Brief Review of the Properties of
Before discussing surfactants and water-soluble products, a brief review of some
of the properties of water might be advisable. From a practical consideration, water is
used in chemical processing and in product formulations because it is abundant and cheap.
Several of the important properties of water can be summarised as follows:
- Water is a good solvent.
- Water has a relatively high boiling point (100°C) or 212°F at 760mm Hg).
- Water is stable.
- Water is a common reaction media for neutralisation and hydrolysis reactions.
These properties are based on water being strongly associated and
polar in nature. The polar nature of water is attributed to any
non-symmetrical molecule schematically drawn as follows:
Because of this structure, the oxygen end of the molecule is more negative than
the hydrogen end. This property makes water a good solvent for other polar and ionised
Water itself has only a slight tendency to ionise or split into oppositely
charged particles. Many acids, bases and salts, however, ionise readily in water solution.
Neutralisation is the reaction of ions of acids and bases generating a salt and usually
water. The process of ionisation is important in the preparation and classification of
surfactants, as will be discussed later. A positively charged particle is called a cation
and a negatively charged particle is called an anion. When the surface active portion of a
molecule is negatively charged, for example - the surfactant is classed as anionic.
Water is relatively stable chemically. It ionises only slightly, but will
hydrolyse or react with a number of materials. It is stable at very high temperatures.
Many reactions are catalysed by the addition of very small amounts of water, with
corrosion or rust being an outstanding example.
The water molecules in the liquid state are so strongly associated that a large
amount of energy in the form of heat is needed to break loose a molecule into the gas
state. As a result, the boiling point and heat of vaporisation of water are very high for
this low molecular weight material. Another property related to this internal attraction
or association of molecules in a liquid is called surface tension. Table One
shows the molecular weight, boiling point, and surface tension of several common
Physical Properties of Water Compared with
Those of Other Compounds
||Boiling Point °C @ 760mm. Hg
||Surface Tension @20°C Dynes per centimetre
|Hydrogen Sulphide (H2S)
(a) Not applicable
The molecule in the centre of a beaker of water is very strongly attracted to
all of its immediate neighbours, and the pull is equal in all directions.
The molecule on the surface, however, does not have any neighbours to speak of, in the air
or gas phase above. It therefore, is being pulled inward. The result is a force applied
across the surface like the skin pulled over a drum. The effect is defined as surface
tension. (See Figure One). The related effect is that the water tends to seek the
minimum surface area per unit of volume, or tends to form spheres of droplets.
Figure One Schematic Sketch of