Foley, PM, A Phimphachanh, ES Beach, JB Zimmerman, and PT Anastas. 2011. Linear and cyclic C-glycosides as surfactants. Green Chemistry http://dx.doi.org/10.1039/C0GC00407C.
Surfactants – the active ingredients in many household products, including cleaners and personal care products – are produced on the scale of millions of tons per year. In nearly every application, after a few minutes of use, they are rinsed with water either down the drain or directly into the environment.
Unsurprisingly, there have been harmful effects from such large-scale releases. When surfactants are slow to break down in the environment, problems range from unsightly foaming to toxic effects on aquatic organisms. Some of these chemicals – for example, the widely used nonylphenol ethoxylates – have been implicated in endocrine disruption. The term is used to describe substances that can alter hormone activity in the body.
There has been a push in recent years toward sugar-based surfactants because of improved biodegradability and lower toxicity. They are also derived from natural and renewable sources, adding another “green chemistry” benefit.
Sugar-based surfactants have been commercially available for more than a decade. Formulations of the alkyl polyglycosides (APGs) are used in a variety of consumer products for laundry, hair and skin care. On an ingredient label they are usually identified as a variety of “glucosides,” for example decyl glucoside or lauryl glucoside. The use of APGs is growing at a faster rate than petroleum-based surfactants.
However, chemists are trying to improve APGs. Many of the sugar-derived chemicals can fall apart when exposed to acids in water because the link between the water-loving and oil-loving ends of the APG molecules is vulnerable. Also, depending on the variety of the APG produced, the manufacturing process relies on high temperature and pressure and energy-consuming purification steps.
Researchers investigated a new process to make a stronger chemical bond in one particularly weak spot of sugar-based surfactant molecules. The researchers transformed a precursor chemical by treating it with a chemical mix that included alkyl aldehydes.
Several sugar derivatives with straight or cyclic tails were produced, depending on the conditions that were used to convert the intermediary chemical. The new chemicals were tested for surface tension and foaming and the results were compared to current APG surfactant performance.
The researchers showed that the new surfactants could be prepared in a two-step reaction under mild conditions, using only a minimum of solvent. It was not necessary to purify the products by column chromatography, a procedure that would consume large volumes of potentially hazardous solvents. This improves the prospects for producing the chemicals on a large scale.
The scientists show that the technical performance of the new surfactants is as good as existing APG technology. This was determined by measuring surface activity – how efficient the chemicals are at reducing the surface tension of water. It is important for surface tension to drop quickly with just a small amount of added chemical, for surfactant applications. The best chemical tested in the study worked at just 40 milligrams (the weight of a few grains of rice) per liter.
The researchers also explored the foaming properties of the new chemicals. Foaming is desirable for some personal care products like shampoos, but would be a disadvantage in laundry applications and some industrial cleaners. The Yale chemicals were low foamers compared to a conventional surfactant, sodium dodecyl sulfate (SDS). But when mixed with SDS, the resulting foam lasted longer. Thus they could be useful in either low- or high-foam formulations.
The biodegradability of the new chemicals was not measured, but U.S. Environmental Protection Agency software suggests that the changes made to improve acid stability will not affect how microbes disassemble the chemicals. The glucose end of the molecule contains many carbon-oxygen bonds that are common places for microbial attack. If the surfactants are made from long, straight-chain aldehydes, that should also provide bacteria with a familiar food source.
The new sugar-based surfactants may offer more stable and sustainable varieties to use in consumer products. The novel chemicals are more stable under harsh conditions and work just as well in laboratory tests as the sugar-based surfactants currently used. In addition, their chemical production is a significant improvement over current methods in that it uses less resources and produces a wider array of chemicals with surfactant properties.
This new family of sugar-based surfactants complements APGs that are already on the market. A wider variety of molecular structures means that manufacturers of green consumer products are more likely to come up with formulations that meet all the goals of function, performance, economy and low environmental footprint.
Replacing the weak spot in APGs with a sturdier alternative allows the sugar-based chemicals to be used in more applications. The two parts of the new molecule are linked with a bond between two carbon atoms instead of a bond between an oxygen and a carbon atom. The stronger carbon-carbon bond is unaffected by strong acid. That robustness could help in industrial applications and heavy-duty household formulations, for example acidic tile cleaners.
The new surfactants are made from glucose, which is widely found in nature. It is one of the components of table sugar and is the repeating chain unit in cellulose, which gives plants their supporting structure. Glucose acts as the water-loving part of the surfactant. The oil-loving part of the surfactant is made from aldehydes. Aldehydes are a diverse set of chemicals; some occur in nature and others are produced from petrochemicals. In this study, the aldehydes could be obtained by treating plant oils.
The researchers say their next step will be to explore algae as a possible source for all of the surfactants’ starting materials. The carbohydrate portion of algal biomass could provide the sugar, and algal oils could give the right kinds of aldehydes. Algae oils are particularly rich in carbon-carbon double bonds that can react with ozone to produce aldehydes. If that approach is successful, surfactant production could supplement algae-to-fuel technology.