the drug particles to assist in the suspension formulation stability. Table 5 shows the characteristic physicochemical properties of the common propellants.

Hydrocarbons have also been considered as potential propellants for pharmaceutical aerosols. To date, concerns regarding flammability seem to have precluded significant developments with propane, isobutane, butane, and mixtures of these alkanes (44).

A cosolvent, typically ethanol, may be used to bring drug into solution. A small number of insoluble surfactants (sorbitan trioleate, oleic acid, and lecithin) may be dispersed in propellant systems and can aid in suspension stability and in valve lubrication.

The significance of a propellant is its ability to generate high-velocity emission as it vaporizes upon equilibrium with atmospheric pressure. Raoult's and Dalton's laws may be applied to estimate the vapor pressure of propellant blends.

Raoult's law states that the partial pressure (f) is equal to the product of the pure vapor pressure (P°) and the mole fraction (X) of the component being considered as follows (45,46):

where for a two-component system (a and b)

and nb

where na and nb are the number of moles of components a and b, respectively, present in the product.

Dalton's law states that the total vapor pressure (fT) over a solution is equal to the sum of partial vapor pressures attributable to each component (a, b, ... n).

An empirical relationship between median droplet diameter, Dh and atomizer conditions has been demonstrated (47). The relationship is described in the following expression.

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