Extension of External Voltage Control of Electroosmosis to High-pH Buffers.

Control of electroosmosis by an applied external voltage field in capillary electrophoresis has been limited to buffer pH approximately 5 or below. This poor control at higher pH is caused by a high density of surface charge induced by chemical equilibrium overwhelming the influence of the external voltage-induced charges within the electric double layer. A tert-butyldiphenylchlorosilane treatment was used on fused-silica capillaries to minimize chemically generated ζ-potential where this treatment allowed for control of electroosmosis over a large pH range (2-10). Blocking the surface with traditional polymer-based surface treatments does not work in this application since the polymers increase the viscosity within the electric double layer and impede electroosmosis. The surface created by this reaction is demonstrated in extremely narrow capillaries, down to 2-μm internal diameter. The treated surface is sterically hindered against acid- and base-catalyzed degradation reactions typically associated with organosilanes. This results in a surface that was stable to experimental buffer pH extremes, from pH 3 to pH 10, and was stable for at least 8 weeks exposed to both solution and air.