I evaluate the thermodynamic curvature $R$ for fourteen pure fluids along their liquid-vapor coexistence curves, from the critical point to the triple point, using thermodynamic input from the NIST Chemistry WebBook. In this broad overview, $R$ is evaluated in both the coexisting liquid and vapor phases. $R$ is an invariant whose magnitude $\left|R\right|$ is a measure of the size of mesoscopic organized structures in a fluid, and whose sign specifies whether intermolecular interactions are effectively attractive ($R<0$) or repulsive ($R>0$). I discuss five principles for $R$ in pure fluids: (1) Near the critical point, the attractive part of the interactions forms loose structures of size $\left|R\right|$ proportional to the correlation volume ${\xi }^3$, and the sign of $R$ is negative. (2) In the vapor phase, there are instances of compact clusters of size $\left|R\right|$ formed by the attractive part of the interactions and prevented from collapse by the repulsive part of the interactions, and the sign of $R$ is positive. (3) In the asymptotic critical point regime, the $R$'s in the coexisting liquid and vapor phases are equal to each other, i.e., commensurate. (4) Outside the asymptotic critical-point regime incommensurate $R$'s may be associated with metastability. (5) The compact liquid phase has $\left|R\right|$ on the order of the volume of a molecule, with the sign of $R$ being negative for a liquidlike state held together by attractive interactions and the sign of $R$ being positive for a solidlike state held up by repulsive interactions. These considerations amplify and extend the application of thermodynamic curvature in pure fluids.

• Received 30 June 2012

DOI:https://doi.org/10.1103/PhysRevE.86.021130