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Colloidal Dispersions Flashcards

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1
Q

Colloidal dispersions

A

If substance A is not soluble in substance B, A can be dispersed in B as particles to form a disperse system

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2
Q

Disperse phase

A

Particles that are dispersed (A)

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3
Q

Continuous phase

A

The medium that the particles are dispersed in

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4
Q

What is the size of the particles

A

Between 1 and 1000nm

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5
Q

What happens when particle size is reduced

A

SA increases and as a result interfacial energy increases. Colloidal particles tend to aggregate due to high interfacial energy
E= gamma A

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6
Q

Lyo means

A

Liquid

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7
Q

Lyophobic vs lyophilic colloids

Thermodynamically stable or unstable

A

Lyophobic unstable

lyophilic stable

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8
Q

Lyophobic vs lyophilic colloids

Redispersible after drying or not

A

Lyophobic not redispersible lyophilic redispersible after dried if media added agin

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9
Q

Lyophobic vs lyophilic colloids

Does dispersion process occur automatically

A

Lyophobic doesn’t occur automatically needs work

lyophilic occurs automatically

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10
Q

Lyophobic vs lyophilic colloids

Strong or weak interaction

A

Lyophobic interaction between medium and particles weak

lyophilic strong attraction between particle and medium due to solvation, hydration

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11
Q

Lyophobic vs lyophilic colloids

Sensitivity to electrolytes

A

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

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12
Q

How are lyophilic colloids prepared

A

Dispersion process is spontaneous upon addition of continuous phase

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13
Q

How are lyophobic colloids prepared

A

Work is needed to break down the disperse phase. Colloid mill, ultra sonication >20k Hz, interfacial energy is increased

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14
Q

Kinetic properties of colloids

What is Brownian motion

A

Colloidal particles are in random collisions with the molecules of the dispersion medium resulting in the irregular motion of the colloidal particles

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15
Q

Kinetic properties of colloids

How does particle size affect Brownian motion

A

The larger the particles the weaker the BM. It is alimony negligible when >=5 um

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16
Q

Kinetic properties of colloids

What is the results of Brownian motion and what is the equation that tells you how fast molecules go

A

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
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17
Q

D is given by one of Einstein’s equations

A

D = kT/f

K is Boltzmann constant
T is absolute temp
F is frictional coefficient

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18
Q

Kinetic properties of colloids

Sedimentation - Stokes law

A

Stokes law equation tells you how fast the particles will settle
Brownian motion distrust the sedimentation process

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19
Q

Kinetic properties of colloids

Stokes law

A

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

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20
Q

Optical properties

Tyndall effect

A

When a beam of light passes through a colloidal dispersion a visible bright cone can be seen from the side

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21
Q

Optical properties

Due to scattered light the dispersion looks..

A 
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
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22
Q

Colloidal particles can acquire charge due to

A

– 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 (-).

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23
Q

Electrical doublelayer – shift of shear plane due to adsorbed polymers

A

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).

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24
Q

Lyophobic colloids – adsorption of surface active charged species

A

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

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25
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
26
Lyophobic colloids - Potential field in the electrical double layer - define shear plane
boundary between the moving particle and the surrounding medium
27
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
28
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)
29
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
30
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.
31
Electrophoresis
Charged particles move against a stationary medium in an electrical field
32
Microelectrophoresis
The movement of the particles is observed using a microscope
33
Electrophoresis can be used to..
Measure zeta potential and the mobility of particles can be detected by laser scattering
34
Physical stability of lyophobic colloids - define aggregation
particles come together forming larger particles
35
Physical stability of lyophobic colloids - define coagulation
the particles come together forming tight clusters. Difficult to re-disperse
36
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
37
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
38
Interparticle forces - all molecules experience
attractive van der Waals forces
39
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)
40
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
41
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
42
Total potential of interaction - draw diagram and VT equation
drawing | VT is calculated from VT= VA + VR
43
Total potential of interaction - primary min
at a small distance from the surface, attraction predominate. So there is a deep well approaching zero distance
44
Total potential of interaction - primary max
in the intermediate distance range, repulsive forces predominate. This is the energy barrier that stabilises the dispersions
45
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
46
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)
47
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
48
Stability of lyophilic colloids
Stabilised by a combination of charge interaction and solvation.
49
lyophilic colloids are less sensitive to
They are less sensitive to electrolytes compared to lyophobic colloids
50
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
51
Define coacervation
the collection of colloid rich layer after the addition of another substance
52
Steric stabilisation
Lyophobic colloid can be stabilised by the addition of polymeric materials such as non-ionic surface active agents, gum and cellulose derivatives
53
Steric stabilisation - polymers can absorb onto..
The polymers can adsorb on to the lyophobic particles to form a protective layer
54
What is a main factor to stabilise the colloids
The hydration of the polymeric materials is a main factor to stabilise the colloids
55
Sterically stabilised colloidal systems are less sensitive to..
Sterically stabilised colloidal systems are less sensitive to electrolytes and sometimes are called protective colloids
56
Define Bridge flocculation
polymer is like a bridge between the particles. Colloidal particles can be linked together by polymers leading to flocculation
57
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)
58
Define association colloids
Surfactants associate together to form micelles which falls into the size range of colloids, and therefore are termed association colloid
59
The associated molecules are in what equilibrium?
The associated molecules are in dynamic equilibrium with the surfactant molecules in the dispersion medium
60
How many molecules associate together to form a micelle
Normally 50 to 100 molecules associate together to form a micelle
61
Sizes of micelles measured in?
Sizes of micelles are in the nanometre range
62
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.
63
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
64
Main role of surfactants (surface active agents)
reduce surface tension
65
What is critical micelle concentration (CMC)
the concentration at which micelles are first formed
66
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)
67
The rate of increase in the number of ‘particles’ ..
The rate of increase in the number of ‘particles’ will change dramatically at CMC
68
What happens to properties at the CMC
Due to the association of molecules to form micelles, many properties change at the CMC
69
What happens to hydrophobic molecules in the hydrophobic cores of the micelles
Hydrophobic molecules can be solubilised in the hydrophobic cores of the micelles
70
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

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