Critical fluctuations and the equation of state of Van der Waals

It is well known that classical equations of state, like the Van der Waals equation, fail in the region near the critical point of a fluid, where the behavior of the thermodynamic properties is strongly affected by density fluctuations. In this paper, we present a theoretical approach to correct classical cubic equations of state for the effects of critical fluctuations. The approach utilizes a transformation deduced from the renormalization-group theory of critical phenomena that was developed earlier for a classical Landau expansion of the Helmholtz-free-energy density. Using the Van der Waals equation as an example, we explain how critical fluctuations lower the critical temperature, flatten the coexistence curve, induce a singularity in the isochoric heat capacity, and lead to apparent critical exponents that depend on the temperature distance from the critical point.