Complex Network and QTAIM Analyses Reveal the Dominance of Structural over Electronic Modulation in Supercritical Water under Ion-Free and Ionic Conditions.

Supercritical fluids play a fundamental role in a wide range of scientific, industrial, environmental, and engineering processes due to the presence of inhomogeneities in their microscopic structure, which characterize a continuous transition between two structurally distinct regimes: a low-density gas-like state and a high-density liquid-like state. However, the relationship between thermodynamic conditions and the microstructural organization of water under supercritical conditions is still under investigation. In this work, we employ criteria based on energetic principles to characterize intermolecular interactions combined with network topological properties to describe the microstructural organization of water across variations in temperature and pressure in the supercritical regime. Analysis based on the quantum theory of atoms in molecules (QTAIM) indicates that intermolecular interactions between water molecules are predominantly noncovalent in nature, although partially covalent contributions may emerge within more extensively connected components under specific conditions. Statistical analyses further demonstrate that thermodynamically induced variations manifest primarily at a collective and topological level of the molecular network without significant changes in the local electronic nature of intermolecular interactions.