Colloid stability

István Bányai,

University of Debrecen

Dept of Colloid and Environmental Chemistry

www.kolloid.unideb.hu

(Stability of lyophilic colloids see: macromolecular solutions)

Stabilities

• 1. Stability of lyophobic colloids („colloid stability”, kinetic stability – Electrostatic

– Steric

– Mixed

• 2. Stability of lyophilic colloids (thermodynamic stability) – Making unstable

Colloidal stability requires a repulsion force:

VR VS

Lyophobic colloid may be stabilized by lyophilic colloid

Molecular origins of van der Waals Attraction (between particles in vacuum.)

Attraction between atoms/ molecules in

vacuum

Dispersion attraction between atoms /

molecules is additive so it effects in case

of macroscopic bodies too.

r

H

6

.A

constV r

r

2A

AV

H

depends on geometry!

12

A

AaV H

H

a

H

A Hamaker constant

Hamaker model: calculates the attraction between particles from molecular attractions

Molecules in

particle 1

Molecules in

particle 2

The attraction of bodies arises from London (dispersion) attraction of molecules (all molecules act independently). The effect is additive; that is, one molecule of the first colloid has a van der Waals attraction to each molecule in the second colloid. This is repeated for each molecule in the first colloid, and the total force is the sum of all of these. An attractive energy curve is used to indicate the variation in van der Waals force with distance between the particles.

depends on geometry!

Attraction: effective Hamaker constant

H

Hamaker constant: A in vacuum depends on material properties: density, polarizability

,AV H J

H

12

A

AaV H

H

An attractive energy curve is used to indicate the variation in van der Waals force with distance between

the particles.

The effective Hamaker constant Aeff also depends on the dispersion medium

12

A

AaV H

H

Similar charged particles: zeta potential

Plane of shear

Positive particle with negative ion atmosphere

expSt x

x ~ distance from surface

ζ

ψSt

St

Electrostatic repulsion between overlapping double layers

The loosely held countercharges form “electric double layers.” The electrostatic repulsion results from the interpenetration of the diffuse part of the double layer around each charged particle.

VR

2

0 expRV H H

H ~ distance between surface

An electrostatic repulsion curve is used to indicate the energy that must be overcome if the particles are to be forced together

The Balance of Repulsion & Attraction

is the sum of the electrostatic repulsion and the dispersion attraction, DLVO theory:

Notice the secondary minimum. The system flocculates, but the aggregates are weak. This may imply reversible flocculation.

The point of maximum repulsive energy is called the energy

barrier. Energy is required to overcome this repulsion . The height of the barrier indicates how stable the system is .The electrostatic stabilization is highly sensitive with respect to surface charge (ζ~ψ~ pH) and salt concentration (κ, z).

VT = VA + VR

exp 12

exp 12

St

St

ze

kT

ze

kT

2 2 2( ) expRV H a kT z H

12

A

AaV H

H

Total Interaction= sum of the attractive and repulsive interactions VT = VA + VR

large sediment height or gel

van der Waals attraction will predominate at small and at large interparticle distances. At intermediate distances double layer repulsion may predominate, depending on the actual values of the forces. In order to agglomerate, two particles on a collision course must have sufficient kinetic energy due to their velocity and mass, to “jump over” this barrier.

The height of the energy barrier depends upon

the zeta potential and 1/

Precipitate, or cake

sol

In the secondary minimum

there is a reversible

flocculation: sol- gel

transformation

coagulation

VT ,VA, VR (J) the total, attractive and repulsive energy of two spherical particles at distance d (m)

Electrostatic stability of dispersions

An increase in electrolyte concentration leads to a compression of the double layer (kappa increases) and so the energy barrier to coagulation decreases or disappears.

If the barrier is cleared, then the net interaction is all attractive, and as a result the particles coagulate. This inner region is after referred to as an energy trap since the colloids can be

considered to be trapped together by van der Waals forces.

What concentration of salt (n0) just eliminates the repulsive barrier?

1 2

Curve 1: Low ionic strength: primary minimum and high maximum → stable colloidal dispersion.

Curve 2: High ionic strength: only primary minimum → unstable colloidal dispersion.

Critical coagulation concentration What concentration of salt (n0) eliminates the repulsive barrier?

If the potential energy maximum is large compared with the thermal energy, kT of the particles, the system should be stable; otherwise, the system should coagulate.

Counter -ion valency

c.c.c (in mol/L) ~z-6

c.c.c. is the concentration of salt just eliminates the repulsive barrier.

Schulze – Hardy Rule

The Schulze – Hardy Rule: the stability depends on the sixth power of the charge on the ions!

c.c.c (in mol/L) ~z-6

1:1/26:1/36=1:0.015:0.0014

What concentration of salt (n0 or c.c.c.) just eliminates the repulsive barrier?

Strength of interparticle forces – Rates of coagulation

If there is an energy barrier, Vmax to coagulate then a fraction (α) of collisions is unsuccessful, so the rate of coagulation slower, ks.

Rates of coagulation can be measured by the change in the number of particles, Smoluchowski equation:

2 28 d

dNDaN k N

dt

kd is the rate of the diffusion limited aggregation or

rapid coagulation (no barrier, Vmax=0)

the stability ratio: d

s

kW

k

The stability of dispersion is increased by: increase in particle radius, increase in surface potential (ζ >25mV), decrease in Hamaker constant, decrease in the ionic strength, decrease in temperature.

maxexpV

kT

t is the time, Np the numbers of single particles per unit volume, D diffusion coefficient, kD rate constant, kBoltzman constant, T temperature, V max

Stability ratio vs. electrolyte conc.

the stability ratio: d

s

kW

k

1W

ln 0W

Stable and instable systems

The larger the negative voltage value of ZP, the more dispersing power it has.

Can you see this happening inside

our bodies? [ A low Zeta Potential

will cause blood cells to clump

together. It is the force that maintains

the discreteness of the billions of

circulating cells, which nourish the

organism ]

Coagulation in the human blood system

Numerical

"Grade"

(arbitrary)

"Degree" of Clump

* (Observed in

Sclera)

Probable ZP of Red

Blood Cells (in

situ) mV

0 Absent –17

1 Slight –16

2 Moderate –15

3 Significant –14

4 Heavy –13

5 Very Heavy –12

6 Terminal (death) –11

8 Fluid gel (5 min.) –7

10 Rigid gel (10 min.) –7

A low Zeta Potential

will cause blood cells to

clump together.

Many types of cardiovascular

disease are manifest in the early

stages as "moderate to

significant" intravascular

coagulation, and in advanced

stages as "heavy to very heavy"

coagulation.

http://www.hbci.com/~wenonah/riddick/chap22.htm

Colloidal stability requires a repulsion force:

VR VS

Lyophobic colloid may be

stabilized by lyophilic colloid

Steric stabilization (Vs potential)

New repulsive force can arise by adsorption of natural or industrial macromolecules or amphiphiles. These stabilizers are in interaction with the medium: hydration, solvation

Thickness of polymer layer

We need to invest work (isotherm reversible) to push them close, within the distance determined by the adsorbed polymer . No repulsion outside of the layer.

Consists of three main components - entropic effect (conformational S) - osmotic effect - enthalpy effect

Importance: Food industry, cooking (soups) fruit juice, cocoa with milk

The details of effects

• Entropy effect

– The degree of freedom decreases when the two layers overlap: S<0 stabilization

– Effective distance H < 2r

– Better stbilization with increasing of the chain length and of the amount adsorbed polymer

– There is an attractive component: restricted volume

The volume available for solvent molecules encreases

Osmotic effects

The sorbed macromolecules on the particles (or amphiphiles) penetrate into each other’s layers and push out solvent molecules. The chemical potential of the solvent will be lower in the cage, so is the chemical potential. As a consequence osmotic pressure arises and stresses apart the two particles. Stabilization

lncage

solvent

bulk

cRT

c

Enthalpy effect

If there is „good solvent” (from the point of view of coating molecules) present, then the water (solvent) molecules are in thermodynamically more stable state hydrating the particles It is an repulsive potential: stabilization.

Steric stabilization, (no other attraction beyond Van der Waals interaction)

Steric stabilization (Adsorption of polymers): 1. not sensitive on the salt concentration 2. works in non-aqueous medium 3. works in concentrated colloid systems It is difficult to plan, a lot of empirical rules exist.

If the energy of attraction is larger (negative) than that of the thermal motion no coagulation happens. If it is larger (negative) then the coagulation takes place

Conditions of the steric stability

Dispersion is stable when the kinetic energy is larger than the energy of attraction in the case of collision. It is fulfilled when the distance is enough large so the attraction is weak. Energy balance (A121 Hamaker constant particle-medium-particle)

kT >A121d/ (48t).

Therefore the thickness of the polymer, t, should be larger than a certain value:

t > A121d / (48kT)

A121(10-21), J A121/48kT, nm

Oil - water 0.5 0.025

Polistyrene-water 1.05 0.05

carbon-water 2.8 0.14

TiO2-water 7.0 0.35

12

A

AaV H

H

Titania spheres (hidroxy-propyl cellulose)

Steric + electrostatic stabilization

– Polyelectrolytes’ (pl. proteines, gelatin) sorption

- Neutral polymers can stabilize charged colloids

VTeljes = VA + VR + VS VTeljes = VA + VR

Thre can be opposite effect: sensitization

Sensitization

• A combination of – long polymer, small concentration

– good solvent, strong adsorption

– Application: water purification (Fey(OH)x(x-3y))

A few ppm cationic poly electrolites can flocculate colloids

Stability of lyophilic colloids: destabilization

The isoelectric point of casein is 4.6.

Lyophilic colloids are liquid loving colloids (Lyo means solvent and philic means loving)

Lyophilic sols’ stability comes from solvation +

charge. If solvation interaction alone is strong

enough the colloids stay stable at its isoelectric

pH if it is not they coagulate at their isoelectric

pH.

Gelatin is stable at its isoelectric condition so

called isostable colloids, but it can be precipitate

with much more salt or dehydration agent

(acetone, alcohol).

Casein is unstable at this isoelectric pH where

there is no charge, this is a isolabile protein.

Casein precipitates at iep where there is no

repulsion.

lyophilic colloids: isostable

no precipitation at iep

isolabile

precipitation at iep

The fermentation of milk sugar (lactose) produces lactic acid, which acts on milk protein to give yoghurt its gel-like texture