The demonstration of a nonlinear optical technique for directly monitoring adsorption of surfactants on the surface of microparticles in colloids is reported. In this approach, dye molecules with strong hyperpolarizability are first adsorbed on the particle surface to give detectable second-harmonic generation. The surfactant is then added to the colloidal solution in competition with dye for adsorption on the surface. The displacement of the dye molecules on the surface results in a decrease of the second-harmonic signal, indicating the adsorption of the surfactant molecules. A continuous flow/titration system in combination with a high-repetition-rate femtosecond laser allows the adsorption to be monitored in real time. This approach was first demonstrated on a methacrylate polymeric surfactant on latex and talc particles in an aqueous solution. The adsorption free energy and surface density of this surfactant on these particles have been determined.

Nonlinear optical probe, second harmonic generation (SHG), of the adsorption of the dye molecule malachite green (MG), in cationic form at pH < or = 5, on polystyrene microspheres in aqueous solution is used to study the effect of surface charge and composition on molecular adsorption. Three types of polystyrene microspheres with different surface composition are investigated: (1) a sulfate terminated, anionic surface, (2) a neutral surface without any functional group termination, and (3) an amine terminated, cationic surface. The cationic dye was found to adsorb at all three surfaces, regardless of surface charge. The adsorption free energies, DeltaG's, measured for the three surfaces are -12.67, -12.39, and -10.46 kcal/mol, respectively, with the trend as expected from the charge interactions. The adsorption density on the anionic surface, where attractive charge-charge interaction dominates, is determined by the surface negative charge density. The adsorption densities on the neutral and cationic surfaces are on the other hand higher, perhaps as a result of a balance between minimizing repulsive charge interaction and maximizing attractive molecule-substrate and intermolecular interactions. The relative strength of the SH intensity per molecule, in combination of a model calculation, reveals that the C(2) axis of the MG molecule is nearly perpendicular to the surface on the anionic surface and tilts away from the surface norm when the surface is neutral and further away when cationic. Changing the pH of the solution may alter the surface charge and subsequently affect the adsorption configuration and SH intensity.

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.