Choice of solvent-nonsolvent system

There are a number of other parameters, in addition to those listed, such as the use of additives (low molecular weight as well as high molecular weight components), the molecular weight distribution, the ability to crystallise or aggregate, the temperature of the polymer solution and of the coagulation bath, etc., that also influence the ultimate structure obtained after phase inversion. These latter factors will not be considered here. 

One of the main variables in the immersion precipitation process is the choice of the solvent/nonsolvent system. In order to prepare a membrane from a polymer by phase inversion the polymer must be soluble. Although one or more solvents may be suitable for the chosen polymer, the solvent and nonsolvent must be completely miscible. Water is frequently used as a nonsolvent but other nonsolvents can also be used. Some solvents for cellulose acetate and polysulfone which are miscible with water are listed in table III.6. The solubility of these organic solvents with water must be considered further. As described in the previous section, the miscibility of components of all kind is determined by the free enthalpy of mixing 

For ideal solutions = 0 and AS = ASn, ijej,). However, mixtures of organic solvents 

The excess free enthalpy of mixing (G ) is the difference between the actual free enthalpy of mixing (AG ,) and the ideal free enthalpy of mixing (AG j 

If G is known, gj 2 can be calculated as a function of composition for any binary mixture (0 refers to the volume fraction in the binary solution and x to the mol fraction, respectively). In fact, represents the non-ideal part of the free enthalpy of mixing and can be expressed as 

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