Domain size polydispersity effects on the structural and dynamical properties in lipid monolayers with phase coexistence

Simulations and integral-equation theories for dipolar density interacting disks

Integral equation theories (IETs) based on the Ornstein-Zernike (OZ) relation can be used as an analytical tool to predict structural and thermodynamic properties and phase behavior of fluids with low numerical cost. However, there are no studies of the IETs for the dipolar density interaction potential in two-dimensional systems, a relevant interdomain interaction in lipid monolayers with phase coexistence. This repulsive interaction arises due to the excess dipole density of the domains, which are aligned perpendicular to the interface. This work studies the performance of three closures of the OZ equation for this novel system: Rogers-Young (RY), modified hypernetted chain (MHNC), and variational modified hypernetted chain (VMHNC). For the last two closures the bridge function of a reference system is required, with the hard disk being the most convenient reference system. Given that in two dimensions there is no analytical expressions for the hard disk correlation functions, two different approximations are proposed: one based on the Percus-Yevick (PY) approximation, and the other based on an extension of the hard spheres Verlet-Weis-Henderson-Grundke (LB) parametrization. The accuracy of the five approaches is evaluated by comparison of the pair correlation function and the structure factor with Monte Carlo simulation data. The results show that RY closure is satisfactory only for low-structured regimes. MHNC and VMHNC closures perform globally well, and there are no significant differences between them. However, the reference system in some cases affects their performance; when the pair correlation function serves as the measure, the LB-based closures quantitatively outperform the PY ones. From the point of view of its applicability, LB-based closures do not have a solution for all studied interaction strength parameters, and, in general, PY-based closures are numerically preferable.

Size-Polydisperse Model Ionic Liquid in Bulk

The static and the dynamic properties of a size-polydisperse model ionic liquid is studied using molecular dynamics simulations. Here, the size of the anions is derived from a Gaussian distribution while keeping cation size fixed, resulting in a system that closely corresponds to IL mixtures with a common cation. We systematically explore the behavior of thermodynamic transition temperatures, spatial ordering of ions, and the resulting screening behavior as a function of polydispersity index, δ. We observe a nonmonotonic dependence of transition temperatures on δ, and this nonmonotonic behavior is also reflected in other properties such as screening length. Furthermore, from the radial distribution function analysis it is found that, upon varying δ, the spatial ordering of cations is affected, while no such changes is seen for anion. On the other hand, the analysis of ion motion through mean-square displacement show that for all δ values considered both inertial and diffusive regimes are observed (as expected in the liquid state). However, in contrast to the neutral counterpart, the overall relaxation time of the polydisperse IL system increases (and hence decreasing diffusion coefficient) with increasing δ.