Criticality of dipolar fluids: Liquid-vapor condensation versus phase separation in systems of living polymers

We consider the strongly dipolar fluid as an equilibrium mixture of self-assembled chains as suggested by the results of recent simulations. The free energy of the system is given by the sum of the free energy of an ideal chain mixture and additional terms arising from the interactions. We discuss the inclusion of such terms, namely, dipole-dipole interactions between monomers, hard core interactions between monomers and between chains, and dispersive interactions between monomers and between chains. We calculate the phase diagrams for several ratios of dispersive to dipolar interactions $\left(\lambda \right)$ and the corresponding critical points. In agreement with the simulation results we have found ordinary liquid-vapor coexistence for 0.34<λ⩽1. When λ is decreased still further, the theory predicts that coexistence obtains for a fluid of chains. The critical density decreases and the mean chain length at the critical point increases exponentially as λ→0. The reasons why this coexistence between chained fluids was not observed in the simulations are discussed.