Miscibility of components in the adsorbed film for the n-butanol (NBA)-dodecyltrimethylammonium chloride DTAC) and tertbutyl alcohol (TBA)-DTAC mixtures were investigated by measuring surface tension of the aqueous solutions. The experimental results were analyzed thermodynamically. Diagrams of the excess Gibbs energy in the adsorbed film g(H,E) vs the composition in the adsorbed film X2(H) were obtained from the phase diagrams of adsorption. The g(H,E) values of the NBA-DTAC system are more negative than those of the TBA-DTAC system. It is concluded that the molecular interaction between NEA and DTAC is stronger than that between TEA and DTAC. With respect to the mean molar area in the adsorbed film A vs X2(H) curve at a fixed surface tension γ, the criterion of ideal mixing is demonstrated to be a straight line connecting the pure values. The A vs X2(H) curves are convex downward in both systems and the deviation from the straight line is attributed to the dependence of g(H,E) on γ.
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