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Expansion property of ammonium montmorillonite

アンモニウム型モンモリロナイトの膨潤性

齋藤 彬人*; 川喜田 竜平* ; 佐久間 博*; 小田 治恵  ; 本田 明; 佐藤 努*

Saito, Akito*; Kawakita, Ryohei*; Sakuma, Hiroshi*; Oda, Chie; Honda, Akira; Sato, Tsutomu*

NH$$_{4}$$型モンモリロナイト(MMT)の膨潤挙動を明らかにするため、Na, K, NH$$_{4}$$型MMTをそれぞれ準備し、その膨潤性の違いを相対湿度(RH)制御XRDによる分析で比較した。NH$$_{4}$$型MMTは、RH30%以上でK型と同様の膨潤挙動を示し、RH30%以下ではNa、K型よりも膨潤性があることが示された。また、分子動力学計算では、低RH条件で、NH$$_{4}$$型はNa、K型よりも層間に1水分子層をもつ状態に達しやすいことが示された。これはNH$$_{4}$$のH原子とMMTの底面O原子の結合に起因すると考えられる。

To understand the swelling behavior of NH$$_{4}$$-montmorillonite (MMT), the expandability of monoionic Na-, K- and NH$$_{4}$$-MMT was investigated by XRD under controlled relative humidity (RH) conditions and molecular dynamics (MD) calculations. XRD results indicate that the expansion properties of NH$$_{4}$$-MMT are similar to that of K-MMT for RH $$>$$ 30%. At RH $$<$$ 30%, however, NH$$_{4}$$-MMT expands further and has a larger basal spacing than both Na- and K-MMT. MD calculation results showed that NH$$_{4}$$-MMT reaches a one-layer hydration state under lower RH conditions than that of Na- and K-MMT, and was entirely consistent with the above observations by XRD. It was also confirmed that an irregular motion of interlayer NH$$_{4}$$$$^{+}$$ could result from bonds between H atoms of the NH$$_{4}$$$$^{+}$$ ion and the base O atoms of MMT. This irregular motion enabled the basal spacing of NH$$_{4}$$-MMT to increase and consequently, to expand further under low RH conditions.

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