Geoscience Reference
In-Depth Information
presence of an electrolyte introduces a localized perturbation of the ''tetrahedral
configuration.'' This perturbation derives from several sequences. Near the ion,
water molecules are dominated by a dense electromagnetic field, resulting in the
formation of the primary solvation shell. In the next zone, called the secondary
solvation shell, water molecules interact weakly with the ion. An example of such
behavior of an electrolyte solution near a clay surface was discussed by Sposito
(
1984
). It was shown that the primary solvation shell of a monovalent cation
contains between three and six water molecules that exchange relatively rapidly
with the surrounding bulk liquid. A secondary solvation shell, if it exists, is very
weakly developed. The primary solvation shell of a bivalent cation contains
between six and eight water molecules, which exchange rapidly with the sur-
rounding bulk liquid. A secondary solvation shell containing about 15 water
molecules develops as the cation concentration decreases, and it also moves with
the cation as a unit.
The configuration of near solid phase water can be altered in close proximity to
the phyllosilicate. The siloxane surface influences the character of the water due to
the nature of their charge distribution and the complexes formed between the
cation and the surface functional groups. Both the type of charge and degree of
charge localization, as well the valence and size of the complexed cations, control
the characteristics of the water molecules near the surface.
Clay minerals with their own surface properties affect the near-surface water in
different ways. The adsorbed water in the case of kaolinite consists only of water
molecules (''pure'' water), whereas water adsorbed on a smectite-type mineral is
an aqueous solution, due to the presence of exchangeable cations on the 2:1 layer
silicate. Sposito (
1989
) noted the generally accepted description that the spatial
extent of adsorbed water on a phyllosilicate surface is about 1 nm (two to three
layers of water molecules) from the basal plane of the clay mineral.
The interlayer water of clay minerals is structurally different from bulk liquid
water or water in aqueous solution (Sposito and Prost
1982
), and chemical reac-
tions in this region are affected by a perturbed water structure. Another ability of
the interlayer water is to diffuse widely over surface oxygens (e.g., smectites); a
strong hydrogen bonding to the layer is not essential. The same patterns also
follow when the cation is large and univalent. Under these conditions, a much less
polar liquid can completely replace the interlayer water (Farmer
1978
). Water is
retained on oxides and hydroxides of aluminum and iron through the hydroxyl
groups, but it can also involve oxide bridges and water coordinated with structural
cations.
Retention of water on organic surfaces also occurs at a molecular level. Farmer
(
1978
) stated that the principal polar sites where water adsorption occurs are likely
to include carboxyl groups, such as phenolic and alcoholic groups, oxides, amines,
aldehydes, and esters. Ionized carboxyilic groups and their associated cations are
likely to have the greatest affinity for aqueous solutes. Recall that, in addition to
polar sites, organic surfaces exhibit important hydrophobic regions, which are
involved largely in the retention of organic contaminants.
