Colloidal Dispersions Flashcards
Colloidal dispersions
If substance A is not soluble in substance B, A can be dispersed in B as particles to form a disperse system
Disperse phase
Particles that are dispersed (A)
Continuous phase
The medium that the particles are dispersed in
What is the size of the particles
Between 1 and 1000nm
What happens when particle size is reduced
SA increases and as a result interfacial energy increases. Colloidal particles tend to aggregate due to high interfacial energy
E= gamma A
Lyo means
Liquid
Lyophobic vs lyophilic colloids
Thermodynamically stable or unstable
Lyophobic unstable
lyophilic stable
Lyophobic vs lyophilic colloids
Redispersible after drying or not
Lyophobic not redispersible lyophilic redispersible after dried if media added agin
Lyophobic vs lyophilic colloids
Does dispersion process occur automatically
Lyophobic doesn’t occur automatically needs work
lyophilic occurs automatically
Lyophobic vs lyophilic colloids
Strong or weak interaction
Lyophobic interaction between medium and particles weak
lyophilic strong attraction between particle and medium due to solvation, hydration
Lyophobic vs lyophilic colloids
Sensitivity to electrolytes
Lyophobic sensitive to addition of electrolytes resulting in flocculation or aggregation
lyophilic not sensitive to electrolytes. Can be salted out at high conc of electrolytes
How are lyophilic colloids prepared
Dispersion process is spontaneous upon addition of continuous phase
How are lyophobic colloids prepared
Work is needed to break down the disperse phase. Colloid mill, ultra sonication >20k Hz, interfacial energy is increased
Kinetic properties of colloids
What is Brownian motion
Colloidal particles are in random collisions with the molecules of the dispersion medium resulting in the irregular motion of the colloidal particles
Kinetic properties of colloids
How does particle size affect Brownian motion
The larger the particles the weaker the BM. It is alimony negligible when >=5 um
Kinetic properties of colloids
What is the results of Brownian motion and what is the equation that tells you how fast molecules go
Diffusion of particles
Dm/ft = -DA dC/dx
Dm is amount of substance diffusing across an area A T is diffusion time Dm/dt is rate of diffusion D is diffusion coefficient dC/dx is the conc gradient
D is given by one of Einstein’s equations
D = kT/f
K is Boltzmann constant
T is absolute temp
F is frictional coefficient
Kinetic properties of colloids
Sedimentation - Stokes law
Stokes law equation tells you how fast the particles will settle
Brownian motion distrust the sedimentation process
Kinetic properties of colloids
Stokes law
The larger the radius the faster the velocity
If the density of the particle is heavier than the medium the molecules will be positive
High viscosity leads to lower sedimentation
Optical properties
Tyndall effect
When a beam of light passes through a colloidal dispersion a visible bright cone can be seen from the side
Optical properties
Due to scattered light the dispersion looks..
Turbid Turbidity is given by I = I0 e power of - turbidity L I0 is intensity of incident light I is intensity of transmitted light L is length of the samples light pass
Colloidal particles can acquire charge due to
– Ion dissolution, e.g. silver iodine. Positive charge (excess Ag+) or negative charge (excess I-)
– Ionisation, e.g. some polymers, -NH3+, -COO-
– Ion adsorption, mostly negatively charged. Cations (+) are generally more hydrated than anions (-).
Electrical doublelayer – shift of shear plane due to adsorbed polymers
The shear plane is pushed further away from the surface of the particles due to the adsorbed polymers. Hence the zeta potential is reduced (ζ2).
Lyophobic colloids – adsorption of surface active charged species
Surface active co-ions (same charge) adsorbed on to the surface increase surface potential.
Surface active counter ions (opposite charge) adsorbed on to the surface reverse SP
Lyophobic colloids - Potential field in the electrical double layer - define counter ions
Counter ions are attracted toward the surface. The closer to the surface, the higher the concentration of the counter ion; the further away rom the surface, the lower the concentration of the counter ion, until it becomes electrically neutral
Lyophobic colloids - Potential field in the electrical double layer - define shear plane
boundary between the moving particle and the surrounding medium
Lyophobic colloids - Potential field in the electrical double layer - define zeta potential
zeta potential (ζ) is the potential at the shear plane. Zeta potential can be reduced due to the adsorption of polymers
Lyophobic colloids - Potential field in the electrical double layer - thickness of the diffuse layer is indicated by
The thickness of the diffuse layer is indicated by 1/κ, the Debye-Huckel length (κ the Debye-Huckel constant)
Lyophobic colloids - Potential field in the electrical double layer - role of electrolytes
Electrolytes compress the double layer, hence reducing the zeta potential. More electrolytes added will lead to a higher concentration of ions, thickness of double layer will get thinner and the potential will drop to neutral a lot faster
Lyophobic colloids - Potential field in the electrical double layer - what does a higher ionic strength lead to
The higher the ionic strength, the thinner the diffuse double layer, consequently the lower the zeta potential.
Electrophoresis
Charged particles move against a stationary medium in an electrical field
Microelectrophoresis
The movement of the particles is observed using a microscope
Electrophoresis can be used to..
Measure zeta potential and the mobility of particles can be detected by laser scattering
Physical stability of lyophobic colloids - define aggregation
particles come together forming larger particles
Physical stability of lyophobic colloids - define coagulation
the particles come together forming tight clusters. Difficult to re-disperse
Physical stability of lyophobic colloids - define flocculation
there are greater spaces between the particles grouped together. The particles form a loose structure and are easy to re-disperse
Physical stability of lyophobic colloids - define DVLO theory
– DerJaguin and Landau, and independently Verwey and Overbeek in the 1940s developed a quantitative approach to illustrate the stability of hydrophobic colloids
– DLVO theory assumes that the potential due to electrical repulsion and van der Waals attraction are additive:
VT = VA + VR
Interparticle forces - all molecules experience
attractive van der Waals forces
Interparticle forces - what must be done to keep particles separate
anopposing repulsive force must be provided between the particles
– Electrical repulsion (due to zeta potential)
– Steric protection (protective polymer layer on the surface of particles)
van der Waals forces equation
VA =-Aa/12H
where A is the Hamaker constant, a is the diameter of the particles, H is the separation between the particles. Negative values of the potential (VA) represent attraction
Repulsive forces
For small values of the surface potential,Ψ VR =2πεaΨ2 exp(-κH)
where ε is the dielectric constant of the medium and κ is the Debye-Huckel constant. Positive values of the potential (VR) represent repulsion
Total potential of interaction - draw diagram and VT equation
drawing
VT is calculated from VT= VA + VR
Total potential of interaction - primary min
at a small distance from the surface, attraction predominate. So there is a deep well approaching zero distance
Total potential of interaction - primary max
in the intermediate distance range, repulsive forces predominate. This is the energy barrier that stabilises the dispersions
Total potential of interaction - secondary min
at a long distance from the surface where repulsive force falls more quickly than attractive force, forming a small (shallow) min. Controlled flocculation can happen at the secondary minimum
The effect of Ionic strength on VR
An Increase in ionic strength means an increase in electrolytes concentration, and consequently an decrease of the thickness of the double layer (1/κ)
• Hence, VR will be reduced at a given distance H from the surface of a particle (e.g. at H1)
The effect of Ionic strength on Vt
An increase in ionic strength results in a lower VR, and consequently lower VT at a given distance H
Stability of lyophilic colloids
Stabilised by a combination of charge interaction and solvation.
lyophilic colloids are less sensitive to
They are less sensitive to electrolytes compared to lyophobic colloids
When can lyophilic colloids become lyophobic
They can be considered to have become lyophobic when the macromolecules are dehydrated by the addition of sufficient quantity of solvent such as alcohol and acetone
Define coacervation
the collection of colloid rich layer after the addition of another substance
Steric stabilisation
Lyophobic colloid can be stabilised by the addition of polymeric materials such as non-ionic surface active agents, gum and cellulose derivatives
Steric stabilisation - polymers can absorb onto..
The polymers can adsorb on to the lyophobic particles to form a protective layer
What is a main factor to stabilise the colloids
The hydration of the polymeric materials is a main factor to stabilise the colloids
Sterically stabilised colloidal systems are less sensitive to..
Sterically stabilised colloidal systems are less sensitive to electrolytes and sometimes are called protective colloids
Define Bridge flocculation
polymer is like a bridge between the particles. Colloidal particles can be linked together by polymers leading to flocculation
What are the 3 conditions that need to be met for bridge flocculation to occure
– The polymer has two or more segments that can adsorb on to the particles
– The polymer molecules are long enough to adsorb on to two particles
– The surface coverage of the particle is low (i.e. low concentration of the polymer)
Define association colloids
Surfactants associate together to form micelles which falls into the size range of colloids, and therefore are termed association colloid
The associated molecules are in what equilibrium?
The associated molecules are in dynamic equilibrium with the surfactant molecules in the dispersion medium
How many molecules associate together to form a micelle
Normally 50 to 100 molecules associate together to form a micelle
Sizes of micelles measured in?
Sizes of micelles are in the nanometre range
Where do surface active agents allign
Surface active agents tend to align at the surface of water with the hydrophobic section pointing away from water, and surface tension is thus reduced.
What happens after the surface is saturated by surfactants
After the surface is saturated by surfactants, they tend to form micelles to shield the hydrophobic core from the aqueous phase
Main role of surfactants (surface active agents)
reduce surface tension
What is critical micelle concentration (CMC)
the concentration at which micelles are first formed
Any further increase in concentration will lead to..
Any further increase in concentration will lead to an increase in the number of micelles. Many molecules form one micelle and one micelle is counted as one “particle” (also behaves as one particle)
The rate of increase in the number of ‘particles’ ..
The rate of increase in the number of ‘particles’ will change dramatically at CMC
What happens to properties at the CMC
Due to the association of molecules to form micelles, many properties change at the CMC
What happens to hydrophobic molecules in the hydrophobic cores of the micelles
Hydrophobic molecules can be solubilised in the hydrophobic cores of the micelles
4 factors that determine CMC
A – Osmotic pressure
B – Solubility of poor water-soluble drugs
C – Turbidity
D – surface tension
ABCD depends on the number of molecules
