illustrates the large effect of ionic strength on the stability of colloids
stabilized by electric charge.
Hydrodynamic Retardation. Smoluchowski assumed in the
derivation of his equations thatDpair¼D 1 þD 2 , but this is not true if the
diffusing particles are relatively close to each other. When two particles
come close, the liquid between them has to flow out of the gap, and this
means that (a) the local velocity gradient is increased and (b) the flow type
becomes ‘‘biaxial elongation’’ rather than simple shear (see Section 5.1).
Both factors cause the effective viscosity to be increased, which means in
turn that the mutual diffusion coefficient of the particles is decreased, the
more so as the particle separation (h) is smaller. The phenomenon is called
theSpielman–Honig effect.
FIGURE13.4 Idealized curves of the stability factorWin perikinetic aggregation
for small particles stabilized by electrostatic repulsion, as a function of the salt
concentrationm. Numbers near the curves denote the valence of the ions in the
solution. The dotted lines indicate the ‘‘critical coagulation concentrations’’mcr.