Suspensions Flashcards
What is the difference between Solutions and Dispersed systems (Suspension, Emulsions)?
Solutions: Homogenous systems
-solute and solvent have interacted very well
-particle size is less than 1 nm (not visible with eyes or microscope)
Dispersed system:
Coarse: 10-50 µm
Fine: 0.5-10 µm
Colloidal: < 0.5 µm (< 500 nm) can’t be seen with eyes, need fancy equipment to make it colloidal, the liquid is not turbid anymore
How is the solute distributed in solutions and dispersed systems?
Solution: SOLUTE is DISSOLVES
Dispersed System: SOLUTE is DISPERSED
Example for Solutions:
Ear drops, Eye drops, Injectables, Syrups, mouth washes,..
What is the purpose of dispersed systems?
Because some drugs are not that hydrophilic -> the drug will not dissolve in a full amount -> to achieve the preferred dose and to formulate a liquid we have to use dispersed systems
What is the size of most Suspensions and Emulsions?
-Coarse system: 10-50 µm
-particles are visible with eyes or microscope
-particle size will settle down eventually
What is the DEFINITION of Suspensions?
Consists of solid particles dispersed in a liquid phase in which the particles are NOT soluble
What are the two phases in a Dispersion -> in this case, Suspension?
Internal phase (SOLID): an insoluble substance that is distributed
External phase (LIQUID): vehicle in which the insoluble substance is distributed
What is the purpose of Suspension?
- Taste masking: the solid form tastes less -> so if we want to have a drug that is bitter and we still want it in a liquid form, we can make a suspension with a poorly soluble salt(f.e. Erythromycin has a bitter taste, the suspension Erythromycin estolate form is tasteless)
- Stability: some drugs are not stable in water bc they will hydrolyze (f.e Aspirin), and can’t be stored as a solution -> formulate such as it is less soluble -> Suspension with poorly soluble salt
- Easier to administer for pediatrics and geriatrics having difficulties taking solids -> make a suspension out of tablets or capsules
IN-CLASS ACTIVITY: Vials #2 and #3 were suspensions,
#3 was thicker and the particles were visible followed by settling down of the particles
WHY weren’t there visible particles in Vial #2?
Because Vial #2 has suspending (thickening) agent that keeps the particles suspended and makes them look nice and uniform
Did the agent reduce the particle size in vial #2 and thereby caused a uniform and stable suspension??
What are the CHARACTERISTICS of Ideal Suspensions?
-Particles settle slowly and the suspension is re-dispersible after shaking
-Narrow size distribution: less likely to form a cake (bigger particle size come together with smaller ones forming a cake)
-Should pour easily from the container, but not as free-flowing as water; we want a certain degree of viscosity, to build a layer in the throat and a convenient feeling
-> PSEUDOPLASTIC behavior, Viscosity goes down with shaking
->THIXOTROPHIC behavior, Viscosity reduction remains during the pouring process
What does Stakes Law try to explain in terms of Dispersed Systems?
-What can be done with drug particles and the liquid dispersion phase so that it will stay stable
-settling velocity of the spherical (round) particles in a fluid medium, considering the forces acting (frictional forces from the medium (H2O) on the particles when it is settling (bc of gravitation)
REMEMBER STOKES EQUATION
Stokes Equation:
Factors that determine the rate of settling
-the numerators are directly proportional to the settling rate, hence when increasing f.e. the particle size it will increase the rate of settling and vice versa
! In the case of suspension we still want particle size in the range of 10-50 µm, because Fine sizes are prone to form a cake
-for particle density it is about the difference between the particle and medium density, we want to keep the difference small for low settling -> BUT by reducing the particle density, not the medium density ?? -> for medium density we change both??
-viscosity is inversely proportional, by increasing the viscosity (Cellulose, Glycerin, Propylene glycol, Sucrose), we reduce the settling
What is the Zeta potential?
Every particle has its own charge (with its own Nertsch potential), and excipients can also have their own charge, when particles (with opposite or same charge, repelling or attracting) come together they determine the charge of the next localized particles
What is the Zeta potential in ideal suspensions and how can a suspension be FLOCCULATED?
-30 mV to build Flocks -> FLOCCULATED
-Flocculated are easy re-dispersible
-We want to induce a limited interaction between particles by building oppositely charged particles, if the potential is too high or low it will cause too much separating (cake formation) or attraction (aggregation)
So what is a Flocculating agent?? Does it change the charge of particles so that it is in the range of 30 mV??
What is the difference between Flocculated and Deflocculated systems?
Flocculated: Narrow size distribution and easily re-dispersible
-Particles in aggregates or flocs
-loosely packed and a scaffold-like structure
-Particles do not bond tightly -> no cake
-Suspension is unsightly
Defloccularted: Wide size distribution and might be not re-dispersible (Cake-formation)
-Particles in separate entities
-closely packed, due to the weight of the upper layers
-pleasing appearance
-Rate of sedimentation is slow
