Probing oscillatory hydration potentials using thermal-mechanical noise in an atomic-force microscope

Intermolecular forces between surfaces and molecules in aqueous solutions at distances below a few nanometers are critically important to the functioning of many systems, from colloidal to tribological and from geological to biological. These forces are not, however, well understood at present. We have made measurements of multiple, metastable states near the hydrophilic cleavage planes of the ionic crystals calcite (CaCO₃) and barite (BaSO₄) using a low-noise atomic-force microscope. The interaction potential of the cantilever tip is determined using the Boltzmann relation on histograms of the tip position as it hops between the metastable states, and is found to be oscillatory, indicating layering of the solvent at the tip or sample. These oscillations are spaced from 0.15 to 0.30 nm apart and are of order 5×10^-21 J in amplitude, which is comparable to the thermal energy k_BT=4×10^-21 J at room temperature.