Spherical Monovalent Ions at Aqueous Liquid–Vapor Interfaces: Interfacial Stability and Induced Interface Fluctuations

Abstract

Ion-specific interfacial behaviors of monovalent halides impact processes such as protein denaturation, interfacial stability, surface tension modulation, and as such, their molecular and thermodynamic underpinnings garner much attention. We use molecular dynamics simulations of monovalent anions in water to explore effects on distant interfaces. We observe long-ranged ion-induced perturbations of the aqueous environment as suggested by experiment and theory. Surface stable ions, characterized as such by minima in potentials of mean force computed using umbrella sampling MD simulations, induce larger interfacial fluctuations compared to non-surface active species, conferring more entropy approaching the interface. Smaller anions and cations show no interfacial potential of mean force minima. The difference is traced to hydration shell properties of the anions, and the coupling of these shells with distant solvent. The effects correlate with the positions of the anions in the Hofmeister series (acknowledging variations in force field ability to recapitulate essential underlying physics), suggesting how differences in induced, non-local perturbations of interfaces may be related to different specific-ion effects in dilute biophysical and nanomaterial systems.

Association of alkanes with the aqueous liquid–vapor interface: a reference system for interpreting hydrophobicity generally through interfacial fluctuations

“…At z = −4.0 Å, the first hydration shell breaks; solvent redistribution and exchange occurs and thus induces more interfacial fluctuations. This behavior is in general reminiscent of the I − anion shown in Figure 6A of reference 21 . Furthermore, there is evidence of water residence times in first hydration shells of various anions (based on Grote-Hynes and reactive flux approaches incorporating quantum nuclear effects) suggesting that for larger anions such as I − , water is not as persistent in the first hydration shell as it is for smaller ions such as Cl − and F − 54 .…”

“…By plotting the maximum interfacial fluctuation $(\Delta G)$ versus the value of the PMF at the position of the minimum (Δ G ) for various spherical monovalent ions, we found a force-field-independent best-fitting line, indicating a threshold of $\Delta h_L^2\approx =1.5$ dividing those ions that are interfacially stable and those that are not 21 . The same analysis is performed here as the main focus of this study is on the correlation of the behaviors of hydrophobic solutes and ions with respect to induced interface fluctuation and interfacial stability, as presented in Figure 7.…”

“…Previously, we reported that near the (hydrophobic) interfaces, ions can be differentiated depending on the associated induced interfacial fluctuations 21,23 . By plotting the maximum interfacial fluctuation $(\Delta G)$ versus the value of the PMF at the position of the minimum (Δ G ) for various spherical monovalent ions, we found a force-field-independent best-fitting line, indicating a threshold of $\Delta h_L^2\approx =1.5$ dividing those ions that are interfacially stable and those that are not 21 .…”

“…Included are results of spherical monovalent ions adapted from Reference 21,23 for comparison. A linear relationship between$⟨\Delta h_{L,\max }^2⟩$ and Δ G is found with a best-fit intercept of 1.51, which is very close to the result obtained for spherical monovalent ions (1.53).…”

“…Black circles and red rectangular mark the results of alkanes in TIP3P and SWM4-NDP. Blue triangles and purple diamonds mark the results of ions using TIP3P and SWM4-NDP, while are adapted from Reference 21, 23 …”