Concentration-dependent restructuring of the Eigen-Zundel ensemble revealed by a solvation-state map of the hydrated proton

Proton transport in aqueous environments underlies processes ranging from biological energy conversion to electrochemical energy storage, but the microscopic mechanisms remain challenging to elucidate. Here, we establish a solvation-state map of aqueous acids that quantitatively links vibrational signatures to concentration-dependent populations of fully solvated ions, solvent-shared ion pairs (SSIPs), and contact ion pairs (CIPs) using vibrational spectroscopy, ab initio molecular dynamics (AIMD) simulations, and quantum mechanical calculations. By identifying the high-frequency O–H stretching band as a sensitive reporter of solvation topology, we show how ion pairing reshapes the free-energy landscape governing proton sharing and localization. Cluster calculations reveal that fully solvated protons adopt a broad conformational ensemble, SSIPs are stabilized in a deep asymmetric potential well, and CIPs collapse into a single-well, Eigen-like state. This framework provides a basis for interpreting proton transport barriers in concentrated electrolytes and aqueous interfaces, offering insight into proton-conducting materials and ion-regulated environments.