Page 206 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soil Water
Soil Water 201

10.5.2 Polarity of the Water Molecule
Another important feature of the water molecule, aside from its tendency to
participate in hydrogen bonding, is that the arrangement of hydrogen ions
creates positive and negative ends to the molecule. Polarity makes water a good
solvent for ionic crystals such as NaCl, by weakening the electrostatic charges that
hold the crystal together. A measure of polarity is the dielectric constant, which is
high for water. Water also decreases the surface hardness of minerals such as
quartz that are only partly ionically bonded. For example, the scratch hardness of
quartz is lower in water than in air. This also applies to glass, which is
disorganized silica molecules, and is more easily scratched with a glass cutter if it
is wet.

10.5.3 Interaction with Cations
Water dipoles are attracted to ions in solution, particularly to positive ions that
are smaller than negative ions, where electrons in outer shells repel one another
and increase the ion diameter. This adsorption is illustrated in Fig. 10.3(c), where
negative ends of water molecules are attracted to a positive Ca 2þ ion in solution.
This kind of attraction also exists between water and cations in the interlayer
space of expansive clay minerals. Because Ca 2þ cations have a more positive hold
þ
on adsorbed water than single-valence Na cations, calcium-smectite is much less
expansive than sodium-smectite.

10.6 DIFFERENTIAL THERMAL ANALYSIS (DTA)

10.6.1 Overview
An early method for studying clay minerals involved measuring thermal delays
occurring in small samples as they are heated. For example, calories are required
to cook off free water at 1008C, which causes a lag in temperature. A clever
arrangement was devised to amplify these differences, heating an inert reference
sample and monitoring the difference in temperature between the test sample and
the reference. This is easily accomplished with two opposing thermocouples
attached in series so that a voltage difference occurs only when one is hotter than
the other. This is the scheme for differential thermal analysis or DTA.

10.6.2 Uses of DTA

While X-ray diffraction has replaced DTA for clay mineral identifications, the
reactions that occur during heating are an indication of the different energy levels
for different classes of soil water. This is illustrated in Fig. 10.4, where a large peak
above 1008C indicates the loss of hygroscopic water from a sample that previously
was air-dried. Such a reaction is described as ‘‘endothermic’’ because it takes heat.
This is followed by a smaller peak at higher temperature, from loss of water held

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