lecture notes 2 nano materials

STABILIZATION MECHANISMS(Prevention of agglomeration)

Two major stabilization mechanisms:

1) Electrostatic stabilization2) Steric stabilization

Electrostatic stabilization is kinetically stable, Steric stabilization is thermodynamically stable

Colloidal stabilityContinuous phase (dispersion medium) Particles Material (dispersed phase)

The stability of colloids is determined by the interaction between the particles during collision.

There are two basic interactions:

Attractive and Repulsive Attraction dominates, particles adhere, finally coalesce Repulsion dominates; system stable & dispersed state

Primary attraction source ‘Van der Waals forces’

Short range interactionsLong range attractions

Surface charged density (electrode potential)

(1) Adsorption of ions(2) Dissociation of surface charged species(3) Substitution of ions(4) Accumulation or depletion of electrons at the surface(5) Physical adsorption of charged species onto the surface.

Electrical double layer structure & electric potential near the solid surface

ELECTROSTATIC STABILIZATION

Counterbalance of VDWL Coulombic repulsion

In liquid dispersion media, ionic groups can adsorb to the surface of a colloidal particle forming charged layer

To maintain electroneutrality, an equal number of Counter-ions with the opposite charge will surround the colloidal particles and give rise to overall charge-neutral double layers.

In charge stabilization, it is the mutual repulsion of these double layers surrounding particles that provides stability.

Conditions for occurrence of electrostatic repulsion between two particles

Electrostatic Repulsion

Electrostatic forces are only present where charged particles are interacting through a polar medium (e.g. water, ethanol)

Electrostatic (or Coulomb‘s) forces are in general stronger and longer range than all other surface forces

Decays exponentially with particle separation

Affected by the ionic strength of the suspension

Limitations of Electrostatic stabilization

(1) Electrostatic stabilization is a kinetic stabilization method.

(2) It is only applicable to dilute systems.

(3) It is not applicable to electrolyte sensitive systems.

(4) It is almost not possible to redisperse the agglomerated particles.

(5) It is difficult to apply to multiple phase systems, since in a given condition, different solids develop different surface charge and electric potential.

DLVO Theory

(Derjaguin, Landau, Venvey and Overbeek)

The total interaction between two particles, which are electrostatic stabilized, is the combination of van der Waals attraction and electrostatic repulsion:

DLVO potential: VA = attractive van der Waals potential, VR = repulsive electrostatic potential.

Assumptions of DVLO Theory:

1. Infinite flat solid surface,

2. Uniform surface charge density,

3. No redistribution of surface charge, i.e. the surface electric potential remains constant,

4. No change of concentration of both counter ions and surface charge determining ions, i.e. the electric potential remains unchanged

5. Solvent exerts influences via dielectric constant only, i.e. no chemical reactions between the particles and solvent

Steric Stabilization (Polymeric stabilization)

It is achieved by attaching (grafting or chemisorption) macromolecules to the surfaces of the particles. The stabilization due to the adsorbed layers on the dispersed particle is generally called steric stabilization.

Schematics of steric stabilization

Solvent & Polymer

Aqueous solvents & Non-aqueous solvents or organic solvents

Protic and aprotic Solvents

Protic solventWhich can exchange proton like methanol and ethanol

Aprotic solventWhich cannot exchange protons like acetone and benzene

Solvent & Polymer

Depending on the interaction between polymer and solid surface, a polymer can be grouped into:

(1) Anchored polymer, which irreversibly binds to solid surface by one end only, and typically are diblock polymer

(2) Adsorbing polymer, which adsorbs weakly at random points along the polymer backbone

(3) Non-adsorbing polymer, which does not attach to solid surface and thus does not contribute to polymer stabilization.

(a) (b)

Interaction between polymers and solid surface:(a) anchored polymer and (b) absorbing polymer.

Adsorption Chemical or physicalInteractions between polymer layers

(a) (b)(a) Schematic of two approaching polymer layers,

(b) Gibbs free energy as a function of the distance between two particles.

Conditions

Interactions of two layers, separation distance, H, less than twice thickness, L, of polymer layers

However, distance reduces to 2L > L interaction occurs

No direct interaction of polymer layer of one particle and solid surface of other particle

Less coverage of solid surface, insufficient polymer concentration therefore interpenetration of polymer layers of approaching particles, increase in Gibbs free energy

High coverage of solid surface, no interpenetration, coil up of layers, increase in Gibbs energy and repel of particles

Interactions between polymer layers

(a) (b)(a) Two approaching polymer layers and (b) Gibbs free energy as a function of the distance between two particles.

Poor solvent, low coverage, L< H < 2L, polymer adsorb on one particle tend to penetrate to layer of approaching particle, coil up of polymer & increase in Gibbs free energy

High coverage, similar to good solvent, increase in overall energy and reductions in distances and repulsive force produces

STERIC STABILIZATION

(ADVANTAGES)

(1) It is a thermodynamic stabilization method, so that the particles are always redispersible.

(2) A very high concentration can be accommodated, and the dispersion medium can be completely depleted.

(3) It is not electrolyte sensitive.

(4) It is suitable to multiple phase systems.

STABILIZATION MECHANISMS

Electrostatic stabilization is kinetically stable, Steric stabilization is thermodynamically stable

ELECTROSTATIC STABILIZATION

Counterbalance of VDWL Coulombic repulsion

In liquid dispersion media, ionic groups can adsorb to the surface of a colloidal particle forming charged layer

To maintain electroneutrality, an equal number of Counter-ions with the opposite charge will surround the colloidal particles and give rise to overall charge-neutral double layers.

Application Involved Principles• Pharmaceutics, cosmetics,

inks, paints, foods, foams, chemicals

• Formation and stabilization of end-use products

• Photographic products, ceramics, paper coatings, catalysts, magnetic media

• Formation of colloids for use in subsequent manufacturing processes

• Pumping of slurries, coating technology, filtration

• Handling properties of colloids, rheology, sintering

• Water purification, fining of wines and beer

• Destruction of unwanted colloidal systems

Metal Oxides Stabilization

The surface charge in oxides is mainly derived from preferential dissolution or deposition of ions.

In the oxide systems, typical charge determining ions are protons & hydroxyl groups & their concentrationsare described by pH (PH = -log [H+]).

As the concentration of charge determining ions varies, the surface charge density changes from positive to negative or vice versa.

The concentration of charge determining ions corresponding to a neutral or zero-charged surface is defined as a point of zero charge (P.z.c.) or zero-point charge (z.P.c.).

At pH > P.z.c., the oxide surface is negatively charged, since the surface is covered with hydroxyl groups, OH-, which is the electrical determining ion.

At pH < P.z.c., H+ is the charge determining ions and the surface is positively charged.

DLVO Theory

(Derjaguin, Landau, Venvey and Overbeek)

DVLO theory suggests that the stability of a particle in solution is dependent upon its total potential energy function VT.

This theory recognizes that VT is the balance of several competing contributions:

VT = VA + VR + VS

VS is the potential energy due to the solvent.

DVLO theory suggests that the stability of a colloidal system is determined by the sum of these van der Waals attractive (VA) and electrical double layer repulsive (VR) forces that exist between particles as they approach each other due to Brownian motion.

Finally, if the particles have a sufficiently high repulsion, the dispersion will resist flocculation and the colloidal system will be stable. However if a repulsion mechanism does not exist then flocculation or coagulation will eventually take place.

DLVO applying conditions

(1) Dispersion is very dilute, so that the charge density and distribution on each particle surface and the electric potential in the proximity next to each particle surface are not interfered by other particles.

(2) No other force is present besides van der Waals force and electrostatic potential, i.e. the gravity is negligible or the particle is significantly small, and there exist no other forces, such as magnetic field.

(3) Geometry of particles is relatively simple, so that the surface properties are the same over the entire particle surface, and, thus surface charge density and distribution as well as the electric potential in the surrounding medium are the same.

(4) The double layer is purely diffusive, so that the distributions of counter ions and charge determining ions are determined by all three forces: electrostatic force, entropic dispersion and Brownian motion.

Total interaction energy between two spherical particles, as a function of the closest separation distance So between their surfaces, for different double layer thickness K-I obtained with different monovalent electrolyte concentrations. The electrolyte concentration is C (mo1.L-I) = 10-15K2 (cm-I)

Electric potential is dependent on the concentration and valence state of counter ions.

Van der Waals attraction potential is almost independent of the concentration and valence state of counter ions.

The overall potential is strongly influenced by the concentration and valence state of counter ions.

An increase in concentration and valence state of counter ions results in a faster decay of the electric potential

As a result, the repulsive barrier is reduced and its position is pushed towards the particle surface

Two ways of understanding repulsion forces

Interaction between the double layers

The repulsion derives from the overlap of electric potentials of two particles. Repulsion is not directly due to the surface charge on solid particles; instead it is the interaction between two double layers.

Osmostic flow

Two particles approach one another, the concentrations of ions between two particles where two double layers overlap, increase significantly, since each double layer would retain its originalconcentration profile.

As a result, the original equilibrium concentration profiles of counter ions and surface charge determining ions are destroyed. To restore the original equilibrium concentration profiles, more solvent needs to flow into the region where the two double layers overlap.

Such an osmotic flow of solvent effectively repels two particles apart, and the osmotic force disappears only when the distance between the two particles

equals to or becomes larger than the sum of the thickness of the two double layers.Mixed Steric and electric interactions

Electrosteric Stabilization: When polymer attached to charged particle, polymer layer forms and in addition an electric potential to the solid surface would retain. Therefore both electrostatic repulsion and steric restriction prevents agglomeration.

(a)

(b)

Schematic representation of electrosteric stabilization:(a) Charged particles with nonionic polymers and

(b) Polyelectrolytes attached to uncharged particles