Freezing Water and Aqueous Solutions: Elucidating the Molecular Ballet Using Computer Simulations

Sapna Sarupria from Clemson University

Nucleation – that is the onset of a new phase from a metastable phase – is a difficult phenomenon to study
both experimentally and computationally. Nucleation relevant spatial and temporal resolution is difficult to
access in experiments. In contrast, while computer simulations are perfectly suited to probe these length
and timescales, sampling statistically relevant number of nucleation events is computationally expensive.
In our research, we use advanced sampling techniques in conjunction with molecular dynamics simulations
to sample hundreds and thousands of nucleation events. We apply these methods to study heterogeneous
ice nucleation and gas hydrate nucleation.
Heterogeneous ice nucleation relates to freezing of water driven by the presence of an impurity
such as a mineral surface. Interestingly, it is not yet understood what about a surface promotes or inhibits
the formation of ice. We use simulations of salt (silver iodide) and mineral (kaolinite) surfaces to provide
insights into the properties of a surface that facilitate ice formation. These findings can help design surfaces
that either inhibit ice nucleation (for example in cases such as wind mills, power lines, and transportation)
or that promote ice nucleation (for example in cases such as food preservation and cryopreservation).
Gas hydrates are crystalline (ice-like) solids formed by water forming cages around guest molecules
such as methane, carbon dioxide and tetrahydrofuran. These form at low temperatures and high pressure
conditions and are a flow assurance problem in the oil and gas industry. In addition, hydrates can be
potential energy source, and be used for gas separation and water desalination. The fundamental question
that remains unanswered is -- how does the solution of guest and water transform into this crystalline solid.
Our simulations provide insights into the mechanisms of hydrate nucleation and highlight its dependence
on the water solubility of the guest molecules. These findings will facilitate design of promoters and
inhibitors of gas hydrates.
Collectively, my talk provides an overview of the intricate behavior of water molecules in
transforming from liquid to the solid phase.