Abstract
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 (σS) data obtained from solid-liquid-air (SLA) contact angle (θLa) 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 (θO) measurements through the oil phase using the inverted sessile drop method over a wide range of interfacial tensions (γo-aq). The results were plotted as γo-aq·cosθO vs. γo-aq, showing a smooth wetting transition from water-wetting to oil-wetting with decreasing γo-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 (σSd, σSp), which required additional SLA θLa measurements using PTFE as the reference surface. However, attempts at predicting θO for systems with high γo-aq resulted in significant deviations, a problem linked to the high σSd values required to fit the wettability of low γo-aq systems (toluene-water-IPA). The extended EQS (e-EQS) method produced reasonable predictions of γo-aq·cosθO for all the available experimental and literature data. The e-EQS method required fitting one of the interfacial energy terms (γS-L). For low surface energy materials, such as PTFE, the γS-o value should be fitted. For high surface energy materials, the γS-aq should be fitted instead. The fitted values of γS-o for PTFE and γS-aq for glass were consistent with the values obtained from Young's equation applied to SLA data.
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