Glutamate Surfactants and Natural Micellar Thickeners

Amino acid-based surfactants have been widely used over the past twenty years in personal care products, household cleaners and industrial cleaners due to numerous advantages. They are very mild [1-3] anionic surfactants, which means they do not cause skin drying [4] and provide protection from the negative effects of surfactants such as SLS and SLES [5,6] through their use as auxiliary surfactants. In addition to good cleansing properties, they exhibit excellent foam formation. As a result, the high microporosity of the foam, its creamy white structure and stability are supported [7]. Amino acid-based surfactants are readily biodegradable [8-10] and exhibit extremely low toxicity [11]. These two important properties are crucial for creating ecologically pure products without creating a negative impact on the environment. Glutamic acid-based surfactants hold a special position in the range of surfactant compounds because they can be obtained entirely from renewable plant sources. In this case, they also provide applications for natural personal care products certified by COSMOS [12] and NaTrue [13]. A fundamental problem was encountered with thickening formulations based on "green" surfactants. The addition of salt in these products did not provide the desired effect. It was necessary to use water-soluble polymers such as xanthan gum or acrylates to achieve the desired consistency and appearance in the finished product.

Recently, high viscosity standard surfactant concentration can be achieved without a thickening agent using a combination of sodium salts of cocoyl or lauryl glutamate or lauryl glucoside.

Based on this information, Rheo2Green, a new product ready for use at mixture concentrations, was developed. It can be easily diluted and does not require the addition of other surfactants. Disodium cocoyl glutamate and lauryl glucoside are "green" surfactants, therefore all products in the Rheo2Green series comply with COSMOS and NaTrue. Both are readily biodegradable and environmentally friendly, making them suitable for finished products certified with Ecolabel (EU or Nordic Swan). Rheo2Green2 contains sufficient amounts of sodium benzoate and potassium sorbate to protect the finished product. The advantages of using Rheo2Green mixtures are as follows: a) Pure lauryl glucoside is a highly viscous liquid that typically requires pre-heating before use. Rheo2Green concentrates are low-viscosity products. They are easily processed without requiring heat during the production stage. b) Rheo2Green simplifies logistical aspects in production, storage, purchasing and dosing compared to separate use of its components. c) Rheo2Green provides advantages due to price and optimization of the production process. As mentioned above, this combination of surfactants provides natural micellar thickening. It is typical that most surfactants form spherical micelles. Only under certain conditions can worm-like, cylindrical micelles be built. At the molecular scale, these structures resemble long polymer molecules. The interaction between these cylindrical micelles provides a thickening effect in a similar manner (Figure 1).     Figure 1: Formation of spherical and cylindrical micelles.        

With Rheo2Green products, the desired viscosity is easily achieved. It should be diluted with water and, when adjusted to pH 4.6-5.3, it instantly acquires consistency. Figure 2 shows the viscosity change versus pH for Rheo2Green1 solutions at different dilutions. It was found that the viscosity of these systems has a maximum. For diluted Rheo2Green1 solutions, in the absence of other components, it is found at constant pH ~4.9. When additional ingredients are present, the pH value at which viscosity is maximum slowly changes in the range of 4.5 to 5.3. Figure 2: Viscosity values of Rheo2Green1 solutions with different pH values at different dilutions measured with Brookfield Sp.3, 5 rpm, RT (W% product - weight percent of raw material; W% active - corresponding concentration of active substance). The effect of components commonly used in personal care products on viscosity in final formulations was tested. Results are summarized in Figure 3. In general, in addition to polyhydric alcohols, preservatives based on sorbate and benzoate (++ or +) can increase viscosity. Other non-hydrophilic surfactants have small effects (+ or 0) on this parameter up to certain amounts. On the other hand, hydrophobic substances such as oils break viscosity at low concentrations. Surprisingly, polymeric thickening agents destabilize the system and precipitate. Figure 3: Compatibility of Rheo2green1 with typical rinse components in terms of homogeneity and viscosity. A basic formula for shampoo was developed with Rheo2Green2 (Figure 4). As mentioned previously, this raw material contains sodium benzoate and potassium sorbate, which act as preservatives in the final product. Microbiological testing was conducted according to Ph.Eur (European Pharmacopoeia) and ISO 11930 confirms that the amount is sufficient for microbiological protection of the finished product when using 30% Rheo2Green2. Figure 4: "Thickened Shampoo Containing Green Product" with Rheo2Green2. The scope of applications for Rheo2Green products is very broad: from skin and hair care products to household and pet care, Rheo2Green is recommended wherever mildness, exceptional foam and good cleansing are desired. The achievement of high viscosity without using thickening agents, excellent flow behavior and clarity are additional advantages of formulations based on Rheo2green concentrates. They can be used conveniently in formulations due to their cost-effectiveness, environmental friendliness and "Natural/Green" certification.

References

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8. Gitte I. Petersen og Trine Thorup Anderse, DHI – Vand og Miljø, Arbejdsrapport fra Miljøstyrelsen Nr. 10 2004: Substitution af overflade aktive stoffer i kosmetiske produkter.

9. Uusitalo T (Ecolabelling Finland), Hirsch T (Ecolabelling Norway), Eskeland M B (Ecolabelling Norway). Revision of the harmonised Detergent Ingredient Database; Final report for DID 2016, Version 2016, May 2016.

10. Husmann M. Surface Active Agents Derived from Naturally-Occurring Amino Acids. CESIO - 7th World Surfactants Congress 2008.

11. ECHA website: https://echa.europa.eu/nl/registration-dossier/-/registered-dossier/11409

12. COSMOS website: http://www.cosmos-standard-rm.org/

13. NATRUE website: http://www.natrue.org/information-for/manufacturers/raw-materials/

  Author: Dr. Yana Bykov Technical Director Schill+Seilacher GmbH Boeblingen, Germany                 Translator/Editor: Tuğçe Güner Research and Development Engineer Cosmer Kimya A.Ş.