Role of Polarity on the Interfaces of Binary Combinations

Abstract

Owing to a large number of applications, starting from pharmaceutical packaging to advancement in nanotechnology and microfluidic devices, wetting characteristics has always been regarded as an essential prerequisite for many phenomenal processes. In the present time, the term ‘wetting’ is not limited to spreading of a liquid on a solid but also portrays displacement ability of a gas over a liquid. To quantify wetting of a solid surface by a liquid, numerous established theories argue that interactions between polar–polar, polar–non-polar and non-polar–non-polar components of surface tension or equivalently, surface energy dictate the final equilibrium contact angle. In this work, the extent to which individual phases of binary liquid−vapor and solid−liquid system interacts, and how such interactions are influenced by the polar and the dispersive components of the surface tension is examined. For liquid−fluid systems, the effect of the polarity of the surrounding (saturated) vapor medium on the equilibrium surface tension is critically investigated. Such measurements being prone to inaccuracy for highly volatile liquids, a standard protocol to obtain equilibrium surface tension with reasonable accuracy has been developed. A wide range of fluids covering polar−polar, polar−non-polar and non-polar−non-polar liquid-vapor combinations are studied and results confirm that the influence of molecular weight of both of the phases (drop and surrounding saturated vapor) must be accounted for in addition to the interactions (polar-polar, polar-non-polar etc.) that occur therein. For the case of polar-polar and non-polar-non-polar combination, observations suggest that the liquid drop interface becomes active only if the molecular weight of vapor is lower than the liquid phase. Further, it is observed that polar component of surface tension of drop (for polar liquid) influences the interaction between equilibrium surface tension and Fowkes’ dispersive interaction. Similar influence of the polar component of the surface tension has been observed for the solid−liquid systems where the percentage polarity of the (gold) substrates are varied by means of functionalization with mixed self assembled monolayer, SAM. Moreover, the surface characterization using X-ray photoelectron spectroscopy (XPS) proved that the mole fraction of the chemicals on the surface is different than that of the immersion solution.