chapter  7
32 Pages

7

I. Introduction ................................................................ 348 II. Dynamics of Surfactant Lyotropic Phases and of

Transitions between Phases...................................... 351 A. Dynamics of the L3 Phase................................... 351 B. Dynamics of Phase Transitions.......................... 354

1. Transition from the L a

Phase to the Bicontinuous Cubic Phase (V1) and V1 Æ La Transition................................. 354

2. Other Phase Transitions............................... 355 III. Dynamics of Phase Transitions in Lipid

Lyotropic Phases ........................................................ 356 A. Dynamics of Lamellar-to-Lamellar

Phase Transitions................................................ 356

B. Dynamics of the Lamellar-to-Inverted Hexagonal (H2) Phase Transition....................... 364

C. Dynamics of Transitions Involving the Inverse Cubic Phases Q2..................................... 367

IV. Kinetics of Shear-Induced Phase Transitions.......... 370 V. Conclusions ................................................................. 372 References ........................................................................... 374

I. INTRODUCTION

In the presence of water, surfactants and lipids give rise to a variety of phases referred to as lyotropic phases or mesophases.1,2 The most important of these phases are the lamellar, hexagonal, cubic micellar, and cubic bicontinuous phases denoted by L, H and V, and Q, respectively (see Figure 1.11 in Chapter 1). The subscripts 1 or 2 attached to these phase symbols indicate that the phase is direct (water continuous) or inverse (discontinuous water domains). Many other lyotropic phases have been identified that differ from the main ones by the state of the alkyl chain (crystalline or disordered) and of the head group arrangement (ordered or disordered).1 In the particular case of the lamellar phase, additional variations come from the possible different orientations adopted by the alkyl chains with respect to the plane of the lamellae (angle of tilt of the chain) and also from the state of the surface of the lamellae that can be planar or rippled.1 Numerous detailed descriptions have been given for the equilibrium state of the various phases that surfactants and lipids can form in the presence of water.1-3

However, relatively few studies have addressed the dynamics of phase transitions of lyotropic phases of surfactants and lipids. The superb book The Aqueous Phase Behavior of Surfactants, by R.G. Laughlin,3 has one chapter dealing with this topic. This chapter is 8 pages long and contains only 25 references, not all dealing with the dynamics of phase transitions, in a book of 558 pages. Likewise, the recent review on the kinetics of phase transitions in surfactant solutions by Egelhaaf4 includes a rather short

paragraph on the kinetics of phase transitions between lyotropic phases. In addition, a literature survey shows that the majority of the reported studies on the dynamics of lyotropic phase transitions concern lipids and not surfactants. Many of these studies deal with the transformation of lipid lamellar phases into other phases and the reverse processes. The main reason for this situation is that the bilayers making up a lamellar phase are a good model for the cell membrane. Therefore, many studies of the rate of lipid phase transitions were undertaken in order to better understand the behavior of biological cells during replication as well as cell membrane lysis and reconstitution. As Laggner et al.5 pointed out, studies of dynamics of phase transitions may provide “methods for prolonging the lifetimes of eventual structural intermediates in the transitions. A benefit from such an achievement could be the better structural description of the intermediates…. Also they are likely to be biomedical benefits from such results since the development of agents modulating the dynamics of membrane transformation, such as fusion, is likely to play an important role in many medical applications, e.g., liposomebased gene therapy, fertility modulation, or percutaneous drug applications.” Most of the reported studies aimed at bringing information on the time scale of the transitions and on the nature of eventual intermediate phases between the known initial and final phases, and presenting mechanisms that account for the observations.