Density anomalies order parameter

A careful investigation of the picrate systems yields a substantial diameter anomaly [87] observed with all reasonable choices of the order parameter (see, however, somewhat contradicting results in Ref. 89). The data are consistent with the predicted (1 — a) anomaly. Large diameter anomalies are expected, when the intermolecular interactions depend on the density, as expected in these cases The dilute phase is essentially nonconducting and mainly composed of neutral ion pairs, while the concentrated phase is a highly conducting ionic melt [68]. However, any general conclusion is weakened by the fact that with Pitzer s system no such anomaly was observed [96]. 

The last anomaly we discuss here is structural anomaly. Initially, this anomaly was introduced via order parameters characterizing the local order in liquid. However, later on the local order was also related to excess entropy of the liquid that is defined as the difference between the entropy and the ideal gas entropy at the same (p, T) point Se% = S — 5id. In normal liquid, excess entropy is monotoni-cally decaying function of density along an isotherm, while in anomalous liquids it demonstrates increasing in some region. This allows to define the boundaries of stractural anomaly at given temperature as minimum and maximum of excess entropy. 

In this work we have simulated a system consisting of rigid dimers in which each monomer interacts with monomers from the other dimers through a core-softened shoulder potential. In order to check how the anisotropy induced by the dimeric structure affects the presence of density, diffusion and structural anomalies, we have obtained the pressure temperature phase diagram, the diffusion constant and the structural order parameter of the system for different A, distance between the bonded particles in each dimer, values. 

Phase transitions can be first-order or second-order (or continuous), and critical energy fluctuations quite often have a significant impact on thermal parameters. First-order transitions are characterized by discontinuous jumps in the first derivatives of the free energy, resulting in finite density p and enthalpy H differences between two distinct coexisting phases at the transition temperature Tp.. For a second-order transition there are no discontinuities in the density or the enthalpy but the specific heat capacity Cp will exhibit either a discontinuous-jump (for mean-field regime) or a critical anomaly... 

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