Formulation guideline for application of rhamnolipids in cosmetics and personal care

A  recently published microrheological study of ternary surfactant‐biosurfactant mixture reveales the effect of formulation conditions on the mixtures rheological responses.

The main objective of the paper by Liangchen Xu and Samiul Amin is to understand the rheological impact of rhamnolipids biosurfactant (mono/dirhamnolipids mixture, CCB) on a common personal care mixed surfactant system: anionic (Sodium laureth sulfate, SLES) and zwitterionic (Cocamidopropyl betaine, CAPB). The ternary biosurfactant/surfactants mixtures were evaluated at three different formulation conditions. The experimental results can provide a formulation guideline when applying rhamnolipids in cosmetics and personal care.

Methods used

Traditional mechanical rheometer was used to measure bulk viscosity and lower frequency rheological behavior of ternary surfactant mixture. Diffusing wave spectroscopy was utilized to access rheological responses at high frequency regime and extract structural parameter such as contour length, entanglement length and persistence length of wormlike micelles in the anionic/zwitterionic/biosurfactant mixtures.

Results of the study

Maxwellian type response is present in SLES/CAPB/CCB system to indicate formation of wormlike micelle. However, contour length of flexibility wormlike micelle formed by SLES/CAPB reduced from 445.8nm to 88.37nm with only 2 wt.% addition of CCB. Consequentially, viscosity decreased, and shorter relaxation time were observed. Addition of sodium chloride can build the viscosity of SLES/CAPB/CCB to an extent by screening charges between surfactant head groups. Highest viscosity of SLES/CAPB/CCB (10 wt%, 2wt% and 4wt% respectfully) were observed when overall sodium chloride is at 4 wt%. In the same SLES/CAPB/CCB system, pH impacts the system’s rheological response significantly. Due to the zwitterionic nature of CAPB, CAPB become more cationic at lower pH while rhamnolipids become more nonionic. SLES still maintains its anionic nature at low pH and this promoted the interaction between CAPB and SLES. At lower pH, overall loss and storage modulus exhibits higher values while longer relaxation times were observed. Increase in micelles contour length under lower pH conditions were shown by the DWS data.

Conclusion

This preliminary rheological and microstructure study of the complex biosurfactant/surfactants ternary system revealed the effect of formulation conditions on the mixtures rheological responses. The paper has been accepted for publication in International Journal of Cosmetic Science https://doi.org/10.1111/ics.12541

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