Colloid Chemistry · Silvia Gross –Chimica dei Colloidi – Laurea Triennale in Chimica Colloid...

Silvia Gross – Chimica dei Colloidi – Laurea Triennale in Chimica

Istituto di Scienze e Tecnologie Molecolari

ISTM-CNR, Università degli Studi di Padova

e-mail: [email protected]

Silvia Gross

La chimica moderna e la sua comunicazione

Dipartimento di Scienze Chimiche

Università degli Studi di Padova

e-mail: [email protected]

http://www.chimica.unipd.it/silvia.gross/

Silvia Gross

Colloid Chemistry


Colloid steric stabilisation

Typically used charged macromolecules (polyelectrolytes)

- Electrostatic effects

- Steric effects

→ Electrosteric stabilisation effects

Relevant role of:

- Polymer/solution thermodynamics

- Polymer/colloid thermodynamics

→ different effects of polymer chains on a dispersion (vide infra)


Colloid steric stabilisation: ruling factors

Flory, P. J. (1953) Principles of Polymer Chemistry. Cornell University Press, Ithaca.

Flory, P. J. (1989) Statistical Mechanics of Chain Molecules. Hanser, Munich.

The Nobel Prize in Chemistry 1974 was awarded to

Paul J. Flory

"for his fundamental achievements, both theoretical

and experimental, in the physical chemistry of the

macromolecules".

Paul J. Flory (1910-1985)



Theta (Flory) (Q) temperature

Symbol θ

The unique temperature at which the attractions and repulsions of a polymer in a solution

cancel each other. A polymer solution at the Flory temperature is called a theta (θ) solution.

At the Flory temperature the virial coefficient B, associated with the excluded volume

(conformations in which different chain segments occupy the same space are not physically

possible) of the polymer, is zero, which results in the polymer chain behaving almost

ideally. This enables the theory of polymer solutions at the Flory temperature to provide a

more accurate description of events than for polymer solutions at other temperatures, even if

the polymer solution is concentrated.

It is not always possible to attain the Flory temperature experimentally.

Oxford Dictionary of Chemistry, 6th Edition



Quality („goodness“) of solvent

Theta (Q)–solvent: interaction between polymer segments is equal to the interaction of

segments with the solvent

Flory-Huggins parameter: c (polymer/solvent interaction parameter)

c< 0.5 : „good“ solvent, repulsive forces act between the polymer segment

polymer coils swell

c> 0.5 : „poor“ solvent, the polymer segments attract each other

polymer coils shrink

Flory, P. J. (1953) Principles of Polymer

Chemistry. Cornell University Press, Ithaca.

Flory, P. J. (1989) Statistical Mechanics of

Chain Molecules. Hanser, Munich.


(1) measured at / extrapolated to 25 °C.


Flory-Huggins c values (refers to polymer/solvent pairs)

Polymer-Solvent Interaction Parameter at Infinite Dilution (1)

Polymer Solvent χ∞

Poly(acrylamide) Water 0.495

Poly(dimethyl siloxane) 2-butanone 0.500

Polyisobutylene n-Pentane 0.480

Poly(methyl methacrylate) Acetone 0.480

Poly(p-chlorostyrene) Toluene 0.475

Polystyrene Cyclohexane 0.509

Polystyrene Benzene 0.465

Poly(vinyl alcohol) Water 0.494



Quality („goodness“) of solvent The swelling of the chain in good solvent results as a balance of the:

• repulsion energy between monomers

• insertion of solvent molecules among chains

• entropy loss due to increased isotropic expansion of the network and deformation of the

swollen network both leading to reduction in entropy



Role of polymer/solvent interaction

The “quality” of solvent (i.e., solvent-polymer interactions) affects the

interaction forces. In a good solvent, polymer segments favor contacts with

the solvent.

Since the compression of the polymer layer by an approaching surface tends

to squeeze out the solvent and force segment-segment interactions, the net

result is a repulsion.

In contrast, poor solvents produce an opposite effect, and a net attraction

is possible for certain range of compression.



Quality („goodness“) of solvent in copolymers

Copolymers having more than one monomer type have more than

one c parameter.

Random copolymers often have intermediate properties between

the two monomers, if they are truly random, and it is possible to

use a weighted c parameter to describe some of their solution

behaviour.



T. Cosgrove, Colloid Science: Principles, Methods and Applications, Wiley-Blackwell 2005.

Effect of solvent quality on steric interactions

silica particles (diameter: 88 nm)

with end-grafted polystyrene of

MW = 26.600 g/mol.

c=0.31

c=0.57

c=0.0

c=0.79

good solvent

poor solvent

good solvent: strong repulsive interactions

solvent quality decreases: stabilising layers shrink repulsions set in at smaller distance

poor solvent: chains are collapsed coagulation

Effect of solvent quality on steric interactions

obtained from mean field theory.

Polystyrene stabilised silica. Solvent c parameter

(top to bottom) 0.0, 0.31, 0.57, 0.79

Achain = interaction energy per polymer chain



Cloud point

Experimentally measured point in the phase diagram of a mixture at which a loss in transparency is

observed due to light scattering caused by a transition from a single to a two phase state.

Notes:

1. The phenomenon is characterized by the first appearance of turbidity or cloudiness.

2. A cloud point is dependent on heating rate or cooling rate.

(see also Krafft temperature)

IUPAC Compendium of Chemical Terminology

http://old.iupac.org/goldbook/CT07275.pdf



Quality („goodness“) of solvent and cloud point


Source of picture: Chem. Commun., 2014, 50, 6556-6570

Effect of polymer additives on the stability of dispersions



Source of picture:

R. J. Hunter, Foundations

of Colloid Science

Effect of polymer concentration on the stability of dispersions


Molecules adsorbed or grafted

on surface

> vdW: impart stability

Free polymer chains extert an

influence: exclusion of polymer

chains (osmotic effect)Demixing p/s

Unfavourable

in good solvents

DG > 0


Steric repulsion

Steric repulsion between polymer-coated

particles.

Overlap of adsorbed polymer layers on close

approach of dispersed solid particles (parts a

and b).

Repulsive interaction energy due to the

overlap of the polymer layers

(dark line in part c).

Depending on the nature of the particles, astrong van der Waals attraction and perhaps

electrostatic repulsion may exist between the

particles in the absence of polymer layers

(dashed line in part c), and the steric repulsion

stabilizes the dispersion against coagulation in

the primary minimum in the interaction potential.


Steric repulsion: ruling factors

- nature and lenght of the (co)-polymers

- ratio of molecular weights (MB enough large to provide steric barrier)

- interaction anchor group-surface (grafted/adsorbed)

- conformation the polymer adopts in solution (theta conditions)

- solvent quality

- surface density/coverage

Criteria for effective stabilisation

- full coverage of particle surface

- strong anchoring

- sufficient extension of chains from particle

- polymer segments in good solvent conditions

grafted polymer adsorbed polymer

(polymer may be (particularly

attached or copolymers)

grown from the

surface)



Influence of the amount adsorbed on the layer thickness (criterion for

choosing the stabilizer)

Hydrodynamic layer thickness increases

with amount adsorbed on the surface.

Maximum layer thickness is determined by

a combination of the adsorbed amount and

molecular weight.

T. Cosgrove, Colloid Science: Principles, Methods and Applications, Wiley-Blackwell 2005.

Adsorbed amount


Influence of the grafting density

Potential energy curves for pairs of sterically stabilized colloids separated by distance h. Grafting density decreases from i to iv.

decreasing of grafting density: magnitude of repulsive interactions decreases

at high surface coverage: repulsion of the particles

at too low density: repulsive interaction is not large enough to overcome attractive

forces aggregation occurs

Ian W. Hamley, Introduction to soft matter: Synthetic and biological self-assembling materials,

Wiley 2007.


Grafting density


Source of picture:


of Colloid Science




on surface





Unfavourable

in good solvents

DG > 0


Flocculation by bridging: practical aspects: behavior of

polymeric materials as flocculants

Bridging flocculation

Bridging flocculation: (A) two particles by one polymer molecule; (B) two

particles by two separately adsorbed polymer molecules.

Source of picture: Jingyu Shi, Steric Stabilization


Flocculation by bridging: practical aspects


Type A flocculation occurs when:

· The polymer molecule has more than two adsorbable segments.

· The chain is long enough to adsorb onto more than one particle.

· The surface coverage by adsorption of polymer is low, so that there are more chances for

adsorption of polymer extending from one particle to another particle.

This bridging flocculation occurs only at low polymer concentrations where the surface coverage of

the particles is less than half of the saturation value


Flocculation by bridging: practical aspects


Type B flocculation occurs when the polymer chains are very long and the surface coverage by

adsorbed polymer is so high that adsorption sites are scarce, and the probability for polymer

extending between the particles is low. Another condition for this type of flocculation is that the affinity

between the interacting chains should be large enough to overcome the repulsion cause by steric

stabilization.

→ very long polymer molecules contribute to bridging flocculation.


Source of picture:


of Colloid Science




on surface





Unfavourable

in good solvents

DG > 0



Attraction forces due to

polymers:Depletion attraction forces.

They occur whenever the surfaces of the particles are immersed

in a solution of a non adsorbing polymer (a polymer which does

not absorb on, or it is repelled by surfaces: free polymer).

Dispersed polymer molecules extert an osmotic pressure on all

sides of particles when they are separated, d > Rg→ no net

interaction among particles (Figure (a))

If d < Rg → depletion of polymer molecules in the interparticle

region otherwise loss of configurational entropy of polymer

chains (Figure (b))

The osmotic pressure forces exterted by molecules in the

external part of the particles > those in the interparticles side: net

attraction.

Lower than dispersion forces but become significant with

increasing:

1) Concentration

2) Molecular weight of polymer


Source of picture:


of Colloid Science

Effect of polymer additives on the stability of dispersions



on surface





Unfavourable

in good solvents

DG > 0


Source of picture: Cosgrove

Depletion interactions


Addition of free (non-adsorbing) polymer in solution induces so-called depletion interactions between

colloidal particles.

Asakura–Oosawa (AO) model of depletion interactions: The particles are considered as hard spheres of

diameter d and the polymers are represented by little spheres of diameter 2L0.

Within this description the polymer coils do not interact, and hence the osmotic pressure of the polymer

solution, P, can be calculated from their number concentration npol using the Van’t Hoff law.

P = npkBT


Source of picture: Cosgrove

Depletion interactions (flocculation)


Polymer coils: have hard sphere interactions with the colloidal particles

→ excluded from a depletion layer with thickness L0 around each particle (avoid entropic loss)

When two particles approach to a surface separation less than 2 L0, the depletion layers overlap. Due to

the polymer osmotic pressure this results in an effective attraction between the particles: solvent displaced

from interparticle region

P = npkBT


Depletion flocculation and stabilisation (non-adsorbing)


Wolf S.E., Gower L.B. (2017) Challenges and

Perspectives of the Polymer-Induced Liquid-

Precursor Process: The Pathway from Liquid-

Condensed Mineral Precursors to Mesocrystalline

Products. In: Van Driessche A., Kellermeier M.,

Benning L., Gebauer D. (eds) New Perspectives on

Mineral Nucleation and Growth. Springer, Cham


Depletion flocculation and stabilisation (non-adsorbing)


1. particles undergo close approach

2. solvent become inaccessible to

polymer

3. to fit: drastic distortion of

conformations

4. overall increase in free energy

5. when a good solvent is not

available: demixing: unven

distribution results in repulsion

6. for stabilisation to occur: energy

barrier ≈ 25 kBT


Depletion stabilisation mechanism (still an open issue)


The polymer-induced repulsion in this regime is therefore primary due to the excess of conformational

free energy of unattached (free) polymer chain

the FPI (free polymer induced) stabilization assisted by soft surface layers is more efficient under better

solvent conditions


Colloid steric stabilisation: summary

Attraction forces due to polymers:Segments-segments attraction in poor solvents

Segments prefer each other over the solvent → attractive forces between polymer segments for both adsorbed and grafted

layers, depending on theta temperature. Above theta-temperature (from poor to good solvent) segment-segment attraction

vanishes, and the force become repulsive at all separations.

Bridging attraction (bridging flocculation)

Polymers having affinity for a surface can establish bridges among surfaces. Availability of free surface sites for adsorprion




When two particles with adsorbed polymer layers approach each other at a distance of less than twice

the thickness of the adsorbed layer, interaction of the two layers takes place. The degree of

stabilization can be defined quantitatively in terms of the energy change occurring upon the

interaction of the adsorbed layers.

The Gibbs free energy change DG of the overlap interaction of the adsorbed layers is expressed

as

DG = DH -TDS .

DG: overlap interaction of the adsorbed layers is expressed as: DG = DH -TDS

DG < 0 upon the overlap of the adsorbed layers, flocculation or coagulation will result

DG > 0 repulsion: stabilization will result.

Under isothermal conditions, the stability is then a function of the enthalpy change, DH and the entropy

change, DS .


Colloid steric

stabilisation:

ruling factors


DGr = 0 overlap 0

Approaching particles experience an

increase of free energy

DGr > 0 overlap increases

(r = repulsion: stability against aggregation)

DGr = DHr - TDSr

Threshold Temperature (CFT)




DGr = 0 no overlap

DGr > 0 repulsion

DGr < 0 spontaneous aggregation (vdW), coagulation

DHr > 0 against aggregation

DSr > 0 favours aggregation

DSr < 0 loss of configurational entropy (fewer possible configurations in

the compressed state than in the uncompressed state): favours repulsion


Critical Flocculation Temperature (CFT)

Upper and Lower Critical Flocculation Temperatures in Sterically Stabilized Nonaqueous Dispersions

Melvin D. Croucher, Michael L. Hair

Macromolecules, 1978, 11 (5), pp 874–879


DGr = DHr - TDSr


R. J. Hunter, Foundations of Colloid Science

Colloid steric stabilisation



Steric stabilization vs electrostatic stabilization:Relative insensitivity to the presence of electrolytes.

For instance, for 1:1 electrolytes a charge-stabilized dispersion will not be stable and coagulate when the concentration of

electrolytes exceeds the 10-1 M limit. The dimensions of polymer chains display no such dramatic sensitivity and sterically

stabilized dispersions are relatively insensitive to the presence of electrolyte.

Equal efficacy in both aqueous and nonaqueous dispersion media.

Charge stabilization is less effective in nonaqueous dispersion media than it is in aqueous media. This is primarily due to the

low relative dielectric constant (<10) of most nonaqueous media. In contrast, steric stabilization is effective in both nonaqueous

media and aqueous media. This explains why steric stabilizion is usually preferred for nonaqueous dispersion media.

Reversibility of flocculation. The coagulation of charge-stabilized particles (induced by the addition of electrolyte) is usually

irreversible by subsequent dilution. In contrast, flocculation of sterically stabilized dispersions (induced by the addition of a

nonsolvent for the stabilizing moieties) can usually be reversed spontaneously by mere dilution of the nonsolvent concentration

to a suitably low value (improving solvent “goodness”). This difference is due to the fact that sterically stabilized dispersions

may be thermodynamically stable while charge stabilized dispersions are only thermodynamically metastable. As a

consequence, for charge stabilized dispersions, the coagulated state represents a lower energy state and the

coagulation can be reversed only after input of work into the system. Another important consequence of the thermodynamic

stability of sterically stabilized dispersions is that they can redisperse spontaneously after drying.



In order to achieve effective steric stabilisation, and for

adsorbing polymers following criteria should be met:

1. high surface coverage, G

2. strong adsorption

3. good solvent for stabilising chain, c < 0:5 (for

repulsion)

4. low free polymer concentration, ceq ≈ 0 (to avoid

depletion)



43

141 nm silica particles- with grafted Poly(methoxytri(ethylene glycol)methacrylate) LCST ~ 48 °C. Pictures were taken at 0 °C and 60 °C.

Phase-transfer of the particles with change in polymer solubility

D. Li, B. Zhao, Temperature-Induced Transport of Thermosensitive Hairy Hybrid

Nanoparticles between Aqueous and Organic Phases, Langmuir 2007, 23, 2208.

Quality of solvent is dependent on temperature:

at high temperatures the polymer expands, at low temperatures the polymer collapses

Ethyl acetate

H2O

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