Structural Properties of Span 80/Tween 80 Reverse\nMicelles by Molecular Dynamics Simulations

The sorbitan monooleate\n(Span 80)/poly­(oxyethylene) sorbitan monooleate\n(Tween 80) reverse micelles (RMs) in the water-in-<i>n</i>-decane microemulsion were studied using the molecular dynamics simulation.\nThe coexistence of the large RMs with the hydrodynamic radii <i>R</i>_h ∼ 10–20 nm and small RMs with <i>R</i>_h ∼ 1–2 nm was previously specified\nfor this system. Models of both surfactants and decane were based\non the united-atom approach to allow us to describe the structural\nproperties of the small RMs. The micelles have been self-assembled\nfrom an initially homogeneous mixture of surfactant, water, and decane\nmolecules. The dependence of the shape of the RMs on the relative\ncontent of surfactants has been established. The inner structure of\nSpan 80, Tween 80, and Span 80/Tween 80 RMs was quantitatively described.\nTween 80 molecules penetrate the water core, whereas Span 80 molecules\nare located on the surface of the RM. The obtained data show that\nthe hydrogen bonds are formed between the surfactant molecules on\nthe surface of the RM and play an important role in the formation\nof RMs. The water hydrogen bond density distribution in individual\nand mixed RMs explains the advantages of the mixed surfactant system\ncompared to an individual surfactant.