92168.pdf

12 The colloidal state

The rate of particle growth depends mainly on the following

factors:

1. The amount of material available.


2. The viscosity of the medium, which controls the rate of diffusion
   of material to the particle surface.


3. The ease with which the material is correctly orientated and
   incorporated into the crystal lattice of the particle.


4. Adsorption of impurities on the particle surface, which act as
   growth inhibitors.


5. Particle-particle aggregation.

Von Weimarn (1908) investigated the dependence on reagent

concentration of the particle sizes of barium sulphate precipitates

formed in alcohol-water mixtures by the reaction

Ba(CNS) 2 + MgSO 4 -> BaSO 4 + Mg(CNS) 2

At very low concentrations, c. 10~^4 to 10~^3 mol dm~\ the
supersaturation is sufficient for extensive nucleation to occur, but
crystal growth is limited by the availability of material, with the result
that a sol is formed. At moderate concentrations, c. 10~^2 to 10"^1 mol
dm""^3 , the extent of nucleation is not much greater, so that more
material is available for crystal growth and a coarse filterable
precipitate is formed. At very high concentrations, c. 2 to 3 mol dm"^3 ,
the high viscosity of the medium slows down the rate of crystal
growth sufficiently to allow time for much more extensive nucleation
and the formation of very many small particles. Owing to their

Precipitate

Concentration

Figure 1.3 The dependence of particle size on reagent concentration for the

precipitation of a sparingly soluble material