Researchers from Queen Mary University of London have developed a novel
computational approach to better understand freezing in different types of
liquids.
The process of freezing, where a liquid turns into a solid, isn't as simple
as it might seem. Many substances, including water and wax, have several
solid states as a result of differences in the arrangement of their atoms
and molecules. However, performing experiments to visualize the exact
molecular arrangements and how they transform between states can be
difficult.
Over the last few decades computational models have increasingly been used
to complement experimental studies, bringing new molecular insights into the
properties of gas and liquid states as well as the transitions between them
(e.g. evaporation).
However denser phases are still a challenge, and the complexity of the
freezing liquids into solids has eluded most methods, especially where there
is more than one possible solid arrangement.
In the study, published in the Journal of Physical Chemistry B, the
scientists developed novel computational approaches to study wax, which is
known to have multiple frozen arrangements. Using their method they were
able to predict its melting point within 2°C of the experimental value.
Comparing performance
When they compared the performance of these methods with most existing
computational techniques, they showed their modeling approach provided a
more realistic view of what happens when liquids freeze and could even
predict some of the more 'exotic' crystal structures formed during this
process.
Dr. Stephen Burrows, Postdoctoral Research Assistant at Queen Mary, said:
"Solid alkanes are unusual because the molecules have a surprising amount of
freedom. If you start from a perfect crystal and increase the temperature,
the molecules suddenly gain the ability to rotate, with a motion similar to
a restless sleeper tossing and turning in bed."
"We have tested the most widely used methods to simulate these 'rotator'
phases, finding that the Williams model from the 1960s was ahead of its
time. Initially impractical due to a lack of computational power, it may now
undergo a renaissance for modern molecular dynamics simulation. With our
newly optimized model, we aim to study the rotator phase of hexadecane,
found in oil, which is hard to observe experimentally because of its
unstable nature."
Real-world applications
Like waxes, oils such as diesel fuel can also freeze at many stages and
exhibit different solid properties. Therefore, methods to predict the
molecular and atomic intricacies of liquid transitions to different types of
'solid' oils could have several potential real-world applications, from
helping better predict freezing of oil pipelines (and preventing oil
spills), to developing better smart insulation and energy storage.
Understanding solid transitions in wax could also lead to lighter,
stronger-than-steel polymers, and help researchers to improve understanding
of newly discovered processes like artificial morphogenesis. These could
enable greener manufacturing processes so we could 'grow' matter as seen in
nature, reducing side or waste products.
Dr. Stoyan Smoukov, Reader in Chemical Engineering at Queen Mary, said:
"Being able to predict the transformation behavior of oils would help us in
our quest to develop sustainable manufacturing processes for the future.
Usual lithographic microfabrication is like sculpturing, cutting/chiseling
away from a slab of marble, generating lots of waste. In our current grant
we are using novel processes to self-shape droplets and use nearly 100% of
the starting material to literally grow shaped particles."
"The process is highly scalable as each droplet shapes itself due to
internal phase transitions. Efficient production of such particles could
revolutionize industries from inkjet printing to drug delivery. And the
modeling tools we've developed will help us tune this control on the
molecular scale."
Reference:
Stephen A. Burrows et al. Benchmarking of Molecular Dynamics Force Fields for
Solid–Liquid and Solid–Solid Phase Transitions in Alkanes, The Journal of
Physical Chemistry B (2021). DOI:
10.1021/acs.jpcb.0c07587