The structure of ionic aqueous solutions at interfaces: An intrinsic structure analysis

Abstract

We investigate the interfacial structure of ionic solutions consisting of alkali halide ions in water at concentrations in the range 0.2-1.0 molal and at 300 K. Combining molecular dynamics simulations of point charge ion models and a recently introduced computational approach that removes the averaging effect of interfacial capillary waves, we compute the intrinsic structure of the aqueous interface. The interfacial structure is more complex than previously inferred from the analysis of mean profiles. We find a strong alternating double layer structure near the interface, which depends on the cation and anion size. Relatively small changes in the ion diameter disrupt the double layer structure, promoting the adsorption of anions or inducing the density enhancement of small cations with diameters used in simulation studies of lithium solutions. The density enhancement of the small cations is mediated by their strong water solvation shell, with one or more water molecules "anchoring" the ion to the outermost water layer. We find that the intrinsic interfacial electrostatic potential features very strong oscillations with a minimum at the liquid surface that is ∼4 times stronger than the electrostatic potential in the bulk. For the water model employed in this work, SPC/E, the electrostatic potential at the water surface is ∼−2 V, equivalent to ∼80 k B T (for T = 300 K), much stronger than previously considered. Furthermore, we show that the utilization of the intrinsic surface technique provides a route to extract ionic potentials of mean force that are not affected by the thermal fluctuations, which limits the accuracy of most past approaches including the popular umbrella sampling technique.

Specific cation effects at aqueous solution−vapor interfaces: Surfactant-like behavior of Li + revealed by experiments and simulations

“…Thus, the surfactant-like behavior of Li + adsorption observed in the LiI solution is also seen in the LiBr and LiCl solutions, albeit to a lesser extent, and decreasing in the order LiI > LiBr > LiCl. The surface enhancement of Li + ions that we observed in lithium halide solutions agrees qualitatively with the results of previously reported MD simulations of 0.2 molal (m) and 1.0 m LiCl solutions (35), as well as an 8.6 m LiBr solution (36). We cannot quantitatively compare the extents of adsorption because of differences in the definition of the interface used to calculate density profiles.…”

“…1C and the integrated area ratios in Fig. 1D provide direct experimental confirmation of the prediction, made on the basis of MD simulations (34)(35)(36), that Li + ions adsorb to the aqueous solution-air interface.…”

“…It has been suggested, based on MD simulations (34)(35)(36), that Li + may be an exception. Presently, Li + is the only metal cation that has been observed in MD simulations to exhibit the "surfactant-like" behavior displayed by certain anions.…”

Long-range specific ion-ion interactions in hydrogen-bonded liquid films

“…49 Thus, our experimental results are consistent with the involvement of such extended networks in LR-SIE at air-liquid interfaces. [60][61][62][63][64][65][66] In summary, we demonstrated that the depth of the interfacial layers sampled in our experiments is controlled by nebulizer gas velocity ν G. We found that the larger, rather than the more polarizable, anions are systematically enriched in the thinner (nanoscopic air-liquid-air) films produced at higher gas velocities in all tested solvents (water, methanol, 2-propanol, and acetonitrile). Addition of third ions (beginning at sub-μM levels) specifically perturbs the I − /Br − ratio in water and methanol solutions but has no effect in acetonitrile or 2-propanol.…”

Lithium Ion–Water Clusters in Strong Electric Fields: A Quantum Chemical Study

“…The interfacial potential predicted by classical simulations can be, locally, much stronger, reaching values of ∼ −2 V, as reported in recent computations using methods that remove the smoothing effect of the interfacial capillary waves. 20 The modification of the interfacial potential upon addition of salt has been considered too, showing a significant dependence with salt concentration.…”