3o8 ORGANISATION IN SPACE AND TIME 



electric point, at which pure proteins carry no net charge, 

 their solubiHty is at its lowest and therefore they are most 

 easily salted out with neutral salts. At pH 4-82 the net charge 

 of gelatin disappears and its hydration is least, but, on the 

 other hand, the charges on the gum arable particles are 

 not neutralised and the mean charge drawing together the 

 particles of gelatin and gum arable is greater than at lower 

 pH levels at which the charges can completely neutralise one 

 another. As the pH falls so the positive charge on the gelatin 

 increases and therefore requires more gum arable for its 

 neutralisation and for the formation of a coacervate. As the 

 pH increases, so the negative charge on the gum arable also 

 increases and it therefore requires the addition of a larger 

 amount of gelatin to form a coacervate. When the pH 

 becomes higher than the isoelectric point of gelatin and 

 both substances are negatively charged, their charges cannot 

 neutralise one another and coacervation cannot occur. In 

 all these cases the formation of a coacervate depends on the 

 mean net charge of the particles (i.e. on the algebraic sum 

 of the positive and negative charges of the associated part- 

 icles), and also on the degree of hydration and the solubility 

 of the particles. The same rules govern the formation of 

 coacervates from two proteins with markedly different iso- 

 electric points, and also from proteins and phosphorylated 

 starches and other such substances. 



As the system becomes more complicated, when coacervates 

 are formed from three components, for example, the condi- 

 tions under which they can arise become more complicated 

 also, as the mutual solubility of substances does not merely 

 depend on their charges, but also on many other factors, 

 hydrogen bonds, the hydration of non-ionising polar gioups 

 (e.g. OH, CO, etc.), the interaction of hydrophobic groups, 

 etc. We have already indicated that the theory of coacerva- 

 tion is very complicated. This is because the mutual solubil- 

 ity of substances is itself very complicated and we have, as 

 yet, no complete theory of solubility. This, however, does 

 not fundamentally alter our approach to the phenomenon of 

 coacervation, which may be considered as the various mani- 

 festations of the limited mutual solubility of substances which 



