18 Sol-Gel Applications

Question 1

Applications of sol-gel materials

Answer 1

– Biosensors’ supports
– Precursors for the synthesis of composites
with controlled chemical doping
– Preparation of synthetic opals
– Synthesis of porous materials

Question 2

Inorganic-organic hybrid material

Answer 2

Sol-gel reactions using more than one precursor
can yield “hybrid” materials. Low-temperature
conditions and availability of precursors makes
sol-gel ideal for synthesizing materials with
unique properties.

Question 3

Embedding of organic molecules

Answer 3

Dissolve organic molecules in the sol-gel
reaction mixture. Once a gel is formed, the
organic molecules are trapped in the gel matrix.
Polymers, dyes, catalysts can be embedded into
the gel matrix.

Question 4

Biosensing

Answer 4

biological species embedded in a porous solid
matrix acts as a biosensor for specific molecules.
Biomolecules are sensitive to pH, temperature
changes, and different solvents. Mild conditions
used in sol-gel reactions are ideal for synthesis
of biosensors.

Question 5

incorporation of organic groups via covalent bonding

Answer 5

Use organotrialkoxysilane and organodialkoxysilane or
more complicated organosilanes for sol-gel processing
(see examples below). Si-C bonds are hydrolytically
stable and are maintained in the final product.

Question 6

Porous materials

Answer 6

Also known as cellular solids. Used for
insulation, shock absorption, catalysis. A porous
material must have cavities with a large aspect
ratio

Question 7

Pore type

Answer 7

Closed pores
Open, blind
Open, through
Open, through

Question 8

Classification of pores

Answer 8

Mobility of molecules within a pore depends on
pore size.

Macropores: Mobility depends on bulk diffusion.

Micropores: Mobility is controlled by
intermolecular interactions between the porous
material and solution.

Question 9

Templating using molecules

Answer 9

Sol-gel synthesis can be used to prepare porous
materials such as zeolites or mesoporous
silicates. The zeolite or silicate grows around a
templating agent producing a porous structure
displaying long-range order

Question 10

Adsorption Isotherms

Answer 10

Measuring surface area

fraction of molecules adsorbed
to a surface as pressure increases (temperature is held
constant, hence isotherm).

Question 11

Freundlich
* Langmuir
* BET (Brunauer, Emmett, and
Teller)
* BJH (Barrett, Joyner, Halenda

Answer 11

There are several different
adsorption models (see below) which are based on
slightly different assumptions.

Question 12

Langmuir equation

Answer 12

𝜃 =𝐾𝑃/(1 + 𝐾𝑃)
θ = number of sites covered with gas molecules
P = pressure, K = equilibrium constant

Only works well at low pressures and assumes
monolayer coverage.

Question 13

BET Adsorption Isotherm

Answer 13

Multiple layer formation on a surface under high
pressures and low temperatures.

Question 14

BJH Adsorption Isotherm

Answer 14

Multiple layer formation on a porous material
under high pressures and low temperatures. It
assumes the pores are cylindrical. Adsorption
into these materials occurs in two stages:
monolayer/multilayer adsorption on the walls,
filling by capillary condensation.

Question 15

Experimental procedure (Surface area)

Answer 15

1. Heat material under vacuum to remove
   adsorbed species.
2. Cool the material under vacuum (77K).
3. An adsorbent (N2
   ) is added in controlled
   increments.
4. Pressure is allowed to equilibrate.
5. Amount adsorbed is calculated.
6. Surface area is determined by fitting into
   different adsorption models

Question 16

Experimental procedure (porosity)

Answer 16

As the pressure in the system increases, liquid
begins to condense in the pores. The pressure is
increased until the pores are saturated with
liquid, and then decreased until the liquid
evaporates. Adsorption, desorption and
hysteresis provide information about pore
volume, size and surface area