(517b) Molecular-Thermodynamic (MT) Modeling of Ellipsoidal Micelles to Predict Surfactant Micellization Properties In Aqueous Solution

In the micellar literature, there have been several contradictory reports on the existence of ellipsoidal micelles in aqueous solution. Although a number of experimental studies using small-angle neutron scattering and light scattering have suggested the existence of ellipsoidal micelles in aqueous surfactant solution (for example, Dupuy et al. showed that N-alkylamino-1-deoxylactitols form oblate ellipsoidal micelles in aqueous solution), many theoretical studies (for example, studies by Israelachvili et al. and Leibner and Jacobus) have claimed that micelles with an ellipsoidal shape cannot exist in solution because they are always free-energetically sub-optimal compared to other micelle shapes like spheres, cylinders, and bilayers. As a result, it is not clear whether micelles with ellipsoidal shapes can exist in solution, and if they can be considered as a transition shape between spheres and cylinders or bilayers.

In this talk, we discuss the development of a Molecular-Thermodynamic (MT) framework to model micelle shapes having varying curvature, for example, biaxial ellipsoidal micelles and discoidal micelles. Unlike many models in the literature that use average geometrical properties of the ellipsoidal shape to compute the equivalent of a micellization free energy, our theoretical framework utilizes concepts from differential geometry, mean field chain-packing theory, and electrostatics to develop expressions for the position dependent, local free energy of micellization. These expressions are then integrated over the entire micelle surface to obtain the average free energy of micellization for these micelles.

The model equations for the biaxial ellipsoidal micelles are used to establish theoretical limits on the size of ellipsoidal micelles that can exist in solution. These limits depend solely on the structure of the surfactant molecule, namely, its tail volume and head area. In addition, use of the model for ellipsoidal micelles in combination with previously developed models for spherical, spherocylindrical, and discoidal micelles sheds light on the solution conditions, including surfactant concentration and salt concentration, under which the biaxial ellipsoidal shape is the preferred micelle shape in solution.

References

Dupuy, C., Auvray, X., Petipas, C., Anthore, R., Costes, F., RicoLattes, I., Lattes, A., Small angle X-ray and neutron scattering study of the micellization of (N-alkylamino)-1-deoxylactitols in water, Langmuir 1996, 12 (13), 3162-3172.

Israelachvili, J. N., Mitchell, D. J., Ninham, B. W., Theory of Self-Assembly of Hydrocarbon Amphiphiles into Micelles and Bilayers, Journal of the Chemical Society-Faraday Transactions II 1976, 72, 1525-1568.

Leibner, J. E., Jacobus, J., Charged Micelle Shape and Size, Journal of Physical Chemistry 1977, 81 (2), 130-135.