Changes in zeta potential and surface free energy of calcium carbonate due to exposure to radiofrequency electric field

Abstract

A calcium carbonate (Fluka; more than 99.0% pure) suspension or a thin layer of its powder deposited on a glass slide was exposed to an r.f. electric field (44 MHz; 60 V peak-to-peak no-load amplitude) for 5–30 min. The zeta potential of the suspension in water (natural pH, 9.9 ± 0.1) or in 10 −2 M NaCl (natural pH, 9.6 ± 0.1) was then measured electrophoretically. The surface free energy components, i.e. non-polar Lifshitz—van der Waals (γ LW S) and polar (acid—base) electron donor (γ − S) and electron acceptor (γ + S) were determined by the “thin-layer wicking technique”. It was found that a negative zeta potential of CaCO 3 in water measured immediately (within 2–3 min) after the exposure to the r.f field changed in an oscillatory fashion depending on the exposure time. For the sample treated for 15 min with the r.f field the zeta potential decreased by 5–7 m V in relation to an untreated sample. Residual changes occurred also in an oscillatory fashion during the 40 min after field removal. A similar behavior was found for the suspension in 10 −2 M NaCl. The observed zeta potential changes were accompanied by changes in the electron donor component of the surface free energy and the appearance of a small electron acceptor component, which resulted from the presence of hydrogen bonding on the calcium carbonate surface. The r.f. field treatment led to a decrease in γ − S from 83 mJ m −2 (untreated sample) to 40 mJ m −2 (15-min-treated sample) and 64 mJ m −2 (25-min-treated sample) and caused the appearance of γ + S components of 1.34 and 1.77 mJ m −2. The observed changes in the zeta potential and the surface free energy components are believed to result from the changes in the surface charge of calcium carbonate and the structure of the water film at the surface.