Particle size dependence solvent evaporation

The importance of particle sizing is that only small particles will reach the lung, since the nose and mouth will remove any larger particles. There is continuing debate as to what maximum size will reach the lung, but consensus is for around 5 to 7 um. For alveoli penetration, a size of 3 lam is required, but very fine particles, below 1 jun, may be exhaled. It is the size of the droplets in the aerosol cloud that is important for dry powders and fully evaporated liquid suspensions, this maybe the original powder particle. Hence, it is essential to have powders in the micron range, and normal micronisation will usually produce a size of 1 to 3 jun. Newer techniques, such as supercritical fluid extraction, can produce smaller particles. Naturally, if the product is a solution, the particle size formed on evaporation of the solvent will depend on solution concentration. 

CdjPj and CdjAs are low band gap semiconductors (0.5 and 0.1 eV, respectively). The bulk materials are black and start to absorb in the infrared. These materials have been prepared as colloids in alkaline solution by precipitation of Cd with phosphine and arsine Depending on the conditions of preparation, particles of different sizes (between about 2 and 10 nm) were obtained, which could also be recovered in the solid state after evaporation of the solvent. The color of these materials ranged from black to colorless with decreasing particle size, with all kinds of intermediate colors in the visible. 

Preparation of nanoparticles can be by a variety of different ways. The most important and frequently used is emulsion polymerization others include interfacial polymerization, solvent evaporation, and desolvation of natural proteins. The materials used to prepare nanoparticles are also numerous, but most commonly they are polymers such as poly-alklcyanoacrylate, polymethylmethacrylate, poly-butylcyanoacrylate, or are albumin or gelatin. Distribution patterns of the particles in the body can vary depending on their size, composition, and surface charge [83-85]. In particular, nanoparticles of polycyanoacrylate have been found to accumulate in certain tumors [86,87]. 

An organosol is the same mixture as described above, with the addition of solvent to reduce viscosity. These find their major applications in coatings. The solvent is evaporated before fusion of the film. Various pigments, colorants, stabilizers and fillers may be added, depending on the desired properties. Emulsion polymerization resins are generally employed because of their fast fusion rates. Coarser particle sized PVC resins would require extended time at the elevated temperature. 

The properties of nanoparticles depend on surface morphology, specific surface area, particle size distribution, bulk density, drug incorporation, capacity, release, hydrophobicity, bioadhesiveness, and biodegradability. Nanoparticles (microspheres) loaded with the drug product can be formulated using copolymers, e.g., poly(lactide-co-glycolide) (PLG) or poly(lactide-co-ethylphosphate), by solvent extraction/evaporation technique. 

Separate large- and small-drop aerosols can be produced by taking advantage of the different stop distances of the primary and satellite droplets. Aerosols of the original pure liquids can be produced in this way with primary droplet diameters ranging from 6 to 3000 / im and liquid flow rates up to 168 cmVmin. When solutions with volatile solvents are used, the solvents can be evaporated, leaving behind particles whose size depends on the... 

Precipitation from a concentrated solution of cations can be performed by solvent evaporation. To ensure that the particle size remains small, the concentrated solution may be atomised at high pressure into fine droplets of 100-500 pm diameter the solvent is rapidly evaporated by an upward stream of hot gas. The particles obtained, which can be as small as 100 nm, are compacted and calcined to produce the ceramic. A schematic representation of the spray-drying process is shown in Fig. 3.4. Several alternative methods are currently under development they are known as aerosol synthesis, aerosol pyrolysis or mist pyrolysis, depending on the specific technique to produce the gaseous suspension of fine particles aerosols are produced in high pressure nozzles and mists are obtained by means of nebulisers. YIG particles (0.25 pm) have been obtained by mist pyrolysis (Matsumoto et ai, 1991) by nebulising an aqueous solution of... 

---