Ammonium sulfate (AS)-methanesulfonic acid (MSA) deep eutectic solvents: a tuneable superacidic platform for lithium...

This study reports the formulation and comprehensive characterization of a novel superacidic Deep Eutectic Solvent (DES) composed of ammonium sulfate (AS) and methanesulfonic acid (MSA). Prepared at molar ratios ranging from 1:5 to 1:9, the structural evolution and chemical environment of these solvents were systematically probed using FTIR, Raman, and ¹H NMR spectroscopy, while UV-vis spectroscopy was employed for precise acidity quantification. Vibrational analysis reveals a complex, cooperative hydrogen-bonded network. This network is characterized by a significant bathochromic shift of the sulfate (SO₄^2-) asymmetric stretch and a corresponding blue shift of the ammonium (NH₄⁺) bending mode. Such interactions were found to be most intense at the 1:5 ratio, suggesting a point of maximum coordination between the salt and the acid. The ¹H NMR results complement these vibrational findings, displaying a pronounced upfield shielding of methyl protons alongside a downfield deshielding of exchangeable protons (5.19-5.29 ppm) as the MSA content increases. Quantitative acidity measurements, determined via the Hammett acidity function (H₀) using UV-vis spectrophotometry, demonstrate that the anhydrous DES exhibits an exceptional superacidic character. The system reaches a peak H₀ value of -6.010 at a 1:9 ratio. This extreme acidity arises from the synergistic coordination of MSA's sulfonic groups and ammonium cations. This mechanism promotes high proton mobility because the weakly basic anions have a low affinity for protons, thereby preventing their sequestration (trapping) within the eutectic matrix and keeping them available for reaction. These findings establish the AS-MSA system as a potent, tuneable medium for mineral processing.