Effect of an external radiofrequency electric field on the surface free energy components of calcium carbonate in the presence of cationic and anionic surfactants

The surface free energy components of calcium carbonate for bare and surfac tant-treated surfaces were studied by the thin-layer wicking technique. The surface precoverage from a 10(-3) M aqueous solution of cetyltrimethylammo nium bromide (CTMABr) and sodium dodecyl sulfate (SDS) caused a decrease in the electron donor component and an increase in the electron acceptor comp onent. The changes were more pronounced in the case of CTMABr than for SDS. Consequently, in the presence of the cationic surfactant, the surface beco mes hydrophobic while the SDS-precovered surface is still slightly hydrophi lic. The presence of a radiofrequency (RF) electric field (44 MHz, 60 V pea k-to-peak no-load amplitude) during wicking experiments perpendicularly to the wicking direction also decreases the electron donor component and incre ases the electron acceptor component. The increase is relatively higher for the surfactant-precovered surfaces. The RF treatment does not change the h ydrophobicity of the bare surface but changes the hydrophobic CTMABr-precov ered surface to slightly hydrophilic. On the contrary, the slightly hydroph ilic SDS-preadsorbed surface becomes slightly hydrophobic in the presence o f RE It is concluded that the free energy changes appearing in the presence of RF are due to reorientation of the adsorbed surfactant molecules and/or changes in the structure of the water molecules hydrating the surface.