Temperature dependence of solvation forces as measured in atomic force microscopy

An atomic force microscope (AFM) has been used to study the effect of temperature on solvation forces in the liquids octamethylcyclotetrasiloxane, n-hexadecane, and n-dodecanol confined between the AFM tip and a graphite surface. Discrete solvation layers can be observed for all three liquids at all the temperatures measured (298-348K). However, with increasing temperature there is a significant decrease in the magnitude of the measured solvation forces and a reduction in the number of solvation oscillations which can be observed. Solvation forces per se are only weakly temperature dependent and the most plausible explanation is that we are measuring how the layers are squeezed from the tip-sample gap. The squeeze out process is a thermally activated phenomenon and gives rise to the large change in the magnitude of the force oscillations with temperature. A simple analysis is provided based on transition rate theory. The large change in solvation force with temperature has implications for the measurement of localized forces using AFM, particularly in interpreting biological interactions and single asperity friction.