Hansen solubility parameters from surface measurements: A comparison of different methods

Abstract

The surface energies (γ), and Hansen solubility parameters (HSP, δ) are all related to the cohesive energy density. Modifications of solid surfaces can affect the surface properties of the solids, such as the surface energy, adhesion characteristics, and also the HSP. In membrane technologies, the use of multi layered membranes such as the thin film composite (TFC) is very common in many applications, such as water treatments and gas separation. In the TFC membranes, the top layer has a different characteristic than the bottom layer including different solubility characteristics as well. The HSP of a material can be measured via various methods such as Hansen dissolution tests, and inverse gas chromatography. However, those methods will not selectively determine the HSP for the surface or top layer of a membrane. The possibility of estimating the HSP of the surfaces of solid materials from measurements of contact angle is investigated. Various relations in literature are presented between HSP and the surface energies of solids. The molar volume Vm - free γ − δ relations were investigated and discussed in terms of the numerous methods used for surface energy evaluation and in terms of their accuracy compared to dissolution tests proposed by Hansen for the estimation of the HSP. The sessile drop test was conducted for the measurements of advancing contact angle for five different polymer films using a set of fourteen different solvents. Five polymeric films, Polyvinyl chloride (PVC), Polyethylene oxide (PEO), Cellulose acetate (CA), Polyvinylidene Fluoride (PVDF-2801), and Polysulfone (PS) were prepared in the lab. The morphology characteristics were studied using atomic force microscopy to determine the relative roughness of the surfaces. The HSP were calculated using HSPiP software using three different solvent ranking methods to categorize the solvents as bad or good solvents for each of the five films studied. The results of the HSPiP from the dissolution tests and visual observation ranking was considered the most accurate and the base case to compare all the results to. The surface energy components were calculated from the equilibrium contact angle for each film and the HSP were estimated using the Vm-free γ − δ relations. The HSP from dissolution experiments and HSP calculated from the Vm- free γ − δ relations were compared and discussed. Results clearly indicated the absence of a direct relation between contact angle and HSP and the use of the contact angle threshold for identifying solvents as bad and good solvent is irrelevant. The HSP can be roughly estimated from the surface energy values, obtained from contact angle measurements, via the Vm- free γ − δ relation with varying accuracy depending on the methods used to calculate the surface energy and on the nature of the solid film. Generally, an error of less than 20% in the estimation of the total HSP and dispersive term of HSP was found when compared to dissolution tests. Higher errors were noticed for the polar and hydrogen bonding components of the HSP for some of the films.