MECHANISMS OF DETERGENCY 365 SUSPENDING ACTION This factor is of vital importance in laundering, and in textile processing in general, where detergent liquors are operated close to the economic limit. It is frequently found that a detergent will remove the dirt but will subsequently allow a portion to be re-adsorbed on to the surface. In serious cases this may be due to loss of detergent by adsorption and absorption by the surface being cleaned, within the material being cleaned (cf. the high/on exchange capacity of wool and human hair), and by the dirt (both surface adsorption and complex formation). If sufficient deter- gent is present to overcome such losses, polar soiling matter, such as fatty acids and alcohols, will finally be effectively solubilised or held within a complex phase. Much non-polar material (e.g. mineral oil) will be removed by the rolling-up process, spontaneously emulsified, etc., when in admixture with polar oils and fats. Later on, such polar material may separate from the oil phase into the aqueous phase by preferential solubilization, or in the case of fatty acids in contact with alkaline detergent solutions, by neutralisation. This will leave the oil drop in a condition to rewet any suitable surface with which it may collide. Pigment or particulate soiling matter, as already mentioned, is often held by wetting forces arising from greasy and oily matter and the detergency processes are essentially those of the oily phase. However. just as this phase is removed from the sub- strate, so it is also removed from the pigment and the final detergent liquor will comprise an emulsion of oil droplets, a solution of solubilized polar oil plus some non-polar oil, a dispersion of some complex material, and finally, a dispersion of pigment matter. The latter will be free to re-adsorb on to any surface with which it collides with sufficient vigour. DEFLOCCULATION The ability of a detergent to maintain dirt in suspension is usually gauged in terms of its performance with respect to pigment matter. The over-riding factor controlling this is the zeta potential of the surfaces of the substrate and dirt. (An additional factor is considered by some to be deposition of a "protective colloid" film of finite thickness, acting as a barrier between the dirt particle and the substrate). As detergent molecules adsorb onto a surface, the zeta potential of the double layer changes accord- ing to the nature of the adsorbing species. Organic surfaces are inherently negative in charge, possibly due to stray carboxyl groups, over a wide range of pH values. Anionic detergents increase this charge to a marked extent •3, whereas cationic surface active agents reduce and then reverse. the charge. Non-ionic detergents are similar to, although not as effective. as, anionic detergents (due to dipole orientation). The formation of a strong zeta potential at the surfaces in contact with the detergent solution

JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS acts as a repulsive barrier and restrains the redeposition of the pigment matter. However, if the two surfaces approach to within a certain distance by direct force or by virtue of an activation in the Arhenius sense during normal Brownian motion, the double layer barrier will be passed and the particle will adsorb on to the surface by London attraction. (Kruyt •4 describes this type of system in considerable detail.) Mechanical agitation as well as assisting removal of dirt also tends to assist this type of re- deposition by increasing the collision rate (cf. butter-churning). The presence of non-surface active ions tends to reduce the zeta potential and lower the suspending power of a detergent. Likewise, increased temperature increases the water affinity of the surface active species, lowers the adsorption, and consequently the surface potential, quite apart from increasing the vigour of Brownian motion. FOAM Lastly, some reference must be made in the context in which this paper is published, to the effect of foam or lather in detergency. McBain 15 once wrote that "soap and beer are popularly appraised by the amount and stability of the foam they produce". Unfortunately, it is now fairly well agreed that foam plays a very minor role indeed. One can add tributyl phosphate to soap to kill the foam (or indeed many immiscible liquids or emulsions 16) without impairing its efficiency as a detergent. Conversely saponin, which foams well, has little detersive power. Niven •7 has attempted to draw an analogy between the froth flotation process for separating minerals and the action of foam. (In this process mineral particles adhere, due to imperfect wetting, to air bubbles and are carried to the surface.) soup Figure 11 However, this process relies on the use of additives to make the mineral surface hydrophobic, whereas detergents have the opposite effect and above the critical miceIlar concentration •8 the contact angle usually becomes zero,