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 189

10 5 MC the system). Thereafter, the simulations proceeded with an additional 2

cycles during which averages were collected.
Snapshots from the simulations at each solvent composition are shown in Fig. 3 .
Although these snapshots represent only a single configuration of the millions
generated during the simulation, they convey a wealth of information about these
systems. First, it is observed that as acetonitrile concentration is increased there is
extension of the alkyl chains and greater amount of solvent sorbed into the
stationary phase. Furthermore, it appears that in the binary solvent systems most
of the solvent within the chain region is the organic modifier and not water. Also
appearing enriched in the organic modifier is the interfacial region between the
alkyl chains and the solvent.
These effects mentioned above can be quantified by examining the ensemble
averaged density profiles, also shown in Fig. 3 . These profiles show the density of
each component of the system as a function of z , or the distance from the silica
surface. As indicated by the solvent densities in the region z

5-15 ˚ , there is
essentially no solvent within bonded chains for system WAT. However, as the
acetonitrile concentration is increased there is a dramatic increase in solvent
penetration. For the binary solvents, most solvent in the alkyl chain region appears
to be the organic modifier and very little water is present.
In all systems examined, there are peaks in the solvent density around z

¼

3-5 ˚ .
These peaks are the result of solvent hydrogen bonded to the residual silanols present
on the silica surface [ 31 , 34 ]. There is a large decrease in the peaks as the fraction of
acetonitrile in the solvent is increased. It appears that water shows a strong

¼

1.2

WAT 33A 67A ACN

0.8

0.4

0

0 0 0
z

0 0 0
z

0 0 0
z

0 0 0 0
z [Å] [Å] [Å] [Å]

Total C18 ACN Water GDS Interfacial region

Fig. 3 Snapshots and density profiles of RPLC systems with varying water/acetonitrile ratios. In
the snapshots the stationary phase is shown as tubes with carbon in black , silicon in yellow , oxygen
in red , and hydrogen in white . The mobile phase is shown in the ball and stick representation with
carbon in cyan , nitrogen in blue , oxygen in red , and hydrogen in white . The analytes are shown as
large spheres with carbon in green , oxygen in red , and hydrogen in white . In the density profiles,
the Gibbs dividing surface (GDS) is a plane that defines the boundary between the mobile and
stationary phase [ 55 , 56 ] and the gray shaded area represents the width of the 10-90 interfacial
region [ 57 ] according to the total solvent density