Improved acyl lactylate surfactants: these hydrolytically-stable, value-added, multifunctional, 100% active additives have applications in skin and hair care formulations.

ACYL LACTYLATES are a class of nitrogen-free surfactants derived from natural resources. These surfactants are made by reacting lactic acid with fatty acid and neutralizing with a base according to the reaction scheme shown in Figure 1. Lactylates are well known in the food industry and are used in personal care applications'-5 to improve skin feel, skin softness and moisturization and reduce tackiness during wet to dry transition after product application.

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Acyl lactylates also function as viscosity builders, emulsifiers, foam boosters and stabilizers and can also be used as secondary surfactants. Due to their chemical composition, current commercial lactylates have undesirable hydrolytic stability. This has led to inferior performance in cosmetic products during prolonged storage, especially at elevated temperatures. By manipulating the chemical composition and employing a new process, Stepan researchers have successfully improved the hydrolytic stability of acyl lactylates (patent pending).

This stability enhancement has also enhanced other performance attributes. This article presents data that demonstrates the improved hydrolytic stability, and how sodium stearoyl lactylate-cosmetic grade (Stepan SSL-CG) can be effectively used in skin cleansing/foaming applications to deliver skin-feel and viscosity-building improvement. Further, we will show that this new lactylate gives better quality oil-in-water emulsions when compared to other commercial lactylates.

Lastly, we will present data that demonstrates how sodium lauroyl lactylate-foam booster (Stepan SLL-FB) can be effectively used as an additive in shampoos to improve viscosity building properties as well as foaming properties including flash foam, foam volume and lather richness.

Improved Hydrolytic Stability

A convenient way to test the hydrolytic stability of acyl lactylates is by measuring pH at elevated storage conditions in a model formulation. When acyl lactylates hydrolyze, lactic acid is produced and pH decreases. Thus, the extent of pH drop is an indication of the degree of acyl lactylate instability.

Stepan's new and improved lactylate was tested versus current commercial lactylates for hydrolytic stability using a body wash model formulation based on 12% active sodium laureth sulfate (SLES-2), 3% active cocamidopropyl betaine (CAPB) and 2% active sodium stearoyl lactylate (SSL) by monitoring the pH as a function of time at 50[degrees]C. The results of this study are shown in Figure 2.

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The results in Figure 2 show that there was a more gradual decrease in pH for body washes with the new and improved sodium stearoyl lactylate (SSL-CG) than the commercial lactylate. Also, the formulation with new and improved sodium stearoyl lactylate had significantly less pH decrease than the body wash with commercial lactylate during the same time interval.

For example, body wash with improved sodium stearoyl lactylate and commercial lactylate had an initial pH of 6.3 and pH of 5.5 after 28 days at 50[degrees]C. On the other hand, a body wash formulation with commercial lactylate had initial pH of 6.1 and pH of 4.7 after 28 days at 50[degrees]C. Similar results are obtained using sodium lauroyl lactylate (SLL-FB) in place of SSL-CG (data not shown).

Viscosity Salt Response

Sodium stearoyl lactylate was evaluated as an additive in the aforementioned model body wash formulation based on SLES-2 and CAPB to determine the viscosity salt response. The results in Figure 3 show that the body wash composition based on 12% active SLES-2 and 3% active CAPB with 2% active SSL-CG has higher viscosity at equal concentration of sodium chloride than the same body wash without the acyl lactylate. In other words, to achieve the same viscosity profile the body wash composition with 2% active SSL-CG requires a significantly lower concentration of sodium chloride electrolyte. Sodium lauroyl lactylate shows a similar viscosity salt response (data not shown).

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Skin Feel Evaluation

Sodium stearoyl lactylate (SSL-CG) and commercial lactylate additives were evaluated for skin-feel properties using the previously mentioned model formulation (12% active SLES-2 and 3% active CAPB) and a blind human expert panel test method described below. The results were recorded in a questionnaire form. Softness, moisturization and foam volume are shown in Figure 4.

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Skin Feel Test Method

1. Panelist's skin type is determined using NOVA DPM Meter (dry skin less than 100, normal skin 100-130, and moist skin greater than 130).

2. Hand wash test is conducted using lukewarm (95[degrees]F-105[degrees]F) Chicago tap water (water hardness 140 ppm).

3. 1ml of the test product is applied to the panelist's wet palm.

4. Panelists wash their hands by working the product into a lather for 30 seconds followed by rinsing for 15 seconds.

5. The washing procedure is repeated one more time.

6. Foam is collected and transferred into a graduated beaker, and foam volume is measured and recorded.

7. Hands are rinsed for 15 seconds and dried using a paper towel.

8. Panelists rank test products for ease of application, wet stage and dry stage (skin-feel) performance properties on a scale from 1 to 5.

9. The data is recorded in the questionnaire below.

Questionnaire

1. Ease of Application: 1 = difficult, 5 = easy

2. Flash Foam/Generation: 1 = difficult, 5 = easy

3. Bubble Size: 1 = rich and creamy, 5 = light, loose

4. Foam Volume: 1 = virtually no foam, 5 = copious amount of foam

5. Foam Feel: 1 = Non-lubricating, 5 = very lubricating

6. Overall Impression: 1 = bad, 5 = good

7. Rinseability: 1 = rinses poorly, 5 = rinses easily and quickly

8. Wet Feel: Squeaky Clean, Clean or Substantive

9. Tackiness During Drying: 1 = tacky; sticky, 5 = not tacky or sticky

10. Skin Tightness When Dry: 1 = very tight, 5 = not tight

11. Skin Moisturizing: 1 = very dry, 5 = not dry

12. Skin Softness: 1 = rough, 5 -very soft

13. Overall Initial Impression: 1 = bad, 5 = good

14. Overall Impression After 2-3 Minutes: 1 = bad, 5 = good

Performance differences in softness and moisturization properties between the control and the model formulations of 0-0.5 are considered insignificant (equal), differences of 0.5-0.75 are considered directional, and differences of greater than 0.75 are considered significant.

The results in Figure 4 show that model body wash formulations based on 12% active SLES-2, 3% active CAPB and commercial lactylates 1 or 2 had insignificant to a directional improvement in softness, moisturization and foam volume when compared to the negative control (12% active SLES-2 and 3% active CAPB). However, the model body wash formulation with improved sodium stearoyl lactylate had directionally better to a significant difference in moisturization when compared to the same negative control.

Emulsification Properties

The emulsification properties of SSL-CG were compared to a competitor's product using the oil-in-water model formulation shown in Table 1. Micrographs of skin lotions prepared with Stepan SSL-CG and competitor's SSL are shown in Figure 5.

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Skin Lotion Mixing Procedure

1. Into a suitable vessel, prepare the water phase by adding water and heating it to 70-75[degrees]C with turbine agitation.

2. In a separate vessel, prepare the oil phase by combining together SSL, isopropyl palmitate, glycerol stearate and cetyl alcohol. Heat to 75-80[degrees]C with agitation.

3. Add the oil phase slowly to the water phase with continuous agitation. Emulsify for 30 minutes with agitation set at high speed.

4. Begin to cool the batch to ambient temperature.

5. At 30[degrees]C add DMDM hydantoin preservative. Adjust the pH if necessary to 5.0+/0.2 with either liquid sodium hydroxide or liquid citric acid. Using the formulas in Table 1, the skin lotion prepared with Stepan SSL-CG had a brighter and whiter appearance and viscosity of 10,000 cps at 25[degrees]C compared to skin lotion prepared with competitor's SSL which had duller and off-white appearance and a viscosity of only 5,000 cps at 25[degrees]C. The micrographs in Figure 5 show that the skin lotion with Stepan SSL-CG had a significantly smaller and more uniform particle size distribution compared to the skin lotion prepared with the competitor's SSL. The smaller and more uniform particle size distribution led to better formulation stability.

Foaming Properties

The foaming properties of shampoo model formulation without and with 2% active SLL-FB as an additive on the top of 12% active SLES-2 and 3% active CAPB were studied in a salon using a standard salon half-head test method. The results of this study are shown in Figure 6 on the next page.

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The results in Figure 6 show that incorporation of 2% active sodium lauroyl lactylate as an additive on top of a shampoo model formulation provided a significant improvement in foaming properties including foam volume and lather richness, with a slight improvement in flash foam.

Conclusion

Sodium acyl lactylates are derived from lactic acid and fatty acid, both of which are natural and biorenewable resources. At 2% active concentration, sodium stearoyl lactylate (SSL-CG) and sodium lauroyl lactylate (SLL-FB) demonstrated better stability in body wash formulations based on 12% active SLES-2 and 3% active CAPB. In body wash cleansing products, SSL-CG and SLL-FB function to enhance skin feel, improve skin softness, impart skin moisturization, and reduce the tacky feel during the wet to dry stage transition following product application. Both SSL-CG and SLL-FB function as viscosity builders when added on top of shampoo formulations that are based on SLES-2/CAPB surfactant system at 2% active concentration. SLL-FB functions as a foam booster and a foam stabilizer in shampoos and skin cleansing products. Furthermore, other results not presented here suggest that SLLFB can be used as a non-nitrogen secondary surfactant in both hair and skin cleansing products.

Acknowledgment

We would like to thank our Stepan colleagues for their participation in skinfeel and other studies and for their contribution to this paper.

References

(1.) U.S. Patent 2,744,825, Acyl Lactylic Acid Products, J. Thompson, and Buddemeyer, C. J. Patterson, May 8, 1956.

(2.) U.S. Patent 3,728,447, L. Osipow, and D. Marra, C. J. Patterson Company, Apr. 17, 1973.

(3.) Baiocchi, F., Jennings, D., and Del Vecchio, A.J., "Use of Acyl Lactylates in Cosmetics and Toiletries," Cosmetics and Perfumery, September, vol. 90, pp. 31-34 (1975).

(4.) Murphy, L.J., "Sorption of Acyl Lactylates by Hair and Skin as Documented by Radio Tracer Studies," Cosmetics & Toiletries, March, vol. 94, pp. 43-49 (1979).

(5.) Cook, J.W., "Acyl Lactylate Index," Cosmetics & Toiletries, October, vol. 112, pp.69-79 (1997).

Table 1: Skin lotion model formulations for Stepan sodium stearoyl
lactylate (SSL) and competitor's SSL

Ingredients                           Functionality

Deionized water                       Diluent, carrier
Stepan SSL-CG                         Emulsifier
Competitor's SSL                      Emulsifier
Stepan IPP (isopropyl palmitate)      Emolllient
Stepan GMS Pure (glycerol stearate)   Emulsifier
Cetyl alcohol                         Thickener, emulsifier

                                          Lotion w/          Lotion w/
                                      Stepan SSL-CG   Competitor's SSL
Ingredients                             %Wt. Active        %Wt. Active

Deionized water                       q.s. to 100.0      q.s to 100.00
Stepan SSL-CG                                   1.0                 --
Competitor's SSL                                 --                1.0
Stepan IPP (isopropyl palmitate)               10.0               10.0
Stepan GMS Pure (glycerol stearate)             3.0                3.0
Cetyl alcohol                                   2.0                2.0