Water activity for n-component mixtures from molecular dynamics simulations via Kirkwood-Buff theory

Kirkwood-Buff (KB) theory connects measurable thermodynamic properties with radial distribution functions and particle-number fluctuations accessible from molecular dynamics (MD) simulations. Although KB theory has been used to determine molar volumes, isothermal compressibilities, and solute or solvent activities from MD data, activity calculations have largely been focused on binary mixtures. In this work, we extend the application of KB theory to the calculation of water activity in multicomponent aqueous solutions with an arbitrary number of independent thermodynamic components. We adapt a generalized formulation to evaluate the required chemical-potential derivatives numerically, avoiding the explicit derivation of increasingly complex analytical expressions. The resulting automated workflow requires only MD trajectories and molecular topologies as input. We evaluate the approach using water-ethanol mixtures, aqueous NaCl solutions, mixed NaCl-LiCl solutions, and sodium decanoate solutions at low and high concentrations, a surfactant of relevance to atmospheric aerosol formation and climate modelling. The calculations reproduce the main experimental trends and illustrate the effects of force-field selection, KBI convergence, and solution microheterogeneity. Rigorous treatment of electrolyte solutions at exact electroneutrality requires an additional neutral-component formulation. Overall, the framework provides an automated route for estimating water activity in complex multicomponent solutions and identifies the principal limitations requiring further methodological development.