Introduction to Surfaces and Colloids - Surface and Colloid Chemistry: Principles and Applications - page 3

phase. The nearer the molecule is to the surface, the greater the magnitude of the
force due to asymmetry . The region of asymmetry plays a very important role. Thus,
when the surface area of a liquid is increased, some molecules must move from the
interior of the continuous phase to the interface. The surface tension of a liquid is
the force acting normal to the surface per unit length of the interface, thus tending to
reduce the surface area. The molecules in the liquid phase are surrounded by neigh-
boring molecules, and these interact with each other in a symmetrical way. In the
gas phase, where the density is 1000 times lesser than in the liquid phase, the inter-
actions between molecules are very weak as compared to those in the dense liquid
phase. Thus, when we pass from the liquid phase to the gas phase, there is a change
in density of factor 1000. This means that, while in the liquid phase, a molecule
occupies a volume that is 1000 times smaller than when it is in the gas phase.
Surface tension is the differential change of free energy with change in surface
area. An increase in surface area requires that molecules from the bulk phase are
brought to the surface phase. This is valid when there are two fluids or a solid and a
liquid, and it is usually designated interfacial tension .
A molecule of a liquid attracts the molecules that surround it, and, in its turn,
it is attracted by them (Figure 1.2). For the molecules that are inside a liquid, the
resultant of all these forces is neutral, and all of them are in equilibrium by react-
ing with each other. When these molecules are on the surface, they are attracted
by the molecules below and by the lateral ones, but not toward the outside. The
resultant is a force directed inside the liquid. In its turn, the cohesion among the
molecules supplies a force tangential to the surface. So, a fluid surface behaves like
an elastic membrane that wraps and compresses the liquid below. The surface ten-
sion expresses the force with which the surface molecules attract each other. It is
common observation that, due to the surface tension, it takes some effort for some
bugs to climb out of the water in lakes. On the contrary, other insects, such as the
marsh treaders and water striders, exploit the surface tension to skate on the water
without sinking (Figure 1.3).
Another well-known example is the floating of a metal needle (heavier than water)
on the surface of water (Figure 1.4.) The surface of a liquid can thus be regarded as
the plane of potential energy. It may be assumed that the surface of a liquid behaves
as a membrane (at a molecular scale) that stretches across and needs to be broken in
order to be penetrated. One observes this tension when considering that a heavy iron
needle (heavier than water) can be made to float on the water surface when carefully
placed (Figure 1.4).
The reason a heavy object floats on water is because in order to sink, it must over-
come the surface forces. This clearly shows that, at any liquid surface, there exists a
tension ( surface tension ) that needs to be broken when any contact is made between
the liquid surface and the material (here the steel needle). A liquid can form three
types of interfaces:

1. Liquid and vapor or gas (e.g., ocean surface and air)

2. Liquid 1 and liquid 2 immiscible (water-oil, emulsion )

3. Liquid and solid interface (water drop resting on a solid, wetting, cleaning
of surfaces, adhesion).