168 The solid-liquid interface
fa) Air
(b) Water(c)
Figure 6.10 Removal of dirt from a solid surface by detergent and mechanical
action^83 , (a) Surface covered with greasy dirt, (b) Water by itself fails to dislodge the
dirt mainly because of its high surface tension and inefficient wetting action, (c)
Detergent is added to the water. The hydrophobic parts of the detergent molecules line
up both on the dirt and on the solid surface, thus reducing the adhesion of the dirt to
the solid. The dirt may now be dislodged by mechanical action, (d) Dirt is held
suspended in the solution because the detergent molecules form an adsorbed layer on
the cleaned surface and around the dirt particles (By courtesy of The Scientific
American Inc.)