From colloidal dispersions to colloidal pastesthrough solid–liquid separation processes

Solid–liquid separation is an operation that starts with a dispersion of solid particles in a liquid and removes some of the liquid from the particles, producing a concentrated solid paste and a clean liquid phase. It is similar to thermodynamic processes where pressure is applied to a system in order to reduce its volume. In dispersions, the resistance to this osmotic compression depends on interactions between the dispersed particles. The first part of this work deals with dispersions of repelling particles, which are either silica nanoparticles or synthetic clay platelets, dispersed in aqueous solutions. In these conditions, each particle is surrounded by an ionic layer, which repels other ionic layers. This results in a structure with strong short-range order. At high particle volume fractions, the overlap of ionic layers generates large osmotic pressures; these pressures may be calculated, through the cell model, as the cost of reducing the volume of each cell. The variation of osmotic pressure with volume fraction is the equation of state of the dispersion. The second part of this work deals with dispersions of aggregated particles, which are silica nanoparticles, dispersed in water and flocculated by multivalent cations. This produces large bushy aggregates, with fractal structures that are maintained through interparticle surface– surface bonds. As the paste is submitted to osmotic pressures, small relative displacements of the aggregated particles lead to structural collapse. The final structure is made of a dense skeleton immersed in a nearly homogeneous matrix of aggregated particles. The variation of osmotic resistance with volume fraction is the compression law of the paste; it may be calculated through a numerical model that takes into account the noncentral interparticle forces. According to this model, the response of aggregated pastes to applied stress may be controlled through the manipulation of interparticle adhesion

High thermal neutron flux effects on structural and macroscopic properties of alkali-borosilicate glasses used as neutron guide substrate

The behaviour of four alkali-borosilicate glasses under homogeneous thermal neutron irradiation has been studied. These materials are used for the manufacturing of neutron guides which are installed in most facilities as devices to transport neutrons from intense sources such as nuclear reactors or spallation sources up to scientific instruments. Several experimental techniques such as Raman, NMR, SANS and STEM have been employed in order to understand the rather different macroscopic behaviour under irradiation of materials that belong to a same glass family. The results have shown that the remarkable glass shrinking observed for neutron doses below 0.5 · 10 18 n/cm 2 critically depends upon the presence of domains where silicate and borate network do not mix

Creating transient gradients in supramolecular hydrogels

The self‐assembly of low molecular weight gelators in water usually produces homogeneous hydrogels. However, homogeneous gels are not always desired. Using a photoacid generator, it is shown how to form gels with a transient gradient in stiffness, proved using cavitation and bulk rheology. Small‐angle neutron scattering is used to show that the gels formed by photoacid are the result of the same structures as when using a conventional pH trigger. Patterned gels can also be formed, again with transient differences in stiffness

Controlling And Understanding Single And Multicomponent Supramolecular Gels

Supramolecular gels can be prepared by the self-assembly of small molecules into fibrous structures. The properties of the resulting gels depend on how the gels are formed, such that gels with very different properties can be prepared from a single gelator if different gelation methods are used. We have been working to understand this, and for example can prepare gels that can or cannot be 3D-printed from the same gelator by varying gelation method. Here, we will focus on explaining the design rules. As specific examples, we will discuss how varying the chirality of our dipeptide-based gelators can be used to control the self-assembled aggregates, leading to differences in the final gels. We will also show how our understanding can be expanded to multicomponent systems, where each component gelator can form gels alone. In these mixed systems, we can control assembly such that self-sorted multicomponent gels are formed. We will show how such systems can be characterised to prove this assembly and how this approach can be used to prepare gels with controlled and specific properties

Using small-angle scattering and contrast matching to understand molecular packing in low molecular weight gels

It is difficult to determine exactly the molecular packing in the aggregates in low molecular weight gels. Attempts to understand the packing have been made using X-ray diffraction, but there are complications with drying and questions as to whether the crystal structures represent the packing in the gel phase. Here, we exploit contrast matching in small-angle neutron scattering experiments. By preparing selectively deuterated analogs of the same molecule, the scattering from that section of the molecule decreases compared with the hydrogenated molecule. We examine packing in the pre-gelled solutions at high pH and in the gels at low pH. The data from the final gels show a lack of specific order in the aggregates that form the gel matrix. The packing in these systems is not well ordered in the gel state and so implies that it is likely that current models and cartoons are not correct

Nonlinear Effects in Multicomponent Supramolecular Hydrogels

Multicomponent low molecular weight gels are useful for a range of applications. However, when mixing two components, both of which can independently form a gel, there are many potential scenarios. There is a limited understanding as to how to control and direct the assembly. Here, we focus on a pH-triggered two-component system. At high pH, colloidal structures are formed, and there is a degree of mixing of the two gelators. As the pH is decreased, there is a complex situation, where one gelator directs the assembly in a “sergeants and soldiers” manner. The second gelator is not fully incorporated, and the remainder forms an independent network. The result is that there is a nonlinear dependence on the final mechanical properties of the gels, with the storage or loss modulus being very dependent on the absolute ratio of the two components in the system

Reversible Photoreduction as a Trigger for Photoresponsive Gels

We present here a new type of photoresponsive, reversible low molecular weight gel. All previous examples rely on a photoisomerisation, ring-closing or dimerization. We show that photoreduction of a perylene bisimide gelator results in the formation of a stable radical anion. The formation of the radical anion results in a change in the packing of the perylene bisimides in the self-assembled aggregates, leading to a change in fibrous network and an increase in the rheological properties of the gels. An increase in the rheological properties is extremely rare for a photoresponsive gel; normally, irradiation results in a gel-to-sol transition, and the gel falling apart. As the radical anion decays, which takes several hours in air, the original gel properties are restored. This photoreduction can be cycled many times. Finally, we show that the mechanical properties are different between irradiated and nonirradiated sections in a patterned gel

Tuning the antimicrobial activity of low molecular weight hydrogels using dopamine autoxidation

We present a method to trigger the formation of dipeptide-based hydrogels by the simple addition of dopamine. Dopamine undergoes oxidation in air, reducing the pH to induce gelation. The production of polydopamine and release of reactive oxygen species such as hydrogen peroxide confers antimicrobial activity. Gel stiffness can be controlled by modulating the initial starting pH of the gelator solution. We can use this method to tune the antimicrobial activity of the gels, with gels that are less stiff demonstrating increased bactericidal efficacy against Gram-positive bacteria