Estimation of the dissociation constants for functional groups on modifiedand unmodified silica gel supports from the relationship between electroosmotic flow velocity and pH

Electroosmotic volume flow was directly observed in a simple instrument con sisting of 1 cm long a home-made support, packed between two polyethylene f rits in the polypropylene tube. Equations relating electroosmotic flow (EOF ) velocity and pH for two functional groups on the surface of the solid mat erials were developed. With these equations, we can estimate the dissociati on constants of two different kinds of functional groups on modified silica gel materials simultaneously. The dissociation constants of silanol groups , benzene sulfonic acid groups, and alkyl quaternary ammonium groups on the modified and unmodified silica gel supports were estimated. The estimated pK values of the silanol groups on the silica gel and modified silica gel s urfaces are between 4.0 and 4.3. The estimated pK values of the benzene sul fonic acid groups and alkyl quaternary ammonium groups on the surface of th e modified silica gel are 2.6 and 3.6, respectively.