Studies on Binary Mixed Monolayers containing Vinyl Polymer with Long Side Chains as One Component. II.

Abstract

The retardation of water evaporation by binary mixed monolayers of polyvinyl stearate with 1-hexadecanol, 1-octadecanol and stearic acid has been studied. The evaporation rate was determined from the progressive increase with time in weight of desiccant (LiCI) suspended from one end of the beam of a balance just above the water surface. The surface pressures were measured simultaneously by the Wilhelmy method. From the measurements with and without a monolayer, the specific evaporation resistance γ which is characteristic of the monolayer was calculated. It was found that the mixed monolayers of polyvinyl stearate with long chain alcohols or acids in a certain proportion suppressed remarkably the evaporation of water, although the specific resistance varied with different spreading techniques. According to the energy barrier theory on the evaporation resistance, for an ideal mixed monolayer, logarithms of the specific resistance, as well as the free energies of activation on water evaporation (ΔG≠), may be expected to be linear functions of the composition. However, the mixed monolayers used in the present work showed marked deviation from the ideal behaviour and in γ-composition curves exhibited maxima. From these results the excess free energies of activation on water evaporation (ΔG≠ex) were evaluated as functions of compositions and these relations were compared with those for apparent excess free energies of mixing (ΔGex) discussed in Part I of this paper. Maximum deviations in both cases of ΔG≠ex (positive) and ΔGex (negative) occur at nearly the same composition, suggesting the close corelation of these two quantities. Condensation effect in the mixed monolayers or closer packing of hydrocarbon chains may contribute to the positive deviation in the activation free energies when molecules of water pass through the monolayers. But the absolute values of ΔG≠ex and ΔGex differ considerably from one another, because ΔGex is concerned with equilibrium state while ΔG≠ex is concerned with activated state for water evaporation process through the monolayer.