92168.pdf

The solid-liquid interface 165

fibres), or a part of the body (skin, hair, teeth). The dirt may be

liquid or solid (usually it is a combination of both); it has many

possible origins (e.g. skin, food, the atmosphere); it may be polar or

non-polar; of small or large particle size; chemically reactive or inert

towards the substrate and/or the detergent. In view of the wide

variety of possible substrate-dirt systems, the extent to which a

general theory of detergent mechanism can be developed is limited.

Moreover, when it comes to the formulation of detergents for various

types of usage, the situation is even more complex, since performance

tends to be judged by criteria which are not wholly related to dirt

removal.

Wetting

The wetting of fabrics, as such, is not a critical issue in detergency,
since the critical surface tension, yc, of fabric surfaces is usually in
excess of 40 mN m"^1 and it is an easy matter to reduce the surface
tension of the aqueous bath to below this value. The rate of diffusion
of surfactant into porous fabric, however, is important and the choice
of surfactant involves a compromise between a small hydrocarbon
chain length for rapid diffusion and a longer hydrocarbon chain
length for better dirt removal and dispersion characteristics. For alkyl
sulphates and alkyl-aryl sulphonates, a chain length of about C!2
usually gives the best all-round performance in this respect^82.

Dirt removal

The removal of solid dirt can be considered in terms of the surface-
energy changes involved. The work of adhesion between a dirt
particle and a solid surface (Figure 6.8) is given by

= Tow + Tsw ~ (6.13)

'SD

DW

Figure 6.8