B1. Difference between Solutions and Dispersions

In contrast to solutions, dispersions are heterogeneous on a microscopic scale. They consist of 2 or more phases., whereby solutions are single-phased. Other basic properties of colloidal systems can be found in numerous textbooks (for an introduction into colloidal science, see ref. [1]). Due to the presence of at least 2 phases, dispersions have interfaces (between the dispersed and the continuous phases), a feature not occurring in solutions.

It should especially been taken into account, that colloidal particles, when approaching a size well below 1 , do have a significant portion of their molecules at their surface (where they behave totally different from molecules in bulk or in solution!). For instance, particles with a diameter of 10 nm (exhibiting a surface area of 300 mē per gram) have about 25% of their molecules being situated at the surface. All those specific properties of colloids, which differentiate them from solutions, arise from the presence and the powerful interactions at and between the interfaces.

Colloids are not a special class of chemicals, but chemical species in a very special state.

There is another major difference between solutions and dispersions, their thermodynamical status. Solutions are systems in thermodynamic equilibrium with a free energy lower than that of the pure components, and can be described with equilibrium thermodynamics [2]. Many solution processes are exothermic (H < 0), all of them are exergon (deliberately proceeding):

(1) G = H - TS = Hmelt + Hsolv - TS (with G < 0).

The driving force for solvation, where H is positive, is the increase of entropy. If H is positive, but its value is smaller than |-TS|, then the polymer will be soluble. If H is >0 and > - TS, the polymer is insoluble.

For real dissolution (and not partial "swelling") every monomer unit must be completely solvated (i.e. surrounded by solvent molecules). This means, that the macromolecule takes up as many solvent molecules as is necessary for solvation. Conventional and "easily" soluble polymers therefore need a volume of solvents which is a factor of up to 1000 higher than their own volume.

Footnotes

[1]for entering this field of science, many textbooks are available, like

a) S. Ross, I. Morrison: "Colloidal Systems and Interfaces", J.- Wiley 1988

b) D. Everett "Basic Principles of Colloid Science", Royal Society of Chemistry, 1988

from which I took some ideas for this paragraph

[2]for a deeper discussion, cf. any good textbook about macromolecular science

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