Closo-dodecaborate-based dianionic surfactants with distorted classical morphology: Synthesis and atypical micellization in water

Hypothesis: To challenge the classical concept of step-like micellization of ionic surfactants with singular critical micelle concentration, novel amphiphilic compounds with bulky dianionic head and the alkoxy tail connected via short linker, which can complex sodium cations, were synthesized in the form of disodium salts.Experiment: The surfactants were synthesized by opening of a dioxanate ring attached to closo-dodecaborate by activated alcohol, which allows for attachment of alkyloxy tails of desired length to boron cluster dianion. The synthesis of the compounds with high cationic purity (sodium salt) is described. Self-assembly of the surfactant compound at air/water interface and in bulk water was studied by tensiometry, light and small angle X-ray scattering, electron microscopy, NMR spectroscopy, MD simulations and by isothermal titration calorimetry, ITC. The peculiarities in the micelle structure and formation were revealed by thermodynamic modelling and MD simulations of the micellization process. Findings: In an atypical process, the surfactants self-assemble in water to form relatively small micelles, where the aggregation number is decreasing with the surfactant concentration. The extensive counterion binding is a key characteristic of the micelles. The analysis strongly indicates complex compensation between the degree of bound sodium ions and the aggregation number. For the first time, a three-step thermodynamic model was used to estimate the thermodynamic parameters associated with micellization process. Diverse micelles differing in size and counterion binding can (co-)exist in the solution over the broad concentration and temperature range. Thus, the concept of step-like micellization was found inappropriate for these types of micelles.

Automated summary

Novel amphiphilic compounds with dianionic head and alkoxy tail were synthesized to challenge the classical concept of step-like micellization. The surfactants self-assembled to form small micelles with decreasing aggregation number at higher concentration. Extensive counterion binding was observed in the micelles, and a three-step thermodynamic model was used to estimate the parameters associated with micellization process. The concept of step-like micellization was found to be inappropriate for these types of micelles.