Abstract
A broad range of manufactured products and biological fluids are colliods.
The ability to understand and control the processes (of scientific,
technological and industrial interest) in which such colloids are
involved relies upon a precise knowledge of the electrical double
layer. The traditional approach to describing this ion cloud around
colloidal particles has been the Gouy-Chapman model developed on
the basis of the Poisson-Boltzmann equation. Since the early 1980s,
however, more sophisticated theoretical treatments have revealed
both quantitative and qualitative deficiencies in the Poisson-Boltzmann
theory, particularly at high ionic strengths and/or high surface
charge densities. This review deals with these novel approaches,
which are mostly computer simulations and approximate integral equation
theories based on the so-called primitive model. Special attention
is paid to phenomena that cannot be accounted for by the classic
theory as a result of neglecting ion size correlations, such as overcharging,
namely, the counterion concentration in the immediate neighborhood
of the surface is so large that the particle surface is overcompensated.
Other illustrative examples are the nonmonotonic behavior of the
electrostatic potential and attractive interactions between equally
charged surfaces. These predictions are certainly remarkable and,
on paper, they can have an effect on experimentally measurable quantities
(for instance, electrophoretic mobility). Even so, these new approaches
have scarcely been applied in practice. Thus a critical survey on
the relevance of ion size correlation in real systems is also included.
Overcharging of macroions can also be brought about by adsorption
of oppositely charged polyelectrolytes. Noteworthy examples and theoretical
approaches for them are also briefly reviewed.
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