Do epitaxy and temperature affect oscillatory solvation forces?

We present temperature-dependent atomic force microscope (AFM) measurements in force−distance mode of confined 1-dodecanol. Upon approach of the AFM-tip toward the highly oriented pyrolytic graphite (HOPG) surface, the final liquid film—only a few nanometers thin—is squeezed out in discrete layers. We find that both the force needed to squeeze out these layers and the number of structured layers strongly increase as the freezing temperature is approached. Surprisingly the force increases nonmonotonically and show a local maximum around 3 degrees and a local minimum at 1 degree above the bulk melting point of the liquid. We attribute this result to changes in epitaxial effects between 1-dodecanol and the HOPG surface close to the melting point of the liquid. To test this hypothesis we performed the same measurements in hexadecane, a similar carbon-chain molecule, and octamethylcyclotetrasiloxane (OMCTS), a quasi-spherical molecule. Hexadecane shows the same maximum in the squeeze-out force at 4−5 degrees and a minimum at 1−2 degrees above the freezing temperature of the liquid, while the squeeze-out of OMCTS was found to be independent of temperature.