Selective adsorption at graphite/hydrocarbon interfaces

Abstract

A study was made of adsorption of n-dotriacontane and n-butanol dissolved in n-heptane onto graphitized carbon black and various ground graphites having surface areas ranging from 5 to 700 m2 g-1. It was established that the adsorption of n-dotriacontane by the graphites is confined entirely to the basal planes of graphite crystals. This is attributed to a remarkable fit between the hydrogen atoms attached to one side of the zig-zag carbon chain in the normal paraffins and the centres of hexagons formed by the carbon atoms in the basal planes of the substrate. The longer the chain the more contacts it can form with the graphite surface and the more strongly it is adsorbed. More detailed studies of the adsorption of n-paraffins on ground graphites have shown that they form close-packed monolayers of horizontally disposed molecules on the basal planes. There is little further adsorption after the monolayers are complete. The heat of adsorption per molecule increases uniformly with the chain length reaching very high values for the normal paraffins having more than 30 carbon atoms. The formation of the close-packed layers has been used for the measurement of the proportion of basal plane surface in different types of graphites. n-Butyl alcohol also forms closely packed monolayers, but on the polar sites of graphites, which can be used for the estimation of their area. The basal plane and polar sites act independently in adsorption and their relative proportions characterize the adsorptive properties of graphites. Examination of graphite ground in n-heptane, which consists of plates of average area of several square micrometres and average thickness of 5 nm, shows that its surface consists predominantly of basal planes having a high adsorptive capacity for n-paraffins.