This review describes the application of solid-state vibrational circular dichroism spectroscopy (denoted as SD-VCD) to the intercalation compounds of layered double hydroxides (LDH’s). The hybrid compounds of chiral molecules and LDH are chosen as a main target. SD-VCD is employed to probe the conformation of an intercalated amino acid. Intermolecular interactions are studied from the VCD spectra on LDH samples co-adsorbing an amino acid and tartaric acid. Notably the enhancement of VCD signals occurs to provide evidence for the stereoselective interactions between the intercalated species. As a final goal, the analysis of a model drug tablet containing LDH is described by means of the microscopic scanning VCD method.
A recent study using SAXS (small-angle X-ray scattering) revealed the presence of large amounts of amorphous nanoparticles in clays, soils, and sediments. In their analysis, all SAXS in the 2θ (Cu Kα) range of 0.12° to 8.00°, except for 001 reflection peak of smectite, was assumed to originate from amorphous nanoparticles. We have, however, shown that some parts of the SAXS come from grains of phyllosilicate clay minerals; the contribution from grains of phyllosilicate clay minerals is about 14 percentage for grains around 200 nm and about 3 percentage for grains around 1000 nm. We have also shown that the amounts of amorphous nanoparticles measured by SAXS are proportional to the specific surface area. This proportional relation confirms that large amounts of amorphous nanoparticles are really present in clays, soils, and sediments because the specific surface area of amorphous nanoparticles are very large compared with those of other minerals such as phyllosilicate clay minerals. This proportional relation also indicates that the amounts of amorphous nanoparticles are proportional to the plasticity index because specific surface area was known to be proportional to the plasticity index. Therefore, it is highly probable that the substance imparting plasticity is not phyllosilicate clay minerals but amorphous nanoparticles. If this is true, then the definition of “clay mineral” must be reconsidered.
In this study, the selectivity for arsenate or arsenite of layered double hydroxide (LDH) consisting of Ni and Al with chloride anions as anion-exchangeable anion species was investigated using LDH with different Ni/Al molar ratios. As a result, LDH with a Ni/Al molar ratio of 2 was highly selective for arsenate, while LDH with a Ni/Al molar ratio of 4 was highly selective for arsenite. From various experimental analyses, arsenate was incorporated into the interlayer space of LDH with a Ni/Al molar ratio of 2 by the anion exchange reaction from chloride to arsenate anions. In the case of LDH with a Ni/Al molar ratio of 4, arsenite anions were incorporated by the anion-exchange reaction, and then chemical bonds were formed with the metal hydroxide layer.