Surface and bulk properties of aqueous decyltrimethylammonium bromide–hexadecyltrimethylammonium bromide mixed system

Abstract

The aqueous mixed system decyltrimethylammonium bromide (C 10TAB)–hexadecyltrimethylammonium bromide (C 16TAB) was studied by conductivity, ion-selective electrodes, surface tension, and fluorescence spectroscopy techniques. The mixture critical micelle concentration, cmc*, aggregation number, N ∗ , and micelle molar conductivity, Λ M cmc , showed that the system aggregation is strongly nonideal. Both cmc* and N ∗ results were analyzed with two different procedures: (i) the regular solution theory on mixed micelles or Rubingh's theory, and (ii) by the determination of the partial critical micelle concentration of the amphiphile component i in the presence of a constant concentration of the other amphiphile component, cmc i ∗ . The Rubingh procedure gives micelles richer in C 16TAB than the overall mixtures, while procedure (ii) gives micelles having the same composition as in the complete surfactant mixture ( α C 10 TAB ). Mixed micelles are larger than pure surfactant ones, with nonspherical shape. Using a literature model, the cause of the synergistic effect seems to be a reduction of the hydrocarbon/water contact at the micelle surface when mixed micelles form. Conductivity and ion-selective electrodes indicate that highly ionized premicelles form immediately before the cmc*. The air/solution interface is strongly nonideal and much richer in C 16 TAB than the composition in the bulk. When micelles form there is a strong desorption from the air/solution interface because micelles are energetically favored when compared with the monolayer.