Density, Viscosity, Surface Tension, and Thermal Stability Behavior of Carvacrol-Based Deep Eutectic Solvents

Abstract Two new type V deep eutectic solvents (DESs), tri-n-octylphosphine oxide:carvacrol (TOCA) and lidocaine:carvacrol (LICA), were physicochemically characterized over a range of HBA:HBD molar compositions and temperatures. Thermal stability, density, viscosity, and surface tension were measured to evaluate their applicability and to generate property data for process design and scale-up. Dynamic and isothermal TGA revealed that thermal stability increased with HBA fraction in both systems and was higher for TOCA than for LICA at comparable compositions. Isothermal TGA at 323.15 K over 720 min demonstrated that dynamic TGA alone overestimates solvent stability, as gradual weight loss occurred during prolonged heating below the dynamic onset degradation temperature. Density and viscosity were measured from 298.15 to 333.15 K. Density decreased linearly with temperature for all compositions, ranging from 0.9185 to 0.8750 g·cm–3 for TOCA and from 0.9917 to 0.9593 g·cm–3 for LICA. Viscosity also decreased with temperature and ranged from 70.69 to 14.14 mPa·s for TOCA and from 102.30 to 10.91 mPa·s for LICA, showing that TOCA was generally less viscous than LICA, while the Vogel–Fulcher–Tammann model correlated viscosity better than Arrhenius equation. Surface tension at 294.4 K was lower for TOCA (24.91–25.17 mN·m–1) than for LICA (28.38–29.19 mN·m).