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The hydration of layered double hydroxides (LDHs) was investigated by changing the interlayer anion species, the Mg/Al ratio of the LDH hosts and the relative humidity (RH). The anions were CO3^2-, Cl^-, Br^-, NO3^-, I^-, SO4^2-, and ClO4^- (listed in the order of ion size, small to large) and LDHs with Mg/Al = 1.90 (LDH2) and 2.91 (LDH3) were used. Their XRD profiles were measured by an XRD diffractometer while controlling the RH in the range 0–95% at 25 °C. Only I^-, SO4^2-, and ClO4^- LDH2s and SO4^2- LDH3 showed a large step-wise basal-spacing expansion, 0.24–0.28 nm, under high RH conditions ((greater than) ca. 60%) probably due to the insertion of one water layer into the interlayer space. Such hydration occurred more favorably for the LDHs with larger anions and those with a higher layer charge (LDH2). Among them, I^- and ClO4- LDH2s exhibited the second staging – alternate stacking of hydrated (H) and non-hydrated (NH) interlayers – in the intermediate RH region.

Astroparticle Physics

102931