Consumer products such as shampoo, soap and detergents, as well as processed foods and drugs, are often laden with extra ingredients. Many of these extra ingredients are surfactants — chemicals responsible for changing the texture and solution properties of consumer products. Surfactants affect how well a shampoo or toothpaste “foams” and how well detergents remove dirt and grease. New research is highlighting how these products, typically derived from petroleum byproducts, can be made from renewable resources such as sugar.
Oil and water don’t mix
Oil separates from water to form a hydrophobic (think phobia of water) layer on the surface because the oil prefers to interact with itself rather than water. This causes difficulties in removing stains from clothing, as the grease on your shirt — much like the oil — will prefer to remain in contact with the clothing, minimizing its contact with water. Surfactants are added to detergents to solve this problem, with different parts of the surfactant molecule interacting with either oil or water.
Linear alkylbenzene sulfonates (LAS, Figure 1) are one of the most common groups of industrially used surfactants. The LAS molecule consists of a hydrophilic “head” (attracted to water), which is a charged region containing sodium and sulfur, and a hydrophobic “tail” (attracted to oil), which is an alkyl group — purely carbon and hydrogen. When a surfactant such as LAS is added to an oil/water mixture, the hydrophilic “head” prefers to interact with water, and the hydrophobic “tail” prefers oil, as well as other hydrophobic tails from neighboring LAS molecules. The result is the formation of spheres, or micelles, as shown in Figure 2. The center of these micelles trap the oil and grease while the “heads” are attracted to water, so the micelles can suspend in solution and lift the trapped grease from your shirt.
The sweet science
Because surfactants are derived from non-renewable petroleum byproducts, there is a need for alternatives. A research team at the Catalysis Center for Energy Innovation (CCEI), an Energy Frontier Research Center, developed a route to make surfactants from sugars derived from plant matter, or biomass. Surprisingly, many of these biomass-derived surfactants — coined “Oleo-Furan” surfactants — are not only renewable, but they have properties superior to their petrochemical brethren, the LAS surfactants. The structure of the Oleo-Furan surfactants is in red in Figure 3.
Scientists tackle optimization of molecules from sugars
Perhaps the most difficult challenge in producing surfactants is that the optimal molecular structure is different depending on the application. The hydrophobic “tail” of the surfactants can be designed with one carbon chain instead of two, or with carbon chains of various sizes. Different geometries are optimal for making stable foams, wetting porous materials and fibers, or withstanding harsh conditions such as high temperature or “hard water” — water containing a high concentration of metal ions. On the industrial scale, it is expensive to produce all these varieties of surfactants, each of which would require different processing conditions. Thus, industry often targets one particular surfactant, such as LAS, and uses additives after production to tailor it towards different target applications. One example is the use of “chelating agents,” which bind to metal ions in solution that would otherwise disrupt the micelle structures of LAS.
Figure 3 is a breakdown of the composition of detergents, with surfactants (blue) and chelating agents (green) making up close to 50 percent of the composition, as well as the majority of the cost. The Oleo-Furan surfactants developed by the CCEI research team (Figure 3, red) have an improved ability to withstand hard water conditions over LAS (blue), without the need for expensive and environmentally damaging chelating agents.
Further, by making minor adjustments to the Oleo-Furan structure, as seen in Figure 3 (right), the CCEI team created surfactants capable of a wide variety of applications, including strong foaming, wetting and stability properties in harsh conditions. This is just one area in which CCEI research is creating routes to modern commodity chemicals by renewable means.
