Judith E. G. J. Wijnhoven

Bio: Judith E. G. J. Wijnhoven is an academic researcher from Utrecht University. The author has contributed to research in topics: Gibbsite & Colloidal crystal. The author has an hindex of 7, co-authored 8 publications receiving 193 citations.

Sedimentation and Phase Transitions of Colloidal Gibbsite Platelets

Abstract: We study the competition between sedimentation, gelation, and liquid crystal formation in suspensions of colloidal gibbsite platelets of five different sizes at three ionic strengths. For large particles (with diameters of 350, 420, and 570 nm) sedimentation is initially the most important factor determining the macroscopic behavior. Only after the main part of the sample has sedimented in an amorphous phase, phase separation takes place. For the smallest particles (diameter 210 and 270 nm), it is the other way around: fast (within one week) phase separation or gelation takes place, after which sedimentation determines the final macroscopic appearance. We distinguish six different scenarios within this two-fold scheme and interpret these on the basis of the previously obtained phase diagram of colloidal gibbsite platelets (van der Beek, D.; Lekkerkerker, H. N. W. Langmuir 2004, 20, 8582).

Seeded growth of monodisperse gibbsite platelets to adjustable sizes.

Abstract: Nanosized hexagonal gibbsite seeds are grown from a mixture of dissolved alumina alkoxides at 85 °C. Centrifugation reduces the polydispersity by 30%. The seeds can be grown further by adding them to a fresh alkoxide mixture and heating it. This procedure was repeated several times to obtain particles of 570 nm ± 11% diameter and a thickness of 47 ± 23%. No indications of a size limit were observed. The thus obtained particles may form easily a columnar phase. Individual gibbsite particles in solution can be seen by confocal microscope.

Crystallization in settling mixtures of colloidal spheres and plates.

Abstract: The behavior of mixtures of silica spheres and smaller silica-coated gibbsite plates is studied by means of confocal microscopy. Addition of plates slows down the crystallization of the settling spheres. Liquidlike microphases of the plates are found in the sediments of the spheres. It is argued that this is due to simultaneous sedimentation of the plates and spheres as well as depletion interaction between both species. Typical length scales in the sediments, derived from Fourier transforms of the confocal images, suggest there are still specific interactions present.

Gelation versus liquid crystal phase transitions in suspensions of plate-like particles

Abstract: Gelation is a common effect in aqueous suspensions of charged colloidal clay platelets at concentrations as low as 1 wt%. However, in systems of charged gibbsite [Al(OH)3] platelets, gelation can be delayed to concentrations as high as 50 wt% depending on the ionic strength. We investigated the phase behaviour of this system approaching the state of gelation in the delicate region between attractive and repulsive states that originate from competition between Coulomb repulsion and van der Waals attraction. As a function of the ionic strength, isotropic-nematic, nematic-columnar and isotropic-columnar phase separations were observed. Moreover, compression by gravitational forces allowed us to observe phase separation that is arrested by gelation in the homogeneous suspensions.

Coating of Gibbsite Platelets with Silica

Abstract: In this work, gibbsite particles are synthesized from aluminum alkoxides in an acidic environment at 85 °C. After centrifugation, coating with silica is investigated following two different methods. Practical aspects and the feasibility to produce significant amounts of material are studied. The first method, analogous to a procedure used for boehmite, is a three-step procedure that starts from the addition of water glass and ends with Stober growth of silica. In the second method, poly(vinylpyrrolidone) is used to stabilize gibbsite in the Stober-like mixture for silica growth. Both methods lead to silica-coated particles. The first method is elaborate, but can be used to make very thin (<4 nm) silica layers. The second method is quick and produces smooth layers. The particles are characterized by scanning and transmission electron microscopy and energy-dispersive X-ray analysis and show a smooth silica layer of 10−25 nm thick. These particles can be used for confocal microscopy and in mixtures with diff...

Engineering shape: the novel geometries of colloidal self-assembly

Abstract: This article investigates the role of shape in colloidal self-assembly and argues for the importance of a tight synergy between particle design and assembly strategies. To this end, we review synthetic methodologies developed to impart colloidal building blocks with anisotropic shapes and self-assembly mechanisms that exploit geometry to direct and control the particles' organization. This paper, which deliberately focuses on micron-scale colloids, is divided into two main sections. Firstly, we discuss the impact of shape on particles' interactions and how this has been exploited to develop heuristic rules for the creation of self-assembling architectures. Secondly, we examine state-of-the-art advances in colloidal synthesis with a clear emphasis on design rules and bulk methods, which are aimed at producing shape-anisotropic particles.

Liquid-crystalline nematic phase in aqueous suspensions of a disk-shaped natural beidellite clay.

Abstract: After size-selection and osmotic pressure measurements at fixed ionic strength, the behavior of aqueous colloidal suspensions of anisotropic disklike beidellite clay particles has been investigated by combining optical observations under polarized light, rheological, and small angle X-ray scattering (SAXS) experiments. The obtained phase diagrams (volume fraction/ionic strength) reveal, for ionic strength below 10−3 M/L, a first-order isotropic/nematic (I/N) phase transition before gel formation at low volume fractions, typically around 0.5%. This I/N transition line displays a positive slope for increasing ionic strength and shifts toward lower volume fraction with increasing particle size, confirming that the system is controlled by repulsive interactions. The swelling laws, derived from the interparticle distances obtained by SAXS, display a transition from isotropic swelling at low volume fractions to lamellar swelling at higher volume fractions. The liquid-crystal properties have then been investigat...

Water-in-Water Emulsions Stabilized by Nanoplates

Abstract: Ultrathin plate-like colloidal particles are effective candidates for Pickering stabilization of water-in-water emulsions, a stabilization that is complicated by the thickness and ultralow tension of the water–water interface. Plate-like particles have the advantage of blocking much of the interface while simultaneously having a low mass. Additionally, the amount of blocked interface is practically independent of the equilibrium contact angle θ at which the water–water interface contacts the nanoplates. As a result, the adsorption of nanoplates is stronger than for spheres with the same maximal cross section, except if θ = 90°.