Phase Behavior of Long-Chain n-Alkanes at One and between Two Mica Surfaces

Abstract

The phase behavior of long-chain n-alkanes (carbon number: m = 14, 16−18) adsorbed at isolated mica surfaces and confined between two mica surfaces has been studied above and below the bulk melting points Tm. Using the Surface Force Apparatus we have measured the thickness of alkane films adsorbed from vapor (0.98 < p/p0 < 0.995), studied the formation and growth of capillary condensates between two surfaces, and monitored phase changes in both the adsorbed films and the condensates. By measuring the growth rate of the capillary condensates we have identified a transition in the lateral mobility of molecules in the adsorbed films. This transition to greater mobility occurs slightly above Tm for n-C16 to n-C18, but several degrees below Tm for n-C14, and is accompanied by a change in wetting properties and a measurable decrease in adsorbed film thickness for n-C17 and n-C18. Capillary condensates that form below Tm remain liquid, but may freeze (crystallize) if the degree of confinement is reduced by separation of the mica surfaces. We conclude that an increase in the area of the liquid−vapor interface relative to that of the liquid−mica interface facilitates freezing in the case of the longer chain alkanes (n-C16 to n-C18), which show surface ordering at the liquid−vapor interface. n-C14, which does not show surface ordering at the liquid−vapor interface, does not freeze under the same conditions.