A theory on the effect of low molecular salts on the conformation of linear polyions

The effect of low molecular salts on the conformation of linear polyions is analyzed on the basis of the additivity law for the thermodynamic properties of polyelectrolytes in the presence of salts. In the case of flexible polyions the theory shows that the logarithm of the conformation parameter changes proportional to A ln (1 + γ s 0 C s /γ p C p ) as a function of the salt concentration c s , where c p is the concentration of counterions from polyelectrolytes and γ p and γ s 0 are the activity coefficient of counterions from polyelectrolytes in the absence of salts and from salts in the absence of polyelectrolytes, respectively. In the case of the discontinuous transition of the conformation of polyions, it is shown that the change of the transition point by salts is proportional to the same factor as the above. The constant A , which determines the magnitude of the effect of salts, is proportional to the sensitivity of the activity coefficient to the conformation of polyions in the absence of salts. A similar analysis is made for the conformational change due to dilution by the addition of solvent. These theoretical results are found to be in good agreement with experimental data on the viscosity of flexible polyions and on the helix–coil transition of biopolymers. The interrelation between these conformational changes and various thermodynamic properties of polyelectrolytes with salts is discussed.