This paper has been submitted at Nov 4th, 1996, for publication in a
scientific journal and is now subject to the reviewing process.
You may ask for a hardcopy with the complete manuscript, which contains all figures and all
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comment back.
This is the 3rd revised version.
Dr. Bernhard Wessling
Zipperling Kessler / Ormecon Chemie
D-22949 Ammersbek
Conductive Polymer / Solvent Systems: Solutions or Dispersions?
2.5 The role of the solvent as solvating or dispersing medium
We can now come to discussing the role of the solvent in either case. In both cases, we start with the pure solvent and a chemical species. We assume
a) it is soluble
b) it is not soluble, but can be dispersed.
Two principally different curves would ideally result when plotting the free energy versus process time during a) dissolution b) dispersion, leading to a) an energy minimum, b) an energy maximum (or a relative minimum after the maximum), resp., as can be seen in fig. 5.
In the case of a solution, it is easy to understand what the solvent does with the molecules: it dissolves it, and this is possible because the system can gain some free energy.
But why does the solvent disperse the other material, which cannot be dissolved? Or, as Shacklette wrote [3]: is it "necessary for the solvent to possess solubility characteristics (parameters) which are a close match to those of the (conductive) polymer"? 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. To repeat the experiment the reader might reduce gold chloride in water with sodium citrate (which he did with phosphorous). After a short time, a blue coloration will appear and then a ruby-red gold dispersion.
There is no question any more since Faraday's time that this system is in fact a gold sol 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 °C)[15]. Or 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 let me ask: 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[16] 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[17].
We should 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 the given material, then it cannot dissolve it.
The surface tension is an expression of these intramolecular 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!
Back to the question why a solvent can act as dispersing medium for insoluble materials. We will compare the amount of surface energy generated (or: energy put in to create the surface) in vacuo and in a liquid medium. In the first case 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 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.
The reduction of surface tension by adsorption is given by
So water will spread on high energy surfaces like hydrated silica or gold (wetting angle 0°), it will wet graphite having a higher, but not orders of magnitude higher surface tension than water, with a wetting angle of 85,7°, and it will not wet PE or PTFE (wetting angle 105° and 115°, 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 (only) capable of lowering the interfacial energy when
interacting with insoluble materials, as also do detergents.
