Phase transition at the surface of mixed micelles of the ganglioside GM1 and dodecylphosphocholine

A temperature-dependent irreversible variation of the average aggregation number of GM1 micelles, with no change in the chemical structure of the molecule, has been observed by light and x-ray scattering. GM1 is an amphiphilic molecule of biological origin, similar to phospholipids but with an extended headgroup, made up of many sugar units. A simple model has been developed to describe the experimental results. It assumes that the polar headgroups of GM1 monomers may exist in two different stable conformations, each of them with a very similar energy, dependent on its own internal structure and displaying preferential interactions with the surrounding heads once inserted in the micelle. The interconversion between the conformational minima is then described as a cooperative transition occurring at the micelle surface, overcoming a naturally emerging barrier due to collective effects. To assess the extent of interactions among the GM1 conformer headgroups different amounts of an amphiphilic spacer have been inserted in GM1 micelles. The thermal hysteresis phenomenon was still observed on mixed micelles until a molar ratio GM1/spacer similar to 1/3 was reached, corresponding to the critical concentration calculated from the mean-field theory of dilute magnetic alloys in a 2D lattice; The observed behaviour, then, appears as a critical phenomenon of topological nature happening in a confined two-dimensional system, that is the micelle surface.