The equilibrium surface tension of the aqueous solutions of benzyl alcohol (BA) -C. I. Basic Yellow 11 (BY11) mixture and
m-cresol (CR)-BY11 mixture was measured as a function of the total molarity (_??_) and the mole fraction of dye (_??_
2). By applying Matomura's thermodynamic equation, the mole fraction of dye in the adsorbed film (_??_
H2) was estimated. The phase diagram of BA-BY11 system, which represents the relationship between _??_
2 and _??_
H2, became a deformed semilunar type, and in comparison with the aqueous solution the adsorbed film abounded in the more surface-active BY11 molecules over the whole _??_
2 range. The phase diagram of CR-BY11 system having a minimum point was analoguous to that of an azeotropic mixture. It was concluded that the adsorbed film of CR-BY11 system abounds in the less surface-active CR molecules, and in the more surface-active BY11 molecules at the higher and the lower _??_
2 region than that at minimum point respectively. The _??_
2 value corresponding to the minimum point decreased along with the lowering of the surface tension. This adsorption behavior differed remakably from that of BA-BY11 system. It can be explained by the strong interaction between CR and BY11 molecules in the adsorbed film due to the phenolic hydroxy group of CR molecule.
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