Solid-liquid-liquid wettability and its prediction with surface free energy models

Understanding wettability in solid-liquid-liquid (SLL or immersed) systems is important for numerous applications. However, predicting SLL wetting behavior on smooth surfaces has received little attention. The objective of this work was to explore alternatives to predict SLL wettability. To this end, we first present a review of solid surface free energy (σ_S) data obtained from solid-liquid-air (SLA) contact angle (θ_L^a) data and a summary of available SLL contact angle data for selected materials. Next, the existing surface free energy models for SLA systems are discussed in terms of their applicability to predict wettability of SLL systems. Finally, the SLL wettability of toluene drops on glass, mica, stainless steel and PTFE immersed in equilibrated Toluene-water-isopropyl alcohol (IPA) solutions was determined via contact angle (θ_O) measurements through the oil phase using the inverted sessile drop method over a wide range of interfacial tensions (γ_o-aq). The results were plotted as γ_o-aq·cosθ_O vs. γ_o-aq, showing a smooth wetting transition from water-wetting to oil-wetting with decreasing γ_o-aq for glass and stainless steel. Mica remained water-wetting, while PTFE oil-wetting. The Geometric (GM) and Harmonic (HM) mean approaches, and the Equation-of-State (EQS), originally developed for SLA systems, were extended to SLL systems. The extended GM and HM approaches could fit the SLL behavior after fitting the dispersive and polar contributions of the solid surface free energy (σ_S^d, σ_S^p), which required additional SLA θ_L^a measurements using PTFE as the reference surface. However, attempts at predicting θ_O for systems with high γ_o-aq resulted in significant deviations, a problem linked to the high σ_S^d values required to fit the wettability of low γ_o-aq systems (toluene-water-IPA). The extended EQS (e-EQS) method produced reasonable predictions of γ_o-aq·cosθ_O for all the available experimental and literature data. The e-EQS method required fitting one of the interfacial energy terms (γ_S-L). For low surface energy materials, such as PTFE, the γ_S-o value should be fitted. For high surface energy materials, the γ_S-aq should be fitted instead. The fitted values of γ_S-o for PTFE and γ_S-aq for glass were consistent with the values obtained from Young's equation applied to SLA data.