B7. The role of the solvent as dispersing medium

In the case of a solution, it is easy to understand what the solvent does with the molecules: it dissolves it. This is possible because the system can gain some free energy and is deliberately moving towards thermodynamic equilibrium.

But why does a solvent disperse another material, which cannot be dissolved? Or is it "necessary for the solvent to possess solubility characteristics (parameters) which are a close match to those of the (conductive) polymer [47]"? In other words: is it possible to determine the solubility parameter and hence the surface tension of an insoluble material by looking at solvent systems in which we can disperse this chemical species?

In 1856, M. Faraday prepared the first colloidal gold dispersion in water. There is no question since Faraday's time that this system is in fact a gold sol (dispersion) and not an elemental gold solution in water. Would we now conclude, that the surface tension of water (ca 70 mN/m) is close to the one of gold? Not at all. Solid gold is estimated to have a surface tension of up to 2.000 mN/m. A gold melt has a surface tension of about 1000 (at 1200 ºC) [1]. Would we conclude that the solubility parameters are "a close match"? No.

A comparison of surface tension values of metals and solvents shows, that they are different by several orders of magnitude. So it may be asked: Is there any solvent for metals at all (a solvent dissolving gold, iron or copper in elemental form, not by oxidizing it to Au3+, Fe2+ or Cu2+)? Synthetic chemists are often using a "sodium solution in ammonia" for reduction purposes - a brilliant blue metal / solvent system. But some older evaluations [2] are showing, that also this system is a dispersion, which is the reason for the sudden increase of conductivity at a certain critical concentration, see fig. 6 - a phenomenon surprisingly parallel to what we are describing for heterogeneous polymer systems and explaining by a sudden dissipative structure formation [3].

We note: if a solvent is not capable of overcoming the intramolecular forces (by introducing the heat of fusion or by delivering more solvation energy than lattice energy) of a given material, then it cannot dissolve it. The surface tension is an expression of these intra-molecular forces. The huge difference of between metals and solvents prevents any solubility of the former. But solvents can disperse metals, as they might reduce their very high surface tension!

The reduction of surface tension by adsorption is given by

(9)   = 0 - ads

For understanding this principle we have to compare the amount of surface energy

 G = 2 0 A

(with 0   being the surface tension and A the surface)

generated (or: the energy necessary for creating the surface) in vacuo and in a liquid medium. In the first case, in vacuo, the energy is necessary for dividing the bulk into the particles, making the surface and separating the particles from each other so far that they will not attract each other any more.

In the latter case, the solvent is filling the space between the particles, is shielding local surface charges and can therefore reduce the attraction forces. Moreover, it will reduce the overall surface energy (interfacial energy). It is a fundamental result of colloidal science that adsorption of a substance lowers the surface tension at that surface. This is broadly known for the water / gas interface when adding a liquid soap (or a drop of oil). It will instantaneously spread over the whole surface because it lowers its surface tension towards the gaseous atmosphere.

So water will spread on high energy surfaces like hydrated silica or gold (wetting angle 0º), it will wet graphite having a higher, but not orders of magnitude higher surface tension than water with a wetting angle of 85,7º, and it will not wet PE or PTFE (wetting angle 105º and 115º, resp.), which have a much lower surface energy than water.

Let us recall: In contrast to solutions, dispersions are in a state of non-equilibrium because of the solvent's inability of overcoming the intramolecular forces (hence H >> 0 and
|-TS|  < H. Therefore, also the necessary energy for forming the interfaces has to be pumped in by process energy. Solvents are capable of lowering the interfacial energy when interacting with the surface of insoluble materials, as also do detergents.

Footnotes

[1]see [6], ref. c) Handbook of Chemistry and Physics

[2]E. Bosch, Z. Phys. 137, 89 - 103 (1954)

[3]B. Wessling, Polym. Sci. Engin. 31 (16), 1200 - 1206 (1991) and also cf. [44b]; for a broader discussion of dissipative structures in such systems, cf. B. Wessling, in: S. Nalwa (ed.) Handbook of Organic Conductive Molecules and Polymers, Vol III, J. Wiley (in press)

[ previous | TOC | next ]



Home