RESIDUAL CHANGES IN THE SURFACE-CHARGE OF TIO2 (ANATASE) SUSPENSIONS AS A RESULT OF EXPOSURE TO A 44 MHZ RADIOFREQUENCY ELECTRIC-FIELD

Potentiometric titrations of TiO2 (anatase) suspensions in solutions o f 10(-2), 10(-3), and 10(-4) M NaCl prior to treatment with a radiofre quency (RF) electric field gave values of pH(pzc) and pH(iep) of 5.75 +/- 0.1 and 5.85 +/- 0.1, respectively, within the range of literature values. Surface charge plots Versus pH for untreated samples gave cur ves with a common intersection point, indicating that NaCl acts as an indifferent electrolyte. Identical suspensions of anatase were then ex posed for 30 min to an RF electric field with a peak-to-peak, no-load amplitude of 34 V and a frequency of 44 MHz. Portions of the treated s uspension were titrated after different time periods following removal of the field (2, 15, 30, and 45 min). At 2 min, the pH(pzc) had shift ed to 6.50 +/- 0.1, and at 15, 30 and 45 min, the curves for the three NaCl concentrations were found to lack a common intersection point, a n effect which became more pronounced with time. It is proposed that R F treatment results in the formation of a porous gel layer of hydrous titanium dioxide on the surface of the anatase particles that subseque ntly undergoes a slow ion-exchange reaction involving ions of the supp orting electrolyte.