Surface forces between hydrophilic self-assembled monolayers in aqueous electrolytes

In this paper we present an atomic force microscopy (AFM) study on electrostatic and acid-base interactions between hydrophilic self-assembled monolayers (SAMs) with monoprotic ionizable acid functional groups (-COOH). As expected for surfaces composed of weak acid groups the surface potential and strength of interaction are sensitive to pH and ionic strength. Repulsive forces 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 strength of adhesion and extent of attractions diminish with increasing pH. As the surface groups ionize, at intermediate pH values, jump-in distances and adhesion grow with ionic strength and at high pH they saturate with increasing electrolyte concentration at values close to those expected for van der Waals interactions. However, the maximum charge is around 5% of the total expected for the surfaces. Thus, allowing sodium ions to bind, with a binding constant of pK_Na = 3.7 and pK_a = 6.3, ionic strength and pH dependencies of the surface charge can be described.