Solvent exclusion and chemical contrast in scanning force microscopy

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(CH2)(10)Y and force measurements are conducted as a function of terminal group Y (Y = CH2CH3, CH2OCH3, CO2CH3, CO(NH2), CO2H, and CH2OH) 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 (gamma(ts)) of like CH3- and CH2OCH3-terminated surfaces and the surface-vacuum interfacial energies (gamma(sy)) of the hydrophilic surfaces. An effective tip radius of similar to 30 nm and contact area of similar to 10 nm(2) (or similar to 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.