Exam 3 Flashcards
Dispersed System
Mixture of one phase in another largely immiscible phase
What is another term for dispersed phase?
Internal phase
What is another term for external phase?
Continuous phase
Suspension
Solid particles dispersed in a liquid medium
Emulsion
Liquid droplets dispersed in a liquid medium
3 classifications of a dispersed system
Molecular, colloidal, coarse
Advantages of suspensions
Preparation of poorly soluble drugs, masks taste, easier to swallow, prepare oral, IV, topical
Disadvantages of suspensions
Instability, non-uniformity, bulkier
3 examples of suspensions
Insulin, magnesium hydroxide, penicillin G procaine
Suspendable
Drug substance is uniformly dispersed after shaking container
Re-dispersable
Settled solid phase is readily and uniformly dispersable
Desired physical properties of suspensions
Suspendable, re-dispersable, transferable, smooth/grit free
Physically stable
State of no aggregation and uniform distribution of particles
Interface
Boundary between existing phases
T/F every surface is an interface
True
If you increase the surface area, what happens to the free energy
increases
T/F Increased surface energy creates a stable system
False, thermodynamically unstable
Decreasing particle size causes what to increase?
Interfacial area
Agglomerate
Stick together
Why do you use wetting agents
Increase solubility and increase affinity to liquid to decrease agglomeration
Define wetting
Solid-air interface is changed to solid-liquid interface
Potential determining ions
Specifically absorbed ions that influence the surface charge
Stern layer
Tightly bound layer of solvent and counter ions on the solid surface
Surface potential
Electrical potential on the surface
What does the presence of counter ions do in the stern layer?
Shields solid surface charge and decreases electrical potential across the stern layer
Shear plane is also called
the slipping plane
Shear plane
Shell of solvent molecules and ions existing around each particle beyond the stern layer
Where is the zeta potential found
Shear plane
Zeta potential
Potential difference between the shear plane and the electrically neutral bulk medium
What does the zeta potential dictate
Magnitude of electrical repulsion
Major forces that suspended particles are subject to
Electrical double layer repulsion and Van der Waals attractive forces
Zeta potential has positive or negative energy of interaction
Positive
Van der Waals attractive forces have positive or negative energy of interaction
Negative
Where does attraction predominate
Small and large distance – Primary and secondary minimums
Where do electrical repulsions predominate
Intermediate distances – primary maximum
Flocculation
Easy to suspend particles
When is a secondary minimum not observed
High repulsive forces
When are flocs formed
At secondary minimum
What do high zeta potentials create
High repulsive forces; primary maximum
What happens when zeta potentials are very high
No secondary minimum, deflocculated suspension
When does coagulation occur
If particles overcome the repulsion barrier and approach primary minimum
Caking
Non dispersible hard sediment
Rheology
Study of the flow behavior of liquids and the deformation of solids
Viscosity
Resistance to flow
What is viscosity a measure of
Molecular friction that resists flow
Shear stress
Tangential force applied per unit area which produces flow
Rate of shear
change in velocity with distance
How do you find viscosity
Shear stress/rate of shear
Newtonian liquid
Rate of shear increases proportionally with shear stress, viscosity is constant
Plastic flow
Not observed until minimum shearing stress is applied (Yield)
What is exhibited below the yield value
Elastic behavior
What happens above the yield value
newtonian behavior
Pseudoplastic flow
No yield point, viscosity decreases with increase in shear stress
Pseudoplastic is what kind of behavior
Shear thinning
Dilatant flow
Rare, increase viscosity when increase rates of shear
What kind of behavior does dilatant flow show
Shear thickening
Stokes law
Rate of sedimentation of uniformly sized particles in dilute suspensions
Deflocculated
Particles exist in suspension as separate particles, stays dispersed longer than flocculated, low sedimentation rate
Flocculated system
Loose aggregates with high sedimentation rates
Sedimentation volume ratio
Equilibrium volume/total volume, want to be 1 (Flocculation with no sediment)
Degree of flocculation equation
Ultimate sediment volume of flocculated suspension/ultimate sediment volume of deflocculated suspension
Degree of flocculation
Measure of the extent of increase in sediment volume achieved by flocculation
What do surfactants do
Decrease interfacial tension, promote wetting of solids, facilitate dispersion
3 types of flocculating agents
Electrolytes, surfactants, polymers
Structured vehicle
Provides viscous medium to slow down sedimentation of suspended particles
Clay vehicles
Provide a 3D network of interacting particles minimizing flocs
Polymeric vehicles
Shear thinning properties make re-dispersion easier and minimize settling
Organleptic additives
Aspects of a product that the patient experiences through the senses
What can thickeners cause in formulations
Microbial growth
Levigation
Triturating powders in a mortar or using a spatula on a tile with a small amount of liquid, reducing particle size and wetting the surface
Gels
Semisolid systems containing gelling agents which are suspensions of small inorganic particles or polymer solutions
Jellies
Gels with high water content in the matrix
Gel network
Interlaced structure increasing the internal friction and provides resistance to flow
4 gel structures
Randomm coils, helix, stacks, house of cards
Random coils
Synthetic polymers such as cellulose derivatives
Helix
Intertwined molecular chain structure such as xanthan gum
Stacks
Cross linking of polymer chains by divalent cations
House of cards
Alignment of positively charged edges with negatively charged flat surface of the particles
Liquid phase gel classifications
Hydrogels and organogels
Gel former/structure classifcation
Single phase and two phase
Thixotropy
Breakdown of structure that does not re-form immediately when stress is removed
How can gelling be induced
Gelling agent, pH, cross linking ions, thermal gelation
Reversible thermogelation
Change in temperature causes viscosity to go one way, but alter returns
Emulsion
Thermodynamically unstable mixture of two immiscible liquids with two phases stabilized by an emulsifier
Two emulsion classifications
O/W, W/O
Bancroft’s rule
The phase in which the emulsifier is more soluble constitutes the continuous phase
Advantages of emulsions
Preparation of two immiscible liquids, administration of non-aqueous liquid phase, masks taste, improved bioavailability, IV administration with oil, external application
Ideal Characteristics of emulsions
Constant, small droplet size, appropriate consistency, redispersible
Physically stable
Product is where we want it, may be thermodynamically unstable
If droplets of an emulsion coalesce, what happens
decrease in free energy, causes settling to be faster
What causes cracking
Combination of creaming and coalescence
Creaming
Oil droplets rise, increasing amount of droplets
Emulsifying agents include
Surfactants, hydrated lyophilic colloids, auxiliary, finely divided solids
What do surfactants do in emulsions
Create monomolecular films, reduce interfacial tension, facilitate breakdown of internal phase, prevent coalescence, creates repulsion
What type of surfactant is not susceptible to pH change
Non ionic
HLB value
Hydrophile lipophile balance, affinity for water and oil phase in non ionic surfactants
High HLB
More soluble in water, o/w emulsion
Low HLB
More soluble in oil, W/O emulsion
Griffin method
Proportion between weight percentages of hydrophilic and lipophilic groups in non ionic surfactant molecule would dictate surfactant behavior
Davies method
Sum of groups + 7
What do hydrated lyophilic colloids do in emulsions
No effect on interfacial tension, form multi molecular films around droplets, creates stability when dissolved in external phase
What do finely divided solid particles do in emulsions
Orient at interface and form coalescence barrier
How do auxiliary emulsifiers function
Thicken or form gel structure to provide coalescence barrier
Emulsion stabilization mechanisms
Repulsion due to like surface charges (Zeta), Tightly packed surfactant molecules to hinder coalescence, adsorbed polymer trans physically hinder, polymer strands form bridges, solid particles at interface hinder coalescence
Destabilization of emulsions
Temperature changes, microbial growth, addition of chemical agents
Semisolids
Viscous preparation that is not pourable and does not flow under low shear stress at room temperature but exhibits spreadability on skin and mucous membrane surfaces
Uses of semisolids
Protect skin/mucous membrane, promote hydration, provide a vehicle
Properties of topical products
Occlusive, humectant, emollient, protective
Occlusive
Promotes retention of water by forming hydrophobic barrier that prevents evaporation of moisture from within skin
Humectant
Causes water to be retained because of its hygroscopic properties
Emollient
softens and reduces irritation
Ointment
Semisolid preparation intended for external use containing less than 20% water
Creams
Semisolid emulsion containing more than 20% water
Pastes
Stiff ointments with high concentration of solid particles in fatty vehicle, more adherent when applied
Types of ointment bases
oleaginous, absorption, water washable, water soluble
Oleaginous bases
Emollient, occlusive, not water washable, hydrophobic, greasy
Absorption base types
hydrophilic anhydrous or w/o emulsion
Absorption base
Emollient and occlusive, greasy, can incorporate water
Water washable bases
O/W, non occlusive, non greasy
Water soluble base
Non greasy, non occlusive, cannot use on weeping skin, anhydrous, water washable
Outermost layer of skin
Stratum corneum, major barrier for drug
Transdermal routes
Transcellular, intercellular, transfollicular
