Does solubility affect photolytic degradation rate?

[banyen]

Does the solubility of a chemical affect the photolytic degradation rate? For example, if the chemical triclosan is more soluble in a higher pH, does that mean that the chemical will degrade faster in a higher pH buffer solution? If that is true, why is that?

[deleted-71417]

Hi,
The answer is it depends. The first step in photolysis is for the molecule to absorb a light photon. This leaves the molecule in an excited state ( with lots of energy rattling around inside the molecule). The next thing that happens determines the answer to your question. Sometimes in some molecules the extra energy is enough to cause the molecule to break at least one bond - when this happens sometimes the atoms in the molecule rearrange and form new bonds to make a new molecule if the same formula or fall apart into two or more new molecules. Other times the excited molecule reacts with a neighboring solvent molecule to form a different compound- this might not happen if the photolysis target is not in solution. If the first case applies the molecule may not need to be in solution to rearrange(an example of this type is a molecule that contains a double bond, which when irradiated opens to a single bond and rotates around the single bond before reforming the double bond, effecting a cis-trans isomerization). Another type might be a molecule containing a nitrogen-nitrogen single or double bond that when excited falls apart to form a nitrogen molecule and the remains of the original molecule then react further, either with itself or anything else nearby. To begin to understand what happens in any specific case you have to determine what the photolysis product molecules are and how the are related to the original molecule and the compounds in the neighborhood (ie the solvent or crystal structure). You also have to realize the molecules react differently when they are ions than when they are neutral, so pH can make a big difference.

This is a very general answer - we would have to investigate the specifics of photolysis of triclosan to try to come with a more specific answer.

Best regards,

Barrett Tomlinson