COMBINED MEASUREMENT OF SURFACE-POTENTIAL AND ZETA POTENTIAL AT INSULATOR ELECTROLYTE INTERFACES

A method is described which allows variations of surface potential and zeta potential at an insulator surface to be measured simultaneously. The samples consist of silicon nitride thin films deposited on silicon substrates. Light-addressable potentiometric sensors (LAPSs) are used to make the simultaneous measurements. The zeta potential follows from the streaming potential, which is monitored as the difference in potential between two LAPS sites at each end of the channel. The output of one of the LAPS sites yields the variations of surface potential. Measurements have been made with Si3N4 surfaces exposed to aqueous electrolytes of varied pH. The results show that zero zeta potential occurs at an electrolyte pH of 3.1. This indicates that the Si3N4 surface is highly oxidized and contains less than 1% of amine sites. The location of the point of zero zeta potential can be used to give absolute values of the surface potential. These data show that upon changing pH, only about 10% of the surface potential change also appears as a change in zeta potential.