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 R_h ∼ 10-20 nm and small RMs with R_h ∼ 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.