Ionic liquids that form micelles have great potential as drug carriers and separating agents for
bioactive
substances. For such applications, a key issue is the distribution of the target substance between
the micelle and its environment. We perform MD simulations to study solubilization of
zwitter-ionic tryptophan in micelles of 1-dodecyl-3-methylimidazolium bromide. We found that the
distribution of tryptophan depends strongly on the degree of counterion binding. A decrease in
binding of bromide counterions leads to a substantial increase of the distribution coefficient. A
dense layer of counterions at the micellar surface impedes the solubilization of the zwitter-ionic
tryptophan but at the same time the presence of such a dense layer obstructs the washout of the
solubilized tryptophan molecules from the micelle. Based on our simulation data, we conclude that
an increase of the distribution coefficient of tryptophan between the micelle and water may be
achieved by several means: by introducing counterions that bind weakly to the micelle (bulky
ions whose charge is not strongly localized) and/or by employing micelle-forming ionic liquids
with shorter alkyl chains to diminish the degree of
counterion binding.