The trend of formulating without sulfates in the personal care industry has been present for almost a decade and has not decreased in popularity since its emergence. It began with press releases issued by activist groups claiming that sulfates in personal care were potentially harmful, and as consumers became more concerned with the safety of sulfates, more manufactures began formulating without them. As a result, formulators are now requesting alternatives to the popular sulfates sodium lauryl sulfate (SLS) and sodium lauryl ether sulfate (SLES) from their raw material suppliers in order to meet the demands of consumers.
Sulfonated alkyl polyglucosides (SAPGs) have evolved in the past five years to become efficacious primary surfactant alternatives to sulfates. As sulfonates, SAPGs can be substituted for sulfates—an unknown fact to many consumers and some formulators. Therefore, consumers and formulators alike must be educated on the benefits of SAPGs in personal care formulations without sulfates.
A previous article1 by Anderson and Smith focused on SAPGs’ mildness to the skin and eyes, sulfate-free claims, foaming properties and the renewability and environmentally friendly nature of the materials. At that time, two versions of SAPGs were introduced, including a decyl version (sodium decylglucosides hydroxypropyl sulfonate (SDHS)a) and a lauryl version (sodium laurylglucosides hydroxypropyl sulfonate (SLHS)b). This paper will discuss the various benefits of SAPGs, specifically SLHS, that have been researched since 2005, including among other benefits their biodegradability, mildness and eco-friendly attributes as recognized by the US Environmental Protection Agency’s Pollution Prevention Award.
The SAPG Reaction
The reaction designed to produce an SAPG, shown in Figure 1, utilizes sodium 3-chloro-2-hydroxypropyl sulfonate, a commercially available chemical intermediate often used in the surfactant industry. This sulfonate intermediate has been used for decades to produce sulfo-betaine surfactants. The 3-carbon portion of this molecule originates from epi-chlorohydrin and is currently the only portion of the SAPG molecule that is petroleum-derived. The renewable carbon index for this product is 0.875; therefore, it is derived from nearly 90% renewable carbon sources. Reaction of alkyl polyglucoside (APG) with the intermediate yields the sulfonated product and sodium chloride as the only by-product.
The starting materials APG and sodium 3-chloro-2-hydroxypropyl sulfonate are both water-soluble, and the APG is supplied as a solution in water, approximately 50% active. The reaction is conducted in water without any other solvents and is typically complete in 4–5 hr at 75–80°C.
A key concern for consumers seeking sulfate-free products is toxicity. Recently, there has been increased consumer awareness on surfactant formulations containing trace levels of 1,4-dioxane—a probable carcinogen by-product from ethoxylation—especially when those formulations are designed for use on children.2 In 2008, a lawsuit filed by the California Attorney General detailed findings by independent laboratories reporting that 46 out of 100 personal care and household cleaning products tested contained 1,4-dioxane, and many of these were above the 20 ppm limit.3 The sulfonated APG product, SLHS does not contain any 1,4-dioxane since no ethoxylation processes are used in the manufacture of SLHS.
It is important to note that a chemical’s toxicity profile can include its toxicity to humans, the environment, and eye and skin irritation, as tested by mammalian toxicity, aquatic toxicity, and eye and skin irritation tests, respectively. However, there are also some less common tests that are important in assessing the toxicity of a chemical, including skin sensitization, genotoxicity and repeat dose mammalian toxicity.
The aquatic toxicity of SLES, SLS and SLHS were assessed and compared in-house by the author’s company. To test aquatic toxicity, the surfactants were tested against daphnia for 48 hr (ESA SOP 101, EC50); against freshwater algae for 72 hr (US EPA Method 1003, IC50); and against fish larvae for 96 hr (ESA SOP 117, EC50). With daphnia, SLHS, SLS and SLES produced 16.3 mg/L, 4.6 mg/L and 5–37 mg/L, respectively. With freshwater algae, SLHS, SLS and SLES produced 52.9 mg/L, no data and > 65 mg/L, respectively. Finally, with fish larvae, SLHS, SLS and SLES produced 27.3 mg/L, 4.1 mg/L and >8.9 mg/L, respectively. These results show that SLHS has better or comparable toxicity to the common anionic surfactants SLS and SLES.
SLS, SLES and SLHS also were tested in-house for eye and skin irritation using both a hen’s egg test-chorio-allantoic membrane assay (HET-CAM) and human patch testing. The HET-CAM results illustrated that SLHS showed no eye irritation, while SLS scored 30.0 and SLES scored 25.5. As a point of reference, baby shampoo scores an 11.0 and standard shampoo scores a 24.3 with this test.
Human patch testing using a 1% active level showed that SLHS presented no potential for dermal irritation, while SLS showed slight to severe irritation4 and SLES showed skin irritation in some individuals.5 SLES has been found to be less irritating than SLS in human patch testing,6 but is still considered to be potentially irritating.
Other related toxicity tests were conducted, including a bacteria reverse mutation assay or Ames assay, an in vitro eye irritation study, and skin sensitization via human repeat insult patch testing. The Ames assay, described by the Organization for Economic Co-operation and Development (OECD), demonstrated no detectable genotoxic activity for SLHS. The in vitro eye irritation studyc on SLHS showed practically no ocular irritation potential in vivo, with a score of zero. The results indicated that SLHS can be classified as nonirritating.
Repeat insult patch testing (RIPT) also indicated that SLHS had no potential for dermal irritation or allergic contact sensitization. This testing consists of repeated application of a 10% active solution of the surfactant to an area on the skin nine times during a three-week period, followed by another application two weeks later. The lack of any irritation in the three-week period of application indicated the product is mild to the skin. Further, the lack of any irritation after the additional application two weeks later indicated the product is not a sensitizer.
Another important characteristic for any household cleaning ingredient is biodegradability. In fact, the EU Detergent Directive established restrictions or bans on the use of surfactants in cleaning formulations based on biodegradability.7 In relation, a third party company tested SLHS using the OECD 301E test method, and the results showed greater than 80% biodegradability after only seven days.8 The biodegradability of SLS and SLES is also well-documented in the literature.
An unexpected property of SLHS is its ability to withstand microbial contamination. As a firsthand observation, in-house samples of the product aged more than four years did not show any propensity for contamination with microbial growth, even without the use of preservatives. This is contrary to the behavior of most surfactants encountered in laboratory work. Without added preservatives, surfactants at a neutral pH and in a 30–50% aqueous solution commonly show signs of microbial growth in less than 12 months.
In order to quantify these anecdotal observations, a two-year old sample of SLHS was sent to an outside laboratory for microbial testing and it showed no growth of Candida albicans, Staph aureus or Gram negative bacteria, as well as < 10 cfu/g of both aerobic bacteria and yeast/mold. While these results confirmed resistance to microbial growth, the sample was further tested to determine its ability to inhibit microbial growth.
A second study was performed at an outside laboratory using the Zone of Inhibition test method, again using a sample that was two years old. This method is a quick means to measure the ability of an antimicrobial agent to inhibit the growth of microorganisms. A microbial suspension is spread evenly over an agar plate and the antimicrobial agent is applied to the center of the agar plate. If antimicrobial activity is present, a zone of no growth around the agent is seen. This is the zone of inhibition, and the size of the zone is usually related to the level of antimicrobial activity.9
SLHS was tested at both 16% and 8% against a Gram-negative bacteria, Pseudomonas aeruginosa; a yeast, C. albicans; and a mold Aspergillus niger. The level of 16% is considered a normal range for a primary surfactant in high-end shampoo formulations. The ability of SLHS to prevent the aforementioned microbes was measured on a scale of 1–8; 1 being excellent, 2 being very good, 3 being good, 4 being moderate, 6 being poor, and 8 being no activity. The 16% active sample showed very good (2) microbial protection against all three microbes, whereas the 8% active showed very good protection against C. albicans, moderate protection against A. niger, and poor protection against P. aeruginosa.
The conclusion, as reported by the outside testing lab, was that the material could be considered as having reasonably good antimicrobial activity at the 16% active level. Further, it was noted that using this material in formulations could contribute to enhancing overall resistance to microbial contamination.
Foaming in aqueous solutions is one of the distinctive properties of surfactants, and a property where consumers have a literal hands-on ability to judge the quality. There are many methods of testing the foam properties of surfactant solutions, and one of the oldest is the Ross-Miles method, described in ASTM D-1173. In the Ross Miles test, a dilute solution of the surfactant is dropped from a fixed height into a pool of the same dilute solution. The height of the foam is then measured for a given period of time.10
Solutions of 1% SLHS, SLS and SLES were prepared using tap water having a hardness of approximately 20 grains and were tested at RT (23°C) using the Ross-Miles method. Foam heights were recorded immediately and after 1 min and 5 min. The foam height of SLHS was 155 mm immediately, 153 mm after 1 min, and 150 mm after 5 min. In comparison, SLS had foam heights of 180 mm, 165 mm and 160 mm while SLES had foam heights of 175 mm, 160 mm, and 155 mm, immediately, and after 1 min and 5 min, respectively.
These results indicate that the initial foam height of SLHS is only slightly less than that of SLS and SLES, and the original foam height is actually maintained longer. A qualitative evaluation of the foam was also conducted on each of these products by the author by pouring the solution into his hands and lathering for about 30 sec. The SLHS foam reportedly had a creamier consistency and a smaller bubble structure, compared with the foam generated from SLS and SLES.
Detergency is the primary function of surfactants in personal care products and household formulations. To measure detergency of SLHS, SLS, SLES and nonoxynol-9, a test was developed to measure the ability of each to effectively emulsify and clean squalene from the hair. In 1982, Koch described the typical lipids present in human hair and their contribution to the perception of oiliness.11 Since squalene is a natural component in hair lipids, it was chosen as the oil component used in this test.
The method is carried out by weighing equal amounts (1.5 g) of squalene and deionized water and pouring them into the same test tube. This gives a two-phase system with a clear interface. The surfactant is diluted to 10% active and is then added dropwise to the test tube, which is shaken. As the surfactant is added, the squalene oil droplets are observed to grow smaller until a uniform, white liquid is obtained. At this point, the squalene oil droplets are completely emulsified.
This test, called the squalene titer (SQ) method, represents the ability of a surfactant to effectively emulsify 1.5 g of squalene. A higher number indicates a better detergent, meaning it takes less of the surfactant to emulsify the squalene. SQ is calculated based on the following equation:
SQ = 1.5 g squalene
g of detergent added *
(activity of detergent)
The described SQ test was performed in-house by the author’s company using SLHS, SLS, SLES and nonoxynol-9, a common nonionic surfactant used in household and industrial applications. Nonoxynol-9 showed an SQ of 37.5, whereas SLHS, SLS and SLES showed a SQ of 18.7, 15.8 and 10.0, respectively. The result for nonoxynol-9 was not unexpected due to is common use in heavy duty cleaning applications. However, the strong result for SLHS was somewhat unexpected due to its mildness on the skin and eyes.12
Building Viscosity in Formulations
A major difference in formulating sulfate-free personal care products is the formulator’s approach to building viscosity. With traditional sulfate formulations, sulfates, amide and/or betaine can be combined and will build viscosity with the simple addition of small amounts (< 5%) of sodium chloride. In sulfate-free formulations, however, one cannot build viscosity in the same manner, and the viscosity will be only slightly affected with the addition of salt. Therefore, other approaches to build viscosity into sulfate-free formulations are required. Adding naturally occurring polymers, e.g. gums, or cellulosic polymers are two methods and while they are acceptable, they can have drawbacks such as a lack in clarity, appearing hazy or even opaque in formulations.
Interestingly, the simple combination of SLHS with a common amphoteric, lauramidopropyl betaine (LMB), was found to build viscosity to nearly 100,000 cps; and a viscosity of 2,000–10,000 cps is generally all that is desired for shampoos and body washes. Studies regarding the effects of these two ingredients, i.e., SLHS and LMB, are shown in Figures 2 and 3.
The first study was conducted using a fixed amount of 35% total surfactant in the solution. The relative amounts of SLHS and LMB were varied to give the curve shown in Figure 2. The second study was carried out using a fixed ratio of SLHS/LMB (65/35) and varied the total amount of surfactant in the solution, from 12% to 35%. It can be seen from this graph that high viscosity (> 30,000 cps) can be achieved at an active content of only 25%.
SLHS is a surfactant shown to exhibit efficacious biodegradability, detergency and some level of antimicrobial activity. In addition, the material was found to be practically nonirritating to the skin and eyes. These results described here show that SLHS provides formulators with a new tool to develop sulfate-free products for both personal care and household cleaning products. In addition, since the material is derived from raw materials that are nearly 90% renewable and of vegetable origin, it is also a raw material that fits well with companies seeking to formulate with eco-friendly or natural raw materials.
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1. D Anderson and D Smith, Sugar and the quest for sulfate-free formulation, Cosmet & Toil 120 8 55 (2005)
2. 1,4-Dioxane Fact Sheet (EPA 749-F-95-010a), US Environmental Protection Agency, Feb 1995, www.epa.gov/chemfact/dioxa-sd.txt (Accessed Feb 23, 2011)
3. California files lawsuit against whole foods, avalon, and others whose products tested positive for carcinogenic 1,4-dioxane in OCA study, Organic Consumers Association, www.organicconsumers.org/articles/article_ 12797.cfm (Accessed Sept. 27, 2010)
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7. Regulation (EC) No 648/2004 of the European Parliament and of the Council of 31 March 2004 on detergents
8. Biodegradability Determination of SugaNate 160 (Ref: M08001671), Silliker Australia, Pty. Ltd., Apr 14, 2008
9. Zone of inhibition test-Kirby-Bauer test, Antimicrobial Test Laboratories, www.antimicrobialtestlaboratories.com/Zone_of_Inhibition_Test_for_Antimicrobial_Activity.htm (Accessed Feb 20, 2011)
10. K Klein, Evaluating shampoo foam, Cosmet & Toil 119 10 32–35 (2004)
11. J Koch et al, Hair Lipids and their contribution to perception of hair oiliness, J Soc Cosmet Chem 33 317–326 (1982)
12. R Coots, Detergency and hair cleaning insights, presentation at the Mid-Atlantic Society of Cosmetic Chemists (Oct 2006)