Dudko Volodymyr, Rosenfeldt Sabine, Siegel Renée, Senker Jürgen, Matejdes Marian, Breu Josef
Department of Chemistry and Bavarian Polymer Institute, University of Bayreuth, Universitätsstr. 30, Bayreuth 95440, Germany.
Department of Inorganic Materials, Faculty of Chemical and Food Technology, Slovak University of Technology, Radlinského 9, Bratislava 812 37, Slovakia.
Langmuir. 2022 Sep 6;38(35):10781-10790. doi: 10.1021/acs.langmuir.2c00965. Epub 2022 Jul 21.
Swelling of clays is hampered by increasing layer charge. With vermiculite-type layer charge densities, crystalline swelling is limited to the two-layer hydrate, while osmotic swelling requires ion exchange with bulky and hydrophilic organic molecules or with Li cations to trigger repulsive osmotic swelling. Here, we report on surprising and counterintuitive osmotic swelling behavior of a vermiculite-type synthetic clay [Na][MgLi][Si]OF in mixtures of water and dimethyl sulfoxide (DMSO). Although swelling in pure water is restricted to crystalline swelling, with the addition of DMSO, osmotic swelling sets in at some threshold composition. Finally, when the DMSO concentration is increased further to 75 vol %, swelling is restricted again to crystalline swelling as expected. Repulsive osmotic swelling thus is observed in a narrow composition range of the binary water-DMSO mixture, where a freezing point suppression is observed. This suppression is related to DMSO and water molecules exhibiting strong interactions leading to stable molecular clusters. Based on this phenomenological observation, we hypothesize that the unexpected swelling behavior might be related to the formation of different complexes of interlayer cations being formed at different compositions. Powder X-ray diffraction and Na magic angle spinning-NMR evidence is presented that supports this hypothesis. We propose that the synergistic solvation of the interlayer sodium at favorable compositions exerts a steric pressure by the complexes formed in the interlayer. Concomitantly, the basal spacing is increased to a level, where entropic contributions of interlayer species lead to a spontaneous thermodynamically allowed one-dimensional dissolution of the clay stack.
层电荷增加会阻碍黏土的膨胀。对于蛭石型层电荷密度，晶体膨胀仅限于两层水合物，而渗透膨胀则需要与体积大且亲水的有机分子或锂离子进行离子交换，以引发排斥性渗透膨胀。在此，我们报道了蛭石型合成黏土[Na][MgLi][Si]OF在水和二甲基亚砜（DMSO）混合物中令人惊讶且违反直觉的渗透膨胀行为。尽管在纯水中的膨胀仅限于晶体膨胀，但随着DMSO的加入，在某个阈值组成时会出现渗透膨胀。最后，当DMSO浓度进一步增加到75体积%时，膨胀如预期的那样再次仅限于晶体膨胀。因此，在二元水 - DMSO混合物的狭窄组成范围内观察到了排斥性渗透膨胀，在此范围内观察到了冰点降低。这种降低与DMSO和水分子表现出强相互作用从而形成稳定的分子簇有关。基于这一现象学观察，我们推测这种意外的膨胀行为可能与在不同组成下形成不同的层间阳离子络合物有关。文中给出了粉末X射线衍射和钠魔角旋转核磁共振证据来支持这一假设。我们提出，在有利组成下，层间钠的协同溶剂化通过层间形成的络合物施加空间压力。同时，基面间距增加到一定程度，此时层间物种的熵贡献导致黏土堆叠体自发地进行热力学允许的一维溶解。
