DETERMINATION OF THE INTRINSIC ACID-BASE DISSOCIATION-CONSTANT AND SITE DENSITY OF IONIZABLE SURFACE GROUPS BY CAPILLARY RISE MEASUREMENTS

A theoretical description is presented for the equilibrium capillary h eight (h(eq)) of a solution between parallel, flat, solid surfaces whi ch contain ionizable groups. The h(eq) is related to the change in gra vitational potential energy, the intrinsic wettability of the un-ioniz ed surface, and the free energy of formation of the ionizable surface in aqueous solution. The theoretical approach takes into account both the electrostatic free energy of charging the surface and the change i n the free energy associated with the acid-base reactions of the surfa ce sites. It is shown that the dependence of h(eq) on pH depends on th e number of ionizable surface sites per unit area, the intrinsic acid- base dissociation constant (K-a(i)) ofthe surface sites, and the backg round electrolyte. The negative free energy change which accompanies t he acid-base reactions dominates over the positive electrostatic free energy of charging the surface. Consequently, the overall free energy of ionization is negative, and an ionized surface is more wettable tha n an un-ionized surface. The theoretical description is applied to exp erimental values of h(eq) as a function of pH, measured between two he ptylamine plasma polymer surfaces in the presence of I mM NaCl. The th eoretical fit to the data indicates that the plasma polymer surface co ntains ca. 10(17) amines/m(2) and the pK(a)(i) of the amine groups is ca. 5. The surface site density is in reasonable accord with values ob tained by both derivatization techniques and contact angle measurement s. The pK(a)(i) is consistent with a low effective dielectric constant for the polymer-water interface.