Interactions between acid- and base-functionalized surfaces.

In this paper we present an AFM force study on interactions between chemically modified surfaces. Surfaces with terminal groups of either NH(2) or COOH were obtained by chemisorption of a silane-based compound (3-amino-propyltriethoxysilane) on silica or a thiol compound (11-mercapto undecanoic acid) on gold. The surfaces were characterized by streaming potential and contact angle measurements. For the NH(2) surfaces the density of functional groups strongly depends on the pretreatment of the silica. Both the NH(2) and COOH surfaces show a steady and ongoing loss of functional groups, but on the time scale of our force measurements they can be considered to be stable. Interaction curves for the various combinations of surfaces in aqueous solutions show a strong correlation with the ionization state of the surface groups. The approach curves can be explained on the basis of electrostatic interactions. On retraction a pH-dependent adhesion is found, the strongest being between NH(2) and COOH surfaces as a result of acid-base interactions. Between NH(2) layers and between COOH layers there is also adhesion, due to the formation of H bonds. The corresponding work of adhesion W was calculated using the DMT (Derjaguin, Muller, Toporov) equation. The values of W obtained are small compared to literature data and suggest that only a fraction of the functional groups in the contact area is involved in acid-base or H-bond interactions. This is attributed mainly to the roughness of the surfaces, which substantially reduces adhesion. A rough estimate is made for the rupture force of the NH(2)-COOH acid-base bond in water.