The adsorption behavior of a mixture of lanthanum chloride (LaCl3) and propylammonium chloride (PAC) at the water/air interface was investigated by measuring the surface tension γ of the aqueous solution as a function of the total molality of LaCl3 and PAC X^, and the mole fraction of PAC in the total electrolyte X^2 at 298.15 K under atmospheric pressure. The total surface density of the mixture X^H and the composition of PAC in the adsorbed film X^2H were evaluated by applying the thermodynamic equations derived previously to the experimental results. It was observed that the sign of X^H value changes from minus to plus at X^2 = 0.160 and further that the X^2H value diverges negatively with increasing X^2 in the composition range below X^2 = 0.160 and positively with decreasing X^2 in the composition range above X^2 = 0.160. This interesting feature was proved to be due to the fact that the absolute value of the negative adsorption of LaCl3 is equal to that of the positive one of PAC at X^2 = 0.160. The results were compared with those of the hexylammonium chloride (HAC)-PAC mixture. It was concluded that the peculiar behavior of the γ vs X^2H curves is caused by the negative adsorption of one constituent.
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