Lyotropic Series

Hofmeister series, a series of ions arranged in ascending or descending order of lyotropic effect, that is, their influence on the properties of the solvent and on the rate and degree of the chemical reactions and physicochemical processes occurring in it.

The most well studied lyotropic effect and the one of greatest practical value is that produced by ions in aqueous media. Typical examples include lyotropic series of single-charged inorganic anions (F^–, C1^–, Br^–, NO^–₃, I^–, CNS^–) and cations of alkaline and alkaline-earth metals (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺), which are arranged in order of increasing adsorptivity from aqueous solutions on activated carbon or other adsorbents. This order is described by decreasing salting out, increasing coagulability, influence on the solubility and swelling of macromolecular compounds in water (proteins, polysaccharides, synthetic polyelectrolytes), and effect on the various physiological functions in an organism.

The sequence of ions in a lyotropic series is determined by their charge, size, and solvation (hydration), that is, their ability to bond molecules of the solvent (water). In a group of identically charged ions solvation increases with a decrease in ionic radius. The ionic sequence of a lyotropic series can be altered, to the point of complete reversal, with changes in the concentration of hydrogen ions (pH), temperature, external force fields, and composition of the solvent medium. Lyotropic series are of value in understanding and regulating a variety of chemical-engineering and biochemical processes.

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