Osmosis
Osmosis is the
diffusion of a liquid through a
semipermeable membrane from a region of low solvent potential to a region of high solvent potential. The semipermeable membrane must be permeable to the solvent, but not to the solute, resulting in a pressure gradient across the membrane. Osmosis is a natural phenomenon. However, it can be artificially opposed by increasing the pressure in the section of high solute concentration with respect to that in the low solute concentration. The force per unit area required to prevent the passage of solvent through a selectively-permeable membrane and into a solution of greater concentration is equivalent to the turgor pressure.
Encyclopedia
Osmosis is the
diffusion of a liquid through a
semipermeable membrane from a region of low solvent potential to a region of high solvent potential. The semipermeable membrane must be permeable to the solvent, but not to the solute, resulting in a pressure gradient across the membrane. Osmosis is a natural phenomenon. However, it can be artificially opposed by increasing the pressure in the section of high solute concentration with respect to that in the low solute concentration. The force per unit area required to prevent the passage of solvent through a selectively-permeable membrane and into a solution of greater concentration is equivalent to the turgor pressure. Osmotic pressure is a
colligative property, meaning that the property depends on the concentration of the solute but not on its identity.
Osmosis is an important topic in
biology because it provides the primary means by which
water is transported into and out of cells.
Basic explanation of osmosis
Consider a permeable membrane, such as
visking tubing, with apertures small enough to allow water molecules, but not larger molecules, to pass through. Suppose the membrane is in a volume of pure water. At a molecular scale, every time a water molecule hits the membrane, it has a defined likelyhood of passing through. In this case, since the circumstances on both sides of the membrane are equivalent, there is no net flow of water through it. However, if there is a solution on the other side, that side will have fewer water molecules and thus fewer collisions with the membrane. This will result in a net flow of water to the side with the solution. Assuming the membrane does not break, this net flow will slow and finally stop as the pressure on the solution side becomes such that the diffusion in each direction is equal.
Osmosis can also be explained via the notion of
entropy, from statistical mechanics. As above, suppose a permeable membrane separates equal amounts of pure solvent and a solution. Since a solution possesses more entropy than pure solvent, the
second law of thermodynamics states that solvent molecules will flow into the solution until the entropy of the combined system is maximized. Notice that, as this happens, the solvent loses entropy while the solution gains entropy. Equilibrium, hence maximum entropy, is achieved when the entropy gradient becomes zero.
Examples of osmosis
Many plant cells perform osmosis. This is because the osmotic entry of water is opposed and eventually equalled by the pressure exerted by the
cell wall, creating a steady state. In fact, osmotic pressure is the main cause of support in plant leaves.
When a plant cell is placed in a hypertonic solution, the water in the cells moves to an area higher in solute concentration, and the cell shrinks and so becomes
flaccid [pron.
flaxid]. .
Osmosis can also be seen very effectively when potato slices are added to a high concentration of salt solution. The water from inside the potato moves to the salt solution, causing the potato to shrink and to lose its 'turgor pressure'. The more concentrated the salt solution, the bigger the difference in size and weight of the potato chip.
In unusual environments, osmosis can be very harmful to organisms. For example,
freshwater and saltwater aquarium fish placed in water with a different salt level will die quickly, and in the case of saltwater fish rather dramatically. Additionally, note the use of table salt to kill
leeches and
slugs.
Chemical potential
When a solute is dissolved in a solvent, the random mixing of the two substances results in an increase in the entropy of the system, which corresponds to a reduction in the chemical potential. For the case of an ideal solution the reduction in chemical potential corresponds to:
Where is the gas constant, is the temperature and is the solute concentration in terms of mole fraction. Most real solutions approximate the
ideal behavior for low solvent concentrations . This reduced potential creates a 'driving' force and it is this force which enables diffusion of water through the
selectively-permeable membrane.
Osmotic pressure
As mentioned before, osmosis is opposed by increasing the pressure in the region of high solute concentration with respect to that in the low solute concentration region. The force per unit area, or pressure, required to prevent the passage of water through a selectively-permeable membrane and into a solution of greater concentration is equivalent to the
osmotic pressure of the
solution, or turgor. Osmotic pressure is a
colligative property, meaning that the property depends on the concentration of the solute but not on its identity.
Increasing the pressure increases the chemical potential of the system in proportion to the molar volume . Therefore, osmosis stops, when the increase in potential due to pressure equals the potential decrease from Equation 1, i.e.:
Where is the osmotic pressure and is the molar volume of the solvent.
For the case of very low solute concentrations, -ln ˜ and Equation 2 can be rearranged into the following expression for osmotic pressure:
Reverse osmosis
The osmosis process can be driven in reverse with solvent moving from a region of high solute concentration to a region of low solute concentration by applying a pressure in excess of the osmotic pressure. Recent advances in pressure exchange and the ongoing development of low pressure membranes have significantly reduced the costs of water produced by reverse osmosis. The reverse osmosis technique is commonly applied in
desalination, water purification, water treatment, and food processing.
Forward osmosis
Osmosis may be used directly to achieve separation of water from a "feed" solution containing unwanted solutes. A "draw" solution of higher osmotic pressure than the feed solution is used to induce a net flow of water through a semi-permeable membrane, such that the feed solution becomes concentrated as the draw solution becomes dilute. The diluted draw solution may then be used directly , or sent to a secondary separation process for the removal of the draw solute. This secondary separation can be more efficient than a reverse osmosis process would be alone, depending on the draw solute used and the feedwater treated. Forward osmosis is an area of ongoing research, focusing on applications in
desalination, water purification, water treatment, and food processing.
See also
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External links
