Solvatochromic Polarity, Physicochemical Properties, and Spectral Analysis of New Triple NADES-Based on Urea–Glycerol

In the present study, ten type-V natural deep eutectic solvents (NADESs) were synthesized and comprehensively characterized, based on urea as a hydrogen-bond acceptor and three different groups of donors-glycerol, organic carboxylic acids, and carbohydrates. Their physicochemical parameters, spectral characteristics (FTIR), surface tension, and solvatochromic properties were determined using Nile Red, betaine 30, and Kamlet-Taft parameters. The densities of the systems (1.243-1.361 g/cm³) and the high values of molar refraction and polarizability indicate the formation of highly organized hydrogen-bonded networks, with the incorporated carboxyl and hydroxyl groups enhancing the structural compactness of the NADES. Surface tension varied significantly (46.9-80.3 mN/m), defining systems with low, medium, and high polarity. Solvatochromic analysis revealed high E_NR, E_T(30), and <i>E_T^N</i> values, positioning all NADES as highly polar media, comparable or close to water, but with distinguishable H-bond donating/accepting ability depending on the third component. The normalized Kamlet-Taft parameters show that the NADES cover a broad solvent spectrum-from highly H-bond accepting to strongly H-bond donating or dipolar systems-highlighting the potential for fine-tuning the solvent according to target applications. The obtained results highlight the applicability of these NADESs as green, tunable media for the extraction and solvation of bioactive compounds.