Page 208 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soil Water
Soil Water 203

Figure 10.5
The finer the
particles the larger
the surface area.

Every surface has unsatisfied bonds that adsorb water and other molecules.
Colloidal suspensions are highly surface-active, which means that they are
sensitive to the presence of chemicals that can act as either dispersing agents or
flocculating agents.

10.7.2 Double-Layer Theory
The stability of suspensions remained somewhat of a mystery prior to the
development of double-layer theory, which was proposed in the nineteenth
century by von Helmholtz and later modified independently by Guoy and
Chapman. The theory is based on a supposition that a surface carries a negative
charge. This can be attributed to the large size and larger surface exposure of
oxygen ions in minerals, and to net charge deficiencies in expansive clay minerals
that are balanced with interlayer cations. In a suspension, negative surfaces
attract positive ions that are accompanied by hydration water as illustrated in
Fig. 10.3(d), so they do not approach closely enough to neutralize the surface
charge on the colloid. The combination of negative surface charge balanced by
cations that can approach but do not attach constitutes a diffuse double layer.This
concept is illustrated in Fig. 10.6.

There also are weak attractive forces between particles or molecules that are called
van der Waals attractions. These forces are relatively constant while the diffuse
double layer changes depending on the hydration, concentration, and electrical
charges of the cations. Therefore by juggling cations in the diffuse double layer a
colloid can be ‘‘stabilized,’’ that is, remain dispersed, or it can become flocculated.
Flocculation is the preferred attribute in soils used for engineering purposes,
although dispersion is utilized in particle size analysis.

A high salt concentration or more highly charged positive ions such as Ca 2þ
compress the double layer and cause flocculation, whereas lower charged and

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