Humidity, water/cosolvent

Computer Methods for Finding Solvent Blend Replacements and for Predicting Water/Cosolvent Evaporation at Any Humidity... 

The chemical composition of the matrix can have either a positive or a negative effect on the results obtained by SFE. One of the most important parameters influencing the extraction is the degree of humidity of the sample. For example, it is generally admitted that a partial dehydration of the sample allows for a faster extraction. Indeed, hydrophylic matrices have a tendency of preventing contact between the supercritical fluid and the analyte. This is particulary true for the extraction of drugs from skin tissue. In specific cases, however, the presence of water can, on the contrary, improve the rate of extraction by acting as an "internal cosolvent."... 

Because of kinetic factors, metastable forms are encountered in temperature ranges outside the thermodynamic range. Crystallization processes generally imply the cooling of concentrated solutions or precipitation by addition of cosolvent. Depending on the relative positions of the solubility curves of the metastable polymorphic forms and the metastable curve of supersaturation, the first nucleous can be a metastable form. Transformation to the stable crystalline form may or may not occur, depending on kinetic factor. Furthermore solvates exist at lower temperatures and their presence should be considered and finally due to the humidity of the air or from water activity of the solvents, hydrates may be formed. Polymorphism of solvates and hydrates is not uncommon. This phenomenon of concomittant polymorphs has been recently reviewed. ... 

The polymers used in waterborne coatings can be either soluble or dispersed in water (or a combination of water and a cosolvent). The problem of formulations containing soluble polymers is that the presence of polar/hydrophilic groups in the polymer makes the final films more sensitive to humidity. For this reason, a better solution is to formulate hydrophobic polymers that are stabilized in water by internal or external surfactants forming emulsions. Emulsified polymers have high molar masses but because they are dispersed in a particulate form, the viscosity of the media is not sensitive to their molar masses. Therefore, the physical properties are expected to be less dependent on the cure reactions. 

Generally, the binders have a low content of organic solvents (ca. 10%) and can be applied over a wide temperature and humidity range. Water and organic cosolvents used in clearcoats applied by wet-on-wet coating evaporate rapidly after predrying with convection or IR radiation. 

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