An ellipsometry study of silica nanoparticle layers at the water surface

We have studied silica nanoparticle layers spread at the air/water interface. The surface pressure of the layers has been determined in a Langmuir trough via two orthogonal Wilhelmy plates. We observed significant differences in surface pressure according to the preparation protocol: layers spread then compressed or layers obtained after successive spreading steps. We also studied the two types of layers by multiple angle of incidence ellipsometry. We introduce a two-layer model which enables us to evaluate the radius of interfacial aggregates and their contact angle with the air/water interface.