Systematics of wetting at the vapor—liquid interface

Abstract

We have measured transition temperatures, T w, from incomplete wetting to complete wetting at the air—liquid interface for five binary mixtures of normal chain alcohols with the fluorocarbon perfluoromethylcyclohexane (C 7F 14). Consolute temperatures, T c, contact angles, and surface tensions were measured for each mixture. T w decreases monotonically as the alcohol chain length increases from 1 (methanol) to 6 ( n-hexanol). The mixture water (chain length = 0) + C 7F 14 is anomalous with the lowest T w (below the freezing point of the aqueous phase). From these measurements and a regular solution free energy, we calculated the lengths of the prewetting lines and the discontinuity in the derivative of the vapor—liquid surface tension dσ/ dT at T = T w. The prewetting line is a line of low to high adsorption transitions extending into the one liquid phase region and has been predicted to occur in mixtures in which there is both a miscibility gap and a wetting transition. The calculated lengths ranged from 38 K (for ethanol + C 7F 14) to 99 K (for n-hexanol + C 7F 14). The calculated discontinuities in dσ/ dT ranged from 0.008 dyn/cm deg K (for ethanol + C 7F 14) to 0.039 dyn/cm deg K (for n-hexanol + C 7F 14) and are roughly correlated with the lengths of the prewetting lines. Among the mixtures studied, the most likely candidate for directly detecting the prewetting transition is n-hexane + C 7F 14 based on a comparison to a regular solution model.