Modelling of artefacts in estimations of particle size of needle-like particles from laser diffraction measurements

Manufacturing of particulate products across many industries relies on accurate measurements of particle size distributions in dispersions or powders. Laser diffraction (or small angle light scattering) is commonly used, usually off-line, for particle size measurements. The estimation of particle sizes by this method requires the solution of an inverse problem using a suitable scattering model that takes into account size, shape and optical properties of the particles. However, laser diffraction instruments are usually accompanied by software that employs a default scattering model for spherical particles, which is then used to solve the inverse problem even though a significant number of particulate products occur in strongly non-spherical shapes such as needles. In this work, we demonstrate that using the spherical model for the estimation of sizes of needle-like particles can lead to the appearance of artefacts in the form of multimodal populations of particles with size modes much smaller than those actually present in the sample. This effect can result in a significant under-estimation of the mean particle size and in false modes in estimated particles size distributions.Comment: 28 pages 8 figures accepted in the journal of Chemical Engineering Scienc

Crystal templating through liquid–liquid phase separation

Controlled induction of crystal nucleation is a highly desirable but elusive goal. Attempts to speed up crystallization, such as high super saturation or working near a liquid–liquid critical point, always led to irregular and uncontrollable crystal growth. Here, we show that under highly nonequilibrium conditions of spinodal decomposition, water crystals grow as thin wires in a template-less formation of “Haareis”. This suggests that such nonequilibrium conditions may be employed more widely as mechanisms for crystal growth control

Validity of particle size analysis techniques for measurement of the attrition that occurs during vacuum agitated powder drying of needle-shaped particles

Analysis of needle-shaped particles of cellobiose octaacetate (COA) obtained from vacuum agitated drying experiments was performed using three particle size analysis techniques: laser diffraction (LD), focused beam reflectance measurements (FBRM) and dynamic image analysis. Comparative measurements were also made for various size fractions of granular particles of microcrystalline cellulose. The study demonstrated that the light scattering particle size methods (LD and FBRM) can be used qualitatively to study the attrition that occurs during drying of needle-shaped particles, however, for full quantitative analysis, image analysis is required. The algorithm used in analysis of LD data assumes the scattering particles are spherical regardless of the actual shape of the particles under evaluation. FBRM measures a chord length distribution (CLD) rather than the particle size distribution (PSD), which in the case of needles is weighted towards the needle width rather than their length. Dynamic image analysis allowed evaluation of the particles based on attributes of the needles such as length (e.g. the maximum Feret diameter) or width (e.g. the minimum Feret diameter) and as such, was the most informative of the techniques for the analysis of attrition that occurred during drying

Formation of valine microcrystals through rapid antisolvent precipitation

In this work we have experimentally studied concentration effects on antisolvent precipitation of valine (an amino acid) from aqueous isopropanol solutions. Our experiments showed that the valine precipitation is very sensitive to both the supersaturation and to the water content in the final solution. Results from spectrophotometric measurements and supersaturation analysis showed that the crystal formation kinetics are strongly dependent on both mixing and concentration profiles in the early stages of the process, even though no visible change in the systems occurs immediately upon mixing with the antisolvent or subsequent dilution. Results from small-angle static light scattering measurements showed that the first visible crystals are of micron size and they grow only little over time, while their number increases gradually. Taken together, these experiments point to intermediate phase separation of (possible amorphous) precursors, being either very small nanoparticles or droplets with their refractive index closely matching that of the continuous phase, which subsequently assemble into micron size valine crystals

Effects of temperature and concentration on mechanism and kinetics of thermally induced deposition from coffee extracts

Production of soluble (instant) coffee powders typically involves extraction of roasted coffee by water followed by evaporation in order to concentrate extracts before spray or freeze drying to produce dry coffee powder. In the course of evaporation, deposition of dissolved material from coffee extracts is a major cause of fouling at the heat exchange surfaces of evaporators. Therefore, in order to improve the design and optimization of evaporation processes of coffee extracts, better understanding of the deposition mechanism and kinetics is needed. In this study, optical waveguide lightmode spectroscopy (OWLS) was used to monitor the initial formation of nanometer scale deposits on surfaces exposed to coffee extracts. OWLS measurements were complemented by light scattering from extract solutions, gravimetry of macroscopic deposits, and scanning electron microscopy imaging of deposited layers. Primary molecular-scale layers of about 1 mg m^−2 were rapidly formed in the first stage of deposition, even at ambient temperature, followed by the secondary deposition with kinetics strongly dependent on temperature. Secondary deposition rates were low and largely independent of the extract concentration at ambient temperature, but became strongly dependent on the extract concentration at elevated temperatures. In particular, activation energies for the deposition between 25◦C and 70◦C were much higher for the original extract (13.3 mass %, solids) than for diluted extracts (up to 1.3 mass %, solids). Furthermore, heating of the original extracts above 60◦C resulted in rapid aggregation of suspended macromolecules into large clusters, while only gradual aggregation was observed in diluted extracts

Nucleation and crystal growth of alpha-glycine

Secondary nucleation is widely present in crystallisation processes and it is often relied upon to attain desirable critical quality attributes of crystalline products, such as polymorphic form and crystal size distribution. This is particularly the case in continuous crystallisation, where a good control of secondary nucleation can be crucial in order to achieve and maintain steady state operation. This work utilises rapid, small-scale experiments in agitated vials with in-situ imaging for crystal counting and sizing (using the Crystalline platform), to quantify nucleation and crystal growth kinetics of α-glycine across a range of supersaturations in aqueous solutions under isothermal conditions. Both seeded and unseeded crystallisation experiments were conducted. It was found that secondary nucleation and crystal growth rates determined from the same vials show a close correlation across the whole range of supersaturations investigated, which suggest a close relationship between the mechanisms of shear induced secondary nucleation and crystal growth in this system

Small-scale experiments supporting the MicroFactory

Small-scale crystallisation experiments allow for rapid screening and the minimisation of material use

Effects of secondary metal carbonate addition on the porous character of resorcinol-formaldehyde xerogels

A deeper understanding of the chemistry and physics of growth, aggregation and gelation processes involved in the formation of xerogels is key to providing greater control of the porous characteristics of such materials, increasing the range of applications for which they may be utilised. Time-resolved dynamic light scattering has been used to study the formation of resorcinol-formaldehyde gels in the presence of combinations of Group I (Na and Cs) and Group II (Ca and Ba) metal carbonates. It was found that the combined catalyst composition, including species and times of addition, is crucial in determining the end properties of the xerogels, via its effect on growth of clusters involved in formation of the gel network. Combination materials have textural characteristics within the full gamut offered by each catalyst alone; however, in addition, combination materials which retain the small pores associated with sodium carbonate catalysed xerogels exhibit a narrowing of the pore size distribution, providing an increased pore volume within an application-specific range of pore sizes. We also show evidence of pore size tunability while maintaining ionic strength, which significantly increases the potential of such systems for biological applications

Scaling of glycine nucleation kinetics with shear rate and glass-liquid interfacial area

The scaling of the nucleation kinetics of glycine was investigated in supersaturated aqueous solutions under isothermal conditions. Induction times were measured in a Couette cell with a wide range of average shear rates γ_avg (25-250 s^-1) and a range of glass-liquid interfacial areas A (2.5-10 cm^2 per ml solution). The probability distributions of induction times were found to scale with shear rate and glass-liquid interfacial area, with the characteristic timescale (γ_avg.A)^-1. Primary nucleation rates and growth times to reach detection (estimated from the probability distributions) were both dependent on this timescale. In-situ dynamic light scattering revealed mesoscale clusters in the solutions that increased in size over time at rates which also depended on this timescale. The increase in size was thought to be due to the shear-enhanced aggregation or coalescence of mesoscale clusters leading to a higher number of larger mesoscale clusters, resulting in higher rates of primary nucleation