I am developing a stick sunscreen with an anhydrous base structured by waxes and vegetable oils. During rheological characterization (viscosity × temperature), I observed that the formulation containing plant extracts showed significantly lower viscosity compared to the base formulation, despite using the same structuring agents and lipid proportions.

I am looking for references or theoretical explanations to justify this reduction in viscosity caused by the presence of the extracts. I have considered the hypothesis that the extracts may act as plasticizers or interfere with the organization of the lipophilic matrix, but so far, I have not found any articles that clearly address this effect, especially in anhydrous systems.

Does anyone have suggestions of studies or books that discuss the influence of plant extracts or bioactive compounds on the viscosity or structuring of cosmetic formulations, particularly oily or anhydrous systems?

Thank you in advance for your help!