[M-IS36_1AM2] Interface- and nano-phenomena on crystal growth

[MIS36-07] Atomic scale in situ observation of solid-liquid interface of calcite

*Yuki ARAKI¹, Katsuo TSUKAMOTO², Yuki KIMURA³, Tomoyuki MIYASHITA⁴, Noriaki OYABU⁵, Kobayashi KEI⁶, Hirofumi YAMADA⁵ (1.Graduate school of Science, Kobe University, 2.Graduate school of Science, Tohoku University, 3.Institute of Low Temperature Science, Hokkaido University, 4.Faculty of Biology-Oriented Science and Technology, Kinki University, 5.Department of Electronic Science and Engineering, Kyoto University, 6.The Hakubi Center for Advanced Research, Kyoto University)

Keywords:Calcite, Hydration, FM-AFM

Calcium carbonate is one of common minerals on the earth. Calcium carbonate crystals are utilized industrially in various fields, so that the control of crystal growth is required. It has been known that organisms control the morphology and polymorph of calcium carbonate crystals by utilizing inorganic and organic additives in biomineralization. Understanding the additive effects on growth of calcium carbonate crystal is necessary to control the crystal growth.The effect of additives on growth of calcite which is a stable polymorph of calcium carbonate has been investigated. The additive effect on calcite surface, such as incorporation of magnesium ions into calcite and pinning of step propagation by organic molecules has been confirmed. On the other hand, the additive effect on hydration of calcite has remained unclear even if that effect has been suggested by the measurement of growth rate of calcite in the presence of additives. Hydration affects adsorption and surface diffusion of ions on calcite surface. Also, the dehydration has been considered as rate-determining process in solution growth by the estimation of energy barriers of solution growth processes. Therefore, hydration is a key to control the kinetics of calcite growth. Hydration at the vicinity of calcite surface has been measured by surface X-ray diffraction. Although this technique made the description of hydration structure clear, it does not show the local difference of hydration structure between on the terrace and the step front which is capture site of ions. Hence, we employed the newly frequency modulation atomic force microscopy (FM-AFM) for in situ observation of local hydration structure in atomic scale. This technique is expected to provide insight into the atomic scale distribution of hydrated water molecules in growth solution even at step front. This study describes the first in situ examination of the additive effect of organic molecules and magnesium ions on local hydration structure of calcite surface in atomic scale utilizing FM-AFM. The hydration images were compared with the growth rate of calcite measured using phase shift interferometry so as to validate the influence of hydration on the growth rate of calcite. The findings are summarized as follows:(1) The synthetic polypeptide, even that with high hydrophilicity, does not affect hydration at the surface of calcite.(2) Combination of magnesium ions and the synthetic polypeptides provides a rigid hydration on calcite surface.(3) Magnesium ions and the synthetic polypeptides influence hydration and the surface pattern of calcite, respectively.(4) Structured water distribution eases the energy gap between the calcite surface and solution. As a result, the interfacial tension between the calcite surface and the solution is decreased.(5) Magnesium ions and the synthetic polypeptide act in unison to accelerate nucleation via changes in hydration structure.(6) Hydration contributes to interfacial energy between the calcite and the solution, but not for the adsorption of ions on the calcite steps.This study demonstrated that additives affect the interfacial tension via altering hydration structure by application of FM-AFM for crystal growth experiment for the first time. Our results also showed that there is hardly any change in the adsorption of ions on calcite surface due to the hydration structure. That suggests that dehydration is not a rate-determining process, an observation that is contrary to the currently prevailing theory. The further observation of hydration of step front will be carried out by FM-AFM to demonstrate the effect of hydration on adsorption of ions. These findings indicate that the control of interfacial tension is possible utilizing the additive effect on hydration. That provides a new knowledge to regulate the polymorphism of calcium carbonate.