Lecture 6 Flashcards
What is silica made up of?
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. This flexibility is what makes silica an excellent glass former!
What is the main method to produce glass and how does that tie back to sillica?
The main method to produce glass is to quickly cool (qunech) the melt. If a materila is a good glass former, like silica, its atoms will not have time, during this rapid cooling, to assemble in the positions they would occupy in a crystals and will instead remain frozen in a metastable phase called glass.
A consequence of the excellent glass-forming characteristics of silica is that most of the silica you will see is in the amorphous (glassy) phase.
What is the reaction to form silanol groups on the surface of silica?
It is an equilibrium reaction that is depicted in Figure 2.1
Why are silanol important for handling silica surfaces?
They are so important because they affect the surface charge and reactivity, which, in turn, determine what chemistry you can do with silica
How can the concentration of silanol on the surface of silica be modified?
They can be modified in the following ways:
- it can be increased by exposure to oxygen plasma or air plasma, or by immersing the surface into a strong acid, possibly with oxidizing characteristics. Both methods break Si-O-Si bonds, increasing temporarily the concentration of the silanols above the equilibrium level. Within a few hours, if exposed to air, the surface will return to the equilibrium state.
- It can be decreased to about 1 per 10nm^s by heating the silica to about 800C; at such temperature the silanols will condense, releasing water, and forming new -Si-O-Si- bridges.
another point for temp is that when you increase the temp high enough water will evaporate driving the equilibrium to the right and decreasing silanols, and vice versa, if we have high humidity the concentration of water will increase causing the equilibrium to move to the left increasing the silanols!
what are the properties of Silanol at pH=7
Before I answer I want to make it clear that the pH of the environment strongly determines whether the silanol is protonated or deprotonated (thats why also acids increase the concentration of silanols!)
Okay in the case of pH 7 the silica surfaces are negatively charged, hence silanols are more DEprotonated than protonated. This makes silica generally hydrophilic (loves water) in normal conditions.
The hydration layer that forms will hardly evaporate at room temperature due to the strong electrostatic and hydrogen-bonding interaction it has with the surface: 300C is often necessary to temporarily remove such adsorbed water, and exposure to water vapor or liquid will reinstate immediately such hydration layer
How is the surface charges of Silanols identified?
The surface charges can be identified by the contact angle of water on the surface; the higher surface charge will correspond to smaller contact angles, or higher wettability, as the water will try to maximize its interaction with the surface.
What is the formal defination of wettability?
How easily a solid surface can be covered by a liquid; related to contact angle
How do silanols determine the surface reactivity?
Silanols can react very easily with chlorides (or alkoxides), as shown in Figure 2.1, and this can be used to covalently attach any molecule to the surface of silica. This allows us to be able to functionalize the surface of silica.
A nice aspect of this reaction is that it can be performed in solution (where the silica sample is immersed in a solution of the alkoxide or the chloride) or in the vapour phase (where the silica sample is placed into a low-pressure chamber containing the alkoxide or the chloride), whenever the precursor is sufficiently volatile.
Note that in Figure 2,1, R is an arbitrary group which can be an ALKY CHAIN, CARBOXYLIC ACID, AMINE for bioconjugation, OR ANYTHING ELSE, even Fullerences.
What interactions do hydroflouric acid (HF) and silia surfaces have?
Silica surfaces have a high susceptibility to hydrofluoric acid (HF) and resistance to all the other common acids like sulfuric acid and hydrochloric acid; Few other materials possess such selective reactivity.
This peculiarity makes it possible to selectively etch silica in the presence of many other materials or vice versa.
How does HF etching of silica happen?
The chemistry of HF etching of silica is shown in Figure 2.1:
HF attacks the Si-O bond by protonating the oxygen and coordinating the silicon atom with fluorine; this leads to the cleavage of the Si-O bond and the formation of a silanol and a Si-F bond, this process continues and, as HF is consumed, H2O and SiF4 (which is a gas at room temp and atmospheric pressure) are produced. The Si-F bond is very stable and the backwards reaction is thus strongly unfavourable.
Under aq condition conditions the F^- anion can also strongly coordinate SiF4 to yeild the stable hexaflourosilicate anion [SiF6]^-2
note that HF is extremely dangerous even though it is a mild acid.
When studying the size of silica, what discoveries are opened for us?
colliod and sol-gel chemsitry
What is the formal defination of colloids?
Colloids are generally defined as mixtures formed from a continuous phase and a dispersed phase whose colloidal stability (i.e., their tendency to remain homogeneously mixed) is determined by the surface energy and charge of both phases.
The term colloid in nanochemistry is most often used to indicate dispersions of solid particles in a liquid continuous phase.
What is polydispersity?
The amount of size inhomogeneity is often defined as the standard deviation of the size distribution (see Figure 2.2).
As a consequence, well defined and homogenous properties will only result if one can limit polydispersity.
(This is generally and can be applied to colloids as homogeneity increases as we limit the colloidal polydispersity)
What is the general sequecence for the production of Colliods?
A colloid is produced in solution by a sequence of controlled nucleation, growth, and precipitation reactions.
How is the partical size controlled.
The control of particle size is mostly exerted through the following three principles:
- By controlling the surface chemistry or charge of the particles, to prevent them from aggregating into a single lump.
- 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 nuclei will yield smaller particles as the SiO2 is distributed to a larger number of particles.
- By supplying such nuclei with enough regent for them to grow to the desired size.
How can we make silica colliods?
Silica colloids can be made in solution using the Stober process which is the main sol-gel method for producing silica collides
Firstly we will explain the chemistry behind sol-gel and then (in the next flash cars) define the Stober process:
Sol-Gel chemistry is based on hydrolysis and condensation reactions of suitable metal-organic precursors, such as alkoxides with formula M(OR)n where M is a metal with oxidation state n and R is an organic group (Often ethyl).
The sequence of hydrolysis and condensation reaction leads to the formation of M-O-M bonds (eventually leading to a network!) with concomitant release of water and alcohol ROH molecules, as shown in Figure 2.2. (dont get confused the metal is basically silica okay!!)
It is important to note that each step of a sol-gel reaction is differently sensitive to humidity, to the nature of the R group, and to Ph. The use of sol-gel chemistry often implies a study of these conditions and how to optimize them to get the reaction to proceed as desired!
What is the stober prrocess
TEOS is first dissolved into a water/ethanol mixture. TEOS reacts very slowly with water in the air at pH 7. The initiation of the reaction is thus performed by adding ammonia which, by increasing the pH, catalyses the process. Once ammonia is added the TEOS will begin to HYDROLYZE by the water, then subsequently CONDENSED by heat to form active nuclei silica centers.
after a few minutes of this mechanism, the dispersion is composed of three changing species: The nuclei that are being formed, the hydrolyzed TEOS which can add to the nuclei, and the not-yet-hydrolyzed TEOS, acting as a neutral bystander (as shown in Figure 2.2). The nuclei, due to the naturally high negative surface charge of silica, electrostatically repel each other maintaining their identity and minimizing aggregation.
With time, the hydrolyzed TEOS molecules add to the nuclei, while the non-hydrolyzed TEOS have time to hydrolyze. Given the amorphous nature of the product, the absence of crystalline facets and any other kind of atomic lattice anisotropy, there is no substantial difference between different spots on the surface of the colloids, so the growth happens on all sides at the same rate, isotropically, and thus yielding perfectly spherical particles.
By the end of the reaction, all of the TOES will have hydrolyzed and added to the particles; their final size will be determined by the nuclei initially formed, and by the amount of TEOS available after that. This principle is used in seeding
Along with Figure 2.2, there is a lecture slide shown to give a better picture.
THIS IS VERY IMPORTANT IT WILL COME IN THE EXAM
What is seeding
It is a process where small silica particles are synthesized separately via Stober synthesis and then used as seeds to grow larger particles.
The advantage of this approach is that you already know how many “nuclei” (seeds) you have in your reaction vessel. Since you can measure how much TEOS will react with them. It is worth mentioning that the seeds can be other nanoparticles like gold nanocrystals or iron oxide nanocrystals, and regrowth with TEOS creates core-shell Particles. Depicted in digital notes.
The sheath of silica can serve to protect the nanomaterial core and also facilitate the chemical functionalization of the silica surface.
What does the efficienct of the electrostatic repulsion between the silica nuclie depend on?
The efficiency of this electrostatic repulsion is depended on the ionic strenght of the continous phase; high ionic strenghts will cause the surface charges on the neighboring silica particles to be screened from each other, reducing the repulsion beyween particles and thus leading to more aggregation.
What is a standard test to see if dispersion in water is a colloid?
add salt to the water and see if it precipitates
What is a general annoyance when it comes to the synthesis of colliods and how can we deal with it?
A reccurring annoyance in the synthesis of silica colloids is the formation of aggregates, such as seen in Figure 2.2; such particles will disturb the self-assembly of said colloids since their size is much larger than average. To alleviate this problem these aggregates can be separated from the product solution by centrifugation, since they are larger and heavier so they will settle faster.
An alternative yet slower process than centrifugation is gravity driven sedimenation.
When do silica gels form pores?
You can understand the origin of this porosity if you relize that the condenstation reaction does not need to be completet for the partice to be a slod. A large number of silanols within the particle will not have had the change to condense with their neighbors thereby leaving empty spaces within the particle. The progressice reticulation of the silica matrix makes further reaction of silanols increasingly diffuclt due to thier diminshing freedon of movement. As the silanols are bound to an increasngly rigid matric they will find it increasingly difficult to move and find another silanol to react with. The properties of many materials will be affected by the degree of porosity, like density, mechanical strenght, dielectric constant, and so on.
How can we levitate the porosity formed in the silica gels?
In order to remove this porosity, so-gel oxides like amorphous silica are ‘calcined’, which means that they are heated to high temperatures in the air. This process leads to a shrinking of the solids as residual silanol groups further condense, according to the equation Si-OH +OH-Si —> Si-O-Si _H2O, causing the material to lose porosity, decrease its volume, and increase its density.
