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Soft Matter Simulations in the Wilson Group |
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Contact Details
Prof. Mark Wilson Department of Chemistry University Science Laboratories South Road Durham DH1 3LE United Kingdom Tel: +44 (0) 191 334 2144 Fax: +44 (0) 191 384 4737 Email: mark.wilson@durham.ac.uk |
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We have recently1 carried out the first detailed study
of the structure and dynamics of a chromonic liquid crystal
(edicol - the food dye sunset yellow). The simulations show
spontaneous self-assembly in water at atomistic detail,
and can be used to determine the free energy of association in
solution. Sunset yellow is seen to form single molecule stacks
with a preference for head-to-tail ordering, which at higher
concentrations form liquid crystal phases in water (as shown in the
picture above). Our work shows that simulation can be
used to understand, predict
and control molecular self-assembly with a view
to the future use of chromonic materials for applications
in self-assembled molecular electronics.
Read More (2010 JACS paper).
ODBP-Ph-C7. The first "true" low molecular weight biaxial nematic.
We have2 used atomistic simulation to study the structure of "the
first" low molecular weight biaxial
liquid crystal,
ODBP-Ph-C7. The sequence shows a sequence of time frames showing
the slow growth of a biaxial phase from a uniaxial starting
point. As the phase grows it develops ferroelectric domains, which are shown
colour-coded. Molecules which are blue and red have transverse dipoles pointing
in opposite directions.
Read More (2006 PRL on biaxial nematics)
We have also (with our experimental colleagues at UEA) been developing techniques to understand dynamics in liquid crystals and to use simulation as a tool to help interpret experimental measurements of dynamical processes.3,4
Read More (Chem Eur. J. 2010).We have developed new methods for calculating material properties in liquid crystal phases from atomistic simulations. For example, the rotational viscosity determines how quickly a nematic liquid crystal can reorientate in a liquid crystal display; and can be determined from our simulations by monitoring the liquid crystal director in the course of a molecular dynamics simulation.5 To carry out this work, we need accurate representations of intramolecular interactions within a molecule and intermolecular interactions between molecules. This is made possible by a new force field for use in materials chemistry, which we have been developing.6
Read More.
A snapshot from a
molecular dynamics
simulation
showing the liquid crystal molecule
PCH5 in a nematic
phase.
We have been developing coarse-grained simulation methods that allow us to understand structure and organisation in a range of molecular materials. We have recently studied liquid crystalline polymers and dendrimers and are working on “molecular engineering” tools to design molecules to have the desired organisation at the nanoscale. This research is being extended to lipid bilayers and vesicles. An important part of our work has involved developing efficient simulation methods that make use of parallel computers, so we are able to simulate extremely large systems composed of many molecules and take advantage of today’s supercomputers.
The figure (left) shows microphase separation in a liquid crystalline side chain polymer to give polymer-rich and liquid crystal-rich regions. Blue: polymer backbone (methylsiloxane), white: liquid crystal group, red: flexible spacer groups (alkyl chain).
The pictures (left) show the structure of a series of mesophases formed by a multiblock copolymer as studied by coarse-grained modelling.7 The phases are nematic (top left), lamellar (top right and middle right for different lengths of rods), gyroid (middle left), micellar (bottom). The bottom left structure show elongated micelles formed from polymer rods.
The second picture shows the formation of a polymer nanowire within a simulation.
Such wires may well have future applications in the area of nanoelectronics. Read More.
When a nematic liquid crystal phase is doped with a small amount of a chiral molecule, the whole phase becomes chiral, and the liquid crystal director (which describes the preferred direction of alignment for the molecules) is twisted through space to form a helix. Some molecules are better than others at inducing this twist, and the effectiveness of an individual molecule is measured by its Helical Twisting Power (HTP). There is considerable industrial interest in making new molecules with very large HTPs, to use as chiral dopants for chiral polymer films. However, it is difficult to relate chemical structure to HTP.
We developed new methods for predicting HTP prior to synthesis.
Our
work suggests that each separate molecular
conformation has a different HTP, and we can
show how the overall HTP value changes with increases in temperature
(as
higher energy conformations with different HTPs start to become
populated).
This mechanism explains the temperature-induced reversal in HTP that
occurs
with some materials. It also explains the fact that some molecules can
have different HTPs in different solvents. The latter is caused by some
conformations being preferentially selected in certain solvents.
Recently, we have applied out methods to "banana molecules" and the results help explain the remarkable observation that bananas can behave as if they are chiral, even though the molecules themselves are achiral.
Read More.
We have been using atomistic simulation to study dynamics in molecular solids.
Our recent work with octafluoronaphthalene and urea inclusion compounds has
indicated that its is now possible to look at dynamical processes over tens
of ns, and study correlated motion in solids. This provides the possiblity of using
simulation to complement and help interpret experimental solid state NMR data,
and raises intriguing questions as to whether we can use simulation to
help engineer
tractable molecular machines for future applications.
We are interested in the properties of amphiphilic molecules and have used simulation to understand the behaviour of amphiphilic polymers at a water/air interface.8 The simulations can be used to interpret the results of complementary neutron diffraction studies by calculating neutron reflectivity curves to match with experiment.
The snapshot (left )is from a simulation study of an amphiphilic polymer at a water-air interface (see cartoon below). At higher surface concentrations the PEO chains are forced down into the aqueous subphase to form a polymer brush.
Similar modelling techniques are being used to study lipid - amino acid interactions. Our recent atomistic work in this area seeks to understand how these interactions (specifically the balance of hydrogen bonding and cation-Pi interactions) can be perturbed by small chemical modifications of the amino acid structure. Such studies seek to aid our understanding of binding between proteins and cell membranes.