Page 47 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

Nanomaterials Fabrication 33

Therefore, condensation can be initiated by varying the pH of the solu-
tion by addition of a base on an acid (element M with z 4) or by addi-
tion of an acid on a base (element M with z 4). Condensation can also
be initiated via redox reactions with elements having several stable oxi-
dation states (Figure 3.1).
Hydrolysis is, strictly speaking, a neutralization reaction carried out
by the water molecule:

z (z h)
O) ] h H O → [M(OH) (H O) ]
[M(H 2 n 2 h 2 n h h H solvated
For this reaction, it has been shown [9]:

1
H (75.2 – 9.6 z) kJ mol , S ( –148.4 73.1 z) J mol 1
and:

(119.5 – 31.35 z) kJ mol 1
G 298
The reaction is spontaneous ( G 0) for elements of charge equal
to or greater than 4. Therefore, at room temperature, tetravalent ele-
ments do not exist as purely aquo complexes, even in strongly acidic
medium. For elements with a charge, z, smaller than 4, G becomes
negative only if the temperature is higher than 298 K. Therefore, it is
necessary to heat the solution in order to carry out hydrolysis of the
cation (forced hydrolysis or thermohydrolysis).
The monomeric, electrostatically charged, hydroxylated species
[M(OH) (H O) n h ] (z h) or [MO n h (OH) ] (2n z h) are generally observed in
2
h
h
solution only at very low concentrations. More often, they condense and
form soluble polynuclear species, polycations (h z), and polyanions (h z)
respectively, in which the cations are bounded by hydroxo or oxo bridges
[10]. These entities are generally of molecular size, although giant polyan-
ions of molybdenum containing up to 368 Mo ions have been recently syn-
thesized! [13] The condensation of neutral complexes with h z is, in
general, not limited and continues to the formation of a solid.
Hydroxylated complexes condense via two basic mechanisms of nucle-
ophilic substitution, depending on the nature of the coordination sphere
of the cations. In all cases, the driving force to the condensation is the
nucleophilicity of the hydroxo ligand. The cation must also have an elec-
trophilic character high enough to be subjected to the nucleophilic
attack. Condensation of aquohydroxo complexes proceeds by elimination
of water and formation of hydroxo bridges (olation):

|δ+ δ– |δ+ δ+ | |
H 2 O M OH + M OH 2 H O M OH M OH 2 + H 2 O
2
| | | |