Adsorption of Dodecyltrimethylammonium Chloride-Decylammonium Chloride Mixtures at Water/Air Interface

Abstract The surface tension of the aqueous solution of dodecyltrimethylammonium chloride and decylammonium chloride mixture has been measured as a function of the total concentration and composition of the surfactants at 298.15 K under atmospheric pressure. It has been observed that the surface tension decreases with an increase in the composition of dodecyltrimethylammonium chloride at a fixed total concentration. A break point has been observed on the surface tension vs. total concentration curve. By applying the thermodynamic relations, the surface densities of surfactants have been evaluated numerically. It has been proved that the phase transition of the mixed adsorbed film takes place from a gaseous to an expanded state at the break point. By comparing the composition in the adsorbed film with that in the bulk phase, the difference between the compositions has been found to diminish as the surface tension decreases. It has been shown that the mutual relation between the compositions of the gaseous and expanded films at the transition point is described by the two-dimensional phase diagram.