Journal of Physical Chemistry, Vol.100, No.14, 5888-5891, 1996
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 (sigma(LV)) and the liquid-liquid interfacial tension (sigma(LL)) were measured at 25 degrees 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 less than or equal to n less than or equal to 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 less than or equal to n less than or equal to 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 sigma(LV) and sigma(LL), plays the role of n in the experiments.
Keywords:3-COMPONENT SURFACTANT SYSTEMS;VAPOR INTERFACE;PHASE-TRANSITIONS;WATER MIXTURES;BINARY-SYSTEMS;BEHAVIOR;CYCLOHEXANE;ADSORPTION;PHENOL