Static Contact Angles of Mixtures: Classical Density Functional Theory and Experimental Investigation

Abstract: This work assesses a classical density functional theory (DFT) model for predicting macroscopic static contact angles of pure substances and mixtures by comparison to own experimental data. We employ a DFT with a Helmholtz energy functional based on the perturbed-chain statistical associating fluid theory (PC-SAFT) for the fluid-fluid interactions and an effective external potential for the solid-fluid interactions. The solid substrate is characterized by adjusting a single solid-solid interaction energy parameter to a single contact angle value of $n$-octane, while all other results are predictions based on Berthelot-Lorentz combining rules. The surface tensions between solid, liquid and vapor phases are determined from one-dimensional DFT calculations, and Young's equation is used to calculate the contact angle. A non-polar polytetrafluoroethylene (PTFE, Teflon) substrate is used, and experiments are carried out using the sessile droplet method with graphical evaluation of the contact angle. Accurate results are obtained for pure substances except for monohydric alcohols. Contact angles for monohydric alcohols are systematically overestimated, and we show that this is partially due to neglecting orientational effects. The approach provides accurate results for mixtures whenever the respective pure substance contact angle is described well. This comprises mixtures of non-polar, polar, and hydrogen bonding substances, including mixtures with water. Our results suggest that DFT based on PC-SAFT provides a fast and accurate method for prediction of contact angles for a wide range of pure substances and mixtures.