Surface adsorption and micelle formation of dodecyltrimethylammonium chloride and decylammonium chloride mixture

The surface tension of the aqueous solution of dodecyltrimethylammonium chloride (DTAC) and decylammonium chloride (DeAC) mixture was measured as a function of the total molality and composition of the surfactants at 298.15 K under atmospheric pressure. To make clear the miscibility of surfactants, the compositions in the mixed adsorbed film and micelle were calculated by use of the thermodynamic equations developed previously. The results indicated that both adsorbed film and micelle are richer in DTAC molecules than the bulk solution. Furthermore, the composition in the adsorbed film at the critical micelle concentration (cmc) was evaluated and compared with that in the micelle. It was found that the composition of DeAC in the micelle is smaller than that in the adsorbed film. This forms a striking contrast to the previous observation of dodecylammonium chloride (DAC)-DeAC mixture. It was concluded that the structure and size of polar head group of surfactants have a significant influence on the miscibility of surfactants.