Phase behavior of the Stockmayer fluid via molecular dynamics simulation.

The gas-isotropic liquid-nematic liquid phase behavior of the Stockmayer fluid is studied using molecular dynamics simulation together with a mean field lattice model. We obtain coexistence curves of the Stockmayer fluid over a wide range of dipole strengths, temperatures, and densities, including the transition from the isotropic liquid to the ferroelectric liquid. In our simulations we do not observe the disappearance of the isotropic gas-isotropic liquid coexistence at high dipole strength contrary to earlier findings based on Monte Carlo techniques. Even though the formation of reversible dipole chains strongly affects the location of the critical point, it does not lead to its disappearance. These results are supported by a mean field lattice model which yields good qualitative, and in parts quantitative, agreement with our simulations. In addition, we also investigate the gas-isotropic liquid phase behavior for different polarizabilities.

Equilibrium polymerization and gas-liquid critical behavior in the Stockmayer fluid.

We develop a simple theory explaining the dependence of the gas-liquid critical point in the Stockmayer fluid on dipole strength. The theory is based on the Flory-Huggins lattice description for polymer systems in conjunction with a transfer matrix model for isolated chains of reversibly assembled dipolar particles. We find that the shift of the critical point as a function of dipole strength, which originally was found in computer simulation, strongly resembles the critical point shift as a function of chain length in ordinary linear polymer systems. In particular, the decrease of the critical density with increasing dipole strength is a consequence of the existence of reversible chains near criticality. In addition we report simulation results for gas-liquid critical points well above the limiting dipole strength found previously.