Diffusion and state of water in mixed nonionic w/o microemulsion from spectroscopic data and molecular dynamics simulation

Knowledge of the location and diffusion of water molecules in microemulsions is essential for establishing the mechanisms of extraction and solubilization, and understanding their phase behavior. In this work, we study the diffusion of water in water/ n- decane microemulsions that contain nonionic surfactants sorbitan monolaurate (Span 80) and poly(oxyethylene20)sorbitan monooleate (Tween 80). We combine nuclear magnetic resonance (NMR) measurements with Fourier transform infrared (FT-IR) spectroscopy and molecular dynamycs (MD) simulations for microemulsions of varying content of water, total surfactant, and Span 80 to Tween 80 ratio. Experimental NMR-data on the diffusion coefficient are reported for the temperature interval 298 – 318 K. FT-IR studies of the extent of hydrogen bonding of water and MD simulations have been performed for 298 K. We confirm the existence of two distinct populations of aggregates: small reverse micelles, consisting predominantly of Span 80, and much larger aggregates containing considerable amounts of Tween 80. The self-diffusion coefficients of the reverse aggregates obtained from MD simulation agree with our NMR diffusomtry data. FT-IR data demonstrate the presence of water molecules in both the core and shell parts of microemulsion droplets and reverse micelles. The number of water molecules in the small reverse micelles and their size remain constant with increasing water content. The size and mobility of the large aggregates depend substantially on the content of water and Tween 80. Large hydrophilic parts of Tween 80 molecules bind substantial amounts of water, massively engaged in hydrogen bonds inside the aggregate’s cores. Accumulation of water leads to swelling of these microemulsion droplets and to an increase in their size. Water pools start to appear in the droplets at ca. 8 wt.% of water.