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Characteristics of drug include:
- Physical description
- Microscopic examination
- Melting point
- The Phase Rule
- Particle size*
What did we find out about particle size and it’s impact on drugs?
- Particle size affects solubility
- Solubility affects dissolution
- Dissolution affects absorption
What can particle size affect?
formulation and efficacy
Particle size impacts the equal distribution of the active ingredient
Physical and Chemical Properties Affected by Particle Size
- Dissolution rate
- Content uniformity
- Sedmentation rates
Unit of matter having a defined physical dimension
Science and technology of small particles. Study of characteristics like particle size and distribution, shape, angle of repose, porosity and others
- Particle size and size distribution
- Pore size distribution
- Particle shape
- Particle surface area
- Angle of repose
- Flow Properties- dusting powders
- True and bulk volumes
- Apparent density and bulkiness
When a pharmaceutical powder contains particles of wide range of sizes and shapes, then it is called polydisperse sample
When a sample has particles of same size and shape, then the powder will be called monodisperse sample
Two most important parameters for pharmaceutical powders are:
- The shape and surface area of individual particles
- The size range and number or weight of particles present
When the number or weight of particles of certain size range is plotted against the size range or particle size...
a frequency distribution curve is obtained
Two samples may have different distributions even though...
their average diameter is the same
How can particle size distribution be plotted?
Using either weight or number of the particles
Equivalent Spherical Diameter:
- This parameter is used to describe particle size of asymmetric particles.
- Equivalent spherical diameter relates size of the particle to the diameter of a sphere having the same surface area, volume, or diameter
Surface diameter (ds)
Diameter of a sphere having the same surface area as the particle in question.
Volume diameter (dv)
Diameter of a sphere having the same volume as the particle in question
Stokes’ diameter (dst):
Equivalent sphere undergoing sedimentation at the same rate as the asymmetric particle.
Projected Area Diameter:
This is depicted as dimension 'C' and is the diameter of a theoretical circle, which would contain the same projected area as the irregular particle.
This is the diameter of a spherical particle that exhibits the same settling velocity in the air as the irregular particle
Techniques to determine Particle Size:
- Coulter Counter
- Dynamic Light
- Laser Diffraction
- Series of standard sieves
- Used for grading coarser particles.
- The larger the mesh number the smaller the particle size of the powder
- Ordinary microscope is used to measure particle size in the range of 0.2 μm to about 100 μm.
- Depth cannot be measured
Particle size in the subsieve range may be obtained by gravity sedimentation as expressed in Stokes’ law Particle diameter corresponding to various time periods is calculated from Stokes’ law
The Coulter Principle states that amplitude of this pulse is directly proportional to the volume of the particle that produced it.
Principle of Laser Diffraction:
- Light from a laser is shone into a cloud of particles (e.g. cement), which are suspended in a transparent gas (e.g. air)
- The particles scatter the light
- Scattered light measured by photodetectors. This is known as the diffraction pattern for the sample.
Importance of Particle Size (surface area)
- Affects the physicochemical properties of a drug and hence the pharmacological properties
- Affects the route of administration
- Formulation consideration – Flow, type, preparation
- Stability of formulation
- Dissolution and absorption characteristics
- Efficacy of therapeutic agent
Particle Size also affects:
- Physicochemical properties like bioavailability, dissolution rate, content uniformity, taste, stability, color and texture
- Affects flow and sedimentation rates
- Delivery of inhalation drug substance to the correct part of the lung at the correct dose
- Packing of powders
How does particle size affect surface area:
- The smaller the size of a particle, the greater is its surface area
- The greater the surface area, the better the dissolution
When is dissolution most important?
Absorption of drugs
How do particles pass intestinal membrane:
Dissolution: Only particles in the solution form pass intestinal membrane into systemic circulation for absorption
What is the rate of absorption limited by?
Rate of absorption for many slightly soluble drugs is limited by rate of dissolution
When rate of solution is slower than the rate of absorption...
then the dissolution process becomes the rate limiting step
If you don’t remember anything else…
Decreased Particle Size: Increases Surface Area
Increased Surface Area: Increases Dissolution
Particle size is an important factor when it comes to topical preparations because...
- Reduced and uniform particle size makes the preparation smooth to touch and non-irritating
Particles of different sizes lead to stratification which in turn leads to...
For internal use, the finer the particle the higher the rate of dissolution thus improving...
Particle size is an important factor when it comes to powders , tablets and capsules for the following reasons:
- Free flow of powders to compress tablets
- Content uniformity
- Drugs with particle size between 1 – 3 µm are deposited into the alveolar region
- Larger particles deposit in the upper airways
- Particles less than 0.5 µm are either expelled or inhaled
Ocular suspensions formulas should...
95% of the particles should have a size 10 micrometers or less
Topical creams, ointments formulation:
Finely divided particles for better texture and feel
Particle size is of great importance due to possible problems with embolism and toxicity
Mechanical process of reducing the size of fine particles or aggregates
- Manual – trituration, levigation, pulverization
- Mechanical – grinders, coffee mills
When should nonporous mortars be used?
i.e. glass, when comminuting dyes and potent ingredients
- Continued rubbing of solids in a mortar with a pestle to reduce the size of the solid’s particles to a desirable degree of fineness.
- Typically w/ dry particles
There are 3 types of mortars and pestles:
- Glass, wedgewood and ceramic
- Glass mortars have smooth sides and so may not be as efficient in reducing particle size in comparison with ceramic and wedgewood
- Reduction in particle size by trituration or by spatulating on an ointment slab with addition of a non-solvent to form a paste
- To improve ease of wetting, the solvent used is generally viscous with a low surface tension
Pulverization by intervention
- Used for compounds that cannot be easily triturated Drug is dissolved in a minimum amount of solvent
- The solution is spread onto sides of a glass mortar or on an ointment slab as a thin layer and the solvent is allowed to evaporate.
- The fine crystals are then scraped off the surface using a metal spatula
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