PHYSICAL ASPECTS OF NONIDEAL MIXING OF AMPHIPHILIC MOLECULES IN SOLUTION - THE INTERESTING CASE OF GANGLIOSIDES

We review the interesting physical effects arising when bifunctional m olecules, consisting of two moieties, one polar and the other apolar, are dissolved in a simple solvent. The peculiarity of such amphiphilic molecules is that they generate by themselves confined structures in which they are embedded and of which they dictate the topology accordi ng to their aggregative properties. Individual molecules are still fre e to redistribute within or across the aggregates. When two or more ty pes of amphiphilic molecule are mixed together, the strong coupling be tween local interactions and the aggregate topology gives experimental access to a variety of non-ideal mixing effects. The couplings betwee n the lateral distribution and the geometrical features of amphiphile assemblies are discussed separately for monolayers, bilayers, multilay ers, and micelles. Concentration and temperature effects are taken int o account. The case of non-ideal mixing of chemically identical molecu les with mutually interchanging conformations is also discussed. It is found that bistability and thermal hysteresis phenomena can be import ant. Theoretical arguments are supported by a variety of experimental observations made of a class of amphiphilic molecules, the ganglioside s, which are particularly suitable subjects for investigation as regar ds such non-ideality effects. Gangliosides, with a double-tail hydroph obic part, like phospholipids, and a bulky saccharidic headgroup, show relevant like-like and like-unlike interactions, and easily form larg e aggregates whose topology is quite sensitive to monomer redistributi on.