Osmosis

Related Topics:
Osmosis_Jones

Osmosis is the diffusion of a liquid (most often assumed to be water, but it can be any liquid solvent) through a partially-permeable membrane from a region of low solvent potential to a region of high solvent potential. The selectively-permeable 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 likelihood 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.

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. (This means the cell has become plasmolysed - the cell membrane has completely left the cell wall due to lack of water pressure on it (the opposite of turgid).

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 (than they are adapted to) 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:

RT \ln(1 - x_2)  \qquad  (1)

Where $R$ is the gas constant, $T$ is the temperature and $x_2$ is the solute concentration in terms of mole fraction. Most real solutions approximate the ideal behavior for low solvent concentrations (At higher concentrations interactions between solute and solute cause deviations from Equation 1). 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 ( $\delta\mu = \delta PV$ ). Therefore, osmosis stops, when the increase in potential due to pressure equals the potential decrease from Equation 1, i.e.:

\delta PV = -RT \ln(1-x_2)\qquad (2)

Where $\delta P$ is the osmotic pressure and $V$ is the molar volume of the solvent.

For the case of very low solute concentrations, -ln(1- $x_2$ ) ≈ $x_2$ and Equation 2 can be rearranged into the following expression for osmotic pressure:

\delta P = RTx_2/V \qquad (3)

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 (as with an ingestible solute like glucose), 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.

External links

Osmosis simulation in Java

Diffusion | Water technology

Gourt Home::Gourt :: Osmosis