Hydrophilic-Hydrophobic Double Layers around Amphiphilic Solutes in Mixed Solvents

Mixed solvents are commonly used in chemistry and technology to enhance various properties and are the default liquids in both biology and the environment. Preferential solvationwhereby some solvent components are in excess in the solute solvation shellmay mechanistically explain why observables such as solubility or reactivity depend nonlinearly on bulk solvent composition, but the local composition and structure of solvation shells have remained elusive for mixed solvents. Using IR-solvation shell spectroscopy and molecular simulations of tert-butyl alcohol in water/1-propanol mixtures, we reveal that solvation shells organize in layers with alternating hydrophilic-hydrophobic character and have different dimensions depending on bulk solvent composition. Solvation shells qualitatively reorganize as a function of solvent composition, with 1-propanol inverting its preferential orientation. Our results are relevant for understanding how alcoholscommonly used cosolventsimpact solubility, reactivity, and viscosity. More generally, these findings challenge simplistic models that consider only the first coordinating layer of the solute or only averaged excess solvation numbers around it to explain nonlinearities in solution properties and highlight the complex behavior of solvation shells in 3-component systems.