46 



FUNDAMENTALS OF SUBMICROSCOPIC MORPHOLOGY 



but. 



whereas in that case the number of HgO molecules round a 



80 

 °C 



60 

 40 

 20 



hydroxyl group could be increased ad libitum, each -O-bridge can 

 only attract a fraction of a water molecule. For that reason, as soon as 

 the amount of water present exceeds a certain limit, the water mole- 

 cules must cluster together. They accumulate into drops and form 

 their own phase. Conversely, a few ether molecules may be dispersed 

 in this phase, but, as will be shown, these ether molecules tend to 

 accumulate in the neighbourhood of the phase boundary. 



Much the same phenomena are observed in the phenol/water sys- 

 tem (Fig. 42). If some phenol is added to 

 water, it is dissolved. Beyond a certain 

 percentage of phenol, however, two co- 

 existent phases are obtained, which do 

 not mix. Similarly, traces of water are 

 soluble in pure phenol, but if the amount 

 of water is increased, a miscibility limit 

 is reached beyond which the two phases 

 no longer mix. As shown in Fig. 42, the 

 miscibility depends not only on the con- 

 centration of the two components but also 

 on the temperature. In the region called 

 the Miscibility gap the system is hetero- 

 geneous. Here two phases are formed, one 

 consisting of phenol saturated with wa- 

 ter and the other of water saturated with 

 phenol. Outside the miscibility gap only 

 a single phase exists, a homogeneous 

 solution with a completely uniform distribution of intermingled 

 phenol and water molecules. When heat is supplied, the miscibility 

 of the two components increases, until at a certain temperature the 

 miscibility gap disappears. At low temperature the hydration layer of 

 the phenolic OH-group is smaller in si2e than the phenylic residue, 

 so that limited miscibility results. With rising temperature the hydration 

 sphere is increased and at 69° C surrounds the whole space of the CgHg- 

 group (comparable to Fig. 4 1 a) causing in this way unlimited miscibility. 

 To sum up, the decisive factor in the solubility of organic substances 

 in water is not only the presence of hydrophilic (i.e., water-attracting) 

 groups, but primarily also their nuwber in comparison with the number 



Water - 



20 



40 



60 



80 100% 



— ^Phenol 



Fig. 42. Diagram of miscibility of 

 the water/phenol system (from 

 Rothmund, 1898). Abscissa: 

 from left to right content of 

 phenol in % of weight. Ordinate: 

 Temperature in °C. 



