APART from their nonspecific electrostatic interactions, ions can produce specific effects on the conformation and solubility of macromolecules in aqueous solution1. The relative effectiveness of anions and cations in altering macromolecular configurations is remarkably independent of the nature of the macromolecule and is generally referred to as the lyotropic or Hofmeister series. To explain the generality of the effects, the influence of the ions on the structure of the solvent has often been invoked, but as yet no theory has been able to link satisfactorily ion–water interactions with the effects of ions on macromolecules. We argue here that it is the effect of the ions on the empty volume of the solvent which underlies the Hofmeister series and we derive a simple thermodynamic equation which accounts for the effect of a salt on the transition temperature for a polymer chain aggregation process. To test the equation we have taken data from the literature relating to the cloud point of polyvinylmethylether (PVME; intermolecular aggregation); and to the thermal transition temperature of ribonuclease (intramolecular aggregation).
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