Sunday, 3 July 2011

Chemical Reactions in Solution: It all happens at the liquid surface.

The actual fact is that everybody was wrong: chemical reactions between dissolved chemicals happen at the surface of liquids more often than not. The mathematical form of chemical reactions and chemical equilibria is properly explained by the fact that it is about the various species involved concentrating near the surface (often in layers at different distances from the surface). When they are sufficiently concentrated to reach out, then they react. The implication is that many processes in the chemical industry which seem to be working properly are simply working in the best possible way for a device designed under the wrong set of assumptions.

It is known that solutes, chemicals dissolved in a solvent, tend to distribute unevenly between the bulk of the liquid and its surface. However, for a long time it has been assumed that their amount at their surface is small (which it is) and its influence unimportant (which, as I show here, is not).

It is well known that solutes, e.g. salts in water behave "ideally" in diluted solutions, but things are not so when they are sufficiently concentrated (which changes from solute to solute). "Ideal" behavior means that any reactions and processes they are involved in can be explained in relation with their concentrations. "Non-ideal" behavior means they appear to be at a different concentration. This is evaluated by calculating "activities" (apparent concentrations) and activity coefficients (ratios activity/concentrations).

Surfactants are a type of chemical composed of fragments with affinity for water and oil. Because of that, they place themselves at the surface of water (or oil, depending on their main affinity) in a more marked way that other solutes. And, because they cause the surface of an oil to have some properties of water and the surface of water to have some properties of oil (mainly, they change the surface tension of the liquid), their content at the surface of a liquid can be measured more easily than with other compounds.

I have recently found that in the case of ionic surfactants (which are salts that dissolve in water giving off a cationic -positive- and an anionic -negative- fragment like other salts)  their content at the surface (surface excess) follows a relationship called the Bjerrum correlation, which predicts when ions of opposite charge are too close to remain free, but get together in what are called ionic pairs. This happens when a type of aggregate called micelle (spherical micelles can be visualized as footballs where instead of patches you have the surfactant fragments with affinity to water if the micelle is in water, or the surfactant fragments with affinity to oil if the micelle is in oil) can form in the surfactant solution.

I have also found out that when ionic surfactants and nonionic surfactants (which have a fragment with affinity to oil and a petroleum derived fragment with affinity with water but do not behave as salts) the ability of ionic surfactants to form ionic pairs changes, and that there are several possible "ionic pairs" (or rather, "ionic groups") forming.If there is little ionic surfactant, then no ionic pairs form, the more ionic surfactant present, more dense ionic pairs form.

This is important, because the solubility of surfactants can be explained from the relative abundance of free surfactant and ionic pairs. The solubility of surfactants is peculiar in that in some conditions the more of a chemical which would cause precipitation (insolubility) of a surfactant there is, the less surfactant may remain in solution. But if the content of the precipitating chemical keeps on raising, then suddenly the amount of surfactant that may remain in solution.

This behavior, which is unlike other salts, cannot be explained from the concentration in the bulk phase of the surfactant, but can be explained if the precipitation of the surfactant is linked to the effective content of surfactant at the surface that CAN precipitate because it is not forming persistent ionic groups.

Until here, this could be considered specific of surfactants. But it is not, because some research has found salts concentrating its positive and negative fragments in two separate layers near the surface of the liquid.

This justifies the idea that precipitation of salts happens at the surface. And that other chemical reactions do the same. And here is where the perverse bit comes in place. It has not been found out before it happens on all surfaces of a liquid. It happens in the walls, the bottom and.... the surface of anything you dip in the liquid. Any sensor which has even been introduced inside a solution has created new surface, where an excess has formed. The readings have always been those of the surface excess.

Now, in dilute solutions there is a linear proportionality between the concentration in the solution and the surface excess. "Linear proportionality" means that one is equal to the other times a constant. Thus, during years solutions on which measures have been made in this "linear proportionality" region have been considered  "ideal solutions", whereas solutions in which the pile-up of chemicals in the "surface excess" prevented the relationship between concentration and surface excess to remain linear have been considered "non-ideal solutions".

The best part is that non-physical means of analysis do not avoid this. Optical means of measuring solutions measure how much light has managed to go through a sample. That is, has not been absorbed by the chemicals in the solution..... which have been concentrated at the surface. And the story repeats itself: there is a region of diluted solutions where the proportion of rejected light to concentration of a solute is linear and a region of concentrated solutions where.... it is not. This would actually show that coloring in solutions exactly follows the behavior of coloring with paints. The more layers of paint are applied, there is a pile up of dye at the surface of the object, until the color with not become more intense. The more concentrated the solution, the pile up of the surface excess will reach a value where the color of the liquid will not change anymore because the surface excess will be saturated.

A paper on this subject has already been rejected in Journal of the American Chemical Society by its Editor and by Langmuir, where 6 out of 8 referees refused to review it (yes, NOT declared the paper worthless, but refused to review). From the other two, one was in strongly in favor and one dead against. In both cases the editorial argument was that the subject was too specialized for a chemical journal. Which is fascinating because it affects the foundations of all the solution chemistry (and, yes, solution chemistry is VERY important in chemistry).

So, just in case, I have decided to make this post.

© Federico Talens-Alesson 2011

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