Anomalous Phase Sequences in Lyotropic Liquid Crystals

Abstract

We present a coarse-grained model in order to describe the unusual sequence of mesophases observed in aqueous solutions of nonionic lipids, such as monoolein. The lipid molecules are modeled as a rigid head and a flexible Gaussian tail, and water is treated explicitly. A key component of the model is thermally reversible hydrogen bonding between the lipid head and water resulting in changes in both head volume and the interactions of the hydrated head with its surroundings. Phase diagrams obtained from unit-cell self-consistent field simulations capture the qualitative thermotropic and lyotropic phase behavior of the monoolein-water system. The unusual phase sequences result from a competition between hydrogen bond formation, changes in head volume and interactions, lipid tail entropy, and the hydrophobic effect. [3,4,7]. While cubic phases are desirable, there is very limited understanding of the relationship between the molecular details of the lipid, its interactions with water, and the presence and placement of cubic phases within the temperature-composition plane. The problem is particularly acute for aqueous solutions of nonionic lipids with a small hydrophilic head, such as the monooleinwater system [8,9]. This class of lipid-water mixtures possesses both a normal phase sequence from inverted hexagonal cylinders to inverted cubic gyroid (H II -Ia 3d II ) and a reverse phase sequence from lamellar to inverted gyroid (L -Ia 3d II ) as the water volume fraction is increased [see Fig. 2(a)]. There is currently no satisfactory theoretical model that explains both the normal and reverse lyotropic phase sequences as well as the changing topology of the phase diagram with temperature. It is this challenge that is addressed in the present Letter.The concept of a critical packing parameter (CCP) [10], defined as the ratio of the volume of a lipid tail to the product of the cross-sectional area of a lipid head and the length of a lipid molecule, is a useful tool for anticipating membrane curvature and the stability of various lipid and surfactant phases in solution. Nonetheless, the CCP approach predicts only the normal sequence of phases, reflecting a transition in which the water-lipid interfaces are curved less towards the water domains as the concentration of water is increased. The framework does not provide insight into a reverse phase transition such as L ! Ia 3d II , where the interfaces go from flat to curved towards water upon increasing the amount of water.A membrane elasticity model for lipid bilayers initiated by Canham and Helfrich [11,12] invokes phenomenological parameters such as spontaneous curvature, bending rigidity and saddle splay modulus. The model does not elucidate the dependence of these parameters on molecular details of the lipid and solvent, but it can be used to anticipate certain trends in phase behavior [13]. At the other extreme, fully atomistic simulations have been attempted for the monoolein-water system using a classical force field and explicit water [14]. While some promising ...

Ordered micellar and inverse micellar lyotropic phases

“…In other words, the fluid lamellar phase is in a state of curvature frustration, which can be geometrically relaxed by adding additional water. Fredrickson and co-workers have developed a self-consistent field theory (SCFT) approach to calculating the temperature-composition phase diagram of weakly amphiphilic, hydrogenbonding lipids such as the monoacylglycerol monoolein, and have thereby gained an understanding of the mechanisms underlying such anomalous phase sequences [11,12]. In this course-grained SCFT approach, the monoolein headgroup can either have water bound to it (by hydrogen-bonding) or be unbound, depending on the water content and the temperature.…”

Self-consistent field theory for lipid-based liquid crystals: Hydrogen bonding effect

“…31 Here, we expand on the details of the model and the computational methods employed and provide a broader set of simulation results. We also compare our theoretical predictions for lattice constants of the mesophases with the literature values obtained from small angle x-ray scattering experiments.…”

Organic Solvent-Based Graphene Oxide Liquid Crystals: A Facile Route toward the Next Generation of Self-Assembled Layer-by-Layer Multifunctional 3D Architectures

“…35 High Gordon parameter represents firstly a higher chance to achieve selfassembled liquid crystalline phases and secondly a higher thermal stability. 36 However, the ultra large size of GO sheets used in this study may push the boundaries toward organic solvents with Gordon parameters which are normally considered to be too low to be able to support solvophobic self-assembly. So far, the lowest reported Gordon parameter that supports amphiphilic self-assembly was G = 0.576 J m -3 for a protic ionic liquid (EAB), 34 which is far beyond the limit predicted by Evans (G ≥1-1.2 J m -3 ).…”