Revisiting the Surface Energy Parameters of Standard Test Liquids with a Corrected Contact Angle Method over Rough Surfaces.

Interfacial free energy is a quantitative basis for explaining and predicting interfacial behavior that is ubiquitous in nature. The contact angle (CA) method can determine the surface free energy (γ) as well as Lifshitz-van der Waals (γ LW ) and Lewis acid/base (γ + /γ - ) components of a solid material from its CAs with a set of known test liquids according to the extended Young-Dupré equation. However, the reliability of the "known" parameters of the test liquids is questioned due to the long-neglected surface roughness effect during calibration of the liquids. This study proposed a simple and practicable two-step approach to correct the energy parameters of several test liquids by incorporating Wenzel's surface roughness relationship into CA measurement. Step 1: water and two apolar liquids (diiodomethane and α-bromonaphthalene) were used as benchmarks to calibrate the surface roughness and energy parameters of two reference solids [apolar poly(tetrafluoroethylene) and monopolar poly(methyl methacrylate)], and step 2: the reference solids were used to calibrate any other test liquids by solving the energy parameters from their CAs in the extended Young-Dupré-Wenzel model. Monte Carlo simulation was used to evaluate error transmission and robustness of the model solutions. The obtained energy parameters (γ LW /γ + /γ - ) of four test liquids (dimethyl sulfoxide, formamide, ethylene glycol, and glycerol) are 28.01/13.68/4.67, 34.95/3.53/37.62, 26.26/7.51/15.74, and 32.99/9.24/26.02 mJ/m 2 , respectively, and different from the literature values. The liquids were applied to characterize an example solid surface with true γ LW /γ + /γ - values of 28.00/1.00/8.00 mJ/m 2 and a roughness index ( r ) of 1.60. Without correction of the liquid parameters, the calculated surface energy, hydration energy, and hydrophobic attraction energy of the solid sample can deviate by 50, 13, and 27%, respectively. This proves the necessity of correcting parameters of the test liquids before they can be used in CA and interfacial energy studies in the presence of surface roughness.