Surface tension of electrolytes: hydrophilic and hydrophobic ions near an interface

We calculate the ion distributions around an interface in fluid mixtures of highly polar and less polar fluids (water and oil) for two and three ion species. We take into account the solvation and image interactions between ions and solvent. We show that hydrophilic and hydrophobic ions tend to undergo a microphase separation at an interface, giving rise to an enlarged electric double layer. We also derive a general expression for the surface tension of electrolyte systems, which contains a negative electrostatic contribution proportional to the square root of the bulk salt density. The amplitude of this square-root term is small for hydrophilic ion pairs but is much increased for hydrophilic and hydrophobic ion pairs. For three ion species, including hydrophilic and hydrophobic ions, we calculate the ion distributions to explain those obtained by x-ray reflectivity measurements.     

Effect of ions on the hydrophobic interaction between two plates

We use molecular dynamics simulations to investigate the solvent mediated attraction and drying between two nanoscale hydrophobic surfaces in aqueous salt solutions. We study these effects as a function of the ionic charge density, that is, the ionic charge per unit ionic volume, while keeping the ionic diameter fixed. The attraction is expressed by a negative change in the free energy as the plates are brought together, with enthalpy and entropy changes that both promote aggregation. We find a strong correlation between the strength of the hydrophobic interaction and the degree of preferential binding/exclusion of the ions relative to the surfaces. The results show that amplification of the hydrophobic interaction, a phenomenon analogous to salting-out, is a purely entropic effect and is induced by high-charge-density ions that exhibit preferential exclusion. In contrast, a reduction of the hydrophobic interaction, analogous to salting-in, is induced by low-charge-density ions that exhibit preferential binding, the effect being either entropic or enthalpic. Our findings are relevant to phenomena long studied in solution chemistry, as we demonstrate the significant, yet subtle, effects of electrolytes on hydrophobic aggregation and collapse.     

The adsorption of alkylpyridinium chlorides and their effect on the interfacial behavior of quartz

This research was directed at understanding cationic surfactant adsorption phenomena on wet-ground natural quartz, mainly with dodecylpyridinium chloride as the model surfactant. How these surfactant ions adsorb at the interface was delineated through measurements of adsorption isotherms, zeta potentials, suspension stability, contact angles, induction times, and flotation response. Hydrocarbon chain association of adsorbed surfactant ions (or self-association) leads to four distinct adsorption regions as the concentration of surfactant is increased in solution. The same four regions manifest themselves in the behavior of all of the interfacial processes studied. At low concentrations, adsorption is controlled primarily by electrostatic interactions, but when the adsorbed surfactant ions begin to associate into hemimicelles at the surface, hydrophobic chain interactions control the adsorption process. The results of experiments with alkylpyridinium chlorides of 12, 14 and 16 carbon atoms can be normalized in terms of their CMCs, which clearly show that surface aggregation phenomena are driven by the same hydrophobic interactions that lead to micelle formation in bulk solution.     

Interfacial water on hydrophobic surfaces recognized by ions and molecules

Recent spectrophotometric and molecular dynamics simulation studies have shown that the physicochemical properties and structures of water in the vicinity of hydrophobic surfaces differ from those of the bulk water. However, the interfacial water acting as a separation medium on hydrophobic surfaces has never been detected and quantified experimentally. In this study, we show that small inorganic ions and organic molecules differentiate the interfacial water formed on the surfaces of octadecyl-bonded (C(18)) silica particles from the bulk water and the chemical separation of these solutes in aqueous media with hydrophobic materials can be interpreted with a consistent mechanism, partition between the bulk water phase and the interfacial water formed on the hydrophobic surface. Thermal transition behaviour of the interfacial water incorporated in the nanopores of the C(18) silica materials and the solubility parameter of the water calculated from the distribution coefficients of organic compounds have indicated that the interfacial water may have a structure of disrupted hydrogen bonding. The thickness of the interfacial water or the limit of distance from the hydrophobic surface at which molecules and ions can sense the surface was estimated to be 1.25 ± 0.13 nm from the volume of the interfacial water obtained by a liquid chromatographic method and the surface area, suggesting that the hydrophobic effect may extend beyond the first solvation shell of water molecules directly surrounding the surfaces.     

Molecular dynamics simulation of aqueous NaF and NaI solutions near a hydrophobic surface

We present results from molecular dynamics simulation of aqueous solutions of alkali halide salts (NaI and NaF) at the interface with hydrophobic objects. The primary objective of this study is to investigate the structural properties of the salt solutions at the hydrophobic surface. An alkane crystal has been taken as the parent model for a hydrophobic surface. A hexagonal hole was created on it, which was half a nm deep and 2.5 nm wide. The density distributions of different species (water, anions, and cations) are studied as a function of distance from the surface. While iodide prefers the interface, the fluoride ions stay inside the bulk water region. The higher concentration of iodide ions at the interface drags sodium counterions to the interface. It also decreases the water density at the interface because of steric effects of the iodide ions. The number of contacts between the surface carbons and water decreases in the case of NaI solutions but is unchanged for NaF solutions. The orientation of the water-ion and the water-water hydrogen bond vector orientations near the interface is discussed in detail.     

十二烷基苯磺酸钠在SiO2表面聚集的分子动力学模拟

采用分子动力学方法研究了阴离子表面活性剂十二烷基苯磺酸钠(SDBS)在无定形SiO2固体表面的吸附. 设置不同的水层厚度, 观察固液界面和气液界面吸附的差异. 模拟发现表面活性剂分子能够在短时间内吸附到SiO2表面, 受碳链和固体表面之间相互作用的影响形成表面活性剂分子层, 并依据吸附量的大小形成不同的聚集结构; 在水层足够厚的情况下, 由于有较多的表面活性剂分子吸附在固体表面,从而形成带有疏水核心的半胶束结构; 计算得到的成对势表明极性头与钠离子或水分子之间的结合或解离与二者之间的能垒有关, 解离能垒远大于结合能垒, 引起更多Na+聚集在极性头周围而只有少数Na+存在于溶液中; 无论气液还是固液界面, 极性头均伸向水相, 与水分子形成不同类型的氢键. 模拟表明, 分子动力学方法可以作为实验的一种补充, 为实验提供必要的微观结构信息.     

Interaction of monovalent ions with hydrophobic and hydrophilic colloids: charge inversion and ionic specificity

Here we study experimentally and by simulations the interaction of monovalent organic and inorganic anions with hydrophobic and hydrophilic colloids. In the case of hydrophobic colloids, our experiments show that charge inversion is induced by chaotropic inorganic monovalent ions but it is not induced by kosmotropic inorganic anions. For organic anions, giant charge inversion is observed at very low electrolyte concentrations. In addition, charge inversion disappears for both organic and inorganic ions when turning to hydrophilic colloids. These results provide an experimental evidence for the hydrophobic effect as the driving force for both ion specific effects and charge inversion. In the case of organic anions, our molecular dynamics (MD) simulations with full atomic detail show explicitly how the large adsorption free energies found for hydrophobic colloids are transformed into large repulsive barriers for hydrophilic colloids. Simulations confirm that solvation free energy (and hence the hydrophobic effect) is responsible for the build up of a Stern layer of adsorbed ions and charge inversion in hydrophobic colloids and it is also the mechanism preventing charge inversion in hydrophilic colloids. Overall, our experimental and simulation results suggest that the interaction of monovalent ions with interfaces is dominated by solvation thermodynamics, that is, the chaotropic/kosmotropic character of ions and the hydrophobic/hydrophilic character of surfaces.     

Charge-tranfer reactions of oxygen ions and titanium ions at titanium oxide films

The partial current densities for the transfer of titanium(IV) and oxygen ions, and of electrons at the interface between the electrolyte and the titanium(IV) oxide layer on titanium were measured as functions of total current density and pH value. It is shown theoretically, how conclusions regarding the mechanisms of the transfer reactions of both ions can be drawn from various relations between the ionic partial current densities and their dependence on solution composition, even if the electric potential difference at the oxide interface cannot be measured. Mechanisms for the transfer reactions of titanium(IV) and oxygen ions are derived from the experiments.     

Structure formation due to antagonistic salts

Antagonistic salts are composed of hydrophilic and hydrophobic ions. In a mixture solvent (water–oil) such ion pairs are preferentially attracted to water or oil, giving rise to a coupling between the charge density and the composition. First, they form a large electric double layer at a water–oil interface, reducing the surface tension and producing mesophases. Here, the cations and anions are loosely bound by the Coulomb attraction across the interface on the scale of the Debye screening length. Second, on solid surfaces, hydrophilic (hydrophobic) ions are trapped in a water-rich (oil-rich) adsorption layer, while those of the other species are expelled from the layer. This yields a solvation mechanism of local charge separation near a solid. In particular, near the solvent criticality, disturbances around solid surfaces can become oscillatory in space. In mesophases, we calculate periodic structures, which resemble those in experiments.     

Ion hydration near air/water interfaces and the structure of liquid surfaces

Negative adsorption of ions, commonly observed at air-water interfaces, is examined in terms of models of restricted polarization of the solvent by ions at the interface and the structure of the liquid interface. The Born and other models of ionic hydration are applied to evaluate the self-energy of the ion arising in the region of solvent near its interface and in the vacuum or vapour beyond. The adsorption energy of an ion varies substantially with distance from the liquid interface so that a distribution of ions arises as a function of distance from the interface. Integration of this distribution gives an expression, and results, for the ionic surface excess. The diffuse-layer potential, which an unequal distribution of cations and anions give rise to, gives a contribution to the surface potential of the electrolyte solution at finite concentrations.Structural aspects of the liquid interface at which ions are negatively adsorbed are discussed in terms of Stefan's ratio and the superficial excess entropies of various liquid surfaces. These entropies are related to the cohesive energy densities of the bulk liquids. Ion solvent-structure co-sphere interactions with structured interfaces will lead to specificity of negative adsorption of ions.     

Transmetalation reaction between hydrophobic silver nanoparticles and aqueous chloroaurate ions at the air-water interface

The transmetalation reaction between a sacrificial nanoparticle and more noble metal ions in solution has emerged as a novel method for creating unique hollow and bimetallic nanostructures. In this report, we investigate the possibility of carrying out the transmetalation reaction between hydrophobic silver nanoparticles assembled and constrained at the air-water interface and subphase gold ions. We observe that facile reduction of the subphase gold ions by the sacrificial silver nanoparticles occurs resulting in the formation of elongated gold nanostructures that appear to cross-link the sacrificial silver particles. This transmetalation reaction may be modulated by the insertion of an electrostatic barrier in the form of an ionizable lipid monolayer between the silver nanoparticles and the aqueous gold ions that impacts the gold nanoparticle assembly. Transmetalation reactions between nanoparticles constrained into a close-packed structure and appropriate metal ions could lead to a new strategy for metallic cross-linking of nanoparticles and generation of coatings with promising optoelectonic behavior.     

Capillary electrophoretic determination of self-dimerization constants of aromatic carboxylate and sulfonate ions.

Self-dimerizations of twenty three aromatic carboxylate and sulfonate ions from their electrophoretic mobilities in aqueous solution were estimated by capillary zone electrophoresis (CZE). The magnitudes of the self-dimerizations ascribed to pi-pi interactions of these aromatic anions were determined by CZE as dimerization constants (KD). Although the largest KD value of 1.2 dm3 mol(-1) for 9-anthracenecarboxylate ion (9-AC) in these aromatic anions was found, almost all of the KD values were zero, or near to zero. It was found that the pi-pi interactions of the aromatic anions were relatively small at zero ionic strength, in which the contribution of an ionic association between the cation and aromatic anions could be excluded from the KD values, since the contribution of the electric repulsion between the aromatic anions on the KD values was large. The relatively large KD value of 9-AC caused that it electro-migrates as its planar shape, and has an anthracene ring of a largely hydrophobic aromatic ring.     

Accounting for diffuse layer ions in triple-layer models

The triple-layer model is one of the most widely used surface complexation models for adsorption on mineral surfaces. In current implementations, the accounting of ions in the diffuse layer may be neglected, resulting in a charge imbalance in the modeled solution as well as errors in mass balance, particularly in low ionic strength solutions when mineral-specific surface area is large. This paper introduces an internally consistent scheme for modeling diffuse layer ions in the triple-layer model. Model calculations illustrate the difference between the proposed and previous implementations using an idealized example. The guarantee of charge balance on both sides of the interface assures that pH is accurately modeled. This may be important in reactive transport simulations, such as modeling adsorption in low ionic strength variable charge soil solutions.     

Chaotropic salts interacting with soft matter: Beyond the lyotropic series

The effects of the chaotropic ions of the Hofmeister series on many systems and phenomena are typically quite pronounced. What happens, however, when one uses chaotropic ions beyond SCN⁻, ClO₄⁻, or guanidinium, which are the usual limiting ions of the lyotropic series considered in most investigations? This review focuses on the extensive but scattered literature that discusses how larger hydrophobic ions and hydrotropic ions interact with soft matter. There are many similarities between hydrophobic and hydrotropic ions; they differ in the fact that the hydrotropes are intrinsically asymmetric with respect to aqueous solvation. Strong specific effects of these ions with a common denominator are found in diverse systems: Hydrophobic ions “stick” to hydrophobic surfaces, or intercalate within soft matter interfaces, becoming a basic component of the structure and often inducing disruption or phase change. In other situations, hydrophobic ions act indirectly by failing to provide adequate screening of electrostatic interactions because of their large size. The hydrophobic and hydrotropic ions discussed here constitute the link between the lyotropic series and the surfactant domain. It is pointed out that, despite the size and breadth of the literature, there is still much work to be done to clarify how these ions interact with soft matter. Many important applications can result from the control of soft matter structure that can be achieved with these ions.     

Cyclic voltammetry of highly hydrophilic ions at a supported liquid membrane

Cyclic voltammetry has been used to study the coupling of ion transfer reactions at a liquid membrane. The liquids are either supported by a porous hydrophobic membrane (polyvinylidene difluoride, PVDF) when the organic solvent is non-volatile (o-nitrophenyloctylether) or are merely a free standing organic solvent layer such as 1,2-dichloroethane comprised between two hydrophilic dialysis membranes supporting the adjacent aqueous phases. The passage of current across the liquid membrane is associated with two ion transfer reactions across the two polarised liquid liquid interfaces in series. It is shown that it is possible to study the transfer of highly hydrophilic ions at one interface by limiting the mass transfer of the other ion transfer reaction at the other interface. Indeed, for systems comprising an ion M in one aqueous phase and a reference ion R partitioned between the membrane and the other aqueous phase, the observed and simulated cyclic voltammograms have a half-wave potential determined by the Gibbs energy of transfer of M transferring at one interface and by the limiting mass transfer of R at the other interface. This new methodology opens a way to measure the Gibbs energy of transfer of highly hydrophilic or hydrophobic ions, which usually limits the potential window at single liquid liquid interfaces (ITIES).     

Physisorption of hydroxide ions from aqueous solution to a hydrophobic surface

We present results from detailed molecular dynamics simulations revealing a counterintuitive spontaneous physical adsorption of hydroxide ions at a water/hydrophobic interface. The driving force for the migration of the hydroxide ions from the aqueous phase is the preferential orientation of the water molecules in the first two water layers away from the hydrophobic surface. This ordering of the water molecules generates an electrical potential gradient that strongly and favorably interacts with the dipole moment of the hydroxide ion. These findings offer a physical mechanism that explains intriguing experimental reports indicating that the interface between water and a nonionic surface is negatively charged.     

Structure at Colloid Interface

Abstract

The electrical double layer at a solid-liquid interface plays a prominent role in the interpretation of many colloidal phenomena. Adsorption of foreign ions at colloidal particle surfaces is influenced by the surface charge density ([sgrave]₀) and the double layer potential (φ₀). The stability of hydrophobic colloidal solutions is largely controlled by the relative strength of repulsive and attractive (van der Waals) forces which come into play when the double layers of two approaching particles overlap.     

Adsorption states of amphipatic solutes at the surface of naturally hydrophobic minerals: a molecular dynamics simulation study

An initial molecular dynamics simulation study regarding interfacial phenomena at selected naturally hydrophobic surfaces is reported. Simulation results show that, due to the natural hydrophobicity of graphite and talc basal planes, the cationic surfactant dodecyltrimethylammonium bromide preferentially adsorbs at these surfaces through hydrophobic interactions. When a model dextrin molecule is considered, the simulation results suggest that the hydrophobic interaction between the naturally hydrophobic surfaces of graphite, talc basal plane, and sulfur and the hydrophobic moieties (C-H and methylene groups) in the dextrin molecule plays a significant role in dextrin adsorption at these surfaces. The hydroxyl group in the dextrin molecule also contributes to its adsorption at the talc basal plane surface. In contrast, dextrin was not found to adsorb at talc edge surfaces.     

An SFG study of interfacial amino acids at the hydrophilic SiO2 and hydrophobic deuterated polystyrene surfaces

Sum frequency generation (SFG) vibrational spectroscopy was employed to characterize the interfacial structure of eight individual amino acids--L-phenylalanine, L-leucine, glycine, L-lysine, L-arginine, L-cysteine, L-alanine, and L-proline--in aqueous solution adsorbed at model hydrophilic and hydrophobic surfaces. Specifically, SFG vibrational spectra were obtained for the amino acids at the solid-liquid interface between both hydrophobic d(8)-polystyrene (d(8)-PS) and SiO(2) model surfaces and phosphate buffered saline (PBS) at pH 7.4. At the hydrophobic d(8)-PS surface, seven of the amino acids solutions investigated showed clear and identifiable C-H vibrational modes, with the exception being l-alanine. In the SFG spectra obtained at the hydrophilic SiO(2) surface, no C-H vibrational modes were observed from any of the amino acids studied. However, it was confirmed by quartz crystal microbalance that amino acids do adsorb to the SiO(2) interface, and the amino acid solutions were found to have a detectable and widely varying influence on the magnitude of SFG signal from water at the SiO(2)/PBS interface. This study provides the first known SFG spectra of several individual amino acids in aqueous solution at the solid-liquid interface and under physiological conditions.     

Determination of the heterogeneous association constants of metal ions to omega-mercaptoalkanoic acids by using double-layer capacity measurements.

The binding of metal ions to ligands in homogeneous solutions and that to the same ligands anchored to metallic surfaces through self-assembled monolayers (SAMs) are expected to differ primarily due to the difference in the degree of freedom of the ligands and the surface potential. We studied the heterogeneous binding of CdII ions to omega-mercaptoalkanoic-acid SAMs on Au. This was accomplished by adding metal ions at a constant pH and following the changes in the double-layer capacity. A mathematical treatment, which is based on calculating the electrochemical-potential differences at the double layer-solution interface, has been developed. Our approach follows that proposed by White et al. and Kakiuchi, who used the acid-base equilibrium at the monolayer-electrolyte interface as a means of calculating the pK of ionizable SAMs. Experimentally, SAMs of omega-mercaptoalkanoic acids, HS(CH2)nCO2H, with different chain lengths (i.e., n=2, 5, and 10) in 0.1 M sodium perchlorate were assembled on Au surfaces and studied. The capacity was measured first in the absence of CdII at different pH values, and then at a constant pH while increasing the concentration of CdII in the solution. We found that the interfacial capacity decreased as the concentration (of either protons or CdII) increased. The results matched the model fairly well, which allowed the extraction of the thermodynamic equilibrium constant that is established at the monolayer-electrolyte interface. The suggested mathematical treatment of this model system is simple and yet very useful for estimating the heterogeneous association constants of metal ions by SAMs.