Environmental Engineering Reference
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Fig. 8
The disjoining pressure
ʠ
of a simple fluid made up of monomers obtained from
GCMC-DPD simulations of the fluid confined by linearly decaying wall forces, as a function
of the distance separating the surfaces. The
inset
shows a schematic diagram of the system model
and how it is calculated. The
top
part of the figure illustrates how the maxima and minima are due
to the arrangement of the fluid's molecules. The axes are shown in reduced units. Adapted from
Gama Goicochea (
2007
)
optimization of known formulations, it is usually required to determine the amount
of material (polymers, surfactants) that needs to get adsorbed on the surfaces of
the colloidal particles. This is traditionally determined from adsorption isotherms,
which can take several weeks to measure and interpret. By contrast, the adaptability
of computer simulations allows one to calculate adsorption isotherms in a relatively
short time, having full control of the thermodynamic variables. For these purposes it
is crucial to perform the calculations at constant chemical potential (and at constant
volume and temperature). Because of the mesoscopic reach of the DPD model and its
success in predicting the behavior of polymers in solution and also confined fluids,
it becomes a well suited tool for the study of adsorption of polymers on colloidal
particles.
Figure
9
shows adsorption isotherms obtained with GCMC-DPD simulations for
two cases. In one case, where the surfaces were implemented as Lennard-Jones (9-3)
forces to represent alumina surfaces (
Al
2
O
3
) that are known to have hydrophilic
character (Esumi et al.
2001
). In the other case, the walls were modeled as soft
DPD walls (see Eq.
13
). In order to represent the hydrophobic nature of silica (
SiO
2
)
surfaces. The fluid confined by these walls is composed of solvent monomers and a
varying number of
PEG
molecules, which are modeled as linear chains with
N
=
7
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