Matthews Lauren, Ruscigno Silvia, Rogers Sarah E, Bartlett Paul, Johnson Andrew J, Sochon Robert, Briscoe Wuge H
School of Chemistry, University of Bristol, Cantock's Close, Bristol, BS8 1TS, UK.
ISIS Muon and Neutron Source, Rutherford Appleton Laboratory, Harwell Oxford, Didcot, OX11 0QX, UK.
Phys Chem Chem Phys. 2021 May 26;23(20):11672-11683. doi: 10.1039/d1cp01370j.
Glyceline, a deep eutectic solvent comprising glycerol and choline chloride, is a green nonaqueous solvent with potential industrial applications. Molecular mechanisms of surfactant self-assembly in deep eutectic solvents are expected to differ from those in their constituent polar components and are not well understood. Here we report the observation of self-assembled SDS fractal dendrites with dimensions up to ∼mm in glyceline at SDS concentrations as low as cSDS ∼ 0.1 wt%. The prevalence of these dendritic fractal aggregates led to the formation of a gel phase at SDS concentrations above ≥1.9 wt% (the critical gelation concentration cCGC). The gel microscopic structure was visualised using polarised light microscopy (PLM); rheology measurements confirmed the formation of a colloidal gel, where the first normal stress difference was negative and the elastic modulus was dominant. Detailed nano-structural characterisation by small-angle neutron scattering (SANS) further confirmed the presence of fractal aggregates. Such SDS aggregation or gelation has not been observed in water at such low surfactant concentrations, whereas SDS has been reported to form lamellar aggregates in glycerol (a component of glyceline). We attribute the formation of the SDS fractal dendrites to the condensation of counterions (i.e. the choline ions) around the SDS aggregates - a diffusion-controlled process, leading to the aggregate morphology observed. These unprecedented results shed light on the molecular mechanisms of surfactant self-assembly in deep eutectic solvents, important to their application in industrial formulation.
甘油胆碱，一种由甘油和氯化胆碱组成的深共熔溶剂，是一种具有潜在工业应用价值的绿色非水溶剂。表面活性剂在深共熔溶剂中的自组装分子机制预计与其组成的极性成分不同，目前尚未得到很好的理解。在此，我们报告了在甘油胆碱中观察到自组装的十二烷基硫酸钠（SDS）分形树枝状晶体，其尺寸可达约毫米级，此时SDS浓度低至cSDS ∼ 0.1 wt%。这些树枝状分形聚集体的普遍存在导致在SDS浓度≥1.9 wt%（临界凝胶化浓度cCGC）以上形成凝胶相。使用偏光显微镜（PLM）观察凝胶微观结构；流变学测量证实形成了胶体凝胶，其中第一法向应力差为负，弹性模量占主导。通过小角中子散射（SANS）进行的详细纳米结构表征进一步证实了分形聚集体的存在。在如此低的表面活性剂浓度下，在水中未观察到这种SDS聚集或凝胶化现象，而据报道SDS在甘油（甘油胆碱的一种成分）中形成层状聚集体。我们将SDS分形树枝状晶体的形成归因于SDS聚集体周围抗衡离子（即胆碱离子）的凝聚——这是一个扩散控制过程，导致观察到的聚集体形态。这些前所未有的结果揭示了表面活性剂在深共熔溶剂中的自组装分子机制，这对其在工业配方中的应用很重要。
