Thermodynamics of the aggregation of imidazolium ionic liquids with sodium alginate or hydroxamic alginate in aqueous solution

Abstract

Ionic liquids (ILs) were examined as new components in polymer/IL mixtures for their potential applications as advanced materials. This work reports the thermodynamics of the aggregation of imidazolium ionic liquids, CnMIMBr (n = 10, 12, 14, and 16), and sodium alginate (ALG) or hydroxamic alginate (AHX) in aqueous solutions. The goal of the study was to evaluate the effect of the concentration and alkyl chain length of the ILs and the concentrations and charge density of the polysaccharides on the formation of polyelectrolyte complexes. The aggregation constant (Kagg) was determined from electrical conductivity measurements, and we found that the polymer–IL interactions increase with increasing alkyl chain length of the ILs. Furthermore, an increase in surface tension close to the critical aggregation concentration (C2) indicates that the adsorbed layer thickness at liquid–air interface is much thicker for the AHX/CnMIMBr systems than for the ALG/CnMIMBr systems (n = 12, 14, and 16). Phase separation was observed for ILs with high hydrophobicity (n = 12,14, and 16) at zeta potentials close to zero. However, in the presence of C10MIMBr, only a slightly turbidity was observed, and the dispersion became transparent with further IL addition. The aggregation of ILs on alginate and its derivative is favorable for longer IL alkyl chains, meaning that the hydrophobic interactions act cooperatively with the electrostatic interactions in the formation of the complexes. The values of the standard Gibbs energy of aggregation (ΔGagg°) indicate that the interactions between C12MIMBr with ALG are favorable compared to those with AHX because the polymer charge density is higher. On the other hand, C14MIMBr and C16MIMBr prefer to aggregate with the more hydrophobic polymer (AHX). These outcomes highlight the role of ILs in the design of new polymer formulations.