Impedance analysis of an electrode-separated piezoelectric sensor as a surface monitoring technique for surfactant adsorption on quartz surface

This paper describes the measurement of the kinetics of adsorption of sodium dodecyl sulfate (SDS), an anionic surfactant, onto a quartz surface with a pre-adsorbed layer of Ca²⁺ as an ion bridge, using an electrode-separated piezoelectric sensor (ESPS). An impedance analysis method was employed to characterize the responses of the ESPS. The impedance and frequency parameters of the ESPS were examined as functions of the conductivity, permittivity, viscosity and density of the liquid. The adsorption process of SDS onto the quartz surface resulted in an increase in both the mass and energy dissipation of the oscillating quartz crystal. The adsorption densities could be estimated by the ESPS method after taking into consideration the effects of surface viscosity and roughness. The adsorption and desorption rate constants of SDS onto the quartz surface were calculated as k_a = (88.1 ± 0.26) mol^-1 L s^-1 and kd = (4.92 ± 0.53) × 10^-3 S^-1 respectively, based on the Langmuir model. ESPS was shown to be a powerful means of examining anionic surfactant adsorption to the solid/liquid interface.