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The physical origin of charged interfaces involving electrolyte solutions is in the thermodynamic equilibrium between the surface reactive groups and certain dissolved ionic species ...in the bulk. This equilibrium is very strongly dependent on the precise local density of these species, also known as potential determining ions in the solution. The latter, however, is determined by the overall solution structure, which is dominated by the large number of solvent molecules relative to all solutes. Hence, the solvent contribution to the molecular structure is a crucial factor that determines the properties of electric double layers. Models that explicitly account for the solvent structure are often referred to as “civilized” as opposed to the “primitive” ones that consider the solvent as a structureless continuum. In the present paper, we demonstrate that for a physically correct description of charged interfaces that involve electrolyte solutions (electric double layers), the full solution structure needs to be taken into account in conjunction with the precise surface chemistry governed by the thermodynamic equilibrium. The analysis shows how the surface charge depends on various experimentally relevant parameters, many of which are outside the realm of simple electrostatics. We present results on the effects of solvent molecular dimensions, ionic solvation, surface chemistry, solvophilicity and solvophobicity.
The electrophoretic mobility of oil droplets, dispersed without any surfactant in the aqueous phase, was measured. Four different oils were studied: xylene, dodecane, hexadecane, and ...perfluoromethyldecalin. Special precautions were undertaken to avoid artifacts caused by the presence of surfactant impurities. The results show that the oil droplets are negatively charged and the magnitude of their ζ-potential strongly depends on pH and the ionic strength of the aqueous phase. The electrophoretic mobility is almost independent of the type of specific nonpolar oil. Series of experiments were performed to check different hypotheses about the origin of the spontaneous charging of the oil−water interfaces. The results lead to the conclusion that hydroxyl ions, released by the dissociation−association equilibrium of the water molecules, adsorb at the oil−water interface. The specific adsorption energy was estimated to be 25kT per ion (kT is the thermal energy). The molecular origin and the implications of this phenomenon are discussed. The ζ-potential decreases in magnitude when poly(oxyethylene) chain nonionic surfactants are adsorbed at the interface.
We have studied the structure of the protein species and the protein–protein interactions in solutions containing two apoferritin molecular forms, monomers and dimers, in the presence of Na
+ and Cd
...2+ ions. We used chromatographic, and static and dynamic light scattering techniques, and atomic force microscopy (AFM). Size-exclusion chromatography was used to isolate these two protein fractions. The sizes and shapes of the monomers and dimers were determined by dynamic light scattering and AFM. Although the monomer is an apparent sphere with a diameter corresponding to previous x-ray crystallography determinations, the dimer shape corresponds to two, bound monomer spheres. Static light scattering was applied to characterize the interactions between solute molecules of monomers and dimers in terms of the second osmotic virial coefficients. The results for the monomers indicate that Na
+ ions cause strong intermolecular repulsion even at concentrations higher than 0.15
M, contrary to the predictions of the commonly applied Derjaguin–Landau–Verwey–Overbeek theory. We argue that the reason for such behavior is hydration force due to the formation of a water shell around the protein molecules with the help of the sodium ions. The addition of even small amounts of Cd
2+ changes the repulsive interactions to attractive but does not lead to oligomer formation, at least at the protein concentrations used. Thus, the two ions provide examples of strong specificity of their interactions with the protein molecules. In solutions of the apoferritin dimer, the molecules attract even in the presence of Na
+ only, indicating a change in the surface of the apoferritin molecule. In view of the strong repulsion between the monomers, this indicates that the dimers and higher oligomers form only after partial denaturation of some of the apoferritin monomers. These observations suggest that aggregation and self-assembly of protein molecules or molecular subunits may be driven by forces other than those responsible for crystallization and other phase transitions in the protein solution.
We have studied molecular interactions in solutions of the protein apoferritin by static and dynamic light scattering. When plotted against the electrolyte concentration, the second osmotic virial ...coefficient exhibits a minimum. The ascending branch of this dependence is a manifestation of a surprisingly strong repulsion between the molecules at electrolyte concentrations about and above 0.2M, where electrostatic interactions are suppressed. We argue that the repulsion is due to the water structuring, enhanced by the accumulation of hydrophilic counterions around the apoferritin molecules, giving rise to so-called hydration forces.
The focus of the present article is on the ionic size variation effects on the properties of charged interfaces involving electrolyte solution, commonly referred to as electric double layers. The ...presence of a well defined charged interface between the solution and a substrate has a profound impact on the local structure of the liquid phase. All solution species are distributed according to the various fluid and surface interactions. The excluded volume and finite dimensions of all ions and the solvent molecules are a major contributor to the detailed liquid structure. The structure determines important properties of the electric double layers such as charge and potential distributions and the surface and in the fluid. Our analysis is based on using classical density functional theory, which allows one to account for a variety of Coulombic and non-Coulombic interactions. The surface charge is determined by the thermodynamic equilibrium with potential determining species in the solution. We demonstrate that the size variation of the background electrolyte ions has a strong impact on the surface chemical equilibrium, as well as on the structure, charge and potential distributions in the electric double layer.
Alkyloxyethylene sulfates are a special class of surfactants that are unusually stable in the presence of multivalent counterions and are not as prone to precipitation as anionic surfactants without ...intermediate ethoxy groups in the molecule. However, formation of micelles, their structure, and the properties of monolayers of these surfactants exhibit very interesting and sometimes unexpected properties depending on the nature of the ions dissolved in the solution. This paper presents a brief overview of our recent efforts to reveal the nature of these properties, including some new results. We show that the strong binding of multivalent (and particularly trivalent counterions) triggers a sphere-to-cylinder shape transition of the micelles and facilitates their further growth, even at very low ionic strength. The properties of surfactant monolayers are coupled to those of the micelles in the bulk and are governed also by multivalent counterion binding. The effect of multivalent counterions on the aggregation and structure formation in anionic surfactant solutions has both fundamental and practical importance.
This report presents a study of electrokinetic transport in a series of integrated macro- to nano-fluidic chips that allow for controlled injection of molecular mixtures into high-density arrays of ...nanochannels. The high-aspect-ratio nanochannels were fabricated on a Si wafer using interferometric lithography and standard semiconductor industry processes, and are capped with a transparent Pyrex cover slip to allow for experimental observations. Confocal laser scanning microscopy was used to examine the electrokinetic transport of a negatively charged dye (Alexa 488) and a neutral dye (rhodamine B) within nanochannels that varied in width from 35 to 200 nm with electric field strengths equal to or below 2000 V m-1. In the negatively charged channels, nanoconfinement and interactions between the respective solutes and channel walls give rise to higher electroosmotic velocities for the negatively charged dye than for the neutral dye, towards the negative electrode, resulting in an anomalous separation that occurs over a relatively short distance (<1 mm). Increasing the channel widths leads to a switch in the electroosmotic transport behavior observed in microscale channels, where neutral molecules move faster because the negatively charged molecules are slowed by the electrophoretic drag. Thus a clear distinction between "nano-" and "microfluidic" regimes is established. We present an analytical model that accounts for the electrokinetic transport and adsorption (of the neutral dye) at the channel walls, and is in good agreement with the experimental data. The observed effects have potential for use in new nano-separation technologies.
The charge and potential distributions in an electric double layer result from various chemical and physical interactions between an interface and the adjacent electrolyte solution. The charge ...typically originates from a chemical equilibrium between the reactive surface and certain potential determining ions in the solution (i.e., surface charge regulation). This surface chemistry however, is strongly dependent on the wide variety of interactions between all species in the solution, as well as with the interface. These interactions could be Coulombic and non-Coulombic, and are system-specific. The focus of this study is on the ion valency and its effect on the Coulombic interactions, their interplay with the molecular excluded volume, and attraction in the fluid phase, as well as with the electric double layer interface.
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