Norio Hirokawa

Heterogeneity in aqueous solutions: electron microscopy of citric acid solutions

Abstract The structures of aqueous citric acid solutions of various concentrations, including supersaturated solutions, were examined directly by scanning electron microscopy (SEM). Electron microscopic samples were prepared by solidifying a droplet of solution in a refrigerant at very high quenching speed, then replicating it with electron-beam-gunned platinum and carbon. Solute clusters and their aggregates were observed in all the samples irrespective of the solute concentration. The solutions showed a double-zone structure in scanning electron microphotographs, consisting of a smooth zone and a compartment of many solute clusters. With the increase in solute concentration, the number density of compartments increased, while their volume decreased. Electron microscopy thus strongly suggested that the solutions examined were heterogeneous, namely, the so-called colloidal mixtures. X-ray diffraction analysis revealed the very important fact that the crystal of citric acid monohydrate contains fine particles which seem to be the remnants of solute clusters in solution. This is supported by the observation in the boundary region between the crystal and solution of fine particles which stretch like elastic to form a network structure.

Solute clusters in aqueous citric acid solutions

Abstract Densities of aqueous citric acid monohydrate solutions are examined at three different concentrations. Two supersaturated solutions exhibit density gradients in a vertical column, but a saturated solution does not. The density gradients can be expressed satisfactorily by use of a theoretical model based on the Brownian diffusion equation for solute clusters under gravity. Electron microscopy also strongly suggests the existence of solute clusters in supersaturated solutions. The cluster size is estimated to be about 16 nm in diameter. Ultraviolet-ray scattering suggests a possible secondary structure of cluster aggregations.

Whisker formation from jet of supercritical fluid solution

Abstract The growth mechanism of a whisker of stigmasterol particles deposited by rapid expansion from supercritical CO 2 solutions has been investigated by scanning electron microscope. Amorphous fine particles were found to be under relatively low pre-expansion pressures while whisker-like crystals were observed under relatively high pre-expansion pressures. The turning point was about 13 MPa at 373 K. It was found that the secondary products grew on a screen in the expansion jet after deposition of primary particles of about 10-nm diameter. The morphology of secondary products was found to strongly depend on the surface condition of the primary particle. Because of the temperature decrease caused by the rapid expansion, the pre-expansion pressure plays an important role in determining the thermal roughening degree on the surface of primary particles, and consequently the morphology of the secondary products.

Mechanism of crystallization of enzyme protein thermolysin

Abstract The mechanism of crystallization of enzyme protein thermolysin was investigated. The size distribution of thermolysin precipitates was measured by dynamic light scattering during precipitation, and the surface and cross section of the finally obtained precipitate were observed by scanning electron microscopy. The thermolysin precipitates obtained at the initial supersaturation ratio of 8.9 to 164 and pH 7.0 were hexagonal rods having an average size of 9.2×1.5 μm, and were composed of a number of small particles of 15 to 200 nm in diameter. The average size of the small particles was 60 nm in diameter, and the formation of the particles was found to be completed in the early stage of precipitation. Observation of the finally recovered thermolysin precipitate by polarizing microscopy revealed that the precipitate is a crystalline solid. From these data, a possible mechanism of thermolysin crystallization was proposed. The crystallization proceeds through two steps: the first step is the formation of primary particles, and the second step is crystal growth by highly ordered aggregation of the primary particles.

Change in microstructure of an aqueous citric acid solution under crystallization

The micro morphology changes of an aqueous citric acid solution with increasing supersaturation ratio S and in the presence of a seed crystal were observed by using a scanning electron microscope (SEM). With increasing S, the number density of particles of ∼ 60 nm in diameter increases to form a network structure. With further increasing S, particles of ∼ 60 nm aggregate to several microns in size and then are rearranged into an ordered crystal structure. When a seed crystal is placed in a supersaturated solution, ∼ 60 nm particles gather and migrate to the crystal surface and are rearranged in a regular structure on it. From these results, it is supposed that the crystallization of citric acid monohydrate proceeds not through a molecular-by-molecular process but by an aggregation process of solute cluster units of ∼ 60 nm in size.

Polymorphic transition of solid-fats dispersed systems — its characterization by a novel method and scanning electron microscopy observation

Abstract Solid-fats dispersed systems, such as margarine, butter and cacao-butter, were characterized by a novel method based on liquid permeation under pressure, for the simultaneous measurement of a solid-content ϵp and an average diameter dp of solid particles (fats crystals) in them. Further, micro-structures of these systems were observed by a scanning electron microscope (SEM). As the result, it has been clarified that the spherical fats crystals of several μm in size appeared in the initial solid-fats products are agglomerates of fine particles of ca. 0.1 μm and that these fine particles are uniformly redispersed during an annealing treatment accompanying the reduction of ϵp and dp. It is strongly suggested that this phenomenon is caused by a transition of fat crystals into a more stable polymorph.