Hofmeister Solute Effects on Hydrophobic Adhesion Forces in SFM Experiments

One of the chief problems currently facing surface scientists is determining the relationship between mechanical forces, such as those measured by surface forces apparatus (SFA) or scanning force microscopes (SFM), and chemical driving forces, such as those governing interactions between molecules. Hydrophobic forces are important at both levels, but there is no clear relationship between their manifestation at molecular and supramolecular scales. To help bridge this gap, we have used an SFM to measure detachment forces between untreated silicon nitride SFM tips, which are moderately hydrophobic, and the highly hydrophobic surfaces paraffin and octadecylsilane−mica in the presence of various 2.0 and 3.0 M salt solutions. The salts were chosen for their strong abilities to promote or inhibit hydrophobically driven phenomena, according to their positions in the Hofmeister series. Thus, this is the first systematic assessment of Hofmeister salt effects on supramolecular hydrophobic adhesion in solution-phase chemical terms. NaCl has no effect on hydrophobic adhesion force relative to pure water, which agrees with previous SFM and SFA work but lies in contrast with the effects of NaCl on solution-phase behavior such as aqueous nonpolar compound solubility. Chaotropes such SCN - and guanidinium +, which promote exposure of molecular nonpolar surfaces to water, decrease adhesion force, whereas kosmotropes such as NH 4 + and SO 4 2-, which promote sequestration of nonpolar molecular surfaces from water, enhance adhesion force. Our results suggest that solvent structures near molecular and supramolecular hydrophobic surfaces are fundamentally different but that the nonideal effects embodied by the Hofmeister series have similar mechanisms in both length scales. Preferential interactions of solutes with hydrophobic surfaces and potential sources of hydrophobic adhesion, such as cavitation, van der Waals interactions, and solvent ordering, are discussed.