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

An atomic force microscope (AFM) was used to directly measure the symmetric interaction forces between solid surfaces in dilute aqueous electrolyte so lutions as the type and concentration of dissolved gas were varied. The gas es studied were air, argon and carbon dioxide. The amount of dissolved gas was reduced by degassing the electrolyte solutions. The solid surfaces stud ied were silica, hydrophobised by either dehydroxylation or methylation. Th e approach of methylated surfaces in electrolyte solutions was typically ch aracterised by a jump into contact from separations sufficiently large so a s to not be attributed to van der Waals forces. The range of these attracti ve jumps was quite reproducible for a given pair of interacting surfaces. H owever the jump distance was found to vary dramatically (between 5 and 75 n m) for differing pairs of interacting surfaces, albeit prepared in an ident ical maner. For surfaces with large jump distances (> 25 nm), degassing of the electrolyte solution caused a significant reduction in the jump distanc e. When smaller jump distances were measured, whether involved methylated o r dehydroxylated surfaces, degassing had no significant effect on the jump distance. The effect of gas type on jump distance was examined for both typ es of surfaces. For dehydroxylated silica surfaces interacting in CO, satur ated electrolyte solutions, the jump distances were significantly greater t han in the presence of air or argon. Overall this interaction behaviour may be explained by the presence of gas bubbles formed on hydrophobic solid su rfaces. The gas bubbles are stabilised by a combination of hydrophobicity a nd chemical heterogeneity.