Specific Interactions in the Model of Mixed Multicomponent Micelles: Predicting Aggregation Behavior and Details of Structure

Abstract

Modeling of multicomponent micellar aggregates is important for screening and selecting formulations in a variety of engineering applications, including micelle-mediated separation, drug delivery and nanoreactors. Typically, the constituent molecules of such aggregates have complex chemical structure and are engaged in specific and nonspecific interactions with one another and with the hydration water. Description of these interactions in the strongly nonuniform aggregates is very difficult, impeding the development of the theoretical models. In this work we extend to the multicomponent aggregates a molecular-thermodynamic model that describes the hydrogen bonding and other specific interactions in the hydrated corona. The model is formulated for an arbitrary number of nonionic and ionic amphiphiles that self-assemble into polydisperse aggregates in an aqueous solution. For describing the aggregation equilibrium together with the material balance in this solution, a simple method is suggested and tested. We illustrate application of our model by predicting aggregation equilibrium and characteristics of the three-component aggregates in the aqueous mixture of the nonionic surfactant Triton X-114, a micelle-forming alkylimidazolium ionic liquid (1-methyl-3-octylimidazolium chloride), and n-octanol. Our calculations suggest that the tails of the octanol molecules are deeply buried in the cores of the mixed micelles with only one methylene group penetrating the coronae and that the entire hydrocarbon tail of the ionic liquid resides in the core. Taking into account the populations of polydisperse spherical and rodlike aggregates, we model equilibrium partitioning of the components between the aggregates and the surrounding aqueous solution. The model predicts that addition of the ionic liquid to the mixture promotes the partitioning of n-octanol into the Triton-rich micelles.