Transcript Document

Sol_Gel PRocessing
Xerogels and Aerogels
Drying Effects
supercritical drying
aerogels
Metal oxide
cellulose, organic, and Carbon
Xerogels applications
Type title here
Templated Sol-Gels
Surfactants
Polymer surfactants
Applications
Encapsulation technology
Technique
Cellulose paper
TiO2 presentation
Stages in the Formation of a
Collodal Particle
The Colloidal Particle is an
Unstable Species
• Thermo dynamics tells us that any particle with high
specific surface area will have an associated high surface
energy.
• If we imagine the work needed to increase the surface of a
particle by a small increment d then the work is the
product of the force resisting increase in area times the
distance it moves.
• All material exhibits a resistance which we define as the
surface tension X. Then the increment of the work dw is
Xd, i.e. dw =X d
• A more rigorous argument is that work of surface creation is
additional to pressure/volume work and if we examine
Helmhotz function relating the change of state of a system to
entropy, p-v work and surface creation, we can express this as
follows:
• Helmholtz function:
= Entropy + Surface Creation + pressure/volume
dA = -S dT – pdV + X d
dA is the Helmholtz function
S is entropy
T is Temperature
P/v is pressure/volume
• All systems try to lower dA thus a system attempts to
lower its internal energy and increase entropy or disorder.
• The additional term in the expression shown above
indicates the desire to decrease surface energy.
• With the Gibbs free energy function we are often more
interested in changes at constant pressure and not constant
volume
• For our purposes, the Helmholtz is preferred because it
considers changes in a system at constant temperature and
volume and more appropriate to consider the work to
increase surface area of a particle.
• From this, it should be apparent that the colloidal
state should not exist at all yet experience tell
otherwise.
• The Stability of a colloid is therefore a very
kinetic one.
• The particles are trying to collaspe and move
towards one another and coalesce to reduce
surface energy.
• Imagine two spherical particles 200 nm diameter.
• Their volumes are 4/3 R13 = 4.189 R3 and surface areas are 4R2
= 12.57R2
•
• For the basis of a simple unit of square area in nm we would then
have:
•
• Volumes, V1 = 4.189 x 106 and areas = 12.57 x 104
•
• The total surface area of the two sphere is = 25.14 x 104
•
• If we dissolve the two sphere into one larger sphere we need to
calculate its new diameter.
•
Therefore 4/3 R23 = 4.189 R23 = 2 x V1 = 2 x 4.189 x 106
•
Therefore R2 = 126.99 and the new area of that sphere is 19.9 x 104
• Thus area of the single combine sphere is much
smaller than two separate spheres for the same
volume.
• Another way of looking at it is the ratio of area to
volume, V/A:
•
• (4R2) / (4/3 R3) = 3/R
•
• therefore A/V as R goes to zero goes to infinity
• This then provides a strong driving force for the
particles to combine.
Mechanism of Combining
• Particles when they pass close enough, will
experience a force of attraction know as van der
Waals force. And differ from electrostatic forces.
Electrostatic
van der Waal Forces
• Unlike electrostatic forces which vary with 1/r2
van der Waal forces obey a higher power law, 1/r6
– Permanent Dipole
– Dipole Induced in a Non-Polar Molecule by a
Permanent Dipole
– Resonant Induction of dipoles
van der Waal Forces
Permanent Dipoles
• Van der Waal forces arise because of the
permanent or induced polarization in adjacent
atoms or molecules even though the normal
valence requirements are satisfied.
Permanent Dipoles
• Molecules with permanent dipoles can orient in
such a way a to produce attractive forces.
• Attractive orientations correspond to a lower
energy state than repulsive ones; hence, in a fluid
the net average orientations cause attraction.
Dipole Induced in a Non-Polar
Molecule by a Permanent Dipole
• Here the electron cloud around the non polar
molecule is distorted and forms an induced
dipole.
Resonant Induction of Dipoles
• If the electron cloud in one molecule resonate, it can
induce a dipole in an adjacent electron cloud leading to
attraction.
• This induced dipole – dipole interaction is sometimes
called london attraction or dispersion force.
• Molecules with permanent dipoles can orient in such a
way a to produce attractive forces.
• Attractive orientations correspond to a lower energy state
than repulsive ones; hence, in a fluid the net average
orientation cause attraction.
• The total attractive force is between molecules is cause by
the sum of all three mechanism above.
• This augument has only considered dipoles but molecules
exist with more complex electron distributions such as
quadrupoles and higher.
• There are many more interactions but all lead to a depence
on 1/r6.
Why do Colloids Exist at All?
• Example:
• Basis: 1cm3 of cube of material with Area = 6 cm3
• Divide this into many 100 nm cubes
– The area increases now to 6000,000 cm3
• Divide this into many 10 nm cubes
– The area increases now to 6000,000 cm3
• Thus, any effects connected with colloids are going
to be surface dominated.
The are Other Forces that
Oppose the Long Range Effects
• It possible for a particle to develop a protective film at its
surfaceby reacting with the solvent.
• Example:
– A platinum sols will react in water to form Pt-(OH)3. This forms a
protective layer around the particle
• Emulsification of fat by soap is another example
• The adsorption of surface active molecules (neutral or
ionic).
– This can lead to kinetic stablization due to electrical charge on the
particle surface and its surrounding ions in solution.
• Electrical Double Layer ( we will discuss in more detail later )
Stages in the Formation of a
Collodal Particle
Sequence of Events leading to
Uniform spheres
(A_C) Time elaspe
over 6 hrs at
90 degrees
(D)
Aging 48 hrs
Oswald Ripening
• Ostwald ripening Derives from the mechanism
driving small particles to combine.
• It is the process by which larger particles (or, for
emulsions, droplets) grow at the expense of
smaller ones due to the higher solubility of the
smaller particles and to molecular diffusion
through the continuous phase.
• Initial formed aggregates restructure through
disolution-reprecipitation to form larger, more
stable particles, thereby consuming the small
primary particles.
Continued:
•On prolonged heat treatment, the precipitates
coarsen to decrease the interfacial free energy
between the precipitate and the matrix.
•During Oswald ripening the volume fraction Vf of
precipitates remains constant and the diameter of the
precipitates increases.
•The final product of Oswald Ripening is
indistinguishable from the structure of a nucleation
and growth process
Continued:
• The fate of primary nanoparticles depends on their
size, as well as on T and pH of the solution
• The Solubility, S, of a particle is related to its
radius, r, by the Oswald-Freundlich equation:
S = S0 exp[(2slVm)/(RgTr)]
Where So is the solubility of a flat plat, sl is the
solid-liquid interfacial energy, Vm is the molar
volume of the solid phase, Rg is the ideal gas
constant, and T is the temperature.
Continued:
• The effect of size on solubility is most important
for nanoparticles with smallest diameters.
• Nanoparticles of silica less than 5 nm will tend to
dissolve and reprecipitate on larger particles
• This process of particle growth will raise the
average particle diameter from 5 to 10 at pH > 7
• At low pH growth will be negligible for particles
larger than 2 to 4 nm.
• The final particle size increases with temperature
and pressure, as both increase the solubility
Continued:
• Since the condensation reation is exothermic, each
Si atom tries to surrounds itself with four siloxane
(i.e. Si-O-Si ) bonds
• For nanoparticles less than 5 nm, more than 50%
of the Si atoms are on the surface, so they must
have one or more silanol ( i.e. Si-OH) bonds
• The interiors can be regarded as dense SIO2
Solubility with Radius of Curvature
Silica Sol Gel Reaction
• Silica particles were precipitated from
solution of:
• 1.7 M tetraethly orthosilica,
• 1.3 M amomonia,
• and 2.0 M H2O in ethanol at 25oC
• SEM pictures of this reaction was taken
with time
Hydrolysis and Condensation
Reaction of TEOS
OR
OR
+
Si
OR
OR
OR
OR
+
Si
OR
OR
HO
Si
OR
OR
OR
Si
OR
OR
Condensation
+
HO
OR
ROH
OR
OR
OR
Si
OR
O
OR
OR
OH
+
Si
HO
OR
OR
OR
Hydrolysis
H2O
+
OR
Si
OR
OR
Si
OR
OR
OR
ROH
OR
Si
O
Si
+
OR
OR
H2O
Model of Silica Particle Growth
Cyclic trisilicic
Cubic octasilic acids
Oxygen
hydrogen
Silica not shown
C and D colloidal particles formed by momoners to form
closed rings until covered with a layer of silanol groups
Polymerization Behavior of Silica
The effect of Salt on Silica
Particle Formation and Growth
• With salt presence aggregation, precipitation
or gelation can occur at pH< 7 or pH 7 to 10.
• In the absence of salt, no chaining or
aggregation occur, because the particles are
mutually repulsive.
• The addition of salt reduces the thickness of
the electrical double layer at a given pH,
dramatically reducing the gel times.
Ripening of Silica Particles
Bar = 100 nm
Grids taken 2,8, 30 and 120 minutes apart after initial reaction.
No salt present.
Continued:
• The effect of size on solubility is most important
for nanoparticles with smallest diameters.
• Nanoparticles of silica less than 5 nm will tend to
dissolve and reprecipitate on larger particles
• This process of particle growth will raise the
average particle diameter from 5 to 10 at pH > 7
• At low pH growth will be negligible for particles
larger than 2 to 4 nm.
• The final particle size increases with temperature
and pressure, as both increase the solubility
Silica Solubility vs its Particle
Diameter
Particles formed at
80 to 100oC, pH 8
Particles formed at
25 to 50oC, pH 2.2
pH Dependence of the Reaction
• The polymerization process may be divided into
three domains: pH2, pH2 to 7, and >pH 7
• pH 2 is a boundary as the point of zero charge and
the isoelectric point ( zero mobility) both fall the
range of pH 1-3
• Ph 7 is a boundary because silica solubility and
dissolution rates are maximized at or above pH7 and
because above this the particles are so ionized that
particle growth occurs with aggregation or gelation
Silica Polymerization, pH 2 to 6
• Since the gel time decreases steadily between pH
2 and ~pH 6, it is generally assumed that above
the isoelectric point the condensation rate is
proportional to [OH-]
Effect of pH on Silica/H20 System
Polymerization Behavior of Silica
Silica Solubility vs pH & T
Solubility with Radius of Curvature
Necking Effects in aging
Aggregates
Sequence of Events leading to
Uniform spheres
(A_C) Time elaspe
over 6 hrs at
90 degrees
(D)
Aging 48 hrs
Growth by Aggregating Spheres
Silica from reacting TEOS
Silica Growth by Sweeping and Aggregation
Aggregate Particle