The adsorption behavior of a mixture of propylammonium chloride (PAC) and hexylammonium chloride (HAC) at the water/air interface was investigated by measuring the surface tension of the aqueous solution as a function of the total molality of PAC and HAC and the mole fraction of HAC in the total salt at 298.15 K under atmospheric pressure. The total surface density of the mixture and the composition of HAC in the adsorbed film were evaluated by using thermodynamic equations derived previously. The HAC molecules were seen to be adsorbed predominantly by the water/air interface. The composition of HAC in the adsorbed film was observed to have a larger value than unity in a region where the mole fraction of HAC in the aqueous solution was large. It was proved that such peculiar behavior is attributable to the negative adsorption of PAC in the adsorbed film, that is, the expulsion of the PAC molecules from the interface by the HAC molecules adsorbed. Further, the thermodynamic equations of mixed adsorbed films were concluded to be applicable not only to the mixed systems of surfactants but to those of various kinds of substances.
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