Disperse Systems: Colloids 5/6 Flashcards
Molecular Dispersion
Less Than < 1nm (10A*)
Invisible by electron microscopy
Pass through ultrafilter/semipermeable membrane
Rapidly diffuse through medium
Colloidal Dispersion
1-500nm
Visible by electron microscopy (but not light)
pass through filter paper (not semipermeable)
Diffuse slowly through medium
*APPEAR transparent to the eye by transmitted light
Coarse Dispersion
500nm - 1mm
Visible by microscopy or even naked eye
do not pass through normal filter paper
Do not diffuse through medium
–> aerosols
Cow’s Milk
EMULSION
not a colloid
although it passes through coarse filter paper
globules are not a uniform size, some are greater than the right size range
What Do Colloids look like?
Many different shapes
Threads –> entagled
Form structure –> Gels
JELLY = colloidal dispersion
Lyophilic Collods
(solvent Loving)
- Consist of porticles that have HIGH AFFINITY for the dispersion medium
- Disperse to form SOLS with ease
- (acacia / gelatin in water)
-
Usually large organic molecules
- proteins / nucleic acids / carbs
- Molecules are Solvated
-
Viscosity of dispersion medium Increased greatly
- –> can form gels
- Stable to all but very high conc of electrolytes
LyoPHOBIC Colloids
(solvent-hating)
- Consist of particles that have poor afinity for dispersion medium
- Disperse with relative difficulty & consist of inorganic particles
- Gold/silver/sulfur
- Viscocity does not change much
- Unstable in presence of even small amounts of electrolytes
Association Colloids
- Consist of particles formed by amphiphilic substances
- SURFACTANTS
- Aggregate to form MICELLES when CMC is exceeded
- ~5nm (50A*) so it is a COLLOID
- Surface tension changes abruptly @ CMC
- Electrical conduction and osmotic pressure also change @CMC
- –> how you distinguish between SOLUTION and COlloidal DISPERSION
Faraday Tyndall Effect
When light passes through a COLLOIDAL SOLUTION
some light is scattered by invisible pathway so you can see the light
Reveals that they are not completely transparent
Exhibit TURBIDITY
turbidity can be measured and is related to partcle size
Raleigh Light Scattering
arises from the interaction of light with individual atomsof small-scale
like the faraday-tyndall scattering but with light not colloids
gives atmosphere its blue color
Brownian Motion
Describes the random movement of particles dispersed in a medium
due to the bombardment of particles by thermally agitated molecules in the dispersion medium
~5um diameter particles
Explains how particles spontaneously diffuse through the medium
Fick’s First Law
Applies to the diffusion of colloidal particles through the dispersion medium
Mass flow per unit area per unit time
Stokes-Einstein Equation
D
- Studies the diffusion particles of colloidal dispersion
- Viscosity
- Absolute temp
- particle weight
-
Increase diffusion by:
- Increasing temp
- decreasing VISCOCITY
Osmotic Pressure (pi)
Increasing Temperature –> Increases Osmotic Presure
Is effected by colloidal dispersions
Stoke’s Law
Higher gravitational force is needed to cause colloidal particles to SEDIMENT
velocity density viscocity in a gravitational field
Sedementation
in colloidal dispersions
Ultracentrifuge can be used to generate the High gravitational fields needed to sedement the particles
Heavier/larger => sedement faster
High Viscoscity => slow sedementation
Viscoscity
Critical to controlling sedementation rate
Particle Shape influences Viscosity
-rod/thread shaped particles INCREASE VISCOSITY
sphericle particles have little effect on viscosity
Making a colloidal dispersion of something –> INCREASES VISCOSITY
Viscosity can help determine what?
Molecular weight of molecules that form LYOPHILIC colloidal dispersions
Electric Double Layer
- POSITIVE CATIONS (1st line) OFFSETTED by:
- 2nd Layer of NEGATIVE ANIONS
- –> edge (shear plane) comes after that
- 2nd Layer of NEGATIVE ANIONS
- Tightly bound layer containing tightly bound solvent molecules
- off-setting anions
- Also a looseley defined region containing the rest of the off-setting anions
- +extra anions/cations
-
Thermal energy keeps the bulk solution well mixed
- Charge balance with the surface –> electroneutrality
Zeta Potential
Potential @ the Shear (Edge) Plane
- Net Positive or Negative Zeta Potential
- –> Repel Each other
- Colloidal dispersions will stay dispersed
- ~coarse dispersions will sediment slowly
-
Zeta potential ~ ZERO
- –> no repulsive forces
-
particles can FLOCCULATE (coalesce)
- due to brownian motion
Flocculation
= Coalescense / clump
- Occurs when Zeta Potential is LOW (close to zero)
- Coagulation becomes easier
- since there are less repulsive forces to overcome attractive forces
-
Colloidal dispersions might sediment
- You do not want flocculation in colloidal dispersion
- hard to redisperse
-
Coarse dispersions will too, but will easily disperse with agitation
- Flocculation is a Good thing for coarse dispersions
Electrokinetic Phenomena
Movement of a charged surface w/ respect to a bulk liquid
underlies 4 related phenomena:
Electrophoresis
ElectroOSMOSIS
Sedementation Potential
Streaming Potential
Electrophoresis
Involves the movement of charged particles through a liquid
under the influence of an applied electrical potential difference
Technique to Seperate macrromolecules (lyophilic colloids)
Rate determining potential is ZETA POTENTIAL of particle
Electroosmosis
Opposite of electrophoresis
Solid is IMMOBILIZED and the fluid is allowed to flow through a
membrane/porous structure
Can be used to obtain Zeta Potential measurements
Sedementation Potential
Backwards of electrophoresis
When charged particles sediment in a gravitational field,
the movement of the charges creates an electrical potential difference
Streaming Potential
Potential created when external pressure is used to force a liquid
containing charged particles through a semipermeable membrane
to allow only the passage of the liquid
Electric Potential Difference = Streaming Potential
Donnan Membrane Equilibrium
Used experimentally as a way to enhance absorption
of permeable drugs across GI tract membrane
by co-administering an impermeable charged excipient
Physical Stability of Colloids
- Presence/absensce of Charge on sthe surface of colloidal particles is a key factor
-
Brownian motion will result in flocculation
- if LOW/Zero Zeta potential
- Expecially problomatic for Lyophobic colloids
-
Brownian motion will result in flocculation
-
Ions disrupt the electric double layer
-
Salts will reduce zeta potential
- –> agglomeration/precipitation
-
Salts will reduce zeta potential
-
Mixing two EQUAL AMOUNTS of colloidal dispersions w/ OPPOSITE charged particles
- –> leads to COAGULATION = a COACERVATE
- physical incompatibility
- if not equal, LARGE EXCESS –> stablize some formulations
Solubilization of Drugs by Surfactants
Three distinct regions within micelles that solubilize drugs:
-
Hydrophobic core
- –> dissolves NONPOLAR DRUGS
-
Region closer to polar heads
- –> SEMIPOLAR drugs are attracted to this region
-
Palisade Region (polar head group themselves)
- –> POLAR DRUGS adsorb to this area
Factors affecting solubilization by surfactants
- Solubilization capacity varies with:
- Type of Surfactant
- Location within the micelle of the dissolved drug
-
Long hydrocarbon chains in Anionic Surfactants (up to C16)
- promote solubilization of non-polar drugs
- non-ionic surfactants are less effective
-
Solution pH effect ionization state
-
Uncharged form is More readily solubilized
- in interior hydrophobic part of micelle
-
Uncharged form is More readily solubilized
Kraft Point
Critical temperature
that a solution has to reach for micelle formation by surfactants
-
Above krafft temperature:
- solubility of surfactant rises sharply
- excess dissolved surfactant –> forms Micelles
-
Below Kraft temp:
- excess solid crytalline remains in equilibrium
Cloud Point
many NONIONIC surfactants decrease in solubility as solutions are heated
As temperature rises to CLOUD POINT,
phase seperation –> 2 immiscible liquid phases begin
1 phase is mostly solvent
Other phase is mostly liquid surfactant
Liposomes
- Artificial cells consisting of:
-
Lipid Bilayers
- aphoteric lipids (lecethins) that are found in cell membranes
-
Lipid Bilayers
- Most liposomes are the same size range as colloids
- have same usual properties
Micelle Delivery Systems
- Most common systems are based on Nonionic Surfactants
-
Pluronic Block Copolymers
- Widely used to deliver poorly soluble small molecule drugs
- As well as to stabilize/solubilize macromolecules
- such as proteins and nucleic acids
-
Pluronic Block Copolymers
Nanoparticles
- Colloidal particles consisting of a thin polymer film surrounding a liquid/solid core
-
Solid lipid nanoparticles
- deliver drugs topically or cosmetics
- size is similar to viruses
- Been tested as gene delivery systems
