The role the charge sign of simple ions plays in determining their surface affinity in aqueous solutions is investigated by computer simulation methods. For this purpose, the free surface of aqueous solutions of fictitious salts is simulated at finite concentration both with nonpolarizable point-charge and polarizable Gaussian-charge potential models. The salts consist of monovalent cations and anions that are, apart from the sign of their charge, identical to each other. In particular, we consider the small Na+ and the large I- ions together with their charge-inverted counterparts. In an attempt to avoid the interference even between the behavior of cations and anions, we also simulate systems containing only one of the above ions, and determine the free energy profile of these ions across the liquid-vapor interface of water at infinite dilution by potential of mean force (PMF) calculations. The obtained results reveal that, in the case of small ions, the anion is hydrated considerably stronger than the cation due to the close approach of water H atoms, bearing a positive fractional charge. As a consequence, the surface affinity of a small anion is even smaller than that of its cationic counterpart. However, considering that small ions are effectively repelled from the water surface, the importance of this difference is negligible. Further, a change in the hydration energy trends of the two oppositely charged ions is observed with their increasing size. This change is largely attributed to the fact that, with increasing ion size, the factor of 2 in the magnitude of the fractional charge of the closely approaching water atoms (i.e., O around cations and H around anions) outweighs the closer approach of the H than the O atom in the hydration energy. Thus, for large ions, being already surface active themselves, the surface affinity of the anion is larger than that of its positively charged counterpart. Further, such a difference is seen even in the case when the sign of the surface potential favors the adsorption of cations.

Budapest University of Technology and Economics Szt Gellért tér 4 H 1111 Budapest Hungary

Department of Chemistry Eszterházy Károly Catholic University Leányka utca 6 H 3300 Eger Hungary

Department of Physical Chemistry University of Chemistry and Technology 166 28 Prague 6 Czech Republic

Institute of Physics and Materials Science University of Natural Resources and Life Sciences Peter Jordan Straße 82 A 1190 Vienna Austria

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Shaw D. J.Introduction to Colloid and Surface Chemistry; Butterworths: London, 1980.

Heydweiller A.About Physical Properties of Solutions in Their Context. II. Surface Tension and Electrical Conductivity of Aqueous Salt Solutions, Ann. Phys. 1910, 338, 145–185 (in German), 10.1002/andp.19103381108. DOI

Weissenborn P. K.; Pugh R. J. Surface Tension of Aqueous Solutions of Electrolytes: Relationship with Ion Hydration, Oxygen Solubility, and Bubble Coalescence. J. Colloid. Interface Sci. 1996, 184, 550–563. 10.1006/jcis.1996.0651. PubMed DOI

Ali K.; Haq A.; Bilal S.; Siddiqi S. Concentration and Temperature Dependence of Surface Parameters of Some Aqueous Salt Solutions. Colloid Surf. A 2006, 272, 105–110. 10.1016/j.colsurfa.2005.07.014. DOI

Chen H.; Li Z.; Wang F.; Wang Z.; Li H. Investigation of Surface Properties for Electrolyte Solutions: Measurement and Prediction of Surface Tension for Aqueous Concentrated Electrolyte Solutions. J. Chem. Eng. Data 2017, 62, 3783–3792. 10.1021/acs.jced.7b00503. DOI

Onsager L.; Samaras N. N. T. The Surface Tension of Debeye-Hückel Electrolytes. J. Chem. Phys. 1934, 2, 528–536. 10.1063/1.1749522. DOI

Markovich G.; Giniger R.; Levin M.; Cheshnovsky O. Photoelectron Spectroscopy of Iodine Anion Solvated in Water Clusters. J. Chem. Phys. 1991, 95, 9416–9419. 10.1063/1.461172. DOI

Markovich G.; Pollack S.; Giniger R.; Cheshnovsky O. Photoelectron Spectroscopy of Cl–, Br–, and I– Solvated in Water Clusters. J. Chem. Phys. 1994, 101, 9344–9353. 10.1063/1.467965. DOI

Perera L.; Berkowitz M. L. Many-Body Effects in Molecular Dynamics Simulations of Na+(H2O)n and Cl–(H2O)n Clusters. J. Chem. Phys. 1991, 95, 1954–1963. 10.1063/1.460992. DOI

Dang L. X.; Smith D. E. Molecular Dynamics Simulations of Aqueous Ionic Clusters Using Polarizable Water. J. Chem. Phys. 1991, 95, 6950–6956. 10.1063/1.465441. DOI

Perera L.; Berkowitz M. L. Structures of Cl–(H2O)n and F–(H2O)n (n=2,3,...,15) Clusters. Molecular Dynamics Computer Simulations. J. Chem. Phys. 1994, 100, 3085–3093. 10.1063/1.466450. DOI

Jungwirth P.; Tobias D. J. Molecular Structure of Salt Solutions: A New View of the Interface with Implications for Heterogeneous Atmospheric Chemistry. J. Phys. Chem. B. 2001, 105, 10468–10472. 10.1021/jp012750g. DOI

Jungwirth P.; Tobias D. J. Ions at the Air/Water Interface. J. Phys. Chem. B 2002, 106, 6361–6373. 10.1021/jp020242g. DOI

Vrbka L.; Mucha M.; Minofar B.; Jungwirth P.; Brown E. C.; Tobias D. J. Propensity of Soft Ions for the Air/Water Interface. Curr. Opin. Colloid. Interface Sci. 2004, 9, 67–73. 10.1016/j.cocis.2004.05.028. DOI

Ottosson N.; Heyda J.; Wernersson E.; Pokapanich W.; Svensson S.; Winter B.; Öhrwall G.; Jungwirth P.; Björneholm O. The Influence of Concentration on the Molecular Surface Structure of Simple and Mixed Aqueous Electrolytes. Phys. Chem. Chem. Phys. 2010, 12, 10693–10700. 10.1039/c0cp00365d. PubMed DOI

Stern A. C.; Baer M.; Mundy C. J.; Tobias D. J. Thermodynamics of Iodide Adsorption at the Instantaneous Air-Water Interface. J. Chem. Phys. 2013, 138, 114709.10.1063/1.4794688. PubMed DOI

Olivieri G.; Parry K. M.; D’Auria R.; Tobias D. J.; Brown M. A. Specific Anion Effects on Na+ Adsorption at the Aqueous Solution–Air Interface: MD Simulations, SESSA Calculations, and Photoelectron Spectroscopy Experiments. J. Phys. Chem. B 2017, 122, 910–918. 10.1021/acs.jpcb.7b06981. PubMed DOI

Hantal G.; Horváth R. A.; Kolafa J.; Sega M.; Jedlovszky P. Surface Affinity of Alkali and Halide Ions in Their Aqueous Solution: Insight from Intrinsic Density Analysis. J. Phys. Chem. B 2020, 124, 9884–9897. 10.1021/acs.jpcb.0c05547. PubMed DOI

Eggimann B. L.; Siepmann J. I. Size Effects on the Solvation of Anions at the Aqueous Liquid-Vapor Interface. J. Phys. Chem. C 2008, 112, 210–218. 10.1021/jp076054d. DOI

dos Santos D. J. V. A.; Müller-Plathe F.; Weiss V. C. Consistency of Ion Adsorption and Excess Surface Tension in Molecular Dynamics Simulations of Aqueous Salt Solutions. J. Phys. Chem. C 2008, 112, 19431–19442. 10.1021/jp804811u. DOI

Horinek D.; Herz A.; Vrbka L.; Sedlmeier F.; Mamatkulov S. I.; Netz R. R. Specific Ion Adsorption at the Air/Water Interface: The Role of Hydrophobic Solvation. Chem. Phys. Lett. 2009, 479, 173–183. 10.1016/j.cplett.2009.07.077. DOI

Bresme F.; Chacón E.; Tarazona P.; Wynveen A. The Structure of Ionic Aqueous Solutions at Interfaces: An Intrinsic Structure Analysis. J. Chem. Phys. 2012, 137, 114706.10.1063/1.4753986. PubMed DOI

Liu D.; Ma G.; Levering L. M.; Allen H. C. Vibrational Spectroscopy of Aqueous Sodium Halide Solutions and Air-Liquid Interfaces: Observation of Increased Interfacial Depth. J. Phys. Chem. B 2004, 108, 2252–2260. 10.1021/jp036169r. DOI

Petersen P. B.; Johnson J. C.; Knutsen K. P.; Saykally R. J. Direct Experimental Validation of the Jones-Ray Effect. Chem. Phys. Lett. 2004, 397, 46–50. 10.1016/j.cplett.2004.08.048. DOI

Padmanabhan V.; Daillant J.; Belloni L.; Mora S.; Alba M.; Konovalov O. Specific Ion Adsorption and Short-Range Interactions at the Air Aqueous Solution Interface. Phys. Rev. Lett. 2007, 99, 08610510.1103/PhysRevLett.99.086105. PubMed DOI

Levin Y; Dos Santos A. P.; Diehl A. Ions at the Air-Water Interface: An End to a Hundred-Year-Old Mystery?. Phys. Rev. Lett. 2009, 103, 25780210.1103/PhysRevLett.103.257802. PubMed DOI

Levin Y. Polarizable Ions at Interfaces. Phys. Rev. Lett. 2009, 102, 14780310.1103/PhysRevLett.102.147803. PubMed DOI

Atkins P.; de Paula J.. Physical Chemistry; Freeman: New York, 2006.

Hofmeister F.On the Effects of Salts. Archiv f. Experiment. Pathol. U. Pharmakol. 1888, 24, 247–260 (in German), 10.1007/BF01918191. DOI

Jungwirth P.; Cremer P. S. Beyond Hofmeister. Nat. Chem. 2014, 6, 261–263. 10.1038/nchem.1899. PubMed DOI

Hantal G.; Sega M.; Horvai G.; Jedlovszky P. Contribution of Different Molecules and Moieties to the Surface Tension in Aqueous Surfactant Solutions. II: Role of the Size and Charge Sign of the Counterions. J. Phys. Chem. B 2021, 125, 9005–9018. 10.1021/acs.jpcb.1c04216. PubMed DOI

Netz R. R.; Horinek D. Progress in Modeling of Ion Effects at the Vapor/Water Interface. Annu. Rev. Phys. Chem. 2012, 63, 401–418. 10.1146/annurev-physchem-032511-143813. PubMed DOI

Lbadaoui-Darvas M.; Idrissi A.; Jedlovszky P. Computer Simulation of the Surface of Aqueous Ionic and Surfactant Solutions. J. Phys. Chem. B 2022, 126, 751–765. 10.1021/acs.jpcb.1c08553. PubMed DOI PMC

McFegan L.; Juhász Á.; Márton P.; Hórvölgyi Z.; Jedlovszky-Hajdú A.; Hantal G.; Jedlovszky P. Surface Affinity of Tetramethylammonium Iodide in Aqueous Solutions – A Combined Experimental and Computer Simulation Study. J. Phys. Chem. B 2023, 127, 5341–5352. 10.1021/acs.jpcb.3c01370. PubMed DOI PMC

Wilson M. A.; Pohorille A.; Pratt L. R. Surface Potential of the Water Liquid-Vapor Interface. J. Chem. Phys. 1988, 88, 3281–3285. 10.1063/1.453923. PubMed DOI

Kathmann S. M.; Kuo I. F. W.; Mundy C. J.; Schenter G. K. Understanding the Surface Potential of Water. J. Phys. Chem. B 2011, 115, 4369–4377. 10.1021/jp1116036. PubMed DOI

Baer M. D.; Stern A. C.; Levin Y.; Tobias D. J.; Mundy C. J. Electrochemical Surface Potential Due to Classical Point Charge Models Drives Anion Adsorption to the Air-Water Interface. J. Phys. Chem. Lett. 2012, 3, 1565–1570. 10.1021/jz300302t. PubMed DOI

Dang L. X.; Chang T. M. Molecular Dynamics Study of Water Clusters, Liquid, and Liquid–Vapor Interface of Water with Many-Body Potentials. J. Chem. Phys. 1997, 106, 8149–8159. 10.1063/1.473820. DOI

Olivieri; Goel A.; Kleibert A.; Cvetko D.; Brown M. A. Quantitative Ionization energies and Work Functions of Aqueous Solutions. Phys. Chem. Chem. Phys. 2016, 18, 29506–29515. 10.1039/C6CP05682B. PubMed DOI

Allen M. P.; Tildesley D. J.. Computer Simulation of Liquids; Clarendon: Oxford, 1987.

Joung I. S.; Cheatham T. E. III Determination of Alkali and Halide Monovalent Ion Parameters for Use in Explicitly Solvated Biomolecular Simulations. J. Phys. Chem. B 2008, 112, 9020–9041. 10.1021/jp8001614. PubMed DOI PMC

Berendsen H. J. C.; Grigera J. R.; Straatsma T. The Missing Term in Effective Pair Potentials. J. Phys. Chem. 1987, 91, 6269–6271. 10.1021/j100308a038. DOI

Kiss P. T.; Baranyai A. A Systematic Development of a Polarizable Potential of Water. J. Chem. Phys. 2013, 138, 204507.10.1063/1.4807600. PubMed DOI

Kiss P. T.; Baranyai A. A New Polarizable Force Field for Alkali and Halide Ions. J. Chem. Phys. 2014, 141, 114501.10.1063/1.4895129. PubMed DOI

Dočkal J.; Lísal M.; Moučka F. Polarizable Force Fields for Accurate Molecular Simulations of Aqueous Solutions of Electrolytes, Crystalline Salts, and Solubility: Li+, Na+, K+, Rb+, F–, Cl–, Br–, I–. J. Mol. Liq. 2022, 362, 11965910.1016/j.molliq.2022.119659. DOI

Abraham M. J.; Murtola T.; Schulz R.; Páll S.; Smith J. C.; Hess B.; Lindahl E. GROMACS: High Performance Molecular Simulations Through Multi-Level Parallelism from Laptops to Supercomputers. SoftwareX 2015, 1-2, 19–25. 10.1016/j.softx.2015.06.001. DOI

Nosé S. A Molecular Dynamics Method for Simulations in the Canonical Ensemble. Mol. Phys. 1984, 52, 255–268. 10.1080/00268978400101201. DOI

Hoover W. G. Canonical Dynamics: Equilibrium Phase-Space Distributions. Phys. Rev. A 1985, 31, 1695–1697. 10.1103/PhysRevA.31.1695. PubMed DOI

Essman U.; Perera L.; Berkowitz M. L.; Darden T.; Lee H.; Pedersen L. G. A Smooth Particle Mesh Ewald Method. J. Chem. Phys. 1995, 103, 8577–8593. 10.1063/1.470117. DOI

Int Veld P. J.; Ismail A. E.; Grest G. S. Application of Ewald Summations to Long–Range Dispersion Forces. J. Chem. Phys. 2007, 127, 144711.10.1063/1.2770730. PubMed DOI

Miyamoto S.; Kollman P. A. Settle: An analytical version of the SHAKE and RATTLE algorithm for rigid water models. J. Comput. Chem. 1992, 13, 952–962. 10.1002/jcc.540130805. DOI

Kolafa J.MACSIMUS (Macromolecule Simulation Software), release 2019-10-08 with blend V2.4a and cook V3.3k. Freely available software from URL: http://www.vscht.cz/fch/software/macsimus/ (last accessed Feb 28, 2023).

Kolafa J.; Lísal M. Time-Reversible Velocity Predictors for Verlet Integration with Velocity-Dependent Right-Hand Side. J. Chem. Theory Comput. 2011, 7, 3596–3607. 10.1021/ct200108g. PubMed DOI

Ewald P. Die Berechnung Optischer und Elektrostatischer Gitterpotentiale. Ann. Phys. 1921, 369, 253–287. 10.1002/andp.19213690304. DOI

de Leeuw S. W.; Perram J. W.; Smith E. R. Simulation of Electrostatic Systems in Periodic Boundary Conditions. I. Lattice Sums and Dielectric Constants. Proc. R. Soc. Lond. A 1980, 373, 27–56. 10.1098/rspa.1980.0135. DOI

Kolafa J.; Perram J. W. Cutoff Errors in the Ewald Summation Formulae for Point Charge Systems. Mol. Simul. 1992, 9, 351–368. 10.1080/08927029208049126. DOI

Yeh I. C.; Berkowitz M. L. Ewald Summation for Systems with Slab Geometry. J. Chem. Phys. 1999, 111, 3155–3162. 10.1063/1.479595. DOI

Kolafa J. Time-Reversible Always Stable Predictor-Corrector Method for Molecular Dynamics of Polarizable Molecules. J. Comput. Chem. 2004, 25, 335–342. 10.1002/jcc.10385. PubMed DOI

Ryckaert J. P.; Ciccotti G.; Berendsen H. J. C. Numerical Integration of the Cartesian Equations of Motion of a System With Constraints; Molecular Dynamics of n-Alkanes. J. Comput. Phys. 1977, 23, 327–341. 10.1016/0021-9991(77)90098-5. DOI

Pártay L. B.; Hantal G.; Jedlovszky P.; Vincze Á.; Horvai G. A New Method for Determining the Interfacial Molecules and Characterizing the Surface Roughness in Computer Simulations. Application to the Liquid–Vapor Interface of Water. J. Comput. Chem. 2008, 29, 945–956. 10.1002/jcc.20852. PubMed DOI

Jorge M.; Jedlovszky P.; Cordeiro M. N. D. S. A Critical Assessment of Methods for the Intrinsic Analysis of Liquid Interfaces. 1. Surface Site Distributions. J. Phys. Chem. C 2010, 114, 11169–11179. 10.1021/jp101035r. DOI

Sega M.; Hantal G.; Fábián B.; Jedlovszky P. PYTIM: A Python Package for the Interfacial Analysis of Molecular Simulations. J. Comput. Chem. 2018, 39, 2118–2125. 10.1002/jcc.25384. PubMed DOI PMC

Pickard C. J. Real-Space Pairwise Electrostatic Summation in a Uniform Neutralizing Background. Phys. Rev. Mater. 2018, 2, 01380610.1103/PhysRevMaterials.2.013806. DOI

Thompson A. P.; Aktulga H. M.; Berger R.; Bolintineanu D. S.; Brown W. M.; Crozier P.; Jint Veld S. P.; Kohlmeyer A.; Moore S. G.; Nguyen T. D.; et al. LAMMPS - a Flexible Simulation Tool for Particle-Based Materials Modeling at the Atomic, Meso, and Continuum Scales. Comput. Phys. Commun. 2022, 271, 10817110.1016/j.cpc.2021.108171. DOI

Sega M.; Fábián B.; Jedlovszky P. Nonzero Ideal Gas Contribution to the Surface Tension of Water. J. Phys. Chem. Lett. 2017, 8, 2608–2612. 10.1021/acs.jpclett.7b01024. PubMed DOI