Lipase-Catalyzed Synthesis of a "Green" Surfactant

Surfactants are molecules that lower the surface tension in water. They behave this way because of their dual nature: a long, hydrophobic carbon tail attached to an ionic end, typically an acid or ammonium group. Surfactants are used in a variety of industrial applications including detergents and wetting agents. A number of widely used surfactants are problematic in that they are non-biodegradable and toxic. PFOA and PFOS are perfluorinated compounds (all hydrogens replaced with fluorines) – the fluorines make the compounds very good surfactants, but also very unreactive (and not biodegradable). They are also known carcinogens. SDS and SLES are not so bad, but still they are irritants and contain sulfonate groups that make them not as easily biodegradable as other compounds.

This article describes the synthesis of an alternative surfactant that is both more biologically friendly (non-toxic and biodegradable) and uses “green” chemistry methods that improve the efficiency of the synthesis and reduce hazardous waste products. N-Acyl palmitoyl ethanolamine is derived from two common constituents of biological lipids: the fatty acid palmitic acid and ethanolamine. Like soap, this surfactant is cheap, non-toxic and biodegradable. Like many industrial surfactants, and unlike soap, it is not prone to forming soap scum with hard water.

They used the enzyme Lipase to catalyze the reaction between ethanolamine and a fatty acid (or its ethyl ester). There are two possible products depending on which end of the ethanolamine reacts: either a hydroxy amide or an amino ester. Perhaps not too surprisingly, the hydroxy amide is formed exclusively. Amides are generally more stable than esters, amines are stronger nucleophiles than alcohols, and as the authors point out themselves amino esters with only two carbons between the N and O will rearrange on their own to the more stable amide form. Lipase was chosen because it is a flexible catalyst and the reaction takes place under mild conditions (no strong acids or bases to neutralize at the end).

They investigated three different sets of reaction conditions:

1. reactants in solution, conventional heat source
2. reactants in solution, microwave heating
3. reactants in solid state, microwave heating
   

The enzyme itself was attached to porous resin beads. For the solution-phase experiments, the enzyme and reactants were added to dioxane. For the solid-phase experiment, the reactants were dissolved in a small amount of solvent which was added to the enzyme and then evaporated to leave the reactants as a film on the enzyme-containing beads. Both microwave-heated reactions went considerably faster than conventional heating, and the solid-phase reaction was faster that the solution.

What exactly makes this "green?" While dioxane is not the most desirable solvent, it can be recovered afterwards and re-used. Otherwise, there is little waste. If fatty acids are used as the starting material instead of the corresponding ester there are no waste byproducts (such as ethanol) to separate and dispose of safely, and no strong acids or bases to neutralize. Even the enzyme is re-usable. And the product hydroxy amide is of low toxicity and should be biodegradable as well. If the solid-phase reaction with microwave heating turns out to be practical, it would greatly improve the efficiency of the process by speeding-up the reaction and improving the yield and purity of the products.

Kidwai, M., Poddar, R., & Mothsra, P. (2009). N-acylation of ethanolamine using lipase: a chemoselective catalyst. Beilstein Journal of Organic Chemistry, 5 DOI: 10.3762/bjoc.5.10