spaces, which compensate for the positive charge of the brucite-type layers [65]. Due
to the structural characteristics and compositional variation, the application of LDHs
in such areas as adsorption/separation of ions [65–69], catalysis [66–68], polymer
additives [66–68], and medical and biochemical uses [66, 68] has been proposed so
far. The chemical composition of the LDHs is expressed as [M(II) 1-X M(III) X (OH) 2 ]
[A
nÀ
X/n ]
XÀ where M(II) ¼ Mg, Co, Ni, etc.; M(III) ¼ Al, Cr, Fe, etc.; and A is an
interlayer anion such as CO 3
2À and Cl
À
.
In addition to the crystalline structures, the particle size and its distributions of
layered solids are key issues in order to achieve optimum performance of layered
solids and their intercalates; accordingly, attention has been paid for the powder
morphology during the syntheses as well as classification [27, 70, 71]. Powders
[36, 72], suspensions [73, 74], and thin films [55, 75–77] have been used for the
evaluation of the photoprocesses, as well as for other application [78]. One of the
unique and attractive properties of smectites is their spontaneous swelling in water.
Platy particles pile up with their ab plane parallel to the substrate to form a film when
the suspension is evaporated on a flat substrate [55, 75, 79]. The preparation of thin
films by the Langmuir-Blodgett technique (LB technique) from exfoliated platelets
of clays has also been reported [80, 81]. Inorganic-organic multilayered films have
also been prepared via alternate adsorption of a cationic species and an anionic sheet
of an exfoliated layered solid (layer-by-layer deposition technique, hereafter abbreviated as LbL technique) [82–86].
3 Surface Modification
In addition to the structural and compositional variation of smectites and other clay
minerals (Table 1), the possible surface modification with organic/inorganic cations
and polymers makes the variation of the material more versatile (Fig. 2) [87]. Longchain alkylammonium ions have been studied most extensively in the chemistry of
organophilic smectites, and the practical application of the organophilic smectites as
adsorbents [25, 88] has been extensively reported. Phospholipids have been utilized
for the construction of environmentally benign organoclay [89, 90]. Several nonionic surfactants have also been used for the surface modification of smectites
[75, 91–93]. Intercalation of alkylammonium ions with more complex structures
into layered silicates to precisely design hydrophobic nanospace, and, recently,
flexibility of the interlayer surfactant aggregates has been discussed based on
quasi-elastic neutron scattering data [57].
Microporous and mesoporous solids have been obtained by crosslinking the
nanosheets. The pioneering example is the pillaring with polyoxocations (e.g.,
[AlO 4 Al 12 (OH) 24 (H 2 O) 12 ]
7+ ) [94, 95]. Nanoporous solids composed of silicate
layers and metal/metal oxide finite particles have been prepared [96–100]. The
microporous solids composed of silicate nanosheet and small organoammonium
cations (e.g., tetramethylammonium ion, TMA) have been prepared and used for
the separation/sensing and other functional materials [101–107]. The adsorptive
256
T. Yamaguchi et al.
to the structural characteristics and compositional variation, the application of LDHs
in such areas as adsorption/separation of ions [65–69], catalysis [66–68], polymer
additives [66–68], and medical and biochemical uses [66, 68] has been proposed so
far. The chemical composition of the LDHs is expressed as [M(II) 1-X M(III) X (OH) 2 ]
[A
nÀ
X/n ]
XÀ where M(II) ¼ Mg, Co, Ni, etc.; M(III) ¼ Al, Cr, Fe, etc.; and A is an
interlayer anion such as CO 3
2À and Cl
À
.
In addition to the crystalline structures, the particle size and its distributions of
layered solids are key issues in order to achieve optimum performance of layered
solids and their intercalates; accordingly, attention has been paid for the powder
morphology during the syntheses as well as classification [27, 70, 71]. Powders
[36, 72], suspensions [73, 74], and thin films [55, 75–77] have been used for the
evaluation of the photoprocesses, as well as for other application [78]. One of the
unique and attractive properties of smectites is their spontaneous swelling in water.
Platy particles pile up with their ab plane parallel to the substrate to form a film when
the suspension is evaporated on a flat substrate [55, 75, 79]. The preparation of thin
films by the Langmuir-Blodgett technique (LB technique) from exfoliated platelets
of clays has also been reported [80, 81]. Inorganic-organic multilayered films have
also been prepared via alternate adsorption of a cationic species and an anionic sheet
of an exfoliated layered solid (layer-by-layer deposition technique, hereafter abbreviated as LbL technique) [82–86].
3 Surface Modification
In addition to the structural and compositional variation of smectites and other clay
minerals (Table 1), the possible surface modification with organic/inorganic cations
and polymers makes the variation of the material more versatile (Fig. 2) [87]. Longchain alkylammonium ions have been studied most extensively in the chemistry of
organophilic smectites, and the practical application of the organophilic smectites as
adsorbents [25, 88] has been extensively reported. Phospholipids have been utilized
for the construction of environmentally benign organoclay [89, 90]. Several nonionic surfactants have also been used for the surface modification of smectites
[75, 91–93]. Intercalation of alkylammonium ions with more complex structures
into layered silicates to precisely design hydrophobic nanospace, and, recently,
flexibility of the interlayer surfactant aggregates has been discussed based on
quasi-elastic neutron scattering data [57].
Microporous and mesoporous solids have been obtained by crosslinking the
nanosheets. The pioneering example is the pillaring with polyoxocations (e.g.,
[AlO 4 Al 12 (OH) 24 (H 2 O) 12 ]
7+ ) [94, 95]. Nanoporous solids composed of silicate
layers and metal/metal oxide finite particles have been prepared [96–100]. The
microporous solids composed of silicate nanosheet and small organoammonium
cations (e.g., tetramethylammonium ion, TMA) have been prepared and used for
the separation/sensing and other functional materials [101–107]. The adsorptive
256
T. Yamaguchi et al.