(705f) Molecular Modeling of Ordering Transitions in Liquid Crystal Thin Films in Contact with Mixtures of Lipid Monolayers

The control of the orientation of liquid crystal thin films at surfactant-decorated interfaces is of primary importance in the development of liquid crystalline devices as well as many applications in biological systems. Highly detailed molecular level modeling of these interfaces is needed to help us to create liquid crystal-based sensors that respond to specific chemical and biological signals. These liquid crystal/surfactant systems have been studied extensively due, in part, to both their birefringence optical properties and orientational sensitivity to surface interactions. It is therefore critical to understand the interplay between the conformational entropy of the surfactants, the rotational entropy of the liquid crystals, the intermolecular and molecule-surface interactions, and the packing at the interfaces to be able to create design platforms for sensing applications using these systems.

In this work, we present theoretical predictions for the phase behavior of liquid crystal thin films in the presence of various lipid mixtures. These results shed light on the interplay between the conformational entropy of the lipids, the penetration of the liquid crystal into the lipid region, the propagation of the interfacial orientation to the bulk phase behavior of the liquid crystal film, and the effects of liquid crystal orientation on the phase transitions of the lipids themselves. 

In the absence of liquid crystal molecules, binary DPPC/DOPC lipid monolayers undergo phase transitions from liquid-expanded to liquid-condensed phases as the lipid areal density increases.  DPPC has two fully saturated fatty acid tails, and DOPC has two monounsaturated fatty acid tails. The area per molecule of this phase transition is highly dependent on the temperature and the composition of the monolayer.  In this work, we present the effect of liquid crystal nematic elasticity on liquid-expanded/liquid-condensed phase diagram.

Finally, we will present the effects of tethering polyethylene glycol (PEG) with various molecular weights to the saturated lipid monolayer. Tethering PEG to the headgroups of the lipids greatly affects the molecular organization of the liquid crystal thin film. By changing the molecular weight of PEG on these lipid surfaces, we can create a mushroom-pancake transition that could be utilized to control lateral phase separation of PEGolated lipid layers.