Dynamic Properties of Amino Acid-Based Surfactants

Dynamic Properties of Amino Acid-Based Surfactants Personal hygiene routines have become an important part of our daily lives and present new challenges for the formulation of these personal care products. Consumers increasingly demand not only perfect cleaning performance, but also a pleasant effect that continues even after rinsing. Additionally, products must meet expectations created by growing environmental awareness and should be as natural as possible. The mildness of formulations is essential, and amino acid-based surfactants such as cocoyl glutamate and lauroyl sarcosinate salts are playing an increasingly important role in this developing market (Figure 1). Both substances consist of a naturally occurring amino acid combined with a long-chain fatty acid as a hydrophobic tail with a hydrophilic head. The ionic structure of the amino acid portion has high affinity for protein-rich surfaces such as skin and hair. Studies have shown that these amino acid-based surfactants form a protective monolayer on the skin and prevent skin dryness. This protective monolayer also keeps more aggressive primary or secondary surfactants that may be present away from the skin surface 1. In contrast to standard surfactants such as SLS, SLES and cocamidopropyl betaine (CAPB), N-acyl sarcosinate and N-acyl L-glutamate show buffering capacity in the pH 5.5 range. This buffering capacity allows the preparation of stable formulations without any negative effects on skin health4. N-acyl glutamates such as Perlastan SC 25 NKW are well tolerated by the skin due to their low tendency to cause skin irritation (hypoallergenic) and the absence of pore-clogging (non-comedogenic) properties. Transepidermal water loss (TEWL) is a measure of the barrier stability of the epidermis and therefore an indicator of the irritant potential of substances. In a clinical study, TEWL of human skin was investigated following the application of two rinse formulations based on sodium cocoyl glutamate and sodium lauroyl sarcosinate as main components. No significant change in skin moisture of the test subjects was detected, which should be interpreted as a positive result. Additionally, the rinse formulations tested showed high acceptance by the test subjects in terms of soft, nourishing, cleansing and foaming effects. As a starting material, glutamic acid has the advantage of being produced through natural fermentation processes. Thus, Perlastan glutamates such as SC 25 NKW, SL and SCG 50 ZPF meet the requirements for certification from various organizations such as COSMOS, ECOCERT, NATRUE, ECO-Label and Nordic Swan. When certification as natural cosmetics is required, they can be easily used as main components or secondary surfactants in formulations. Their mild and particularly skin-friendly character is combined with distinct foaming behavior and excellent aerobic and anaerobic biodegradability6. These positive properties make the range of N-acyl sarcosinate and N-acyl glutamate surfactants preferred to meet consumers' increasingly complex needs in terms of naturalness, health and performance. In this article, the performance of amino acid-based surfactants and formulations has been examined using various methods in terms of cleaning power, foaming capacity and foam structure and compared with conventional standard surfactants. Cleaning Performance To evaluate the cleaning performance of surfactants, the determination of critical micelle concentration (CMC) can be measured using a ring tensiometer. The maximum reduction in surface tension of an aqueous surfactant solution is measured as a function of surfactant concentration. The lower the CMC value, the better the surfactant performance. Measurements made using the ring tensiometer method showed that the standard anionic surfactants SLS and the milder SLES have CMC values of 1.2 g/l (SLS) and 0.3 g/l (SLES), respectively (Figure 2). The relatively high value for SLS corresponds to poor cleaning performance that can only be compensated for in formulations through higher levels of surfactant use.   Figure 2: Static surface tension as a function of concentration. CMC of sodium cocoyl glutamate (Perlastan SC 25 NKW), sodium lauroyl glutamate (Perlastan SL), sodium lauroyl sarcosinate (Perlastan L-30) and other common surfactants. For this reason, it is common practice to add an additional surfactant or surfactants with particularly low CMC to a formulation to reduce the CMC of the overall formulation. A commonly used and suitable formulation of cleaning products is a mixture of the anionic surfactant SLES and the amphoteric CAPB (CMC 0.12 g/l). However, the strong ionic structure of the sulfate group in SLES increases the potential for situations that can be demonstrated as skin irritation and may lead to intolerance, particularly in individuals with sensitive skin. In this case, the use of amino acid-based surfactants such as sodium cocoyl glutamate (CMC 0.4 g/l), sodium lauroyl glutamate or sodium lauroyl sarcosinate (CMC 0.4 g/l) can reduce irritant effects and produce a much milder formulation. To produce significantly milder effects with a positive impact on the health of the skin surface, for example SLES and amphoteric surfactants (betaines) can be added at very low levels to formulations. The protective monolayer reduces the adsorption of aggressive surfactants on the skin and also prevents skin moisture loss. When surfactants are mixed in equivalent proportions with the nonionic surfactant coco glucoside (CMC 0.045 g/l), the total CMC is reduced from 0.4 g/l to 0.12 g/l or 0.2 g/l (Figure 3). This means that a lower surfactant concentration is required to achieve the same cleaning performance with the product's excellent foaming capacity and mildness.

Foaming Behavior

One of the most important visible indicators of good cleaning performance for a consumer is good foam formation. Rich foaming is associated with good cleaning capacity, low foaming is not. Good foaming also visibly demonstrates that the product is ready to spread over the skin. To investigate the foam properties of Perlastan surfactants, manual Ross-Miles testing was performed (Figure 4). Figure 4: Ross-Miles test with sodium cocoyl glutamate (Perlastan SC 25 NKW), sodium lauroyl sarcosinate (Perlastan L-30) and other surfactants (3 g/l active content). This test was performed to determine the foaming capacity and foam stability of surfactant solutions with active content of 3 g/l (above CMC). A defined foam volume was created by a foam disperser and foam height was measured as a function of time. The foam height developed by Perlastan surfactants is generally equivalent to high-foaming standard surfactants such as sulfates and betaines. A positive additional effect is that these surfactants form extremely stable and micro-porous foams in structure. In an additional test in which defined foam was created by means of a foam disperser, the amount of liquid drained from the foams was measured as a function of time. This is an indicator of the amount of liquid in the foam. The results show that sarcosinates and more particularly glutamates provide a stable foam with high water content that remains stable for a longer period (Figure 5). Figure 5: Foam Stability: Drainage amount over time of sodium cocoyl glutamate (Perlastan SC 25 NKW), sodium cocoyl glutamate/coco glucoside/xanthan gum, sodium lauroyl sarcosinate (Perlastan L-30) and other surfactants (active content 3 g/l).

Conclusion

Amino acid-based surfactants provide many positive properties that offer major advantages for the formulation of sulfate-free cleaning products. They particularly show distinct foaming behavior with high moisture content foam. This makes them very attractive for rinse applications. In formulations, more aggressive surfactants such as SLS or SLES can be combined with Perlastan surfactants. The irritant potential of the main surfactant is greatly reduced and the resulting formulation benefits from much milder properties. Additionally, the buffering capacity of glutamate and sarcosinate surfactants in the pH 5.0 and pH 6.5 range keeps the skin in a healthy condition. Furthermore, the cleaning power of amino acid-based surfactants can be optimized through combination with other surfactants offering a lower CMC. Due to excellent aerobic and anaerobic biodegradability, Perlastan surfactants are classified as environmentally friendly. Sodium acyl glutamates obtained from products such as Perlastan SC 25 NKW, Perlastan SC 25 NKPF, Perlastan SCG 50 ZPF and Perlastan SL meet the different criteria of a wide range of certification procedures and can be easily applied in natural cosmetic formulations.

References

1. Sugar M, Schmukker R. Reduzierung der Hautadsorption von Sodium Laureth Sulfat: Ein neuer Weg, die Hautfeuchtigkeit nach Anwendung von Duschprodukten zu erhöhen, SOFW Journal 2001; 127: 3-5.
2. Kanari M, Kawasaki Y, Sakamoto K. J. Soc. Cosm. Chem. 1993; 27: 498.
3. Nnanna IA, Xia J. Protein-Based Surfactants. Surfactant Science Series 2001; 101: 261-270.
4. Husmann M, Weisse J, Wragg P, Wasko J. Perlastan Surfactants Derived from Naturally occurring Amino Acids. Cosmetic Science Technology 2008; 194-201.