Chapter 2: Water- Detergents & Chaotropes.

Detergents, sometimes called surfactants, are molecules that are both hydrophilic and hydrophobic; they usually have a hydrophobic chain at least 12 carbon atoms long and an ionic or polar end. Such molecules are said to be amphipathic.

One of the synthetic detergents most commonly used in biochemistry is sodium dodecyl sulfate (SDS), which contains a 12-carbon tail and a polar sulfate group.

Sodium Dodecyl Sulfate. Click on image for credit.

Some ions such as thiocyanate (SCN) and perchlorate (CIO4) are called chaotropes, meaning they are structures that disrupt the structure of water, so as to promote activities inhibited by the water molecules. These ions are poorly solvated compared to ions such as ammonium (NH4+), sulfate (SO42-) and dihydrogen phosphate (H2PO4).

Ammonium.

Sulfate

Dihydrogen phosphate

Chaotropes enhance the solubility of nonpolar compounds in water by disordering the water molecules (there is no general agreement on how chaotropes do this). Enzymes are usually amphipathic chaotropes.

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