Predicting Microemulsions Stability, Viscosity and Cloud Points of Micelles via Spontaneous and Effective Packing Competition

The spontaneous packing parameter is a scalar linked to accessible structures formed in solution by adjacent molecules1. The effective packing parameter of any microemulsion or micelle can be measured by scattering2. The difference between effective and spontaneous packing creates a « frustration energy » that can moreover be evaluated in kJ/Mole by first principles3.

Comparing to different types of instability in surfactant solutions allows to predict the temperature range of standardized « fish-type » phase diagrams4,5 or the differences observed upon branching of the chains in the maximum accessible temperature without micro-phase separation known as cloud-points6.
The elastic energy originating from a mismatch between spontaneous and effective packing is equivalent to the more recently defined HLD when taken as an average over a whole sample7. When giant branched micelles form viscous solution, the frustration energy must be considered in terms of connexion points, end-caps and cylindrical parts of micelle-based gels that are common feature in formulated shampoos. Comparing probabilities of forming cylindrical micelles, branching points, end caps and entanglements is a general approach that can predict viscosity variations versus addition of electrolytes as well as perfumes8.
Practical examples with large collection of surfactants will be shown and illustrate the parameter-free predictive power of this approach based on geometry for the curvatures and on thermodynamics with the effective “rigidity” linked to the entropy of the film, as for example in the hyperbranched surfactants 9 that are even effective in supercritical CO2.