ABSTRACT
In addition to contact adhesion from van der Waals attraction and hydrogen bonding, the electrostatic forces acting between surfaces outside contact area can be understood by taking into account the complex chemical compositions of surfaces. The usual pATa value for amine surfaces is in the range of 7-8 [19]. The hydrophilized silicon wafer in water is, in fact, composed of a relatively thin layer of silicon dioxide (SiC>2 bonds, pKa = 2-3), some of them being hydrolyzed to become silanol groups (Si-OH, p^ a approx. 6). As a result, silica wafer surfaces exhibit essentially zwitterionic behavior due to significant changes of electrostatic interactions caused by shifts of acid-base equilibria of different types of surface groups [20]. It is, however, known that the actual p £ a values for chemical groups on surfaces are, usually, lower compared with pKa values of the same groups in solution [21]. The difference is attributed to the difficulty in forming charged state of the ionizable groups at constrained surfaces due to the influence of nearby charged sites. Considering the pKa values and adhesion forces, it can be inferred that the amine surface bears strong positive charges in the pH range from 3 to 6, and becomes predominantly neutral over solution pH between 8 and 10. In contrast, the silica surface, which exhibits zwitterionic behavior due to the presence of Si02 and Si-OH, is predominantly negatively charged in the solution of pH from 3 to 6, and becomes increasingly more negative at pH 7-8 and fully negatively charged in solutions of pH from 8 to 10. The pH dependence of surface charging mechanism interpreted here is in line with the observations of zeta potential measurements using corresponding surfaces [22]. Thus, the variation of both amine and silica surface charges can be considered as the reason for the observed variation of adhesion forces with solution pH. The charging states of amine-terminated probe and silica surfaces at various pH values are shown schematically in Fig. 3. At very low pH, the probe surface is strongly positively charged due to protonation of amine groups, while silica surface carries negative charges. In addition to the non-compensated strong van der Waals forces within the contact area, the opposite charges on the two interacting surfaces result in an electrostatic attraction. As a result, a maximum adhesion force was measured. An increase in pH causes a steady decrease in adhesion forces due to a gradual decrease in protonation of surface amine groups at an intermediate solution pH. The reduced protonation translates into a reduced positive surface charge density on the probe particle, diminishing the attractive electrostatic contribution which, in turn, leads to a decrease in adhesion forces. At higher pH, above 10, both amine-terminated probe and silica surfaces bear strong negative charges due to ionization of both silanol and amine groups, inducing a strong electrostatic repulsion between highly negatively charged surfaces, which compensates attractive van der Waals forces. More importantly, such ionization
