COLLOIDAL CHEMISTRY OF SOAP. 



PART I.— SOLUTIONS. 



By J. W. McBain, Leverhulme, Professor of Physical Chemistry, 



University of Bristol. 



I. — Brief Resume. 



Recent investigations of aqueous soap solutions culminating in 

 1914, have revealed soaps as a prototj'pe of a great class of colloids 

 remarkable alike for their theoretical interest and their industrial 

 importance. The substances belonging to this class have been defined 

 by McBain and Salmon^* as " colloidal electrolytes." 



Colloidal electrolytes are salts in which one of the ions has been 

 replaced bj^ a heavily charged, heavily hydrated ionic micelle which 

 exhibits equivalent conductivity that is not only comparable with 

 that of a true ion but may even amount to several times that of the 

 simple ions from which it has been derived. In other words, this 

 ionic micelle is a typical but very highly charged colloidal particle 

 of very great conductivity. 



The conductivitj' of such a colloidal electrolyte is quite comparable 

 with that of an ordinary electroh^t«. On the other hand since the 

 ionic micelle exhibits only the osmotic effect characteristic of an 

 ordinary colloid, the total osmotic acti^dt}^ of the colloidal electrolyte 

 is correspondingly deficient and may be distinctly less than that of 

 a non-electrol}^^. Thus high conductivity goes hand in hand with 

 only moderate osmotic effects. 



Some of the very numerous substances which must be recognised 

 as belonging to this group are the protein and gelatine salts 

 (T. Brailsford Robertson's well-known alternative hypothesis, which 

 is to tl\e ert'ect that thei'e are no ordinaiy ions present,, but that the 

 protein salts ionise into two colloidal ions resembling ordinary slow 

 complex ions, appears to have been built up upon early E.M.F. data 

 which Pauli, Manabe, and Matuli have since shown to be erroneous), 

 dyes, such as Congo-red, indicators, the higher sulphonic acids and 

 hydrochlorides, tellurates and many inorganic substances ; in fact, 

 most substances of high molecular weight or containing long carbon 

 chains which are capable of spHtting off an ordinary ion. 



The soaps are a particularly interesting case for investigation 

 in that their chemical formulae are well ascertained, tautomerism 

 does not occur, true reversible reproducible equilibrium is established 

 in all solutions, and finally the definite transition from typical simj^le 

 electrolyte through colloidal electrolyte to neutral colloid may be 

 observed in all its stages. This transition from crystalloid to colloid 

 is exhibited not only in passing from salts of the lower to those of 

 the higher fatty acids, but may be demonstrated in any one of the 

 higher members merely upon change of temperature and concentration. 



This striking and wholly unexpected combination of properties 

 on the part of the ionic micelle, w&s plausibly explained in 1913 by 

 the wTiter, on mechanical grounds. This conception is based upon 

 the consideration of the application of the principle contained in 



