Characterization of the rapid expansion of supercritical solutions by Fourier transform infrared spectroscopy in situ

Abstract

A new apparatus for the in situ characterization of the rapid expansion of supercritical solutions ͑RESS͒ by Fourier transform infrared spectroscopy is presented. The infrared characterization is complemented by particle sizing with a scanning mobility particle sizer, by three-wavelengths-extinction measurements, and by scanning electron microscopy. Several examples show that a wide range of information about particle properties can be obtained with this setup. One new aspect is the possibility to expand into the vacuum which also allows us to investigate the conditions in the collision-free region before the Mach disk. These investigations elucidate that in the free jet region the solvent CO 2 condenses to particles with mean radii Ͼ50 nm for pre-expansion pressures between 100-400 bar and temperatures between 298-398 K.

Infrared analysis of CO ice particles in the aerosol phase

“…Also fundamental applications to retrieve the optical constants of solids producible only at low temperatures, to test the processes involved in homogeneous nucleation and to determine the surface energy of particles, are of importance. To generate clusters or aerosol particles, several techniques can be used, such as jet expansion, 1,2 supercritical solutions, 3 electrospray, 4 and collisional cooling, [5][6][7][8] depending on the nature of the monomers and the required temperature. Collisional cooling offers the advantage of operating in thermal equilibrium with high lifetimes of the aerosol particles reaching up to hours.…”

Physical Chemistry of Dust: Weak Intermolecular Interactions in Molecular Nanoparticles

“…These 'ice' particles are of relevance in the atmosphere and in the interstellar dust. They are also important for the generation of non-volatile particles by rapid expansion of supercritical solutions (see Section 5) where the solvent forms ice particles during the expansion process [12]. One of the first questions that comes to mind in this context is whether the finite size leads to any characteristic features in infrared spectra.…”

“…Conventional particle preparation methods for non-volatile substances, such as milling or spray drying, are not suitable to form submicron particles. Such particles can rather be generated by rapid expansion of supercritical solutions (RESS) [12] [13] or by electrospraying [14].…”

“…There is also a lot of interest in producing pharmaceutical or cosmetic agents in the form of micro-or nanoparticles [3] [12][13] [16]. The micronization of pharmaceuticals is a means to increase their bioavailability.…”

“…Furthermore it is sensitive, non-invasive, and can be used for various kinds of particulate samples under a broad range of different experimental conditions. The usage of specially designed in situ techniques [12][15] enables us, for example, to observe the particle formation in the aerosol phase and to probe the properties of particles in supersonic expansions. We complement the results from in situ infrared spectroscopy with particle sizing using a scanning mobility particle sizer, three-wavelengths-extinction measurements, and electron microscopy.…”