Methods Nucleation and Particle Growth

In principle one can prepare a colloidal particle from a molecular species by forming molecular complexes of increasing size until the colloid size range is reached, and this is the way in which very many dispersions are formed. Usually the molecular (or atomic) species are formed by a chemical reaction, and being virtually insoluble in the dispersion medium they aggregate into particles of increasing size. Among familiar examples are the formation of colloidal sulphur by the interaction of sodium 

This process is, however, not as simple as this brief description implies, and stable colloidal dispersions are obtained only if the conditions are properly controlled. 

In the first place, passage from the molecular state to a particle involves passing through the intermediate size range of 1—10 nm in the process of nucleation. We can see rather generally what this involves in the following way a more rigorous alternative treatment will be mentioned in Chapter 11. 

We have already seen (Chapter 2) that the surface tension of a pure substance can be equated to the difference per unit area in free energy between bulk and surface molecules, so that in this 

Now for a saturated solution x =, ts u and ptb — pi° + kTIn that equation (4.9) becomes xsal, so 

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