"Hot aqueous fluids are key agents in geological transport processes in the Earth’s crust, redistributing geothermal heat as well as dissolved components. Thereby, they become alternative energy resources, create valuable mineral deposit and fundamentally control the composition of the oceans. To quantitatively understand these geochemical processes requires an accurate method to evaluate the thermodynamic properties of the fluids including their dissolved components (solutes), over wide ranges of temperature, pressure and fluid density from liquid- to vapor-like. Solute thermodynamics is commonly modeled using the Helgeson-Kirkham-Flowers (HKF) equation of state, but this empirical model becomes unreliable in the vicinity of the critical point of water. This limitation restricts modeling hydrothermal systems, where the importance of near-critical and low-density fluids in heat and mass transfer is increasingly recognized.The goal of this study is to pioneer a new equation of state describing the standard state properties of aqueous solutes over wide ranges of pressure and temperature, valid for low- to high-density fluids including near-critical conditions. This study will use recent advances in the theory of solute thermodynamics in compressible fluids to derive the mathematical form of the equation of state, and combine these results with recently emerging experimental data for low-density fluids at high temperature. Molecular simulations provide atomic-scale insights into solute-solvent interactions and will be used to complement the experimental constraints. The new model will improve our ability to understand high temperature fluid-rock interactions and thus promote multi-disciplinary research at the nexus of hydrothermal geochemistry, economic geology, geothermal energy and chemical engineering."