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 187

GEMC/CBMC simulations were also successfully applied to investigate the
contribution of adsorption at the mobile/stationary phase interface (here the station-
ary phase is represented as a two-dimensional film) [ 22 ] and the influence of analyte
overloading (here the analyte concentration is sufficiently high to result

in

departures from Henry's law behavior) [ 23 ] in gas-liquid chromatography.

4 Reversed-Phase Liquid Chromatography

RPLC is similar in principle to GLC in that both techniques rely on the differing
affinities of analyte molecules for the mobile and stationary phases to enact a
separation. The distinct difference between RPLC and GC is complexity. In
RPLC, the stationary phase typically consists of dimethyl octadecyl silane chains
grafted to the surfaces of highly porous silica particles and the mobile phase is a
binary solvent containing water and an organic modifier, most commonly methanol
or acetonitrile. The complex interplay between the stationary phase, solvent, and
analyte has made a molecular-level description of RPLC extremely difficult.
Before focusing on the application of Monte Carlo techniques to RPLC systems,
we would like to highlight some important contributions that employed the molec-
ular dynamics approach. In 1994, Schure investigated the bonded-chain conforma-
tion and solvent structure in an alkylsilane RPLC system [ 24 ]. A few years later,
Klatte and Beck [ 25 ] and Slusher and Mountain [ 26 ] provided quantitative data on
the transfer of a methane solute from the mobile phase to the stationary phase in
model RPLC systems. More recently, Zhao and Cann [ 27 ] investigated chiral RPLC
phases and obtained qualitative data on the retention of ten analytes, Fouqueau et al.
[ 28 ] investigated the adsorption of acridine orange at the mobile/stationary-phase
interface in an RPLC system, and Melnikov et al. [ 29 ] probed the influence of
residual (protonated and deprotonated) silanol groups on solvent and ion distribu-
tion in an RPLC system.
Over the past 6 years we have used the GEMC/CBMC methodology to provide
much needed molecular-level insight into the stationary-phase structure and the
retention mechanism in various RPLC systems. For alkylsilane stationary phases
we have investigated the effects of mobile phase composition for water/methanol
and water/acetonitrile mixtures [ 30 - 34 ], of alkylsilane coverage [ 35 , 36 ], of
alkylsilane chain length [ 37 , 38 ], and pressure and pore shape [ 37 ]. In addition,
the effects of embedded polar groups (ether and amide) have been explored [ 39 ]. To
illustrate some of the insights that can be gleaned from the GEMC/CBMC
simulations, this review will focus on mobile-phase effects for water/acetonitrile
mixture on bonded-phase structure and retention of small analyte molecules
(a more complete discussion can be found in [ 34 ]), and briefly mention studies
involving the retention of large polycyclic aromatic hydrocarbon (a more complete
discussion can be found in [ 40 ]).