17 Sol-Gel Synthesis

Question 1

Parameters affecting sol-gel reactions

Answer 1

– Type of precursor
– Type of catalyst (pH)
– Type of solvent
– Alkoxy group to water ratio (Rw
)
– Temperature
– Precursor concentration

Question 2

Steric factors:

Answer 2

Rate of hydrolysis decreases as
sterics increase:

Question 3

Inductive effects

Answer 3

Stabilize or destabilize
transition states. Electron density on the silicon
atom (in black) increases from O-Si to R.

Question 4

Acid- vs. base-catalyzed reaction

Answer 4

Because of inductive effects, acid-catalyzed
reactions produce linear structures
(condensation favored at terminal sites). Basecatalyzed reactions produce branched structures
(condensation favored at central sites).

Question 5

pH dependence

Answer 5

At pH <5 hydrolysis is favored and condensation
is the rate-determining step. At pH > 5,
hydrolysis is the rate-determining step. In other
words, at pH >5 hydrolyzed species are
consumed quickly after being formed.

Question 6

Alkoxy group to water ratio (Rw)

Answer 6

Rw
is the alkoxy group to H2O mole ratio.
Consider the reaction of TMOS with water t

Question 7

Rw

Answer 7

Decreasing Rw
(increasing water content) favors
the formation of silanols over Si-O-Si. Rw = ≤ 2
favors hydrolysis. Rw&raquo_space; 2 favors condensation

Question 8

Solvent

Answer 8

Polar protic solvents are generally used because
they stabilize polar species during hydrolysis and
condensation. Non-polar solvents are used for
non-polar silanes (organotrialkoxysilanes,
R’Si(OR)3
) that do not completely hydrolyze.

Question 9

Sol-gel reactions of metal oxides

Answer 9

Metal alkoxides are stronger Lewis acids than
silicon alkoxides. For this reason, hydrolysis
rates are strongly increased. In addition, as
opposed to Silicon, metals have higher
coordination numbers

Question 10

Percolation theory

Answer 10

A model to describe the formation of a gel from a sol.
Each sol particle is denoted by a dot on a grid. A line is
drawn between 2 neighbouring dots to indicate that
they have aggregated.

Question 11

Percolation theory and Gelation

Answer 11

Key points:
– At the gel point (when a spanning cluster is
formed), unbound clusters are still present.
– Gelation is NOT a special thermodynamic
event

Question 12

Aging

Answer 12

During aging, condensation of particles or
monomers within the pore liquid changes the
structure and properties of the gel. For
instance, liquid is expelled due to network
contraction (syneresis).

Question 13

Conventional drying

Answer 13

Involves the direct transition from liquid to gas,
by reducing pressure or increasing temperature

Question 14

Drying

Answer 14

Two steps occur during conventional drying
1. Gel deformation/shrinkage. Further
condensation reactions occur.
2. Gel cracks due to uneven pressure at the
surface and interior or in pores of different
sizes.
15
Xerogels are obtained by conventional drying. Powders and thinfilm production are the most common applications of xerogels

Question 15

Supercritical drying

Answer 15

Liquid  supercritical fluid  gas. Supercritical drying
is performed to significantly reduce the surface tension
of the pore liquid. This prevents cracks from forming as
the material dries.

At the critical temperature, the liquid/vapour
interface does not exist. This allows drying of
the gel without collapse of the solid structure.

Question 16

Aerogels

Answer 16

If drying is done under supercritical conditions,
the material is called an aerogel. Using
supercritical drying, the surface tension of the
pore liquid is reduced. Upon removal of the
pore liquid, the resulting structure experiences
minimal shrinkage or cracking. This type of
structure is called an aerogel.