Cleaning up cleaners with bio-surfactants
When you wash your hands, or your clothes, or your kitchen floor, you almost always will use a commercial product that came in a brightly colored box.
This box probably will promise you that the substance in it will do all the work for you, and also protect you from almost certain death by bacterial infection. That product likely will be either a soap, traditionally made from animal fat and lye, or a synthetic detergent.
The most important ingredients in these cleaners are the so-called surfactants. Surfactants are compounds that lower the surface tension of a liquid, the interfacial tension between two liquids or the interfacial tensions between a liquid and a solid. Because of this characteristic, they are very good at liberating dirt from where it is not wanted.
In the latter case, say your dirty floor, these chemicals will orient themselves at the boundary between the liquid of the solvent they are suspended in (such as water) and any solids (such as dirt).
Their molecules have hydrophobic tails and hydrophilic heads, and therein lies their usefulness: The water-avoiding tails will attach to the dirt, and the water-loving heads will orient toward the water, thereby lifting the dirt into suspension where it can be carried away.
When such “amphipathic” molecules circle up into a sphere, it is called a micelle. This same behavior is exhibited throughout living organisms, as the orientation of glycolipids or phospholipids into so-called bilayers forms membranes.
Indeed, it has been supposed that this self-organizing behavior had a key role to play in the very start of complex life on earth.
The basic building blocks of these molecules are lipids, comprising a wide range of fats, waxes, sterols, fat-soluble vitamins and other compounds. A glycolipid is a fat molecule joined to a sugar. In your body, they are important as an energy source. Also, they exist on the outside of cell membranes, where they are essential to chemical recognition, membrane stability and the joining of cells to form tissue.
Surfactants made up of these building blocks are not just useful for cleaning. This same mechanism allows the easier spreading and application of paints on surfaces, fertilizers on crops, lubricants on machine parts and medicines into our bodies.
In 2008, the world produced 13 million tons of surfactants, fueling a multi-billion dollar market.
Back to basics
It is only in the past 40 years that synthetic chemicals have replaced the natural ingredients in soaps. Today, most surfactants are made from non-renewable petroleum sources.
Billions of pounds of these are manufactured every year in the U.S., and the majority goes into detergents for cleaning. Many do not break down easily once disposed of — some for 70-100 years — and there have been concerns raised about long-term toxicity.
It is impossible to imagine that the modern world will return to rendering fat from animals for many of these products, but it just might return to using more natural ingredients.
A research group at the University of Arizona (UA), led by Jeanne Pemberton, is investigating just that. They are studying the use of glycolipids, or sugar-based fats, as a green alternative to the ingredients now lining our grocery shelves and filling our industrial warehouses.
Importantly, they are investigating a synthesis process for bio-surfactants found in common bacteria that could scale to the requirements of mass production and allow formulas to be tuned for specific applications.
The team at UA is interdisciplinary and relies on the expertise of Pemberton, an analytical chemist, and her colleagues — an environmental microbiologist and a toxicologist — to systematically design, synthesize and characterize a wide array of new glycolipid surfactants.
The team first reviews the structure of a wide array of glycolipids, choosing good candidates with best surfactant properties in solution or on surfaces. Synthetic formulations are devised using univariate green metrics, and test formulas are then assessed for biodegradability and toxicity.
Final candidates are screened further in an in vitro model of ocular irritation. The goal of this is to discover new structure-function relationships and to push molecular design forward.
Responding to NSF’s Science Nation program, Pemberton claimed that the biggest hurdle to using bio-surfactants was that they were difficult to produce in large quantities.
The UA group is therefore looking at how to scale up this production to satisfy industrial demand. The two basic approaches to scale up production are to either cook bacteria and a feedstock in a fermentation tank, or to synthesize the molecules. The UA team has chosen the latter path.
They are also looking at how to customize these surfactants.
“It’s very exciting because it gives you the opportunity to tailor molecular properties for the applications you need,” Pemberton said. “You can control properties by control of the molecular structure.”
Catering to ‘cosmeceuticals’
Environmental microbiologist Raina Maier sees a wide field of opportunity for the development of not just more bio-surfactants, but new classes of the molecules.
“We are just … skimming the surface with the bio-surfactants that we’ve studied so far,” she said on the recent Science Nation program.
She has speculated that the first common uses of these molecules will be in niche markets, such as cosmetics or medical cleaners.
Indeed, the cosmetics and personal care market is the focus of a company Maier and Pemberton have formed with Chett Boxley, who serves as CEO. Glycosurf LLC was founded in 2013, after receiving development funds from Tech Launch Arizona at the University of Arizona.
It offers batches of rhamnolipids as research feedstock to companies that have greener criteria in their R&D programs for “cosmeceutical” products such as moisturizers, anti-oxidants, skin-lightening creams, anti-aging creams and sunscreen.
Rhamnolipids are lipids joined to a naturally occurring deoxy sugar molecule, Rhamnose. A deoxy sugar is one that has had its hydroxyl group replaced with hydrogen. In nature, it is produced by a common bacteria, Pseudomonas aeruginosa, as well as by plants including the Buckthorn (Rhamnus), from which it gets its name.
Rhamnolipids are used in areas beyond cosmetics and cleaners, as they facilitate bioremediation of heavy metal and waste oil sites using Pseudomonas aeruginosa. They are in common use in industrial processes and are typically produced using bacteria in a fermentation process.
They are usually produced on soybean oil soapstock, spent soybean oil or chicken fat as a carbon source, but can be produced using all sorts of agricultural waste.
At Glycosurf they are produced using a proprietary synthetic process, and the company claims subsequent advantages such as quality control (95 percent pure product) and lower manufacturing costs for client companies because the more common steps of fermentation are eliminated.
It uses rhamnose sugar as a base and then customizes molecules by attaching chosen lipids. Rhamnose sugar is used in the food flavoring industry, particularly as a raw material for synthetic strawberry flavor. Sources include the processing of the dried buds of the Chinese Sophora tree, and a typical bulk cost for less than 1,000 liters is in the $1 per liter range.
GlycoSurf has reported to the Green Chemicals Blog that the global market for surfactants is expected to generate revenues of more than $41 billion by 2018, with growth of 4.5 percent per year.
Biosurfactants are expected to comprise a growing portion of this growth as consumers, especially in niche markets, become more concerned about the materials absorbed into their bodies and the wider environment.
Seeing the use of sugars as surfactants in the wider world of cleaning, lubrication and material application would be very sweet.