Structural definition of water activity reveals near-ideal thermodynamic behavior in electrolyte solutions

Chemical activity is central to thermodynamics and, through the chemical potential, governs phase equilibria and transport. While activity appears formally in thermodynamics, it is typically accessed empirically through the activity coefficient or indirectly via free-energy differences. Here, we show that electrolyte solutions traditionally classified as strongly non-ideal exhibit near-ideal behavior when activity is expressed in terms of the appropriate microscopic species. Using molecular dynamics simulations, we define water activity as the quotient of unbound solvent molecules (e.g., free waters) to the sum of unbound solvent molecules and solvent-solute clusters, both identified directly from solvation structure. Applied to NaCl solutions, this framework quantitatively reproduces experimental activities over the entire solvation range without free-energy calculations or fitting parameters, which is the first time to our knowledge. These results establish a structural foundation for chemical activity and suggest that apparent non-ideality arises from an incomplete molecular description of mole fraction rather than a breakdown of ideal thermodynamics.