Formulation of rhamnolipid-containing cosmetics without sulfate surfactants: A rheology and stability study

Sulfate surfactants in cosmetic products raise concerns due to their skin irritating impact and sustainability. Rhamnolipid, a green and safe biosurfactant, was studied from the perspective of rheology to help develop a formulation with natural and effective ingredients for cosmetics. The viscosity of rhamnolipids was investigated by changing pH, concentration and adding additives like inorganic salts (NaCl, MgCl₂) and polymers (Carbopol® Ultrez 20, Carbopol® Aqua SF-1, Carbopol® 940 and cationic guar gum), showing that the viscosity of rhamnolipid had a positive correlation with the increase of concentration, a negative correlation with the increase of pH, and that polymers instead of inorganic salts were suitable for thickening rhamnolipid. Further rheology tests indicated that Carbopol® Aqua SF-1 imparted gel properties of rhamnolipid while cationic guar gum determined the liquid properties. In addition, the rhamnolipid-containing moisturizing cream consisting of stearic acid/squalane ratios all showed shear-thinning and gel behavior the same as the commercial ones. Visual appearance and particle size analysis indicated that all formulations exhibited high storage stabilities. The knowledge gained from this study may be useful for designing “sulfonate-free” shampoo and cream with rheological properties that can be tailed for specific commercial cosmetic applications.