Ellipsometry is a classical optical technique, which is based on the change of the polarization state from linear to elliptical polarization, which a light beam experiences upon reflection at an interface.

The raw experimental data are the so-called ellipsometric angles and , from which the refractive index and/or the thickness of an interface layer can be calculated. The sensitivity of this method is proportional to the optical contrast (refractive index gradient) at the interface, and therefore this method is used mostly for studies of solid surfaces in contact with air or with an aqueous solution as the ambient incidence medium. Since most of the adsorption and surface processes we are interested in take place at solid/liquid interfaces of organic solvents, we have adapted and optimized ellipsometric measurements for studies at nonaqueous liquid/solid interfaces.

The solid substrate is mounted at the bottom of a trapezoidal glass cell, which is filled with an adsorbate solution and allows real-time monitoring of surface adsorption processes with a time resolution of about 1s.

Kinetic and structural information about film formation at the substrate/solution interface, which are difficult to obtain with other techniques, become accessible with this in-situ ellipsometric instrumentation.

As an example, a two-step adsorption process on a gold surface is shown. In the first step, a monolayer of 16-hydroxyhexadecanethiol (HHDT) is adsorbed from a dilute n-hexane solution. A densely packed monolayer film of thiolate molecules is thereby created, which is anchored to the substrate surface by Au-S bonds and is terminated by surface hydroxyl groups. The latter can be used to bond a second layer of octadecylsiloxane via hydrolysis and condensation of octadecyltrischlorosilane precursors:

Both reactions can be followed directly at the gold/hexane interface and reveal information about the rate of adsorption as well as the surface coverage.