Influence of van der Waals forces from metallic substrates on fluids supported on self-assembled monolayers formed from alkanethiols

We report that contact angles of Liquids supported on self-assembled monolayers (SAMs) formed from alkanethiols on metallic substrates are measurably influenced by van der Waals forces that act between the liquids and metallic substrates (through the SAMs). Van der Waals forces between gold and hexadecane separated by SAMs are evaluated using Lifshitz theory and found to quantitatively describe the measured decrease in contact angles of hexadecane supported on SAMs formed from alkanethiols (CH(3)(CH(2))(m)SH, 2 < m < 17) of decreasing length. Lifshitz theory is also used to predict an increase in the contact angles of drops of perfluorononane placed under hexadecane on SAMs formed from alkanethiols of decreasing length; our experimental measurements confirm this prediction. The effects of different substrates (silver, copper, and silicon dioxide) on contact angles of hexadecane supported on SAMs are also calculated and compared to measurements reported in the literature. Our results demonstrate that van der Waals forces from metallic substrates of SAMs should be considered when developing structure-property relationships for these surfaces.