Consistency of Ion Adsorption and Excess Surface Tension in Molecular Dynamics Simulations of Aqueous Salt Solutions

Abstract

Molecular dynamics simulations of the liquid−vapor interface of aqueous solutions of sodium fluoride and of sodium iodide have been carried out using nonpolarizable force fields for ions and water molecules. Despite the absence of explicit polarizability, the tendency of iodide ions to show an enhanced concentration at the surface that was reported for polarizable force fields (Jungwirth, P.; Tobias, D. J. J. Phys. Chem. B 2001, 105, 10468) is reproduced, while sodium and fluoride ions prefer the interior of the bulk liquid. These observations are confirmed by the contributions of the different species to the surface potential. The systems we study here are much larger than the ones investigated in previous simulations by other authors, which enables us to calculate the adsorption of ions at the interface from the density profiles and subsequentlyvia Gibbs’ adsorption isothermthe corresponding excess surface tension over that of pure water. The so-obtained values for the surface tension are compared with the results calculated directly from the normal and lateral components of the pressure tensor in the simulation. Consistency is found among the data, but the directly obtained values have significantly larger error bars and are intrinsically more scattered. The Gibbs adsorption isotherm thus not only is a thermodynamic requirement to be met but also offers a reliable and less error-prone way of calculating the surface tension increment from density profiles.

Origin of Interfacial Nanoscopic Gaseous Domains and Formation of Dense Gas Layer at Hydrophobic Solid–Water Interface

“…As shown in Table 3, the vapor-liquid interfacial tension for water is similar to values previously reported for SPC/E water from both MC and MD simulations 45,58,59 . For the water-graphite interfaces, the calculated Young's angle (88.8±1.8 degrees) is close to the "measured" contact angle by cylindrical water droplet on the graphite surface.…”

“…Normally, 0 z is a reference point in the charge-free vapor region, ( 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 If we chose a 0 z value of 2.5 nm for pure water system at 300 K, as shown in Figure 2, we obtain a surface potential difference value, φ ∆ , of -600 ± 10 mV which is close to the simulation values reported by other groups 32,43,45 but is significantly larger than the surface potential values reported from air-bubble experiments in the range from -40mV to -65mV 27,46,47 . However, there is another electrical field zero point at z = 3.2 nm as the consequence of the reversal of electrical field in which corresponds to the minimum value of the potentials.…”

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

“…Indeed, recent computer simulations have convincingly shown a correlation between ion size/charge and ion selectivity. [21][22][23][24][25][26][27] Similarly, the sensitivity of the ion-ion potential of mean force to the force-field parameters had been noted previously, 28 and has been confirmed more recently, 17 indicating the subtle role that the ion size, and the dispersion interactions, have in determining the structure of aqueous interfaces.…”

Pairwise-additive force fields for selected aqueous monovalent ions from adaptive force matching

“…However, the error bar remains fairly large. As has been pointed out in the literature, the surface tension is notoriously 73,74 This is partly due to the difficulty in converging the concentration profiles especially at lower concentrations. Our results support an increase of the surface tension as the concentration of the salts increases, consistent FIG.…”