Adsorption at a Carbon Black Microparticle Surface in Aqueous Colloids Probed by Optical Second-Harmonic Generation

The surface properties of carbon black microparticles in aqueous solution are characterized through adsorption of a hydrophobic ion (malachite green) monitored by optical second-harmonic generation (SHG). Carbon black particle surfaces with 14 different O/C ratios (from 0.028 to 0.270) have been examined. The adsorption isotherm of MG on the surface of carbon black particles weighing as little as 0.010 g can be measured using SHG measured during continuous titration of the MG molecule into the solution. The SHG data allows determination of the adsorption free energy and reveals coherent interference between the SH fields from the surface-adsorbed MG molecules and the carbon black particle. The change in the coherent interference implies a change of adsorption orientation of MG on surfaces with different O/C ratios. There is strong correlation found between the observed adsorption characteristics and the O/C ratio of the surfaces. Furthermore, MG adsorption isotherms of surfaces with higher O/C ratios appear similar to the ones of surfaces preadsorbed with surfactants that can increase surface hydrophilicity and dispersion in aqueous solution. All observations indicate that particle surfaces with higher O/C ratios appear more hydrophilic. This study demonstrates once again the many advantages of SHG as an effective surface analytical technique of colloidal particles.