Waals forcesnearly always cause attraction. The interaction depends on the
materials involved, which dependence is given by the Hamaker constant;
besides the material of the particles, that of the fluid between them is
involved. Generally, the attraction across air is much stronger than that
across water. Besides, the geometry of the system affects the interaction. It is
strongest forh&0, and is inversely proportional toh(spherical particles) or
toh^2 (platelets).
Repulsive forcesare generally caused by substances adsorbed onto the
particles, as illustrated in Figure 12.12. In all cases, the interaction force
increases with increasing surface excess (number of adsorbed molecules per
unit interfacial area).
Electric charge on the surface induces anelectric potential. Counter-
ions accumulate near the surface to neutralize the charge, and they thus
form an electric double layer. The potential thus decreases when going away
from the surface. If the particles become close, their double layers start to
overlap, causing a locally increased osmotic pressure, hence a repulsive free
energy acts, trying to drive the particles apart. In practice, the repulsion
increases with increasing surface charge (as affected, e.g., by pH), and the
range over which the repulsion acts decreases with increasing ionic strength.
In theDLVO theory, van der Waals attraction and electrostatic repulsion
are added to calculate the totalV(h), from which the stability of the system
can be predicted.
FIGURE12.12 Illustration of materials being adsorbed onto fluid interfaces (O–W
or A–W), thereby causing repulsion between two of such interfaces. (a) Anions, (b)
soaps, (c) Tween-like surfactants, (d) neutral polymers, (e) proteins. Highly
schematic; the straight lines represent aliphatic chains. The repulsion is electrostatic
(a and b), steric (c and d), or mixed (e).