Recent development of the technique for measurement of solid-liquid interface led direct observation of the hydration structure on crystal surface. In this study, we introduced our results of the atomic-scale observation of hydration structure on growing surfaces of calcite using frequency modulation atomic force microscopy (FM-AFM). We visualized the difference of hydration structure on terraces and on step edges, and the effect of additives. While the two hydration layers were formed on the terraces of calcite, a few hydrated water molecules were observed at the step edges. The hydration layers on the terraces increased to four layers in the existence of magnesium ions and the hydration force also increased in the coexistence of hydrophilic polypeptide and magnesium ions. On the other hand, the adsorption of the additives was not observed on the calcite surfaces. These results suggest that hydration would not disturb the step propagation, but would affect the ion substitution and adsorption of the polypeptide on the terrace. The clarification of the influences of the hydration on solution growth is expected through the atomic-scale measurement of the solid-liquid interface.
Mineral/water interfaces control ion substitution, dissolution, precipitation, and crystal growth. Atomic-scale structure of mineral/water interfaces is important for understanding the interaction among mineral surface, water, dissolved ions and molecules. The interfacial structures have been revealed by surface X-ray scattering measurements by comparing the results with molecular simulations. In this review, we explain the methods and interpretation of the X-ray scattering profiles and molecular simulations applied for muscovite and calcite/water interfaces. Different hydration structures of K^+ and Cs^+ on a muscovite surface is revealed by the method. Wettability alteration of a calcite surface is discussed based on a molecular simulation. We show the results that ion substitution of the outermost layer of the calcite surface can be stable, though the incorporation of the foreign ions in bulk crystal is unstable. These results should be important for understanding various phenomena occurring at the crystal surface during the growth.
For hydrated proteins, hydration waters largely contribute to the structure and properties of proteins. Therefore, the information of the dynamics of hydration waters in proteins is very important for understanding unique structure, function and crystal growth of protein. In the present study, the change in the rotational motion of water molecules around dynamical transition point (T_D〜220 K) was investigated for hydrated lysozyme using solid-state ^2H NMR. An Arrhenius-type temperature dependence of correlation time for the rotational motion of water molecules was observed above and below T_D. Although the water molecules at the surface of proteins undergo fast isotropic rotation above T_D, 180° flip becomes main motion of water molecules below T_D. Thus, the dynamics of hydrated water is found to be suppressed below T_D. The large distribution of the electric field gradient at ^2H observed below T_D indicates the glassy state of hydration water in protein.
Precise understandings of hydration and dehydration processes in crystallization processes are prerequisites for deeper insights into fundamentals of crystal growth in aqueous solutions. In this review, we present the great potentialities of thermodynamic and kinetic analyses of the dependence of solubilities and crystal-growth-rates on pressure on experimentally accessible information of hydration and dehydration processes in crystallization processes. The dependence of solubility of crystals on pressure provides molar volume change of dissolution. Positive and negative slopes of solubility curves with pressure result in the decrease and increase in the molar volume of solutes during dissolution processes, respectively. Activation volumes during solute incorporation processes at kink sites are calculated using the dependence of step kinetic constant on pressure. In practice, solubilities of alkali halide, amino acid, and protein crystals, and step velocity of glucose isomerase crystals are discussed.
Control of crystal growth is rather important in the field of materials, because the shape directly affects their functional abilities. Our research group engages in hydrothermal synthesis of titanium dioxide, which is regarded as one of the representative ceramics, using water-soluble titanium complexes. The high chemical stability and peculiar molecule structures of these titanium complexes allowed us to use various compounds which play a role in control of crystal growth, resulting in the selective synthesis of four titanium dioxide polymorphs, that is, rutile, anatase, brookite and TiO_2(B), with a controlled shape. This paper summarizes our past study and reported researches on synthesis of titania polymorph crystals by classification of zero-, one-, two-, and three-dimensional structures.
Extensive molecular dynamics simulations have been performed to study the phase behavior of water confined in quasi-one-dimensional hydrophobic nanopores, namely carbon nanotubes. We provide unambiguous evidence for solid-liquid critical points by investigating (i) isotherms in the pressure-volume plane, (ii) the spontaneous solid-liquid phase separation below a certain temperature, (iii) diverging heat capacity and isothermal compressibility as a certain point is approached, (iv) continuous change of dynamical and structural properties above the point. Furthermore, the result combined with the study of confined Lennard-Jones particles suggests that the solid-liquid critical point is not uncommon in quasi-one-dimensional fluids.