Emulsifiers, Detergents, and Surfactants in Green Synthesis

Cetyl trimethyl ammonium bromide- A Detergent

As part of the increased popularity of ‘green chemistry’, many persons have considered doing organic chemical synthesis using water as solvent. This may be a good idea or it may be simplistic. Using water avoids organic solvents, but even when there are only small amounts of dissolved or entrained organics, cleaning up waste water so that it can be sent to sewage is neither simple nor green. Destroying water by combustion is expensive. Whatever the outcome of this controversy, detergents, emulsifiers or surfactants, whatever you choose to call them, can cause homogeneous, bulk water to hold substantial amounts of lipophilic materials. Chemicals can react in such media, sometimes with rate accelerations. It is not clear whether these accelerations are caused principally by miscelle formation or phase transfer catalysis or some combination of the two.  

Generally the presence of these amphiphilic substances makes isolation procedures more difficult, because chemical separation at the molecular level is achieved by phase separation at the macroscopic level and an emulsifying agent makes the two most common immiscible liquid phases oil and water, more miscible; therefore, it seems necessary that the surfactant should be destructible so that it loses its ability to emulsify once the reaction is complete and before the separation phase of the process is begun.

An article by F.M. Menger, J.U. Rhee and H.K. Rhee published [J. Org.Chem., 40, 3803 (1975)] is one of the early preliminary explorations. In one experiment, they compared the oxidation of piperonal, mp 36∞ C, using potassium permanganate in water at 55∞C, with and without 0.01M cetyltrimethylammonium bromide.  The authors observed about 33-37% yield without surfactant and 64-74% yield with surfactant.  Surprisingly, the yield appeared to be independent of the reaction time; whether 70, 100 and 150 minutes was used.  This was not further explored even though the authors were aware that in Organic Synthesis Coll. Vol. II this same reaction had been done without emulsifier at 80∞C but with vigorous agitation of the water and molten piperonal phases giving a 97% yield. It may have been that the permanganate was degrading as the reaction proceeded and the oxidizing capacity was over after 70 minutes or less. Another complicating possibility is that at the reaction temperature the permanganate also attacks the bromide in the cetyltrimethyl ammonium salt.  The permanganate could oxidize bromide to bromine and this could in turn brominate the piperonal leading to brominated by-products. This latter suggestion is supported by the fact that in one of two runs of 70 minutes duration, where the emulsifier was mixed with the room temperature potassium permanganate and water and added dropwise to the warm piperonal solution, a yield of 74% was achieved compared to 66% when the emulsifier was all placed completely in the hot water and piperonal mixture before starting to add oxidant.

These authors noted that the time saved from the increased reaction rate with emulsifier present was spent in the extended time needed to get phase separation in the extractive isolation. The experimental portion of the article states that when surfactant was present, 2 hours were allowed for phase separation.

In another trial of the methodology, α,α,α-trichlorotoluene was hydrolyzed by 20% aqueous sodium hydroxide at 80∞C.  Using 0.01M cetyltrimethylammonium bromide the reaction gave a 98% yield in 1.5 hours while without catalyst there was zero yield.  In this reaction, the non-ionic block polymer emulsifier  Brij 35 [C₁₂H₃₅(OCH₂CH₂)₂₃OH ] reduced the time to 11 hours for a 97% yield. No detergent such as sodium lauryl sulphate was tried in these reactions.

David Jaeger worked on this possibility in the 1980s 

Jaeger, D.A.; and Frey, M.R., J. Org. Chem. 47, 311 (1982).

Jaeger, D.A. and Ward, M.D. J. Org. Chem. 47, 2221 (1982). 

Jaeger, D.A. and, Martin, C.A. and Golich, T.G. ????

Craig, A. Timothy G. Golich and David A Jaeger, J. of Colloid and Interfacial Science, 99, 561 (1984).

In reaction using a cleavable surfactant, the type of surfactant and the reagent and conditions for cleavage need to be selected in advance so that the product is inert. 

It is still necessary to be able to cleanly separate the cleaved lipophilic portion of the surfactant from the hydrophobic portion.

If the hydrophilic portion is a di-quarternary ammonium salt it is a possibility that it can be precipitated as the embonate salt. This might be converted cleanly back into a more soluble quarternary ammonium species.

It is also possible, if the lipophilic portion is a straight chain alcohol, it might be separated as a urea complex by crystallizing the complex out from a mixture of urea and methanol.

This problem has been approached in a different aspect by persons who were asking how long chain quarternary ammonium salts could be metabolized.  In these cases the long chain amphiphile was interrupted by an ester which under physiological conditions could be hydrolyzed.

NOTE: This blog article was started at least forty years ago so there is a lot of literature not taken into consideration.  It is probably pertinent whether any degradable emulsifiers are now commercial.