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

This work assesses classical density functional theory (DFT) for predicting static contact angles of pure substances and mixtures by comparison with new experimental data. A Helmholtz energy functional based on perturbed-chain statistical associating fluid theory (PC-SAFT) describes fluid–fluid interactions, combined with an effective external potential for solid–fluid interactions. The solid is characterized by adjusting a single solid–solid interaction parameter to the contact angle of n-octane; all other results are predictions. Surface tensions between solid, liquid, and vapor phases are determined from one-dimensional DFT, and Young’s equation yields the contact angle. Experiments employ the sessile droplet method on a nonpolar polytetrafluoroethylene (Teflon) substrate. Accurate results are achieved for pure substances except monohydric alcohols, where neglecting orientational effects contributes to systematic overestimation. For mixtures, predictions are reliable whenever the respective pure substances are described well, including polar and hydrogen-bonding systems. Overall, DFT based on PC-SAFT enables fast and accurate contact angle predictions.