Sodium-Ion Interactions with Polyions in Aqueous Salt-Free Solutions by Diffusion

Abstract

An unambiguous way to study the interactions of counterions with polyelectrolytes in solution is by determining the counterion and coion radioactive tracer diffusion coefficients inthe presence of a polyelectrolyte with a common counterion. This has been done principally by FernandezPrini, et al. 1,2 , Magdelenat, et al. 3,4 and Ander, et al. 5-13 . A capillary diffusion technique was employed. Aqueous solutions have been used in almost all studies. This brief review will focus on the experimental findings of the author. Equations have been developed by Manning for counterion and coion diffusion coefficients in polyelectrolyte solutions in the absence and presence of simple salts for a line charge model for the polyelectrolyte. 14-20 DIFFUSION MEASUREMENTS To determine the self-diffusion coefficients of ions, the open-end capillary method originally introduced by Anderson and Saddington 21 was employed without stirring. Due to the high viscosity of polyelectrolyte solutions, a non-stirring technique was found to be more reliable than one employing stirring because of the considerable change of viscosity of the solution as concentration varies. Fawcett and Caton 22 analyzed the source of error in the capillary method of measuring diffusion coefficients. They concluded that the serious source of systematic error results from convective loss of diffusant from the mouth of the capillary, and that accurate results require the use of fine bore capillaries from which convective loss is minimal.In our laboratory all diffusion measurements were carried out in a constant temperature bath at 25.00 + 0.01°C. Precision bore capillaries of 1.00 + 0.005 mm diameter and 2.82 + 0.005 cm length were filled with the desired polyelectrolyte solution containing tracer amount of radioactively labeled ion under investigation. The outside of the filled capillaries were then wiped dry and each capillary was placed into an individual test tube (100 mm χ 13 mm), which was subsequently filled with a radioactively inert solution of identical composition to the solution inside the capillary. Since the solutions inside and outside the capillary were the same polyelectrolyte concentration and the same simple salt concentration, no concentration