Microrheological study of ternary surfactant-biosurfactant mixtures

The main objective of this paper is to understand the rheological impact of rhamnolipids biosurfactant (mono/dirhamnolipids mixture, CCB) on a common personal care mixed surfactants 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 products.

Traditional mechanical rheometer was used to measure bulk viscosity and lower frequency rheological behaviour of ternary surfactant mixtures. Diffusing wave spectroscopy (DWS) was utilized to access rheological responses at high frequency regime. Structural parameters such as contour length, entanglement length and persistence length of wormlike micelles in the anionic/zwitterionic/biosurfactant mixtures can be extracted from the rheological data gathered by DWS.

Maxwellian type response was present in SLES/CAPB/CCB system which indicated the formation of wormlike micelle. Contour length of wormlike micelle formed by SLES/CAPB reduced from 445.8 to 88.37 nm with only 2 wt% addition of CCB. In the same system, viscosity decreased and shorter relaxation time were observed. Addition of sodium chloride built 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) was observed when sodium chloride concentration was at 4 wt%. In the same SLES/CAPB/CCB system, pH impacted the system's rheological response significantly. Due to the zwitterionic nature of CAPB, it became more cationic at lower pH while rhamnolipids became more nonionic. SLES still maintained its anionic nature at low pH and this promoted the interaction between CAPB and SLES. At lower pH, overall loss and storage modulus exhibited higher values while longer relaxation times were observed. Increase in micelles contour length under lower pH conditions were shown by the DWS data.

This preliminary rheological and microstructure study of the complex biosurfactant/surfactants ternary systems revealed the effect of formulation conditions on the mixtures rheological responses.