Role of spatial ionic distribution on the energetics of hydrophobic assembly and properties of the water/hydrophobe interface

Abstract

We present results from all-atom molecular dynamics simulations of large-scale hydrophobic plates solvated in NaCl and NaI salt solutions. As observed in studies of ions at the air-water interface, the density of iodide near the water-plate interface is significantly enhanced relative to chloride and in the bulk. This allows for the partial hydration of iodide while chloride remains more fully hydrated. In 1M solutions, iodide directly pushes the hydrophobes together (contributing −2.51 kcal/mol) to the PMF. Chloride, however, strengthens the water-induced contribution to the PMF by ~ −2.84 kcal/mol. These observations are enhanced in 3M solutions, consistent with the increased ion density in the vicinity of the hydrophobes. The different salt solutions influence changes in the critical hydrophobe separation distance and characteristic wetting/dewetting transitions. These differences are largely influenced by the ion-specific expulsion of iodide from bulk water. Results of this study are of general interest to the study of ions at interfaces and may lend insight to the mechanisms underlying the Hofmeister series.

Free energetics of carbon nanotube association in aqueous inorganic NaI salt solutions: Temperature effects using all‐atom molecular dynamics simulations

“…The propensity of larger halide anions to the water–hydrophobe interface is consistent with the scenario of anions at liquid–vapor interfaces (which represent the limit of the most hydrophobic interface) . Recent studies have begun to consider differential perturbations of liquid–vapor interfacial fluctuations induced by different ions along with ionic surface affinities.…”

“…Larger ionic charge density species may amplify the hydrophobic interaction between nonpolar solutes and show different effects with smaller ionic charge density species . In a biological context, these specific ion effects such as those observed by Hofmeister relating to the “salting‐in” (increasing the solubility of proteins) and “salting‐out” (antisolvent crystallization, precipitation crystallization) of proteins in aqueous electrolyte solutions of varying ionic species and concentrations have been pursued from theoretical, modeling, and experimental approaches for decades; studies continue to probe the fundamental phenomenology of this effect ,, . Horinek et al investigated the free energetics for Na + , Cl –, Br – , and I − to transfer from bulk aqueous solution to a hydrophobic self‐assembled monolayer (SAM)‐water interface in an infinite dilution and reported that soft polarizable monovalent anions (I − and Br – ) prefer to accumulate around the hydrophobic interface.…”

Free Energetics of Carbon Nanotube Association in Pure and Aqueous Ionic Solutions

“…These parameters are adopted from previous studies 46 , which have been validated to model hydrophobic effects in understanding the nature of surface friction in nanoscale fluid transport. Although the addition of a water hydrogen-tube LJ interaction may influence the structure of water near the tubes, the inclusion of this term seems arbitrary as the water model used in this study and many other previous study 17,42,47–50 do not include LJ interactions. For two SWNT’s in pure water, a bulk slab consisting of 4800 water molecules, which are represented by the extended simple point charge (SPC/E) model 51 , results in two liquid-vapor interfaces.…”

Temperature Dependence and Energetics of Single Ions at the Aqueous Liquid–Vapor Interface

“…Ions were treated as non-polarizable particles with interaction parameters based on those by Lamoureux and Roux 65 and validated for use with TIP4P-FQ 66-70 . We acknowledge that the use of a mixed polarizable water model with non-polarizable anion representation may appear unorthodox, but we consider that the combination of this empirical model is well-validated and reproduces many of the currently-accepted experimental observables upon which the quality of such force fields are based.…”