Controlled sequential assembly of metal-organic polyhedra into colloidal gels with high chemical complexity

Assembling many chemical components into a material in a controlled manner is one of the biggest challenges in chemistry. Particularly porous materials with multivariate character within their scaffolds are expected to demonstrate synergistic properties. In this study, we show a synthetic strategy to construct porous networks with multiple chemical components. By taking advantage of the hierarchical nature of a colloidal system based on metal-organic polyhedra (MOPs), we developed a two-step assembly process to form colloidal networks; assembling of MOPs with the organic linker to the formation of MOP network as a colloidal particle, followed by further connecting colloids by additional crosslinkers, leading to colloidal networks. This synthetic process allows not only for the use of different organic linkers for connecting MOPs and colloidal particles, respectively, but for assembling different colloidal particles formed by various MOPs. The proof-of-concept of this tuneable multivariate colloidal gel system offers an alternative to developing functional porous soft materials with multifunction.

Evaluation of Aloe elegans Mucilage as a Suspending Agent in Paracetamol Suspension

Background. There are various natural excipients which have been used as suspending agents in pharmaceutical suspensions due to the presence of mucilage in their specialized cells and their capacity to form a colloidal gel in an aqueous medium. Objective. The purpose of this study was to evaluate the suspending capacity of Aloe elegans mucilage in suspension formulations. Materials and Methods. Aloe elegans mucilage (AEM) was evaluated as a suspending agent in comparison with xanthan gum (XG) in paracetamol suspensions at 1, 2, 3, 4, and 5% ( w / v ) concentrations. The resulting suspensions were evaluated for their sedimentation volume, apparent viscosity, flow rate, rate of redispersibility, pH, assay, and dissolution profile. Results. The volume of sedimentation, apparent viscosity, and redispersibility rate of the formulations were significantly increased ( p < 0.05 ), with the concentration of the suspending agents. Meanwhile, the apparent viscosity for all formulations has significantly decreased ( p < 0.05 ) with an increase in shear rates. Volume of sedimentation, apparent viscosity, and redispersibility degree of the formulations prepared with AEM were significantly ( p < 0.05 ) lower than XG-containing formulations at the same concentration. Nevertheless, the sedimentation volume of all formulations with AEM was significantly ( p < 0.05 ) higher than the suspension without any suspending agent. With regard to drug content and pH values, all formulations showed an acceptable result with the standards. All formulations showed a release of greater than 85% of drug content within 45 min. Conclusion. Aloe elegans mucilage could have a potential to be utilized as an alternative suspending agent in pharmaceutical suspensions.

Life and death of colloidal bonds control the rate-dependent rheology of gels

Colloidal gels exhibit rich rheological responses under flowing conditions. A clear understanding of the coupling between the kinetics of the formation/rupture of colloidal bonds and the rheological response of attractive gels is lacking. In particular, for gels under different flow regimes, the correlation between the complex rheological response, the bond kinetics, microscopic forces, and an overall micromechanistic view is missing in previous works. Here, we report the bond dynamics in short-range attractive particles, microscopically measured stresses on individual particles and the spatiotemporal evolution of the colloidal structures in different flow regimes. The interplay between interparticle attraction and hydrodynamic stresses is found to be the key to unraveling the physical underpinnings of colloidal gel rheology. Attractive stresses, mostly originating from older bonds dominate the response at low Mason number (the ratio of shearing to attractive forces) while hydrodynamic stresses tend to control the rheology at higher Mason numbers, mostly arising from short-lived bonds. Finally, we present visual mapping of particle bond numbers, their life times and their borne stresses under different flow regimes.

Time–connectivity superposition and the gel/glass duality of weak colloidal gels

Colloidal gels result from the aggregation of Brownian particles suspended in a solvent. Gelation is induced by attractive interactions between individual particles that drive the formation of clusters, which in turn aggregate to form a space-spanning structure. We study this process in aluminosilicate colloidal gels through time-resolved structural and mechanical spectroscopy. Using the time–connectivity superposition principle a series of rapidly acquired linear viscoelastic spectra, measured throughout the gelation process by applying an exponential chirp protocol, are rescaled onto a universal master curve that spans over eight orders of magnitude in reduced frequency. This analysis reveals that the underlying relaxation time spectrum of the colloidal gel is symmetric in time with power-law tails characterized by a single exponent that is set at the gel point. The microstructural mechanical network has a dual character; at short length scales and fast times it appears glassy, whereas at longer times and larger scales it is gel-like. These results can be captured by a simple three-parameter constitutive model and demonstrate that the microstructure of a mature colloidal gel bears the residual skeleton of the original sample-spanning network that is created at the gel point. Our conclusions are confirmed by applying the same technique to another well-known colloidal gel system composed of attractive silica nanoparticles. The results illustrate the power of the time–connectivity superposition principle for this class of soft glassy materials and provide a compact description for the dichotomous viscoelastic nature of weak colloidal gels.