Solvent Exclusion and Chemical Contrast in Scanning Force Microscopy

Abstract

The importance of a solvent in regulating the adhesion forces between surfaces is studied quantitatively with scanning force microscopy. Both samples and tips are coated with alkyl thiolate monolayers of type HS(CH 2 ) 10 Y and force measurements are conducted as a function of terminal group Y (Y = CH 2 CH 3, CH 2 OCH 3, CO 2 CH 3, CO(NH 2 ), CO 2 H, and CH 2 OH) and solvent (water, ethanol, and n -hexadecane). Adhesive forces in water span the greatest range (0.30−12.5 nN), with hydrophobic surfaces adhering most strongly and hydrophilic surfaces most weakly. In ethanol the adhesive forces are substantially smaller and in n -hexadecane they are negligible. In water, these adhesive forces are consistent with the work required to exclude solvent from the tip−sample interface, indicating that solvent exclusion dominates adhesion. Such macroscopic solvent exclusion cannot fully explain the adhesive forces in ethanol. This force data is used to evaluate the tip−sample interfacial energies (γ ts ) of like CH 3 - and CH 2 OCH 3 -terminated surfaces and the surface−vacuum interfacial energies (γ sv ) of the hydrophilic surfaces. An effective tip radius of ∼30 nm and contact area of ∼10 nm 2 (or ∼50 contacting molecules) is estimated from the adhesion between methyl groups in water. Since solvent exclusion regulates adhesion between these model organic surfaces, it provides a source of chemical contrast in force imaging. We explore this chemical contrast with friction force measurements of co-block polyethylene glycol−polyamide polymer surfaces.