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
greek symbols.
Everybody
is invited to comment to this paper. I will consider every argument and
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?
2.2 Solution properties of salts and metals
Up to here, we only discussed solubility from a viewpoint, as if Polyaniline (ES) or other organic metals/conductive polymers were polymers like all the others we are dealing with in our daily life and work. PAni might be "a little more polar" and "only unmoldable", that's what people would agree.
But in fact, ES is principally different from any other polymers, it is
· a salt: in ES every second monomer unit is protonated, and hence ES contains 0.5 Mol equivalent counteranions (Cl-, pTs- or other X-)
· a metal: we have shown[7] that PAni behaves like a typical mesoscopic metal[8]; PAni consists of mesoscopic particles of around 9.6 nm diameter with a metallic core of 8 nm [7].
All other salts which we know have a lot of comparable, but not identical properties in common; the same with metals: they are widely different in their properties (cf. Hg, Fe, Pt, Na, H2 under pressure), but they have those properties in common which we assign to be characteristic for metals. PAni is both: it is comparable in some sense with salts, in some sense also with metals (and in other sense: also with polymers). No other material class does this.
The solubility parameter concept has not yet been applied for salts, i.e.,
it was not yet possible to generalize this concept in a way that it would
describe the solubility behaviour of salts, too [6]. Therefore, the
solubility parameter of only a few salts has been determined, i. e. for salts
which occur as ion pairs (not as completely dissociated and solvated ions) in
solution. Some more, although still few, surface tension values of salts are
published (note: it is not possible to use a 
-
-relation for calculating
solubility parameters from surface tension data from salts).
Salts have in general a much higher surface tension than neutral (inorganic or organic) compounds. Also salts occurring as ion pairs in solution are showing rather high values, like 124 mN/m (KBr) or 135 (KCl). Salt melts have 114 (NaCl at 1035 K) or 140 mN/m (LiCl at 883 K). The corresponding solids will have an order of magnitude higher values. Also note: the surface tension increases with decreasing temperature.
We should have in mind when comparing with the situation for PAni that crystals generally have a much higher surface tension than the corresponding amorphous compound or its melt.
But the solubility of, e.g. NaCl, in H2O is ruled by a completely different factor: the lattice energy. For dissolving NaCl in H2O it is necessary to separate the ions to an infinite distance. The energy required for this is the lattice energy. It can be determined by using the Born-Haber-cycle.
Only those salts will be dissolved in a solvent like H2O if the hydration energy (or for other solvents: solvation energy) is higher than the lattice energy. For some salts, this is (in kcal/Mol)[9,10]:
salt lattice hydrat. hydrat. hydration difference
energy energy A+ energy X- energy
NaCl -183.1 Na+: -93.2 Cl-: -91.8 -185.0 -1.9: soluble
LiF -240.1 Li+: -119.3 F-: -109.3 -228.6 +11.5:
insoluble
AgCl -208.7 Ag+: -102.2 Cl-: -91.8 -194 +14.4:
insoluble
Note: The lattice energy is an energy specific for salts, and a requirement for solubility with a much higher restriction as we had discussed earlier, when we talked about the necessary melt energy: here, we have to separate the ions from each other, and not only to destroy a crystal arrangement, where the ions are still in some order.
The hydration energy comes from the energy gain when the charge is going to
be transferred from a medium (vacuum) with low dielectric constant
and a
polarizability 
of Zero into one with a significantly higher and , like
water.
Here we find the reason why water is the best solvent for salts: it has the
much higher (78.54) compared to even the most polar organic solvents, like
- formic acid: 58
- tri-chloro-acetic acid: 4.6
- dioxane: 2.2
- m-cresol: 11.8
- (and note for aniline: 6.9).
If water does not dissolve a salt, organic solvents will do the job even less.
This leads us to conclude that there are at least 2 contributions to a significant decrease of solubility for PAni (ES) compared to low molecular weight analogues: the polymeric and the salt character. We should assume that both contributions will even work synergistically is we cannot separate the ions completely from each other (in order to completely solvate them) as the positive charges are linked together via the polymer chain.
A comparison of the situation in some other inorganic substances, liquid elements, will give us some more aspects to think about. Non-metals like sulfur (60.9 mN/m at melt T), selen (92.4 at 217 °C) or NH3 (23.4 at 11 °C) compared to metals like
· Ag: 1000 mN/m at 900 °C
· Au: 1200 at 1070 °C
· Cu: 1100 - 1400 at melt T
· Fe: 1500 - 1700 at 1400 - 1600 °C
have a much lower surface tension. Softer metals have a lower surface tension as harder ones, e.g. K (110 at melt T), Li (400/180 °C), Na (200/123 °C), but still significantly higher than non-metals.
We will summarize: metals, crystals, salts and polymers have a much higher surface tension, a much lower solubility or additional restrictions for solubility than non-metals, amorphous or neutral compounds and low molecular weight materials. OM/ICPs are all of it together: PAni (ES) is a polymer, a metal, a salt and crystalline. And such a material is said to have the same range of surface tension or solubility like Aniline, Morpholine or Benzylamine [3]?
In more general words: the higher the intramolecular forces, the higher the surface tension. We should therefore not be too surprised when finding surface tension or solubility values for PAni (ES) and other OM/ICPs significantly higher than for any other polymer, and significantly higher that estimated by Shacklette [3] or measured by us [18].
It should also be considered in this context, that the metallic conductivity in organic metals like polyaniline is based on a metallic conduction band 3-dimensionally extended over many polymer molecules (within an about 8 nm metallic core). Metallic conductivity is a phenomenon based on supramolecular properties and intramolecular interactions. This will result in very strong intramolecular forces.
