Role of sodium alginate on the modification of the interfacial, micellization and thermodynamic properties of two imidazolium-based surface active ionic liquids in water

A combined theoretical and experimental approach has been used to study the influence of a biopolymer sodium alginate (NaAlg) on the aggregation, interfacial, and thermodynamic phenomena of two surface active ionic liquids (SAILs), 1-decyl-3-methylimidazolium chloride (DMeImCl), and 1-hexadecyl-3-methylimidazolium chloride (HDMeImCl) with a view to explore the effects of the tail lengths of the SAILs and the temperature. Density Functional Theory (DFT) in conjunction with conductometry, tensiometry, and vapor pressure osmometry has been employed. DFT studies indicate that the geometries of the SAIL-NaAlg coordinated structures become more stabilized compared to the geometries of NaAlg, DMeImCl and HDMeImCl themselves, and that an increase in the alkyl chain length of the SAILs results in a lowering in the optimization energy for SAIL-NaAlg coordinated structures. Surface activity of the SAILs in presence of NaAlg has been found to be improved as the tail length of the SAILs increases. Evaluation of the Gibbs minimum free energy demonstrates a poorer thermodynamic stability of the interfacial growth of the SAIL molecules as the temperature is elevated. SAIL monolayer has been found to become more compact for the HDMeImCl-NaAlg system compared to the DMeImCl-NaAlg system arising from improved van der Waals attraction among the hydrophobic tails. The pC20 values also indicate more interfacial adsorption in the HDMeImCl system within the investigated temperature range. Two characteristic concentrations, e.g., the critical aggregation concentration (cac) and polymer saturation concentration (psc), prior to the appearance of free SAIL micelles above the critical micellar concentration (cmc) in each of the SAIL-NaAlg solutions have been identified. Micellization of both the SAILs has been found to be thermodynamically spontaneous in presence of NaAlg. As the temperature increases or the surfactant tail gets longer, micellization becomes more spontaneous. The geometrical shapes of both DMeImCl and HDMeImCl micelles in presence of the NaAlg have been predicted to be spherical irrespective of the experimental temperatures. The results have been discussed to elucidate the SAIL-NaAlg interactions considering diverse forces prevailing in these mixed systems.