3o6 ORGANISATION IN SPACE AND TIME 



in composition between the two layers decreases, the surface 

 tension of the droplets also decreases and, finally, at a particu- 

 lar temperature, known as the critical temperature of solubil- 

 ity, the boundary between the drops and the surrounding 

 medium disappears and a homogeneous solution is formed. 

 For systems of water and phenol this temperature is 65-9° C 

 and for mixtures of methanol and cyclohexane 491° C, etc. 



When the temperature falls very slightly below this critical 

 temperature (only o-i -0-2° C), statistical variations in density 

 occur and the system begins once more to differentiate. 

 However, as the difference in composition between the 

 phases and the magnitude of the surface tension (some hun- 

 dredths of a dyne /cm. at these temperatures) are not so 

 marked as at room temperature, the droplets have no great 

 tendency to coalesce and they form a stable drop-coacervate 

 of a high degree of dispersion. When the temperature falls 

 further the droplets will once more coalesce and the two 

 substances dissolved in one another will separate out into 

 two layers. If we change the relative mutual solubility of the 

 two substances, for example by the addition of salt or 

 naphthalene, this correspondingly alters the temperature 

 relations of their mixtures or drop-coacervates. 



Thus, in this simple example of two liquids of low mole- 

 cular weight which are partially soluble in one another, 

 the formation of drop-coacervates is limited to a narrow 

 region in which the surface tension is very low. If it in- 

 creases the liquids separate out completely ; if it approaches 

 too close to zero they mix completely. 



In the systems with which we are concerned, containing 

 protein-like substances and others of high molecular weight, 

 the phenomenon must be more complicated and the ' play ' 

 of surface tension will not, by itself, be enough to determine 

 the formation of drop-coacervates. Nevertheless, the essential 

 condition for coacervation, the limited mutual solubility of 

 substances, remains just as important as before. 



Bungenberg de Jong obtained simple coacervates from 

 aqueous solutions of gelatin by adding dehydrating agents 

 such as ethanol or sodium sulphate, which decrease the 

 hydration of the particles of gelatin and thereby decrease 



