Surface forces between hydrophilic self-assembled monolayers in aqueous electrolytes

In this paper we present an atomic force microscopy (AFM) study on electros tatic and acid-base interactions between hydrophilic self-assembled monolay ers (SAMs) with monoprotic ionizable acid functional groups (-COOH). As exp ected for surfaces composed of weak acid groups the surface potential and s trength of interaction are sensitive to pH and ionic strength. Repulsive fo rces grow with the degree of ionization. At low fractional ionization, the surfaces experience attractions with a range larger than that expected for van der Waals interactions and the strength of adhesion is large. The stren gth of adhesion and extent of attractions diminish with increasing pH. As t he surface groups ionize, at intermediate pH values,jump-in distances and a dhesion grow with ionic strength and at high pH they saturate with increasi ng electrolyte concentration at values close to those expected for van der Waals interactions. However, the maximum charge is around 5% of the total e xpected for the surfaces. Thus, allowing sodium ions to bind, with a bindin g constant of pK(Na) = 3.7 and pK(a) = 6.3, ionic strength and pH dependenc ies of the surface charge can be described.