Mathematical Analysis of the van der Waals Equation

Emil M. Prodanov

Published 2022-01-07, updated 2022-06-11Version 2

The parametric cubic van der Waals polynomial $\,\, p V^3 - (R T + b p) V^2 + a V - a b \,\,$ is analysed mathematically and some new generic features (theoretically, for any substance) are revealed: the temperature range for applicability of the van der Waals equation, $T > a/(4Rb)$, and the isolation intervals, at any given temperature between $a/(4Rb)$ and the critical temperature $8a/(27Rb)$, of the three volumes on the isobar-isotherm: $\,\, 3b/2 < V_A \le 3b$, $ \,\, 2b < V_B < 4b/(3 - \sqrt{5})$, and $\,\, 3b < V_C < b + RT/p$. The unstable states of the van der Waals model have also been generically localized: they lie in an interval within the isolation interval of $V_B$. In the case of unique intersection point of an isotherm with an isobar, the isolation interval of this unique volume is also determined. A discussion on finding the volumes $V_{A, B, C}$, on the premise of Maxwell's hypothesis, is also presented.