Adsorption of solutes at liquid–vapor interfaces: insights from lattice gas models

Self Cite

The adsorption of ions to aqueous interfaces is a phenomenon that profoundly influences vital processes in many areas of science, including biology, atmospheric chemistry, electrical energy storage, and water process engineering. Although classical electrostatics theory predicts that ions are repelled from water/hydrophobe (e.g., air/water) interfaces, both computer simulations and experiments have shown that chaotropic ions actually exhibit enhanced concentrations at the air/water interface. Although mechanistic pictures have been developed to explain this counterintuitive observation, their general applicability, particularly in the presence of material substrates, remains unclear. Here we investigate ion adsorption to the model interface formed by water and graphene. Deep UV second harmonic generation measurements of the SCN − ion, a prototypical chaotrope, determined a free energy of adsorption within error of that for air/water. Unlike for the air/water interface, wherein repartitioning of the solvent energy drives ion adsorption, our computer simulations reveal that direct ion/graphene interactions dominate the favorable enthalpy change. Moreover, the graphene sheets dampen capillary waves such that rotational anisotropy of the solute, if present, is the dominant entropy contribution, in contrast to the air/water interface.specific ion effects | graphene | SHG spectroscopy | molecular dynamics | adsorption O ver a decade ago, detailed simulations predicted that certain simple ions would exhibit enhanced concentrations at the air/water interface (1), reinvigorating a century-old debate primarily addressing the origin of the famous Hofmeister effects in protein chemistry (2). Experiments subsequently verified these predictions (3, 4). Surface enhancement of simple ions has since been widely studied [e.g., see the review by Jungwirth and Tobias (5)] and attributed to various properties of the ions including size, polarizability, dispersion forces, hydration free energy (6-8), and the degree of interfacial roughness (9-11). Previous temperaturedependent deep UV (DUV) second harmonic generation (SHG) experiments from the Saykally group have determined the enthalpic and entropic contributions to the adsorption of the prototypical pseudohalide, the thiocyanate (SCN − ) ion, showing that a negative enthalpy change drives the ion adsorption, whereas a negative entropy change impedes it (9). Using molecular simulations performed in the same study, the following underlying mechanism was proposed: first, when the ion moves from the bulk to the interface, weakly interacting water molecules are displaced from both the surface and the ion solvent shell into the bulk solution, where they form stronger water-water bonds, leading to the negative enthalpy change. Second, the presence of an ion at the interface dampens its capillary wave fluctuations, leading to the negative entropy change. Although a consensus has yet to be reached regarding the complete theory of interfacial ion adsorption, these collective, detailed studie...

Section: Discussionmentioning

confidence: 99%

Mechanism of ion adsorption to aqueous interfaces: Graphene/water vs. air/water

McCaffrey

Nguyen

Cox

et al. 2017

Self Cite

“…As examples of its utility, we compute the free energy profile for a solute crossing the air-water interface, as well as the thermodynamic cost of evacuating the space between extended nanoscale surfaces. These calculations suggest that a highly reduced model for aqueous solvation can enable efficient multiscale modeling of spatial organization driven by hydrophobic and interfacial forces.hydrophobic effect | lattice model | water Hydrophobic forces play a crucial role in biological selfassembly, protein folding, ion channel gating, and lipid membrane dynamics [1,2,3,4,5,6,7,8,9]. The origin and strength of these forces are well understood at extreme length scales, based on the recognition that accommodating an ideal volume-excluding hydrophobe in water carries the same thermodynamic cost as evacuating solvent from the corresponding volume.…”

mentioning

confidence: 99%

“…For the parameterization we have described ( /T = 1.35, δ = 1.84Å), which ensures T > TR, interfaces of this lattice gas exhibit a spectrum of capillary modes comparable to that of a natural liquid-vapor interface. Fluctuations and response of these capillary modes may contribute significantly to solvation structure and thermodynamics [4,5,6].To assess the importance of capillary fluctuations, we examine a different parameterization of Eq.[ 1 ] ( /T = 6, δ = 4Å), for which T < TR. This lattice gas too supports interfaces with the correct surface tension.…”

mentioning

confidence: 99%

“…To make this connection explicit, we have computed the excess chemical potential µex(z) of a spherical hydrophobe as a function of its perpendicular displacement z from the air-water interface (with z → ∞ indicating bulk vapor and z → −∞ bulk liquid). Such interfacial free energy profiles have been determined from molecular simulations for a variety of solutes [4,5,21,22], including small ions, which can exhibit a surprising tendency to adsorb to the liquid-vapor phase boundary [5]. While charged solutes are distinct in important ways from hydrophobes, the cost of cre-ating solute-sized cavities has been implicated as a key factor in their surface affinity [4,5].…”

mentioning

confidence: 99%

See 1 more Smart Citation

Necessity of capillary modes in a minimal model of nanoscale hydrophobic solvation

Vaikuntanathan

Rotskoff

Hudson

et al. 2016

Self Cite

Modern theories of the hydrophobic effect highlight its dependence on length scale, emphasizing the importance of interfaces in the vicinity of sizable hydrophobes. We recently showed that a faithful treatment of such nanoscale interfaces requires careful attention to the statistics of capillary waves, with significant quantitative implications for the calculation of solvation thermodynamics. Here, we show that a coarse-grained lattice model like that of Chandler [Chandler D (2005) Nature 437(7059):640-647], when informed by this understanding, can capture a broad range of hydrophobic behaviors with striking accuracy. Specifically, we calculate probability distributions for microscopic density fluctuations that agree very well with results of atomistic simulations, even many SDs from the mean and even for probe volumes in highly heterogeneous environments. This accuracy is achieved without adjustment of free parameters, because the model is fully specified by well-known properties of liquid water. As examples of its utility, we compute the free-energy profile for a solute crossing the air-water interface, as well as the thermodynamic cost of evacuating the space between extended nanoscale surfaces. These calculations suggest that a highly reduced model for aqueous solvation can enable efficient multiscale modeling of spatial organization driven by hydrophobic and interfacial forces.hydrophobic effect | self-assembly | coarse-grained model | lattice models H ydrophobic forces play a crucial role in biological self-assembly, protein folding, ion channel gating, and lipid membrane dynamics (1-9). The origin and strength of these forces are well understood at extreme length scales, based on the recognition that accommodating an ideal volume-excluding hydrophobe in water carries the same thermodynamic cost as evacuating solvent from the corresponding volume. On the scale of a small molecule like methane, density fluctuations that enable such evacuation are Gaussian-distributed to a very good approximation, even far from the mean (10). Linear response theories, such as Pratt-Chandler theory, can thus be quite accurate for assessing solvation of individual small hydrophobic species.Solvation at much larger scales is, by contrast, dominated by water's proximity to liquid-vapor coexistence. Hydrophobic forces involving extended substrates are shaped by the physics of interfaces and are quantified by macroscopic parameters like surface tension. In between these extremes, a rich variety of hydrophobic effects results from the combined importance of nearby phase coexistence and details of intermolecular structure. Capturing this interplay, for instance, near a biological macromolecule, presents a significant challenge for theory.Lum-Chandler-Weeks (LCW) theory represents the modern understanding of hydrophobic solvation, providing a conceptual and mathematical framework to couple interfacial forces with the short-wavelength density fluctuations that determine solvation of small molecules (10, 11). The theory's physical pers...

“…The net energetic contribution can be favorable or unfavorable, depending on the differences between ion-water, ion-ion, and water-water interactions in bulk and at the interface. The entropic contribution is typically unfavorable due to the restriction of water molecules in the hydration shell of the ion and the corresponding pinning of capillary fluctuations at the interface (7,13,14). Solvent structure and fluctuations at the interface are also known to play an important role in ion dissociation pathways in the transport of ions across liquid-liquid interfaces (15).…”

mentioning

confidence: 99%

Water-mediated ion–ion interactions are enhanced at the water vapor–liquid interface

Venkateshwaran

Vembanur

Garde

2014

108

There is overwhelming evidence that ions are present near the vapor-liquid interface of aqueous salt solutions. Charged groups can also be driven to interfaces by attaching them to hydrophobic moieties. Despite their importance in many self-assembly phenomena, how ion-ion interactions are affected by interfaces is not understood. We use molecular simulations to show that the effective forces between small ions change character dramatically near the water vapor-liquid interface. Specifically, the water-mediated attraction between oppositely charged ions is enhanced relative to that in bulk water. Further, the repulsion between like-charged ions is weaker than that expected from a continuum dielectric description and can even become attractive as the ions are drawn to the vapor side. We show that thermodynamics of ion association are governed by a delicate balance of ion hydration, interfacial tension, and restriction of capillary fluctuations at the interface, leading to nonintuitive phenomena, such as watermediated like charge attraction. "Sticky" electrostatic interactions may have important consequences on biomolecular structure, assembly, and aggregation at soft liquid interfaces. We demonstrate this by studying an interfacially active model peptide that changes its structure from α-helical to a hairpin-turn-like one in response to charging of its ends. T raditional models of an air-water interface of a salt solution present a picture in which salt ions are excluded from the interfacial region (1). However, recent simulations and experiments have shown that certain chaotropic ions, such as iodide, azide, and thiocyanate, can adsorb to the air-water interface (2-7). Even when ions are depleted from the interface, the extent of depletion is limited to a nanometer length scale (8). Charged species can also be driven to an air-water interface by covalently attaching them to hydrophobic moieties, as in ionic surfactants, or interfacially active proteins (9, 10). Thermodynamics of ion adsorption to interfaces are complex, determined by a balance of energetic and entropic contributions (7,11,12). The net energetic contribution can be favorable or unfavorable, depending on the differences between ion-water, ion-ion, and water-water interactions in bulk and at the interface. The entropic contribution is typically unfavorable due to the restriction of water molecules in the hydration shell of the ion and the corresponding pinning of capillary fluctuations at the interface (7,13,14). Solvent structure and fluctuations at the interface are also known to play an important role in ion dissociation pathways in the transport of ions across liquid-liquid interfaces (15). How these factors govern the effective ion-ion interactions near aqueous interfaces and, in turn, influence interfacial self-assembly and aggregation is, however, not understood.We present results from extensive molecular simulations of ion hydration and ion-ion interactions near a water vapor-liquid interface. Our principal results are that solvent-mediated ...