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824 - Dosage Forms > Colloids > Flashcards

Colloids Flashcards

1
Q

What drug delivery systems employ colloidal systems?

A
  • Hydrogels
  • Microparticles
  • Emulsions
  • Liposomes
  • Micelles
  • Nanoparticles/Nanocrystals
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2
Q

Colloids Definition

A
  • Particles < 0.5 um dispersed in another phase
  • Liposomes, nanoparticles, dendrimers…
  • It is now recognized that nano-sized drug delivery systems may have significantly improved performance (i.e. treating cancer)
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3
Q

What particle size range is in colloidal systems?

A
  • 10 A - 5000 A
  • 1A = 10^8 cm = 10^-10 m = 0.1 nm
  • 1nm - 500nm (0.5 um)
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4
Q

What does a small particle size mean?

A

A small particle size means a large interfacial area and a system in which interfacial properties are important.
dG = surface tension x area

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5
Q

Characteristics of Colloids

A
  • Large SA
  • Drive to decrease surface free energy
  • Tendency for aggregation
  • Unequal distribution of phases - tendency to move to top or bottom
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6
Q

Surface Area

A
  • A cube w/1 cm edge has a volume of 1 cm^3 and surface area of 6 cm^2
  • If the material in the cube is subdivided into smaller cubes having an edge of 100um: 1) volume stays the same. 2) SA is 600,000 cm^2 (↑ 105 fold)
  • Surface phenomena are critical for colloidal systems
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7
Q

3 Types of Colloids

A

1) Lyophilic
2) Lyophobic
3) Association

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8
Q

1) Lyophilic Colloids

A
  • Strong and extensive interactions between dispersed phase and dispersion medium
  • Thermodynamically stable systems
  • I.e. gelatin in water, solution of chromosomal DNA
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9
Q

2) Lyophobic Colloids

A
  • Thermodynamically unstable systems
  • Little interaction between dispersed phase and dispersion medium (low adhesional forces)
  • I.e. inorganic particles in water, charcoal in water
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10
Q

3) Association Colloids

A
  • Thermodynamically stable systems
  • The dispersed phase consists of aggregates of amphiphilic molecules or ions (i.e. micelles)
  • At low concentration, these molecules exist in true solution
  • At high concentration, they aggregate to form micelles (CMC)
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11
Q

3 Types of Colloidal Properties

A

1) Optical
2) Kinetic
3) Electrical

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12
Q

1) Optical Properties

A
  • Cannot see them using a light microscope

- Faraday-Tyndall Effect: light may be absorbed, scattered, polarized or reflected by the dispersed phase of a colloid

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13
Q

2) Kinetic Properties

A
  • Brownian Movement: particles are generally small enough to be influenced by the collision with molecules of the dispersion medium
  • When particles are observed, they are seen to move in a random, erratic manner
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14
Q

Consequences of Brownian motion

A
  • Stable colloids are systems in which the dispersed particles do not settle, because the force of gravity is counteracted by Brownian movement
  • Colloids will diffuse from a region of high concentration to a region of low concentration
  • Fick’s Law of Diffusion
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15
Q

Fick’s Law

A
  • dq = amount of substance
  • dt = time
  • S = plane of area
  • dc = change of concentration
  • dx = distance travelled
  • D = diffusion coefficient
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16
Q

Stokes-Einstein Equation

A
  • k = Boltzman constant
  • T - absolute temperature
  • n = viscosity of solution
  • R = hydrodynamic radius of the particle
    D = kT/6πnR
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17
Q

What is the effect of INCREASING PARTICLE SIZE on the rate of diffusion of a colloid?
D = kT/6πnR

A

Decreases rate of diffusion

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18
Q

What is the effect of INCREASING TEMPERATURE on the rate of diffusion of a colloid?
D = kT/6πnR

A

Increases rate of diffusion

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19
Q

What is the effect of INCREASING VISCOSITY OF THE DISPERSION MEDIUM on the rate of diffusion of a colloid?
D = kT/6πnR

A

Decreases rate of diffusion

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20
Q

3) Electrical Properties

A
  • Interface in contact with an aqueous phase usually acquires a charge
  • Ionization of surface acidic or basic groups
  • Adsorption of dissolved ions and polymers
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21
Q

What are cations ions in solution called?

A

Counterions

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22
Q

What are anions in solution called?

A

Similions

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23
Q

Bulk Solution

A

Electrically neutral - equal concentration of similions and counterions

24
Q

Diffuse Layer

A
  • “Adsorbed” ions on the surface of the colloid
  • Diffuse layer with an excess of counterions to neutralize surface charge
  • Surface charge: -7
  • Net charge in the diffuse layer: +7
25
Q

Double Diffuse Layer

A

Electrically neutralizes the particle surface charge

26
Q

Zeta Potential

A
  • THE HIGHER THE # THE MORE STABLE
  • Measures the voltage difference between the double diffuse layer and the neutral region of the solution
  • Measures the degree of attraction and repulsion between adjacent particles
  • Agglomeration of colloids/suspensions
  • Stability of colloids/suspensions
27
Q

Surface Tension

A

The amount of energy required (similions) or released (counterions) when an ion is moved from bulk into contact with the surface

28
Q

Physical Stability

A
  • Frequent collision between colloidal particles (Brownian Motion)
  • Irreversible destruction (Coagulation/coalescence)
  • Temporary aggregation (flocculation)
  • Electrical and molecular forces
29
Q

What are 2 predominant forces that act between colloidal properties?

A

1) Van der Waals attractive forces

2) Electrostatic repulsive forces

30
Q

1) Van der Waals forces

A
  • Dependent on: (1) the size of particles and (2) distance between particles
31
Q

2) Electrostatic repulsive forces

A
  • Dependent on: (1) surface charge of particle and (2) thickness of the electrical double layer, diffuse layer
32
Q

Coagulation

A

Attractive forces overwhelm repulsive forces

33
Q

Aggregation

A

Collection of colloids into groups

34
Q

Coagulation/Coalescence

A

Colloids are closely aggregated and difficult to redisperse (particles are at the primary minimum on the energy curve)

35
Q

Flocculation

A

Aggregates that have an open structure and where the particles remain a small distance apart (secondary minimum)

36
Q

How are colloids unstable? And how do you stabilize them?

A
  • Colloids are unstable with respect to forming an aggregate

- To stabilize: (1) Electrostatic stabilization and (2) Steric stabilization

37
Q

1) Electrostatic stabilization

A
  • Particles repel each other due to surface charges

- Lyophobic solutions are stabilized using this method

38
Q

2) Steric stabilization

A

Polymers absorbed to the surface make it difficult for particles to approach each other

39
Q

Electrical Properties of Lyophobic Colloids

A

Generally, the larger the diffuse double layer the greater the repulsive force and the more stable the colloid

40
Q

Extent of the Double Diffuse Layer

A

1/K = “thickness of the DDL”

- Increasing concentration of electrolyte decreases 1/K and leads to coagulation

41
Q

Effect of Electrolyte Valence on DDL

A

Polyvalent counterions will have a much more significant effect on the thickness of the DDL than monovalent ions

42
Q

Double Layer Theory

A

Addition of polyvalent counterions results in compression of the diffuse double layer

43
Q

Steric Stabilization

A
  • Use of macromolecules to stabilize lyophobic colloids
  • Adsorb to particle surface
  • Prevent colloids from approaching each other sterically
44
Q

Polymer Bridging

A
  • Polymer bridging occurs when a polymer promotes an opposite effect that occurs in steric stabilization
  • Thus, instead of stabilization of the particles, the polymer promotes flocculation
45
Q

What makes adsorbed polymers to either stabilize or bridge colloidal systems?

A
  • Bridging happens at very low polymer concentration

- In steric stabilization, due to high polymer concentration, the polymer covers all or most of the particle surface

46
Q

Electrostatic Stabilization

A

Combination of both steric and electrostatic stabilization

47
Q

Micelles

A

Aggregates of amphiphilic molecules

48
Q

Critical Micelle Concentration (cmc)

A

Concentration of amphiphiles above which micelles begin to form

49
Q

Affect of Association Colloids on SURFACE TENSION

A

-

50
Q

Affect of Association Colloids on SOLUBILITY

A

-

51
Q

Hydrogels

A
  • Gel = colloid with liquid as dispersion medium and solid as dispersed phase
  • Hydrogel = water is the dispersion medium
  • Natural and synthetic (gelatin and polyethylene glycol hydrogels)
  • Wound healing, tissue engineering, drug delivery systems (responsive to environment (pH, temp, etc), sustained release, biocompatible and degradable)
52
Q

Microparticles

A
  • 0.2 - 5 um microspheres loaded with drug
  • Polymers
  • Carriers for vaccines, chemotherapeutics
  • Improve release profiles
  • Targeting
53
Q

Emulsions

A
  • O/W emulsion: oil is discontinuous phase and water is continuous phase (vice versa)
54
Q

Liposomes

A
  • Consist of outer membrane(s) and an inner liquid core
  • Can be loaded with drugs/excipients
  • Lipophilic drugs associate with membrane
  • Hydrophilic drugs associate with inner core
55
Q

Micelles

A
  • Similar to liposomes but do not have inner liquid structures
  • Used as microcontainers for poorly soluble hydrophobic drugs
  • Surface can be modified (like liposomes)
56
Q

Nanoparticles

A

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824 - Dosage Forms (4 decks)

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