Capillary tube with reduced protein interactions and controllable electroosmotic flow

A method for treating capillary walls exposable to protein solutes is provided that reduces interactions of the protein solutes with the wall and that permits control of electroosmotic flow within the capillaries by selection of solution pH. Thus, a capillary tube has an interfacial layer covalently bonded to the inner wall of the tube. This interfacial layer is effective to reduce interactions between the inner wall and protein solutes and preferably includes a hydratable amphoteric phase. The amphoteric phase is a reaction product of a protein, peptide or an ampholyte and an oxygen or nitrogen nucleophile. Electroosmotic flow magnitude and/or flow direction may be controlled when capillaries of the invention are used for electrophoretic separations.