ABSTRACT
The interactions between the carboxylate-terminated probe and silica surface cannot be explained by considering only the electrostatic force, van der Waals forces and the ionic binding mechanism, all compounded in the JKR theory. Considering the pK values for the carboxylate-terminated microsphere and hydrophilic silica surface, one would expect similar or weaker pull-off forces than that of the amine-terminated microsphere interacting with silica surface. However, the pulloff forces are almost an order of magnitude higher for hydrophilic silica interacting with carboxylate-terminated than with amine-terminated sphere under the similar conditions. Hydrogen bonding appears to be a controlling factor in determining adhesion between the carboxylate-terminated probe and the hydrophilic silica wafer [23]. The pKa value of carboxylate-terminated surfaces (p^ a around 8) has been determined to be several units higher than that of carboxylate in solution [24]. The increase in adhesion force below pH 8 is attributed to hydrogen bonding between the proton of the carboxylic acid group [||RC(0)OH] on the sphere and the oxygen atom on the silica wafer [||SiO], and between proton of silanol group on silica wafer [||SiOH] and oxygen of both carboxylate [||COO~] and carboxylic acid [||RC(0)OH] groups on the sphere. Symbol [| in this paper emphasizes the species on a surface. Evidence of hydrogen bonding was further substantiated by a rapid decrease in pull-off forces at pH above 8 where the carboxylic groups and hydrolyzed silica surfaces are substantially deprotonated to become ||C(0)0~ and ||SiO~, respectively, a system with diminishing hydrogen bonding potency. Further increase in solution pH to 10 results in a strong electrostatic repulsion between highly negatively charged surfaces without hydrogen bonding and hence a zero adhesion force. It is clear that the adhesion between carboxylate-terminated sphere and hydrophilic silica surfaces in an electrolyte solution is controlled collectively by electrostatic interactions, van der Waals forces and hydrogen bonding. Because of this nature, the adhesion forces between the carboxylate-terminated probe and silica surfaces (Figs 5 and 6) were determined to be much stronger than that between the amine-terminated probe and silica surfaces (Figs 3 and 4).
