[M-IS17] Interface- and nano-phenomena on crystal growth and dissolution

[MIS17-07] Time lapse imaging of pH/ionic concentration distribution for understanding the dissolution/formation process of minerals in aqueous solution

*Jun Kawano¹, Makoto Yazaki², Shinji Matsumoto², Takashi Toyofuku^3,4, Yukiko Nagai^3,5, Kazuya Ohno², Takaya Nagai¹ (1.Faculty of Science, Hokkaido Univ., 2.Graduate School of Science, Hokkaido Univ., 3.JAMSTEC, 4.TUMSAT, 5.NMNS)

Keywords:imaging, calcium carbonate, pH, dissolution

Recent works have revealed that growth or dissolution mechanism of minerals in aqueous solution is more complicated than we had imagined. To understand these mechanisms, analyzing the local conditions of the solution around the dissolving or forming minerals is important. Recently, we succeeded in visualizing the distribution of pH around a calcium carbonate crystal dissolving in aqueous solution using a fluorescent probe. However, the ionic concentration is another important factor to describe the formation/dissolution reaction of calcium carbonate. In this study, we attempted to visualize the distribution of Ca²⁺ concentration along with the pH around a dissolving calcium carbonate crystal in aqueous solution.

By simultaneously using HPTS (8-hydroxypyrene-1,3,6-trisulfonic acid) and Rhod-FF as fluorescent probes, pH and Ca²⁺ could be measured independently and their distributions around the dissolving calcium carbonate in an aqueous solution were successfully visualized. Tendencies of their distributions were basically similar, but the pH was found to increase and saturate rapidly just above the surfaces, while the Ca²⁺ concentration changed more slowly and its 2D features seemed to be different from those of pH. Rough estimation of the solution composition just above the surface showed that concentration of bicarbonate ions was comparable with Ca²⁺ concentration, which suggest that bicarbonate ions play an important role in the surface reaction.

Here we will also report the result of pH/ionic concentration imaging for the calcium carbonate formation process in gel media. These results could provide new insights into the knowledge of the dissolution/formation reaction of minerals in aqueous solutions.