The surface tension of the aqueous solution containing dodecylammonium chloride and decylammonium chloride has been measured as a function of total concentration at 298.15°K under atmospheric pressure. It has been observed that the surface tension decreases with increasing the composition of dodecylammonium chloride at a fixed total concentration and the surface tension vs concentration curve has a distinct break point. By applying the thermodynamic relations developed, the surface densities of surfactants have been evaluated numerically. Examining the relation of the composition in the bulk phase to the one in the adsorbed film at constant surface tension, the difference in the surface activities of surfactants has been found to be pronounced at high surface tension. It has been proved that the break point corresponds to the phase transformation of the adsorbed film from a gaseous to an expanded state. It has been shown that the phase transition is expressed by a two-dimensional phase diagram.
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