The Interface between Water-Solvent Mixtures and a Hydrophobic Surface.

The mechanism behind the stability of organic nanoparticles prepared by liquid anti-solvent precipitation (LAS) without specific stabilizing agent is poorly understood. In this work, we propose that the organic solvent used in the LAS process rapidly forms a molecular stabilizing layer at the nanoparticles' interface with the aqueous dispersion medium. To confirm this hypothesis, n-octadecyltrichlorosilane (OTS)-functionalized silicon wafers in contact with water-solvent mixtures were used as a flat model system mimicking the solid-liquid interface of the organic nanoparticles. We studied the equilibrium structure of the interface by X-ray reflectometry (XRR) for water-solvent mixtures (methanol, ethanol, 1-propanol, 2-propanol, acetone, and THF). The formation of an organic solvent rich layer at the solid-liquid interface was observed. The layer thickness increases with organic solvent concentration and correlates with the polar and hydrogen bond fraction of the Hansen solubility parameters (HSP). We developed a self-consistent adsorption model via complementing adsorption isotherms obtained from XRR data with molecular dynamics (MD) simulations.