The transition from salt-in-water to water-in-salt nanostructures in water solutions of organic ionic liquids relevant for biological applications.

The nanostructure in water solutions of three organic ionic liquids relevant for biological applications has been studied by molecular dynamics simulations based on empirical force fields. The three compounds consisted of two different triethylammonium salts, known to affect the fibrillation kinetics of Aß peptides, and a phosphonium dication, which has been shown to possess a marked bactericidal activity. The structure of solutions spanning a wide concentration range (from 25 to 75 wt%) has been analysed by computing several combinations of partial structure factors, measuring the fluctuation of the ion and water distribution in space. At moderate salt concentration, the results reflect the formation in water of salt-rich domains of nanometric size. With salt concentration increasing beyond 50 wt%, the system enters the so-called water-in-salt regime, in which the aggregation properties of water become relevant, giving origin to water-rich domains in the nearly uniform salt environment. The persistence over a wide concentration range of nearly integer (∼6; ∼4) water-ion coordination numbers suggests the formation of stoichiometric liquid ionic hydrates.

Absorption of Phosphonium Cations and Dications into a Hydrated POPC Phospholipid Bilayer: A Computational Study.

Molecular dynamics (MD) based on an empirical force field is applied to investigate the effect of phosphonium cations ([P6,6,6,6]+) and geminal dications ([DxC10]2+) inserted at T = 300 K into the hydration layer separating planar POPC phospholipid bilayers. Up to high concentration, nearly every added cation and dication becomes absorbed into the lipid phase. Absorption takes place during several microseconds and is virtually irreversible. The neutralizing counterions ([Cl]-, in the present simulation) remain dissolved in water, giving origin to the charge separation and the strong electrostatic double layer at the water/lipid interface. Incorporation of cations and dications changes the properties of the lipid bilayer such as diffusion, viscosity, and the electrostatic pattern. At high ionic concentration, the bilayer acquires a long-wavelength standing undulation, corresponding to a change of phase from fluid planar to ripple. All these changes are potentially able to affect processes relevant in the context of cell biology. The major difference between cations and dications concerns the kinetics of absorption, which takes place nearly two times faster in the [P6,6,6,6]+ case, and for [DxC10]2+ dications displays a marked separation into two-stages, corresponding to the easy absorption of the first phosphonium head of the dication and the somewhat more activated absorption of the second phosphonium head of each dication.