On the Degree to which the Contact Angle is Affected by the Adsorption onto a Solid Surface of Vapor Molecules Originating from the Liquid Drop

ABSTRACT

From surface tensions of liquids and Lifshitz-van der Waals (LW) and Lewis acid-base (AB) surface tension components and the AB electron-acceptor γ⁺ and electron-donor γ^˙ parameters determined by contact angle (θ) measurements (using the Young-Dupré equation for polar systems), the interfacial work of salvation (W_st) between various contact angle liquids (L) and a moderately polar solid (S), such as polymethylmethacrylate (PMMA) could be determined. From these W_SL -values the maximum values of the equilibrium association constant, K_a, are obtained for the adsorption of molecules of liquids, L, onto a solid substratum, S. From the K_a-values and the vapor pressures of the various liquids, the maximum number of liquid molecules adsorbed from the gaseous phase onto the solid surface can be determined, at 20^°C and 76cm Hg ambient atmospheric pressure. This yields the maximum value for the fraction, ϕ, of the surface area of the solid that will be covered by molecules of the liquid, L, emanating from the liquid drop, via the gaseous state. From these ϕ-values, using Cassie's approach, the maximum amount, Δθ, can be determined by which the observed contact angle is lower than the ideal contact angle, as a consequence of the coverage of the solid substratum by adsorbed molecules originating from the contact angle liquid.

For most of the contact angle liquids used, the maximum deviation, Δθ, is well under 1^°; for water on PMMA it is about 1½^°.