284 HYDROGEN ION CONCENTRATION 



83. The influence of electrokinetic potentials upon the boundary 



surface tension 



For the sake of completeness we must mention here a mechanical 

 manifestation of electrokinetic potentials, a more comprehensive 

 discussion of which will be reserved for the chapter on colloids in a 

 later volume of this work. According to the observations of Lipp- 

 mann, in conjunction with the discovery of the capillary electrometer 

 and with the Helmholtz theory relating to it, the double layer at a 

 phase boundary exercises an influence upon the surface tension. 

 Every individual stratum of the double layer, because it consists of 

 similarly charged particles, tends to expand or to to extend itself, 

 thus working in opposition to the ordinary surface tension. Con- 

 sequently the surface tension at an interphase boundary must depend 

 upon the potential difference. Now, when one of the phases is soUd 

 or at least much more viscous than the other, this expansion can occur 

 only in that ionic layer which is in the more fluid or less viscous 

 phase. This tendency to expand on the part of the layer at the 

 surface of the liquid is antagonistic to the ordinary surface tension. 

 The total tension is therefore dependent upon the absolute value of 

 the electric density of the double layer. It is at a maximum when 

 this density = 0; this is the case when the potential of the double 

 layer is also = 0, or hence, at the isoelectric point. The theory was 

 extended by Bredig^* to colloidal solutions, and Michaelis^^ showed, 

 for colloidal solutions of protein substances, that, in fact, the iso- 

 electric point (determined by cataphoresis experiments) is identical 

 with the maximum of the surface tension (as determined by estima- 

 tions of flocculation optima). This aspect will be further discussed 

 in the following volume. 



84. A summary of electrokinetic phenomena 



In conclusion it may be useful to present a systematic summaiy 

 of all electrokinetic phenomena. Let us imagine a soUd wall and a 

 fluid movable in reference to this wall. If an external potential is 

 apphed in a direction tangential to the wall, a displacement of the 

 fluid along the wall will occur. When the wall is not mechanically 

 displaceable (a capillary or a diaphragm), the fluid is displaced along 



*' Bredig, Anorgan.Fermente. Leipzig 1901. 



*' Summarized by L. Michaelis, Nernst-Festschrift. Halle 1913. 



