Hofmeister effects: interplay of hydration, nonelectrostatic potentials, and ion size

The classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory of colloids, and corresponding theories of electrolytes, are unable to explain ion specific forces between colloidal particles quantitatively. The same is true generally, for surfactant aggregates, lipids, proteins, for zeta and membrane potentials and in adsorption phenomena. Even with fitting parameters the theory is not predictive. The classical theories of interactions begin with continuum solvent electrostatic (double layer) forces. Extensions to include surface hydration are taken care of with concepts like inner and outer Helmholtz planes, and "dressed" ion sizes. The opposing quantum mechanical attractive forces (variously termed van der Waals, Hamaker, Lifshitz, dispersion, nonelectrostatic forces) are treated separately from electrostatic forces. The ansatz that separates electrostatic and quantum forces can be shown to be thermodynamically inconsistent. Hofmeister or specific ion effects usually show up above ≈10(-2) molar salt. Parameters to accommodate these in terms of hydration and ion size had to be invoked, specific to each case. Ionic dispersion forces, between ions and solvent, for ion-ion and ion-surface interactions are not explicit in classical theories that use "effective" potentials. It can be shown that the missing ionic quantum fluctuation forces have a large role to play in specific ion effects, and in hydration. In a consistent predictive theory they have to be included at the same level as the nonlinear electrostatic forces that form the skeletal framework of standard theory. This poses a challenge. The challenges go further than academic theory and have implications for the interpretation and meaning of concepts like pH, buffers and membrane potentials, and for their experimental interpretation. In this article we overview recent quantitative developments in our evolving understanding of the theoretical origins of specific ion, or Hofmeister effects. These are demonstrated through an analysis that incorporates nonelectrostatic ion-surface and ion-ion dispersion interactions. This is based on ab initio ionic polarisabilities, and finite ion sizes quantified through recent ab initio work. We underline the central role of ionic polarisabilities and of ion size in the nonelectrostatic interactions that involve ions, solvent molecules and interfaces. Examples of mechanisms through which they operate are discussed in detail. An ab initio hydration model that accounts for polarisabilities of the tightly held hydration shell of "cosmotropic" ions is introduced. It is shown how Hofmeister effects depend on an interplay between specific surface chemistry, surface charge density, pH, buffer, and counterion with polarisabilities and ion size. We also discuss how the most recent theories on surface hydration combined with hydrated nonelectrostatic potentials may predict experimental zeta potentials and hydration forces.     

CO oxidation on Pt nanoclusters, size and coverage effects: a density functional theory study

CO oxidation on Pt nanoclusters of approximately 1 nm in size was studied using density functional theory (DFT). Reaction barriers on various sites of a cuboctahedral 55-atom cluster and of several two-layer plane clusters representing (111) and (100) facets of the 147-atom cluster have been calculated at various coverage. The effect of atomic structure of various clusters was discussed. It was concluded that the 147-atom cuboctahedral cluster reveals properties of the Pt single crystal surfaces, while a 55-atom cluster cannot be fully described in terms of Pt single crystal surfaces. It was found that CO oxidation may occur faster at higher coverage and that for cluster sizes up to a few nanometers in size, larger platinum clusters can be more efficient in CO oxidation than the smaller clusters. The size effect was found to depend upon coverage.     

Surfactant-colligend particle size effects on ion flotation: Influences of mixing time,temperature, and surfactant chain length

Summary The ion flotation of Cr(VI) from 0.926 x 10^–3 M aqueous suspensions at pH 4.1 is related to particle size distribution data, obtained by filtration of the surfactant-Cr(VI) suspensions prior to flotation. The effects of surfactant-Cr(VI) mixing (precipitation and aggregation) time and of temperature are established over 10–45 °C, with particle size and flotation generally increasing with temperature. Five, rather high purity, quaternary ammonium surfactants are used, with chain lengths from C₁₀ to C₁₈. The optimum chain length at 13° and 23 °C is C14 and at 33° and 43 °C is C16. An increase in the molar surfactant/Cr(VI) ratio in the initial suspension improves flotation until values of the ratio of 1.1, 1.2, and 2.2 are exceeded for C₁₆, C₁₈, and C₁₄ surfactants, respectively. The four roles of a surfactant in ion flotation — as a precipitant, as a dispersant (with surface adsorption opposing aggregation), as a collector, and as a frother — are discussed in terms of flotation, particle size distributions, and surface charge measurements.

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Zusammenfassung Die Flotation von Cr(VI)-Ionen aus 0.926 × 10^–3 M wässeriger Suspension bei pH 4.1 steht in Beziehung zu der Verteilung der Teilchengröße, die sich durch Filtration einer Suspension von Benetzungsmittel-Cr (VI) vor der Flotation ergibt. Die Wirkung von Benetzungszeit (=Fällung und Zusammenballen) sowie der Temperatur im Bereich von 10–45 °C wurden untersucht; Teilchengröße und Flotation nehmen im allgemeinen mit steigender Temperatur zu. Fünf quaternäre Ammonium-Benetzungsmittel hoher Reinheit und mit Kettenlängen von C₁₀ bis C₁₈ wurden benutzt. Die optimale Kettenlänge bei 13° und 23 °C ist C₁₄, bei 33° und 43 °C dagegen C₁₆. Eine Vergrößerung des Verhältnisses von Benetzungsmittel:Cr(VI) in der anfänglichen Suspension ist für die Flotation so lange vorteilhaft, bis die Verhältniswerte 1.1 (C₁₆), 1.2 (C₁₈) bzw. 2.2 (C₁₄) überschritten werden. Die vier Aufgaben des Benetzungsmittels in einer Ionenflotation — als Fällungsmittel, Verteilungsmittel (die Oberflächenadsorption wirkt der Zusammenballung entgegen), Sammler und Schaummittel — werden in Bezug auf die Flotation, die Verteilung der Teilchengröße und Messungen der Oberflächenladung diskutiert.

     

Numerical study supplemented with simplified model on electrophoresis of a hydrophobic colloid incorporating finite ion size effects and ion-solvent interactions

We consider a modified electrokinetic model to study the electrophoresis of a hydrophobic particle by considering the finite sized ions. The mathematical model adopted in this study incorporates the ion steric repulsion, ion-solvent interactions as well as Maxwell stress on the electrolyte. The dielectric permittivity and viscosity of the electrolyte is considered to vary with the local ionic volume fraction. Based on this modified model for the electrokinetics we have analyzed the electrophoresis in a single as well as mixture of electrolytes of monovalent and non-$z:z$ electrolytes. The dependence of viscosity on local ionic volume fraction modifies the hydrodynamic drag as well as diffusivity of ions, which are ignored in existing studies on electrophoresis. A simplified model for electrophoresis of a hydrophobic particle incorporating the ion steric repulsion and ion-solvent interactions is developed based on the first-order perturbation on applied electric field. This simplified model is established to be efficient for a Debye layer thinner than the particle size and a smaller range of slip length. This model can be implemented for any number of ionic species as well as non-$z:z$ electrolytes. It is established that the ion steric interactions and dielectric decrement creates a counterion saturation in the Debye layer leading to an enhanced mobility compared to the standard model. However, experimental data for non-dilute cases often under predicts the theoretically determined mobility. The present modified model fills this lacuna and demonstrate that the consideration of finite ion size modifies the medium viscosity and hence, ionic mobility, which in combination lowers the mobility value.     

Matrix-induced peak enhancement of pesticides in gas chromatogrtaphy: Is there a solution?

In this study, it is shown that calibration solution prepared in control matrix extrix extract can be used to compensate for matrix-induced chromatographic response enhancement observed for certain pesticedes. This phenomenon is characterized by enhanced chromatographic response for certain pesticides in the presence of matrix du to reduced analyte loss during injection. Unacceptably high recoveries are seen for affected pesticides when maatrix-free solutions are used for reference and calibration. The effects of matrix concentration on overall enhancement and linear response were determined. When used as reference standards for the determination of recovery, matrix-standard solutions were found to provide acceptable recoveries for pesticides subject to matrix-induced chromatographic response enhancement along with other pesticides. General guidelines are presented for using matrix-standard calibration solutions in pesticide residue analysis.     

Theoretical study on oligothiophene N-succinimidyl esters: size and push-pull effects

We report a theoretical study on the optical properties of bithiophene and terthiophene N-succinimidyl esters, which have been functionalized with a methylsulfanyl group in the alpha or the beta positions. Time-dependent density functional theory (TD-DFT) and approximate coupled-cluster singles and doubles with the resolution of identity technique (RI-CC2) calculations have been performed in the ground and excited states. The RI-CC2 results for absorption and fluorescence energies are in better qualitative agreement with experiments, whereas TD-DFT does not correctly describe the higher energy part of the absorption spectra of beta-substituted bithiophenes, due to the presence of charge-transfer states. Systems functionalized at the alpha position show a large red-shift of the main absorption and fluorescence band and a larger Stokes-shift compared to the unsubstituted species. These effects are in most cases less pronounced for the beta-substituted structures. In particular, we found that the Stokes-shift of the alpha-substituted structures is larger than the one of the beta-substituted species due to a more planar orientation of the methylsulfanyl group with respect to the neighbouring thiophene in the excited state.     

Electrostatics of B-DNA in NaCl and CaCl2 solutions: ion size, interionic correlation, and solvent dielectric saturation effects

The epsilon-modified Poisson-Boltzmann (-MPB) equations ( J. Phys. Chem. B, 2007, 111, 5264) have been solved on a three-dimensional grid for an all-atom geometry model of B-DNA. The approach is based on the implicit solvent model including finite sizes of hydrated ions and a dielectric approximation of the ion hydration shell. Results were obtained for the detailed geometry model of B-DNA in dilute and moderately concentrated solutions of NaCl and CaCl(2). All -MPB parameters of ions and dielectric medium were extracted from published results of all-atom molecular dynamics simulations. The study allows evaluations of the ion size, interionic correlation, and the solvent dielectric saturation effects on the ion distributions around DNA. It unambiguously suggests that the difference between the -MPB and Poisson-Boltzmann distributions of ions is low for Na(+) counterions. Such a difference in the case of divalent counterions Ca(2+) is dramatic: the dielectric saturation of the ion hydration shell leads to point-like adsorption of Ca(2+) on the phosphate groups of DNA. The -MPB equations were also applied to calculate the energy of interaction between two B-DNA molecules. Results agree with previously published simulations and experimental data. Some aspects of ion specificity of polyelectrolyte properties are discussed.     

Equilibrium properties of charged spherical colloidal particles suspended in aqueous electrolytes: finite ion size and effective ion permittivity effects

The adsorption behaviour and mechanism of As(III) and Se(IV) oxyanion uptake using a mixed inorganic adsorbent were studied. The novel adsorbent, based on Fe(III)-Mn(III) hydrous oxides and manganese(II) carbonate, was synthesised using a hydrothermal precipitation approach in the presence of urea. The inorganic ion exchanger exhibited a high selectivity and adsorptive capacity towards As(III) (up to 47.6 mg/g) and Se(IV) (up to 29.0 mg/g), even at low equilibrium concentration. Although pH effects were typical for anionic species (i.e., the adsorption decreased upon pH increase), Se(IV) was more sensitive to pH changes than As(III). The rates of adsorption of both oxyanions were high. Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) studies showed that the ion exchange adsorption of both anions took place via OH(-) groups, mainly from Fe(III) but also Mn(III) hydrous oxides. MnCO(3) did not contribute directly to As(III) and Se(IV) removal. A higher adsorptive capacity of the developed material towards As(III) was partly due to partial As(III) oxidation during adsorption.     

A theoretical study of substituent effects on allylic ion and ion pair SN2 reactions

An ab initio study of ionic and ion pair displacement reactions involving allylic systems has been carried out at the RHF/6-31+G* level. The geometries and natural charges show the absence of conjugative stabilization in the ionic transition states, thus differing from traditional explanations. The high reactivity of allyl halides is explained by electrostatic polarization of the double bond. Substituent effects were also studied; in general, electron-withdrawing groups lower the barriers of the ionic S(N)2 reactions but increase the barriers of the ion pair reactions. The allylic reactions are compared with related benzylic systems. Hammett correlations give rho of opposite sign for the ionic and ion pair displacement reactions, in agreement with some experimental results.     

Specific ion effects on the electrophoretic mobility of small,highly charged peptides: A modeling study

In this work, we use coarse‐grained modeling to study the free solution electrophoretic mobility of small highly charged peptides (lysine, arginine, and short oligos thereof (up to nonapeptides)) in NaCl and Na₂SO₄ aqueous solutions at neutral pH and room temperature. The experimental data are taken from the literature. A bead modeling methodology that treats the electrostatics at the level of the nonlinear Poisson Boltzmann equation developed previously in our laboratory is able to account for the mobility of all peptides in NaCl, but not Na₂SO₄. The peptide mobilities in Na₂SO₄ can be accounted for by including sulfate binding in the model and this is proposed as one possible explanation for the discrepancy. Oligo arginine peptides bind more sulfate than oligo lysines and sulfate binding increases with the oligo length.     

The coupling effects of hexapole and octopole fields in quadrupole ion traps: a theoretical study

A theoretical method, the harmonic balance method, was introduced to study the coupling effects of hexapole and octopole fields on ion motion in a quadrupole ion trap. Ion motion characteristics, such as ion motion center displacement, ion secular frequency shift, nonlinear resonance curve and buffer gas damping effects, have been studied with the presence of both hexapole and octopole fields. It is found that hexapole fields have bigger impacts on ion motion center displacement, while octopole fields dominate ion secular frequency shift. Furthermore, the nonlinear features originated from hexapole and octopole fields could enhance or cancel each other, which provide us more space in a practical ion trap design process. As an example, an ion trap with improved performance was designed using a specific combination of hexapole and octopole fields. In this ion trap, a hexapole field was used to achieve efficient ion directional ejection, while an octopole field was added to correct the chemical mass shift and resolution degradation introduced by the hexapole field. Copyright © 2013 John Wiley & Sons, Ltd.     

The formation of thymidine-based T-tetramers with remarkable structural and metal ion size effects

We present direct ESI Q-TOF MS and X-ray evidence for remarkable structural and metal ion size effects on the formation of thymidine-based T-tetramers. The conventional H-bond acceptors on the ribose and deoxyribose may disfavor the formation of T-tetramers, and in the series of alkali metal ions, lithium did not induce T-tetramer due to its small ion size. Sodium, potassium, rubidium and caesium could produce thymidine-based T-tetramers. Furthermore, rubidium and caesium could induce T-pentamers and dimeric T-pentamers probably due to their larger ion sizes.     

Computational study of iminium ion formation: effects of amine structure

Density functional calculations are used to explore the formation of iminium ions from secondary amines and acrolein and the subsequent reactivity of the resulting iminium ions. After establishing a feasible profile for this reaction in simulated experimental conditions, we focus on the effect of variation in amine structure on calculated barriers. This analysis shows that incorporation of a heteroatom (N or O) in the alpha-position to the reactive amine results in significantly reduced energy barriers, as does an electron-withdrawing group (carbonyl or thiocarbonyl) in the beta-position. Electron density analysis is used to monitor reactions at a detailed level, and to identify important intermolecular interactions at both minima and transition states. Barriers to reaction are linked to calculated proton affinities of secondary amines, suggesting that the relative ease of protonation-deprotonation of the amine is a key property of effective catalysts. Moreover, barriers for subsequent Diels-Alder reaction of iminium ions with cyclopentadiene are lower than for their formation, suggesting that formation may be the rate determining step in the catalytic cycle.     

Substituent effects in benz[a]anthracene carbocations: a stable ion, electrophilic substitution (nitration, bromination), and DFT study

A series of novel carbocations were generated from isomeric monoalkylated and dialkylated benz[a]anthracenes (BAs) by low-temperature protonation in FSO(3)H/SO(2)ClF. With the monoalkyl derivatives (5-methyl, 6-methyl, 7-methyl, and 7-ethyl) as well as the D-ring methylated analogues (9-methyl, 10-methyl, and 11-methyl), the C-7 or the C-12 protonated carbocations were observed (as the sole or major carbocation) in all cases. Protonation of the 12-methyl derivative (9) gave the C-7 protonated carbocation (9H+) as the kinetic species and the ipso-protonated carbocation (9aH+) as the thermodynamic cation. With the 12-ethyl derivative (10), relief of steric strain in the bay-region greatly favors ipso-protonation (10aH+). With 3,9-dimethyl (14), C-7 protonation (14H+) is strongly favored (with <10% protonation at C-12), and with 1,12-dimethyl (15) the sole species observed is the C-7 protonated carbocation (15H+). For 7-methyl-12-ethyl, 7-ethyl-12-methyl, and 7,12-diethyl derivatives (16, 17, and 18), two ipso-protonated carbocations were initially formed (C-7/C-12), rearranging in time to give the C-12 protonated carbocations exclusively (16aH+, 17aH+, and 18aH+). Protonation outcomes are compared with the computed relative energies by DFT. Charge delocalization paths in the resulting carbocations were deduced based on the magnitude of Deltadelta13C values. For the thermodynamically more stable C-12 protonated carbocations, the charge delocalization path is analogous to those derived based on computed NPA charges for the benzylic carbocations formed by 1,2-epoxide (bay-region) and 5,6-epoxide (K-region) ring opening. Nitration (and bromination) of the 4-methyl, 7-methyl, 7-ethyl, 3,9-dimethyl, and 1,12-dimethyl derivatives resulted in isolation and characterization of several novel derivatives. Excellent agreement is found between low-temperature protonation selectivities and the regioselectivities observed in model substitution reactions.     

Structural properties of a calcium aluminosilicate glass from molecular-dynamics simulations: a finite size effects study

We study a calcium aluminosilicate glass of composition (SiO(2))(0.67)-(Al(2)O(3))(0.12)-(CaO)(0.21) by means of molecular-dynamics simulations, using a potential made of two-body and three-body interactions. In order to prepare small samples that can subsequently be studied by first principles, the finite size effects on the liquid dynamics and on the glass structural properties are investigated. We find that finite size effects affect the Si-O-Si and Si-O-Al angular distributions, the first peaks of the Si-O, Al-O, and Ca-O pair correlation functions, the Ca coordination, and the oxygen atoms' environment in the smallest system (100 atoms). We give evidence that these finite size effects can be directly attributed to the use of three-body interactions.     

Critical nucleus size effects on nanoparticle formation in microemulsions: a comparison study between experimental and simulation results

Monte Carlo simulations were performed to study the influence of critical nucleus size on nanoparticle formation in microemulsions. It was found that critical nucleus size strongly affected nucleation and growth rates, as well as final nanoparticle sizes. An increase of critical nucleus leads to a slower nucleation process. In contrast, it gives rise to acceleration of the growth process. Final nanoparticle sizes increase as the critical nucleus value increases. It is predicted that this dependence will be less pronounced when a high reactant concentration is used. We have compared the simulation results with experimental data taken from different authors. Good agreement between the two kinds of results supports the conclusions of this paper.     

MRCISD studies of the dissociation of vinylhydroperoxide, CH2CHOOH: there is a saddle point

Multireference ab initio methods are used to investigate the dissociation of vinylhydroperoxide CH(2)CHOOH into vinyl oxide and hydroxide radicals. In contrast to some previous studies, which claim the reaction has no saddle point, our calculations confirm that the dissociation is associated with a kinetic barrier (transition state). We further propose the existence of a hitherto undiscovered radical-radical complex on the reaction path. The computed reaction energetics are used to estimate VHP dissociation rates, and the results are discussed in the context of atmospheric ozonolysis pathways. Qualitative aspects of the dissociation of larger, substituted vinylhydroperoxides are also discussed.     

The significance of the alkene size and the nature of the metal ion in metal-alkene complexes: a theoretical study

Cation interactions with π-systems are a problem of outstanding contemporary interest and the nature of these interactions seems to be quite different for transition and main group metal ions. In this paper, we have systematically analyzed the contrast in the bonding of Cu(+) and main group metal ions. The molecular structures and energetics of the complexes formed by various alkenes (A = C(n)H(2n), n = 2-6; C(n)H(2n- 2), n = 3-8 and C(n)H(2n + 2), n = 5-10) and metal ions (M = Li(+), Na(+), K(+), Ca(2+), Mg(2+), Cu(+) and Zn(2+)) are investigated by employing ab initio post Hartree-Fock (MP2/6-311++G**) calculations and are reported in the current study. The study, which also aims to evaluate the effect of the size of the alkyl portion attached to the π-system on the complexation energy, indicates a linear relationship between the two. The decreasing order of complexation energy with various metal ion-alkene complexes follows the order Zn(2+)-A > Mg(2+)-A > Ca(2+)-A > Cu(+)-A > Li(+)-A > Na(+)-A > K(+)-A. The increased charge transfer and the electron density at (3,-1) intermolecular bond critical point corroborates well with the size of the π-system and the complexation energy. The observed deviation from the linear dependency of the Cu(+)-A complexes is attributed to the dπ→π* back bonding interaction. An energy decomposition analysis via the reduced variational space (RVS) procedure was also carried out to analyze which component among polarization, charge transfer, coulomb and exchange repulsion contributes to the increase in the complexation energy. The RVS results suggest that the polarization component significantly contributes to the increase in the complexation energy when the alkene size increases.