CHAPTER 2 LITERATURE REVIEW2.1 Layered Double HydroxideLayered double hydroxide (LDH) is an inorganic anionic material formed from two or more types of metal cations and has positively charged layers and exchangeable anions. LDHs can be called anionic clays in comparison with cationic clays which contain the cation in the interlamellar region. In LDH, the species that constitutes the intermediate layers is the anion.Retracing the historical background of LDH:(Rives, 2001)The structure of LDH derives from the parent structure of brucite, Mg(OH2) through isomorphic substitution of divalent cations. Brucite consists of a tight hexagonal packing of hydroxy ions with alternating layers of octahedral sites occupied by divalent cations. The metal cation Mg2+ is then replaced by the trivalent cation such as Al3+ thus creating a basal hydroxide layer with a net positive charge. The charge density M3+/(M2++M3+) is proportional to the trivalent metal ratio and the formation of pure LDH usually occurs for stoichiometric intervals of 0.20 < x < 0.33. These charges are electrically balanced by the intercalation of anionic species in the interlayer region while water molecules reside in the remaining space. These M2+/M3+ (OH)6 octahedral units share the edge and lead to the formation of two-dimensional infinite sheets. These sheets can be stacked on top of each other and connected via hydrogen bonding between the hydroxyl groups of the adjacent sheet (Cavani et al., 1991; Rives, 2001). Figure Schematic representation of hydroxide layers in hydrotalcite (Palmer et .... .. middle of paper ...... the choice of host species can be based on the resulting properties of the desired product (Ozin, 1992). Therefore, it is possible to create a variety of multifunctional composites with highly tunable nanometric range materials interlamellar composition coupled to the choice of the organic moiety (Leroux & Besse, 2001) There are several routes for the incorporation of the host material into the nanohybrid such as exchange, co-precipitation, hydrothermal treatment , in situ polymerization, surface-mediated insertion, restacking or reconstruction techniques (Leroux&Besse,2001) In this study, host and guest anions are included in the mother liquors and followed by the aging process to lead to the formation of the well-ordered nanolayered structure. by spontaneous self-assembly or co-precipitation ( Messersmith et al., 1995 ; Hussein et al., 2005).2.5 Controlled release