Aggregation of Mixed (Cationic + Anionic) and (Cationic + Nonionic) Surfactant Systems within a (Lanthanide Salt:Urea) Deep Eutectic Solvent

Deep eutectic solvents (DESs) are emerging as versatile media to support and facilitate various molecular aggregation processes, including surfactant self-assembly. The effect of a type IV DES consisting of La(NO₃)₃·6H₂O and urea in a 1:3.5 molar ratio named (La:U) as a solubilizing medium on the self-aggregation of (cationic + anionic) and (cationic + nonionic) surfactant mixtures with <i>N</i>-dodecyltrimethylammonium bromide (DTAB), sodium dodecyl sulfate (SDS), and Triton X-100 (TX-100) as cationic, anionic, and nonionic surfactants, respectively, is investigated. The surfactant mixtures are found to readily self-aggregate within the (La:U) DES across the complete composition range under ambient conditions. The critical aggregation concentrations (CACs) of both (DTAB + SDS) and (DTAB + TX-100) surfactant mixtures lie between the CACs of the respective surfactants in the mixture, indicating that, owing to its ionic nature, the (La:U) DES effectively screens the charges present on the surfactant headgroups. While for the (DTAB + SDS) mixture, this observation is in contrast to that in aqueous media, it is similar to that in water for the (DTAB + TX-100) mixture. Both investigated surfactant mixtures exhibit a mostly negative interaction parameter (β) within the DES, implying the presence of predominantly attractive interactions between DTAB and SDS/TX-100. The thermodynamic parameters estimated for both surfactant mixtures within the (La:U) DES indicate aggregation to be a spontaneous process. While it appears to be both enthalpically and entropically driven, it is predominantly entropically driven. Using fluorescence spectroscopic methods, the aggregation number (<i>N</i>_agg) of the self-assembly of mixed surfactant systems is estimated and the solute solvation microenvironment afforded by these aggregates within the (La:U) DES is also ascertained. The outcomes will help expand the application potential of such polar DESs by establishing them as alternate and effective media for mixed surfactant aggregation.