Cetyl betaine is an amphoteric surfactant that is made by reacting naturally-derived alkyl dimethyl tertiary amine with sodium monochloroacetate. In general, betaines are known to possess multifunctional properties in personal care hair and skin cleansing products including: viscosity building, foam volume enhancement, foam stability, improved skin and eye mildness and skin feel. Although mid-chain alkyl dimethyl and alkyl amidopropyl betaine surfactants are well known and extensively used in personal care products with conventional primary surfactants such as alkyl sulfates and alkyl ethoxy sulfates, they have not been found to function as efficient thickeners when combined with new generation sulfate-free surfactants. Sulfate-free personal care products have become very popular recently as evidenced by introduction of several new commercial brands, as well as new products under existing brands.
According to Mintel and Chain DrugReview market research reports,1, 2 global hair care items boasting a sulfate-free formulation showed exponential growth since their launch in January 2006 through November 2009, and this was particularly evident in North America. In the U.S., L’Oréal introduced an EverPure sulfate-free product line with claims of anti-fade system, 100% sulfate-free and no harsh salt. From a unit sales perspective, these lines of shampoos and conditioners were the top sellers in the U.S. sulfate-free hair care market. Increased demand for sulfate-free products is further supported by a statement published in Happi, “More marketers are trying to develop mild cleaners that are free of sulfates.”3
Sulfate-free surfactants include the salts of methyl ester sulfonates (MES) and salts of sulfonated fatty acids (SFA). They have been used in detergents as primary surfactants for many years. These surfactants possess excellent foaming and cleansing properties in liquid dishwashing detergents and heavy duty liquids, as described in detail in several U.S. patents.4-6 However, due to the hydrotropic properties of salts of MES and salts of SFA, products containing these materials usually have viscosity of less than 1,000 centipoises (cps). Building viscosity of personal care finished products based on salts of MES, salts of SFA and alpha olefin sulfonates (AOS) has been a challenge in the surfactant industry for many years. Amphosol CDB Special was developed to meet these technical, economic and market challenges.
The chemicals used in the study are all available from Stepan Company. Alpha-Step PC-48 (PC-48, sodium salt of methyl ester sulfonates and sodium salt of sulfonated fatty acid), Bioterge AS-40 AOS (sodium C14-16 olefin sulfonate), Steol CS-230 (SLES-2, sodium laureth-2 sulfate), Amphososl HCA (CAPB, cocamidopropyl betaine), Stepan-Mild LSB (LSB, sodium lauryl sulfoacetate and disodium laureth sulfosuccinate), Stepan-Mild PCL (PCL, sodium methyl-2 sulfolaurate, disodium 2-sulfolaurate, sodium lauryl sulfoacetate), Amphosol CDB Special (CDB, cetyl betaine), Stepanol WA-Extra (SLS, sodium lauryl sulfate).
All formulations were prepared in DI water. The pH of all experimental formulations was adjusted to 5~6 using citric acid or sodium hydroxide. Viscosity was measured with a Brookfield LVT viscometer using spindle number 3 or 4, 12rpm at 25ºC.
Foam volume was measured at 0.2% surfactant active concentration in Chicago tap water at 25ºC. A sample solution of 100.0g was slowly added to a 500ml graduated cylinder to minimize the foam. Castor oil (2.0g) was added to the graduated cylinder as appropriate. Cylinders were placed into a cylinder inversion machine (customer made) and inverted 10 times. Cylinders were allowed to settle for 15 seconds. The total foam volume was then recorded.
Zein protein solublization was carried out using a 1% active surfactant solution at 45ºC.Zein protein (1.5g) was added to 25ml of surfactant solution and mixed at 300rpm for 1 hour. The residual protein was filtered, rinsed with DI water and dried. The weight of unsolublized protein was measured and the solubility was calculated. A zein score was obtained by comparing the solubility of zein protein in each surfactant to the solubility of sodium lauryl sulfate (control) standardized at a value of 100.
Hair color wash fastness was carried out using European blonde hair (about 8 inches long and ~5g). Garnier 660, Intense Auburn Vibrant Colors by Nutrisse, was the colorant. Hair was colored according to the product instructions. Hair swatches were allowed to dry completely at room temperature before washing and then washed by applying 1ml of the experimental shampoo to the hair. The shampoo was worked into a lather using gloved fingers for 30 seconds, followed by rinsing under running tap water (~98°F) for 30 seconds. The hair was pat-dried with a paper towel. The washing procedure was repeated three times and then 10 times. Following washing procedure, hair swatches were allowed to air dry completely at room temperature. The hair swatches were then combed, and the color reflectance measurements were taken using a spectrophotometer (CM-2600 from Minolta). The difference in color intensity was calculated using the following formula:
∆E = ∆Eunwashed - ∆Ewashed.
Results and Discussion
Cocamidopropyl betaine (CAPB) has been extensively used as a secondary surfactant in personal care applications. CAPB helps to attain desired viscosity and improves foam of cleansing products based on primary surfactants such as alkyl sulfates or alkyl ether sulfates. However, when CAPB is used in combination with MES/SFA, it did not show any significant viscosity building property (Figure 1). Amphosol CDB Special is a modified cetyl betaine. It is a 30% active clear liquid in water with viscosity ~200cps. Therefore, Amphosol CDB Special does not pose the handling problem associated with the traditional cetyl betaine that has extremely high viscosity and stringy consistency. When used as a secondary surfactant in a sulfate-free system, CDB Specialshows significant improvement in viscosity building property when compared to CAPB.
Figure 1 demonstrates viscosity-building benefits for Alpha-Step PC-48 using Amphosol CDB Special. The total active surfactant concentration is held constant at 15% for all experimental formulations, but the ratio between PC-48 and CDB Special varies. In order to build viscosity of a composition based on PC-48 primary surfactant, the ratio between PC-48 and CDB Special must be optimized. If the PC-48:CDB Special ratio is 4:1 (more PC-48 is present), there is no appreciable viscosity building, even with 3% sodium chloride. However, if the ratio is 2:1 or higher (more CDB Special is present), there is a significant viscosity increase with the addition of small concentration of NaCl salt. For example, a gel product consistency can be obtained at 1.5:1 of PC-48:CDB Special ratio. The viscosity benefit was only observed with CDB Special system.The PC-48:CAPB system did not show such efficient viscosity increase under the same conditions. There was no significant viscosity increase even when the ratios between PC-48 and CAPB were in favor of CAPB with sodium chloride.
The efficiency of CDB Special on the viscosity building of formulated systems based on PC-48 was further explored at lower surfactant active concentrations. Figure 2 shows surfactant systems with 8~12% total active surfactant concentration. The results clearly show that even for an 8% surfactant system, viscosity of more than 6,000cps can be achieved with 1.5% sodium chloride. The efficiency of CDB Special to build viscosity of compositions at very low total surfactant active range makes this technology suitable for use in economy personal care cleansing formulations.
In addition to PC-48, CDB Special was also tested in various Stepan sulfate-free products, such as LSB, PCL and AOS. The viscosity-salt response at 15% total active and foaming results are presented in Figures 3 and 4 respectively. It should be noted that the ratio between the primary surfactant and CDB Special varies depending on the system. For example, LSB to CDB Special was chosen at a 12:3 ratio, while AOS to CDB was chosen at an 11:4 ratio. The ratio selection was based on the viscosity building property of each surfactant system (Figure 3).
Figure 3 shows that CDB Special can effectively build viscosity of more than 5,000cps of these sulfate-free surfactant systems with less than 1.5% sodium chloride. These systems are typically difficult to thicken with a secondary surfactant and NaCl. For example, AOS/CAPB system often requires more than 4% salt to achieve thedesired viscosity.7 Furthermore, AOS:CAPB at 11:4 ratio and 15% active concentration has viscosity of ~300cps with 1.5% NaCl. However, when CAPB is replaced in the same formulation with CDB Special, the resulting viscosity is ~ 22,000cps. In this case, CDB Special is about 70 times more efficient in viscosity building when compared to CAPB. CDB Special not only improves the viscosity building property of sulfate-free systems but also enhances foaming when used as a secondary surfactant. The foaming properties of CDB Special in combination with the sulfate-free surfactants are presented in Figure 4. The foam volume was obtained with 0.2% active surfactant solution and compared to a SLES:CAPB control under the same testing conditions. The results show that sulfate-free systems with CDB Special perform equal to or better than the control in foam volume.
Preliminary results show that CDB Special works effectively with other sulfate-free mild surfactant systems such as sarcosinate, sulfosuccinate and glutamate. These surfactants are commonly used in personal cleansing products and most of them have hydrotropic properties, which makes it difficult to build viscosity especially when these surfactants are used as primary surfactants.The experimental formulations based on sulfate-free surfactant systems were also tested for hair color washing fastness property. The results of this study are shown in Figure 5. The hair color washing fastness was carried out according to the procedures given in the experimental section. The total color difference before and after washing was recorded as ΔE (Formula 1). The smaller the ΔE indicates less color change or stated another way, better color washing fastness. The sulfate-free systems containing CDB Special had better hair color washing fastness compared to the SLES-2: CAPB (control) at 4:1 ratio.
L* = LightnessFormula 1
a* = +red/-green
b* = +red/-blue
b* = +red/-blue
Another benefit of CDB Special is that it improves the mildness of anionic surfactants. The mildness was evaluated using in-vitro zein protein solubilization test. The results are presented in Figure 6. The amount of protein solubilized by a single surfactant or a surfactant system correlates to its mildness. A mild surfactant solubilizes less protein. Since the sodium lauryl sulfate (SLS) is well known as an irritating surfactant, it was used as a positive control during the test. Its solubility for zein protein was normalized to a score of 100. The solubility of other surfactants was compared to SLS. A lower zein score indicates better mildness. The results in Figure 6 show that the surfactant systems with CDB Special had lower zein protein scores, which indicates that they were milder than the anionic surfactant alone. In another words, CDB Special improves the mildness of the anionic surfactants.
Amphosol CDB Special is a mild surfactant derived from natural-based feedstocks. CDB Special was found to be an efficient viscosity builder for sulfate-free anionic surfactants specifically designed for cold processing and easy handling. It works effectively with various hydrotropic surfactants to improve viscosity, foaming and mildness. Alpha-Step PC-48:Amphosol CDB Special system allows formulation of viscous sulfate-free, mild, naturally-derived hair and skin cleansing products.
More Information:Stepan, Northfield, IL, Tel: 847-446-7500;Website: www.stepan.com; Email: email@example.com
1. Mintel Insight and Impact. Examining Sulfate-free Formulations in Hair Care. December 2009 Report.
2. Chain Drug Review. Sulfate-free Items Catch On. High-Beam Research, March 16, 2009.
3. Shoaib, Arif, Sulfate-Free Personal Cleansers, Happi, Vol. 45(9), 93-96, September, 2008.
4. Sajic et al., U.S. Patent 5,616,781, Liquid Detergent Compositions Comprising Salts of Alpha Sulfonated Fatty Acid Esters and Anionic Surfactants, Apr. 1, 1997.
5. Sajic et al., U.S. Patent 5,637,758, Liquid Detergent Compositions Comprising Salts of Alpha Sulfonated Fatty Acid Esters and Anionic Surfactants, Jun. 1, 1997.
6. Sajic et al., U.S. Patent 5,945,394, Heavy Duty Liquid Detergent Compositions Comprising Salts of α-Sulfonated Fatty Acid Methyl Esters and Use of α-Sulfonated Fatty Acid Salts to Inhibit Redeposition of Soil on Fabric, Aug. 31, 1999.
7. Sulfate-Free Personal Cleansers, Shoaib Arif, Presentation on SCC Teamworks Tradeshow, March, 2008.