Clean Manufacturing: Surfactants in Surface Cleaning
A Surface Active Agent can be described as a substance that can modify the surface properties of liquids or solids. In cleaning applications, these agents work at the boundary layer between soil and solvent.
How Does it Work?
For aqueous based products, surfactants aid water overcoming its difficulty in dissolving oils and greases. By design, surfactant molecules have two chemical groups as shown in Figure 1. On one end, there is an hydrophobic component that is attracted to the oil/grease. The other group is hydrophilic and compatible with water. Due to the strong interactions between the water molecules arising from dispersion forces and hydrogen bonding acting cooperatively, the hydrocarbon tail is squeezed out of the water. Hence the tail is usually termed hydrophobic.
In a cleaning solution, the hydrophobic end of the surfactant molecule orients toward the soil, which can be seen in Figure 2. Many surfactant molecules will attack the soil, breaking it up into small pieces and completely surrounding it. The hydrophilic ends of the surfactant molecules project into the solvent (i.e. water), causing the soil to be broken up, removed from the surfaces, lifted and suspended into the cleaning solution. The outside ends on the detergent molecule chains are attracted to water, and the inside ends prefer oil, forming a cluster of surfactant molecules around the oil. In addition to the assembly at the interfaces, surfactants can undergo a self-assembly process known as micellization (see Figure 3) resulting in a sequestering of the hydrophobic end.
Surfactant technology represents a vibrant and challenging area of physicochemical science where the exact mechanisms of well-established processes are often not fully understood or are at least the subject of continued debate. This is partly due to the complex nature of even the simplest commercially available surfactants—there is still a degree of empiricism when selecting surfactants for certain applications.1
Modern technology can produce many different types of surfactants by changing the chemical composition of the hydrophobic and hydrophilic ends of the molecule. By changing the chemical composition, we can create surfactants that have greater or lesser abilities in different areas:
Detergency: the ability to break the bond between soil and the surface;1 D. R. Karsa. New Product and Applications in Surfactant Technology, Vol. 1, Annual Surfactant Review, (1998).2 Dawn Chemical, Inc. Presents - The Chemistry of Cleaning©, The Science Of Soil Removal. http://www.riponcomputer.com/dawnchemical/chemistryofcleaning.htm (2001).3 D. R. Karsa. New Product and Applications in Surfactant Technology, Vol. 2, Annual Surfactant Review, (1998). 4 M. McLaughlin. Aqueous Cleaning Guide Book, (The Morris-Lee Publishing Group, 2000).5 C. LeBlanc. The Search for Safer and Greener Chemical Solvents in Surface Cleaning: A Proposed Tool to Support Environmental Decision-Making, Thesis to obtain the degree of Doctor from Erasmus University, Rotterdam, The Netherlands, (December 2001).6 The Surface Solutions Laboratory is part of the Massachusetts Toxics Use Reduction Institute located at the Unicersity of Massachusetts Lowell. The Institute was created to promote reduction in the use of toxic chemicals and the generation of toxic by-products in industry and commerce in the state of Massachusetts.7 Bama Chem Surfactant Test Kits. Bama Chem 10445 Pioneer Road, Theodore, AL 36582-7443.
* Penetrating and wetting: allows water to surround soil particles that would otherwise repel the water;
* Foaming: creation of bubbles that lift dirt from the surface;
* Emulsifying: the ability to break up greasy petroleum soils into small droplets that can be dispersed thoroughly;
* Solubilizing: dissolving a soil so that it is no longer a solid soil particle;
* Dispersing: spreading the minute soil particles throughout the solution to prevent them from sticking to a mop, bucket or back onto the cleaned surface.2
Based on the premise that all commercially available surfactants are at best simple blends and that many, in reality, are a complex mixture of surface-active species and minor non-surface active components, it becomes apparent that the prediction of performance-structure relationships is not easy.3
What are Nonionic Surfactants?
Nonionic surfactants have no ions. These chemicals derive their polarity from having an oxygen-rich portion of the molecule at one end and a large organic molecule at the other end. The oxygen component is usually derived from short polymers of ethylene oxide or propylene oxide. As in water, the oxygen provides a dense electron-rich atom that gives the entire molecule a partial net-negative charge that makes the whole molecule polar and able to participate in hydrogen bonding with water. Examples of nonionic surfactants are alcohol ethoxylates, nonylphenoxy polyethylenoxy alcohols, and ethylene oxide/propylene oxide block copolymers.4
How Much Is Enough?
While surfactant concentrations are well below 10% for most aqueous cleaners as shipped, they constitute the chief active ingredients of the detergent industry and are selected and/or formulated for the final product’s targeted application and markets. The major chemical manufacturers are hesitant about the release of specific information concerning these formulas with and without patenting and will frequently list them as, for example, a “nonionic proprietary surfactant” on Material Safety Data Sheets (MSDSs).5
In an attempt to address this “unknown “ about cleaning products, the Surface Solutions Laboratory (SSL)6 evaluates surfactant levels using a titration test kit formulated specifically for non-ionic surfactants. While this test can answer the quantitative question about relative amounts of surfactant in a product, it does not shed any light on the quality of cleaning product. Therefore, SSL has coordinated its surfactant measurements with the lab’s cleaning assessment program.
The Cleaning Program
SSL’s mission is to test and evaluate the effectiveness of greener cleaning chemicals and related equipment on a variety of substrates and soils. The Lab’s goal is to identify, develop and promote safer alternatives to hazardous materials such as chlorinated and other solvents. This investigative work involves the typical industrial practices of surface preparation, cleaning, rinsing, drying and analysis.
SSL uses the following procedure, Table 1, as the preliminary evaluation of the performance of chemical cleaners purported to be safer and greener.
The Surfactant Measurement System
As mentioned, SSL uses a simple titration kit to assess the level of nonionic surfactant in a cleaning formulation. The methodology used is provided in Table 2.
SSL searched its database of over 300 cleaning formulations. Using vendor supplied data on the ingredients, the lab initially identified products that contained any type of surfactant. This action reduced the number down to 53. Of the formulations with surfactants, 23 did not explicitly define the surfactant type (nonionic, cationic, anionic). Twenty-one contained nonionic surfactant, five contained both anionic and nonionic, and four were listed with anionic surfactant only.
Initially, surfactant measurement was made on the nonionic aqueous products. Following these were the nonionic semi-aqueous products. The last group evaluated contained the mixed anionic and nonionic cleaners.
The lab also evaluated a known amount of a nonionic surfactant as a control of the test method. Using the procedure documented in Table 2, several concentrations of the known nonionic surfactant were evaluated. From the resulting calculations, listed in Table 3, a correlation was made between the expected results and the measured values. This relationship is demonstrated in Figure 4.
The formula, Y = 1.0162X - 2.221, derived from the known surfactant had a correlation factor of 0.998. This high value provided the lab with a significant level of confidence in the results of this methodology. The formula was used to correct the surfactant levels measured for the unknown nonionic surfactants. Table 4 lists the products tested, the measured surfactant level, the corrected surfactant concentrations, vendor ranges (when supplied) and the type of surfactant(s).
The corrected laboratory surfactant levels approximated the vendor-supplied levels. Using this information, the actual surfactant concentrations for the products without vendor information would be around the laboratory measured and corrected values. The product marked with an asterisk in the table encountered interference during the titration. This solution was the only product with an acidic pH, which may have been the source of the interference.
As stated, SSL’s major focus is to evaluate the effectiveness of alternatives to hazardous solvents. For the past eight years, the lab has performed over 800 cleaning trials using 357 different cleaning products. The lab has compiled the test results into a database (Simple Solutions for Surface Cleaning) designed to make finding replacements for the traditional solvents.
The 22 products listed in Table 4 have been used to in over 170 trials on 15 different types of contaminants. Table 5 lists the effective and ineffective attempts for each formulation.
Looking at these 22 products, only six formulations account for about 60% of the testing performed at the SSL. The lab has reviewed the components of each cleaner as supplied by vendors. Again based on the supplied vendor information, there were no major similarities between the formulations. Products that were manufactured by the same company had comparable components.
One of the surfactants was used in multiple products. Nonyl Phenoxy Polyethoxy Ethanol (9016-45-9) was in a quarter of the products from five different companies. Alkyl-polyglycoside surfactant (110615-47-9) was used in two products, both from the same vendor. Similarly, an ethoxylated alcohol, C7-C2 (68991-48-0) was used in two products, both from the same vendor. Octylphenoxypolyethoxyethanol (9036-19-5) was in two formulations from two separate companies.
Okay, But Now What?
Surfactants play a major role in the cleaning industry. Identifying a substitute for solvents in cleaning applications is not an easy task. There are thousands of formulations to choose from. The variability of the literature for these products changes from vendor to vendor and from product to product making the search for an applicable substitute nearly impossible. With hundreds of possible formulations, the main questions that remain are: Which one to use? How much to use? Through SSL’s research efforts these obstacles may have been lessened due to performance to ratio identified for many products.
By using the above information provided about surfactants and the various cleaning products, a comparison of the data provided about your selected cleaning formulation can be done to determine if your cleaning product is the right one for you.
Looking for More Information?
If you still have questions about surfactants and their use in the cleaning field, try going this extensive website about surfactants, www.surfactant.net .