Interlayer region

Chitosan-clay bio-nanocomposites are very stable materials without significant desorption of the biopolymer when they are treated with aqueous salt solutions for long periods of time. In this way, they act as active phases of electrochemical sensors for detection of ions (Figure 1.8). The particular nanostructuration of the biopolymer in the interlayer region drives the selective uptake of monovalent versus polyvalent anions, which has been applied in electrode arrays of electronic tongues [132]. 

A key feature of the chemistry of layered materials is their ability to locate and orient guest molecules of interest within constrained interlayer regions. Once intercalated, the guest molecules can demonstrate a range of interesting properties [3,66]. In... 

Bronsted acid sites) or metal atoms with unsatisfied coordination (Lewis acid sites) react with water to form surface charge (13). Isomorphic substitution in the interlayer region of layered silicates results in a negative surface charge. In each case chemical "exchange" of ions between phases results in the formation of surface charge and the development of an electrical potential. 

In general, the 2 1 clays are not very simple systems in which to study the interaction of water and surfaces. They have complex and variable compositions and their structures are poorly understood. Water occurs in several different environments zeolitic water in the interlayer regions, water adsorbed on the external surfaces of the crystallites, water coordinating the exchangeable cations, and, often, as pore water filling voids between the crystallites. Thus, there are many variables and the effects of each on the properties of water are difficult to separate. 

The reaction of benzene with Cu(II) and Fe(III)-exchanged hectorites at elevated temperatures produced a variety of organic radical products, depending on the concentration of water in the reaction medium and the reaction time (90). The formation of free radicals was accompanied by a reduction in oxidation state of the metals, a process that had a zero-order dependence on the metal ion concentration. Under anhydrous conditions the free radicals appeared to populate sites in the interlayer region, the activation energies under these conditions being lower than in the hydrated samples. 

Spectroscopic techniques such as electron spin resonance (ESR) offer the possibility to "probe" the chemical environment of the interlayer regions. With the ESR technique, an appropriate paramagnetic ion or molecule is allowed to penetrate the interlayer, and chemical information is deduced from the ESR spectrum. Transition metal ions, such as Cu2+, and nitroxide radical cations, such as TEMPAMINE (4-amino-2,2,6,6-tetramethylpiperidine N-oxide) have been used as probes in this manner (6-14). Since ESR is a sensitive and non-destructive method, investigations of small quantities of cations on layer silicate clays at various stages... 

In the case of stearate ions intercalated in Mg/Al LDHs, the electron density shows a pronounced minimiun in the center of the interlayer region, which is consistent with the presence of a bilayer of guest molecules [151,152], The electron density distribution for an anionic azobenzene derivative intercalated in an Mg/Al LDH has also been reported [ 153]. 

The presence of color in many industrial effluent streams is highly undesirable. LDHs have been found to be particiflarly effective at removing various synthetic dyes (Table 1) [158]. For example. Acid Blue 29 could be adsorbed on the surface or enter the interlayer region of the LDH by anion exchange an equilibrium time of 1 h with 99 % dye removal was obtained. Furthermore,... 

Ambrogi et al. [ 186] intercalated the drug ibuprofen (IBU) in the interlayer region of an Mg/Al - Cl LDH by an anion-exchange method in order... 

The interlayer region of LDHs can provide a novel environment for photochemical reactions of guest molecules [201-204]. For example, Takagi et al. reported that the controlled photodimerization of a variety of unsaturated carboxylate species intercalated in the interlayer galleries of Mg/Al LDH could occur between the layers [203]. Syn head-to-head cyclodimers were selectively formed in the irradiation of intercalated cinnamate ions, whereas two isomers of syn head-to-head and syn head-to-taU cyclodimers were formed for the case of phenylethenylbenzoates. The product selectivity was shown to be controlled by the Mg/Al ratio of the host LDH, and hence the packing density of the anions in the interlayer region [204]. 

The peculiar layer structure of these clays gives them cation exchange and intercalation properties that can be very useful. Molecules, such as water, and polar organic molecules, such as glycol, can easily intercalate between the layers and cause the clay to swell. Water enters the interlayer region as integral numbers of complete layers. Calcium montmorillonite usually has two layers of water molecules but the sodium form can have one, two, or three water layers this causes the interlayer spacing to increase stepwise from about 960 pm in the dehydrated clay to 1250, 1550, and 1900 pm as each successive layer of water forms. 

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