Effect of coating electrolytes on two-tailed surfactant bilayer coatings in capillary electrophoresis

Surfactants such as dioctadecyldimethylammonium bromide (DODAB) form semi-permanent coatings that effectively prevent adsorption of cationic proteins onto the fused silica capillary in capillary electrophoresis (CE). The bilayer coating is generated by flushing the capillary with a 0.1 mM surfactant solution. However, formation of the bilayer is strongly dependent on the coating electrolyte. The effect of counter-ions, electrolyte concentrations and buffer co-ions were monitored based on: the separation of basic model proteins; the adsorption kinetics of DODA⁺ onto fused silica; and dynamic light scattering (DLS) to determine vesicle size. Low concentrations (≤10.0 mM) and/or weakly associating buffers such as phosphate (pH 3.0), acetate (pH 4.0) and chloride should be used for DODAB coating solutions. Dissolving the surfactant in strongly associating electrolyte, such as phosphate at pH 7.0, results in poor coating of the capillary surface. Effective cationic bilayer coatings are formed if the buffer conditions favor formation of vesicles with diameters < 300 nm. Monitoring turbidity at 400 nm provides a convenient means of verifying vesicle diameter variation of <5 nm; that is, that the coating solution is effective.