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1.3.2.2. Colloidal solutions and dispersions

Colloidal solutions and dispersions

In living beings, the liquid state is made up of dispersions of many types of dispersed molecules or solutes and a single type of dispersing or solvent phase, which is water.

According to the molecular weight of the solutes we distinguish:

True solutions

If the solutes are of low molecular weight (they are called crystalloids ) such as, for example, sodium chloride (MW = 58.5) and glucose (MW = 180). These dispersions are called true solutions or simply solutions .

In solutions, water-soluble mineral salts are dissociated into their ions and form part of the internal intracellular and extracellular media.

  • The most frequent negatively charged ions or anions in living matter are: chlorides (Cl-), phosphates (PO43-), monoacid phosphates (HPO42-), carbonates (CO32-), the bicarbonates (HCO3-) and nitrates (NO3-).
  • The most abundant positively charged ions or cations in living matter are sodium (Na+), calcium (Ca2+), magnesium (Mg2+), iron (Fe2+ and Fe3+ ) and the potassium (K+).

Colloidal dispersions

If the solutes are of high molecular weight, of the order of several thousand (they are called colloids), as, for example, the proteins of the albumin type (MW between 30,000 and 100,000). These solutions are called colloidal dispersions.

These dispersions can, in turn, present two physical states:

  • Sol. A colloid in the form of a sol has a liquid appearance, since the solute molecules that make up the dispersed phase are found in less quantity than those of the liquid dispersant phase.
  • Gel. A colloid in gel form has a semi-solid and gelatinous appearance. The solvent molecules are, to a lesser extent, among the solute molecules, which intertwine to form a continuous network that acts as a dispersing phase. The network prevents the solvent from flowing, so the gel behaves like a soft solid that is easy to deform.

In cells, the states of sol and gel change according to variations in the concentration of colloidal particles and the places where they are found. Variations in temperature, pH, pressure or concentration increase the reactivity of the micelles, in such a way that the reactions that occur between them can modify the state of the colloidal dispersions, going from sol to gel. Not in all cases this process is reversible; for example, egg white (sol) coagulates due to heat and cannot return to this state.

There are also hydrophobic colloidal dispersions, in which the solutes are not related to water. These dispersions are not stable, and the dispersed particles tend to coalesce and form a separate phase from the water. Hydrophobic dispersions can be stabilized by forming so-called emulsions when substances that prevent the bonding between dispersed particles act. This is how fats are present in milk, and it is some proteins that stabilize the emulsion.