Phase behavior of long-chain n-alkanes at one and between two mica surfaces

The phase behavior of long-chain n-alkanes (carbon number: m = 14, 16-18) a dsorbed at isolated mica surfaces and confined between two mica surfaces ha s been studied above and below the bulk melting points T-m. Using the Surfa ce Force Apparatus we have measured the thickness of alkane films adsorbed from vapor (0.98 < p/p(0) < 0.995), studied the formation and growth of cap illary condensates between two surfaces, and monitored phase changes in bot h the adsorbed films and the condensates. By measuring the growth rate of t he capillary condensates we have identified a transition in the lateral mob ility of molecules in the adsorbed films. This transition to greater mobili ty occurs slightly above T-m for n-C-16 to n-C-18, but several degrees belo w T-m for n-C-14, and is accompanied by a change in wetting properties and a measurable decrease in adsorbed film thickness for n-C-17 and n-C-18. Cap illary condensates that form below T-m remain liquid, but may freeze (cryst allize) if the degree of confinement is reduced by separation of the mica s urfaces. We conclude that an increase in the area of the liquid-vapor inter face relative to that of the liquid-mica interface facilitates freezing in the case of the longer chain alkanes (n-C-16 to n-C-18), which show surface ordering at the liquid-vapor interface. n-C-14, which does not show surfac e ordering at the liquid-vapor interface, does not freeze under the same co nditions.