Molecular self-assembly in a model amphiphile system

The physical origin of the large and negative excess entropy of mixing of alcohols and water remains controversial. In contrast to standard explanations that evoke concepts of water structuring, recent work has shown that, at ambient conditions, it can be quantitatively explained in terms of molecular scale partial demixing of the two components. Here, we estimate the negative excess entropy (Delta S-E) of aqueous methanol at low temperature and high pressure using experimentally-derived structural data and a recently introduced cluster model. On cooling to 190 K the cluster sizes increase, but the change in Delta S-E, which according to this method of calculation depends on the surface area to volume ratio of the clusters, is not significant, suggesting that the topology of the clusters must change with decreased temperature. On compression the cluster sizes also increase, and Delta S-E is now positive, suggesting an even more pronounced change in cluster topology with increased pressure. This work suggests that it is the amphiphilic nature of a molecule that determines aggregation and self-assembly processes in aqueous solution. The results therefore give useful insight into the processes of cold and pressure denaturation of proteins.