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This module presents an interactive molecular simulation, which can be used to study the process of osmosis and reverse osmosis. Osmosis is very important in biological systems and has other important uses too as is explained in the section titled "Background". Reverse osmosis is widely used in water purification (desalination of sea water), and in other industrial applications requiring water purification or removal of contaminants from a wide range of solvents. The osmometer is used to measure the osmotic pressure. This module illustrates the governing principles of osmosis and reverse osmosis. In addition solution chemistry plays an important role in both osmosis and reverse osmosis and in the water module, the important role of solution chemistry is also examined.

The simulated apparatus is in effect an osmometer, which is a container holding the solution (solute dissolved in solvent) separated from a pure solvent by the semi-permeable membrane (represented by an atomic wall). Because of periodic boundary conditions used in molecular simulations, it appears as if they are two solution compartments. In fact there is only one. The instantaneous and average osmotic pressure in the direction normal to the semi-permeable membrane is measured as the difference in pressure between the left hand side wall and the right hand side wall, each of which is determined by the number of collisions on the respective wall. The diameter of the particles, the number of solute and solvent particles can be adjusted. The simulation is of the molecular dynamics type with LJ potential being used to model the solute and solvent in one module, while in the other (water) module, water is represented by the SPC model, and the ions by the primitive model. Many of the parameters used in these simulations can be varied by the user, and in both the examples and problems, the role these parameters play in osmosis and reverse osmosis have been illustrated. The membranes used are tethered using a simple harmonic motion to their chosen positions. The spring constant of the tethered molecules can be adjusted, while the size of the atoms that form the membrane can be varied. In addition the thickness of the membrane can be changed and in the case of the water module, the width can also be adjusted. If you want to read more about osmosis or reverse osmosis you can consult many books available in most libraries. If you want to learn more about the computer program on which this module is based, you can consult the following papers:

S. Murad and J. G. Powles, "A Computer Simulation of the Classic Experiment on Osmosis and Osmotic Pressure", Journal of Chemical Physics, 99, 7271-2 (1993).

J. Lin and S. Murad, "A Computer Simulation Study of the Separation of Aqueous Solutions Using Thin Zeolite Membranes", Molecular Physics, 99, 1175-81 (2001).

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