Interactional behaviour of surface active ionic liquids with gelling biopolymer agarose in aqueous medium

Abstract

Interactions of surfactant-like ionic liquids (ILs), 1-butyl-3-methylimidazolium octylsulfate [C₄mim][C₈OSO₃] and 3-methyl-1-octylimidazolium chloride [C₈mim][Cl] with the neutral biopolymer agarose (Ag) has been investigated in aqueous media through an array of techniques, viz. tensiometry, isothermal titrimetric calorimetry (ITC), turbidimetry, dynamic light scattering (DLS), conductometry, scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). Tensiometric and calorimetric profiles of the studied systems showed three transitions corresponding to the critical aggregation concentration C₁(cac), the saturation concentration C₂(sc), and the critical micelle concentration C₃(cmc). The tensiometric behaviour of both the systems is found to be more like charged polymer–surfactant systems because of the large number of electron rich oxygens in the Ag chain. Interactions in the Ag–[C₄mim][C₈OSO₃] system were found to be weaker when compared with the Ag–[C₈mim][Cl] system because of higher cation–anion interactions in the former as evidenced by low a van’t Hoff's factor (∼1.0) of [C₄mim][C₈OSO₃] in the studied concentration regime. ITC measurements showed that both the ILs interact exothermically below 6 mmol kg⁻¹ with Ag and endothermically thereafter until C₃(cmc) is reached. Turbidimetric and DLS profiles have shown coacervate formation of Ag with both the ILs. The complex formation has been verified by SEM images, which have shown that Ag nanoplates change into spherical complexes upon interaction with the ILs. FTIR spectra indicated hydrogen bonding between IL ions and Ag in various concentration regimes. The study can be useful in understanding the importance of ion–dipole, hydrogen bonding and hydrophobic interactions in Ag sol–gel systems, which are often used as matrices for electrophoresis of charged molecules.