Chemistry Reference
In-Depth Information
the stationary phase is either a solid or a liquid film adhering to the inside of a
capillary tube. In reversed-phase liquid chromatography (RPLC) the mobile phase
is an aqueous/organic solvent and the stationary phase is most often composed of
alkyl chains chemically bonded to the surfaces of highly porous silica particles.
2 Multiscale Monte Carlo Techniques
To enable the simulation of chromatographic systems, a combination of two Monte
Carlo techniques that allows for the sampling of processes across different length
and timescales is used. These are the Gibbs ensemble Monte Carlo (GEMC) and the
configurational-bias Monte Carlo (CBMC) methods. The Gibbs ensemble [
1
,
2
]
involves the concurrent simulation of multiple phases that are thermodynamically
coupled through special Monte Carlo moves, but do not require consideration of
explicit interfaces between these phases. One of the special GEMC moves allows
for direct particle transfers (as opposed to slow diffusive processes) between two or
more separate phases, i.e., the molecule may transfer from one bulk phase to
another bulk phase without having to encounter an interface. Nevertheless, the
GEMC technique can also be used to connect phases that exhibit different charac-
teristic length scales (e.g., a nanoporous sorptive phase and a bulk solution phase)
and may involve explicit interfaces. Such a set-up is used in our Monte Carlo
simulation of RPLC systems. As illustrated in Fig.
1
, these GEMC simulations use a
three-box setup. The first simulation box contains the stationary phase and is
elongated in the
z
-direction with
L
x
¼
90.0
˚
. The
20.0,
L
y
¼
26.0, and
L
z
¼
center of this box contains a five layer slab of
-cristobalite with its two (1 1 1)
surfaces exposed. To these surfaces, octadecylsilane chains (ODS) are grafted. In
contact with this stationary phase, and connected through the periodic boundaries
used in the simulations, is the mobile phase solvent sorbed in the pore. This box is
periodically replicated in all three dimensions. Thus, the setup in this first box
b
Box 1:
Stationary phase in contact with solvent
90 Å
Box 2:
Bulk solvent
Box 3:
Vapor phase
Fig. 1 Three-box Gibbs
ensemble simulation setup
~30 Å
~100 Å
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