Ultrafiltration of a polymer-electrolyte mixture

We present a mathematical model to describe the ultrafiltration behaviour of polymer-electrolyte mixtures. The model combines the proper thermodynamic forces (pressure, chemical potential and electrical potential differences) with multicomponent diffusion theory. The model is verified with experimental data on the ultrafiltration of aqueous solutions of PEG-4000 and potassium phosphate. The single solute rejection of PEG-4000 goes through a maximum as also found by others. The single solute rejection of potassium phosphate depends on the ionic strength of the solution. At low ionic strength rejections are found of 50%. Solutions containing a high concentration of PEG-4000 and potassium phosphate show a negative rejection for potassium phosphate. This is caused by the strongly non-ideal behaviour of these aqueous solutions. The model predicts the behaviour of single solute experiments quite well, but some deviations are found with the mixed solute experiments. However, negative rejections found in the mixed solute experiments are predicted by the model.     

Studies on the Electrochemical Characterization of Cellulose Acetate and Dowex-50 Membranes for Uni-Univalent Electrolytes in Aqueous Solutions

Membrane potential and bi-ionic potential studies using cellulose acetate and Dowex-50 membranes and sodium chloride and potassium chloride aqueous solutions have been carried out. The results have been used to estimate solute permeability, ionic transport numbers, fixed charge density, and surface charge density of both the membranes. Both membrane potential and bi-ionic potential are affected by adsorption of ions. Adsorbed ions affect the surface potential as well as solute retention capacity of the membranes. Solute retention expressed in terms of the “maximal retention” of membranes could thus be estimated. Furthermore, a correlation between permselectivity of the membrane and effective concentration with the dielectric constant of the membrane has also been established.     

On the influence of ionic strength on the equilibrium constant of copper-murexide interaction

The concentration formation constant for the 1:1 complex between copper(II) and murexide in aqueous solutions has been measured as a function of ionic strength by spectrophotometry at 25 degrees . There is an inverse relationship between the K(c) values and ionic strength. The formation constant at infinite dilution was found to be 6.16 x 10(4). The distance of closest approach for the Cu(2+) ion was estimated as 4.3 +/- 0.3 A.     

Specific ion effects in aqueous eletrolyte solutions confined within graphene sheets at the nanometric scale

The underlying mechanisms of specific ion effects on structure and dynamics of aqueous solutions have been long debated. On the other hand, the role of polarization at hydrophobic interfaces when aqueous electrolytes are present is of great importance, as it has been observed at the air-vapor interface. In this work, we have explored influence of ionic species on microscopical properties of aqueous sodium halide solutions constrained inside a double layer graphene channel, as a model for a realistic hydrophobic interface. Our systems have been simulated by molecular dynamics techniques, explicitly including polarization in water molecules and ions. Water and ionic density profiles showed the tendency of ionic species to occupy the whole space available, in good agreement with spectroscopic experimental data. The exception to this general behavior was fluoride, which preferred to stay away from interfaces. Two main regions were defined: interfaces and the central part of the slab, the bulklike region. Ionic hydration numbers at interfaces were lower than those at the bulklike area by about one to two units. We have also analyzed water-ion orientations and polarization distributions and obtained a marked dependence on ionic concentration. Residence time of anions suffered important fluctuations and tended to be largest at interfaces. Large variations of the static permittivity between interfacial and bulklike regions were observed. Ionic diffusion was found to be between 10(-5) and 10(-6) cm(2) s(-1) and showed to be mainly dependent on the concentration, whereas the type of anion considered and the polarizability had significantly less relevance. Conductivities were found to be dependent on ionic concentrations and the polarizabilities of anions, as well as on the spatial direction considered.     

Simple and accurate computations of solvatochromic shifts in pi --> pi* transitions of aromatic chromophores.

A new approach is introduced for calculating the spectral shifts of the most bathochromic pi --> pi* transition of an aromatic chromophore in apolar environments. As an example, perylene in solid and liquid n-alkane matrixes was chosen, and all shifts were calculated relative to one well-defined solid-inclusion system. It was shown that a simple two-level treatment of the solute using Hückel theory yields spectral shifts in excellent agreement with experimental results for the most prominent inclusion sites of perylene in solid n-alkane surroundings and for the dilute solutions in liquid n-alkanes. The idea is general enough to be applied to any aromatic chromophore in a nonpolar solvent matrix. In contrast to earlier treatments, this approach is based on geometry- and environment-dependent polarizabilities, employs an r(-4) dependence for the dispersion energy, and is conceptually very simple and computationally very efficient.     

Calculations of Freezing Point Depression,Boiling Point Elevation,Vapor Pressure and Enthalpies of Vaporization of Electrolyte Solutions by a Modified Three-Characteristic Parameter Correlation Model

A method was proposed for calculating the thermodynamic properties, freezing point depression, boiling point elevation, vapor pressure and enthalpy of vaporization for single solute electrolyte solutions, including aqueous and nonaqueous solutions, based on a modified three-characteristic-parameter correlation model. When compared with the corresponding literature values, the calculated results show that this method gives a very good approximation, especially for 1-1 electrolytes. Although the method is not very suitable for some solutions with very high ionic strength, it is still a very useful technique when experimental data is scarce.     

Reconciliation of Solute Concentration Data with Water Contents and Densities of Multi-Component Electrolyte Solutions

Density and chemical masses are two of the most important parameters tracked in chemical plant flowsheets. Unfortunately, chemical plant laboratories commonly avoid density and solvent concentration measurements. Without these data, it is difficult to reconcile solute concentrations reported by the laboratories with the total mass and volume tracked in flowsheets. In this paper, the Laliberté-Cooper density model is used in conjunction with a numerical algorithm to simultaneously estimate both density and water content from measured solute concentrations for aqueous electrolyte solutions. The algorithm numerically optimizes the water content until the sum of the water and solute concentrations (in mass per volume units) equals the density predicted by the Laliberté-Cooper model for that composition. The algorithm was tested against an experimental dataset of simulated nuclear waste supernatant solutions containing mixtures of ten different electrolytes with total ionic strengths up to 8 mol⋅L⁻¹. The algorithm was able to predict the measured densities with an R² of 0.9912 and an average relative percent error of just 0.05%. The model error was not correlated to the estimated water content or any of the electrolyte concentrations. Thus, the algorithm can be successfully used to simultaneously predict density and water content of aqueous electrolyte solutions containing many electrolytes at high concentrations from analytical data reported in moles or mass of solute per volume.     

Molecular dynamics simulations of ion transport through carbon nanotubes. III. Influence of the nanotube radius, solute concentration, and applied electric fields on the transport properties

The present investigations continue previous research on transport in aqueous ionic solutions through carbon nanotubes. Specifically, the effects of the nanotube radius, solute concentration, and applied external electric fields on the transport properties are investigated in terms of mobilities, currents, and pairing times of the solute ions. The simulated transport features are corroborated with general theoretical results of nanofluidics (such as the linear log-log regime of the nanochannel conductance as function of the solute concentration and the current-voltage curve of the channel). Discontinuities in the partial ionic currents are explained on the basis of a recent theoretical model of quantized ionic conductance in nanopores, developed by Zwolak et al. Correlations between the structural and dynamic properties are established, linking causally the highly structured spatial density profiles, the ion pairing phenomenon and the ionic currents.     

Role of desorption kinetics in determining marangoni flows generated by using electrochemical methods and redox-active surfactants

We report quantitative measurements of Marangoni flows generated at the surfaces of aqueous solutions by using water-soluble redox-active surfactants in combination with electrochemical methods. These measurements are interpreted within the framework of a simple model that is based on lubrication theory and the proposition that the kinetics of the desorption of redox-active surfactants from the surfaces of aqueous solutions plays a central role in determining the strength of the Marangoni flow. The model predicts that the leading edge velocity of the Marangoni flow will decay exponentially with time and that the rate constant for the decay of the velocity can yield an estimate of the surfactant desorption rate constant. Good agreement between theory and experiments was found. By interpreting experimental measurements of electrochemically generated Marangoni flows within the framework of the model, we conclude that the desorption rate constant of the redox-active surfactant Fc(CH(2))(11)-N(+)(CH(3))(3)Br(-), where Fc is ferrocene, is 0.07 s(-)(1). We also conclude that the ionic strength of the aqueous solution has little effect on the desorption rate constant of the ferrocenyl surfactant.     

Ion hydration and structural properties of water in aqueous solutions at normal and supercooled conditions: a test of the structure making and breaking concept

We study with the method of molecular dynamics simulation the structural properties of aqueous solutions of NaCl, KCl and KF salts at ambient conditions and upon supercooling at constant pressure. The calculations are performed at increasing concentration of the salt starting from c = 0.67 mol kg(-1) up to 3.96 mol kg(-1). We investigate the modifications of the hydration shells and the changes in the water structure induced by the presence of the ions. The oxygen-oxygen structure is strongly dependent on the ionic concentration while it is almost independent from the cation. The hydrogen bonding is preserved at all concentrations and temperatures. The main effect of increasing the ionic concentration is the tendency of the water structure to assume the high density liquid form predicted for pure water upon supercooling. An important consequence of our analysis is that the concept of an ion as a structure maker or a structure breaker must be revisited to take into account the other ionic species, the ionic concentration and more generally the thermodynamic conditions of the solutions.     

Salt effects on solute exchange in sodium dodecyl sulfate micelles

We describe the influence of sodium chloride on the rate of solute exchange in aqueous SDS micelles for a water-insoluble solute, a pyrene-containing triglyceride 1. The initially prepared solutions contained a small fraction of micelles containing two molecules of 1 and a large excess of empty micelles. These solutions showed a measurable excimer emission (of intensity I(E)) that was stable for days to weeks in the absence of added salt. Following additions of salt, I(E) decayed exponentially (rate constant, k(obs)) accompanied by an increase in pyrene monomer emission. Values of k(obs) increased strongly with ionic strength (k(obs) similar [Na(+)](4)). There was no contribution of the empty micelle concentration beyond its contribution to the sodium ion concentration. We conclude that the solute exchange involves spontaneous fragmentation of the SDS micelles into two submicelles, each bearing a molecule of 1, which then grow back to normal micelles through condensation of SDS monomers. We propose a model for the fragmentation process in which large amplitude surface fluctuations "pinch off" a subunit that becomes a submicelle. These fluctuations bring sulfate headgroups into close proximity. Fluctuations leading to fission become important only in the presence of sufficient counterion concentration to reduce the electrostatic repulsion between neighboring headgroups.     

Modified McKay–Perring Equation and Its Two Particular Solutions

Zdanovskii’s rule is the simplest isopiestic molality relation of mixed electrolyte aqueous solutions and the McKay–Perring equation is a differentio-integral equation particularly suitable for calculating solute activity coefficients from isopiestic measurements. However, they have two unsolved problems, which have puzzled solution chemists for several decades: (1) Zdanovskii’s rule has been verified by precise isopiestic measurements. But, several scientists suggested that the rule contradicts the Debye–Hückel limiting law for extremely dilute unsymmetrical mixtures. (2) In the McKay–Perring equation, a solute activity coefficient is multiplied by a solute composition variable. Different scientists have suggested that the composition variable may be the total ionic strength, common ion concentration, total ionic concentration, or an additive function with arbitrary proportionality constants. But, the different choices of the composition variable may lead to different activity coefficient results. Here, I derive a modified McKay–Perring equation in which the composition variable has the exclusive physical meaning of total ionic concentration for mixed electrolyte solutions (or of total solute particle concentration for the mixed solutions containing nonelectrolyte solutes). I also demonstrate that Zdanovskii’s rule is consistent with the Debye–Hückel limiting law for extremely dilute unsymmetrical mixtures. I derive two particular solutions of the modified McKay–Perring equation: one for the systems obeying Zdanovskii’s rule and another for the systems obeying a limiting linear concentration rule. These theoretical results have been verified with literature experiments and model calculations.     

Temperature behavior of thermo-responsive poly-N-vinylcaprolactam and poly-N-isopropylmethacrylamide in aqueous solutions involving organic solutes

Poly-N-vinylcaprolactam (PVCL) and poly-N-isopropylmethacrylamide (PIMA) undergo reversible sharp phase separation in water as a function of temperature. Phase separation behaviour was studied for aqueous solutions of these polymers containing amides and alcohols as a function of solute concentration and structure. It is observed that the temperature behaviour of PIMA is not sensitive to solute structure. For PVCL, solute structure affects the phase separation temperature, and possible reasons for the experimental observations are considered.     

Effect of alkyl chain length and temperature on the thermodynamic properties of ionic liquids 1-alkyl-3-methylimidazolium bromide in aqueous and non-aqueous solutions at different temperatures

The alkyl chain length of 1-alkyl-3-methylimidazolium bromide ([Rmim][Br], R = propyl (C₃), hexyl (C₆), heptyl (C₇), and octyl (C₈)) was varied to prepare a series of room-temperature ionic liquids (RTILs), and experimental measurements of density and speed of sound at different temperatures ranging from (288.15 to 308.15) K for their aqueous and methanolic solutions in the dilute concentration region (0.01 to 0.30) mol · kg^?1 were taken. The values of the compressibilities, expansivity and apparent molar properties for [C_nmim][Br] in aqueous and methanolic solutions were determined at the investigated temperatures. The obtained apparent molar volumes and apparent molar isentropic compressibilities were fitted to the Redlich–Mayer and the Pitzer’s equations from which the corresponding infinite dilution molar properties were obtained. The values of the infinite dilution molar properties were used to obtain some information about solute–solvent and solute–solute interactions. The thermodynamic properties of investigated ionic liquids in aqueous solutions have been compared with those in methanolic solutions. Also, the comparison between thermodynamic properties of investigated solutions and those of electrolyte solutions, polymer solutions, cationic surfactant solutions and tetraalkylammonium salt solutions have been made.     

Ions in solutions: Determining their polarizabilities from first-principles

Dipole polarizabilities of a series of ions in aqueous solutions are computed from first-principles. The procedure is based on the study of the linear response of the maximally localized Wannier functions to an applied external field, within density functional theory. For most monoatomic cations (Li(+), Na(+), K(+), Rb(+), Mg(2+), Ca(2+) and Sr(2+)) the computed polarizabilities are the same as in the gas phase. For Cs(+) and a series of anions (F(-), Cl(-), Br(-) and I(-)), environmental effects are observed, which reduce the polarizabilities in aqueous solutions with respect to their gas phase values. The polarizabilities of H((aq)) (+), OH((aq)) (-) have also been determined along an ab initio molecular dynamics simulation. We observe that the polarizability of a molecule instantaneously switches upon proton transfer events. Finally, we also computed the polarizability tensor in the case of a strongly anisotropic molecular ion, UO(2) (2+). The results of these calculations will be useful in building interaction potentials that include polarization effects.     

A generalized setchenov approach to the effect of polar additives on ionic micellar solutions

It is shown that the variation of the critical micelle concentration (CMC) of two ionic surfactants (trimethyldodecylammonium bromide and sodium dodecyl sulfate) with the addition of a number of polar solutes in aqueous solutions follows a generalized type of Setchenov equation which allows the definition of a micellization constant K_M specific to each solute. This constant is a sum of a number of terms: the classical salting constant, an electrical term, and a free energy term. Using a precise vapor pressure method, the salting constant k_s was determined for a number of polar solutes in aqueous trimethyldodecylammonium bromide solutions. Using CMC determinations from the literature, it was shown that for solutes such as dimethylformamide, acetamide, urea, and dimethylurea, k_s = K_M, that is, the increase of the CMC is entirely attributable to the change in the medium due to the solute + monomer surfactant interactions; furthermore the change of the electrical terms upon addition of the solutes is negligible. In other cases, like acetone, dioxane, thiourea, or 1-alkanols, a partition coefficient may be easily calculated from a comparison between K_M and k_s.     

The dielectric response of interfacial water—from the ordered structures to the single hydrated shell

Water is the universal solvent in nature. Does this imply, however, that its interaction with its environment is also a universal feature? While this question maybe too fundamental to be answered by one method only, we present evidence that the broadening of the dielectric spectra of water presents universal features of dipolar interactions with different types of matrixes. If in aqueous solutions the starting point of water’s state can be considered as bulk, with only partial interactions with the solute, then the state of water adsorbed in heterogeneous materials is determined by various hydration centers of the inhomogeneous material (the matrix) and it is significantly different from the bulk. In both cases, the dielectric spectrum of water is symmetrical and can be described by the Cole–Cole (CC) function. The phenomenological model that describes a physical mechanism of the dipole–matrix interaction in complex systems underlying the CC behavior has been applied to water adsorbed in porous glasses. It was then extended to analyses of the dynamic and structural behavior of water in nonionic and ionic aqueous solutions. The same model is then used to analyze the CC relaxation processes observed in clays, aqueous solutions of nucleotides, and amino acids.     

Acid-base equilibria in aqueous solution of cis-bis(trimethylphosphine)platinum(II) dinitrate at 25 degrees C, 0.2 M ionic strength. A potentiometric study

Acid-base equilibria in aqueous solutions of cis-bis(trimethylphosphine)platinum(II) dinitrate at 25 degrees C, 0.2 M ionic strength (KNO(3)), have been investigated by potentiometry with a glass electrode. The procedure consisted of multiple addition of the investigated analyte to a supporting electrolyte solution ("multiple sample addition") and subsequent titration with strong base. For the treatment of potentiometric multiple sample addition data, a new linearization procedure, suitable for an acid dissociation equilibrium whose product dimerizes, has been devised and tested. The potentiometric results have been interpreted with the support of NMR data. By dissociation of the first acidic function of the solute diaquo cation, cis-[(PMe(3))(2)Pt(OH(2))(2)](2+), a dimeric ampholite, cis-[(PMe(3))(2)Pt(mu-OH)](2)(2+), is quantitatively formed which, in turn, can be converted into the di-hydroxo derivative cis-(PMe(3))(2)Pt(OH)(2). The two acid dissociation steps involving two molecules of solute and condensation of ampholyte have pK(a1(c)) = 3.89 and pK(a2(c)) = 22.17.     

A small-angle scattering study on equilibrium clusters in lysozyme solutions

We use small-angle scattering experiments to investigate the structural properties of aqueous lysozyme solutions under conditions where the existence of equilibrium clusters has recently been demonstrated (Nature 2004, 432, 492). We also discuss the possible emergence of a low angle scattering contribution, which recently attracted interest due to its appearance in solutions of various proteins. We demonstrate that in lysozyme solutions under our experimental conditions such rising low q intensities can only be observed under special circumstances and can thus not be attributed to the existence of a universal long-range attraction. We then focus on the structural properties of the equilibrium clusters as a function of protein concentration, temperature, and ionic strength. We show that the experimental structure factors obtained from the scattering measurements exhibit the typical cluster-cluster peak q(c) reflecting the mean distance between charged clusters as well as a monomer-monomer peak q(m), which represents the nearest neighbor shell of monomers within a single cluster. The underlying principle for the formation of these structures is the coexistence of two opposing forces, a short-range attraction and a long-range repulsion due to residual charges. We can quantitatively analyze our scattering data by applying a simple equilibrium cluster model and calculate an average cluster aggregation number, N(c). The thus obtained cluster aggregation number increases linearly with volume fraction. We also observe an increasing N(c) as temperature decreases and as the screening of residual charges increases. We point out the importance of the existence of equilibrium clusters and the universality of this phenomenon for self-assembling processes observed in nature. Finally, we discuss the limitations of our simple globular cluster model in view of recent findings from computer simulations.     

Polarizable Poisson-Boltzmann equation: the study of polarizability effects on the structure of a double layer

We incorporate ion polarizabilities into the Poisson-Boltzmann equation by modifying the effective dielectric constant and the Boltzmann distribution of ions. The extent of the polarizability effects is controlled by two parameters, γ(1) and γ(2); γ(1) determines the polarization effects in a dilute system and γ(2) regulates the dependence of the polarizability effects on the concentration of ions. For a polarizable ion in an aqueous solution γ(1) ≈ 0.01 and the polarizability effects are negligible. The conditions where γ(1) and/or γ(2) are large and the polarizability is relevant involve the low dielectric constant media, high surface charge, and/or large ionic concentrations.