Effect of radio frequency electric fields on the surface free energy and zeta potential of Al2O3

The effects of a high-frequency electric field (44 MHz, 60 V peak-to-peak n o-load amplitude) on surface free energy components and zeta potentials of an alumina (Al2O3) surface were studied. The samples of alumina with preads orbed OH-, H+, Ca2+, Mg2+, La3+ and Al3+ ions were also investigated. All t hese ions essentially reduce the electron donor component of the alumina su rface, and the most drastic changes occurred in the presence of La3+ and Mg 2+ cations. The radio frequency (RF) field caused further changes in the fr ee energy components, electron donor and electron acceptor, of the surface with the preadsorbed ions. These changes can be explained based on thermody namic functions of the ion hydration. The RF field also affected the zeta p otential of an alumina suspension in water or a solution of the investigate d cation. After 15 min of the suspension irradiation with the field, its ze ta potential fluctuated, even up to 50 min. If the suspension was outgassed prior to the zeta potential measurements (with a help of water pump), the RF field effect was much smaller. It supports the literature observations t hat nanosize bubbles present in the system are responsible for so-called "m emory effect" presence, which may last even a couple of hours after the fie ld removal. However, in the presence of trivalent La3+ and Al3+ cations in the outgassed suspension the RF field effect was still present. Therefore, it was concluded that the memory effect must be also due to the field energ y absorption, which causes a disturbance in the adsorption-desorption equil ibrium at the surface/solution interface. Return to the equilibrium state r an in a fluctuating way. Also, a static electric field (24 kV de) caused a decrease in the negative zeta potential of an alumina suspension exposed to the field for 20 min, which lasted during 45 min of the experiment. More e xperiments are needed to explain the observed RF field effects.