A continuous topological change during phase transitions in amphiphile water systems

Amphiphiles are molecules such as surfactants or lipids that have a polar h ead group (hydrophilic) attached to nonpolar hydrophobic alkyl chains, Beca use of this characteristic they self-assemble in,vater and give rise to a w ide range of phases with different structures and properties. Aqueous dispe rsions of amphiphiles are present in every aspect of day-to-day life-e.g., forming biological cell membranes, stabilizing emulsified food, or being us ed as soap. Time-resolved x-ray diffraction has been used to study the hexa decylhexa(oxyethylene glycol) ether (C16EO6)/water system, which shows an i ntermediate phase whose structure depends on the thermal path between lamel lar and hexagonal structures. Heating the hexagonal phase from roam tempera ture leads to a lamellar phase via an Ia3d cubic structure. Cooling from th e lamellar phase initially leads epitaxially to an intermediate <R(3)over b ar>m before the hexagonal phase is reached. Both cubic and <R(3)over bar>m phases are formed by very similar rod units, but the overall structures dif fer because of their spatial distribution and they both bridge morphologica lly the hexagonal and lamellar phases. The Ia3d does so on heating, whereas the <R(3)over bar>m does on cooling. The structural path during the phase transitions is determined by topological similarities between the forming p hase and the one from which it originates. Although the estimated curvature energies for these two phases are similar, on cooling, kinetics and topolo gy are initial factors determining the path for the phase transitions, wher eas on heating energy is the dominant factor.