Chemistry Reference
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the use of polymeric, as opposed to monomeric, bonded phases. The effect of
grafting density will be examined here.
To discern the molecular causes of shape selectivity in some traditional RPLC
systems, GEMC/CBMC simulations were carried out with a water/acetonitrile
mobile phase containing
67 mol% acetonitrile and a stationary phase with ODS
chains at surface coverages of 1.60, 2.88, and 4.15
mol/m
2
at a temperature of
308 K [
40
]. In addition to the stationary and mobile phase entities, numerous PAH
analytes were present ranging in size from benzene up to the four ring isomers
shown in Fig.
7
. Although the focus of this study was the larger PAH analytes since
they exhibit the largest shape effects, the smaller PAH analytes are needed as
intermediates for the identity exchange moves mentioned in Sect.
2
. It would be
extremely difficult to sample the spatial distribution of the large PAH analytes
by traditional means. However, by “growing in” these large molecules starting
from a small molecule like benzene, a much higher sampling efficiency can be
achieved [
40
].
As mentioned above, an important aspect of simulation is validation against
experimental results. For this purpose, the selectivities between the five different
four-ring isomers (see Fig.
7
) computed from simulation are compared to experi-
ment [
71
] in Table
2
. The selectivity
m
for each PAH is computed as the ratio of its
capacity factor
k
0
to the capacity factor for triphenylene, the least retained of the
four-ring isomers:
a
k
0
x
=
k
0
Triphenylene
:
a
x
¼
(4)
The capacity factor can be determined experimentally from retention data and is
exactly equivalent to the average number of analyte molecules in the stationary
phase divided by the average number in the mobile phase. Thus, this quantity is
easily measured in a GEMC/CBMC simulation.
As can be seen in Table
2
, the selectivities computed from simulation are in
excellent agreement with experiment. This agreement is a testament to the precision
of the simulation method and the accuracy of the force field. For example, the
selectivity between chrysene and triphenylene at a coverage of 2.8
mol/m
2
m
is
Table 2 Simulated selectivities of C
18
H
12
PAH isomers relative to triphenylene compared to
experiment at different surface coverages
a,b
Benzo[
c
]
pyrene
Coverage (
m
mol/m
2
) Naphthacene Chrysene Benz[
a
]-anthracene
Simulation
4.15
1.56
16
1.14
08
1.16
10
1.02
04
Experiment
4.20
1.70
1.13
1.13
1.08
Simulation
2.88
1.27
14
1.12
08
1.07
09
1.06
05
Experiment
2.84
1.58
1.08
1.12
1.06
Simulation
1.60
1.11
10
1.12
09
1.01
11
1.06
10
Experiment
1.60
1.19
0.97
1.02
1.02
a
Subscripts indicate the standard error of the mean in the final two digits
b
Experimental data from Sentell and Dorsey [
71
]
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