Anomalous Phase Sequences in Lyotropic Liquid Crystals

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.

Figure 1

(a) Chemical structure of a monoolein molecule. (b) Schematic diagram of a lipid-water model.

Figure 2

(a) Experimental phase diagram of the monoolein-water system reprinted from Ref. 9 with permission from Elsevier. (b) Mean-field phase diagram with $v_{h0}/v_w=1.8$, $v_t/v_w=1.0$, $N=5$, and $C/B=0.5$. The reference point is $-F_b=4.5$, ${\chi }_{h0,t}=1.0$, ${\chi }_{h1,t}=1.45$, ${\chi }_{h0,w}=0.1$, ${\chi }_{h1,w}=0.5$ and ${\chi }_{t,w}=4.2$. The ratio, ${\chi }_{i,j}/{\chi }_{t,w}$, is maintained constant over all values of $-F_b$. Labeled phases are $FI$ (lipid-rich homogenous phase), $H_{II}$ (inverted hexagonal phase), $L_{\alpha }$ (lamellar phase), $Ia\overline{3}{d}_{II}$ (inverted gyroid phase), and 2 phase (region of coexistence with nearly pure water).

Figure 3

(a) Species volume fractions versus $-1/F_b$ at fixed ${\varphi }_{w,\mathrm{tot}}=0.175$. (b) Volume fractions versus ${\varphi }_{w,\mathrm{tot}}$ at $-F_b=4.2$.