ELECTRICAL EQUIVALENT-CIRCUIT PARAMETERS FOR MONTMORILLONITE CLAY FILM-COATED QUARTZ CRYSTAL-OSCILLATORS IN CONTACT WITH AIR, MOISTURE AND ELECTROLYTE-SOLUTIONS

The piezoelectric admittance measurements of the quartz crystal oscillators with and without a montmorillonite clay coating have been carried out over the frequency range covering the area of the resonant frequency of each oscillator system. These oscillators are in contact with air, moisture and Na2SO4 aqueous media of various concentrations (0.01-1.0 M). The results have been represented in the form of conductance and susceptance spectra, ie plots of the real and imaginary components of the admittance vs. frequency. By use of electrical equivalent circuit model conventionally proposed for a piezoelectric quartz crystal, the relevant circuit parameters resistance R, inductance L, capacitance C and static capacitance C0, have been evaluated, especially as a function of the concentration of Na2SO4. R, C, L and C0 correspond to a loss in the crystal mechanical energy, a mechanical elasticity of the vibrating body, a measure of the vibrating mass and a capacitance between the two parallel-plate metal electrodes on the quartz surfaces, respectively. R was found to depend largely on the kind and property of the medium in contact with the oscillator. The values of R obtained for the oscillator with the clay coating were larger in lower concentrations of Na2SO4 electrolyte than in higher ones, in contrast to the case of the oscillator without the clay coating where R increases with the electrolyte concentration as expected from a hitherto proposed theory. These results have been explained on the basis of the electrolyte concentration-dependent swelling of the clay film as well as Derjaguin-Landau-Verwey-Overbeek theory being proposed for the interaction forces between colloidal particles. It is demonstrated that in addition to the conventional measurements of the resonant frequency of the quartz oscillator, the estimation of its electrical equivalent circuit parameters provides information concerning the chemical and physical processes on the oscillator.