<p>Twelve alkyl sulfonic acid functionalized Brønsted Ionic Liquids (BAILs) and their aqueous mixtures (BAIL:Water) were synthesized and evaluated as solvents and acid catalysts for the hydrolysis of microcrystalline cellulose (MCC) to produce cellulose nanocrystals (CNCs). The functionalized BAILs synthesized include various anions (Cl⁻, HSO₄⁻, H₂PO₄⁻), two different cations (imidazolium and pyridinium), and cationic alkyl chains of different lengths (C3, C4). This work examined how structural features influence physicochemical properties, such as acidity and polarity, and in turn, CNC yield and nanomaterial characteristics. Moreover, the presence of the sulfonic acid group (–SO₃H) attached to the cation is emphasized as a key factor in enhancing the acid-catalyzed conversion of cellulose. A Taguchi Design of Experiments (DOE) identified optimal experimental conditions (60&#xa0;°C, 2&#xa0;h, 90:10 BAIL:Water Ratio (BWR), 10 wt% MCC loading). Under these conditions, BAILs were systematically assessed, showing that structural variations and water content modulate solvent polarity and acidity, thereby affecting hydrolysis efficiency. The CNCs produced were characterized by FTIR spectroscopy, thermogravimetric analysis (TGA), X-Ray diffraction (XRD) and Transmission electron microscopy (TEM). The results indicate that changes in solvent structure and physicochemical parameters impact not only CNC yield but also the final properties of the nanomaterial. These insights provide a solid foundation for the rational design of more efficient solvents and catalysts for CNC production.</p> Graphical abstract <p></p>