(775e) The Importance of Intra- and Inter-Molecular Interactions in the Development of Coarse-Grained Models for Chain Fluids Using SAFT-? Mie in Molecular Simulations | AIChE

(775e) The Importance of Intra- and Inter-Molecular Interactions in the Development of Coarse-Grained Models for Chain Fluids Using SAFT-? Mie in Molecular Simulations


Rahman, S. - Presenter, Imperial College
Braga, C., Imperial College London
Lobanova, O., Imperial College London
Raptis, V., Imperial College London
Galindo, A., Imperial College London
Jackson, G., Imperial College London
Muller, E. A., Imperial College London


In common coarse-graining (CG) approaches one starts from an atomistic,
bottom-up, description of molecules and proceed to integrate degrees of
freedom, simplifying the force field in order to speed up the computer
simulation of complex molecules and mixtures. [1-4] By contrast, the CG
intermolecular potentials can also be obtained with a top-down approach by
reproducing the macroscopic configurational properties of the system with a
molecular-based equation of state. The latter approach is considered here, in
particular, the use of the SAFT-γ Mie version of the Statistical
Associating Fluid Theory (SAFT) which is based on interactions of the Mie form
and has successfully been applied to simple organic molecules [5-7]. In our
current work we explore the role of inter- and intra-molecular interactions in
SAFT-γ Mie force fields of the linear alkane homologous series.


Despite its success in correctly reproducing the thermodynamic
properties of complex fluids, SAFT based approaches do not take into account
the structure and flexibility of molecular systems explicitly, apart from the
assumption that chains of m segments
are formed [8]. We have addressed this issue by combining the SAFT-γ Mie
top-down approach with a parameterization of the CG intramolecular (bonding)
potentials on the basis of atomistic simulations.

As prototypical molecules we first perform the analysis for long chain n-alkanes. In order to obtain a CG
model, three consecutive backbone carbon atoms and their bonded hydrogen's are
all grouped together to form a "super-atom", or CG moiety.  The inter-molecular parameterization of
the CG beads with the SAFT-γ Mie methodology then leads to two
possibilities, depending on the recognition (or not) of the distinct identity
of the terminal end CH3 group. In the first model (homonuclear) the
beads representing the end and intervening CG alkyl groups are taken as
identical; in the second model (heteronuclear) a distinction is made between
the interactions of the two types of CG beads. (Fig 1)


Figure 1 Schematic of the CG strategy for n-nonane, lumping three backbone carbon atoms into single beads.
For the homonuclear model (left) all beads are treated on an equal footing, and
for the heteronuclear model (right) the CH2CH2CH3
beads that contain the CH3 end group and the CH2CH2CH2
beads that make up the intervening groups are distinguished.

To address the importance of chain flexibility, we have performed single
component molecular dynamics simulations of an equivalent united-atom (UA)
model using the NERD force field [9]. From the set of atomistic trajectories
the CG intermolecular force field was computed using the potential of mean
force of the corresponding CG degrees of freedom, e.g., CG bond distances and
angles. The CG bond stretching and bond angle bending interactions were then
superimposed to the CG intermolecular parameters. The full model includes both
inter- and intra-molecular contributions.

An assessment of the different contributions to the full
intermolecular potential is made by comparing the results obtained from the
molecular dynamics simulations of fluid phase equilibria as well as the
interfacial, transport and structural properties. An example of how the
structural properties of nonane are affected when using different intra- and
inter-molecular parameters can be seen in Figure 2. The g(r) results obtained
for the homonuclear model was the same as the heteronuclear, so has not been
included to keep the figure simple.


2: Centre-to-centre radial distribution function for n-nonane comparing the heteronuclear SAFT-γ Mie CG
descriptions (cf. Figure 1) with completely flexible (pearl-necklace) model or
the restrained bonding angle model.

3. Conclusion

inclusion of intra- molecular interactions CG models allows for an improved
description of thermodynamic and structural properties without a detriment to the speed
of the calculations. An additional advantage of the heteronuclear description
is that they provide fully transferable CG models for linear alkanes.


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[9] S.K.Nath,
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