Comparative analysis of thermodynamic approaches and diagnostic liquids for determination of contact angle-derived physicochemical parameters of solids coated with conditioning films: A practitioner's perspective

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Abstract
The equation of state (ES), the Lifshitz-van der Waals acid-base approach (LWAB), and the geometric mean equation (GME) were evaluated for calculation of physicochemical parameters from contact angles of the four diagnostic liquids: water (w), formamide (f), α-bromonaphthalene (α-br), and diiodomethane (di) measured on clean and conditioning film-coated substrata (stainless steel, germanium, polypropylene, perspex). Surface free energy and its components (dispersive and polar components in GME, as well as Lifshitz-van der Waals, electron-donor and electron-acceptor components in LWAB) were assessed in all approaches with all liquid combinations possible. The Gram-negative bacterium SW8 grown under carbon-limited conditions was used to calculate the solid-bacterium interfacial free energy and the free energy of adhesion of SW8 to solid substrata. These two parameters and the solid-liquid interfacial free energy were calculated only for combinations of diagnostic liquids that included water contact angles. Actual values of physicochemical parameters on high and low energy interfaces often differed between approaches or within an approach when different diagnostic liquids or combinations thereof were used. However, the same values of surface free energy were obtained on high and medium energy surfaces with the combinations LWAB w/f/di, LWAB w/f/α-br, ES f, GME f/α-br, and GME f/di. Approaches involving polar liquids produced similar trends for surface free energy and solid-liquid interfacial free energy. Results obtained with ES on high and low energy substrata were not consistent with fundamental premises of the theory and the dispersive component of surface free energy was generally underestimated in the GME approach. LWAB w/f/di was found to be the most suitable approach for analysis of the surfaces investigated in this work.

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