Ammonium acetate (AAc)–methanesulfonic acid (MSA) deep eutectic solvents (DESs) represent a promising class of green, tunable media with properties that can be tailored for future hydrometallurgical applications. This work reports the formulation and systematic characterisation of AAc–MSA DESs across molar ratios from 1:5 to 1:9 (AAc:MSA), using Fourier Transform Infrared (FTIR) spectroscopy, Raman spectroscopy, Proton Nuclear Magnetic Resonance ( 1 H NMR) spectroscopy, and Ultraviolet–Visible (UV–Vis) spectroscopy to investigate molecular interactions and acidity profiles relevant to potential metal extraction applications. FTIR and Raman analyses indicate the structural transition from hydrogen-bond-dominated networks to ionically enriched environments with increasing MSA concentration due to interactions between AAc's ammonium (NH 4 + )/acetate (CH 3 COO − ) moieties and the sulfonate group (–SO 3 − ) of MSA. Key vibrational shifts (N–H/O–H stretching at 3219.68–3231.91 cm −1 ; SO 2 stretching at 1137.80–1154.11 cm −1 ) confirm weakened hydrogen bonding and increased ionic character with rising MSA content. The 1 H NMR supports this structural transition, showing upfield shifts in NH 4 + protons (7.30 to 6.79–7.04 ppm) and downfield shifts in –SO 3 − protons (12.86 to 13.26 ppm from 1:5 to 1:8), indicating an MSA-dominated ionic environment. The Hammett acidity function ( H 0 ) demonstrates strong tunability and the emergence of superacidity, ranging from H 0 = −5.385 to −5.991 in pure DESs, and from 1.569 to −0.096 in the diluted systems, depending on concentration. These findings demonstrate that AAc–MSA DESs possess the structural and acidity properties required for proton-driven mineral breakdown, highlighting their potential for sustainable metal extraction and selective ion recovery. • Novel AAc–MSA DESs formulated and characterized for sustainable metal recovery. • Spectroscopic analysis via FTIR, Raman, and 1 H NMR reveals a structural transition from hydrogen-bonded to ionic networks. • Superacidity of pure AAc–MSA DESs with Hammett function ( H 0 ) values reaching −5.991. • Tunable acidity demonstrated in aqueous environments ( H 0 = 1.569 to −0.096), enabling selective metal recovery. • Sustainable alternative to mineral acids validated for high-performance, proton-driven metallurgical leaching.
Ghaedi et al. (Sun,) studied this question.