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Now showing items 1 - 16 of 94

  • Heteroaggregation between Charged and Neutral Particles

    Trefalt, Gregor   Cao, Tianchi   Sugimoto, Takuya   Borkovec, Michal  

    Experimentally determined heteroaggregation rates between charged and neutral colloidal particles are reported for the first time. Different positively and negatively charged polystyrene latex particles are investigated. The neutral particles are obtained through adsorption of an appropriate amount of oppositely charged additives, such as aliphatic oligoamines, iron cyanide complexes, or alkyl sulfates. Heteroaggregation rates were measured with time-resolved multiangle light scattering. One observes that heteroaggregation between charged and neutral particles is always fast and diffusion controlled. These experimental values are compared with calculations of the Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory, whereby one finds that this heteroaggregation process is highly sensitive to charge regulation conditions. The comparison with experiments shows unambiguously that the surface of the neutral particles regulates strongly and probably behaves close to a constant potential surface. This observation is in line with direct force measurements on similar systems and further agrees with the fact that for neutral surfaces the capacitance of the diffuse layer is expected to be much smaller than the one of the inner layer.
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  • Forces between solid surfaces in aqueous electrolyte solutions

    Smith, Alexander M.   Borkovec, Michal   Trefalt, Gregor  

    This review addresses experimental findings obtained with direct force measurements between two similar or dissimilar solid surfaces in aqueous electrolyte solutions. Interpretation of these measurements is mainly put forward in terms of the classical theory of Derjaguin, Landau, Verwey, and Overbeek (DLVO). This theory invokes a superposition of attractive van der Waals forces and repulsive double layer forces. DLVO theory is shown to be extremely reliable, even in the case of multivalent ions. However, such a description is only successful, when appropriate surface charge densities, charge regulation characteristics, and ion pairing or complexation equilibria in solution are considered. Deviations from DLVO theory only manifest themselves at distances of typically below few nm. More long-ranged non-DLVO forces can be observed in some situations, particularly, in concentrated electrolyte solutions, in the presence of strongly adsorbed layers, or for hydrophobic surfaces. The latter forces probably originate from patch-charge surface heterogeneities, which can be induced by ion-ion correlation effects, charge fluctuations, or other types of surface heterogeneities. (C) 2019 Elsevier B.V. All rights reserved.
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  • Measuring Inner Layer Capacitance with the Colloidal Probe Technique

    Smith, Alexander M.   Maroni, Plinio   Borkovec, Michal   Trefalt, Gregor  

    The colloidal probe technique was used to measure the inner layer capacitance of an electrical double layer. In particular, the forces were measured between silica surfaces and sulfate latex surfaces in solutions of monovalent salts of different alkali metals. The force profiles were interpreted with Poisson-Boltzmann theory with charge regulation, whereby the diffuse layer potential and the regulation properties of the interface were obtained. While the diffuse layer potential was measured in this fashion in the past, we are able to extract the regulation properties of the inner layer, in particular, its capacitance. We find systematic trends with the type of alkali metal ion and the salt concentration. The observed trends could be caused by difference in ion hydration, variation of the binding capacitance, and changes of the effective dielectric constant within the Stern layer. Our results are in agreement with recent experiments involving the water-silica interface based on a completely independent method using X-ray photoelectron spectroscopy in a liquid microjet. This agreement confirms the validity of our approach, which further provides a means to probe other types of interfaces than silica.
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  • Attractive non-DLVO forces induced by adsorption of monovalent organic ions

    Smith, Alexander M.   Maroni, Plinio   Borkovec, Michal  

    Direct force measurements between negatively charged colloidal particles were carried out using an atomic force microscope (AFM) in aqueous solutions containing monovalent organic cations, namely tetraphenylarsonium (Ph4As+), 1-hexyl-3-methylimidazolium (HMIM+), and 1-octyl-3-methylimidazolium (OMIM+). These ions adsorb to the particle surface, and induce a charge reversal. The forces become attractive at the charge neutralization point, but they are stronger than van der Waals forces. This additional and unexpected attraction decays exponentially with a decay length of a few nanometers, and is strikingly similar to the one previously observed in the presence of multivalent ions. This attractive force probably originates from coupled spontaneous charge fluctuations on the respective surfaces as initially suggested by Kirkwood and Shumaker.
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  • The persistence length of adsorbed dendronized polymers

    Grebikova, Lucie   Kozhuharov, Svilen   Maroni, Plinio   Mikhaylov, Andrey   Dietler, Giovanni   Schluter, A. Dieter   Ullner, Magnus   Borkovec, Michal  

    The persistence length of cationic dendronized polymers adsorbed onto oppositely charged substrates was studied by atomic force microscopy (AFM) and quantitative image analysis. One can find that a decrease in the ionic strength leads to an increase of the persistence length, but the nature of the substrate and of the generation of the side dendrons influence the persistence length substantially. The strongest effects as the ionic strength is being changed are observed for the fourth generation polymer adsorbed on mica, which is a hydrophilic and highly charged substrate. However, the observed dependence on the ionic strength is much weaker than the one predicted by the Odijk, Skolnik, and Fixman (OSF) theory for semi-flexible chains. Low-generation polymers show a variation with the ionic strength that resembles the one observed for simple and flexible polyelectrolytes in solution. For high-generation polymers, this dependence is weaker. Similar dependencies are found for silica and gold substrates. The observed behavior is probably caused by different extents of screening of the charged groups, which is modified by the polymer generation, and to a lesser extent, the nature of the substrate. For highly ordered pyrolytic graphite (HOPG), which is a hydrophobic and weakly charged substrate, the electrostatic contribution to the persistence length is much smaller. In the latter case, we suspect that specific interactions between the polymer and the substrate also play an important role.
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  • Quantitative Nano-characterization of Polymers Using Atomic Force Microscopy

    Radiom, Milad   Kozhuharov, Svilen   Kong, Phally   di Giannantonio, Michela   Ayer, Mathieu A.   Maroni, Plinio   Kilbinger, Andreas F. M.   Fromm, Katharina M.   Weder, Christoph   Borkovec, Michal  

    The present article offers an overview on the use of atomic force microscopy (AFM) to characterize the nanomechanical properties of polymers. AFM imaging reveals the conformations of polymer molecules at solid liquid interfaces. In particular, for polyelectrolytes, the effect of ionic strength on the conformations of molecules can be studied. Examination of force versus extension profiles obtained using AFM-based single molecule force spectroscopy gives information on the entropic and enthalpic elasticities in pN to nN force range. In addition, single molecule force spectroscopy can be used to trigger chemical reactions and transitions at the molecular level when force-sensitive chemical units are embedded in a polymer backbone.
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  • Measuring slow heteroaggregation rates in the presence of fast homoaggregation

    Cao, Tianchi   Trefalt, Gregor   Borkovec, Michal  

    Homoaggregation and heteroaggregation involving amidine and sulfate latex particles in the presence of the anionic surfactant octyl sulfate (OS) is studied by light scattering. This surfactant causes a charge reversal of the amidine particles. This reversal induces a rapid homoaggregation near the charge reversal point. In the presence of the same surfactant, the sulfate particles remain negatively charged and stable. The heteroaggregation process is probed in mixed suspensions of amidine and sulfate latex particles with multi-angle time-resolved dynamic light scattering. This technique allows differentiating between the contributions of homoaggregation and heteroaggregation, and permits to measure the heteroaggregation rate. By optimally choosing the sizes of the particles, one can optimize the contrast and extract heteroaggregation stability ratio over a wide range. The heteroaggregation rate is fast at low OS concentrations, where the two particles are oppositely charged. This rate slows down at higher OS concentrations due to double layer repulsion between the negatively charged particles. However, the onset of this slow heteroaggregation occurs at lower OS concentrations than for homoaggregation. The reason for this shift is that the double layer repulsion between two OS-decorated amidine particles is weaker than between one sulfate particle and one OS-decorated amidine particle. These measurements compare favorably with calculations with the theory by Derjaguin, Landau, Verwey, and Overbeek (DLVO). These calculations suggest that constant potential boundary conditions are more appropriate than the ones of constant charge. In the system studied, the present light scattering technique permits to extract heteroaggregation stability ratios over almost three orders of magnitude. This study is the first of its kind, where such a large range is being probed. (C) 2020 Elsevier Inc. All rights reserved.
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  • The intrinsic view of ionization equilibria of polyprotic molecules

    Borkovec, Michal   Koper, Ger J. M.   Spiess, Bernard  

    The intrinsic approach describing microscopic ionization equilibria is presented. This description massively reduces the number of parameters needed to characterize microequilibria. Particularly, by exploring molecular symmetries and group transferability, this approach is capable of resolving such equilibria even for rather complex molecules. Intrinsic constants are assigned to each ionizable group and interactions between these sites are introduced. These interactions involve pairs or triplets of sites. The strength of these interactions decreases rapidly with the distance between the sites. Once these parameters are known, one can obtain macroconstants, microconstants, microstate mole fractions, and overall or site-specific titration curves. These quantities provide insight into the protonation of the molecules in question. The knowledge of such properties is relevant for a wide range of phenomena, including receptor ligand interactions, action of drugs, or geochemical processes.
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  • Mechanism of chitosan adsorption on silica from aqueous solutions.

    Tiraferri, Alberto   Maroni, Plinio   Rodriguez, Diana Caro   Borkovec, Michal  

    We present a study of the adsorption of chitosan on silica. The adsorption behavior and the resulting layer properties are investigated by combining optical reflectometry and the quartz crystal microbalance. Exactly the same surfaces are used to measure the amount of adsorbed chitosan with both techniques, allowing the systematic combination of the respective experimental results. This experimental protocol makes it possible to accurately determine the thickness of the layers and their water content for chitosan adsorbed on silica from aqueous solutions of varying composition. In particular, we study the effect of pH in 10 mM NaCl, and we focus on the influence of electrolyte type and concentration for two representative pH conditions. Adsorbed layers are stable, and their properties are directly dependent on the behavior of chitosan in solution. In mildly acidic solutions, chitosan behaves like a weakly charged polyelectrolyte, whereby electrostatic attraction is the main driving force for adsorption. Under these conditions, chitosan forms rigid and thin adsorption monolayers with an average thickness of approximately 0.5 nm and a water content of roughly 60%. In neutral solutions, on the other hand, chitosan forms large aggregates, and thus adsorption layers are significantly thicker (10 nm) as well as dissipative, resulting in a large maximum of adsorbed mass around the pK of chitosan. These films are also characterized by a substantial amount of water, up to 95% of their total mass. Our results imply the possibility to produce adsorption layers with tailored properties simply by adjusting the solution chemistry during adsorption. =20
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  • The intrinsic view of ionization equilibria of polyprotic molecules

    Borkovec, Michal   Koper, Ger J. M.   Spiess, Bernard  

    The intrinsic approach describing microscopic ionization equilibria is presented. This description massively reduces the number of parameters needed to characterize microequilibria. Particularly, by exploring molecular symmetries and group transferability, this approach is capable of resolving such equilibria even for rather complex molecules. Intrinsic constants are assigned to each ionizable group and interactions between these sites are introduced. These interactions involve pairs or triplets of sites. The strength of these interactions decreases rapidly with the distance between the sites. Once these parameters are known, one can obtain macroconstants, microconstants, microstate mole fractions, and overall or site-specific titration curves. These quantities provide insight into the protonation of the molecules in question. The knowledge of such properties is relevant for a wide range of phenomena, including receptor ligand interactions, action of drugs, or geochemical processes.
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  • Probing Colloidal Particle Aggregation by Light Scattering

    Trefalt, Gregor   Szilagyi, Istvan   Oncsik, Tamas   Sadeghpour, Amin   Borkovec, Michal  

    The present article reviews recent progress in the measurement of aggregation rates in colloidal suspensions by light scattering. Time-resolved light scattering offers the possibility to measure absolute aggregation rate constants for homoaggregation as well as heteroaggregation processes. We further discuss the typical concentration dependencies of the aggregation rate constants on additives. Addition of simple salts containing monovalent counterions leads to screening of the electrostatic repulsion of the charged particles and a transition from slow to rapid aggregation. Addition of salts containing multivalent counterions may lead to a charge reversal, which results in a sequence of two instability regions. Heteroaggregation rates between oppositely charged particles decrease with increasing salt level. This decrease is caused by screening of the electrostatic attraction between these particles.
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  • Recording stretching response of single polymer chains adsorbed on solid substrates

    Grebikova, Lucie   Radiom, Milad   Maroni, Plinio   Schluter, A. Dieter   Borkovec, Michal  

    Various techniques to measure the force-extension relationship of individual polymer chains with the atomic force microscope (AFM) are compared. Reliable stretching force profiles can be obtained with the nano-handling technique, which involves imaging of the measured individual polymer molecules prior and after the force experiment. Results originating from the classical pulling technique, which relies on dense adsorbed polymer layers, must be interpreted with care. Comparison with the nano-handling technique reveals that the stretching response of individual molecules can be obtained with the pulling technique, provided the adsorbed polymer layer is dilute. For denser adsorbed layers, such experiments may not reflect the correct stretching response. The discrepancies seem to be related to polymer epolymer interactions and entanglements within the adsorbed layer. AFM imaging of the adsorbed layers can provide an indication concerning the relevance of such entanglement effects. Similar caveats apply to fishing experiments, where polymers are adsorbed to the AFM tip and not to the substrate. (C) 2016 Elsevier Ltd. All rights reserved.
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  • Heteroaggregation of oppositely charged particles in the presence of multivalent ions

    Cao, Tianchi   Sugimoto, Takuya   Szilagyi, Istvan   Trefalt, Gregor   Borkovec, Michal  

    Time-resolved dynamic light scattering is used to measure absolute heteroaggregation rate coefficients and the corresponding stability ratios for heteroaggregation between amidine and sulfate latex particles. These measurements are complemented by the respective quantities for the homoaggregation of the two systems and electrophoresis. Based on the latter measurements, the stability ratios are calculated using Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. In monovalent salt solutions, the two types of particles investigated are oppositely charged. In the presence of multivalent ions, however, one particle type reverses its charge, while the charge of the other particle type is hardly affected. In this region, the heteroaggregation stability ratio goes through a pronounced maximum when plotted versus concentration. This region of slow aggregation is wider than the one observed in the corresponding homoaggregation process. One also finds that the onset of this region sensitively depends on the boundary conditions used to calculate the double layer force. The present results are more in line with constant potential boundary conditions.
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  • Towards Angstrom Resolution with Dynamic Light Scattering

    Seyrek, Emek   Hierrezuelo, Jose   Szilagyi, Istvan   Sadeghpour, Amin   Borkovec, Michal  

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  • Colloidal Stability in Asymmetric Electrolytes:Modifications of the Schulze-Hardy Rule

    Trefalt, Gregor   Szilagyi, Istvan   Tellez, Gabriel   Borkovec, Michal  

    The Schulze Hardy rule suggests a strong dependence of the critical coagulation concentration (CCC) on the ionic valence. This rule is addressed theoretically and confronted with recent experimental results. The commonly presented derivation of this rule assumes symmetric electrolytes and highly charged particles. Both assumptions are incorrect. Symmetric electrolytes containing multivalent ions are hardly soluble, and experiments are normally carried out with the well-soluble salts of asymmetric electrolytes containing monovalent and multivalent ions. In this situation, however, the behavior is completely different whether the multivalent ions represent the counterions or co-ions. When these ions represent the counterions, meaning that the multivalent ions have the opposite sign than the charge of the particle, they adsorb strongly to the particles. Thereby, they progressively reduce the magnitude of the surface charge with increasing valence. In fact, this dependence of the charge density on the counterion valence is mainly responsible for the decrease of the CCC with the valence. In the co-ion case, where the multivalent ions have the same sign as the charge of the particle, the multivalent ions are repelled from the particles, and the surfaces remain highly charged. In this case, the inverse Schulze Hardy rule normally applies, whereby the CCC varies inversely proportional to the co-ion valence.
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  • Adsorption of poly(L-lysine) on silica probed by optical reflectometry

    Jiang, Min   Popa, Ionel   Maroni, Plinio   Borkovec, Michal  

    Optical reflectometry was used to study adsorption behavior of poly(L-lysine) (PLL) on the water-silica interface for different pH and concentrations of the monovalent salt NaBr. At low salt level and low pH, the adsorbed amount increases with increasing salt concentration and pH This adsorption behavior can be qualitatively explained by electrostatic Interactions between the negatively charged substrate and the positively charged polyelectrolyte. At higher salt levels and pH, the adsorbed amount passes through a maximum. This maximum is probably related to the competition between adsorbing monovalent canons and the polyelectrolyte. The initial adsorption kinetics is mainly controlled by the mass transport to the surface (C) 2010 Elsevier B.V. All rights reserved
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