphthol on Alumina
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Figure 4. Ratio of adsorped amount of SDS to C10E6 on alumina for different surfactant mixtures; 25 °C, 10 mmol dm-3 NaCl, pH 3.5.
Figure 5. Adsolubilization of 2-naphthol on alumina for SDSC10E6 mixtures as a function of total surfactant equilibrium concentration; 25 °C, 0.4 mmol dm-3 2-naphthol, 10 mmol dm-3 NaCl, pH 3.5.
Figure 6. (a) Ratio of adsolubilized amount and surfactant adsorbed amount on alumina for SDS-C10E6 mixtures as a function of total surfactant adsorbed amount; (b) ratio of experimental and calculated adsolubilization on alumina for SDS-C10E6 mixtures as a function of total surfactant adsorbed amount; 25 °C, 0.4 mmol dm-3 2-naphthol, 10 mmol dm-3 NaCl, pH 3.5.
electrostatic adsorption of SDS provides a number of hydrophobic sites that is sufficient to enable hydrophobic adsorption of C10E6. The adsorption isotherms of SDS from different mixtures with C10E6 are also shown in Figure 3. The adsorption of SDS is only enhanced at low concentration for SDS:C10E6 ) 3:1, whereas the adsorption of SDS decreases with a drop in SDS content in the mixtures. The incremental adsorption of SDS should occur by reducing the lateral electrostatic repulsion between the ionic headgroups of SDS. On the other hand, the decrease in the adsorption of SDS upon the addition of C10E6 is attributed to the competition between the bulky nonionic C10E6 and SDS. The ratio of the adsorption amount of SDS to that of C10E6 at the alumina-water interface is plotted in Figure 4 as a function of the total surfactant equilibrium
concentration. The figure shows that the ratio of SDS to C10E6 in the adsorption at the alumina-water interface changes over the entire concentration range and decreases with a drop of SDS content in the initial mixtures. For example, in the case of SDS:C10E6 ) 1:3, the ratio of SDS to C10E6 in the adsorption is close to unity. In the case of SDS:C10E6 ) 3:1, the ratio ranges between 3 and 3.6. Figure 5 shows the adsolubilized amount of 2-naphthol as a function of total surfactant equilibrium concentration. Because the adsolubilization of 2-naphthol is not observed on alumina without surfactants, it is apparent that 2-naphthol is incorporated into the surfactant adsorbed layer that exhibits a hydrophobic property. It is seen that the adsolubilized amount of 2-naphthol is very low by adsorption of C10E6 alone, but becomes greater with an increase in the SDS content of the initial mixtures, from
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SDS:C10E6 ) 1:3 to 3:1. The adsolubilized amount of 2-naphthol by adsorption of SDS alone ranges between that of SDS:C10E6 )1:1 and 3:1. Here, in the case of C10E6 alone the decrease in the adsolubilization at above 0.6 mmol dm-3 C10E6 can be explained by a partition of 2-naphthol into the surfactant adsorbed layer and C10E6 micelles. To compare the efficiency of adsolubilization by surfactant, we compared the ratio of adsolubilized amount to surfactant adsorbed amount. The ratios of adsolubilized amount (mmol g-1) to surfactant adsorbed amount (mmol g-1) are plotted with the total adsorbed amount of surfactant in Figure 6(a). In the case of SDS, the proportions are about 0.1 in a whole SDS adsorption concentration, whereas those for C10E6 range between 0.02 and 0.06, suggesting that the efficiency of adsolubilization is low for both SDS and C10E6 single adsorptions. On the other hand, the ratios for both SDS:C10E6 ) 1:3 and 1:1 decrease with an increase in the total adsorbed amount of surfactant and approach 0.1, while that for SDS:C10E6 ) 3:1 increases gradually and also approaches 0.1. It is suggested that for SDS:C10E6 ) 3:1, the efficiency of adsolubilization is predominantly controlled by the adsorption of SDS. Thus, it is found that the efficiency of adsolubilization is increased when the SDS content in the initial mixtures drops below surfactant adsorption of 0.2 mmol g-1. Interestingly, a relationship exists between the efficiency of adsolubilization and the ratio of SDS to C10E6; when the ratio of SDS to C10E6 becomes small, the efficiency of adsolubilization becomes greater. This result might be explained by a view that the mixed surfactant adsorbed layer is more compact because of a shield of electrostatic repulsion of SDS adsorbed by incorporation of C10E6, in particular for the case of SDS:C10E6 ) 1:3. The ratio of experimental to calculated adsolubilization is examined in Figure 6(b) as a function of the total adsorbed amount of surfactant. The calculated adsolubilization is obtained by a sum of (0.1 (SDS adsorbed amount)) and (0.04 (C10E6 adsorbed amount)). The coef-
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ficients, 0.1 and 0.04, were taken from the average of the single surfactant system of SDS and C10E6. It is apparent that the ratio becomes large when the content of SDS in the initial mixtures decreases. For adsolubilization, this result also shows the importance of the properties of the mixed surfactant adsorbed layer at the alumina-water interface. Accordingly, it can be said that the composition of the mixed bilayer, consisting of first layer of SDS and a second layer of C10E6, changes with SDS contents in the initial mixtures, and that the efficiency of adsolubilization of 2-naphthol increases when the proportion of SDS in the mixed bilayer decreases. To understand the adsolubilization by adsorption of surfactant mixtures in detail, it is necessary to characterize the structure of surfactant mixtures adsorbed on alumina. Conclusions Adsolubilization results obtained with mixtures of a anionic (SDS) and a nonionic (C10E6) surfactant on alumina have shown complex behavior compared to those obtained with a single-surfactant system. C10E6 itself showed a very low adsolubilization of 2-naphthol, whereas SDS had an appreciable adsolubilization capacity. A mixed-surfactant adsorbed layer at the alumina-water interface enhanced the adsolubilization of 2-naphthol compared to the SDS single-surfactant system. We suggest that the ratio of SDS to C10E6 in the adsorption layer changes with the SDS content in the initial mixtures and is important for adsolubilization. In particular, we have demonstrated that the surfactant adsorbed layer for both SDS:C10E6 ) 1:3 and 1:1 provides greater efficiency of adsolubilization than that for SDS alone. Thus, surfactant mixtures of anionic and nonionic surfactants with low concentrations have great possibilities for enhancing adsolubilization of toxic compounds. LA000543M
