This paper has been submitted at Nov 4th, 1996, for publication in a
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This is the 3rd revised version.
Dr. Bernhard Wessling
Zipperling Kessler / Ormecon Chemie
D-22949 Ammersbek
3. Surface tension of polyaniline
Let us assume, conductive polymers, and more specifically: polyaniline, were just dispersible but not soluble (we will discuss the other option again later). Then the solvents would act as described above: they would lower the surface tension of PAni.
The only measurement of surface tension of polyaniline or any intrinsically conductive polymer (in "doped" form), that I am aware of, is our own measurement on layers of pure PAni -HCl deposited from dispersion[18]. We found a surface tension of 69.4 mN/m. As this is the only experimental determination of it until now, we will deal with it carefully, but one should also not just discard the value and state, that the value seems being too high, without measuring, as was done before [3].
Such a value could be translated into a solubility parameter of about 45 (J/m3)1/2 if we were allowed to use this relation even though the protonated form of PAni (ES) is a salt. This is close enough for making the reasonable assumption, that water, having a solubility parameter of 47.9 should be able to dissolve polyaniline. But this is not the case, nobody claims that this is possible.
Now, according to the majority, solvents with a much lower solubility parameter should be able to dissolve it? How that? This would be in basic contradiction to solution thermodynamics as discussed above.
Let us compare our value for polyaniline with surface tension values of some solvents (in mN/m):
aniline morpholin pyrrolido DMSO m-cresol NMP DMF p-xylene
e ne
42.9 23,5 47.7 24 32.5 25 25 28.4
These solvents were considered to be "single solvents for polyaniline tosylate" [3]. We consider some of them being efficient dispersion media for polyaniline, in accordance with our assumptions for this discussion paragraph. That means: these solvents would act as reducing the surface tension of PAni . But what is the real surface tension of PAni? Shacklette deducted from his "dissolution" experiments a value of 49.6 mN/m for PAni-Tosylate. We had found 69.4 mN/m experimentally for PAni-HCl. Any solvent - especially those miscible with water, but not limited to this - could reduce the surface tension found by us to any lower value.
So, Shacklette's "solvents" would act as dispersion medium. This is still in accordance with a value of 69.4. If pure polyaniline tosylate is not soluble, but dispersible, and if it has a rather high surface tension, it would adsorb other materials in order to reduce its surface tension. What would be adsorbed? I propose: preferentially water. polyaniline tosylate is polymerized in water medium, is washed with water, and dried below 100 °C; when dried very carefully, it takes up water very easily, about 1 - 2 % - so it is highly probable that water is adsorbed on the polyaniline particle surface. Let's look at its surface tension:
water: 72.9 mN/m
Considering the arguments above, water would first reduce the surface tension of polyaniline, so that we [7] measured only the surface tension of a material which had water adsorbed (which dissolves some electrolytes like HCl or tosylate) - so we measure the surface tension of the modified water surface! And PAni might have a surface tension of at least 200 mN/m (like salts) up to 2000 mN/m (like metals).
This would explain another fact: it is not so easy to prepare dispersions of polyaniline in water. Why? If water is adsorbed preferentially, leading to a surface tension of 69.4 mN/m[19], then water would not be able to disperse this material, as it cannot reduce its surface tension any more[20].
Polyaniline prepared in other media, like 2-butoxyethanol[21] would not adsorb water preferentially, but 2-butoxyethanol. Such a polyaniline type would be dispersible in other media, and in fact, it is. The company claims[22] that their polyaniline is "soluble" in xylene (surface tension 28 - 29 mN/m), butylacetate (ca 24), methylene chloride (ca. 26.5), and toluene (ca 27.5 mN/m).
2-Butoxyethanol has a surface tension of ca 27 mN/m. Probably, the surface tension of the PAni particles polymerized in this medium will be somewhat higher due to the influence of water and electrolytes present. The a.m. solvents are then capable of lowering the surface tension of this material, which has a different adsorbed layer and hence a lower surface tension compared with polyaniline prepared in water media.
Polyaniline doped with other "dopants" like dodecylbenzene-sulfonic acid (DBSA) are showing a different behavior: they are said[23] to be soluble in unpolar solvents like xylene. This would be rather surprising, if we would really find true solutions.
In the context of this paragraph, we propose to consider that the dopants like DBSA or camphersulfonic acid (CSA, which also changes the "solubility" characteristics drastically [27]) are partially located at the particle surface with their unpolar tail directed to the outside (cf. fig. 13). If this model is correct, then several appropriate solvents would wet these surface types better than others, which are better suited for polyaniline tosylate particles.
m-Cresol acts slightly different in our dispersion picture: first it protonates EB then it disperses the formed ES. It acts then by reducing the increased surface tension of the resulting ES Pani-cresolate. m-Cresol cannot dissolve neither disperse other PAni types. J. Frommers first observation of "dissolution" of PPS-AsF5 in AsF3 (cf. 4.1) is to be understood in the same way.
