Constituent- and Composition-Dependent Surfactant Aggregation in (Lanthanide Salt plus Urea) Deep Eutectic Solvents

Due to the ease of tailoring the physicochemical properties by simply changing a constituent or composition, deep eutectic solvents (DESs) possess widely varying capabilities for surfactant self-assembly that could depend on the surfactant headgroup charge. The self-aggregation process of three surfactants, sodium dodecylsulfate (SDS), cetyltrimethylammonium bromide (CTAB), and Triton X-100 (TX-100), dissolved in DESs composed of a lanthanide salt (Ln) and urea (U) is investigated. The role of the identity of the metal salt is assessed by using [La(NO3)3 center dot 6H2O] (La) and [Ce(NO3)3 center dot 6H2O] (Ce) and that of the composition is deciphered by systematically changing the mole ratio of the metal salt and urea in (La/U) DESs. The response to a fluorescence probe pyrene-1-carboxaldehyde along with electrical conductance and surface tension measurements is used to obtain the critical aggregation concentration (CAC). While the CACs in 1:3.5 (Ln/U) for SDS are significantly lower than that in water, the values are marginally higher for CTAB and TX-100. The CACs for all three surfactants are similar in 1:3.5 (La/U) and (Ce/U) DESs, implying that the identity of the metal in the salt is not so important. Increasing the urea composition in (La/U) DESs results in increased CAC for SDS and CTAB; however, a minimal decrease in CAC is observed for TX-100. From the temperature dependence of CAC, thermodynamic parameters, Delta Gagg 0 , Delta Hagg 0 , and Delta Sagg0, of the surfactant self-aggregation process are estimated. These parameters reveal that while at a lower urea content, the SDS/CTAB self-assembly process is enthalpically driven, it becomes entropically favored at higher urea concentrations. The TX 100 self-aggregation in these DESs is found to be strongly enthalpically favored and entropically un-favored. These parameters are explained as a combination of passage of the solvophobic surfactant chain from the bulk DES to the aggregate pseudo-phase and differential orientation/organization of DES constituents around surfactant monomers and/or aggregates.

Automated summary

Due to the ease of tailoring the physicochemical properties, deep eutectic solvents (DESs) have varying capabilities for surfactant self-assembly depending on the surfactant headgroup charge. This study investigated the self-aggregation process of three surfactants (SDS, CTAB, and TX-100) dissolved in DESs composed of a lanthanide salt and urea. The critical aggregation concentration (CAC) was obtained using a fluorescence probe, electrical conductance, and surface tension measurements. The results showed that the CACs varied for different surfactants and compositions of DESs, and the thermodynamic parameters revealed the driving forces for self-aggregation.