The influence of the underlying surface on the cataphoretic mobility of adsorbed proteins

Our knowledge of the structure of the electrical double layer at the interface between a solid and a liquid is so indefinite that it is difficult to give a precise definition or to make an exact measurement of electrokinetic potential. In the liquid in the immediate vicinity of the solid surface, there will be an excess of ions of opposite charge to that on the surface, forming an electrical double layer which is approximately a molecular diameter in thickness. All the potential drop, however, is not confined to this first double layer ; the excess ions in the solution are not rigidly held to the surface, they are able to break away and to form a diffuse layer which extends some distance into the solution. Experience has shown that gentle stirring of the solution can influence the distribution of these ions, and it is probable that the diffuse layer extends as far as 10-5 to 10-4 cm. from the solid surface. It is this diffuse layer of more or less loosely held ions which is responsible for the electrokinetic properties of liquids in contact with solid surfaces. No exact relation has been established connecting the diffuse, or electrokinetic potential, with the total potential drop between a solid and a liquid. Usually they are of the same sign, but there is evidence that the potential drop across the diffuse layer can even be opposite in sign from the main potential, indicating a twofold double layer. The presence of polar molecules adsorbed at the interface may have a profound effect on the distribution of the diffuse layer without sensibly affecting the total potential difference. Further work on the relationship between these quantities would be valuable.