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Dr. André Moreira
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Simulating charged systems: the electric double layer
One of the rules-of-thumb of colloid and surface physics is that most
surfaces
(organic or inorganic) are charged when in
contact with water. This is the case, for instance, in
charge-stabilized colloidal
suspensions, where the surface of the colloidal particles are
generally highly charged,
monolayers of ionic surfactants at an air--water interface (where the
water-loving
head groups become charged by releasing counterions), or cell
membranes, which usually
contain charged phospholipids. Generally speaking, the problems above
can be described by one charged meso- or macroscopic object with a
surrounding
cloud of microscopic counterions. The latter form a so-called "double
layer".
These lectures will be devoted to some of the computational techniques
applied to study the equilibrium properties of charged systems,
with particular emphasis on the double layer problem. As we will see,
the main challenge in the simulation of these systems comes from the
long range character of the Coulomb interaction. We will introduce the
techniques used to circumvent this problem and
look at the behavior of the counterions when in the presence of one
charged wall (both when the charge at the wall is smeared out and when
it is formed
by discrete charges) and when the counterions are confined between two
equally
charged walls (interaction between two double layers). Although
the models used are quite simple, they show a rich behavior that
sometimes can only be captured by computer simulations.
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