Mixtures of Isobutyric Acid and Water Confined in Cylindrical Silica Nanopores Revisited: A Combined Solid-State NMR and Molecular Dynamics Simulation Study

The phase behavior of a mixture of isobutyric acid and water confined in mesoporous SBA-15 silica material is studied by a combination of solid-state NMR spectroscopy and molecular dynamics (MD) simulations. The combination of these two techniques shows that the iBA-rich phase is close to the pore wall and the water-rich phase is in the center of the pores. The MD simulations reveal that this surprising phase behavior is mainly the result of the minimization of the hydrogen-bonding enthalpy. Moreover, the simulations show also that upon increasing temperature the relative water concentration increases at the pore surface, which is attributed to entropic contributions which lead to a more thorough miscibility. These results solve a long-standing problem of the microphase separation inside these materials.