Mesoscopic equation of state of the heterophase fluid and its application to description of the liquid-gas asymmetry

In the vicinity of critical point neither the nominal liquid nor the nominal gas is a single, homogeneous phase. This is a consequence of the presence of heterophase fluctuations. Theoretical models accounting for such fluctuations are necessary to estimate their role and importance at the gas-liquid criticality. Two mesoscopic models of the heterophase fluid are formulated here to solve this problem. The first one is a mean-field mesoscopic lattice model of fluid in which the lattice cells are occupied by “bubbles” and “droplets” called liquid-like and gas-like fluctuons respectively. In this model an order parameter is introduced to characterize the two-state mesoscopic structure. Its relation to the Van der Waals order parameter is found, an equation of state is deduced, and the phase coexistence curve is determined. The Yang-Yang anomaly amplitude and the amplitudes of the leading singular terms of the diameter of the density-temperature coexistence curve (density diameter) are determined in terms of the mesoscopic parameters of asymmetry. In the second model, which generalizes the first one, the fluctuons have no fixed shape and volume. Criticality of the irregular array of fluctuons is then studied.