  A combined experimental/theoretical approach has been used to investigate the
role of water in modifying the microscopic interactions characterizing the
optical response of butyl-ammonium nitrate (BAN) water solutions. Raman
spectra, dominated by the signal from the protic ionic liquid, were collected
as a function of the water content, and the corresponding spatial organization
of the ionic couples, as well as their local arrangement with water molecules,
was studied exploiting classical molecular dynamics calculations. High quality
spectroscopic data, combined with a careful analysis, revealed that water
affects the vibrational spectrum BAN in solution: as the water concentration is
increased, peaks assigned to stretching modes show a frequency hardening
together with a shape narrowing, whereas the opposite behavior is observed for
peaks assigned to bending modes. Calculation results clearly show a nanometric
spatial organization of the ionic couples that is not destroyed on increasing
the water content at least within an intermediate range. Our combined results
show indeed that small water concentrations even increase the local order.
Water molecules are located among ionic couples and are closer to the anion
than the cation, as confirmed by the computation of the number of H-bonds which
is greater for water-anion than for water-cation. The whole results set thus
clarifies the microscopic scenario of the BAN-water interaction and underlines
the main role of the extended hydrogen bond network among water molecules and
nitrate anions.
