Molecular Simulations of Retention in Chromatographic Systems: Use of Biased Monte Carlo Techniques to Access Multiple Time and Length Scales - Multiscale Molecular Methods in Applied Chemistry - page 186

Figure 2 illustrates the ability of GEMC/CBMC simulations using the TraPPE
force field [ 3 , 16 - 18 ] to predict (with some exceptions) rather reliable Kovats
indices for various combinations of analytes and stationary phases. It should be
noted here that the GEMC/CBMC methodology affords computations of Kovats
indices of multiple analyte molecules in the same simulation box with a statistical
uncertainty (standard error of the mean) of less than 10 Kovats units, an impressive
achievement when one considers that the computation of a Kovats index requires
partition constants (or transfer free energies) of three analytes. The retention
order and Kovats indices of six branched alkanes (2-methylpentane, 3-
methylpentane, 2,2-dimethylpentane, 3-ethylpentane, 2,5-dimethylhexane, and
3,4-dimethylhexane in order of increasing I ) were correctly reproduced [ 19 ], i.e.,
the simulations are able to pick up differences caused by displacing a methyl branch
along the backbone or by replacing two methyl branches with a single ethyl branch.
Benzene and toluene are more retained in a squalane phase than n -hexane and
n -heptane, respectively. This separation involves a subtle balance of enthalpic
contributions (larger polarizability of arenes, but lack of significant quadrupole-
multipole interactions for arene/squalane pairs) and entropic contributions (smaller
partial molar volume for benzene vs n -hexane). The data in Fig. 2 illustrate that the
TraPPE force field with explicit quadrupole for arenes yields accurate predictions
for the Kovats retention indices of benzene and toluene at 323 and 383 K where the
temperature increase results in an increase of the I values by about 20 Kovats units
[ 20 ]. Figure 2 also depicts the I values for benzene, toluene, and the three xylene
isomers in a polyethylene oxide phase. In this case, the quadrupole-dipole
interactions result in stronger retention for arenes with I values close to those of n -
alkanes with four additional carbon atoms [ 21 ]. Although the simulations predict the
correct retention order for alcohols (methanol, 2-propanol, ethanol, 1-propanol, and
1-butanol in order of increasing I values), it consistently overpredicts the I values by
about 100 Kovats units; this is an indication that the TraPPE force field overestimates
the strength of hydrogen bonds between alcohols and the polyether [ 21 ].

1200

1000

800

Fig. 2 Predicted Kovats
retention indices for various
analytes in gas-liquid
chromatography with
squalane and polyethylene
oxide stationary phases. See
text for description of the
analyte molecules

alkane/squalane
arene/squalane
arene/PEO
alcohol/PEO

600

600 800 1000 1200

I exp