Density functional theory study of mesophase formation in model lipid systems

The emergence of mesophases in phospholipids is a fascinating phenomenon, which finds application in a wealth of research fields, from designing of building blocks to drug delivery. Here, we investigate structural properties and phase behavior of a coarse-grained model for phospholipid systems, represented by a dipolar Gay-Berne potential, within the framework of Percus-Yevick (PY) integral equation theory and classical density functional theory (DFT), with the aim of shedding light on the appearance of mesophases. Despite the well-known efficiency of PY theory in predicting structure, thermodynamics, and phase behavior of fluids at relatively low computational cost, we are not aware of any previous applications to phospholipid systems modeled through this potential. We focus on the role played by density, temperature, and dipolar head group strengths on the pair correlation functions of the phospholipid molecules, assessing our theoretical predictions against molecular dynamics simulations. Then, we utilize such correlation functions as structural input for DFT in order to locate the liquid crystalline phase transitions, with a particular emphasis on isotropic-nematic and nematic-smectic A transitions. The results highlight that enhanced dipolar interactions are able to stabilize ordered phases at low densities and high temperatures. This is indicated by the increase in orientational and translational order parameters and by the heightened compressibility in the smectic phase. Three different phase diagrams, corresponding to the different head group strengths investigated, are also drawn. A qualitative agreement with simulations is found, even though the densities at which transitions take place are typically underestimated.