Ordered layers of aliphatic alcohols and carboxylic acids at the pure liquid/graphite interface

Abstract

The surface excess mass of pure hexadecan-1-ol and of three pure monocarboxylic acids (decanoic, tetradecanoic, hexadecanoic) at the liquid/Graphon interface has been measured as a function of temperature, using a dilatometric technique. The hexadecanol/Graphon system is characterized by an order/disorder type transition about 36 K above the melting point of the liquid. This feature has previously also been found for lower homologue alcohols and been interpreted by a “melting” of a monolayer. The carboxylic acids give a more diffuse transition at lower relative temperatures than the alcohols. When the freezing point is approached, hexadecanol exhibits a further steep increase of the surface excess, probably caused by the formation of at least two further ordered layers. The carboxylic acids do not show such an interfacial prefreezing effect. The dependence of these order effects on the chain length of the hydrocarbon tail of the molecules is discussed, and a comparison of the interfacial behaviour of n-paraffins, aliphatic alcohols and carboxylic acids at the liquid/Graphon interface is made, using the results of previous papers.