The Rayleigh−Gans−Debye model, adapted for nonlinear optical phenomena, is used to describe the second harmonic scattering from the surface of spherical particles in colloids. Specifically, the effect of the size of the particle on the efficiency of second harmonic generation from Malachite Green (MG) molecules adsorbed on polystyrene particles is examined. The model is found to be adequate for describing scattering patterns from smaller particles with diameters ≤200 nm, but less so for larger particles with diameters approaching 1 μm. From the model fit of experimentally measured data (Part I of this series, J. Phys. Chem. A 2009, 113, 4758) it is determined that χ⊥|||| S is the dominant susceptibility element. This result suggests that the MG molecules with a dominant β z′′x′′x′′ component adsorb on the surface of the spherical nanoparticles with the C2 axis nearly perpendicular to the surface.

The size of the colloidal particle has a dramatic effect on the efficiency of optical second harmonic generation (SHG) from the particle surface. This effect is illustrated with the second harmonic intensity measured from submicrometer polystyrene beads adsorbed with malachite green dye molecules in aqueous solution, both in the forward direction and as a function of the scattering angle in the horizontal plane. In the forward scattering direction, the SHG efficiency initially increases rapidly with the particle size. The efficiency peaks at approximately 1 mum diameter and then decreases with the particle size. The majority of the SH light generated from the particle surface scatters away from the forward direction for smaller particles. In particular for particles with diameter smaller than 100 nm the position of the maximum is nearly 90 degrees away from the forward direction. A straightforward phenomenological model is used to predict that the angle of maximum SH efficiency tilts more toward the forward direction for particles with larger diameter as well as higher refractive index.

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.