The interpretation of mixing behaviour in carboxylic acid monolayers adsorbed on graphite using a regular solution description

Mixing behaviour in monolayers of carboxylic acids physisorbed onto graphite is analysed using a regular solution model. Generally, experiments indicate that binary mixtures of acids with shorter alkyl chain lengths show extensive phase separation, while mixtures of longer and more similar acids mix non-ideally. Here the similarity of crystal structures is expressed quantitatively in terms of an isomorphism model and the degree of mixing by a regular solution interaction parameter. It is found that acids with more similar monolayer structures have a lower excess enthalpy of mixing. In addition, the extent of preferential adsorption is also investigated and it is found that the longer homologue is generally preferentially adsorbed but to a much lower extent than the corresponding alkane mixtures.