Khokhar Vaishali, Pandey Siddharth
Department of Chemistry, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India.
Langmuir. 2023 Apr 11;39(14):5129-5136. doi: 10.1021/acs.langmuir.3c00189. Epub 2023 Mar 31.
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(NO)·6HO] (La) and [Ce(NO)·6HO] (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, Δ, Δ, and Δ, 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.
由于通过简单改变成分或组成就能轻松调整物理化学性质，低共熔溶剂（DESs）对表面活性剂自组装具有广泛不同的能力，这可能取决于表面活性剂头基电荷。研究了三种表面活性剂，即十二烷基硫酸钠（SDS）、十六烷基三甲基溴化铵（CTAB）和 Triton X - 100（TX - 100）溶解在由镧系盐（Ln）和尿素（U）组成的 DESs 中的自聚集过程。通过使用[La(NO₃)₃·6H₂O]（La）和[Ce(NO₃)₃·6H₂O]（Ce）评估金属盐种类的作用，并通过系统改变（La/U）DESs 中金属盐与尿素的摩尔比来解读组成的作用。利用对荧光探针芘 - 1 - 甲醛以及电导率和表面张力测量的响应来获得临界聚集浓度（CAC）。虽然 SDS 在 1:3.5（Ln/U）中的 CAC 显著低于在水中的 CAC，但 CTAB 和 TX - 100 的该值略高。在 1:3.5（La/U）和（Ce/U）DESs 中，所有三种表面活性剂的 CAC 相似，这意味着盐中金属的种类并非那么重要。增加（La/U）DESs 中的尿素组成会导致 SDS 和 CTAB 的 CAC 增加；然而，观察到 TX - 100 的 CAC 有最小程度的降低。根据 CAC 的温度依赖性，估算了表面活性剂自聚集过程的热力学参数ΔH、ΔS 和ΔG。这些参数表明，在较低尿素含量时，SDS/CTAB 自组装过程由焓驱动，而在较高尿素浓度时则由熵主导。发现这些 DESs 中 TX - 100 的自聚集在焓方面强烈有利而在熵方面不利。这些参数被解释为疏溶剂性表面活性剂链从本体 DES 进入聚集体假相以及 DES 成分围绕表面活性剂单体和/或聚集体的不同取向/组织的综合结果。
