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

The aqueous mixed system decyltrimethylammonium bromide (C10TAB)–hexadecyltrimethylammonium bromide (C16TAB) was studied by conductivity, ion-selective electrodes, surface tension, and fluorescence spectroscopy techniques. The mixture critical micelle concentration, cmc*, aggregation number, N∗N∗, and micelle molar conductivity, ΛMcmc, showed that the system aggregation is strongly nonideal. Both cmc* and N∗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, cmci∗. The Rubingh procedure gives micelles richer in C16TAB than the overall mixtures, while procedure (ii) gives micelles having the same composition as in the complete surfactant mixture (αC10TABαC10TAB). 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 C16TABC16TAB 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.

Graphical abstractThe aqueous mixed system C10TAB–C16TAB was studied by conductivity, ion-selective electrodes, surface tension, and fluorescence spectroscopy. Results were analyzed by the regular solution theory, and by the partial critical micelle concentration of one component in the presence of a constant concentration of the other one.Figure optionsDownload full-size imageDownload as PowerPoint slide