Question

I am interested in the salts used in the gel electrophoresis buffers. As I understand it, one of their roles is to change the conductivity of the medium, because otherwise the electric field would be too weak inside the medium to move the negatively charged sample.

How does this work from a mechanistic perspective? I found one lead here, but I don't see how the diagram explains the effect of the rearrangements on the overall electric field. (It seems to imply that the effects on the saline-associated electric field compared to pure water would be net zero to me, since the fields from the charge separation cancel out with the ones from the water.)
Using simplifications like point charges and intuition, is it possible to explain why the dielectric constant becomes lower in saline solutions compared to pure water?

Answer 1

Salts like NaCl and KCl dissociate into ions when dissolved in water, thus enhancing the solution's conductivity and affecting the dielectric constant.

Step-by-step explanation:

When salts like NaCl are dissolved in water, they dissociate into ions which can conduct electrical current in the solution, impacting the conductivity of the medium. This is fundamental in techniques such as gel electrophoresis, where the movement of charged particles is essential.

In pure water, the slightly negative oxygen and slightly positive hydrogens of water molecules can stabilize charges through their dipole moment, but when salts are present, the ionic strength of the solution increases due to the dissociation of the salt into its component ions. These ions enhance conductivity because they carry electrical charge through the solution.

The dielectric constant of a medium is a measure of its ability to reduce the electric field within it. Water has a high dielectric constant, which means it can reduce the electric field effectively; when salts are added, the ions they form reduce the dielectric constant, resulting in a stronger electric field for the same applied voltage.

This change in the dielectric constant affects the degree to which charges within the solution are screened from each other. As salts like KCl dissolve, ion-dipole forces help to solvate and separate the ions, reducing strong interionic forces and permitting their independent motion, thus contributing to the overall electric field in the solution.

The Debye-Hückel theory also explains that solvated ions and residual interionic attractions impact the actual behavior of ions in solution, affecting conductivity and the electric field.