BaSO 4 Surface properties of nanoparticle • the surface of suspended nanoparticles is electrically charged (in many cases) • counter ions are adsorbed onto the surface, more or less to compensate the electrical charges • the layer of surface charges + the layer of counter ions Origin of surface charges: • lattice defects by substituted atoms • adsorption of ions onto surface of the solid particle • adsorption of molecules with functional groups which have electrical charges and/or are dissociable • chemical (e.g. acid / basic) reactions on the surface of the solid particles (e.g. by dissociation) Ba 2+ Ba 2+ Ba 2+ Ba 2+ BaSO 4 model of the electrochemical double layer on the particle surface O O O O Si Si Al O O O O solid particle solid particle
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Surface properties of nanoparticle - OVGU · Electrochemical potential electrical neutrality of charges in infinite distance from particle for practical calculations : the thickness
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BaSO4
Surface properties of nanoparticle
• the surface of suspended nanoparticles is electrically charged (in many cases) • counter ions are adsorbed onto the surface, more or less to compensate the
electrical charges • the layer of surface charges + the layer of counter ions
Origin of surface charges:
• lattice defects by substituted atoms • adsorption of ions onto surface of the solid particle • adsorption of molecules with functional groups
which have electrical charges and/or are dissociable • chemical (e.g. acid / basic) reactions on the surface
of the solid particles (e.g. by dissociation)
Ba2+
Ba2+
Ba2+
Ba2+
BaSO4
model of the electrochemical double layer on the particle surface
O O O O
Si Si Al
O O O O
solid particle
solid particle
Stabilisation of nano - sized titanium dioxide
Zeta - potential of TiO2 ranging from + 20 mV to + 40 mV (pH < 3.0)
TiO--
O-
O-
O-
base
+ OH-
TiOH
OH
+H+
OH2+
OH2+
OH2+
OH2+
OH
Tiacid
OH
Mechanism of redispersion (peptization) and stabilisation
Acid / basic reactions on surface of solid particles by dissociation
of metal oxide nanoparticles (titanium dioxide)
Inner and outer Helmholtz layer
e.g. negatively charged solid nanoparticle Nernst - potential
• after particle formation / suspending an adsorption of negatively / positively
charged counter ions onto the particle surface
• while adsorbing anions lost their hydrate envelope, van der Waals forces
dominates inner Helmholtz - layer
• positively charged cat ions are bound onto the negative mono layer of ani-
ons, electrostatic and van der Waals forces dominate
outer Helmholtz - layer
• inner and outer Helmholtz - layer
Stern - layer Stern - potential
Gouy - Chapman layer
e.g. negatively charged solid nanoparticle
• bound counter ions in the Stern - layer compensate the electrical charge of the solid particle only partially
• for a whole charge compensation of particle surface there is the need of
more counter ions (condition of electrical charge neutrality)
• counter ions form a diffusive cloud around the particle, counter ions can move independently, concentration of counter ions grows in direction to the particle surfaces
• Gouy - Chapman layer begins on the firm bound Stern - layer and ends in
the infinite (totally compensation of particle surface charge)
• destabilisation of suspension e.g. with addition of Fe3+ or Al3+ ions
compression of diffuse double layer
van der Waals attraction higher than
electrostatic repulsion
∑εε
=δκ 220
iiA
r
zcNe
Tk
Gouy-Chapman layer radii in nm for different salt types in water at 298 K
ζ - p
oten
tial
potential
φi
φS
φ0
φS/e
distance from particle surface
shear zones with different shear velocities
Determination of the surface potential of nano particles
measuring of electrical potential :
• suspended particles seems to be neutral, charges of the diffuse double layer are compensated
• by a displacement of a part of the charge cloud
arises a measurable potential difference
• shear forces caused by diffusion only are too low, there is the need of a higher shear velocity
• a removing of the complete double layer is
impossible, only approximately the Stern potential measurable
measurable potentials potential at the shear plane = zeta potential ≈ stern potential
0Ev
ε⋅εη
⋅rr
Determination of the zeta - potential for nanoparticle characterisation
Zeta potential: electrical potential of a charged particle at the shearing plane
A charged particle in motion caused by an electrical field or by diffusion loses a portion of
its counter ions of the electrical double player
It is assumed that the zeta - potential corresponds to the Stern - layer potential
Measurement of the zeta – potential: determination of the electrophoretical mobility
Helmholtz – Smoluchowski equation:
ζ =
ζ zeta - potential
E electrical intensity
v particle velocity
η viscosity
ε·ε0 dielectric constant
Sterical stabilisation of nanoparticles
on the surface there are polymers with hydrophilic groups, polymers form short “hairs” towering into the dispersant stabilisation entropic effects, numbers of possible configurations would be lowered
by coagulation energetic effects, polymers have in the dispersant a lower energy con-
tent than being in contact each other
Electrostatic stabilisation of nanoparticles
DLVO theory: developed by Derjaguin, Landau, Overbeek and Verwey
combines van der Waals attraction and electrostatic repulsion forces
Boris Derjaguin Lew Landau Evert J.W. Verwey J.T.G. (Theo) Overbeek
B. Derjaguin, L.Landau (1941): Theory of the stability of strongly charged lyophobic sols and of the adhesion of strongly charged particles in solutions of electrolytes, Acta Phys. Chem. USSR 14, 633 E.J.W. Verwey, J.T.G. Overbeek (1948): Theory of the stability of lyophilic colloids. The interaction of sol parti-cles having an electric double layer, Elsevier Publishing Company
δ+ δ-
δ-δ+δ-δ+
dipole nonpolar molecule
dipole induced dipole
Stabilisation of disperse systems
a suspension of nanoparticles is then stable, if primary particles stay isolated – the stability is influ-
enced by the interaction of attractive and repulsive forces
Interaction energy between two nanoparticles Question: Why do we have a repulsion force between electrically equal charged nanoparticles? Please remem-ber, the opposite electrically charged counter-ions are screening the charge of the central nanoparticle. Answer: When the ionic double layers start to overlap, there is an excess of ions in the overlap region respect to the electrochemical potential. To compensate this, an os-motic pressure develops between the bulk solution and the overlap region.
xexp1xexp1ln
FzRT2)x(
0
0
Grahame equation
RT2)x(FzsinhCRT8 r0
0surf
Interaction energy between nanoparticles - osmosis and osmotic pressure