High Temperature Microelectrophoresis Studies of the Rutile/Aqueous Solution Interface

A recently developed high temperature microelectrophoresis cell was employed for ζ-potential measurements at the rutile/aqueous solution interface over a wide range of pH at temperatures of 25, 120, and 200 °C. Water, 0.001 mol kg -1 NaCl(aq), and 0.01 mol kg -1 NaCl(aq) solutions were tested at these temperatures. The desired pH values were attained by adding either HCl(aq) or NaOH(aq). The obtained experimental data allowed us to estimate isoelectric point pH values (pH iep ) of 5.26 (±0.45) at 25 °C, 5.13 (±0.37) at 120 °C, and 4.50 (±0.55) at 200 °C. This decrease in pH iep values with increasing temperature agrees with the decease in point of zero net proton charge (pH znpc ) pH values (as determined by potentiometric titration) observed for similar rutile powders in our previous studies. ζ-potential data were combined with the proton charge results and rationalized using a surface complexation modeling approach. Modeling results indicate that ζ-potentials are expressed at a distance that is generally equal to (within error) the so-called diffuse double layer thickness ( κ - 1 ).