Studies Pertaining to the Cation Absorption Mechanism of Plants in Soil

THE CONCEPT OF THE INTERMICELLAR LIQUID IN A COLLOIDAL SUSPENSION T elementary constituents of a colloidal solution are the micells. The micell has been denned as consisting of the colloidal particle plus the electric double layer which surrounds it (i). The intermicellar liquid has been thought of simply as the liquid between the micells. This definition of the intermicellar liquid, although very convenient from the point of view of discussion, meets with serious theoretical difficulties in the light of modern concepts of the electric double layer. The original Helmholtz rigid double layer picture for ions adsorbed on colloidal surfaces has in recent decades been replaced by the diffuse double layer picture which was first introduced by Gouy (2) and further developed and extended by Debye and Hiickel (3) and others. Today, the electric double layer theory of Debye and Hiickel probably remains the most satisfactory one from the point of view of generality and the interpretation of experimental observation. This theory was originally designed to explain certain anomalies in the properties of dilute solutions of strong electrolytes and has in recent years been applied to colloidal suspensions. In essence the Debye-Hiickel theory postulates that the adsorbed ions of a colloidal micell constitute a diffuse swarm which, theoretically, extends to infinity (or rather to the actual limits of the volume of the sol). Consequently, no matter how dilute a colloidal solution, on theoretical grounds it is to be expected that there will always be an intermingling of the diffuse double layers. This means that the ion swarms extend into the solution between the particles to such an extent that they mutually overlap. Thus at least theoretically the assumption of the diffuse double layer picture precludes the concept of the intermicellar liquid. On the other hand, for dilute sols in which the ion swarms are not extremely diffuse, it is convenient to retain the concept of the intermicellar liquid and its boundaries may be approximately defined. Let us consider the distribution of the ion swarm about a particle in such a system. With increasing distances from the solid particle wall the electric density of the ion swarm will decrease until at some distance d, which is characteristic of the sol, it is not measurably greater than zero. Thus for the interparticular liquid which lies at greater distances than d from the particles, the time average concentration of cations will be very nearly equivalent to that of the anions. This liquid, which is in effect uniform in composition, may be defined as the intermicellar-liquid. In this manner the concept of the intermicellar liquid may be retained in the case of colloidal solutions in which the characteristic distance d is small compared with the average distance between the particles. Moreover, the intermicellar liquids of these systems will correspond very nearly with the initial ultrafiltrates or centrifugates of the sols. Numerous experimental observations on soil suspensions strongly suggest the usefulness of the idea of the intermicellar liquid or soil solution. For example, the experiments of Schofield (4) and his coworkers on the osmotic pressure and freezing point lowering of homionic soil suspensions indicate that even with very concentrated suspensions the effects of double layer interpenetration is negligible. For these reasons we have retained the concept of the intermicellar liquid for the dilute sols used in this research and have identified it with the supernatant liquid of the centrifuged suspension.