I ORGANIZATION OF SOLS 21 



In the coacervate the distribution of colloid particles is statistically 

 uniform, as in the original sol, although their concentration has been 

 increased. If the colloid particles are considered as dispersed phase, their 

 state has not been changed in the coacervation process; and yet 

 clearly a new phase boundary is formed between a layer rich in colloid 

 and one poor in colloid. This example shows how vague is the con- 

 cept of phase in colloid chemistry. For that reason the hydrophilic 

 sols and the coacervates originating from them are sometimes called 

 quasi-homogeneous phases, since the distribution of the particles is 

 completely uniform and the particle size is liable to decrease to mole- 

 cular dimensions. 



The dehydration of colloid particles illustrated in Fig. 20 can be 

 brought about in various ways; for instance, a rise in temperature, 

 which accentuates the contrast between bound and freely moving 

 water, will often suffice. Usually, however, use is made of dehydrating 

 substances such as sahs (salting out) or aliphatic alcohols or acetone. 

 Such substances, which disturb the stability of the sol and increase 

 the tendency to separate, are called sensitizers. Besides salts and organic 

 liquids, colloid solutions may also be used as sensitizers if they com- 

 pete with the particles of the original sol to bind the free water and 

 thus cause dehydration. 



The dispersing medium which is separated from the coacervate is 

 called the equilibrium liquid (Fig. 21), for, following changes in tempera- 

 ture or composition in the system, water is taken up or given oflF by 

 the coacervate. The situation is, therefore, analogous to the separation 

 in a mixture of phenol and water (p. 46). Coacervates can be regarded 

 as a solution of water in the colloid (swelling) and the equilibrium 

 liquid must then represent a solution of a small amount of colloid in 

 water. In the example given in Fig. 21, however, the gelatin is insoluble 

 in alcohol-water and the concentration of the colloid in the equilibrium 

 liquid is practically zero. Here the analogy therefore ceases since, in- 

 stead of a reciprocal solubility, there exists only a one-sided adsorption 

 of water by the colloid. The reason why gelatin is completely in- 

 soluble below its melting point will be made clear on p. 73. Coacervates 

 of homopolar substances have been studied by Mme Dobry (1938, 

 1940). 



Discbarge. In biological systems the colloid particles are seldom 

 neutral; usually they are electrically charged. Particles carrying opposite 



