Particle Size and Size Distribution

**Particle Size and Size Distribution** Micromeritics is the science and technology of small particles. Knowledge of particle size and size distribution is important in pharmacy because it affects the physical, chemical, and pharmacological properties of drugs In a collection of particles of more...

**Particle Size and Size Distribution** Micromeritics is the science and technology of small particles. Knowledge of particle size and size distribution is important in pharmacy because it affects the physical, chemical, and pharmacological properties of drugs In a collection of particles of more than one size (in other words, in a polydisperse sample), two properties are important, namely, (*a*) the shape and surface area of the individual particles and (*b*) the size range and number or weight of particles present and, hence, the total surface area. Colloidal dispersions are characterized by particles that are too small to be seen in the ordinary microscope, whereas the particles of pharmaceutical emulsions and suspensions and the ―fines‖ of powders fall in the range of the optical microscope. The size of a sphere is readily expressed in terms of its diameter. As the degree of asymmetry of particles increases, however, so does the difficulty of expressing size in terms of a meaningful diameter. Thus an [equivalent spherical diameter], which relates the size of the particle to the diameter of a sphere having the same surface area, volume, or diameter. Polydispersity index (PDI) is used as a measure of broadness of molecular weight distribution. The larger the PDI, the broader the molecular weight.  **[Methods for Determining Particle Size]** +-----------------------------------+-----------------------------------+ | |  | +===================================+===================================+ | |  | +-----------------------------------+-----------------------------------+ | |  | +-----------------------------------+-----------------------------------+ | |  | +-----------------------------------+-----------------------------------+ | | | +-----------------------------------+-----------------------------------+ |  | | +-----------------------------------+-----------------------------------+ |  | | +-----------------------------------+-----------------------------------+ | |  | +-----------------------------------+-----------------------------------+ |  | | +-----------------------------------+-----------------------------------+ 1. ***Angle of repose*** - The particles will begin to slide when the angle of inclination is large enough to overcome frictional forces. Conversely, an object in motion will stop sliding when the angle of inclination is below that required to overcome adhesion/cohesion. The sides of the heap formed makes an angle with the horizontal called the angle of repose and is a characteristic of the internal friction. - The value of the angle of repose will be high if the powder is cohesive and low if the powder is non-cohesive. - If the powder is very cohesive, initially the interparticle cohesion causes a very steep cone, but on the addition of further powder, this tall stack may suddenly collapse, causing air to be entrained between particles and partially fluidizing the bed, making it more mobile. **Flow Property** **Angle of Repose** ------------------- --------------------- Excellent 25-30° Good 31-35° Fair 36-40° Passable 41-45° Poor 46-55° Very poor 56-65° Very, very poor ≥65° **Angle of repose is measured by: tan Ø = h/r** **h** is the height of the powder cone and 2. **True volume** The true volume, V, of a powder is the space occupied by the powder exclusive of spaces greater than the intramolecular space. 3. **Bulk volume** It is the total volume occupied by powder and voids together. It is equal to true volume plus void volume (porosity). Bulk volume V~bulk~ = V + porosity. 4. **Porosity** It is the void volume exclusive of powder material. Porosity = V~bulk~ -- V/ V~bulk~ 5. ***Particle density (true density)*** As powders normally flow under the influence of gravity, dense particles are generally less cohesive than less dense particles of the same size and shape. 6. **Bulk density** **It is the weight of bulk divided by its bulk volume.** **Bulk density = weight of bulk/** V~bulk~ 7. **Tapped density** The tapped density is an increased bulk density attained after mechanically tapping a container containing the powder sample. **Tapped density = weight of bulk/** V~tapped~