Thermal Phase Behavior, Structure, Acidity, and Quantum Chemical Insights into Novel Hydroxylamine Hydrochloride (HACl)-Methanesulfonic Acid (MSA) Deep Eutectic Solvents

Abstract The present study undertakes the first characterization of hydroxylamine hydrochloride (HACl)-methanesulfonic acid (MSA) deep eutectic solvents (DESs), a new generation of sustainable, dual-functional solvents with exceptionally strong acidic and inherently reduced properties. Employing FTIR, Raman, and NMR spectroscopy, the nature of HACl-MSA DESs’ molecular interactions was probed with varying molar ratios between 1:4 and 1:8. Differential scanning calorimetry (DSC) revealed composition-dependent phase behavior, transitioning from an amorphous glass-forming liquid at 1:4 (Tg = −89 °C, peak at 85.87 °C) to congruent melting at 1:5 (Tm = +5.43 °C). For 1:6 to 1:8 formulations, invariant eutectic behavior was observed with a shared solidus transition at about −53.80 °C and a clear melting range ending between −45.91 and −39.21 °C. Spectroscopic analyses confirmed the formation of a homogeneous, single-phase protic DES characterized by a robust, shielded hydroxylammonium core ([NH3OH]+) at 9.87–9.88 ppm and a dynamic acid proton pool that shifts downfield from 13.19 to 13.63 ppm. This shifting resonance directly reflects a tunable macroscopic acidity driven by the expansion of the MSA-MSA hydrogen-bonded network. Quantitative Hammett measurements (H0), determined via UV–vis spectroscopy, demonstrated significant acidity, reaching −6.116 at the 1:8 ratio. Density functional theory (DFT) analysis also confirmed the results, revealing a greater internal thermodynamic stability for the 1:5 ratio with a |ΔEint| = 4.280 eV. However, conceptual DFT descriptors, including a smaller energy gap (7.324 eV), a larger global softness value of 0.137 eV–1, and a more negative chemical potential (−4.506 eV), highlight the 1:4 ratio as a promising theoretical candidate for further evaluation in metal coordination processes. While these electronic metrics suggest a highly polarizable microenvironment, direct extraction yields, leaching kinetics, viscosity, and selectivity require future experimental validation. Through correlations established between molecular structure, frontier orbital analysis, and macroscopic Brønsted acidity trends, HACl–MSA DESs are highlighted as promising candidate platforms for potential application in hydrometallurgical leaching processes. However, definitive claims regarding selective metal extraction, long-term stability, recyclability, and environmental sustainability require comprehensive future experimental evaluation.