Synthesis and characterization of water-swellable LDH (layered double hydroxide) hybrids containing sulfonate-type intercalant

MgAl-LDH hybrids containing various short-chain organic sulfonate (RSO3-) anions were synthesized, and among them, LDH hybrids containing isethionate (Ise, 2-hydroxyethanesulfonate, HO(CH2)(2)SO3-) exhibited swelling behavior in water. On adding water, these LDH hybrids (Ise-MgAl-LDH3 (Mg/Al = 3) and Ise-MgAl-LDH2 (Mg/Al = 2)) immediately swelled to form a viscous gel and further addition of water led to the formation of semi-transparent colloidal suspensions. The powder X-ray diffraction (PXRD) profiles of the gels showed only a broad feature for the loss of the sharp reflections. On drying the gel, the reflections reappeared in the PXRD profile, indicating that the LDH nanosheets restacked to form the original stacked structure. Atomic force microscopy (AFM) observations revealed the existence of nanosheets with a thickness of 1.5-2 nm having the same morphological features as the initial LDHs. Ise was incorporated into other LDHs-NiAl-LDHs and CoAl-LDH-with different metal ions and various divalent metal/trivalent metal ratios. The resulting Ise-LDH hybrids also showed water-swelling property and formed viscous gels on contact with water. This indicates that the incorporation of Ise in the LDH interlayer makes the resulting LDH hybrid water-swellable.