Relative humidity-​induced reversible hydration of sulfate-​intercalated layered double hydroxides

Layered double hydroxides (LDH) are extremely important materials for industrial processes. In the environment, LDH physicochem. behavior depends in large part on their hydration state, but characterization of these hydration effect on their properties is incomplete. This work examd. interpoly-​type transitions induced by variations in ambient humidity among LDH. The cooperative behavior of intercalated water mols. resulted in a sudden, single-​step, reversible dehydration of the [Zn-​Cr-​SO4] LDH. The [Zn-​Al-​SO4] LDH provided an interesting contrast with: the coexistence of hydration cycle end-​members at a 40-​20​% relative humidity range during the dehydration cycle; and a random inter-​stratified intermediate in the hydration cycle. These observations showed the [Zn-​Al-​SO4] LDH offered sites with a range of hydration enthalpies, where at crit. hydration levels (20-​40​%)​, non-​uniform swelling of the structure resulted in an inter-​stratified phase. Domain size variation during reversible hydration was also responsible for differences obsd. in hydration vs. the dehydration pathways. This behavior was attributed to distortion in the OH-​ array which departs from hexagonal symmetry due to cation ordering as shown in structure refinement by the Rietveld method. This distortion was much less in [Zn-​Cr-​SO4] LDH, where the nearly hexagonal OH-​ array offered sites of uniform hydration enthalpy for intercalated water mols. On this case, all water mols. experienced the same force of attraction and dehydrated reversibly in a single step. Changes in basal spacing were also accompanied by interpoly-​type transitions involving rigid translations of metal hydroxide layers relative to one another