Chemistry Reference
In-Depth Information
4 Reversed-Phase Liquid Chromatography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187
4.1 Mobile Phase Solvent Effects and Small Molecule Retention Mechanisms . . ..... 188
4.2 Shape Selectivity in RPLC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196
5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199
1
Introduction
A multiscale method refers to a computational technique that spans multiple levels
of length (resolution) and/or timescales. Most commonly, the term refers to multi-
ple levels of theory being utilized in an energy or force calculation (e.g., a small part
of the system is treated using expensive electronic structure calculations, whereas
less expensive molecular mechanics force fields are used for the surrounding) or the
presence of multiple timesteps in a molecular dynamics simulation where forces
arising from short-range interactions are calculated more frequently than those
arising from long-range interactions. The term “multiscale” is also often used to
describe a set of separate calculations or simulations where one level provides
information used in the next level.
The main goal in a molecular simulation is to sample the important regions of the
statistical-mechanical phase space, i.e., those configurations that contribute most to
the thermodynamic averages for the system. Sampling inefficiencies occur when
these important regions of phase space are separated by free energy barriers, which
manifest themselves in long relaxation times. In a Monte Carlo simulation, special
(sometimes termed “unphysical” or biased) moves can be deployed that allow one
to hop over these kinetic barriers. As multiple different move types can be
employed in a Monte Carlo simulation, multiple timescales can also be accessed,
and the most efficient computation of equilibrium properties is achieved when the
vastly different timescale for different types of motion (e.g., bond stretching vs
transfer of a molecule over large distances) are merged into a common computa-
tional timescale. The Monte Carlo technique also allows for the use of open
ensembles where multiple phases (each handled in a separate simulation box) are
thermodynamically connected via special Monte Carlo moves but do not share a
direct interface. These phases may also represent different length scales, e.g., a
phase confined in a nanopore and a bulk phase.
In this review we discuss the application of multiscale Monte Carlo simulations
to explore various chromatographic systems. Chromatography is a collective term
for a set of techniques used for the separation of chemical mixtures. The common
theme in these techniques is that the mixture to be separated is dissolved in a mobile
phase which is passed through a stationary phase usually placed in an elongated
column. The separation of the mixture is based upon the differential partitioning of
the components in the mixture between the two phases. When a molecule has a
higher affinity for the stationary phase, it is retained in this phase longer and can be
separated from the less retained components that spend a relatively longer time in
the mobile phase. In gas chromatography (GC), the mobile phase is an inert gas and
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