Electrophoresis of nanoparticles in high temperature aqueous solutions

Metal oxides are abundant in geochemical and bio-environmental systems, and are also widely used in industry. What determines the behavior of oxide materials in these environments, particularly in aqueous solutions, are the protonation and adsorption reactions at their surface. Furthermore, the effect of the solid/aqueous solution interaction becomes more important when the surface area of the contacting phases is high (e.g., colloids or porous media). Surface protonation and adsorption reactions have been widely studied for a variety of oxides but mainly at room temperature (25°C) or occasionally below 100°C, and studies in the hydrothermal regime are, unfortunately, scarce. A high temperature electrophoresis cell utilizing dark-field microscopy was developed in this study capable of reaching temperatures up to 260°C and pressures up to 70 bar. The dark-field microscopy setup developed here allows the visualization of sub-micron and nanometer-sized particles suspended in aqueous solutions. The particle size visualization limit was found by using commercially available SiO2 particle size standards 50 +/- 10 nm and 100 +/- 30 nm, respectively. Utilizing the high temperature electrophoresis cell, the electrophoretic mobility of SiO 2 and SnO2 was obtained as a function of pH and temperature at ionic strengths of 5 x 10--3 and 1 x 10 --3 kg mol--1, respectively. A new methodology was adopted that allows obtaining the electrophoretic mobility with increased accuracy from the measured electrophoretic velocity as a function of electric field strength. Zeta potentials were obtained from the measured electrophoretic mobilities by using the numerical treatment developed by O'Brien and White (1978) which takes into account the particle size.Isoelectric points (IEPs) were obtained for SiO2 at 25, 100, and 150°C, and for SnO2 at 25, 125, 150, 200, and 260°C by fitting the obtained zeta potential data as function of pH. The IEPs experimentally obtained where in excellent agreement with theoretical predictions. Many semi-theoretical models rely on available experimental data for calibration, thus we have compiled and analyzed available high temperature electrophoretic data on SiO2 , SnO2, ZrO2, TiO2, and Fe3O 4 to calibrate semi-theotical models that predict the standard surface protonation constants of oxide materials. Two new expressions were derived, based on the crystal chemistry and solvation theory approach developed by Sverjensky and Sahai (1998), dependent on the relative permittivity of the oxides, epsilonr. The obtained expressions were used to calculate the standard protonation enthalpy of several oxides and the results agree well with available experimental data.