Wetting of solids by surface-active agents: The effects of unequal adsorption to vapor-liquid and solid-liquid interfaces

Abstract

The role of surfactant adsorption in the wetting of relatively low-energy solids was evaluated from contact angle measurement using various aqueous surfactant solution-solid combinations. Analysis of the resulting data indicates that with many systems, pure liquids of low surface tension and aqueous surfactant solutions having the same surface tension do not produce the same contact angles; pure liquids often are better wetting agents. For hydrocarbon surfactants these effects are more significant for semipolar solids such as polymethyl methacrylate than for nonpolar solids. For a fluorocarbon surfactant, perfluoro-n-octanoic acid, the effects were found to be important for semipolar solids as well as nonpolar solids such as paraffin. These differences can be explained by invoking a lower adsorption of surfactants to the solid-liquid interface in comparison to the vapor liquid interface. Independent evidence from adsorption data justifies this explanation both for hydrocarbon and fluorocarbon surfactants. Using directly determined adsorption data on Graphon for both types of surfactants and surface tension values of the aqueous solutions involved, changes in the contact angle for Graphon were calculated. These simulated wetting curves reproduce many features of the observed wetting data for nonpolar solids for similar surfactants.