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Models and mechanisms of Hofmeister effects in electrolyte solutions, and colloid and protein systems revisited
Full text linkSpecific effects of electrolytes have posed a challenge since the 1880's. The pioneering work was that of Franz Hofmeister who studied specific salt induced protein precipitation. These effects are the rule rather the exception and are ubiquitous in chemistry and biology. Conventional electrostatic theories (Debye–Hückel, DLVO, etc.) cannot explain such effects. Over the past decades it has been recognised that additional quantum mechanical dispersion forces with associated hydration effects acting on ions are missing from theory. In parallel Collins has proposed a phenomenological set of rules (the law of matching water affinities, LMWA) which explain and bring to order the order of ion–ion and ion–surface site interactions at a qualitative level. The two approaches appear to conflict. Although the need for inclusion of quantum dispersion forces in one form or another is not questioned, the modelling has often been misleading and inappropriate. It does not properly describe the chemical nature (kosmotropic/chaotropic or hard/soft) of the interacting species. The success of the LMWA rules lies in the fact that they do. Here we point to the way that the two apparently opposing approaches might be reconciled. Notwithstanding, there are more challenges, which deal with the effect of dissolved gas and its connection to ‘hydrophobic’ interactions, the problem of water at different temperatures and ‘water structure’ in the presence of solutes. They take us to another dimension that requires the rebuilding of theoretical foundations
Collins's rule, Hofmeister effects and ionic dispersion interactions
No full textModels based solely on electrostatics cannot explain ion specific properties of electrolyte solutions. We give calculations of dispersion interactions of ions with other ions and with water molecules. These are done via ab initio Symmetry Adapted Perturbation Theory with Density Functional Theory (DFT-SAPT). The calculations establish the substantial, specific contribution of dispersion interactions to ionic interactions. They explain several puzzling properties of electrolyte solutions: the variation in solvation energy among ions of the same size, the small repulsion of iodide from the air-water interface, and the affinity of large ions for each other in water embodied in Collins's rules
A continuum solvent model of the multipolar dispersion solvation energy
No full textThe dispersion energy is an important contribution to the total solvation energies of ions and neutral molecules. Here, we present a new continuum model calculation of these energies, based on macroscopic quantum electrodynamics. The model uses the frequency dependent multipole polanzabilities of molecules in order to accurately calculate the dispersion interaction of a solute particle with surrounding water molecules. It includes the dipole, quadrupole, and octupole moment contributions. The water is modeled via a bulk dielectric susceptibility with a spherical cavity occupied by the solute. The model invokes damping functions to account for solute-solvent wave function overlap. The assumptions made are very similar to those used in the Born model. This provides consistency and additivity of electrostatic and dispersion (quantum mechanical) interactions. The energy increases in magnitude with cation size, but decreases slightly with size for the highly polarizable anions. The higher order multipole moments are essential, making up more than 50% of the dispersion solvation energy of the fluoride ion. This method provides an accurate and simple way of calculating the notoriously problematic dispersion contribution to the solvation energy. The result establishes the importance of using accurate calculations of the dispersion energy for the modeling of solvation
Ab initio molar volumes and gaussian radii
No full textAb initio molar volumes are calculated and used to derive radii for ions and neutral molecules using a spatially diffuse model of the electron distribution with Gaussian spread. The Gaussian radii obtained can be used for computation of nonelectrostatic ion-ion dispersion forces that underlie Hofmeister specific ion effects. Equivalent hard-sphere radii are also derived, and these are in reasonable agreement with crystalline ionic radii. The Born electrostatic self-energy is derived for a Gaussian model of the electronic charge distribution. It is shown that the ionic volumes used in electrostatic calculations of strongly hydrated cosmotropic ions ought best to include the first hydration shell. Ionic volumes for weakly hydrated chaotropic metal cations should exclude electron overlap (in electrostatic calculations). Spherical radii are calculated as well as nonisotropic ellipsoidal radii for nonspherical ions, via their nonisotropic static polarizability tensors
Ion Interactions with the Air-Water Interface Using a Continuum Solvent Model
No full textExplaining and predicting the distribution of ions at the air water interface has been a central challenge of physical chemistry for nearly a century. In essence, the problem amounts to calculating the change in the solvation energy of an ion as it approaches the interface. Here, we generalize our recently developed model of ionic solvation energies to calculate this interaction. The change in the Born energy as well as the static polarization response of the ion is included by using the conductor-like screening model (COSMO), which treats the ions quantum mechanically. Approximate expressions for the dispersion repulsion, cavity attraction, and surface potential contributions are also included. This model reproduces the surface tensions of electrolyte solutions and is consistent with ab initio molecular dynamics (MD) simulation. The model provides clear physical insight into iodide's adsorption. Unlike alternative models, no parameters are deliberately adjusted to reproduce surface tensions, and all of the important contributions to the interactions are included. Solving this problem has important direct implications for atmospheric chemistry and bubble properties. It also has important indirect implications for the more complex interactions of ions with protein and mineral surfaces. These play a fundamental role in a vast number of biological and industrial processes. The model is conceptually simple and has low computational demand, which facilitates its extension to these important applications
Nonelectrostatic ionic forces between dissimilar surfaces: A mechanism for colloid separation
No full textThe interaction between two dissimilar surfaces across an electrolyte is re-examined. The focus is on effects of ion-specific dispersion forces missing from classical electrostatic double-layer theory. The pressure between two flat surfaces is derived by two alternate methods (midpoint and whole domain approaches). Significant differences emerge from expectations of classical theory. These are illustrated by model interactions across electrolytes of mica and oil-like surfaces. A novel consequence that emerges from inclusion of ionic dispersion forces is the possible separation of mixed colloidal suspensions at moderate (0.1 M) concentrations of divalent salt. Repulsion between the model oil and mica surfaces is found to be due to entropic repulsion driven by high adsorption of both counterions and co-ions at the mica surface. Co-ion adsorption is a consequence of electric field reversal ("charge reversal"), caused by attractive ionic dispersion interactions of the counterion to the mica surface. Charge reversal is also found with monovalent electrolyte but only at impracticably high concentrations
A Continuum Solvent Model of the Partial Molar Volumes and Entropies of Ionic Solvation
No full textContinuum solvent models of electrolyte solutions are extremely useful. However, before we can use them with confidence, it is important to test them by comparison with a range of experimental properties. Here, we have adapted our recently developed(1,2) simple continuum solvent model of ionic solvation free energies to calculate the solvation entropies and partial molar volumes of a group of monovalent and monatomic ions. This procedure gives good quantitative agreement for larger ions, and reproduces key qualitative features, such as the shift to positive entropies of solvation for iodide and the shift to negative partial molar volumes for small cations. Small ions require a correction to account for dielectric saturation effects, which brings them also into good agreement with experiment. We argue that this model does not require ad hoc corrections, and uses parameters that have good external physical justification. This work therefore establishes that our continuum solvent model can provide a satisfactory understanding of ionic solvation. It can thus serve as a foundation for improved models that explain and predict more complex ion specific effects
Nonelectrostatic interactions between ions with anisotropic ab initio dynamic polarisabilities
No full textIon specific effects are common in colloidal and biological systems. Bubble lifetime before coalescence depends on the electrolyte. The strength of different salts at precipating proteins leads to Hofmeister series. Theories of ion and colloid interactions based on electrostatics alone, including the Debye-Huckel theory of electrolytes or the DLVO theory of colloids, are unable to predict these ion specific effects. Rather, the theories need to be modified to account for quantum mechanically derived nonelectrostatic dispersion interactions [1].
The dynamic polarisability (i) of an ion lies at the heart of its dispersion interactions. In general it may be written as a sum over many quantum modes, each with frequency n. Approximations in the past have reduced it to a single mode with modal frequency derived from the ionisation potential (IP) of the ion [2]. We have calculated the exact dynamic polarisabilities of a wide range of ions 131 using ab initio quantum mechanics and present here comparisons against the single-mode IP approximation.
The error in calculated dispersion energies due to the single-mode IP approximation averages around 40%, and reaches as high as 86% error for halide ions. Ionic self-energies, and ion-ion and ion-surface interaction energies are calculated. Applications to activity coefficients in the bulk electrolyte are presented. Hofmeister series in the activity coefficients of alkali halides consistent with experiment are found.
We also discuss further development of the theory of dispersion interactions with the aim of obtaining a quantitatively, not merely qualitatively, predictive theory. The steps include (i) taking into account the nonspherical anisotropic features of the ions and (ii) using a nonlocal description of the solvent to include solvent spatial structure. Step (i) should be crucial in resolving the adsorption of anisotropic ions such OH to interfaces, which is currently subject of debate between theory and experiment [4,5]
A continuum model of solvation energies including electrostatic, dispersion, and cavity contributions
No full textPhysically accurate continuum solvent models that can calculate solvation energies are crucial to explain and predict the behavior of solute particles in water. Here, we present such a model applied to small spherical ions and neutral atoms. It improves upon a basic Born electrostatic model by including a standard cavity energy and adding a dispersion component, consistent with the Born electrostatic energy and using the same cavity size parameter. We show that the well-known, puzzling differences between the solvation energies of ions of the same size is attributable to the neglected dispersion contribution. This depends on dynamic polarizability as well as size. Generally, a large cancellation exists between the cavity and dispersion contributions. This explains the surprising success of the Born model. The model accurately reproduces the solvation energies of the alkali halide ions, as well as the silver(I) and copper(I) ions with an error of 12 kJ mol(-1) (+/- 3%). The solvation energy of the noble gases is also reproduced with an error of 2.6 kJ mol(-1) (+/- 30%). No arbitrary fitting parameters are needed to achieve this. This model significantly improves our understanding of ionic solvation and forms a solid basis for the investigation of other ion-specific effects using a continuum solvent model
Importance of accurate dynamic polarizabilities for the ionic dispersion interactions of alkali halides
No full textAb initio quantum mechanical calculations of the dynamic polarizability of alkali metal and halide ions are performed as a function of imaginary frequency. Electron correlation is shown to provide a significant correction to ionic polarizabilities. Ab initio ion-surface dispersion coefficients are compared, with single- and multimode London approximations. The commonly employed single-mode model with the characteristic frequency taken from the ionization potential of the ion is shown to be inadequate, underestimating dispersion forces with an average error around 40% or as high as 80% for halide ions. Decomposition of the polarizability data into five modes covers the major modes of each ion adequately (four modes for Li+). Illustrative calculations of surface potentials at the mica surface in aqueous alkali halide electrolytes are made. Charge reversal is obtained with the more polarizable cations, K+ and Rb +. The error in the single-mode ionization potential models is seen as a strong shift in the surface potential from negative toward positive values
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