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
Silvia Gross – Chimica dei Colloidi – Laurea Triennale in Chimica
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)
Silvia Gross – Chimica dei Colloidi – Laurea Triennale in Chimica
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)
Silvia Gross – Chimica dei Colloidi – Laurea Triennale in Chimica
Colloid steric stabilisation: ruling factors
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
Silvia Gross – Chimica dei Colloidi – Laurea Triennale in Chimica
Colloid steric stabilisation: ruling factors
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.
Silvia Gross – Chimica dei Colloidi – Laurea Triennale in Chimica
(1) measured at / extrapolated to 25 °C.
Colloid steric stabilisation: ruling factors
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
Silvia Gross – Chimica dei Colloidi – Laurea Triennale in Chimica
Colloid steric stabilisation: ruling factors
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
Silvia Gross – Chimica dei Colloidi – Laurea Triennale in Chimica
Colloid steric stabilisation: ruling factors
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.
Silvia Gross – Chimica dei Colloidi – Laurea Triennale in Chimica
Colloid steric stabilisation: ruling factors
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.
Silvia Gross – Chimica dei Colloidi – Laurea Triennale in Chimica
Colloid steric stabilisation: ruling factors
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
Silvia Gross – Chimica dei Colloidi – Laurea Triennale in Chimica
Colloid steric stabilisation: ruling factors
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
Silvia Gross – Chimica dei Colloidi – Laurea Triennale in Chimica
Colloid steric stabilisation: ruling factors
Quality („goodness“) of solvent and cloud point
Silvia Gross – Chimica dei Colloidi – Laurea Triennale in Chimica
Source of picture: Chem. Commun., 2014, 50, 6556-6570
Effect of polymer additives on the stability of dispersions
Colloid steric stabilisation: ruling factors
Silvia Gross – Chimica dei Colloidi – Laurea Triennale in Chimica
Source of picture:
R. J. Hunter, Foundations
of Colloid Science
Effect of polymer concentration on the stability of dispersions
Colloid steric stabilisation: ruling factors
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
Silvia Gross – Chimica dei Colloidi – Laurea Triennale in Chimica
Source of picture:
R. J. Hunter, Foundations
of Colloid Science
Effect of polymer concentration on the stability of dispersions
Colloid steric stabilisation: ruling factors
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
Silvia Gross – Chimica dei Colloidi – Laurea Triennale in Chimica
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.
Silvia Gross – Chimica dei Colloidi – Laurea Triennale in Chimica
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)
Silvia Gross – Chimica dei Colloidi – Laurea Triennale in Chimica
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)
Silvia Gross – Chimica dei Colloidi – Laurea Triennale in Chimica
Colloid steric stabilisation: ruling factors
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
Silvia Gross – Chimica dei Colloidi – Laurea Triennale in Chimica
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.
Colloid steric stabilisation: ruling factors
Grafting density
Silvia Gross – Chimica dei Colloidi – Laurea Triennale in Chimica
Source of picture:
R. J. Hunter, Foundations
of Colloid Science
Effect of polymer concentration on the stability of dispersions
Colloid steric stabilisation: ruling factors
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
Silvia Gross – Chimica dei Colloidi – Laurea Triennale in Chimica
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
Silvia Gross – Chimica dei Colloidi – Laurea Triennale in Chimica
Flocculation by bridging: practical aspects
Colloid steric stabilisation: ruling factors
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
Silvia Gross – Chimica dei Colloidi – Laurea Triennale in Chimica
Flocculation by bridging: practical aspects
Colloid steric stabilisation: ruling factors
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.
Silvia Gross – Chimica dei Colloidi – Laurea Triennale in Chimica
Source of picture:
R. J. Hunter, Foundations
of Colloid Science
Effect of polymer concentration on the stability of dispersions
Colloid steric stabilisation: ruling factors
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
Silvia Gross – Chimica dei Colloidi – Laurea Triennale in Chimica
Colloid steric stabilisation: ruling factors
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
Silvia Gross – Chimica dei Colloidi – Laurea Triennale in Chimica
Source of picture:
R. J. Hunter, Foundations
of Colloid Science
Effect of polymer additives on the stability of dispersions
Colloid steric stabilisation: ruling factors
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
Silvia Gross – Chimica dei Colloidi – Laurea Triennale in Chimica
Source of picture: Cosgrove
Depletion interactions
Colloid steric stabilisation: ruling factors
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
Silvia Gross – Chimica dei Colloidi – Laurea Triennale in Chimica
Source of picture: Cosgrove
Depletion interactions (flocculation)
Colloid steric stabilisation: ruling factors
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
Silvia Gross – Chimica dei Colloidi – Laurea Triennale in Chimica
Depletion flocculation and stabilisation (non-adsorbing)
Colloid steric stabilisation: ruling factors
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
Silvia Gross – Chimica dei Colloidi – Laurea Triennale in Chimica
Depletion flocculation and stabilisation (non-adsorbing)
Colloid steric stabilisation: ruling factors
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
Silvia Gross – Chimica dei Colloidi – Laurea Triennale in Chimica
Depletion stabilisation mechanism (still an open issue)
Colloid steric stabilisation: ruling factors
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
Silvia Gross – Chimica dei Colloidi – Laurea Triennale in Chimica
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
Silvia Gross – Chimica dei Colloidi – Laurea Triennale in Chimica
Colloid steric stabilisation: ruling factors
Colloid steric stabilisation: ruling factors
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 .
Silvia Gross – Chimica dei Colloidi – Laurea Triennale in Chimica
Colloid steric
stabilisation:
ruling factors
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)
Silvia Gross – Chimica dei Colloidi – Laurea Triennale in Chimica
Colloid steric stabilisation: ruling factors
Colloid steric stabilisation: ruling factors
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
Silvia Gross – Chimica dei Colloidi – Laurea Triennale in Chimica
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
Colloid steric stabilisation: ruling factors
DGr = DHr - TDSr
Silvia Gross – Chimica dei Colloidi – Laurea Triennale in Chimica
Critical Flocculation Temperature (CFT)
Silvia Gross – Chimica dei Colloidi – Laurea Triennale in Chimica
Critical Flocculation Temperature (CFT)
Silvia Gross – Chimica dei Colloidi – Laurea Triennale in Chimica
R. J. Hunter, Foundations of Colloid Science
Colloid steric stabilisation
Silvia Gross – Chimica dei Colloidi – Laurea Triennale in Chimica
Colloid steric stabilisation: summary
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.
Silvia Gross – Chimica dei Colloidi – Laurea Triennale in Chimica
Colloid steric stabilisation: summary
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)
Silvia Gross – Chimica dei Colloidi – Laurea Triennale in Chimica
Critical Flocculation Temperature (CFT)
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