||The present work is the result of theoretical investigations and calculations which were performed, with some interruptions, in the years 1940—1944. As conditions during the occupation of our country prevented earlier publication, and as a description of the collected results seemed to exceed the limits set to articles in periodicals we decided to present them in book form after the war.
The science of colloids appears to be entering upon a new stage, which is less empirical, and where the experimental study of better defined objects will be guided rather by more quantitative theories than by qualitative "rules" or "working hypotheses". The theory of the stability of lyophobic colloids, as developed in this book, may serve as an example of this development. This stability problem has been placed on a firmer physical basis by the introduction of the concept of Van der Waals— London dispersion forces together with the theory of the electrolytic or electro-chemical double layer. In the present work, too, these theories form the starting points of our considerations.
For the main part this book gives an. account of our own work on the interaction of colloidal particles. It has not been written, however, on the assumption that the whole of the relevant literature is known to the reader. Accordingly we have added a number of chapters which may serve as an introduction to the main problem of the book. These chapters contain, for instance, a brief survey of some aspects of lyophobic colloids, a concise summary of the theory of the double layer, and a short introduction to the subject of Van der Waals— London forces.
At the time of writing this work (1944/"45) the literature which has appeared during the war in the countries fighting, on the side of the Allies was not at our disposal. To some of the Chapters a few paragraphs were added later on (1947) in order to bring the text in accordance with recent developments.
We desire to acknowledge the assistance we received, in a considerable part of the numerical calculations, from Mr K. van N с s, chem. docts., now once again of the laboratory of the Bataafsche Petroleum Maatschappij, Amsterdam, to whom we arc also indebted for valuable contributions in the final presentation of the results of the theory of Part II.