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

A temperature-dependent irreversible variation of the average aggregation n umber of GM1 micelles, with no change in the chemical structure of the mole cule, has been observed by light and x-ray scattering. GM1 is an amphiphili c molecule of biological origin, similar to phospholipids but with an exten ded headgroup, made up of many sugar units. A simple model has been develop ed to describe the experimental results. It assumes that the polar headgrou ps of GM1 monomers may exist in two different stable conformations, each of them with a very similar energy, dependent on its own internal structure a nd displaying preferential interactions with the surrounding heads once ins erted in the micelle. The interconversion between the conformational minima is then described as a cooperative transition occurring at the micelle sur face, overcoming a naturally emerging barrier due to collective effects. To assess the extent of interactions among the GM1 conformer headgroups diffe rent amounts of an amphiphilic spacer have been inserted in GM1 micelles. T he 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 magn etic alloys in a 2D lattice; The observed behaviour, then, appears as a cri tical phenomenon of topological nature happening in a confined two-dimensio nal system, that is the micelle surface.