Experimental and Computational Studies of Hydrophobic Eutectic Mixtures Based on Tetrabutyl Ammonium and Phosphonium Salts and Alcohols

Hydrophobic eutectic mixtures of tetrabutylammonium bromide (TBABr) or tetrabutylphosphonium bromide (TBPBr) with 1-decanol or 1-dodecanol were investigated using a combination of thermal, rheological, spectroscopic, and quantum-chemical methods. Phase diagrams demonstrate that TBPBr-based mixtures exhibit pronounced negative deviation from ideal behavior and form eutectic minima at a 1:1 composition, while TBABr-based systems behave nearly ideally. Viscosity and density were measured over the temperature range of 293.15 to 343.15 K. FTIR spectroscopy indicates that the addition of small amounts of salt perturbs the hydrogen-bond network of the alcohols, while higher salt fractions partially restore it. Quantum-chemical calculations and QTAIM analysis show that TBPBr forms stronger O–H···Br hydrogen bonds and more compact ion-pair structures than TBABr, consistent with the enhanced intermolecular association and the higher viscosity observed experimentally. The physicochemical properties of these hydrophobic eutectic mixtures arise from the cumulative effect of multiple weak interactions, primarily hydrogen bonding and dispersion forces, rather than a single dominant interaction. These results advance the molecular-level understanding required to design hydrophobic eutectic solvents with tailored properties.