Temperature Dependency of the Apolar Surface Tension Component for Water and Its Role in Classifying Apolar and Polar Interfacial Interactions

The theory of surface tension/energy components has been widely used to predict interfacial interactions between liquids and between liquids and solids. One such model, the van Oss–Chaudhury–Good (vOCG) model, which characterizes interactions as being polar/nonpolar, has been successfully applied to predict interfacial interactions at ambient temperature in a variety of applications/fields. Despite widespread use of the vOCG framework, data for the polar and apolar components of surface tension/energy are only available at room temperature, and the temperature dependence of these components is not well understood. We report detailed measurements to characterize the temperature dependency of the Lifshitz–van der Waals (LW, nonpolar) and the Lewis acid–base (AB, polar) components of surface tension/energy in the vOCG model. Our approach leverages the pendant-droplet method to measure the surface and interfacial tension for water and a group of water-immiscible liquids with surface tension values over a large range (12–46 mN/m) and different molecular structure/compositions, and measurements were made over a wide range of temperatures (≈30–65 °C). We observed that the components of surface tension were dependent on temperature (≈30–65 °C); however, the relative contributions of the apolar and polar components of surface tension to the total surface tension remained constant over this temperature range. For water, this relative contribution was 30% and 70% for the apolar and polar components, respectively. What is interesting to note however is that some fluids (e.g., 1-bromonapthalene, diiodomethane, and silicone oil) which are typically assumed to be nonpolar within the vOCG framework, demonstrated polar interfacial behavior when interacting with water. Our measurements indicate that fluids with a relative polarity exceeding a nominal threshold of ≈0.065 may not be assumed to be nonpolar within the vOCG model framework. Our findings reveal the influence of fluid molecular polarity on determining polar interfacial interactions and the temperature-dependency of surface tension components. Additionally, these findings provide insight into interfacial interactions outside ambient temperature that can further enable the design and development of different technology.