92168.pdf

166 The solid-liquid interface

The action of the detergent is to lower -yDW and ysw, thus decreasing

WSD and increasing the ease with which the dirt particle can be

detached by mechanical agitation.

If the dirt is fluid (oil 6r grease), its removal can be considered as a

contact-angle phenomenon. The addition of detergent lowers the

contact angle at the triple solid-oil-water boundary. If B = 0, the oil

will detach spontaneously from the solid substrate. If 0 < 6 < 90°, the

oil can be removed entirely by mechanical means (Figure 6.9a); but if

90° < 0 < 180°, only part of the oil can be detached by mechanical

means and some will remain attached to the solid substrate (Figure

6.9b), A different mechanism, (e.g. solubilisation) is required to

remove this residual oil. Relating to this roll-up mechanism,

increasing the temperature has a marked effect on detergent

efficiency up to about 45°C (most fats melt below this temperature)

and little effect between about 45°C and just below the boiling point.

(]} HI :Waterz_~z_~: Detergent solution

(2) n;~Water~7~"L~~ jz Detergent solution~ z_~_z~

Figure 6.9 Detachment of oily dirt from a solid surface. The sequences (left to right)
show: (a) the substrate/dirt system in contact with pure water, (b) the lowering of
contact angle caused by detergent [ (1) 0 < 90°, (2) 8 > 90°], and (c) and (d)
mechanical (hydraulic) detachment of oil droplets

It can be seen that surfactants which adsorb at the solid-water and
dirt-water interfaces will be the best detergents. Adsorption at the
air-water interface with the consequent lowering of surface tension
and foaming is not necessarily an indication of detergent effectiveness;
for example, non-ionic detergents usually have excellent detergent