Layered Double Hydroxides and the Environment: An Overview - Advanced Materials for Agriculture, Food and Environmental Safety - page 10

In terms of thermodynamics, ion exchange in LDHs depends mainly
on the electrostatic interactions between the positively-charged sheets and
the interlamellar anion and, to a lesser extent, on the free energy involved
in the changes of hydration [60]. Moreover, the equilibrium constants in
the anionic-exchange process depend on the size of the anion. Hence, the
anion exchange is favored for anions with high charge density (i.e., high
charge and small ionic radius) [61].
h e ionic exchange for simple inorganic divalent anions decreases in the
order of CO 3 2- > HPO 4 2- > SO 4 2- and for monovalent anions, OH - > F - > Cl - >
Br - > NO 3 - > I - . Synthesis of LDHs using ion exchange methods have been
carried out by a number of researchers [5, 39, 62-65].
1.4.6 RehydrationMethod
h e rehydration method was reported by Miyata, who found that at er cal-
cination of the LDHs at 500-800°C, the mixed metal oxide rehydrated and
reformed back to a LDH in the presence if anions. h is unique “memory
ef ect” of LDHs provides an ef ective synthesis route to obtain LDHs with
desired inorganic and organic anions, and avoids the incorporation of
competing inorganic counter anions [66-73].
1.4.7 MiscellaneousMethods
In addition to the above methods, several other methods which have been
applied in the preparation of LDHs are presented below.
1.4.7.1 Salt-OxideMethod
In this method M(III) solution is slowly added into a suspension of M(II)
metal oxide or hydroxide. h e mixture is heated at room temperature for a
few days in order to favor the reaction between the precursors. h ismethod
has been used to prepare LDHs having dif erent combinations of divalent
and trivalent cations and anions such as ZnCr-Cl, ZnCr-NO 3, ZnAl-Cl and
ZnAl-NO 3 .
1.4.7.2 TemplateSynthesis
In recent years, this method has attracted more attention in the i eld of
Materials Science. Using self-assembled aggregates as a template, inorganic
materials can be directed to an ordered structure with specii c morphology
and size. h ere have only been a few reports of the synthesis of LDHs using
this method [73].