Wetting transition at the liquid-air interface of methanol-alkane mixtures

Wetting transition (T_w) and consolute (CT) temperatures close to the upper critical solution temperature (UCST) have been determined for methanol + n-alkane mixtures where n = 6-12. T_w and the CT were also measured for noninteger n values, i.e., for mixtures of methanol + a binary mixture of two normal alkanes. The liquid-vapor surface tension (σ_LV) and the liquid-liquid interfacial tension (σLL) were measured at 25°C. For all mixtures studied the methanol rich-phase is denser than the alkane rich-phase. It is found that while the CT increases continuously, the T_w first increases and then decreases as a function of n. This is the first time that, for a homologous series of mixtures, this behavior has been found. For 6 ≤ n ≤ 8.25 the observed wetting transition was from partial wetting to nonwetting (the alkane rich-phase intrudes between the methanol rich-phase and the vapor) while for 8.5 ≤ n ≤ 12 the transition was from partial to total wetting (the methanol-rich phase intrudes between the alkane rich-phase and the vapor). Both the change of T_w and the inversion in the nature of the wetting phase with n are in qualitative agreement with the predictions of a recently developed mean-field model where a normalized parameter b, whose calculation involves σ_LV and σ_LL, plays the role of n in the experiments.