Highly volatile solvents

The type of solvent used depends on the binder. There is normally more than one type of solvent in paint, particularly for spray application. Highly volatile solvents are needed to reduce the viscosity during atomization and then disperse as quickly as possible, but lower volatile solvents are necessary to remain momentarily to ensure that there is sufficient flow to form a continuous film. 

Formulators can use the tendency of creams, gels, and other systems to evaporatively concentrate to advantage. Solvents are chosen and blended so that the drug remains soluble in the formed film long after application is made. This can be accomplished by replacing a fraction of the water or other highly volatile solvent found in these systems with solvents of far lower volatility. As previously pointed out, 5-15% propylene glycol is found in many topical corticosteroid creams and lotions just for this reason. 

Traces of Highly Volatile Solvents (Acetone, Ethanol)... 

Snyder columns are designed to allow highly volatile solvents to escape while retaining semivolatile analytes of interest. Snyder columns are generally fitted onto the tops of flasks containing extracts, and column design permits solvent to escape as the flask is heated. The analytes of interest condense from a gas to a liquid phase and fall back into the solvent reservoir. The Kudema-Danish concentrator is a Snyder column with a removable collection tube attached to the bottom. As solvent is evaporated, the extract is collected in the collection tube. 

B. Sealed Tube Preparations. Sealed tube reactions and purifications afford good exclusion of the atmosphere and permit operations at moderate pressures. The latter capability makes it possible to work with highly volatile solvents, such as liquid ammonia, at room temperature. Since the sealed tube is often loaded on a high-vacuum line, the details on these techniques are given in Chapter 9. 

The transport of caibon dioxide in polymers has historically been analyzed in the same manner as other simple gases (1). Recent studies have shown, however, that the effects of CX>2 on polymers include scrnie features commonly associated with organic solvents, including swelling (2-5). and depression of glass transition temperatures, i.e., plasticization (6-8). Moreover, CO2 can be handled as a liquid at room temperature under ratho moderate pressures its critical temperature is 31 0 and its saturated vapor pressure at 25" C is 64.6 atm (950 psi). For these reasons it seems appropriate to consider near-critical CX>2 as a highly volatile solvent, rather than as a gas, in its interactions with polymers. 

Modern hairspray consists of a solution of long, chainlike molecules (called polymers) in a highly volatile solvent. Some brands may also contain oils such as resins and lanolin. In general, a volatile substance is one whose state is unstable at room temperature and may readily change from liquid to gas form. Thus, hairspray is in liquid form within the can, as air pressure has been removed. The can is frequently composed of compounds (e.g., aluminum monobloc or tri-layered steel) that allow for a decreased likeliness of puncturing. Spraying the product results in the deposit of a polymer layer around each hair afrer evaporation of the volatile solvent. The web of polymer molecules on the hairs yields a stiff texture and allows the hairs to resist changing shape. 

While aqueous effluents containing highly volatile solvents can be stripped using air or steam preparatory to being discharged, the less volatile and particularly those that are polar, are difficult to strip and are more economically removed from dilute solution using activated carbon- or ion-exchange resins. 

Crystallization of a Fatty Acid Mixture Usii Spray Evaporation of Highly Volatile Solvents... 

Qystallization of tatty acids using spray-evaporation of highly volatile solvents has been examined to obtain solids without inclusion of liquid. Hie powdery solids were collected in the spray column, and they were usually obtained on the glass tube near the nozzle and also on the bottom of the column. The mole fraction of first crystallizing component of the solids in the column decreases from the top to the bottom of the column. Hie percent of the solids collected on the bottom were correlated with the Weber Number. The more volatile solvents such as propane or isobutane reduced the percent of solids and the purity of solids on the bottom. Hie possibility of a separation process using spray-evaporation was suggested. 

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