Chapter 2: Silica

Question 1

Which building blocks is Silica constituted of?

Answer 1

Silica is constituted of tetrahedral [SiO4]4-building blocks; these tetrahedra are rigid, but they can connect via the oxygen atoms (forming Si-O-Si bridges) at fairly flexible angles ranging from linear to tetrahedral.

Question 2

What does metastable mean?

Answer 2

A system is defined as metastable if it is thermodynamically unstable but kinetically stable.

Question 3

What does thermodynamically unstable mean?

Answer 3

Its free energy is not the lowest possible for that system.

Question 4

What does kinetically stable mean?

Answer 4

The times that it takes the system to progress towards the energetic minimum are large compared to our experiment.

Question 5

What is silanol?

Answer 5

Si-OH group, usually found at the surface of silica.

Question 6

What is dynamic equilibrium?

Answer 6

When two reactions are balanced such that there is no net charge; the reaction rate is equal in both directions.

Question 7

What is plasma?

Answer 7

A state of matter, a partially ionised gas.

Question 8

What is surface charge?

Answer 8

The average electronic charge at the surface of a material.

Question 9

Equilibrium is dependent on which three factors?

Answer 9

• Temperature (increasing temperature will increase water evaporation effectively driving the equilibrium towards the right)
• Humidity (high humidity will in general move the equilibrium to the left as it will increase the concentration of water in the system)
• pH (presence of acids and bases will break -Si-O-Si bridges and thus drive the equilibrium to the left)

Question 10

Why are silanols so important for handling silica surfaces?

Answer 10

They affect the surface charge and reactivity, which, in turn, determine what chemistry you can do with silica.

Question 11

Is amorphous silica porous or non-porous?

Answer 11

Amorphous silica is porous due to its disordered lattice.

Question 12

How can the concentration of silanols on the surface of silica be increased?

Answer 12

It can be increased by exposure to oxygen plasma or air plasma, or by immersing the surface into a strong acid, possibly with oxidising characteristics (HNO3, or H2SO4 + H2O2).

Both methods break -Si-O-Si bonds, temporarily increasing temporarily the concentration of silanols above the equilibrium level.

Question 13

How can the concentration of silanols on the surface of silica be decreased?

Answer 13

It can be decreased to about 1 per 10nm^2 by heating the silica to about 800C; at such high temperatures the silanols will condense, releasing water, and forming new -Si-O-Si bridges.

Question 14

What is wettability?

Answer 14

How easily a solid surface can be covered by a liquid; related to the contact angle.

Question 15

What happens to silanols at pH 7?

Answer 15

At pH 7, silica surfaces are negatively charged; hence, the silanols are more deprotonated than protonated.

Question 16

How can the surface charge be identified on silica?

Answer 16

The surface charge can be identified by the contact angle of water on the surface.

Question 17

What does higher surface charge correspond to?

Answer 17

Higher surface charge will correspond to smaller contact angles, or higher wettabilities, as the water will try to maximise its interaction with the surface.

Question 18

What is the contact angle?

Answer 18

This is the angle formed between a liquid droplet and its substrate.

Question 19

What is the hydration layer?

Answer 19

A layer of chemisorbed or physiosorbed water on the surface of a substrate.

Question 20

Are silica surfaces hydrophilic or hydrophobic under normal conditions?

Answer 20

Under normal conditions,, silica surfaces are mildly hydrophilic an they usually retain around 10 nm of chemisorbed / physiosorbed water on them.

Question 21

Why does the hydration layer hardly evaporate at room temperature?

Answer 21

This hydration layer hardly evaporates at room temperature due to the strong electrostatic and hydrogen-bonding interactions it has with the surface: 300C are often necessary to temporarily remove such adsorbed water, and exposure to water vapor or liquid water will reinstate immediately such a hydration layer.

Question 22

What is the benefit of silanols reacting easily with chlorides (or alkoxides)?

Answer 22

To covalently attach any molecule to the surface of silica.

Question 23

What is surface functionalisation?

Answer 23

Changing the properties of a surface by changing its chemical composition, usually by adding or substituting a chemical moiety.

Question 24

What is etching?

Answer 24

Creating a pattern at the surface of a solid by locally dissolving or vaporising the material; usually using chemistry (e.g., acids) , or physics (e.g., radiation).

Question 25

What is HF?

Answer 25

Hydrofluoric acid; chemically mild, but extremely dangerous to organisms as it binds calcium strongly and interferes with metabolic processes.

Question 26

What particular aspect of silica allows it to selectively etch silica in the presence of many other materials?

Answer 26

Silica surfaces are susceptible to hydrofluoric acid (HF) and their resistance to all other common acids like sulfuric acid and hydrochloric acid.

Question 27

What are colloids?

Answer 27

Homogeneous mixtures formed from a continuous phase and a dispersed phase.

Question 28

What is polydispersity?

Answer 28

The amount of size inhomogeneity.

Question 29

What are the three ways in which particle size is controlled?

Answer 29

1. by controlling the surface chemistry or charge of the particles, to prevent them from aggregating into a single lump;
2. by controlling the initial supersaturation, which determines the number of nuclei that are formed at the beginning of the reaction: if the reaction proceeds to the same extent, having more nucei will yield smaller particles as the SiO2 is disturbed to a larger number of particles;
3. by supplementing such nuclei with enough reagent for them to grow to the desired size.

Question 30

How can silica colloids be made in solution?

Answer 30

Using the Stober process.

Question 31

What is sol-gel chemistry?

Answer 31

It is based on the hydrolysis and condensation reaction of suitable metal-organic precursors, such as alkoxides with the formula M(OR), where M is a metal with oxidation state n and R is an organic group (often ethyl).