General Patterns of Biocompatible Microemulsions Formed with Di-rhamnolipid and Alkanediols

Sustainable biosurfactants, such as rhamnolipids (RL), have been attracting increasing attention because of their enhanced biocompatibility and high biodegradability compared to conventional synthetic surfactants. To enable large-scale application of rhamnolipids, such as cleaning or washing, we studied symmetric, biocompatible microemulsions of the type H2O (KCl brine)-isopropyl myristate (IPM)-di-RL (Rha2C10C10)-alkane-1,2-diol. We present a systematic study on the influence of major formulation parameters, namely temperature, salinity, pH, and co-surfactant chain length, on the phase behavior and nanostructure. At pH = 8.0 ≫ pKa, Rha2C10C10 behaves similarly to double-tail anionic surfactants, including AOT, and does not form oil-continuous microemulsions at low salinities. At higher salinities, a classical fish phase diagram with a three-phase body was observed, implying that the addition of octane-1,2-diol causes an inversion of the amphiphilic film curvature from being curved around IPM to around brine. Lowering the pH to pH = 6.0 ∼ pKa leads to the partial protonation of the di-RL and a more hydrophobic behavior. Longer chain alkanediols substantially improve the solubilization efficiency of the amphiphile mixture. Small-angle neutron scattering (SANS) measurements showed an interaction peak at low q with values of the amphiphilicity factor fa around -0.7, confirming the presence of well-structured microemulsions. Data analysis at high q with a modified Porod law revealed surprisingly low values of blurriness t, in view of the di-RL's large headgroup, and a geometrical factor a significantly larger than predicted by models of the bicontinuous structure. Both observations might be linked to a scattering contribution of the bulky sugar head groups. Finally, at high pH and low salinity, an additional high-q scattering feature is observed, which could be related to the packing of the rhamnose moieties or to the local segregation of di-RLs and alkane-1,2-diols in the amphiphilic film.