Determination of contact angles of nanosized silica particles by multi-angle single-wavelength ellipsometry

The determination of nanoparticle wettability is an area of great practical importance to materials science and engineering fields. There has been some recent interest in using spectroscopic analysis to indirectly categorize behaviour of monolayers, using the reflective and refractive properties of immersed particles. A method is developed here to calculate specific contact information for nanoparticles at an air-water interface, using single wavelength ellipsometry. A two-layer model is used that considers the refractive index of the immersed and non-immersed particle portions. Pressure-area isotherms and Brewster angle images were used to confirm and categorize the nature of a packed monolayer, and psi and Delta measurements taken of the interface. Immersion thickness was calculated and related to an equilibrium contact angle, and combined thickness data for the two layers was successfully related to the diameter of the particles.