Dynamics of Structural Transformations between Lamellar and Inverse Bicontinuous Cubic Lyotropic Phases

The liquid crystalline lamellar ($L_{\alpha }$) to double-diamond inverse bicontinuous cubic ($Q_{\mathrm{II}}^D$) phase transition for the amphiphile monoelaidin in excess water exhibits a remarkable sequence of structural transformations for pressure or temperature jumps. Our data imply that the transition dynamics depends on a coupling between changes in molecular shape and the geometrical and topological constraints of domain size. We propose a qualitative model for this coupling based on theories of membrane fusion via stalks and existing knowledge of the structure and energetics of bicontinuous cubic phases.

Figure 1

In these images we show the structural transformations observed in our experiments by sketching the bilayer midplane. The figures are scaled relative to each other to reflect our experimental observations. A spherical onion vesicle consisting of concentric bilayers is shown including the swollen cubic phase present at the center. Thermal undulations in the lamellar phase (a) disappear as interlamellar attachments (ILAs) are formed (b). This occurs because the bilayer’s area and the volume it circumscribes are fixed in an onion vesicle and hence forming contacts exerts a lateral tension. When the tension reaches a critical value, ILAs rupture and form a disordered network of funnels (c) through which water flows from the center of the vesicle. The disordered sponge structure soon resolves into ordered bicontinuous cubic structures such as the swollen $P$ cubic (d).

Figure 2

A pressure jump from 1100 to 260 bar, $T=46.7°\mathrm{C}$ (a) and a temperature jump from 30 °C to 60 °C, $p=1\mathrm{\text{bar}}$ (b) are shown. Each 2D image is integrated to produce a 1D plot of intensity versus scattering vector, $s$, which is then stacked as a function of time. Regions a, b, c, and d correspond to the structural intermediates depicted in Fig. 1.

Figure 3

The change in lattice parameter with time is shown following the pressure jump shown in Fig. 2a and the temperature-jump shown in Fig. 2b. Two experiments having the same jump amplitude are plotted for each. In each case the inset is a close-up view of the behavior of the lamellar phase immediately following the jump.