Energetics and Population of Molecules at Microscopic Liquid and Solid Surfaces

A new application of second harmonic generation to selectively probe the surface of microscopic size centrosymmetric structures in centrosymmetric bulk media has been applied to polymer beads in aqueous solution and to oil droplets in an oil/water emulsion. The free energies of adsorption and the surface densities of organic molecules adsorbed from aqueous solution to 1 μm polystyrene bead surfaces and to 230 nm tetradecane oil droplet surfaces have been determined. The potential use of this method as a chemical and biochemical sensor without the need for a fluorescent or magnetic tag is noted.