DLVO Model Research Articles

Rheological and zeta potential behaviour of kaolin and bentonite composite slurries

The results of the yield stress–pH and zeta potential–pH behaviour of pure kaolin and bentonite slurries, and mixed bentonite–kaolin slurries were evaluated. A high and low Ca(II) content kaolin clays were used. The location of the pH of maximum yield stress of the two pure kaolin slurries is very different, pH 9 for the high Ca(II) kaolin and 2.5 for the low. The trend of the variation of the yield stress with pH for the bentonite–kaolin composite slurries was similar to that of pure high Ca(II) kaolin irrespective of bentonite content. For the low Ca(II) kaolin, the maximum yield stress of the composite suspensions is located at the same low pH as that of pure low Ca(II) kaolin and bentonite suspensions, at pH∼2.5. Bentonite content was found to determine the magnitude of the yield stress. The sequence of KOH addition, whether during or after the preparation stage, did have a significant effect on the yield stress but only for the composite slurries with a high bentonite content. A higher degree of swelling and delamination of the bentonite particles in one of the preparation methods was attributed as the cause. Zeta potential results showed that both the pure and mixed clays are negatively charged at all pH. However the magnitude is reduced by Ca (II). Low Ca(II) kaolin suspension was found to obey the yield stress–DLVO model. This study has provided an excellent approach of controlling and tailoring rheological properties of kaolin suspension with bentonite or vice versa for specific applications.

Forces of Interactions between Bare and Polymer-Coated Iron and Silica: Effect of pH, Ionic Strength, and Humic Acids

The interactions between a silica substrate and iron particles were investigated using atomic force microscopy-based force spectroscopy (AFM). The micrometer- and nanosized iron particles employed were either bare or coated with carboxymethyl cellulose (CMC), a polymer utilized to stabilize iron particle suspensions. The effect of water chemistry on the forces of interaction was probed by varying ionic strength (with 100 mM NaCl and 100 mM CaCl₂) or pH (4, 5.5, and 8) or by introducing 10 mg/L of humic acids (HA). When particles were uncoated, the forces upon approach between silica and iron were attractive at pH 4 and 5.5 and in 100 mM CaCl₂ at pH 8, but they were negligible in 100 mM NaCl buffered to pH 8 and repulsive in water buffered to pH 8 and in HA solutions. HA produced electrosteric repulsion between iron particles and silica, likely due to its sorption to iron particles. HA sorption to silica was excluded on the basis of experiments conducted with a quartz-crystal microbalance with dissipation monitoring. Repulsion with CMC-coated iron was attributed to electrosteric forces, which were damped at high ionic strength. An extended DLVO model and a modified version of Ohshima's theory were successfully utilized to model AFM data.

The Swelling Pressure of Na–Bentonite: Study with a Density Functional Approach

Abstract The combination of the fundamental measure theory (FMT) and the weighted correlation approach (WCA) within the framework of the density functional theory, i.e., the FMT/WCA approach, is applied to study the swelling pressure of Na–bentonite at different salt conditions. Good agreement between the FMT/WCA simulations and the results from both experiments and a Donnan-equilibrium-based DLVO model suggests that the hydration repulsion between charged surfaces can be well accounted by the ion–ion correlations effect.

The Electrochemical Surface Forces Apparatus: The Effect of Surface Roughness, Electrostatic Surface Potentials, and Anodic Oxide Growth on Interaction Forces, and Friction between Dissimilar Surfaces in Aqueous Solutions

We present a newly designed electrochemical surface forces apparatus (EC-SFA) that allows control and measurement of surface potentials and interfacial electrochemical reactions with simultaneous measurement of normal interaction forces (with nN resolution), friction forces (with μN resolution), and distances (with Å resolution) between apposing surfaces. We describe three applications of the developed EC-SFA and discuss the wide-range of potential other applications. In particular, we describe measurements of (1) force-distance profiles between smooth and rough gold surfaces and apposing self-assembled monolayer-covered smooth mica surfaces; (2) the effective changing thickness of anodically growing oxide layers with Å-accuracy on rough and smooth surfaces; and (3) friction forces evolving at a metal-ceramic contact, all as a function of the applied electrochemical potential. Interaction forces between atomically smooth surfaces are well-described using DLVO theory and the Hogg-Healy-Fuerstenau approximation for electric double layer interactions between dissimilar surfaces, which unintuitively predicts the possibility of attractive double layer forces between dissimilar surfaces whose surface potentials have similar sign, and repulsive forces between surfaces whose surface potentials have opposite sign. Surface roughness of the gold electrodes leads to an additional exponentially repulsive force in the force-distance profiles that is qualitatively well described by an extended DLVO model that includes repulsive hydration and steric forces. Comparing the measured thickness of the anodic gold oxide layer and the charge consumed for generating this layer allowed the identification of its chemical structure as a hydrated Au(OH)(3) phase formed at the gold surface at high positive potentials. The EC-SFA allows, for the first time, one to look at complex long-term transient effects of dynamic processes (e.g., relaxation times), which are also reflected in friction forces while tuning electrochemical surface potentials.

Surface-modified cerium oxide nanoparticles synthesized continuously in supercritical methanol: Study of dispersion stability in ethylene glycol medium

Dispersion stability of surface-modified cerium oxide (CeO 2) nanoparticles in ethylene glycol is examined and the experimental stability results are compared with an extended DLVO model consisting of electrostatic, van der Waals, and hydrophobic/hydrophilic interactions. Unmodified, decanoic acid-modified and oleic acid-modified CeO 2 nanoparticles are synthesized continuously in supercritical methanol (scMeOH). The surface charge of the surface-modified CeO 2 particles changes from positive to negative with an increment in the medium pH while the surface charge of the unmodified CeO 2 particle does not change with varying pH. Long-term dispersion stability test (up to 100 days) shows that the oleic acid-modified nanoparticle with a concentration of 0.3 M retains most stable dispersion in ethylene glycol. The unmodified and decanoic acid-modified nanoparticles with a concentration of 0.03 M precipitate within 7–15 days. In contrast, initial short-term stability evolution reveals different stability behavior compared to the long-term stability. The unmodified and the decanoic acid-modified nanoparticles with a concentration of 0.03 M were less attractive than the oleic acid-modified nanoparticle with 0.3 M. The experimental short-term stability data is in good agreement with the computational results of energy profiles for the CeO 2 nanoparticle suspension.

Silver nanoparticles in simulated biological media: a study of aggregation, sedimentation, and dissolution

Nanoparticles, the building blocks of many engineered nanomaterials, can make their way into the environment or into organisms, either accidentally or purposefully. The intent of this study is to provide some insight into the complex environmental, health, and safety issues associated with engineered nanomaterials. In particular, here the state of commercially manufactured silver nanoparticles—i.e., will silver nanoparticles be present as isolated particles, agglomerates, or dissolved ions—in two simulated biological media is explored. Two different commercially manufactured silver nanoparticle samples, one that has been surface modified with a thick polymer coating to render them more water-soluble and the other, with a sub-nanometer surface layer, are studied. The experimental results and the extended DLVO model calculations show that silver nanoparticles have a propensity to settle out in high ionic strength media independent of surface modification. Furthermore, single nanoparticles as well as aggregates/agglomerates are present together in these solutions. Silver ion release in these simulated biological buffers with pHs of 4.5 and 7.4 is negligible after 96 h.

Mathematical modeling of DNA-mediated selective aggregation of CdS quantum dots

We report a mathematical modeling of the DNA-mediated selective aggregation of CdS quantum dots. Addition of hybridized double-stranded DNAs into the suspensions of CdS quantum dots at the optimal salt concentration causes a selective aggregation and a fluorescence-quenching phenomenon depending on the target DNA sequence. We monitor the aggregation process with quasi-elastic light scattering (QELS), zeta potential, and conductivity measurements at different salt concentrations. To model the aggregation process, we use the constant-number Monte Carlo method with the aggregation kernel that accounts for the interparticle interaction from the classical DLVO model. We find that the calculated results are in good agreement with the experiments, and that the fractal dimension of the quantum dot aggregates is 2.3. Modeling also allows us to estimate that the total number of initial quantum dots in aggregates at the beginning of the fluorescence-quenching phenomenon is approximately 200. The insights gained in this study should be useful in the design of biosensors based on the fluorescence-quenching phenomenon caused by the quantum dot aggregation.

Fluorescence quenching caused by aggregation of water-soluble CdSe quantum dots

We investigate the fluorescence quenching phenomenon that water-soluble CdSe quantum dots exhibit upon aggregation. First, we synthesize the hydrophobic CdSe quantum dots and convert them water-soluble using surface-ligand exchange with mercaptoacetic acid. We then induce aggregation of CdSe quantum dots by adding electrolyte to suspension. To monitor the change in photochemical and physical properties upon aggregation, various experimental techniques such as quasi-elastic light scattering (QELS), transmission electron microscopy (TEM), zeta potential measurement, and UV–vis and photoluminescence spectroscopy are employed. We find that addition of electrolyte to water-soluble quantum dot suspension reduces the colloidal stability of quantum dots, causing aggregation and hence the fluorescence quenching of quantum dots. Increasing electrolyte concentration accelerates aggregation of quantum dots, causing faster fluorescence quenching phenomenon. We rationalize the experimental results with the classical DLVO model that accounts for the interparticle interaction and colloidal stability in suspension. We expect that the fluorescence quenching phenomenon caused by quantum dot aggregation can be applied in many fields including molecular sensing and imaging probes.

Colloidal Dispersion Stability of CuPc Aqueous Dispersions and Comparisons to Predictions of the DLVO Theory for Spheres and Parallel Face-to-Face Cubes

The dispersion stability and the zeta potentials of nonspherical crystalline (beta-form) copper phthalocyanine (CuPc) particles of hydrodynamic diameter d(h) approximately = 90 nm were investigated at 25 degrees C in water and in aqueous solutions of NaNO(3). The electrolyte concentrations c ranged from 1 to 500 mM and the particle concentrations ranged from 50 to 10,000 ppm (0.005 to 1 wt %). In each case, the Fuchs-Smoluchowski stability ratio W was determined from dynamic light scattering (DLS) data and the Rayleigh-Debye-Gans (RDG) scattering theory. The data suggest that electrostatic effects play a major role in the stability of CuPc-based dispersions. The calculated particle charge z per CuPc particle based on the zeta potential data and the area of the particles (assumed to be cubical) suggests that there is preferential adsorption of NO(3)(-) ions on the uncharged CuPc surface, and the surface charge increases with increasing electrolyte concentration. Furthermore, two new models of the DLVO theory, for spheres and for parallel face-to-face cubes, were reformulated in dimensionless form. The Hamaker constant of CuPc particles was calculated by the same authors on the basis of theoretical models in J. Chem. Theory Comput. (2010, in press). The key dimensionless group is the ratio N of the electrostatic double layer energy to the Hamaker constant. The two DLVO models were used to predict the value of a dimensionless maximum potential energy Phi(max), the conditions when it may exist, and then the value of W. In water, the DLVO model for spheres overpredicted the stability, while the model for cubes underpredicted the stability. At c = 1 mM, both models overpredicted the stability. At c = 10 mM, the model for spheres underpredicted the stability, whereas the model for cubes overpredicted the stability. Hence, there seems to be some significant shape effects on the electrostatic stabilization of CuPc particle dispersions. At c = 100 and 500 mM, both models underpredicted the stability substantially, suggesting the existence of additional short-range repulsive forces, which may primarily control the stability. Simple sensitivity analysis on the calculations supported these conclusions.

Mechanism of the Cooperative Adsorption of Oppositely Charged Nanoparticles

Quartz crystal microbalance experiments were performed to study the kinetics of surface adsorption from solutions containing oppositely charged nanoparticles. A theoretical model was developed according to which formation of dense nanoparticle (NP) monolayers is driven by a cooperative process, in which the already-adsorbed NPs facilitate adsorption of NPs from solution. The kinetic rate constants change with the NP solution concentration and can be used to backtrack adsorption free energies. These energies agree with the predictions of a simple DLVO model.

Probing Surface Charge Potentials of Clay Basal Planes and Edges by Direct Force Measurements

The dispersion and gelation of clay suspensions have major impact on a number of industries, such as ceramic and composite materials processing, paper making, cement production, and consumer product formulation. To fundamentally understand controlling mechanisms of clay dispersion and gelation, it is necessary to study anisotropic surface charge properties and colloidal interactions of clay particles. In this study, a colloidal probe technique was employed to study the interaction forces between a silica probe and clay basal plane/edge surfaces. A muscovite mica was used as a representative of 2:1 phyllosilicate clay minerals. The muscovite basal plane was prepared by cleavage, while the edge surface was obtained by a microtome cutting technique. Direct force measurements demonstrated the anisotropic surface charge properties of the basal plane and edge surface. For the basal plane, the long-range forces were monotonically repulsive within pH 6-10 and the measured forces were pH-independent, thereby confirming that clay basal planes have permanent surface charge from isomorphic substitution of lattice elements. The measured interaction forces were fitted well with the classical DLVO theory. The surface potentials of muscovite basal plane derived from the measured force profiles were in good agreement with those reported in the literature. In the case of edge surfaces, the measured forces were monotonically repulsive at pH 10, decreasing with pH, and changed to be attractive at pH 5.6, strongly suggesting that the charge on the clay edge surfaces is pH-dependent. The measured force profiles could not be reasonably fitted with the classical DLVO theory, even with very small surface potential values, unless the surface roughness was considered. The surface element integration (SEI) method was used to calculate the DLVO forces to account for the surface roughness. The surface potentials of the muscovite edges were derived by fitting the measured force profiles with the surface element integrated DLVO model. The point of zero charge of the muscovite edge surface was estimated to be pH 7-8.

Fractal aggregates induced by liposome-liposome interaction in the presence of Ca2+

We present a study of the fractal dimension of clusters of large unilamellar vesicles (LUVs) formed by egg yolk phosphatidylcholine (EYPC), dimyristoylphosphocholine (DMPC) and dipalmitoylphosphocholine (DPPC) induced by Ca2+ . Fractal dimensions were calculated by application of two methods, measuring the angular dependency of the light scattered by the clusters and following the evolution of the cluster size. In all cases, the fractal dimensions fell in the range from 2.1 to 1.8, corresponding to two regimes: diffusion-limited cluster aggregation (DLCA) and reaction-limited cluster aggregation (RLCA). Whereas DMPC clusters showed a typical transition from the RLCA to the DLCA aggregation, EYPC exhibited an unusual behaviour, since the aggregation was limited for a higher concentration than the critical aggregation concentration. The behaviour of DPPC was intermediate, with a transition from the RLCA to the DLCA regimes with cluster sizes depending on Ca2+ concentration. Studies on the reversibility of the aggregates show that EYPC and DPPC clusters can be re-dispersed by dilution with water. DMPC does not present reversibility. Reversibility is evidence of the existence of secondary minima in the DLVO potential between two liposomes. To predict these secondary minima, a correction of the DLVO model was necessary taking into account a repulsive force of hydration.

Different Interactions between the Two Sides of Purple Membrane with Atomic Force Microscope Tip

Atomic force microscopy (AFM) is known to be capable of measuring local surface charge density based on the DLVO model. However, it has failed to distinguish charge density difference between the extracellular and cytoplasmic sides of purple membrane (PM) in previous studies. In this paper, tapping-mode AFM with thioglycolate-modified tips was used to image PM in buffers of different salt concentrations. When imaged in 25 mM KCl buffer, the topography of membranes appeared to be of two different types, one flat and the other domelike. Such a difference was not observed in buffers of high salt concentrations. This suggests that the topography variation results from differences in electrostatic interaction between the AFM tip and the different membrane surfaces. With images of papain-digested PM and high-resolution images of membrane surface structure, we proved that the membrane surfaces with flat topography were on the extracellular side while the surfaces with domelike topography were on the cytoplasmic side. Hence, this provides a straightforward method to distinguish the two sides of PM without the requirement of high-resolution imaging. Force-distance curves clearly demonstrated the different tip-sample interactions. The force curves recorded on the extracellular side of PM were consistent with the DLVO model, so its surface charge density can be estimated well. However, the curves recorded on the cytoplasmic side had a much longer decay length, which is supposed to be relevant to the flexibility of the C-terminus of bacteriorhodopsin (bR).

Temperature effect on the stability of bentonite colloids in water

The stability of natural bentonite suspensions has been investigated as a function of temperature at pH 9 and ionic strength 10 −3 M. The sedimentation rate of the particles is directly related to their stability. The sedimentation kinetics was determined by examining the variation of particle concentration in solution with time. The observed kinetics for sedimentation is discussed quantitatively in terms of the potential energy between particles. The ζ-potential of the particles was measured and the DLVO theory was used to calculate attractive and repulsive potentials. Experimental observations are consistent with DLVO model predictions and show that the stability of bentonite colloids increases with temperature. Differences with other colloidal systems can be attributed to the temperature dependence of the surface charge of bentonite particles.

Small-Angle X-ray Scattering and Light Scattering on Lysozyme and Sodium Glycocholate Micelles

Small-angle X-ray scattering (SAXS) together with static (SLS) and dynamic light scattering (DLS) measurements were carried out on aqueous solutions of lysozyme (LY) and of the ionic biological detergent sodium glycocholate (NaGC). Apparent diffusion coefficients (D app), excess Rayleigh ratio, and SAXS spectra were measured for 0.1 M NaGC solutions at different ionic strengths (0.05-0.30 M NaCl). The same data were collected for LY in sodium acetate buffer 50 mM without and with 92 mM NaCl as a function of protein concentration (10-80 g L(-1)). A correlated analysis of SLS data and SAXS spectra was first tested on the LY samples and then extended to the interpretation of the NaGC data to infer information on particle structure and interaction potential. A hard-core (HC) interaction shell of uniform thickness, a screened Coulomb potential of the electric double layer (EDL) or the complete DLVO potential were alternatively used to represent the long-range tail of the interaction potential. Whenever an essentially repulsive tail is expected, all the representations give reasonable results, but the data analysis does not allow the discrimination between the oblate and the prolate symmetries of the NaGC aggregates. The DLVO model allows the interpretation of the data even when the attractive component determines the tail character. With this model an overall fit of the micelle data at all the NaCl concentrations was successfully performed by assuming a simple spherical symmetry of the micelles and invariant values of their ionization degree and Hamaker constant, thus considering just the screening effect of the added electrolyte. Whatever model is used, the results point out that the aggregates are quite hydrated (26-38 water molecules per monomer) and very slightly grow by increasing the NaCl concentration. When spherical symmetry is assumed the aggregate radii for all the samples fall in the range 15-16 A. From the SAXS and SLS, best fitting geometrical parameters, and interparticle structure factor, a D app value was calculated for each sample. An excellent consistence is achieved for LY results. On the contrary, calculated D app values systematically lower than the experimental values are always obtained for the NaGC micelles. Micelle polydispersity and internal dynamics seem to be the most probable reasons of the bad agreement.

Vibrational spectroscopic studies of sol–gel derived physical and chemical bonded ORMOSILs

Fourier transform infrared (FT-IR) and FT-Raman Spectroscopy have been utilized to characterize the structure of physical and chemical bonded ORMOSILs (organically modified silicates) and to compare with the physical properties, reported earlier. The classic and sono ORMOSILs were synthesized by sol–gel method via traditional- and sono-catalysis routes, respectively. The physical and chemical bonding is established by Poly(ethylene glycol) (PEG) and Poly(dimethoxysilane) (PDMS), respectively. Existence of band at ∼560 cm −1 (FT-IR) and ∼490 cm −1 (FT-Raman) for the sono and classic gels with 5–10 wt% of PEG indicates the presence of structural defect due to the four-membered siloxane rings. Application of long molecular PEG chain and PDMS increases this defect. Modification in the intensity and peak position of siloxane (Si–O–Si) bands is found correlative with the physical properties. The xerogels with 5–10 wt% of PEG possess a large number of residual surface silanol groups than the xerogels with long molecular PEG chains and PDMS and is found related with the ν(OH) band at ∼3470 cm −1. These results are attributed to the way of hydrolysis–condensation reactions and discussed with the help of Percolation and DLVO models.

Modeling Effective Interactions of Micellar Aggregates of Ionic Surfactants with the Gauss-Core Potential

Micellar aggregates of ionic surfactants are known to possess a rich variety of interesting thermodynamic as well as structural properties, which are essentially dominated by simple effective interactions between the aggregates. Because of their technological relevance enormous efforts have been invested to understand and characterize their interactions in solution with the goal of developing substances with novel material’s properties. On a theoretical level several approaches have been proposed to describe their effective interactions adequately, generally based on the DLVO theory. However, these approaches do not take into account aspects of stability of the aggregates and therefore fail in the description of several important characteristics, such as, e.g., the re-entrant behavior of the apparent molal heat capacity appearing with increasing density of the micelles. In this paper we study the effective interactions of these systems by investigating the suitability of the Gauss-core model, to reproduce the relevant thermodynamic properties. To this end, we discuss the Gauss-core model in comparison to the standard DLVO model and demonstrate its aptitude to reproduce the results from calorimetric experiments of the ionic surfactant sodium decanoate in water.

Size Distribution and Intermolecular Interaction of Laminin-1 in Physiological Solutions

The diffusion coefficient and hydrodynamic radius of laminin-1 (LN-1) in aqueous solutions were investigated by using a dynamic light scattering (DLS) technique. The autocorrelation functions could be analyzed assuming two relaxation modes, fast and slow, with exponential decay. We primarily focused on the fast mode, because the mass of it represented 99% of the total mass. The unit corresponding to the fast mode had a hydrodynamic radius of 20.6 ± 0.5 nm in both tris- and MOPS-buffered solutions of 150 mM NaCl and pH 7.2 at 25 °C. Intermolecular interactions between these essential units were attractive, and the Hamaker constant in tris-buffered solution was estimated as 5.2 kBT according to DLVO theory. When the magnesium was added to the tris-buffered solutions, the attractive force strongly increased with increasing magnesium concentration. This change was not explained by the charge-screening effect of magnesium ions in the framework of the classical DLVO model.

Characterization of Particle Interaction Energy through Incipient Turbulent Aggregation

A technique for estimating the particle–particle interaction energy in a colloidal system through incipient aggregation in an isotropic turbulent field is proposed. With this technique, we first determine experimentally the critical (i.e., minimum) energy dissipation rate that induces aggregation in a given stable colloidal system. Then, assuming DLVO interparticle potentials, we can estimate the surface charge and potential by determining the critical relative trajectory of two particles leading to aggregation. It is worth noting that the interparticle interaction potential estimated by this technique accounts in an effective manner also for all the other non-DLVO interparticle interactions that may be present in the system. In other words, using the surface charge value estimated by this technique in the frame of a DLVO model allows us to reproduce correctly the conditions under which the colloidal system was found experimentally to undergo aggregation in a turbulent field. An experimental verification of the proposed technique is shown using a fluorinated polymer latex. It was found that the obtained value of the surface charge is very close to that measured by conductometric titration and is practically constant in the investigated range of particle volume fraction and ionic strength values.

A review of non-DLVO interactions in environmental colloidal systems

The interaction and behavior of surfaces orcolloids is of quantitative significance inunderstanding the transport and fate ofcompounds and microorganisms in environmentalsystems. Historically, the DLVO model ofcolloid stability has described theseinteractions. This model finds its basis in aforce (energy) balance that comprisesattractive van der Waals and repulsiveelectrostatic interactions. Recently, the DLVOmodel has been found unable to fully describebiotic and abiotic colloidal behavior inaqueous media. The suspending phase (commonlywater) is often treated as a force (energy)transmitting or propagating medium. It isreasonable to believe that the structure ofwater may participate in a more significantfashion. Moreover, other moieties (sorbed anddissolved) may also have non-DLVO effects. Significant work has been focused on extendingthe precepts of the traditional DLVO model toaccommodate these non-DLVO forces (energies). This paper reviews many of the interactionsthat play a role in environmental systems andare not commonly subsumed by the traditionalDLVO model: e.g., hydrogen bonding and thehydrophobic effect, hydration pressure,non-charge transfer Lewis acid baseinteractions, and steric interactions.