[NO₃]⁻. At low IL concentrations (≤1.5 mol L⁻¹), [BMIM]⁺-based ILs exhibit stronger preferential solvation compared to [EMIM]⁺. However, at higher concentrations (≥2.5 mol L⁻¹), the protein surface sites that differentiate the cations become saturated. The relative preferential solvation is, then, determined by the properties of the bulk solution, in particular by the lower water molarity of solutions with [BMIM] + . These findings provide insights into the interplay between IL composition and protein hydration, with implications for understanding protein stability and design in IL-rich environments.