Jan Vermant

Directed Self-Assembly of Nanoparticles

Within the field of nanotechnology, nanoparticles are one of the most prominent and promising candidates for technological applications. Self-assembly of nanoparticles has been identified as an important process where the building blocks spontaneously organize into ordered structures by thermodynamic and other constraints. However, in order to successfully exploit nanoparticle self-assembly in technological applications and to ensure efficient scale-up, a high level of direction and control is required. The present review critically investigates to what extent self-assembly can be directed, enhanced, or controlled by either changing the energy or entropy landscapes, using templates or applying external fields.

Living on a surface: swarming and biofilm formation

Swarming is the fastest known bacterial mode of surface translocation and enables the rapid colonization of a nutrient-rich environment and host tissues. This complex multicellular behavior requires the integration of chemical and physical signals, which leads to the physiological and morphological differentiation of the bacteria into swarmer cells. Here, we provide a review of recent advances in the study of the regulatory pathways that lead to swarming behavior of different model bacteria. It has now become clear that many of these pathways also affect the formation of biofilms, surface-attached bacterial colonies. Decision-making between rapidly colonizing a surface and biofilm formation is central to bacterial survival among competitors. In the second part of this article, we review recent developments in the understanding of the transition between motile and sessile lifestyles of bacteria.

Exploiting particle shape in solid stabilized emulsions

Pickering–Ramsden emulsions and other forms of particle stabilized soft materials have received quite some attention recently because of the relative ease of formulation and the possibility to create novel materials. There is, however, a clear need for approaches that are versatile and efficient. In the present work the effect of aspect ratio of particles on the stability of both water-in-oil and oil-in-water emulsions is investigated experimentally. Two types of non-spherical particles are used. Hydrophobic prolate ellipsoids with aspect ratios ranging from 1 to 9 are obtained by stretching polystyrene latex particles. Hydrophilic spindle type hematite particles have been synthesized with aspect ratios ranging from 1 to 6. A strong dependence of emulsion stability on the aspect ratio of the particles is observed. Optical as well as cryogenic scanning electron microscopy are used to visualize the droplet morphology and particulate structure and reveal fairly densely packed monolayers of ellipsoids, consistent with the mechanism of limited coalescence. Yet stable emulsions are only obtained for particles with a sufficient aspect ratio. Surface rheology on planar monolayers demonstrates the pronounced effect of aspect ratio on the surface moduli. The magnitude of the interfacial viscoelastic properties is shown to strongly depend on the aspect ratio at a given surface coverage. This is most probably due to an increased effective coverage and the occurrence of strong attractive shape induced capillary interactions. The dependence of the surface rheological properties on the aspect ratio of the particles rationalizes the observed emulsion stability as the surface rheological properties play a role in the coalescence process. The results demonstrate that interfaces with controlled surface rheology, as obtained by using shape induced capillary forces and packing effects, can be used for the rational design of Pickering emulsions and other types of high interface materials.

Quantifying dispersion of layered nanocomposites via melt rheology

Rheological measurements are used to compare clay nanocomposites prepared through melt mixing using two different polypropylene matrices. Steady state and transient nonlinear rheological experiments are employed to separate the contributions of flow induced orientation of the tactoids and particulate network build-up. The conditions under which the rheological properties are dominated by the aggregate network are subsequently identified. Under these conditions, the low frequency linear viscoelastic behavior is analyzed using scaling concepts for fractal networks to determine the degree of network formation by exfoliation. Moreover, the high frequency behavior of the moduli can be used to quickly assess the dispersion quality. The results from the analysis of the linear viscoelastic data are compared to structural features extracted from electron microscopy and small angle X-ray scattering data.

Flow-induced structure in colloidal suspensions

We review the sequences of structural states that can be induced in colloidal suspensions by the application of flow. Structure formation during flow is strongly affected by the delicate balance among interparticle forces, Brownian motion and hydrodynamic interactions. The resulting non-equilibrium microstructure is in turn a principal determinant of the suspension rheology. Colloidal suspensions with near hard-sphere interactions develop an anisotropic, amorphous structure at low dimensionless shear rates. At high rates, clustering due to strong hydrodynamic forces leads to shear thickening rheology. Application of steady-shear flow to suspensions with repulsive interactions induces a rich sequence of transitions to one-, two-and three-dimensional order. Oscillatory-shear flow generates metastable ordering in suspensions with equilibrium liquid structure. On the other hand, short-range attractive interactions can lead to a fluid-to-gel transition under quiescent suspensions. Application of flow leads to orientation, breakup, densification and spatial reorganization of aggregates. Using a non-Newtonian suspending medium leads to additional possibilities for organization. We examine the extent to which theory and simulation have yielded mechanistic understanding of the microstructural transitions that have been observed.

Ordered mesoporous silica material SBA-15: a broad-spectrum formulation platform for poorly soluble drugs.

Encapsulating poorly soluble drugs in mesoporous silicates is an emerging strategy to improve drug dissolution. This study evaluates the applicability of the ordered mesoporous silicate SBA-15 as an excipient to enhance dissolution, for a test series of 10 poorly soluble compounds with a high degree of physicochemical diversity (carbamazepine, cinnarizine, danazol, diazepam, fenofibrate, griseofulvin, indomethacin, ketoconazole, nifedipine, and phenylbutazone). A generic solvent impregnation method was used to load all model compounds. The target drug content was 20%. The physical nature of the formulations was investigated using differential scanning calorimetry (DSC) and the pharmaceutical performance evaluated by means of in vitro dissolution. Aliquots of each formulation were stored at 25 degrees C/52% RH for 6 months, and again subjected to DSC and in vitro dissolution. The target drug content of 20% was attained in all cases. DSC data evidenced the noncrystalline state of the confined drugs. All SBA-15 formulations exhibited an enhanced dissolution as compared to their corresponding crystalline materials, and the high pharmaceutical performance of all formulations was retained during the 6 months storage period. The results of this study suggest that encapsulation in SBA-15 can be applied as a dissolution-enhancing formulation approach for a very wide variety of poorly soluble drugs.

Self-Assembly and Rheology of Ellipsoidal Particles at Interfaces

Colloidal particles confined at liquid interfaces have important applications, for example in the stabilization of emulsions and foams. Also the self-assembly of particles at interfaces offers potential for novel applications and structured particle films. As the colloidal interactions of colloidal particles at interfaces differ from those in bulk, colloidal microstructures can be achieved at an interface which cannot be produced in bulk. In the present work the particle shape, surface charge, and wetting properties are varied, and the resulting self-assembly of particles at a fluid interface is studied. Model monodisperse micrometer-sized ellipsoidal particles were prepared by a mechanical stretching method. These particles were chosen to be well-suited for investigation by optical microscopy. When deposited at an interface between two fluids, shape-induced capillary interactions compete with the electrostatic repulsion. Changing the surface charge and the position at the interface can be used to manipulate the experimentally observed self-assembly process. The initial microstructure of charged ellipsoids at a decane-water interface consists of individual ellipsoids coexisting with linear chains of ellipsoids, connected at their tips. The aggregation behavior in these monolayers was investigated by optical microscopy combined with quantitative image analysis and a dominant tip-tip aggregation was observed. Microstructural information was quantified by calculating the pair-distribution and orientation-distribution functions, as a function of time. Compared to particles at an oil-water interface, particles of the same surface chemistry and charge at an air-water interface seem to have weaker electrostatic interactions, and they also have a different equilibrium position at the interface. The latter leads to differences in the capillary forces. The subsequent change in the balance between electrostatic and capillary forces gave rise to very dense networks having as a typical building block ellipsoids connected at their tips in triangular or flower-like configuration. These networks were very stable and did not evolve in time. The resulting monolayers responded elastically and buckled under compression. Furthermore, the mechanical properties of these monolayers, as measured by surface shear rheology, showed that the monolayer of ellipsoids exhibit a substantial surface modulus even at low surface coverage and can be used to create more elastic monolayers compared to aggregate networks of spheres of the same size and surface properties.

A screening study of surface stabilization during the production of drug nanocrystals

In order to establish a knowledge base for nanosuspension production, a screening was performed on 13 different stabilizers at 3 concentrations for 9 structurally different drug compounds. Concerning the stabilizers tested, the group of semi-synthetic polymers was the least performant (stable nanosuspensions were obtained in only 1 out of 10 cases). For the linear synthetic polymers, better results were obtained with povidones, however poly(vinyl alcohol) did not result in adequate stabilization. The synthetic copolymers showed even higher success rates, resulting in nanosuspensions in two out of three cases when applied at a 100 wt% concentration (relative to the drug weight). Finally, the surfactants gave the best results, with TPGS being successful at concentrations of 25 or 100 wt% of the drug weight for all compounds tested. From the point of view of drug compound, large differences could be observed upon evaluation of the relative number of formulations of that compound resulting in nanosuspensions. It was found that the hydrophobicity of the surfaces, as estimated by the adsorbed amount of TPGS per unit of surface area of nanosuspensions stabilized with 25 wt% TPGS, was decisive for the agglomeration tendency of the particles and hence the ease of nanosuspensions stabilization.

Quorum signal molecules as biosurfactants affecting swarming in Rhizobium etli

Swarming motility is suggested to be a social phenomenon that enables groups of bacteria to coordinately and rapidly move atop solid surfaces. This multicellular behavior, during which the apparently organized bacterial populations are embedded in an extracellular slime layer, has previously been linked with biofilm formation and virulence. Many population density-controlled activities involve the activation of complex signaling pathways using small diffusible molecules, also known as autoinducers. In Gram-negative bacteria, quorum sensing (QS) is achieved primarily by means of N-acylhomoserine lactones (AHLs). Here, we report on a dual function of AHL molecules in controlling swarming behavior of Rhizobium etli, the bacterial symbiotic partner of the common bean plant. The major swarming regulator of R. etli is the cinIR QS system, which is specifically activated in swarming cells by its cognate AHL and other long-chain AHLs. This signaling role of long-chain AHLs is required for high-level expression of the cin and rai QS systems. Besides this signaling function, the long-chain AHLs also have a direct role in surface movement of swarmer cells as these molecules possess significant surface activity and induce liquid flows, known as Marangoni flows, as a result of gradients in surface tension at biologically relevant concentrations. These results point to an as-yet-undisclosed direct role of long-chain AHL molecules as biosurfactants.

Direct Measurements of the Effects of Salt and Surfactant on Interaction Forces between Colloidal Particles at Water−Oil Interfaces

The forces between colloidal particles at a decane-water interface, in the presence of low concentrations of a monovalent salt (NaCl) and the surfactant sodium dodecyl sulfate (SDS) in the aqueous subphase, have been studied using laser tweezers. In the absence of electrolyte and surfactant, particle interactions exhibit a long-range repulsion, yet the variation of the interaction for different particle pairs is found to be considerable. Averaging over several particle pairs was hence found to be necessary to obtain a reliable assessment of the effects of salt and surfactant. It has previously been suggested that the repulsion is consistent with electrostatic interactions between a small number of dissociated charges in the oil phase, leading to a decay with distance to the power -4 and an absence of any effect of electrolyte concentration. However, the present work demonstrates that increasing the electrolyte concentration does yield, on average, a reduction of the magnitude of the interaction force with electrolyte concentration. This implies that charges on the water side also contribute significantly to the electrostatic interactions. An increase in the concentration of SDS leads to a similar decrease of the interaction force. Moreover, the repulsion at fixed SDS concentrations decreases over longer times. Finally, measurements of three-body interactions provide insight into the anisotropic nature of the interactions. The unique time-dependent and anisotropic interactions between particles at the oil-water interface allow tailoring of the aggregation kinetics and structure of the suspension structure.