Thesis defense of Mike Moron

Adsorption is a phenomenon occurring at interfaces, in which molecules attach to an interface via weak interactions. Adsorption plays an important role in biological and industrial processes, such as the adsorption of proteins during blood coagulation, heterogeneous catalysis or the oxidation of industrial sludge in supercritical water. A supercritical fluid (SCF) is a fluid that is above its critical pressure pcrit and its critical temperature Tcrit. At the critical point, there are no longer differences in density or viscosity between the gaseous and liquid phases in the volume of the phase. This leads to SCFs having similar densities and solvent properties to liquids while having the viscosity of a gas. Understanding the adsorption properties of fluids below and above the critical point at solid interfaces is essential to optimise processes in numerous applications. However, studying the adsorption of SCFs poses technical challenges for a suitable sample environment, as the critical points of many fluids, for example water (Tcrit = 374 ◦C, pcrit = 221 bar) or ethanol (Tcrit = 241 ◦C, pcrit = 63 bar), at high pressures and temperatures. Therefore, within the scope of this work, in cooperation with the company Dieckers GmbH & Co. KG and SITEC-Sieber Engineering AG, a sample cell was designed and manufactured by SITEC-Sieber Engineering AG that withstands pressures of up to 1000 bar at temperatures up to 500 ◦C and thus enables the interface-sensitive analysis of a variety of supercritical fluids. The sample cell was successfully commissioned at beamline ID 31 at the ESRF (Grenoble, France) and initial measurements showed, that the supercritical region of water is accessible. In this work, the adsorption of the fluids C2F6, C3F8 and C4F10 on silicon wafers coated with octadecyltrichlorosilane (OTS) was investigated as a function of the distance of a given temperature or pressure from the critical point, varying the distance from the critical point by the choice of the fluids. In particular, the development of roughness as a function of layer thickness of the adsorbed layers was analysed. By recording X-ray reflectivities (XRR), it could be shown that the increase in roughness of the adsorbed layers with increasing pressure is stronger the closer the fluid is to its critical temperature. Even above the critical point, an adsorbed layer remained on the OTS wafer that showed the greatest roughness. In contrast, below the critical point, the layer thicknesses diverge as the saturation vapour pressure is reached, since the ambient fluid condenses and a macroscopically thick liquid layer forms on the OTS wafer. To adequately evaluate the adsorbed layers, the OTS wafers were analysed for their sensitivity to X-rays and high temperatures. The XRR measurements suggested that no severe damage to the OTS layer is expected for the investigated fluids at low temperatures. Furthermore, it was found that damage to the OTS layer occurs at higher photon energies at higher temperatures, so that higher photon energies are generally more suitable for studying adsorption by XRR. Furthermore, it was shown that a gas layer located above the OTS wafer significantly reduces the stability of the OTS layer. The XRR measurements carried out in this work at the interface between the investigated fluids and OTS not only contribute to a better understanding of the adsorption of fluorocarbons on hydrophobic surfaces, but also provide important insights into the adsorption behaviour of SCFs. This has potential applications in various fields, from surface coating to the development of environmentally friendly technologies.

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