Distribution of zwitter-ionic tryptophan between the micelles of 1-dodecyl-3-methyl imidazolium and aqueous medium from molecular dynamic simulation.

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.

Structural Properties of Span 80/Tween 80 Reverse Micelles by Molecular Dynamics Simulations.

The sorbitan monooleate (Span 80)/poly(oxyethylene) sorbitan monooleate (Tween 80) reverse micelles (RMs) in the water-in- n-decane microemulsion were studied using the molecular dynamics simulation. The coexistence of the large RMs with the hydrodynamic radii Rh ∼ 10-20 nm and small RMs with Rh ∼ 1-2 nm was previously specified for this system. Models of both surfactants and decane were based on the united-atom approach to allow us to describe the structural properties of the small RMs. The micelles have been self-assembled from an initially homogeneous mixture of surfactant, water, and decane molecules. The dependence of the shape of the RMs on the relative content of surfactants has been established. The inner structure of Span 80, Tween 80, and Span 80/Tween 80 RMs was quantitatively described. Tween 80 molecules penetrate the water core, whereas Span 80 molecules are located on the surface of the RM. The obtained data show that the hydrogen bonds are formed between the surfactant molecules on the surface of the RM and play an important role in the formation of RMs. The water hydrogen bond density distribution in individual and mixed RMs explains the advantages of the mixed surfactant system compared to an individual surfactant.