(30c) Isotropic-Polar Phase Transition in An Amphiphilic Fluid

We present Monte Carlo simulations of the isotropic-polar (IP) phase transition in an amphiphilic fluid carried out in the isothermal-isobaric ensemble. Our model consists of Lennard-Jones spheres where the attractive part of the potential is modified by an orientation-dependent function.  This function gives rise to an angle dependence of the intermolecular attractions corresponding to that characteristic of point dipoles. Our data show a substantial system-size dependence of the dipolar order parameter. We analyze the system-size dependence in terms of the order-parameter distribution and a cumulant involving its first and second moments. The order parameter, its distribution, and susceptibility observe the scaling behavior characteristic of the classical 3D-Heisenberg universality class. Because of this scaling behavior and because all cumulants have a common intersection irrespective of system size we conclude that the IP phase transition is continuous. Considering pressures 1.3≤ P≤3.0 we demonstrate that a line of continuous phase transition exists which is analogous to the Curie line in systems exhibiting a ferroelectric transition. Our results are can be explained semi-quantitatively by a simple mean-field theory adapted from the theory of IP phase transitions in fluids in which molecules carry an electromagnetic point dipole.