Thermodynamic and structural studies of layered double hydroxide

The preparation of layered double hydroxides via titration with sodium hydroxide was thoroughly investigated for a number of M(II)/M(III) combinations. These titration curves were examined and used to calculate nominal solubility product constants and other thermodynamic quantities for the various LDH chloride systems.;It was found that as the solubility of the LDH varies directly with the solubility of its component hydroxides. For the systems containing Al or Fe as the trivalent metal, the relative solubilities are $rm Alsp{3+} < Fesp{3+}$ and $rm Mgsp{2+} < Mnsp{2+} < Cosp{2+} approx Nisp{2+} < Znsp{2+}$. Additionally, the LDHs were strongly thermodynamically favored over the separate hydroxides under the conditions studied here, this preference also increasing in the order $rm Mgsp{2+} < Mnsp{2+} < Cosp{2+} approx Nisp{2+} < Znsp{2+},$ but being stronger for Al$sp{3+}$ than for Fe$sp{3+}$. The pK$sb{rm sp}$ values range from 52.12 for Mg(II)/Al(III) to 63.84 for Zn(II)/Fe(III).;When the trivalent metal is Cr(III) and the divalent metal is a late first-row transition metal, 2/1 LDHs were formed directly from solution without the intermediacy of a trivalent metal hydr(ous) oxide phase. The thermodynamic quantities of these "one-step" LDHs also followed the same relative trends seen in the aluminum and iron cases with the exception that all three are expected to be more stable than a mixture of simple hydroxides, even at high pH. It was also shown that Zn/Cr LDHs with Zn:Cr ratios other than 2 may be prepared.;The intercalation of cyanide-containing anions into Mg/Al LDH was investigated by means of powder xrd and oriented FTIR techniques. It was found that octahedral anions tend to orient themselves in LDHs with their three-fold axis perpendicular to the hydroxyl sheets. By using an oriented infrared investigation, it was also shown that nitroprusside decomposes in the LDH interlayer gallery to give intercalated ferrocyanide and free nitrosyl groups.;Computational methods were used to investigate the interaction of a hydrogen atom with water and various anions present in LDH systems. These calculations provided evidence that a hydrogen atom is a hydrophobic entity. The presence of a hydrogen atom in small water clusters seems to disrupt the hydrogen bonding network usually seen such clusters. The hydrogen atom seems to interact favorably with carbonate and hydroxide anions but it is unclear whether such interactions would occur in the presence of solvated anions.