Modifying the structure and flow behaviour of aqueous montmorillonite suspensions with surfactant

Sugar-coated proteins: the importance of degree of polymerisation of oligo-galacturonic acid on protein binding and aggregation

We have simplified the structural heterogeneity of protein-polysaccharide binding by investigating protein binding to oligosaccharides. The interactions between bovine beta-lactoglobulin A (βLgA) and oligo-galacturonic acids (OGAs) with various numbers of sugar residues have been investigated with a range of biophysical techniques. We show that the βLgA-OGA interaction is critically dependent on the length of the oligosaccharide. Isothermal titration calorimetry results suggest that a minimum length of 7 or 8 sugar residues is required in order to exhibit appreciable exothermic interactions with βLgA - shorter oligosaccharides show no enthalpic interactions at any concentration ratio. When titrating βLgA into OGAs with more than 7-8 sugar residues the sample solution also became turbid with increasing amounts of βLgA, indicating the formation of macroscopic assemblies. Circular dichroism, thioflavin T fluorescence and small angle X-ray/neutron scattering experiments revealed two structural regimes during the titration. When OGAs were in excess, βLgA formed discrete assemblies upon OGA binding, and no subsequent aggregation was observed. However, when βLgA was present in excess, multi-scale structures were formed and this eventually led to the separation of the solution into two liquid-phases.

Three-Phase Coexistence in Colloidal Rod-Plate Mixtures

Aqueous suspensions of clay particles, such as montmorillonite (MMT) platelets and sepiolite (Sep) rods, tend to form gels at concentrations around 1 vol %. For Sep rods, adsorbing sodium polyacrylate to the surface allows for an isotropic-nematic phase separation to be seen instead. Here, MMT is added to such Sep suspensions, resulting in a complex phase behavior. Across a range of clay concentrations, separation into three phases is observed: a lower, nematic phase dominated by Sep rods, a MMT-rich middle layer, which is weakly birefringent and probably a gel, and a dilute top phase. Analysis of phase volumes suggests that the middle layer may contain as much as 6 vol % MMT.

Controlling Clusters of Colloidal Platelets: Effects of Edge and Face Surface Chemistries on the Behavior of Montmorillonite Suspensions

The structural and rheological consequences of adsorbing pyrophosphate anions to the edges and polyetheramines to the faces of montmorillonite platelets in aqueous suspension were investigated. Oscillatory rheology and scattering experiments showed that the two surface treatments act in different regions of the phase diagram and that this can be attributed to modifications of local particle interactions resulting in changes to the behavior and morphology of platelet clusters. The polyetheramine was found to neutralize surface charge, reducing electrostatic repulsion between platelets and therefore allowing them to come into closer proximity. This reduces the effective volume fraction of the clusters and reverses jamming in low ionic strength arrested phases. Conversely, the adsorption of pyrophosphate was found to introduce a high concentration of negative charge to the particle edge, resisting the formation of bonded percolating gels at high ionic strength. The two separate surface chemistries can be applied in parallel with no adverse effects and thus have the potential to be applied to dual functionalization of two-dimensional colloids such as platelets. This has implications for finer formulation design where targeted rheology modification could be achieved by careful selection of chemistry at one surface accompanied by an additional function at the other.

Smectite clay--inorganic nanoparticle mixed suspensions: phase behaviour and rheology

Smectite clay minerals and their suspensions have long been of both great scientific and applications interest and continue to display a remarkable range of new and interesting behaviour. Recently there has been an increasing interest in the properties of mixed suspensions of such clays with nanoparticles of different size, shape and charge. This review aims to summarize the current status of research in this area focusing on phase behaviour and rheological properties. We will emphasize the rich range of data that has emerged for these systems and the challenges they present for future investigations. The review starts with a brief overview of the behaviour and current understanding of pure smectite clays and their suspensions. We then cover the work on smectite clay-inorganic nanoparticle mixed suspensions according to the shape and charge of the nanoparticles - spheres, rods and plates either positively or negatively charged. We conclude with a summary of the overarching trends that emerge from these studies and indicate where gaps in our understanding need further research for better understanding the underlying chemistry and physics.

Liquid crystal phase transitions in suspensions of mineral colloids: new life from old roots

A review is given of the field of mineral colloidal liquid crystals: liquid crystal phases formed by individual mineral particles within colloidal suspensions. Starting from their discovery in the 1920s, we discuss developments on the levels of both fundamentals and applications. We conclude by highlighting some promising results from recent years, which may point the way towards future developments.