Langmuir 1998, 14, 5347-5354
5347
Miscibility and Nonideality of Mixing of Heptanol and Octylsulfinylethanol in the Adsorbed Film and Micelle Hidemi Iyota,*,† Takako Tomimitsu,† Kinsi Motomura,‡ and Makoto Aratono‡ Department of Life and Environmental Science, Kagoshima Prefectural College, Shimoishiki, Kagoshima 890-0005, Japan, and Department of Chemistry, Faculty of Science, Kyushu University, Fukuoka 812-8581, Japan Received January 23, 1998. In Final Form: May 27, 1998 The surface tension of the aqueous solution of a mixture of 1-heptanol and 2-(octylsulfinyl)ethanol (OSE) was measured as a function of the total molality and composition of the mixture at 298.15 K under atmospheric pressure. The total surface density and the compositions of the adsorbed film and micelle were evaluated by applying thermodynamic equations to the surface tension, and then the phase diagrams of adsorption and micelle formation were drawn. Heptanol and OSE molecules are miscible with each other in the adsorbed film and micelle. A negative azeotropy of adsorption (i.e., the phase diagram of adsorption with a minimum) was observed. The occurrence of the azeotropy was attributed to the similarity of the surface activities of heptanol and OSE, (in contrast, the chemical structures of the molecules are different from each other). The nonideality of mixing in the adsorbed film was quantitatively shown by the negative values of the excess Gibbs free energy and area of adsorption calculated from the activity coefficient in the adsorbed film. The nonideality was ascribed to the fact that the interaction between heptanol and OSE molecules in the adsorbed film is larger than those between the same component molecules in the pure films. The activity coefficient and excess chemical potential of OSE indicated that heptanol and OSE molecules mixed almost ideally in the micelle. A large discrepancy in composition was observed between the adsorbed film and micelle coexisting at the critical micelle concentration. This discrepancy is attributed to the difference both in size and property of the headgroups of heptanol and OSE molecules and in the geometric shape between the adsorbed film and micelle.
Introduction Adsorption and micelle formation of a surfactant in the presence of an alkanol have been extensively studied from the viewpoint of the additive effect of the alkanol.1-8 To obtain a better understanding of and further information about the physicochemical states of the adsorbed film and micelle, we have thermodynamically investigated the systems of 2-(octylsulfinyl)ethanol (OSE, C8H17SOC2H4OH) with 1-hexanol and 1-octanol from the viewpoints of the mixed adsorption and micelle formation.9,10 This approach11-15 enabled us to determine the miscibility of * Author to whom correspondence should be addressed. E-mail:
[email protected]. Telephone: (081) 099-220-1111. Fax: (081) 099-220-1115. † Department of Life and Environmental Science. ‡ Department of Chemistry. (1) Herzfeld, S. H.; Corrin, M. L.; Harkins, W. D. J. Phys. Chem. 1950, 54, 271. (2) Shinoda, K. J. Phys. Chem. 1954, 58, 1136. (3) Hayase, K.; Hayano, S. J. Colloid Interface Sci. 1978, 63, 446. (4) Zana, R.; Yiv, S.; Strazielle, C.; Lianos, P. J. Colloid Interface Sci. 1981, 80, 208. (5) Vollhardt, D.; Czichocki, G. Colloids Surf. 1984, 11, 209. (6) Nishikido, N. J. Colloid Interface Sci. 1986, 112, 87. (7) Muto, Y.; Yoda, K.; Yoshida, N.; Esumi, K.; Megro, K.; BinanaLimbele, W.; Zana, R. J. Colloid Interface Sci. 1989, 130, 165. (8) Attwood, D.; Mosquera, V.; Rodriguez, J.; Garcia, M.; Suarez, M. J. Colloid Polym. Sci. 1994, 272, 584. (9) Matsuki, H.; Kanda, T.; Aratono, M.; Motomura, K. Bull. Chem. Soc. Jpn. 1990, 63, 2159. (10) Iyota, H.; Aratono, M.; Motomura, K. J. Colloid Interface Sci. 1996, 178, 53. (11) Motomura, K.; Matsukiyo, H.; Aratono, M. In Phenomena in Mixed Surfactant Systems; Scamehorn, J. F., Ed.; ACS Symposium 311, American Chemical Society: Washington, DC, 1986; p 163. (12) Aratono, M.; Kanda, T.; Motomura, K. Langmuir 1990, 6, 843. (13) Todoroki, N.; Tanaka, F.; Ikeda, N.; Aratono, M.; Motomura, K. Bull. Chem. Soc. Jpn. 1993, 66, 351. (14) Motomura, K.; Aratono, M. In Mixed Surfactant Systems; Ogino, K., Abe, M., Eds.; Marcel Dekker: New York, 1993; p 99.
the alkanols and OSE in the adsorbed films and micelles in terms of their compositions. It has been found that the surface activity plays a significant role in the miscibility in the adsorbed film and there exists a large interaction between the alkanol and OSE molecules in the adsorbed film. The surface activity determined from the surface tension versus concentration plots increases in the order 1-octanol > OSE > 1-hexanol.9,10 Therefore, it is interesting to examine the miscibility of 1-heptanol and OSE in the adsorbed film because the surface activity of 1-heptanol is very similar to that of OSE. Furthermore, the heptanolOSE system is highly useful to determine how the difference in the polar headgroup between heptanol and OSE molecules affects the miscibility in the adsorbed film because it has been shown that heptanol and octanol molecules mix ideally in the adsorbed film.16 With respect to the micelle formation, we have shown that hexanol and octanol form mixed micelles with OSE over each limited range of composition, although the alkanols do not form their own micelles.9,10 Hence, heptanol is expected to be miscible with OSE in the micellar state and the results of the heptanol-OSE system are expected to give us the dependence of the miscibility in the micelle on the alkyl-chain length of alkanol. Furthermore, our thermodynamic treatment of surfactant mixtures also affords the difference in miscibility between the micelle and adsorbed film coexisting in equilibrium at the critical micelle concentration (cmc). Therefore, it is of interest to elucidate how the difference in the polar headgroup between heptanol and OSE molecules affects the difference in miscibility between the micelle and adsorbed film. (15) Ikeda, N.; Sanefuji, N.; Lu, K.-K.; Aratono, M.; Motomura, K. J. Colloid Interface Sci. 1994, 164, 439. (16) Ikeda, N., et al., unpublished data.
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5348 Langmuir, Vol. 14, No. 19, 1998
Iyota et al.
Figure 2. Critical micelle concentration and total molality at transition versus composition curves: (1) C and (2) mt.
Figure 1. Surface tension versus total molality curves at constant composition: X2 ) (1) 0, (2) 0.1408, (3) 0.2894, (4) 0.4012, (5) 0.4967, (6) 0.6046, (7) 0.7503, (8) 0.8843, (9) 0.9005, (10) 0.9197, (11) 0.9407, (12) 0.9702, and (13) 1; (b) X2 ) 0, (.) turbid solution.
Experimental Section 1-Heptanol (Nacalai Tesque’s guaranteed reagent) was purified by distillation under reduced pressure. OSE is the same sample as that used in a previous study.10 Water, deionized and distilled, was further purified by distilling twice from dilute alkaline potassium permanganate solution. The equilibrium surface tension was measured by the previously described drop volume technique;17,18 the error was
