Contact Angles of Aqueous Solutions on Copper Surfaces Bearing Self-Assembled Monolayers

Abstract

A laboratory exercise suitable for physical chemistry students at university level is described for the evaluation of contact angles. Students are required to modify copper surfaces by the adsorption of a self-assembled monolayer. The monolayers are formed from ethanolic solutions of long-chain alkanethiols that are terminated with either a methyl or a carboxylic acid group. Mixed monolayers are easily formed by immersion of the copper plates in solutions of mixed composition. The surface charge of monolayers bearing carboxylic acid groups can be modified by controlling the pH of the test solution. By these means the composition and degree of dissociation of the monolayer is manipulated, bringing about changes in interfacial energy. This results in a change in the wetting behavior, observed as a change in the measured contact angle. The results are interpreted in relation to the Young equation. The short range of the surface forces influencing the contact angle is highlighted. The experiment requires litt...