The molecularly thin layer of water in direct contact with bio-molecules in a physiological environment plays a major role in determining their properties and functions. In this context, water is a shorthand notation for “water electrolyte solution”, since almost without exception a variety of ions dissolved in water are needed to ensure the stability of bio-systems, greatly contributing to their complex behaviours.In recent years, the development of compounds of the so-called room-temperature ionic liquid (RTIL) family has enormously expanded the number of ionic systems that could be used to modify the properties of the interfacial water, and thus to affect the behaviour of bio-systems.Our study concerns the microscopic mechanisms underlying RTIL effects on biosystems (e.g. phospholipid bilayers, proteins, and nucleic acids) through their hydration water, and relies on the combination of neutron scattering and molecular dynamics (MD) simulations.We present the results for the interaction of imidazolium-based RTILs with phospholipid bilayers [1,2]. Neutron reflectometry and MD simulations confirm the tendency of cations to be absorbed into the lipid phase, enhancing the penetration of water into the bilayer. Neutron scattering and MD reveal apparent changes in the relaxation time of water in close contact of the lipid head upon addition of RTILs, that reflect phase changes in the structure and dynamics of the system.[1] A. Benedetto, et al. J. Phys. Chem. B, 2014, 118, 12192.[2] A. Benedetto, et al. J. Chem. Phys., 2015, 142, 124706.