92168.pdf

The solid-liquid interface 167

action yet are poor foaming agents, and the psychological tendency to

correlate these two properties has somewhat restricted their accept-

ance for household usage.

Redeposition of dirt

Redeposition of dirt can be prevented by the charge and hydration

barriers which are set up as a result of detergent molecules being

adsorbed on to the cleaned material and on to the dirt particles. Since

the substrate and dirt surfaces tend to be negatively charged (see

page 175), anionic detergents tend to be more effective than cationic

detergents. Non-ionic detergents are also effective in this respect as a

result of strong hydration of the poly(ethylene oxide) chains. Mixed

anionic plus non-ionic detergents usually out-perform anionics alone.

The most successful detergents are those forming micelles, and this

originally led to the opinion that micelles are directly involved in

detergent action, their role probably being that of solubilising oily

material. However, detergent action is dependent upon the concen-

tration of unassociated surfactant and practically unaffected by the

presence of micelles (other than as a reservoir for replenishing the

unassociated surfactant adsorbed from solution). It appears, therefore,

that the molecular properties of surfactants associated with good

detergent action also lead to micelle formation as a competing rather

than as a contributing process^2.

Detergent additives

It is general practice to incorporate 'builders', such as silicates,
pyrophosphates and tripolyphosphates, which are not surface-active
themselves but which improve the performance of the detergent.
Builders fulfil a number of functions, the most important being to
sequester (form soluble non-adsorbed complexes with) Ca2+ and
Mg^24 " ions and act as deflocculating agents, tTius helping to avoid
scum formation and dirt redeposition. The builders also help by
producing the mildly alkaline conditions which are favourable to
detergent action. Alternatives to phosphates are being sought for
environmental reasons.
Sodium carboxymethyl cellulose improves detergent performance
in washing textile fabrics, particularly cotton, by forming a protective
hydrated adsorbed layer on the cleaned fabric which helps to prevent