The influence of dissolved gas on the interactions between surfaces of different hydrophobicity in aqueous media Part I. Measurement of interaction forces

Abstract

An atomic force microscope (AFM) was used to directly measure the symmetric interaction forces between solid surfaces in dilute aqueous electrolyte solutions as the type and concentration of dissolved gas were varied. The gases studied were air, argon and carbon dioxide. The amount of dissolved gas was reduced by degassing the electrolyte solutions. The solid surfaces studied were silica, hydrophobised by either dehydroxylation or methylation. The approach of methylated surfaces in electrolyte solutions was typically characterised by a jump into contact from separations sufficiently large so as to not be attributed to van der Waals forces. The range of these attractive jumps was quite reproducible for a given pair of interacting surfaces. However the jump distance was found to vary dramatically (between 5 and 75 nm) for differing pairs of interacting surfaces, albeit prepared in an identical maner. For surfaces with large jump distances (>25 nm), degassing of the electrolyte solution caused a significant reduction in the jump distance. When smaller jump distances were measured, whether involved methylated or dehydroxylated surfaces, degassing had no significant effect on the jump distance. The effect of gas type on jump distance was examined for both types of surfaces. For dehydroxylated silica surfaces interacting in CO₂ saturated electrolyte solutions, the jump distances were significantly greater than in the presence of air or argon. Overall this interaction behaviour may be explained by the presence of gas bubbles formed on hydrophobic solid surfaces. The gas bubbles are stabilised by a combination of hydrophobicity and chemical heterogeneity.