Osmosis : a molecular dynamics computer simulation study
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Date
28/11/2013Author
Lion, Thomas
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Abstract
Osmosis is a phenomenon of critical importance in a variety of processes ranging from
the transport of ions across cell membranes and the regulation of blood salt levels
by the kidneys to the desalination of water and the production of clean energy using
potential osmotic power plants. However, despite its importance and over one hundred
years of study, there is an ongoing confusion concerning the nature of the microscopic
dynamics of the solvent particles in their transfer across the membrane. In this thesis
the microscopic dynamical processes underlying osmotic pressure and concentration
gradients are investigated using molecular dynamics (MD) simulations.
I first present a new derivation for the local pressure that can be used for determining
osmotic pressure gradients. Using this result, the steady-state osmotic pressure is
studied in a minimal model for an osmotic system and the steady-state density gradients
are explained using a simple mechanistic hopping model for the solvent particles. The
simulation setup is then modified, allowing us to explore the timescales involved in the
relaxation dynamics of the system in the period preceding the steady state. Further
consideration is also given to the relative roles of diffusive and non-diffusive solvent
transport in this period.
Finally, in a novel modi cation to the classic osmosis experiment, the solute particles
are driven out-of-equilibrium by the input of energy. The effect of this modi cation
on the osmotic pressure and the osmotic
ow is studied and we find that active solute
particles can cause reverse osmosis to occur. The possibility of defining a new "osmotic
effective temperature" is also considered and compared to the results of diffusive and
kinetic temperatures.