B5. Solution properties of salts and metals

In par B3. and B4., we were treating the case solutions containing isolated, completely solvated chains with the counterions attached at the charged chain backbone. So we looked from a viewpoint, as if Polyaniline (ES) or other organic metals / conductive polymers were polymers like all other polymers we are dealing with in our daily life and work.

But in fact, ES is principally different from any other polymers, it is

The solubility parameter concept has not been applied for salts yet, i.e., it was not possible to generalize this concept in a way that it would describe the solubility behavior of salts, too. 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).

In general salts have 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.

It should be kept in mind that a crystal generally has a much higher surface tension than the corresponding amorphous compound or its melt, and that surface tension increases with decreasing temperature.

The solubility of, e.g. NaCl, in H2O is ruled by the lattice energy. This is the energy required for separating the ions to an infinite distance. 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) [3], [4]:
 

salt lattice energy hydrat. energy 
A+		 hydrat. energy  
X-		 hydration energy difference
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
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

If water does not dissolve a salt, organic solvents will do the job even less.

An intermediate summary shows: 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.

A comparison of the situation for 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 ºC) or NH3 (23.4 at 11 ºC) have a much lower surface tension compared to metals like

Softer metals have a lower surface tension as harder ones, e.g. K (110 at melt T), Li (400/180 ºC), Na (200/123 ºC), but still significantly higher than non-metals.

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, because their intermolecular interactions are stronger. OM/ICPs are all of it together: PAni (ES) is a polymer, a salt, a metal and crystalline. In other words: the higher the intramolecular forces, the higher the surface tension. It does not seem very likely that such a material should have the same solubility characteristics like aniline, morpholine or benzylamine [47].

Footnotes

[1]R. Pelster, G. Nimtz, B. Wessling, Phys. Rev. B 49, 12718 - 12 723

[2]G. Nimtz, P. Marquardt, H. Gleiter, J. Cryst. Growth, 86, 66 (1988)

P. Marquardt, G. Nimtz, Phys. Rev. B, 40, 7996 (1989)

[3]Hollemann, Wiberg "Anorganische Chemie", W. de Gruyter, 1971

[4]Cotton, Wilkinson, "Anorganische Chemie", Verlag Chemie 1970

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