Page 235 - Introduction to Colloid and Surface Chemistry

224 Colloid stability

measured attractive forces were consistent with those predicted by
theory, provided that retardation was allowed for.
Recently, measurements in the non-retarded range have been
95 108
made, the most notable being those of Tabor and co-workers ' , on
the attraction between cleaved layers of mica stuck to two crossed
cylinders. In addition to providing successful tests of the distance
dependence of the van der Waals attraction, the effects of adsorbed
monolayers have also been studied, again giving reasonable agree-
ment with theoretical predictions.

Structural interactions

Both Hamaker and Lifshitz theories of van der Waals interaction
between particles are continuum theories in which the dispersion
medium is considered to have uniform properties. At short distances
(i.e. up to a few molecular diameters) the discrete molecular nature
of the dispersion medium cannot be ignored. In the vicinity of a solid
surface, the constraining effect of the solid and the attractive forces
between the solid and the molecules of the dispersion medium will
cause these molecules to pack, as depicted schematically in Figure
8.5. Moving away from the solid surface, the molecular density will
show a damped oscillation about the bulk value. In the presence of a
nearby second solid surface, this effect will be even more pronounced.
The van der Waals interaction will, consequently, differ from that
expected for a continuous dispersion medium. This effect will not be
significant at liquid-liquid interfaces where the surface molecules can
overlap, and its significance will be difficult to estimate for a rough
solid surface.
Israelachvilli and co-workers 95 have measured directly the forces
between molecularly smooth cleaved mica surfaces separated by
organic liquids and have observed a corresponding periodicity of
force with separation. The extent to which these short-range
interactions (solvation or structural forces) may influence colloid
stability and other related phenomena is not entirely clear.

Particle adhesion 105

The adhesion of colloidal particles to solid substrates is of funda-
mental and technological importance (e.g. pneumatic transport of
powders, printing, filtration, detergency, air pollution). In general,