DEMat MATERIAIS NANOESTRUTURADOS E NANOTECNOLOGIASweb.ist.utl.pt/ist12456/Special Nanomaterials Colloid.pdf · materiais nanoestruturados e nanotecnologias m. clara gonçalves demat
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Summary
• Introduction to Colloid Science
Electrostatic Stabilization
Steric Stabilization
•Further Reading
Summary
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Introduction to Colloid Science
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Introduction to Colloid Science
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Introduction to Colloid Science
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Introduction to Colloid Science
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Introduction to Colloid Science
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Introduction to Colloid Science
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i) COMBINING INDIVIDUAL
NANOSTRUCTURES
TOGETHER TO FORM LARGE
STRUCTURES SO AS TO
REDUCE THE OVERALL
SURFACE AREA;
• AT THE INDIVIDUAL NANOSTRUCTURE LEVEL,
THE SURFACE ENERGY CAN BE REDUCED THROUGH:
ii) AGGLOMERATION OF
INDIVIDUAL
NANOSTRUCTURES WITHOUT
ALTERING THE INDIVIDUAL
NANOSTRUCTURES;
Introduction to Colloid Science
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Introduction to Colloid Science
• THE SURFACE ENERGY CAN BE REDUCED THROUGH
AGGREGATION / FLOCULATION / COALESCENCE
THERMODINAMIC METASTABLE
KINETIC STABLE SYSTEM
THERMODINAMIC STABLE
SYSTEM
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IN AGGLOMERATES, MANY NANOSTRUCTURES ARE ASSOCIATED WITH ONE
ANOTHER THROUGH CHEMICAL BONDS AND PHYSICAL ATTRACTION
FORCES AT INTERFACES.
AGGLOMERATES ARE VERY DIFFICULT TO DESTROY.
THE SMALLER THE INDIVIDUAL NANOSTRUCTURES ARE, THE STRONGER
THEY ARE ASSOCIATED WITH ONE ANOTHER, AND THE MORE DIFFICULT TO
SEPARATE
ii) AGGLOMERATION OF INDIVIDUAL NANOSTRUCTURES
WITHOUT ALTERING THE INDIVIDUAL NANOSTRUCTURES;
Introduction to Colloid Science
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Introduction to Colloid Science
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TO MIX OR NOT TO MIX?
DISPERSIONSSOLUTIONS
Mixing is SPONTANEOUS.
Mixtures are THERMODINAMICALLY
STABLE
Inhomogeneities on molecular level.
Mixing is REVERSIBLE.
Properies of solutions are independent
on the way they are prepared.
Mixing is NON-SPONTANEOUS.
Mixtures are THERMODINAMICALLY
UNSTABLE.
Inhomogeneities on length scales large
compared to molecular dimensions.
Mixing is IRREVERSIBLE.
Empirical preparation methods.
Introduction to Colloid Science
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THERMODINAMICALLY STABLE, UNSTABLE
Dispersions are thermodinamically UNSTABLE.Solutions are thermodinamically STABLE.
A most important physical property of
colloid dispersions is the tendency of
particles to aggregate.
SLOW KINETICS ⇒ KINETIC
STABLE SYSTEM
Protect dispersion particles against
aggregation / flocculation / coalescence by
repulsive interactions.
Introduction to Colloid Science
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DLVO theory
Introduction to Colloid Science
Electrostatic Stabilization
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Introduction to Colloid Science
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AS THE DIMENSION OF NANOSTRUCTURE MATERIALS REDUCES, VAN DER
WAALS ATTRACTION FORCE BETWEEN NANOSTRUCTURED MATERIALS
BECOMES INCREASINGLY IMPORTANT.
THERE ARE TWO MAJOR STABILIZATION MECHANISMS:
ELECTROSTATIC STABILIZATION
STERIC STABILIZATION
ii) AGGLOMERATION OF INDIVIDUAL NANOSTRUCTURES
WITHOUT ALTERING THE INDIVIDUAL NANOSTRUCTURES;
Introduction to Colloid Science
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COLOIDAL STABILITY: DLVO theory
i) ATRACTIVE INTERACTIONS
• London and van der Waals
forces between atoms and
molecules.
• Attraction between atoms /
molecules is additive so it
effects in case of macroscopic
bodies too!
( ) 6
.R
constRVA −≈
HR
( )H
AaHVA 12−≈
• Hamaker constant in vacuum
depends material properties:
density and polarizability.
H
( ) 2HAHVA −≈
Electrostatic Stabilization
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• Hamaker constant in vacuum
depends on material properties:
density and polarizability.
H
( ) 2HAHVA −≈
H ~ 10-20 – 10-21 J
i) ATRACTIVE INTERACTIONS
COLOIDAL STABILITY: DLVO theory
Electrostatic Stabilization
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Electrostatic Stabilization
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WHEN A SOLID EMERGES IN A POLAR SOLVENT OR AN ELECTROLYTE SOLUTION, A
SURFACE CHARGE σ0 (E0) WILL BE DEVELOP THROUGH ONE OR MORE OF THE
FOLLOWING MECHANISMS:
PREFERENTIAL ADSORPTION OF IONS
DISSOCIATION OF SURFACE CHARGED SPECIES
ISOMORPHIC SUBSTITUTION OF IONS
ACUMULATION OR DEPLETION OF ELECTRONS AT THE SURFACE
PHYSICAL ADSORPTION OF CHARGED SPECIES ONTO THE SURFACE
Electrostatic Stabilization
ii) REPULSIVE INTERACTIONS
COLOIDAL STABILITY: DLVO theory
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FOR A GIVEN SOLID SURFACE IN A GIVEN LIQUID MEDIUM, A FIXED SURFACE
ELECTRICAL CHARGE DENSITY OR ELECTRODE POTENTIAL, E, WILL BE ESTABLISHED,
WHICH IS GIVEN BY THE NERST EQUATION:
E0 – standard electrode potential when the concentration of ions is unity
ai – activity of ions
ni – valence state of the atom
R – gas constant
T – temperature
F – Faraday’s constant
ii
aFn
RTEE ln0 +=
Electrostatic Stabilization
COLOIDAL STABILITY: DLVO theory
ii) ELECTRICAL DOUBLE LAYER REPULSIVE
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Electrostatic Stabilization
http://dragon.unideb.hu/~kolloid/coll_ph_9.pdf
• A solid surface in contact with a solution of an
electrolyte usually carries an electric charge, σ0. This
gives rise to an electric potential, ψ0, at the surface,
and a decreasing potential, ψ, as one moves through
the liquid away from the surface.
• Two regions will be formed: the Stern layer
immediately adjacent to the surface where ion size is
important; and outside this is a diffuse layer.
• Because of the difference in charge bertween the
diffuse layer and the solid surface, movement of one
relative to the other will cause charge separation and
hence generate a potential difference. The relative
movement of the solid surface and the liquid occurs at
the surface shear. The potential at the shear plane is
known as the zeta potential, ζ.
ψ
ψD ψD
ψD
σ0
ψ0
ζ
COLOIDAL STABILITY: DLVO theory
ii) ELECTRICAL DOUBLE LAYER REPULSIVE
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Electrostatic Stabilization
COLOIDAL STABILITY: DLVO theory
ii) ELECTRICAL DOUBLE LAYER REPULSIVE
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Electrostatic Stabilization
WHEN A SURFACE CHARGE
DENSITY OF A SOLID SURFACE IS
ESTABLISHED, THERE WILL BE AN
ELECTROSTATIC FORCE BETWEEN
THE SOLID SURFACE AND THE
CHARGED SPECIES IN THE
PROXIMITY TO SEGREGATE
POSITIVE AND NEGATIVELY
CHARGED SPECIES.
COLOIDAL STABILITY: DLVO theory
ii) ELECTRICAL DOUBLE LAYER REPULSIVE
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PLANE OF ZERO SHEAR
Because of the difference in
charge bertween the diffuse
layer and the solid surface,
movement of one relative to the
other will cause charge
separation and hence generate a
potential difference.
επζ qk 14 −
=
Electrostatic Stabilization
COLOID PARTICLE
STERN LAYER
PLANE OF ZERO SHEARZETA POTENTIAL
ζ
COLOIDAL STABILITY: DLVO theory
k-1
ii) ELECTRICAL DOUBLE LAYER REPULSIVE
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Electrostatic Stabilization
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POINT OF ZERO CHARGE (P.Z.C.) – THE CONCENTRATION OF CHARGE DETERMINING IONS
CORRESPONDING TO A NEUTRAL OR ZERO-CHARGED SURFACE
( )[ ]F
pHczpRTE −=
...303.2
( )[ ]pHczpE −≈ ...06.0
AT ROOM TEMPERATURE
ELECTROSTATIC STABILIZATION
Electrostatic Stabilization
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THE SURFACE CHARGE IN OXIDES IS MAINLY DERIVED FROM PREFERENTIAL
DISSOLUTION OR DEPOSITION OF IONS.
IONS ADSORBED ON THE SOLID SURFACE DETERMINE THE SURFACE CHARGE – CO-IONS.
IN THE OXIDE SYSTEMS, TYPICAL CHARGE DETERMINING IONS ARE PROTONS AND
HYDROXYL GROUPS (pH).
ELECTROSTATIC STABILIZATION
Electrostatic Stabilization
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POINT OF ZERO CHARGE (P.Z.C.) – THE CONCENTRATION OF CHARGE DETERMINING IONS
CORRESPONDING TO A NEUTRAL OR ZERO-CHARGED SURFACE
ELECTROSTATIC STABILIZATION
pH < p.z.c. pH > p.z.c.
THE SURFACE IS COVERED WITH OH- GROUPS,
THE OXIDE SURFACE IS NEGATIVELY CHARGED
H+ IS THE CHARGE DETERMINING ION,
THE SURFACE IS POSITIVELY CHARGED
ξ MINIMUM ξ MAXIMUMξ = 0
pH
p.z.c.
POINT OF ZERO CHARGE(ZERO CHARGED SURFACE)
Electrostatic Stabilization
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Electrostatic Stabilization
ii) REPULSIVE INTERACTIONSi) ATRACTIVE INTERACTIONS +
COLOIDAL STABILITY: DLVO theory
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Electrostatic Stabilization
ii) REPULSIVE INTERACTIONSi) ATRACTIVE INTERACTIONS +
COLOIDAL STABILITY: DLVO theory
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Electrostatic Stabilization
COLOIDAL STABILITY: DLVO theory
THERMODINAMIC METASTABLE
KINETIC STABLE SYSTEM
THERMODINAMIC STABLE
SYSTEM
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Electrostatic Stabilization
COLOIDAL STABILITY: DLVO theory
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Electrostatic Stabilization
COLOIDAL STABILITY: DLVO theory
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Electrostatic Stabilization
COLOIDAL STABILITY: DLVO theory
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Electrostatic Stabilization
COLOIDAL STABILITY: DLVO theory
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Steric Stabilization
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AS THE DIMENSION OF NANOSTRUCTURE MATERIALS REDUCES, VAN DER
WAALS ATTRACTION FORCE BETWEEN NANOSTRUCTURED MATERIALS
BECOMES INCREASINGLY IMPORTANT.
THERE ARE TWO MAJOR STABILIZATION MECHANISMS:
ELECTROSTATIC STABILIZATION
STERIC STABILIZATION
ii) AGGLOMERATION OF INDIVIDUAL NANOSTRUCTURES
WITHOUT ALTERING THE INDIVIDUAL NANOSTRUCTURES;
Steric Stabilization
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THERE ARE TWO MAJOR STABILIZATION MECHANISMS:
ELECTROSTATIC STABILIZATIONSTERIC STABILIZATION
http://www.malvern.com/LabEng/industry/colloids/dlvo_theory.htm
Steric Stabilization
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THERE ARE TWO MAJOR STABILIZATION MECHANISMS:
Steric Stabilization
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Steric Stabilizationhttp://www.pall.com/images/hcp_fig_min_fig9a.gif
http://www.chem.uh.edu/Faculty/Perry/images/adsorption1.jpg
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Steric Stabilization
STERIC STABILITY
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Steric Stabilization
STERIC STABILITY
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Steric Stabilization
STERIC STABILITY
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Steric Stabilization
STERIC STABILITY
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Steric Stabilization
STERIC STABILITY
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Steric Stabilization
STERIC STABILITY
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Steric Stabilization
STERIC STABILITY
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Steric Stabilization
STERIC STABILITY
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Steric Stabilization
STERIC STABILITY
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Steric Stabilization
STERIC STABILITY
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Steric Stabilization
STERIC STABILITY
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Steric Stabilization
STERIC STABILITY
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Steric Stabilization
STERIC STABILITY
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Steric Stabilization
STERIC STABILITY
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Steric Stabilization
STERIC STABILITY
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Further Reading
• Nanostructures and Nanomaterials. Synthesis, Properties & Applications, G.
Cao, ICP Imperial College Press, 2007 (ISBN 1-86094-480-9).
•The Colloidal Domain. Where Physics, Chemistry, Biology, and Technology
Meet, D. Fennell Evans and H. Wennerström, Wiley-VCH, 1999, ISBN 0-471-
24247-0
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