Surface Affinity of Alkali and Halide Ions in Their Aqueous Solution: Insight from Intrinsic Density Analysis

Single Particle Dynamics at the Free Surface of Imidazolium-Based Ionic Liquids

In this work, we carry out a systematic computer simulation investigation of the single particle dynamics at the free surface of imidazolium-based room temperature ionic liquids by applying intrinsic surface analysis. Besides assessing the effect of the potential model and temperature, we focus in particular on the effect of changing the anion type, and, hence, their shape and size. Further, we also address the role of the length of the cation alkyl chains, known to protrude into the vapor phase, on the surface dynamics of the ions. We observe that the surface dynamics of ionic liquids, being dominated by strong electrostatic interactions, is about 2 orders of magnitude slower than that for common molecular liquids. Furthermore, the free energy driving force for exposing apolar chains to the vapor phase "pins" the cations at the surface layer for much longer than anions, allowing them to perform noticeable lateral diffusion at the liquid surface during their stay there. On the other hand, anions, accumulated in the second layer beneath the liquid surface, stay considerably longer here than in the surface layer. The ratio of the mean surface residence time of the cations and anions depends on the relative size of the two ions: larger size asymmetry typically corresponds to larger values of this ratio. We also find, in a clear contrast with the bulk liquid phase behavior, that anions typically diffuse faster at the liquid surface than cations. Finally, our results show that the surface dynamics of the ions is largely determined by the apolar layer of the cation alkyl chains at the liquid surface, as in the absence of such a layer, cations and anions are found to behave similarly with respect to their single particle dynamics.

Spatial organization of the ions at the free surface of imidazolium-based ionic liquids

HYPOTHESIS

Experimental information on the molecular scale structure of ionic liquid interfaces is controversial, giving rise to two competing scenarios, namely the double layer-like and "chessboard"-like structures. This issue can be resolved by computer simulation methods, at least for the underlying molecular model. Systematically changing the anion type can elucidate the relative roles of electrostatic interactions, hydrophobic (or, strictly speaking, apolar) effects and steric restrictions on the interfacial properties.

SIMULATIONS

Molecular dynamics simulation is combined with intrinsic analysis methods both at the molecular and atomic levels, supplemented by Voronoi analysis of self-association.

FINDINGS

We see no evidence for the existence of a double-layer-type arrangement of the ions, or for their self-association at the surface of the liquid. Instead, our results show that cation chains associate into apolar domains that protrude into the vapour phase, while charged groups form domains that are embedded in this apolar environment at the surface. However, the apolar chains largely obscure the cation groups, to which they are bound, while the smaller and more mobile anions can more easily access the free surface, leading to a somewhat counterintuitive net excess of negative charge at the interface. Importantly, this excess charge could only be identified by applying intrinsic analysis.

Relationship between Molecular Structure and Surface Activity of Ionic Surfactants

A new model is developed to quantify the surface tension of ionic surfactants from surface affinity and ionization equilibrium. The model successfully predicts two important molecular structure-surface activity factors: the length of single-branch homologues and the nature of counterions. The modeling results also clarify the underlying mechanisms of the two processes. Changing the counterion only affects the ionization, not the affinity. On the other hand, increasing carbon length dramatically increases the affinity while having a small effect on ionization. The modeling framework consistently resolves structure-activity observations, some of which have been reported since the 19th century. The model can be extended for surfactants with more than one ionic state and surfactant/electrolyte mixtures.

Theoretically grounded approaches to account for polarization effects in fixed-charge force fields

Non-polarizable, or fixed-charge, force fields are the workhorses of most molecular simulation studies. They attempt to describe the potential energy surface (PES) of the system by including polarization effects in an implicit way. This has historically been done in a rather empirical and ad hoc manner. Recent theoretical treatments of polarization, however, offer promise for getting the most out of fixed-charge force fields by judicious choice of parameters (most significantly the net charge or dipole moment of the model) and application of post facto polarization corrections. This Perspective describes these polarization theories, namely the "halfway-charge" theory and the molecular dynamics in electronic continuum theory, and shows that they lead to qualitatively (and often, quantitatively) similar predictions. Moreover, they can be reconciled into a unified approach to construct a force field development workflow that can yield non-polarizable models with charge/dipole values that provide an optimal description of the PES. Several applications of this approach are reviewed, and avenues for future research are proposed.

Does the Sign of Charge Affect the Surface Affinity of Simple Ions?

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.

Surface Affinity of Tetramethylammonium Iodide in Aqueous Solutions: A Combined Experimental and Computer Simulation Study

The surface affinity of tetramethylammonium iodide (TMAI) in aqueous solutions is investigated by surface tension measurements and molecular dynamics computer simulations. Experiments, performed in the entire composition range of solubility using the pendant drop method with two different setups, clearly reveal that TMAI is a weakly capillary active salt. Computer simulations performed with the AMBER force field reproduce the experimental data very well, while two other major force fields (i.e., CHARMM and OPLS) can still reproduce the experimental trend qualitatively; however, even qualitative reproduction of the experimental trend requires scaling down the ion charges according to the Leontyev-Stuchebrukhov correction. On the other hand, the GROMOS force field fails in reproducing the experimentally confirmed capillary activity of TMAI. Molecular dynamics simulation results show that, among the two ions, iodide has a clearly larger surface affinity than tetramethylammonium (TMA+). Further, the adsorption of the I- anions is strictly limited to the first molecular layer beneath the liquid-vapor interface, which is followed by several layers of their depletion. On the other hand, the net negative charge of the surface layer, caused by the excess amount of I- with respect to TMA+, is compensated by a diffuse layer of adsorbed TMA+ cations, extending to or beyond the fourth molecular layer beneath the liquid surface.

Dynamics of Aqueous Electrolyte Solutions: Challenges for Simulations

This Perspective article focuses on recent simulation work on the dynamics of aqueous electrolytes. It is well-established that full-charge, nonpolarizable models for water and ions generally predict solution dynamics that are too slow in comparison to experiments. Models with reduced (scaled) charges do better for solution diffusivities and viscosities but encounter issues describing other dynamic phenomena such as nucleation rates of crystals from solution. Polarizable models show promise, especially when appropriately parametrized, but may still miss important physical effects such as charge transfer. First-principles calculations are starting to emerge for these properties that are in principle able to capture polarization, charge transfer, and chemical transformations in solution. While direct ab initio simulations are still too slow for simulations of large systems over long time scales, machine-learning models trained on appropriate first-principles data show significant promise for accurate and transferable modeling of electrolyte solution dynamics.

Computer Simulation of the Surface of Aqueous Ionic and Surfactant Solutions

The surface of aqueous solutions of simple salts was not the main focus of scientific attention for a long while. Considerable interest in studying such systems has only emerged in the past two decades, following the pioneering finding that large halide ions, such as I^-, exhibit considerable surface affinity. Since then, a number of issues have been clarified; however, there are still several unresolved points (e.g., the effect of various salts on lateral water diffusion at the surface) in this respect. Computer simulation studies of the field have largely benefited from the appearance of intrinsic surface analysis methods, by which the particles staying right at the boundary of the two phases can be unambiguously identified. Considering complex ions instead of simple ones opens a number of interesting questions, both from the theoretical point of view and from that of the applications. Besides reviewing the state-of-the-art of intrinsic surface analysis methods as well as the most important advances and open questions concerning the surface of simple ionic solutions, we focus on two such systems in this Perspective, namely, the surface of aqueous mixtures of room temperature ionic liquids and that of ionic surfactants. In the case of the former systems, for which computer simulation studies have still scarcely been reported, we summarize the theoretical advances that could trigger such investigations, which might well be of importance also from the point of view of industrial applications. Computer simulation methods are, on the other hand, widely used in studies of the surface of surfactant solutions. Here we review the most important theoretical advances and issues to be addressed and discuss two areas of applications, namely, the inclusion of information gathered from such simulations in large scale atmospheric models and the better understanding of the airborne transmission of viruses, such as SARS-CoV-2.

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

Understanding the role of the counterion species in surfactant solutions is a complicated task, made harder by the fact that, experimentally, it is not possible to vary independently bulk and surface quantities. Here, we perform molecular dynamics simulations at constant surface coverage of the liquid/vapor interface of lithium, sodium, potassium, rubidium, and cesium dodecyl sulfate aqueous solutions. We investigate the effect of counterion type and charge sign on the surface tension of the solution, analyzing the contribution of different species and moieties to the lateral pressure profile. The observed trends are qualitatively compatible with the Hofmeister series, with the notable exception of sodium. We point out a possible shortcoming of what is at the moment, in our experience, the most realistic nonpolarizable force field (CHARMM36) that includes the parametrization for the whole series of alkali counterions. In the artificial system where the counterion and surfactant charges are inverted in sign, the counterions become considerably harder. This charge inversion changes considerably the surface tension contributions of the counterions, surfactant headgroups, and water molecules, stressing the key role of the hardness of the counterions in this respect. However, the hydration free energy gain of the counterions, occurring upon charge inversion, is compensated by the concomitant free energy loss of the headgroups and water molecules, leading to a negligible change in the surface tension of the entire system.

Single-Particle Dynamics at the Intrinsic Surface of Aqueous Alkali Halide Solutions

The distribution of ions in the proximity of the liquid-vapor interface of their aqueous solution has been the subject of an intense debate during the last decade. The effects of ionic polarizability have been one of its salient aspects. Much less has been said about the corresponding dynamical properties, which are substantially unexplored. Here, we investigate the single-particle dynamics at the liquid-vapor interface of several alkali halide solutions, using molecular dynamics simulations with polarizable and nonpolarizable force fields and intrinsic surface analysis. We analyze the diffusion coefficient, residence time, and velocity autocorrelation function of water and ions and investigate how these properties depend on the molecular layer where they reside. While anions are found in the first molecular layer for relatively long times, cations are only making quick excursions into it, thanks to thermal fluctuations. The in-layer residence time of ions and their molar fraction in the layer turned out to be linearly dependent on each other. We interpret this unexpected result using a simple two-state model. In addition, we found that, unlike water and other neat molecular liquids that show a different diffusion mechanism at the surface than in the bulk of their liquid phase, ions do not enjoy enhanced mobility in the surface layer of their aqueous solution. This result indicates that ions in the surface layer are shielded by their nearest water neighbors from being exposed to the vapor phase as much as possible. Such positions are available for the ions at the negatively curved troughs of the molecularly rugged liquid surface.