Wetting Transition at the Liquid−Air Interface of Methanol−Alkane Mixtures

Abstract

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 (σ LL ) 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.