Semipermeable Membrane

Definition - What does Semipermeable Membrane mean?

A semipermeable membrane is one which only certain molecules can pass through, depending on properties. Some semipermeable membranes only allow molecules if they are bonded to another molecule. It is usually artificially created and used for specific functions, such as water desalination and purification.

This membrane controls what molecules can and cannot pass through it, based on characteristics such as:

  • Molecule size
  • Chemistry
  • Solubility
  • Other specific properties

A semipermeable membrane is also known as a selectively permeable membrane, partially permeable membrane or differentially permeable membrane.

Corrosionpedia explains Semipermeable Membrane

A semipermeable membrane is a barrier that allows only certain molecules or ions to pass through, while blocking others. Molecules pass through the membrane via osmosis, which is the passage of any substance through diffusion, or the tendency of molecules to move from an area where they exist in higher concentrations to an area of lower concentration.

Semipermeable membranes in cells also allow some molecules to pass through via facilitated diffusion. In this case, the molecules cannot pass through the membrane under normal conditions but must bond to another molecule first.

One practical use of such membranes is reverse osmosis, often used in water purification. In reverse osmosis, an artificial membrane is used to remove undesirable substances from water using the natural actions of a semipermeable membrane and osmotic pressure. It often makes use of two membranes that are permeable to water but not permeable to other molecules, such as salt or other minerals. This process commonly is used to desalinate water or to purify well water, which can contain a number of chemicals or minerals leached into the water from the surrounding soil.

A saturated coating could become a semi-permeable membrane that subsequently allows only specific formula ingredients to pass freely through the coating to the metal under the coating. In some instances, the material passing through the coating could be significantly more corrosive and cause pitting corrosion.

Pitting corrosion can produce pits with a semi-permeable membrane of corrosion products.

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