Electrokinetic properties of the methylated surface of quartz capillaries

The surface charge and electrokinetic potential of quartz capillaries with hydrophobized (methylated) surface are measured. It is shown that, during t he flow of an electrolyte solution through the capillary, the potential low ers with time due to formation of gas bubbles on the surface screening the surface charge. In a certain period of time, drastic jump of the zeta poten tial to its initial value is observed, which is interpreted as a detachment of bubbles by the flowing solution. The formation of bubbles is provoked b y the roughness of the methylated surface with asperities of up to 20 nm in height. The profiles of the methylated surface were obtained by the atomic force microscopy. An addition of a nonionic surfactant hydrophilizing the hydrophobic surface to the electrolyte solution lowers the zeta -potential, which could be caused by the suppression of the slip effect. Estimations m ade on the basis of an earlier developed theory lead to the values of slip coefficient that are close to those measured for micron-sized hydrophobic c apillaries.