Environmental Engineering Reference
In-Depth Information
them wettable. The mode of action of these compounds is complex, and
understanding it requires analysis of forces associated with the behaviour
of water molecules. Besides gravity, forces of cohesion, which attract
water molecules to each other, and forces of adhesion, which
attract water molecules to other substances, play deciding roles in which
surfaces can and which cannot be wetted. The effects of these forces can
be illustrated by placement of a drop of water on a piece of filter paper
and another drop on a piece of waxed paper. On the filter paper, the force
of adhesion between the water molecules and the paper molecules is
greater than the force of cohesion that holds the water molecules to-
gether. As a result, the water droplet spreads out and soaks into the
paper. On the waxed paper, the water droplet remains on the surface
because water molecules cohere more strongly to each other than to the
wax. In this case adhesive forces between water molecules and the
surface are weaker than the cohesive forces between water molecules.
A surface with these characteristics repels water and is called hydropho-
bic. Hydrophobic characteristics are found in many types of soils covered
by turf-grass, and appearance of hydrophobic localized dry spots, espe-
cially on golf course greens and other areas mowed low, is quite com-
mon. In numerous studies, microscopic analyses of soil particles taken
from these spots have revealed that they are coated with substances that
repel water, much like a wax. The coating material appeared to be a
complex organic compound produced by a fungus. The localized dry
spots become a serious turf-management problem, especially during
periods of drought. Despite frequent irrigation, the soil in these spots
resists wetting, resulting in patches of dead or severely wilted turf. The
water applied wets the turf but does not adequately penetrate the soil
surface to reach the root zone.
The use of wetting agents on water-repellent soils can improve their
ability to absorb water and thus improve the appearance of the turf
(Fig. 7.12). Long-term residual surfactants actively reduce forces of
water cohesion in the soil for a period of 6-12 months and should be
Strong forces of cohesion between
water molecules prevent wetting of a
hydrophobic surface.
Wetting agents reduce the forces of
cohesion between water molecules,
allowing them to wet a hydrophobic
surface.
Water droplet
Hydrophobic surface
Hydrophobic surface
Fig. 7.12. Wetting agents reduce the effect of soil hydrophobicity.
