DEMat MATERIAIS NANOESTRUTURADOS E NANOTECNOLOGIASweb.ist.utl.pt/ist12456/Special Nanomaterials...

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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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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;


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• 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;


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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.


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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.


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DLVO theory



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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




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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 −≈


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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



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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


ii) REPULSIVE INTERACTIONS


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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 +=



ii) ELECTRICAL DOUBLE LAYER REPULSIVE

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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



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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.



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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 −

=


COLOID PARTICLE

STERN LAYER

PLANE OF ZERO SHEARZETA POTENTIAL

ζ


k-1


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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



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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).



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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


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)


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ii) REPULSIVE INTERACTIONSi) ATRACTIVE INTERACTIONS +


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THERMODINAMIC METASTABLE

KINETIC STABLE SYSTEM

THERMODINAMIC STABLE

SYSTEM

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AS THE DIMENSION OF NANOSTRUCTURE MATERIALS REDUCES, VAN DER

WAALS ATTRACTION FORCE BETWEEN NANOSTRUCTURED MATERIALS

BECOMES INCREASINGLY IMPORTANT.



STERIC STABILIZATION




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ELECTROSTATIC STABILIZATIONSTERIC STABILIZATION

http://www.malvern.com/LabEng/industry/colloids/dlvo_theory.htm


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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

http://www.chem.uh.edu/Faculty/Perry/images/adsorption1.jpg

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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

DEMat MATERIAIS NANOESTRUTURADOS E NANOTECNOLOGIASweb.ist.utl.pt/ist12456/Special Nanomaterials...

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