Aqueous micelle solutions have applications ranging from home cleaning products to drug delivery to environmental remediation. The fundamentals of micelle formation and solubilization are well understood and both experimental and theoretical details are increasing in sophistication. One area where the research is less abundant is the effect of hydrophobic counterions on the micellization process. Here we show the results of pyrene fluorescence experiments and molecular dynamics simulations on the effects of a series of tetraalkylammonium counterions on undecylenic acid-based micelles in aqueous solutions. The fluorescence results show decreasing trends in both critical micelle concentration (CMC) and micelle aggregation number with increasing hydrophobicity of the counterions. The molecular dynamics simulations indicate spherical micelles with an average diameter of 3.0 nm, independent of the counterions. The least hydrophobic counterions, tetramethylammonium and tetraethylammonium, show diffuse radial distribution functions for the counterion with respect to the center of mass of the micelles. In contrast, the more hydrophobic tetrapropylammonium and tetrabutylammonium counterions show two distinct peaks corresponding to a primary layer at the surface of the micelle and a secondary layer at about 0.4 nm from the surface. We are currently investigating the effect of this unusual structure on the polarity of the micelle microenvironment.

Effect of hydrophobic counterions on undecylenic acid-based micelle formation

2019 Fall National Meeting

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