Evaporation Condensation Methods

Methods of stable aerosol generation by evaporation of the bulk material and subsequent condensation to the aerosol form have generally evolved from the method developed by Sinclair and La Mer [8]. The aerosol was formed from the mixing of a flow of condensation nuclei with condensable vapor produced from heating a liquid with a relatively high boiling point and subsequent cooling. The output from this original method of aerosol generation was typically 105-106 particles cm"3 with particle sizes from 0.1 to several micrometers, but this was not very stable or reproducible. Enhancements and improvements to the stability and reproducibility have since been developed (see Ref. [3]). The evaporation-condensation method of aerosol generation that has evolved is depicted in Fig. 1.

The primary requirement is a source of vapor and a less volatile material that serves for condensation nuclei. Sodium chloride is commonly used to produce nuclei in the 10-nm-size range. This can be accomplished by heating the bulk material or by atomizing a dilute solution and drying the aerosol. The nuclei source can be a component (impurity) of the bulk material used, such as in the Rapaport-Weinstock generator, in which the impurity in the nebulized liquid serves as the condensation nuclei [9]. The use of heterogeneous nucleation for the formation of particles leads to a substantially more controllable and monodisperse aerosol than without the use of nuclei, that is, homogeneous nucleation.

The size of the particles is determined by the particular material selected and the vapor concentration used. In practice, limited variation in particle size can be achieved for a particular aerosol material because conditions for stable aerosol formation require a particular set of thermal and vapor concentration conditions. The monodispersity of the aerosol can be improved by revaporization and recondensation. In systems in which the condensation occurs in a container with a high ratio of volume to surface areas, relatively monodisperse particles can be obtained (sg ~ 1.1). Otherwise, the particle size varies with the proximity to the wall. In cylindrical or tubular systems, such as in the condensation aerosol generator developed by Liu et al. [10] or the falling-film generator, the particle size that is produced varies radially (see Ref. [3]). A more monodisperse aerosol can be produced by extracting the central portion of the flow, which is less subject to wall effects. Liu et al. [10] found that the monodispersity improved from a sg value of 1.35 to 1.15 by using only the central 5% of the aerosol flow. A commercial version of a modified Sinclair-LaMer generator is available with particle size control suited for inhalation studies [11].

Figure 1 Schematic of evaporation-condensation method of aerosol generation.

The bulk material used for the vapor source is not restricted to liquids, because solids and even refractory materials have been used [12]. The difficulty is the requirement of higher temperatures (high-temperature furnaces) and a suitable material for the nuclei. Solid particles produced by the evaporation-condensation method tend to be spherical.

Temperature control is essential for maintaining a constant vapor or nuclei concentration. The vapor flux is controlled by the exposed surface area of the bulk material in addition to the temperature. If the vapor source is derived from heating the bulk material, the exposed surface area of the bulk material used in the furnace will change as the material is consumed. After the furnace is turned off, condensation of the vapor will occur on the walls. Consequently, stability and reproducibility problems usually occur in generator systems using furnaces. These problems can be overcome by using a constant-output atomizer or nebulizer system, but a liquid is required. The vapor is obtained by heating the aerosol. Prolonged heating of the bulk liquid, which may lead to decomposition, is avoided. A low concentration of a less volatile substance is usually added to liquid to serve as a stable nuclei source. Impurities in the liquid may also act as nuclei [11]. Otherwise, an external source of nuclei is used. Condensation to form the aerosol takes place in a long tube, where laminar flow is established. Use of the central core of the aerosol flow may be required for a mere monodisperse aerosol. Long-term operation of several hours can easily be achieved by using a large liquid reservoir for the atomizer. Additional control of the vapor source can be obtained by using a solution instead of a pure liquid. Hence, solids can vaporized by this method if a suitable solvent is available. A diffusion dryer may be required to lower the vapor concentration of the solvent. Buildup of the aerosol material on the tube wall of the condensation section will occur from condensation, and particle deposition from long duration operation. Degradation in the output may occur, and the section will have to be cleaned.

The evaporation-condensation method of aerosol generation can be used to produce particles with coatings. Espenscheid et al. [13] generated coated particles by condensing linolenic acid on silver chloride particles. Several other researchers have used similar techniques for the generation of coated particles. More recently, Lee et al. [14] generated diesel particles coated with polycyclic aromatic compounds. In principle, any aerosol particle can be coated by condensing a more volatile substance on it. The thickness of the coating will depend on the vapor concentrations and the coating time. In addition to the increase in particle size after coating, a less monodisperse aerosol can be expected. However, a recent technique developed by Couper et al. [15] produced a monodisperse aerosol (sg = 1.1) of hydrophobic particles with a hydrophilic core. The standard coating technique of using two separate components was combined into a single step by using a modified Rapaport-Weinstock generator with a collision atomizer to disperse a solution of the two components in ethanol.

A different technique was used by Durand Keklikian and Partch [16] to generate particles with a surface coating. Previously, a more volatile substance was coated on a less volatile particle. For their case, oil droplets coated with metal oxide were generated. This was accomplished by nebulizing solutions of titanium or aluminum alkoxides in oil. Hydrolysis of the alkoxide to the oxide occurred in the presence of water vapor, forming a solid shell encapsulating the oil droplet.

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