Sedimentation coefficient ionic atmosphere effect

In the discussion on the latter, that is, on the limiting values, too, the effect of ionic atmosphere should be taken into account in addition to the change of radius of gyration which is only important for the limiting sedimentation and diffusion coefficients of non-ionic polymers. 

Equation (12) predicts that the sedimentation potential decreases as the polyion concentration decreases. Therefore, the sedimentation coefficient of the corresponding discharged polymer may be obtained as the limit of the observed sedimentation coefficient at the infinite dilution of polymer. However, this procedure does not completely eliminate the unknown contribution to from the ionic atmosphere, as discussed later. Furthermore, it is to be noted that the proposed extrapolation procedure is valid only when the sedimentation coefficients of both ions constituting the added-salt are equal, that is, if S + =S. If S + S, the difference S + — S leads to another sedimentation potential which effects the sedimentation coefficient of the polyion even in the limit of infinite dilution of polymer (secondary salt effect) [1]. 

The Effect of the Ionic Atmosphere on the Limiting Sedimentation and Diffusion Coefficients... 

In general, the diffusion coefficient of an electrolyte is a function of concentration. One of the reasons for the concentration dependence of diffusion coefficient is the electrolyte effect of ionic atmosphere around each ion. A similar effect of ionic atmosphere should be observed for diffusion and sedimentation coefficients of a poly electrolyte. Moreover, the effect of ionic atmosphere should remain even at the limit of infinite dilution of polyelectrolyte if the concentration of added neutral salt is kept constant, since the ionic atmosphere does not disappear. 

If the macro-ion is a linear polyelectrolyte, the sedimentation velocity is changed not only by the effect of the ionic atmosphere on the flow of solvent about the polymer, but also by the change in conformation of the polymer chain as the ionic strength is changed. According to Flory [12], the sedimentation coefficient is related to the expansion factor, a, defined by the ratio of the radius of gyration of the polymer to... 

Beside the effect of ionic atmosphere, there are various factors which may affect o Po. ThQ first may be the effect of drainage. If there is partial free drainage of solvent through the polymer coil, the powers of a in Equations (35) and (36) may be considerably different from 1 and 3 [18]. However, we expect that 0q Pq will be constant over a wide range of a, because the effect of partial free drainage on and [rj ] tend to compensate one another (e.g. as in Equation (37)). The second may be that, even in the limit of non-draining sphere, the sedimentation coefficient of Equation (35) and the intrinsic viscosity on Equation (36) may be proportional to lower powers of a than 1 and 3, respectively, as first pointed out by Kurata and Yamakawa [18]. Although there are various experimental works, the dependence of the sedimentation coefficient on a has not yet been completely solved [19, 19a]. 

---