Compression of a Stearic Acid Surfactant Layer on Water Investigated by Ambient Pressure X-ray Photoelectron Spectroscopy

We present a combined Langmuir–Pockels trough and ambient pressure X-ray photoelectron spectroscopy (APXPS) study of the compression of stearic acid surfactant layers on neat water. Changes in the packing density of the molecules are directly determined from C 1s and O 1s APXPS data. The experimental data are fit with a 2D model for the stearic acid coverage. Based on the results of these proof-of-principle experiments, we discuss the remaining challenges that need to be overcome for future investigations of the role of surfactants in heterogeneous chemical reactions at liquid–vapor interfaces in combined Langmuir–Pockels trough and APXPS measurements

Spreading of volatile fluids on swelling hydrophobic polymer brush layers

Polymer brushes are highly responsive materials with application areas in a broad spectrum. Therefore understanding their interfacial behaviour is essential. Under good solvent conditions, drop spreading causes changes in wettability on the brush surface and results in a finite contact angle. This feature was observed by placing a hexadecane drop on a dry hydrophobic poly lauryl methacrylate (PLMA) brush layer. These polymer brushes were synthesized via ARGET-ATRP to obtain high and low grafting densities and different chain lengths. In a closed chamber, that contained an excess of hexadecane, the spreading of the drop on the polymer brushes was followed by an optical microscope. As a result of the complex dynamics between the drop and the substrate, a halo of a few millimetres in width forms around the macroscopic contact line. A numeric model was extended to simulate such a droplet spreading on a swollen brush for understanding the coupled spreading, evaporation, condensation and wicking dynamics. A continuous gradual flux from liquid to brush, liquid to vapor, and brush to vapor can explain the partially wetting of the polymer brushes in this non-equilibrium state. Understanding the spreading dynamics of a droplet on hydrophobic surfaces will be crucial to explaining the mechanism of the halo formation