Mixing behaviour of carboxylic acids adsorbed on graphite

Abstract

Solid monolayer formation by all the linear carboxylic acids from C₆ to C₂₀ adsorbed from their liquids to a graphite surface is demonstrated. In addition, we present the two-dimensional phase behaviour of linear monocarboxylic acid mixtures adsorbed on graphite from their liquid mixtures, determined using differential scanning calorimetry. All acid mixtures with alkyl chains that differ by two or three methylene groups (Δn = 2 or 3) exhibit a significant degree of phase separation in the monolayer. Generally, the mixing tendency increases with increasing alkyl chain length for a given Δn, and with more similar chain lengths (e.g. Δn = 1). We report neutron diffraction data that confirms the formation of solid acid monolayers. This structural data also allows us to compare the mixing results with a recent quantitative model for 2-D mixing. The general form of the observed behaviour agrees well with the model, although the characteristic parameters that separate complete mixing, partial mixing and phase separation are different from those found with alkane and alcohol monolayer mixtures.