Screening Methods for Optimizing Biosurfactant Formulations

Sophorolipids are biosurfactants with potential applications in various industries, including cosmetics and fabric home care. However, formulation and characterization of sophorolipids can be a complex process. Product development and optimization requires systematic methodological procedures: Determination of the interaction parameter and application of the hydrophilic-lipophilic deviation (HLD) concept provides practical insights aiding in the selection of surfactant combinations to achieve the desired product characteristics.

The interaction parameter in a nonideal mixed micelle model is derived from critical micelle concentration (CMC) measurements performed on binary systems of surfactants and represents the net pairwise interactions within the mixed micelle. CMC measurements are carried out on different mixed binary surfactant systems and subsequently compared with numerical solutions of the nonideal mixed micelle model. The interaction parameter is adjusted until the model matches the experimentally measured CMCs. How sophorolipids interact with commonly used surfactants will be discussed to guide the formulation process.

Furthermore, the use of 2-phase titrations as a quick screening method for emulsion design using sophorolipids is demonstrated. The method involves formulating a middle phase optimum microemulsion of the surfactant with known oils at 25°C. By determining the salinity at optimal conditions (Phase Inversion Concentration, PIC) and using the HLD-equation, the critical curvature (cC) can be calculated. The calculated value can be cross-checked by measuring the Phase-Inversion-Temperature (PIT) at various salt concentrations once the scaling factor for the surfactant headgroup k is known. It is found that the lactonic sophorolipid LF is more hydrophobic, while the acidic sophorolipid is more hydrophilic. However, both sophorolipids require a co-formulant to formulate microemulsions at 25 °C.

In conclusion, the interaction parameter and HLD concept offer quick and efficient titration methods for screening sophorolipids, allowing for the identification of suitable co-formulants. This systematic approach can aid in the development and optimization of sophorolipid-based formulations for various applications.