Diverse Applications of Surface and Colloid Chemistry in Science and Industry - Surface and Colloid Chemistry: Principles and Applications - page 226

to reduce loss of life much more than in early wars. The powder is made of porous
mineral zeolites. It is mostly surface chemical reactions that determine their func-
tion. Currently, much research is being carried out on these treatment applications.
10.5.2.2.1 Nucleation and New Phase Formation
When any liquid is cooled below its freezing point, it would be expected to simply
freeze and form a solid phase. However, this phase transition is not so simple in real-
ity. A liquid phase, on cooling below the freezing point, will change to a solid phase,
or, a gas phase, on cooling, would change over to a liquid phase at the appropriate
temperature and pressure. The changing of water vapor to liquid may well be a cen-
tral concern to humans as adequate fresh water sources become scarcer.
However, one finds that, in cooling a liquid below its freezing point, the liquid
may not always turn into solid phase at the freezing point. In fact, in some cases,
such as water, even at around −40°C, liquid water does not turn into a solid phase.
It stays in what is called a supercooled state. A major phenomena is the freezing of
supercooled clouds. However, if certain so-called nucleating agents are used, then
the clouds would turn into liquid droplets (and form rain). The nucleation process is
a surface phenomena and is observed in transitions from
Gas to liquid
Liquid to solid
10.5.2.2.2 Electrokinetic Systems
The variation of charge potential near an interface induces many unique proper-
ties in charged particles (such as colloids, emulsion drops, solid-liquid interfaces).
The electro double layer (EDL) at the interface of a solid and liquid was described
earlier. Electric fields can be used to generate bulk fluid motion (electroosmosis) and
to separate charged species (molecules and particles). Most solid surfaces acquire a
net charge when brought into contact with an electrolyte solution. This happens, for
instance, on the surface of material such as silica and glass. This net charge attracts
nearby ions of opposite charge (i.e., positive ions) and, at the same time, repels ions
of like charges. The typical thickness of this region (called the Stern layer) is 1 to 20
nm (Figure 10.5).