Brief Review of the Properties of Water

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 chemicals.

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 chemicals.

Physical Properties of Water Compared with
Those of Other Compounds

Table One

(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 Surface Tension