Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India.
Langmuir. 2011 Nov 1;27(21):13276-83. doi: 10.1021/la202876g. Epub 2011 Oct 7.
Surfactant-intercalated layered double-hydroxide solid Mg-Al LDH-dodecyl sulfate (DDS) undergoes rapid and facile delamination to its ultimate constituent, single sheets of nanometer thickness and micrometer size, in a nonpolar solvent such as toluene to form stable dispersions. The delaminated nanosheets are electrically neutral because the surfactant chains remain tethered to the inorganic layer even on exfoliation. With increasing volume fraction of the solid, the dispersion transforms from a free-flowing sol to a solidlike gel. Here we have investigated the sol-gel transition in dispersions of the hydrophobically modified Mg-Al LDH-DDS in toluene by rheology, SAXS, and (1)H NMR measurements. The rheo-SAXS measurements show that the sharp rise in the viscosity of the dispersion during gel formation is a consequence of a tactoidal microstructure formed by the stacking of the nanosheets with an intersheet separation of 3.92 nm. The origin and nature of the attractive forces that lead to the formation of the tactoidal structure were obtained from 1D and 2D (1)H NMR measurements that provided direct evidence of the association of the toluene solvent molecules with the terminal methyl of the tethered DDS surfactant chains. Gel formation is a consequence of the attractive dispersive interactions of toluene molecules with the tails of DDS chains anchored to opposing Mg-Al LDH sheets. The toluene solvent molecules function as molecular "glue" holding the nanosheets within the tactoidal microstructure together. Our study shows how rheology, SAXS, and NMR measurements complement each other to provide a molecular-level description of the sol-gel transition in dispersions of a hydrophobically modified layered double hydroxide.
表面活性剂插层层状双氢氧化物固体 Mg-Al LDH-十二烷基硫酸盐（DDS）在非极性溶剂（如甲苯）中迅速且容易地剥离为其最终组成部分，即纳米厚度和微米尺寸的单层，形成稳定的分散体。剥离的纳米片是电中性的，因为即使在剥离时，表面活性剂链仍与无机层相连。随着固体体积分数的增加，分散体从自由流动的溶胶转变为固态凝胶。在这里，我们通过流变学、SAXS 和（1）H NMR 测量研究了疏水性改性 Mg-Al LDH-DDS 在甲苯中的分散体的溶胶-凝胶转变。流变学-SAXS 测量表明，凝胶形成过程中分散体粘度的急剧上升是由纳米片堆叠形成的拟薄片状微结构引起的，层间分离为 3.92nm。导致形成拟薄片状结构的吸引力的起源和性质是通过一维和二维（1）H NMR 测量获得的，该测量提供了与末端甲基相连的 DDS 表面活性剂链与甲苯溶剂分子缔合的直接证据。凝胶形成是甲苯分子与锚定在 opposing Mg-Al LDH 片上的 DDS 链的尾部之间的吸引力分散相互作用的结果。甲苯溶剂分子作为分子“胶”将纳米片保持在拟薄片状微结构内。我们的研究表明，流变学、SAXS 和 NMR 测量如何相互补充，为疏水性改性层状双氢氧化物分散体的溶胶-凝胶转变提供了分子水平的描述。
