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 me thanol + n-alkane mixtures where n = 6-12. T-w and the CT were also me asured for noninteger n values, i.e., for mixtures of methanol + a bin ary mixture of two normal alkanes. The liquid-vapor surface tension (s igma(LV)) and the liquid-liquid interfacial tension (sigma(LL)) were m easured at 25 degrees C. For all mixtures studied the methanol rich-ph ase is denser than the alkane rich-phase, It is found that while the C T increases continuously, the T-w first increases and then decreases a s a function of n. This is the first time that, for a homologous serie s 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 fr om partial wetting to nonwetting (the alkane rich-phase intrudes betwe en the methanol rich-phase and the vapor) while for 8.5 less than or e qual to n less than or equal to 12 the transition was from partial to total wetting (the methanol-rich phase intrudes between the alkane ric h-phase and the vapor). Both the change of T-w and the inversion in th e nature of the wetting phase with n are in qualitative agreement with the predictions of a recently developed mean-field model where a norm alized parameter b, whose calculation involves sigma(LV) and sigma(LL) , plays the role of n in the experiments.