Stern Potentials Research Articles

Control of nanostructures through pH-dependent self-assembly of nanoplatelets

HypothesisThe self-assembly of platy clay particles is influenced by anisotropic surface charging properties of the particles. The Stern potentials of particle surfaces are controlled by solution pH. Consequently, the energy-favorable particle association varies with pH, leading to different self-assembly structures. Therefore, it should be possible to form desired nanostructure of final clay product through self-assembly of clay particles by controlling the surface charge through adjusting solution pH. ExperimentsThree different surfaces of kaolinite clay nanoparticles were selectively exposed for Stern potential determination using an atomic force microscope. Aggregation structures through particle self-assembly were predicted based on the analysis of interaction energies between various types of surfaces of kaolinite particles. The structures were visually confirmed using the freeze-dried scanning electron microscopy technique. FindingsBy reducing pH of a concentrated kaolinite suspension from 8 to 5 and 3, the dispersed kaolinite particles were self-assembled to a well-stacked configuration and card-house structure, respectively. Current study demonstrates that the pH-dependent surface properties of platy kaolinite nanoparticles can be successfully used to understand the macroscopic behavior (rheology) of kaolinite nanoparticle suspensions and design nanostructures of clay products (catalysts and sorbents). The pH-dependent self-assembly is also applicable to other platy particles of anisotropic surface (charging or wettability) properties.

AFM measurements of Hofmeister effects on clay mineral particle interaction forces

Interaction forces between clay minerals determine particle aggregation and dispersion of clay suspension. Strong Hofmeister effects have been found in surface reactions of clay mineral particles, clay aggregation kinetics, and clay aggregate stability. However, Hofmeister effects observed so far lack experimental evidence from direct measurements of clay mineral particle interaction forces. Through introducing a parameter of “effective radius” for clay mineral particle with a rough surface, AFM was successfully employed to determine the interaction forces between montmorillonite (with roughly surface) and mica (with smooth surface), and strong Hofmeister effects on particle interaction forces were directly observed. The directly measured repulsive forces between the two particles followed the sequence of Li+ > Na+ > K+ > Cs+, and the force strength in solution with 10−4 mol L−1 Li+ was about 3.6 times that in solution with 10−4 mol L−1 Cs+. Correspondingly, the determined average Stern potential of montmorillonite and mica decreased in the sequence of Li+ > Na+ > K+ > Cs+, and the Stern potential in solution with 10−4 mol L−1 Li+ was about 2.58 times that in solution with 10−4 mol L−1 Cs+. Analyses of AFM measurements showed that the cation surface adsorption energy determined the ability of cations to shield the electric field around clay mineral particles, which led to the Hofmeister sequence of the Stern potentials of clay mineral particle and the electrostatic repulsive forces between the two clay mineral particles.

Applying a modified Donnan model to describe the surface complexation of chromate to iron oxyhydroxide agglomerates with heteromorphous pores

In this study, a modified Donnan model (mDM) is incorporated into surface complexation model (SCM) to better understand the physicochemical processes for adsorption of hexavalent chromium, Cr(VI), on porous iron oxyhydroxide agglomerates (IOAs). The mDM includes a chemical potential term μatt to account for ionic transport and electrostatic interaction in micropores (dmi<2nm) which is usually neglected in typical Donnan model (tDM). To estimate the parameters of mDM in a simple and accurate way, a rigorous protocol was presented. First, the prepared IOAs was characterized with a heteromorphous pore structure (i.e., co-existing micropores and macropores, dma>2nm) demonstrating high Cr(VI) adsorption in a broad range of ionic strengths. The batch data was then fitted with Donnan model in PHREEQC to obtain Stern (ψS) and Donnan (ψD) potentials used for μatt calculation. The decreasing μatt values with ionic strength indicated obstructing effect of electrolyte ions on Cr(VI) uptake in micropores. Finally, the ionic activity coefficients and reaction constants were corrected using Pitzer model due to the high level electrolytes accumulated in the Donnan layer through osmotic and electrostatic attraction. Results of this study have captured the effects of inner structure of IOAs on Cr(VI) uptake and quantitatively discerned the contribution of micropores and macropores for adsorption reactions at different ionic strengths.

Multi-salt coagulation of soft pitch colloids

The dynamic coagulation of colloidal pitch was quantified under orthokinetic conditions. The effects of single salt, multiple salt and cation valency on the stability of the colloidal pitch were investigated as a function of salt concentration. Critical coagulation concentrations (CCCs) were determined for a range of individual cations. The CCC in the presence of a number of divalent or trivalent cations was investigated to gain an understanding of the effect of multiple salts normally found in industrial systems.Electrostatic destabilisation of wood resin colloids by a single salt is strongly influenced by salt valency (z) and mostly independent of the individual cation (at constant z). Addition of a second cation to solution resulted in a decrease in the CCC for both calcium and aluminium ions in the presence of sodium ions. The decrease in the CCC for the wood resin colloids was non-linear and showed restabilisation of the colloids above the CCC, unlike the effects observed for a single salt. Comparison of the experimental CCC results with the DVLO theory indicates that a higher Hamaker constant than reported for the interaction of abietic acid with talc in water (used here as a model interaction) is needed. This suggests that the aggregation process of the wood resin colloids involves a stronger interaction than abietic acid with talc. Experimental CCC values for divalent and trivalent ions were also lower than those predicted theoretically. To obtain agreement between theory and experimental results, reduced Stern potentials and higher Hamaker constants were required, suggesting specific ion adsorption of the multivalent cations is occurring to reduce surface charge of the colloid.

On the origin of the Hofmeister effect in anion-selective potentiometric electrodes with tetraalkylammonium salts

Abstract A new, adsorption-based mechanism of potentiometric response of the plasticized PVC membranes is proposed. In contrast to previous mechanistic studies of the membrane-based Ion-Selective Electrodes (ISE), the membranes in this study did not contain any purposely-added additives: lipophilic salt or ionophore. Cationic or anionic response of such “blank” ISEs was observed upon contact with ionic surfactants: cationic cetyltrimethylammonium bromide (CTAB) and anionic sodium dodecyl sulfate (SDS), respectively. Such behavior cannot be explained using a classical Interface between Two Immiscible Electrolyte Solutions (ITIES) formalism, based on the Nernst equation for ion partitioning between the aqueous and the membrane phases. Moreover, the “blank” membranes responded anionically (with a negative slope) to the presence of sodium salts of bromide, nitrate and fluoride at constant concentration of CTAB. Also in this case, the theoretical Galvani potential difference vs. added salt concentration, calculated with the ITIES formalism did not fit well the measured potential differences. Better agreement was obtained when assuming that the potentiometric response stems from partitioning between the aqueous phase and the interface (adsorption), and not between the aqueous phase and the membrane phase (bulk partition), as in the classical approach. The Stern layer potentials were calculated from interfacial tension isotherms for CTAB, both alone and in the presence of sodium bromide, nitrate and fluoride using the Surface quasi-Two Dimensional Electrolyte model (STDE). The calculated Stern potentials compare well with those obtained experimentally using ISE.

Studies on Bitumen−Silica Interaction in Aqueous Solutions by Atomic Force Microscopy

The forces between spin-coated bitumen on a silica wafer and a silica particle in aqueous solutions were measured with an atomic force microscope. The effect of solution pH, salinity, divalent ion addition, and temperature on the interaction/adhesion forces was studied. The results showed that higher solution pH and temperature and lower salinity and calcium concentration resulted in a system of a stronger long-range repulsive force and a weaker adhesive force, which is favorable for bitumen detachment from the silica surface and the subsequent stabilization. The long-range interaction forces between bitumen and silica can be well described with the classical Derjaguin−Landau−Verwey−Overbeek theory, suggesting that the electrostatic forces play a dominant role in a bitumen−silica colloidal system. The best-fitted Stern potentials of bitumen and silica were in excellent agreement with the corresponding zeta potential values measured independently using an electrophoresis technique. An additional repulsive ...

Colloidal interactions: contact limiting laws, double-layer dissociation, and "non-DLVO" (Derjaguin–Landau–Verwey–Overbeek) forces

The conceptual basis and main results of a new model of the dissociative electrical double layer augmented with Lubetkin–Middleton–Ottewill (LMO) dissociative law are summarized. The main results are: the derivation of fundamental limiting laws of planar repulsions in the limit of surface contact, and their transition to the Debye–Huckel limiting laws at infinite separation; the prediction of stronger and longer-range electrostatic forces, quantitatively and consistently accounting for repulsions in colloidal montmorillonites; the discovery of the classical Debye–Huckel interionic effect being operative in increasing double-layer dissociation by lowering the activities of diffuse ions via the LMO law; the prediction of maxima of Stern potentials in spherical symmetry without any further assumptions, and the prediction of no maxima for platelike montmorillonites. In the light of these new insights, the concepts of "hydration forces" and other close-range "non-DLVO" forces may need to be revised, as well as the origin of the maxima of electrophoretic mobilities (Stern potentials) for spherical colloids.

Theory of colloid stability and particle nucleation kinetics in emulsion polymerization

The DLVO theory of colloid stability is applied to the uncatalyzed and catalyzed agglomeration of small primary particles formed in the earliest stages of emulsion polymerization in the manufacture of a carboxylic styrene/butadiene latex. The inverse 6th-power relationship between the stability ratio and the cation concentration revealed experimentally in an earlier work can be confirmed theoretically using variable Stern potentials. The Stern potential changes as parabolic function of log cation concentration. From the viewpoint of DLVO and Stern theory it is suggested that a spacer effect is crucial for the activity of the catalyst used.

Electrostatic calculations on colloids with planar and spherical charge distributions

A model of the electrical double layer has been derived by assuming that a fraction of double layer counter-ions remains associated with the charged surface (Nature 1984 310: 396). The magnitude of this fraction, i.e. the degree of counter-ion association, can be derived from experimental data. E.g. the electrostatic repulsions in clay gels can be quantitatively explained with the degree of counter-ion association that is independent of plate separation. The dependence of the degree of counter-ion association on bulk ionic strength is obtained from Lubetkin'set al data (Philos Trans R Soc London (1984) A311∶133), which permits the derivation of a new limiting law that predicts large and ionic strength independent repulsive forces at close surface separations. Such forces have been usually attributed to “hydration forces”. Within the DLVO framework the theory perdicts reasonable values of the equilibrium separations of swelling clays with divalent counter-ions. The Stern potentials of clays are also consistently accounted for. New calculations for spherical surfaces predict that Stern potentials can have maxima at certain ionic strengths. These maxima, which are often observed in the case of polymer latices, disappear in the limit of infinite radius.

Kinetics of heterocoagulation 1. A comparison of theory and experiment

Experimentally determined stability coefficients for the coagulation of hematite dispersions consisting of uniform spherical particles, and for the deposition of the same solids on glass beads, are compared with theoretically predicted values by the Hogg, Healy and Fuerstenau (HHF) approximation and the recently developed Overbeek formulation. Significant discrepancies are found between the measured coefficients for slow processes and those calculated by either the HHF or the Overbeek expressions. The effect of the linearization incorporated in the HHF approximation on the computation of stability coefficients is also assessed by the comparison with the Overbeek treatment. It is found that considerable deviations are generated when Ψ S1 ≈ Ψ S2 > 25 mV, where Ψ S1 and Ψ S2 are the Stern potentials for surfaces 1 and 2 respectively, whereas good agreements ensue when Ψ S1 ⪡ Ψ S2, even though Ψ S2 is as high as 100 mV.

Surface potential in charged synthetic amphiphile vesicles

The surface potential in charged synthetic amphiphile vesicles at low ionic strength is determined. pH and salt effects on vesicle size measured by photon correlation spectroscopy and particle microelectrophoresis agree with calculations from monolayer data using a self-assembly mode. {Xi}-potentials calculated using the O`Brien and White theory agree with estimated Stern potentials. 30 refs., 7 figs., 2 tabs.

Effect of sodium dodecyl sulfate concentration on the attachment between mercury and argon bubbles in aqueous solutions

The attachment between mercury and argon gas bubbles was investigated at various polarization potentials E (volt vs SCE) of a mercury electrode in aqueous sodium dodecyl sulfate (SDS) solutions in the presence of 0.01 M NaClO 4. It was found that the range of E for sponteneous attachment was about −0.3 ∼ −0.6 V when SDS concentration was less than 5 × 10 −6 M, while no sponteneous attachment was observed at SDS concentrations higher than 5 × 10 −6 M. Under the conditions of spontaneous attachment, the Stern potentials of both mercury and bubbles were negative and the surface coverages of DS − ions on the two surfaces were less than 10% and 2%, respectively, assuming a cross-sectional area of the alkyl chain of 18.5 Å 2. Experimental results are discussed in terms of the heterocoagulation theory. The existence of an additional attractive hydrophobic force is suggested and an attempt is made to reveal the dependence of the hydrophobic force on the surface coverage of the surfactant.

Acidic and reductive dissolution of magnetite in aqueous sulfuric acid: Site-binding model and experimental results

The open circuit dissolution of ionic metal oxides in mineral acids is modelled assuming that the rate is controlled by the transfer of metal ions in hydrolytic equilibrium with bulk metal ions, from the metal oxide surface to the Stern plane. The site-binding model of the double layer metal oxide/electrolyte solution is used to obtain the pH dependence of surface and Stern potentials. The nature of the active sites is discussed and their surface concentration is assumed to be proportional to suface charge σ 0. Again, the site-binding model is used to detemine the pH dependence of σ 0. It is thus shown that the rate order in c H + is essentially defined by the potential dependence of the charge transfer process, for oxides with points of zero charge near neutrality that dissolve in mildly or strongly acidic solutions. The role of surface complexation is also discussed in terms of the site-binding model and the difficulties in interpreting dissolution experiments under constant external applied potential are discussed in terms of the complexity of the semiconductor oxide/electrolyte solution interfacial region in magnetite. An experimental study of the open circuit dissolution of magnetite in sulfuric acid is presented and interpreted according to the proposed model. The reductive dissolution of magnetite is modelled by extension of the Valverde-Wagner model of oxide dissolution. Experimental results are presented to demonstrate that the reductive dissolution rate of magnetite in ferrous containing solutions is controlled by the rate of electron transfer from adsorbed Fe(II) to Fe(III) surface states of magnetite.

Colloid Stability of Clays Using Photon Correlation Spectroscopy

AbstractPhoton correlation spectroscopy (PCS), a dynamic light-scattering technique for particle size measurement, was used to determine the coagulation rates of aqueous dispersions of relatively monodisperse South Carolina Peerless kaolinite, Silver Hill, Montana, illite, Wyoming montmorillonite, and Florida palygorskite. This technique allows quantitative measurement of the rate of coagulation for clay particles where the traditional turbidity method gives only a qualitative measure. The critical coagulation concentrations for KCl at pH = 10.0 were: 0.199 M for kaolinite, 0.202 M for illite, 0.290 M for montmorillonite, and 0.034 M for palygorskite. The effective Hamaker constants, calculated using Derjaguin-Landau-Verwey-Overbeek theory, were: 3.1 × 10−20 J for kaolinite, 2.5 × 10−20 J for illite, 2.2 × 10−20 J for montmorillonite, and 1.63 × 10−19 J for palygorskite. Stern potentials at the critical coagulation concentration at pH 10.0 were: −42.7 mV for kaolinite, −40.7 mV for illite,‒21.2 mV for montmorillonite, and −66.9 mV for palygorskite.