Hofmeister solute effects on hydrophobic adhesion forces in SFM experiments

One of the chief problems currently facing surface scientists is determinin g the relationship between mechanical forces, such as those measured by sur face forces apparatus (SFA) or scanning force microscopes (SFM), and chemic al driving forces, such as those governing interactions between molecules. Hydrophobic forces are important at both levels, but there is no clear rela tionship 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 hydrophob ic, and the highly hydrophobic surfaces paraffin and octadecylsilane-mica i n the presence of various 2.0 and 3.0 M salt solutions. The salts were chos en for their strong abilities to promote or inhibit hydrophobically driven phenomena, according to their positions in the Hofmeister series. Thus, thi s is the first systematic assessment of Hofmeister salt effects on supramol ecular hydrophobic adhesion in solution-phase chemical terms. NaCl has no e ffect on hydrophobic adhesion force relative to pure water, which agrees wi th previous SFM and SFA work but lies in contrast with the effects of NaCl on solution-phase behavior such as aqueous nonpolar compound solubility. Ch aotropes such SCN- and guanidinium(+), which promote exposure of molecular nonpolar surfaces to water, decrease adhesion force, whereas kosmotropes su ch as NH4+ and SO42-, which promote sequestration of nonpolar molecular sur faces from water, enhance adhesion force. Our results suggest that solvent structures near molecular and supramolecular hydrophobic surfaces are funda mentally different but that the nonideal effects embodied by the Hofmeister series have similar mechanisms in both length scales. Preferential interac tions of solutes with hydrophobic surfaces and potential sources of hydroph obic adhesion, such as cavitation, van der Waals interactions, and solvent ordering, are discussed.