Hemimicelle formation of cationic surfactants at the silica gel–water interface

Abstract

The effects of added inorganic salts (NaCl and NaBr), alcohols (C₂H₅OH and n-C₄H₉OH) and urea on the adsorption of alkylpyridinium halides from aqueous solution on silica gel have been investigated at 25 °C. The results show that all the adsorption isotherms exhibit two plateaux either with or without various additives. In the first plateau the surface-active cations are adsorbed as individual ions on the negatively charged silica gel surface through both electrostatic attraction and specific adsorption. Then, at a particular concentration known as the hemimicelle concentration (h.m.c.), the adsorption increases dramatically as hemimicelles form on the adsorbent through association between hydrocarbon chains of the adsorbed molecules. As the concentration increases further, the adsorption reaches the second plateau somewhere above the c.m.c. of the surfactant. Based on the assumption that each adsorbed surface-active cation in the first plateau is an active centre for surface aggregation, the minimum average aggregation number of hemimicelle, n_hm, is equal to the ratio between the adsorption amounts of two plateaux, Γ^∞_2nd and Γ^∞_1st, or, more generally, between Γ_cmc and Γ_hmc, the values of adsorption at the c.m.c. and h.m.c. respectively. The results show that the n_hm for alkylpyridinium halides in various media studied in this work is mostly in the range 6–18. The addition of inorganic salts and alcohols will increase and decrease the value of n_hm, respectively. The approach for determination of the number of surfactant ions in a hemimicelle from the adsorption isotherms proposed by Chander et al. [S. Chander, D. W. Fuerstenau and D. Stigter, in Adsorption From Solution, ed. R. H. Ottewill, C. H. Rochester and A. L. Smith (Academic Press, London, 1983), p. 197] has been questioned.