FRONTIERS ARTICLE Specific ion adsorption at the air/water interface: The role of hydrophobic solvation Dominik Horinek a, * , Alexander Herz a , Lubos Vrbka b , Felix Sedlmeier a , Shavkat I. Mamatkulov a,c , Roland R. Netz a a Physik Department, Technische Universität München, 85748 Garching, Germany b Institut für Physikalische und Theoretische Chemie, Universität Regensburg, 93040 Regensburg, Germany c Heat Physics Department of Uzbekistan Academy of Sciences, 28 Katartalstr., 700135 Tashkent, Uzbekistan article info Article history: Received 24 July 2009 In final form 24 July 2009 Available online 28 July 2009 abstract Classical force fields for molecular simulations of aqueous electrolytes are still controversial. We study alkali and halide ions at the air/water interface using novel non-polarizable force fields that were opti- mized based on bulk thermodynamics. In qualitative agreement with polarizable force-field simulations, ion repulsion from the interface decreases with increasing ion size. Iodide is even enhanced at the inter- face, which is rationalized by hydrophobic solvation at the interface, but exhibits a smaller surface pro- pensity than in previous polarizable simulations. Surprisingly, lithium is less repelled than other cations because of its tightly bound hydration shell. A generalized Poisson–Boltzmann approach that includes ionic potentials of mean force from simulation almost quantitatively matches experimental interfacial tension increments for 1 molar sodium halides and alkali chlorides. We conclude that properly optimized non-polarizable force fields are transferable to interfacial environments and hold the potential for unrav- elling ion-specific effects even in biological situations involving peptidic surfaces. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction The behavior of ions at the air/water interface is a classic problem of physical chemistry with many important implications and still puzzling questions [1]. Experiments on ion adsorption at the surface of water are notoriously difficult, specially for low salt concentra- tions, because of the susceptibility to small traces of surface-active contaminants. On the other hand, surface tension measurements al- low to directly gain information of the surface activity of salts and give reliable account of the interfacial thermodynamics [2]. Experi- mentally, it is observed that salt addition to water typically in- creases the surface tension, which implies that there is a negative surface excess (i.e. depletion) in the sum of the density profiles of anions and cations. However, there have been experiments that indicate an increased density of some anions in the interfacial layer. The majority of these experiments is based on sum frequency gener- ation [3] or second harmonic generation nonlinear optical techniques [4]. Direct evidence of iodide being enhanced at the air/water interface was obtained with a completely different exper- imental probe using grazing incidence X-ray fluorescence [5]. In their classical model, Onsager and Samaras identified the interfacial forces acting on ions with electrostatic image forces, which repel all ions from the interface between water ð r ¼ 78Þ and air ð r ¼ 1Þ [6]. This theory relies on a continuum description of water and cannot explain or describe ion-specific effects. Since then, analytic models were refined and generalized to include effects due to ion size [7] or dispersion interactions [8] and thus can allow to treat ion-specific effects on a heuristic level. Using molecular simulations, it was recognized that the discrete nature of the water molecules crucially determines the molecular struc- ture in the interfacial region [9]. As a consequence, solvation forces acting on ions in the interfacial region cannot be described by simple continuum models [10]. Nevertheless, information on the properties of such complex systems are accessible by molecular simulation methods, either Monte Carlo or molecular dynamics (MD), employing suitable atomistic force fields. Soon after the seminal works on the simulation of water at a hydrophobic surface [11] and at the air/water interface [9], a simulation of a sodium ion at the air/water interface was reported [12]. Polarizable force fields for ions and water were applied to bulk water [13,14] and ions in small water clusters [15]. During the last decade, there has been a tremendous amount of simulation work on ions at interfaces. Starting with the work of Jungwirth and Tobias [16], such polariz- able force fields for ions and water have been extensively applied for studies of ions at the air/water interface [17,18], and have also been used for the study of ions at a solid hydrophobic self-assem- bled monolayer [10]. The common rationale behind using polariz- able force fields is the idea that ion affinity for an interface arises from the favorable interaction of the polarizability of an ion [18] with the interfacial electric field due to the mean orientation of interfacial water molecules [19,11]. This view was supported by 0009-2614/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2009.07.077 * Corresponding author. E-mail address: dominik.horinek@ph.tum.de (D. Horinek). Chemical Physics Letters 479 (2009) 173–183 Contents lists available at ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett