micrometrics

It is the science and technology of small particles. Knowledge and control of the size and the size range of particles are of significant importance in pharmacy because the size and surface area of a particle related to the physical, chemical and pharmacologic properties of a drug.

Importance of Particle Size Particle size of drugs in pharmaceutical products can affect: 1) Release & Dissolution Particle size & surface area influence the release of a drug from a dosage form that is administered orally, rectally parenterally & topically. Higher surface area brings about intimate contact of the drug with the dissolution fluids in vivo & increases the drug solubility & dissolution. 2) Absorption & Drug Action Particle size & surface area influence the drug absorption & subsequently the therapeutic action. Higher the dissolution, faster the absorption & hence quicker & greater the drug action.

3)In the area of tablet and capsule manufacture, control of the particle size is essential in achieving the necessary flow properties and proper mixing of granules and powder 4) Physical Stability Micromeritc properties of a particle i.e the particle size in a formulation influences the physical stability of the suspensions & emulsions. Smaller the size of the particle, better the physical stability of the dosage form owing to the Brownian movement of the particles in the dispersion

Particle Size and Size Distribution In a collection of particles of more than one size, two properties are important, namely. 1.The shape and surface are of the individual particles. 2.The particle size and size distributions (The size range and number or weight of particles).

Equivalent Spherical Diameter The size of a sphere is readily expressed in terms of its diameter. The Surface diameter, ds, is the diameter of a sphere having the same surface area as the particle. The Volume diameter, dv, is the diameter of a sphere having the same volume as the particle. The Projected diameter, dp, is the projected diameter of a sphere having the same observed area as the particle. The Stokes diameter, dst, is the diameter which describes an equivalent sphere undergoing sedimentation at the same rate as the asymmetric particle.

Methods for determining particle size: Many methods available for determining particle size such as 1. Optical microscopy (range: μm). 2. Sieving (range: μm). 3. Sedimentation (range: μm). 4. Particle volume measurement (range: μ m).

Sedimentation (range: μm) By measuring the terminal settling velocity of particles through a liquid medium in a gravitational centrifugal environment using Andreasen appartus. Different particle sizes settle at different rates through suspending medium The particle size in the subsieve range can be obtained by gravity sedimentation

Coulter Counter Method (Particle Volume Measurement) (range: μm) determine particle volume and particle size based on the conductivity measurement

Particle Shape andSurfaceArea Particle sizes combined with particle shape affect the packing properties and flow of the powder, and they also influence the surface area. It is generally accepted that the flowability of powders decreases as the shapes of particles become more irregular. The specific surface of a powder is defined as the surface area per unit volume (Sv) or per unit weight (Sw)

SURFACE AREA DETERMINATION METHODS 1-Adsorption Method: Particles with a small particle size (large specific surface) are good adsorbents for the adsorption of gases and of solutes from the solution. The amount of gas or solute adsorbed on the sample of powder to form a monolayer calculated, and from this data, the surface area of the powder is determined.

Air permeability method This method is based on the principle that the resistance offered to the flow of a fluid, such as air, through a plug of compacted powder is proportional to the surface area of the powder. The greater the surface area per gram of the powder, the greater is the resistance to flow. Surface area determination by the air permeability method is generally carried out with an instrument called the Fisher subsieve sizer

Porosity Porosity is a measure of the air spaces or voids in a material. The volume of space between particles may vary depending on the size, shape and density of the particles. Suppose a powder is placed in a graduated cylinder and total volume noted the total volume occupied is known as the bulk volume Vb. bulk volume = true volume + volume of spaces between particles. The volume of the spaces, the void volume, V = Vb - Vp Vp is the true volume of particles. The porosity or voids ε of powder is determined as the ratio of void volume to bulk volume. Porosity = ε = Vb – Vp/Vb = 1 – Vp/Vb Porosity is generally expressed in percent ε x 100

Densities of particles: Density is defined as weight per unit volume (W/V). Types of densities: A- true density:, or absolute density of a sample excludes the volume of the pores and voids within the sample.

bulk density (w/v) the bulk density : value includes the volume of all of the pores within the sample. is defined as the mass of a powder divided by the bulk volume. The determination of bulk density is used to check the uniformity of bulk in given sample. It is important in capsule manufacturing.

Granule density, p g, is determined by liquid displacement method. Mercury is used since it fills the void spaces but fails to penetrate into the internal pores of the particles.