Secondary drying diffusion

After a multicomponent aqueous solution has been freeze-concentrated to the limit and the ice has been sublimed, any residual unfrozen water must be removed from the remaining solid solution by diffusion, desorption and evaporation (transfer to the condenser). This process is termed secondary drying . For an amorphous preparation, the amount of unfrozen water remaining after the removal of ice may be typically 20-30% w/w, but much higher values, even >50% w/w, have been found in some formulations. Attempts are on record to measure the amount of water that remains in the freeze-concentrated solution phase at 7, mainly by differential scanning calorimetry but such... 

Although the process of unfrozen water removal from the material, subsequent to ice sublimation, is commonly referred to as desorption , this is misleading. It has been shown that the residual water forms a mobile component of a solid solution, so that diffusion more correctly describes the mechanism of its removal. Compared to the diffusion rate of water from the bulk to the surface, its eventual desorption from the surface is rapid and can in practice almost be neglected. As will be shown later, secondary drying kinetics can be modelled adequately by standard treatments of diffusive processes. The effects of many variables on the kinetics of secondary drying may be found in Pikal et al ... 

Two workers died while applying dry sodium metabisulfite in a ship hold. Postmortem examination showed diffuse pulmonary edema consistent with death secondary to asphyxia and visceral congestion. 

Oxidation of Anionic Polymers In the Solid State The ability of the macroradical and of the macroions to diffuse In the mixture, and to interreact Is responsible for the secondary products formation coupling reaction and alcoholate synthesis. To prevent the diffusion phenomenon, we have carried out the deactivation In the solid state. The living polymers have been prepared In benzene, with or without a solvating agent (THF or TMEDA) and the solution has been freeze dried before the oxygen introduction. The experimental results are collected in Table VII. 

In the case of a diffusion-limited reaction, hydrodynamic factors, such as turbulent flow, may influence the dissolution process. Humidity fluctuations also may alter the solute concentrations in the moisture film at the rock surface, and they may result in the dissolution and recrystallization, not only of the carbonates, but also of the secondary minerals, such as calcium sulfate. Wetting and drying cycles may lead to measureable changes in fluid composition at the stone surface. 

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