Order-disorder transitions at the liquid/solid interface. Volumetric behaviour of primary aliphatic alcohols near the graphon surface

Abstract

The volumetric behaviour at the liquid/Graphon interface has been investigated as a function of temperature for the following alcohols in the pure state: hexan-1-ol (from –38°C to 60°C), and octan-1-ol, decan-1-ol, dodecan-1-ol (from the melting point to 60°C). The results are expressed in terms of the surface excess mass of the liquids.

The surface excess mass of the alcohol/Graphon systems does not decrease steadily when the temperature is increased (as in the alkane/Graphon systems) but exhibits a step with a point of inflection at a temperature roughly 50°C above the melting point of the alcohol. This behaviour is attributed to an order-disorder transition occuring in the boundary layer of the alcohol. The change in surface excess mass involved in this transition amounts to 2.60 × 10^–9, 2.37 × 10^–9, and 2.34 × 10^–9 g cm^–2, for hexan-1-ol, octan-1-ol, and decan-1-ol, respectively. An analysis of these results suggests that the transition is caused by the “melting” of a monolayer of horizontally disposed molecules on the graphite basal planes of Graphon. It is thought that the molecules in this monolayer are held in position by linear chains of intermolecular hydrogen bonds like in the crystal and that this stabilization causes the difference in the interfacial behaviour of the n-alcohol/Graphon and n-alkane/Graphon systems.

A further ordering beyond a monolayer occurs at temperatures close to the melting point of the alcohol. The magnitude of this effect increases with the chain length of the hydrocarbon tail of the molecules. This effect is similar to the behaviour of the longer chain n-alkane/Graphon systems.