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The origin of excess charge at the water/hydrophobic interfaces

Final Report Summary - EXCHARGEHYD (The origin of excess charge at the water/hydrophobic interfaces)

a summary description of the project objectives:
The aim of this project is the theoretical investigation of the salt adsorption process at the interface of water/hydrophobic substrates taking into account impurities, dissolved gases using thermodynamically consistent force fields for ions by molecular dynamics methods.

Despite on intensive studies directed on solving the problem of origin of charge on the interface of water/hydrophobic substrate remains open and future studies on the origin of charge in the interface of water/hydrophobic substrates should be focused hydroxyl ions whose are the most suitable candidates for the explanation of source of the excess charge at the interfaces.
Moreover, many important chemical reactions in aqueous solution are pH dependent, because they involve gaining or losing a proton. For example, the pH dependence of surface charge densities, which control the behavior of colloids in water – and therefore most biological interactions– is particularly strong. In biology, protons have a special importance because proton gradients, which serve as intermediate energy storage in mitochondria, are being considered as the origin of complex life. There are still many remaining questions surrounding the properties of H3O+ and OH− in water, such as the molecular origin of their high mobility and their surface activity. The success of the simple rigid water model SPC/E, that has neither molecular nor atomic polarizability, at reproducing water properties is remarkable. Most notably, the water’s structure factor and dielectric response function are captured accurately based on careful optimization of the Lennard-Jones parameters and partial charges only. Also for monovalent and divalent ions, optimization of the Lennard-Jones parameters and combination rules suffices to reproduce solvation free energy, solvation enthalpy and activity coefficients. Nevertheless, a similar force field for the water ions has been lacking up to now. Traditionally, ion parameters were chosen in a somewhat unsystematic way to reproduce the solvation free energy and to give the correct ion size when compared with scattering results. Which experimental observable one chooses to reproduce should in principle depend on the context within which the ionic force field is going to be used. For this aim we use the solvation free energy in conjunction with the solvation entropy to construct thermodynamically sound force fields for the OH-, H3O+ and sulphate ion for the simulation of ion-specific effects in aqueous environment. To optimize the force field of H3O+ and OH− we performed a thermodynamic integration for different values of the Lennard-Jones parameters of the oxygen and the partial charge on the hydrogen atom δ and optimize the force field with respect to the solvation free energy and activity coefficient.

- a description of the main results achieved so far,
During the reporting period, we developed force field parameters of the divalent anion SO4-2, hydroxyl ion OH- and hydronium ion H3O+. We performed Molecular Dynamics simulations with explicit water, using the simple point charge-extended (SPC/E) water model. The scheme we propose for the derivation of the ionic force fields is based on a simultaneous optimization of single-ion and ion-pair properties.

It has been shown that nonpolarizable models of sulfate ion significantly overestimate the degree of association in the aqueous solution of Na2SO4 and MgSO4. However, for polarizable models, only a modest degree of association is observed, which is consistent with the activity coefficients of real sodium sulfate solutions. Despite of many arguments in favor of polarizable force fields, the necessity of including polarizability for the accurate prediction of biomolecular properties, such as ligand-binding affinities, is still in debated. At the moment, non-polarizable force fields are still more widely used and quite successful in predicting binding affinities. The idea is that multi-body effects can be included effectively via the optimization of Lennard-Jones (LJ) parameters based on liquid state properties. It turns out that the effective ion size is crucial for accurately capturing ion specific effects and must be correctly represented by the 6- 12 LJ potential, which is commonly used for the modeling of ions, together with combination (or mixing) rules to describe interactions between different atoms or ions.
The force field for sulfate ion we optimized to reproduce the ion solvation free energy and activity of sulfate ion in conjuction with Na+ and Mg2+ cations.
We show that for Na2SO4 the LJ pair combination exists that simultaneously matches experimental solvation free energies of single ions and activity data of the salt. This is not the case for MgSO4. Here we introduce a scaling factor in the cation anion LJ interaction that quantifies deviations from the standard combination rule for the effective LJ diameter and allows to reproduce the activity coefficient without modifying the single Mg2+ and sulfate ion solvation free energy.

• the expected final results and their potential impact and use (including the socio-economic impact and the wider societal implications of the project so far).
At present time molecular dynamics simulations are not only important from an academic point of view but they also find use in pharmaceutical industry in drug design. To have accurate force-fields for biologically relevant ions will thus lead to higher reliability of the simulated results. The optimized parameters are used within the project to study the adsorption of ions at non-polar surfaces and the effect of ions on stability and solubility of proteins. Molecular dynamics simulations in this area are challenging owing to the complex nature and inability of individual models to reproduce a wide variety of parameters for ions.

2. Please provide an overview of the project objectives for the reporting period in question, as included in Annex I of the Grant Agreement. These objectives are required so that this report is a stand-alone document.
Please include a summary of the recommendations from the previous reviews (if any) and indicate how these have been taken into account.

The objective of the proposal is to investigate salt adsorption phenomena at the water-hydrophobic interfaces by molecular dynamics methods. Several questions are addressed. The first one is the optimization of interaction parameters of divalent anion SO4-2, hydroxyl ion OH- and hydronium ion H3O+ in conjunction of SPC/E water model. We propose a new scheme for the derivation of the ionic force fields is based on a simultaneous optimization of single-ion and ion-pair properties. The solvation free energy and the effective radius of the ions are the single ion properties used in our approach. As a probe for ion pair properties we compute the activity derivatives of salts in aqueous solutions. In this respect, the obtained optimized force fields for ions are capable to describe the thermodynamic properties of single ions in water, as well as the electrolyte activity derivatives at finite ion concentrations in agreement with experimental data. The results of this line will be submitted to J.Chem.Phys. in February 2015.