Jp. Cleveland et al., PROBING OSCILLATORY HYDRATION POTENTIALS USING THERMAL-MECHANICAL NOISE IN AN ATOMIC-FORCE MICROSCOPE, Physical review. B, Condensed matter, 52(12), 1995, pp. 8692-8695
Intermolecular forces between surfaces and molecules in aqueous soluti
ons at distances below a few nanometers are critically important to th
e functioning of many systems, from colloidal to tribological and from
geological to biological. These forces are not, however, well underst
ood at present. We have made measurements of multiple, metastable stat
es 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 th
e Boltzmann relation on histograms of the tip position as it hops betw
een the metastable states, and is found to be oscillatory, indicating
layering of the solvent at the tip or sample. These oscillations are s
paced from 0.15 to 0.30 nm apart and are of order 5 X 10(-21) J in amp
litude, which is comparable to the thermal energy k(B)T=4 X 10(-21) J
at room temperature.