Pharmaceutical powders Flashcards
What is a powder?
Its a pool of solid particles. They can have the same composition or be a mixture of different chemical compositions ( powder blend)
In drug formulation, most of the time, we will be working with mixtures of different powders. In most cases, particle sizes will be below 100 micrometers.
What similar behaviours do powders show to solids, liquids and gases?
Powders FLOW like LIQUIDS
Powders RESIST DEFORMATION ( to a point ) like SOLIDS
Powders CAN BE COMPRESSED like GASES
What is the benefit of the concept equivalent sphere?
- Simplify size analysis for powders
- Allow comparisons based on a single value.
This sphere requires a powder particle, independently of its shape, to be re imagined as an hypothetical sphere. Then it is possible to estimate particle size.
What are the key points of the equivalent sphere?
The equivalent sphere does NOT provide the true particle size, but an acceptable approximation. This approximation will be enough to allow the formulation scientist to predict the behaviour of the powder.
Different methods will generate different values for the diameter of powder particles.
What do Microscopy-based methods determine?
- Projected diameters which can be either:
- Independent of particle orientation: Projected perimeter diameter (dp), Projected area diameter ( da)
- Dependent on particle orientation: Martin’s diameter ( dm)
- feret’s diameter ( df)
How do you calculate Martins and Ferets diameter?
What is Sieving and how is equivalent sphere diameter obtained through it?
In lab, sieves of different sizes are stacked up on top of a reciever pan in decreasing order of mesh aperature size. Or an airjet sieve can be used.
The equivalent sphere diameter is obtained through a seiving experiment called the sieving diameter ( ds ) and corresponds to the minimum aperture size that will let the particle through.
What happens when an incident light hits a particle?
Describe laser diffraction in powders.
Particle size range: 1-1000 micrometers
sample type: dry powder or suspended solids
Equivalent sphere diameter obtained: Projected area diameter ( da); volume diameter ( dv) after mathematical extrapolation
Principle: Particles in a liquid or dry suspension are illuminated with a laser beam at a known wavelength, resulting in light scattering. When light hits a particle, it will be diffracted at an angle that varies with particle size. For large particles, the light will be diffracted, but with only a small change in angle compared to the incident beam. In contrast, small particles will diffract light at a much wider angle . The intensity of the diffracted light also varies as a function of particle size, with larger particles generating stronger intensities.
Describe dynamic light scattering.
Dynamic light scattering:
Particle size rangeL 2-2000 nm ( colloidal particles)
Sample type: Suspended solids
Equivalent sphere diameter obtained: Hydrodynamic diameter
Principle: Fluctuations in the intesity of scattered light are measured. Small particles will move faster, creating rapid flucuations in the signal. Similar to laser diffraction, particle size will have an impact on the intensity of the light scatterd with larger particles associted with a higher dynamic light scattering intensity.
Describe coulter counter.
- The electrical resistance of a conducting liquid is increased by the addition of an insulating material. Particles are assessed individually. To obtain adequate sensitivity, the volume of liquid measured must be similar to the volume of the particle.
Describe the sedimentation method.
- The larger the particles the faster the sedementation
- The law can be used to estimate the largets particle size still expected to be in suspension at a specific time point
What is size distribution based on?
The number of particles within each size class ( number distribution)
This is the type of distribution generated from, eg, microscopy-based methods
The weight of particles within each size class ( weight distribution)
Sieving analysis
Length, surface and volume distributions are also possible depending on the method used for analysis.
What are the two forces flowability depends on?
- Forces that drive flow ( flow-promoting) such as : Gravity, Particle mass, Angle of inclination, Applied mechanical force
- Forces that resist ( drag flow) such as: Adhesion, Cohesion ( both forces will vary with particle size and moisture content), Electrostatic interactions, Surface forces.
Will a powder flow if the flow promoting forces outweight the drag force?
Yes, it will. In many cases flowabaility is affected by the properties of powder particles.
What are the better flow properties for powders?
For powders made of uniform, spherical particles, cubic and rhombohedral packing. Which of these is the loosest and tightest?
What is porosity in terms of powders?
It is the measure of how loosely and tightly particles are packed. For particles arranged in a cubic geometry, ca. 48% of the powder volume is void. This proportion drops to 26% for rhombohedral packing.
How do you calculate bulk density and bulk volume ?
Wat is the angle of repose and how do you calculate it?
The angle of repose (AOR) is determined after a powder is left to flow and forms a cone on a flat surface.
The flatter the cone, the smaller the AOR, the better the flow.
However, because there are different ways to measure the AOR, this indirect method may not be best to assess flowability.
How do you calculate poured density from poured volume?
M =mass of the powder
V0 = poured volume.
It is important NOT to distrub the powder at this stage as this could affect the reading.
How do you calculate Tapped density from tapped volume?
m=mass of powder
VF= Final tapped volume
How do you calculate Hausner ratio and Carr’s index?
What are hydroscopic, deliquescent and efflorescent powders?
