Impact of surface charges on the solvation forces in confined colloidal solutions

Combining computer simulations and experiments we address the impact of charged surfaces on the solvation forces of a confined, charged colloidal suspension (slit-pore geometry). Investigations based on the colloidal-probe atomic-force-microscope technique indicate that an increase in surface charges markedly enhances the oscillations of the force in terms of their amplitude. To understand this effect on a theoretical level we perform grand-canonical Monte-Carlo simulations (GCMC) of a coarse-grained model system. It turns out that various established approaches of the interaction between a charged colloid and a charged wall, such as linearized Poisson–Boltzmann (PB) theory involving the bulk screening length, do not reproduce the experimental observations. We thus introduce a modified PB potential with a space-dependent screening parameter. The latter takes into account, in an approximate way, the fact that the charged walls release additional (wall) counterions which accumulate in a thin layer at the surface(s). The resulting, still purely repulsive fluid-wall potential displays a nonmonotonic behavior as function of the surface potential with respect to the strength and range of repulsion. GCMC simulations based on this potential reproduce the experimentally observed charge-induced enhancement in the force oscillations. We also show, both by experiment and by simulations, that the asymptotic wave- and decay length of the oscillating force do not change with the wall charge, in agreement with predictions from density functional theory

Effect of particle size and Debye length on order parameters of colloidal silica suspensions under confinement

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.Using atomic force microscopy (AFM) and small angle X-ray scattering (SAXS), we show a full comparison between structuring of nanoparticles in confinement and in bulk in order to explain the effect of confinement on characteristic lengths and the scaling law of the characteristic lengths. Three different-sized particle suspensions are used to check the generalization and the correlation between the characteristic lengths and the system parameters, like particle diameter and Debye length. The two characteristic lengths obtained from AFM force curves, the oscillatory wavelength λ, which is related to the average particle distance, and the decay length ξ, which measures how far particle correlates to obtain periodic oscillations, are in good agreement with the mean particle distance 2π/qmax and the correlation length 2/Δq in bulk, respectively, obtained from the structure peaks of SAXS diagrams. Although confinement causes layering of nanoparticles parallel to the confining surfaces, the characteristic lengths in the direction perpendicular to the confining surfaces follow the bulk behavior. The wavelength scales as ρ−1/3 with the particle number density ρ irrespective of the particle size and the ionic strength and shows a pure volume effect. Upon comparing with literature results, the λ = ρ−1/3 scaling law can be applied more generally for charged particles, as long as the repulsive interaction is sufficiently long-ranged, than the previous expression of λ = 2(R + κ−1), which only approaches the value of average particle distance under specific conditions. The decay length ξ is controlled both by the particle size and the ionic strength of the suspensions, and ξ = R + κ−1 is proposed in the paper. In addition, the interaction strength, the force amplitude and maximum scattering intensity, increases linearly with particle concentration. On the other hand, the Monte Carlo (MC) simulations and approximate hypernetted chain (HNC) closure calculation based on Derjaguin-Landau-Verwey-Overbeek (DLVO) potential are employed to study the characteristic lengths from the theoretical point of view. The experimental wavelengths are in good agreement with the theoretical counterparts and the experimental decay lengths show the same qualitative behavior as theoretical ones on the particle size and ionic strength.DFG, SPP 1273, KolloidverfahrenstechnikDFG, GRK 1524, Self-Assembled Soft-Matter Nanostructures at Interface

Strukturbildung geladener kolloidaler Teilchen in begrenzter Geometrie

In dieser Arbeit wird die Strukturbildung in Filmen geladener, kolloidaler Suspensionen untersucht, welche durch ungeladene bzw. geladene Schlitzporen räumlich begrenzt sind. Die Kolloide in solchen Systemen sind durch eine Oberflächenladung proportional zu ihrer Größe charakterisiert. Die Gegenionen, welche von den Teilchenoberflächen in das Lösungsmittel dissoziieren, ergeben eine effektive Abschirmung der elektrostatischen Repulsion zwischen den Teilchen. Das Ziel dieser Arbeit ist den Einfluss bestimmer Systemparameter auf charakteristische Längen, Korrelationen und Flüssig-Fest-Übergänge zu verstehen. Solche Parameter sind Salzkonzentration, Dichte, Teilchengröße und Oberflächenladung der begrenzenden Wände. Kolloidale Suspensionen und ihre strukturellen Eigenschaften erhalten große Aufmerksamkeit in den Naturwissenschaften und der Technologie wie z. B. bei der Herstellung von Membranen und photonischen Kristallen. Mit Hilfe von Monte-Carlo-Simulationen betrachten wir das System auf einer theoretischen Ebene, wobei die Teilchen-Teilchen-Wechselwirkung auf der Grundlage der Derjaguin-Landau-Verwey-Overbeek-Theorie beschrieben wird. Ein spezieller Fokus in dieser Arbeit ist der Einfluss von Oberflächenladungen der begrenzenden Wände, welche zusätzliche Gegenionen abgeben, auf die Strukturbildung. Unsere theoretischen Vorhersagen werden dabei durch experimentelle Resultate unter Verwendung eines Rasterkraftmikroskops gestützt. Ein bekannter Effekt in kolloidalen und molekularen Systemen ist die Schichtenbildung der Teilchen parallel zu den Oberflächen, welche sogenannte Strukturkräfte erzeugt. Dieser Effekt zeigt eine signifikante Abhängigkeit von Systemparametern. Dies beinhaltet eine Amplitudenverringerung der oszillierenden Kräfte durch eine Erhöhung der Salzkonzentration als auch eine Erhöhung der Kraft bei steigender Wandladung innerhalb eines für das Experiment relevanten Bereiches. Außerdem zeigen wir, dass Flüssig-Fest-Phasenübergänge durch die begrenzte Geometrie beeinflusst werden. In diesem Kontext diskutieren wir das Einsetzen von Kristallisation bei steigender Dichte und das abwechselnde Auftreten von (seitlich versetzten) hexagonalen und quadratischen Strukturen bei kleiner werdendem Wandabstand. Diese Phänomene, welche durch die Wandladungen stark beeinträchtigt werden, zeigen ein ladungsinduziertes Gefrieren. Außerdem werden diese Untersuchungen auf geladene binäre Mischungen ausgeweitet. Dabei beobachten wir einen sogenannten strukturellen Übergang in homogenen Systemen und diskutieren den Einfluss der Wände auf die Zusammensetzung der Mischung. Unsere Resultate tragen zum fundamentalen Verständnis der Strukturbildung, sowohl in kolloidalen Suspensionen als auch in anderen Systemen wie komplexen Plasmen und molekularen Systemen, bei. Sie sind eine Grundlage für weitere theoretische und experimentelle Arbeiten, welche das Wissen für zukünftige Anwendungen liefern und zum Verständnis der Natur beitragen.The structure formation in films of charged, colloidal suspensions confined by uncharged and charged slit pores is investigated in the present thesis. The colloids in such systems are characterized by a surface charge corresponding to their size. The counterions, which dissolve from their surface into the solvent, yield an effective screening of the electrostatic repulsion between the particles. The aim of this work is to understand the impact of the system parameters on characteristic lengths, correlations, freezing and melting phenomena. Such parameters are salt concentration, density, particle size, and surface charges of the confining walls. Colloidal suspensions and their structural properties receive great attention in nature sciences and technology as, e. g., the preparation of membranes and photonic crystals. We consider the system on a theoretical level by employing Monte-Carlo simulations where the particle-particle interactions are modeled via the Derjaguin-Landau-Verwey-Overbeek theory. A special focus in this work is the influence of surface charges of the confinement, providing additional counterions, on the structure formation. Our theoretical predictions for small particle densities are verified by colloidal-probe atomic-force microscope experiments. A prominent effect of confined colloidal and molecular particles is the particle layering parallel to the surfaces yielding the so-called structural forces. This effect exhibits a significant dependence on the system parameters. It involves the decrease of the amplitudes of the oscillating forces by increasing the salt concentration and a force enhancement by increasing the wall charge in a parameter range relevant for the experiment. Furthermore, we show that freezing and melting phenomena are influenced by confinement. In this context, we discuss the onset of crystallization for increasing density and the alternation of (staggered) hexagonal- and square-like structures for narrowing the confinement. These phenomena, affected crucially by wall charges, show a surface-charge-induced reentrant freezing. Finally, the investigations are extended to charged, binary mixtures where we find a so-called structural crossover in homogeneous systems and discuss the impact of the confinement on the composition of the mixture. Our results contribute to the fundamental understanding of the structure formation in colloidal suspensions as well as in other systems like dusty plasmas and molecular systems. They are a base for further theoretical and experimental studies providing the knowledge for future applications and for understanding nature