Kinetics of lyotropic phase transitions involving the inverse bicontinuouscubic phases

Time-resolved-X-ray diffraction was employed to monitor transitions, induce d by pressure jumps, between inverse lyotropic liquid crystalline phases of the mixed lipid system lauric acid/dilauroylphosphatidylcholine (2:1 molar ratio) in water. We studied transitions between the gyroid, Q(II)(G), and the double diamond, Q(II)(D), inverse bicontinuous cubic phases and between Q(II)(D) and the inverse hexagonal, H-II, phase at a fixed water compositi on of 50 wt %. The transition Q(II)(G) --> Q(II)(D) occurred within the 0.5 s time resolution of our X-ray measurements; This transition occurs withou t any change in water content within the mesophase but with a small change in the total topology. The transition Q(II)(D)-->(Q(II)(D)+H-II+excess wate r) was observed at 59.3, 62.8, and 66.9 degrees C, The process appeared to follow first-order kinetics, with rate constants of 0.11, 0.17, and 0.26 S- 1, respectively. The reverse transition occurred much more quickly, being c ompleted in <1 s. In both cases, intermediate structures were observed, inc luding the appearance of the inverse bicontinuous cubic phase based on Schw arz's P minimal surface, Q(II)(P). We found that the latter intermediate ha d, to first order, the same curvature elastic energy as the Q(II)(D) phase but a greater water content. Building on earlier work (Erbes, J.; Winter, R .; Rapp, G, Ber. Bunsen-Ges. Phys. Chem. Chem. Phys. 1996, 100, 1723-1722), we consider the rate to depend on the difference between the final pressur e and the pressure at the phase transition boundary and suggest that this c an explain both the temperature dependence of the transition Q(II)(D)-->(Q( II)(D)+H-II+excess water) and the higher rate of the reverse process.