CHANGES IN ZETA-POTENTIAL AND SURFACE FREE-ENERGY OF CALCIUM-CARBONATE DUE TO EXPOSURE TO RADIOFREQUENCY ELECTRIC-FIELD

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 held (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-pol ar Lifshitz-van der Waals (gamma(S)(LW)) and polar (acid-base) electro n donor (gamma(S)(-)) and electron acceptor (gamma(S)(+)) were determi ned by the ''thin-layer wicking technique''. It was found that a negat ive zeta potential of CaCO3 in water measured immediately (within 2-3 min) after the exposure to the r.f. field changed in an oscillatory fa shion depending on the exposure time. For the sample treated for 15 mi n with the r.f. field the zeta potential decreased by 5-7 mV in relati on to an untreated sample. Residual changes occurred also in an oscill atory fashion during the 40 min after held removal. A similar behavior was found for the suspension in 10(-2) M NaCl. The observed zeta pote ntial changes were accompanied by changes in the electron donor compon ent of the surface free energy and the appearance of a small electron acceptor component, which resulted from the presence of hydrogen bondi ng on the calcium carbonate surface. The r.f. held treatment led to a decrease in gamma(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) a nd caused the appearance of gamma(S)(+) components of 1.34 and 1.77 mJ m(-2). The observed changes in the zeta potential and the surface fre e energy components are believed to result from the changes in the sur face charge of calcium carbonate and the structure of the water film a t the surface.