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Aerosols



 Aerosols are a type of pharmaceutical vehicle that consists of a propellant, and a product concentrate portion. The product concentrate is made up of a liquid or solid active ingredient within a solvent vehicle. There may also be other ingredients added such as surfactants, antioxidants and stabilizers. Propellants are typically liquified gases that are liquified by cooling or by pressurizing. Once liquified and placed within a closed system such as a canister, a gas-to-liquid equilibrium establishes, with a liquid portion on the bottom, and a gas phase of the propellant above the liquid. This gas phase exerts a constant pressure in every direction. A tube known as a "dip tube" is a tube which goes from down within the liquid, up to the nozzle where the substance is released. When the nozzle or canister is pressed, this positive pressure expels the liquid substance(containing the liquified propellant and the product concentrate) out of the nozzle at a high rate, at which point the propellant evaporates into its gas phase and the product concentrate is delivered to the lungs, skin, or other surface. As more of the liquid is released, the pressure remains constant because more of the liquid phase turns to gas from the pressure change, reaching its equilibrium. The liquified gas often serves as the solvent.

 Aerosols differ by the amount of propellant, and by the level of pressure within the container. Space sprays are sprays intended to propel the product concentrate particles into the air, as opposed to surface sprays which are intended to be sprayed onto a surface. Space sprays include room deodorizers and insecticides, and surface sprays include perfumes and antiseptic sprays. Space sprays have a smaller particle size (below 50 um), a higher pressure(30-40psig), and a higher percentage of propellant(up to 85%), as where surface sprays can have larger particles, a lower percentage of propellant(30-70%) and lower pressure(25-55psig).

 Foam aerosols involve the emulsification of the propellant into the product concentrate portion. Because the propellant is not soluble in water, the use of surfactants or emulsifying agents are used in order to help the propellant disperse throughout the product concentrate phase.

 Some aerosols consist of three phases. In addition to the two phases previously described, a third phase which consists of insoluble propellant forms a liquid layer separate from the product concentrate phase. In these systems the dip tube must be carefully placed to ensure that it is in the product concentrate phase, and not in the liquid propellant phase.

Propellants

CFC's or chloroflurocarbons are not used as commonly as they used to be due to environmental concerns(decreasing the level of ozone in the atmosphere). They are only used to special cases.

Carbon dioxide

Fluorinated hydrocarbons

Nitrogen gases



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