Readings Flashcards
What is the wavelength equation for a particle in a well?
Lambda = 2*L/n ; n=1,2,3…
Amplitude = 0 at walls
How do position and momentum relate on the quantum level?
The more precisely something’s position is determined, the less precisely its momentum is known, and vice versa.
What is a Quantum Dot?
- Semiconducting
- Confined in all three dimensions
- 1 to 10 nanometer scale
What is a Quantum Wire?
- Semiconducting
- Confined in two dimensions
- 1 to 10 nanometer scale
What is a Quantum Well?
- Semiconducting
- Confined in one dimension
- 1 to 10 nanometer scale
Explain how an Exciton is formed
An electron is promoted from the valence band to the conduction band
- This electron leaves a “hole” behind in the valence band
- “hole” is thought of as a particle with a +1 charge and some mass
The promoted electron and the “hole” are bound together and considered a quasiparticle - Exciton!
Periodic Trend of Bandgap Energy of a solid (Eg)
As you go down the periodic table, the bandgap energy of the solid atom decreases
Techniques used to Measure Quantum Dot Size
- Transmission Electron Microscopy (TEM)
- Line-broadening of X-ray Diffraction lines of Quantum Dot powders
- Electronic Absorption Spectroscopy
What are the two ways that Quantum Dots can be made?
- Colloidal solutions
- Grown on solid substrates
Creaming
- When the density of the solvent is > the density of the particle
- The particle will move upward in solution
What is the friction factor of a spherical particle?
f = 6pimu*R
What is the Boundary Zone?
- The area between sedimented particles and space above which contains no particles.
- The C at the base»_space;» C in the upper zone
How does an Ultracentrifuge work?
- Cell is rotated at very high speeds in a horizontal position
- g is increased by a factor of ~10^5
- Used for characterization of colloidal materials
Explain the difference between Analytical and Preparative ultracentrifuge techniques.
Analytical:
- Requires small samples (.1-1mL)
- Optical systems are used to measure the Concentration or change in Concentration
Preparative:
- Requires larger samples (5-100mL)
- Samples removed after time
- fractionation for preparation or purification of samples
Surface Active Agent
Known as:
- Surfactant
- Lipids (fatty acids)
- amphiphiles
Compounds that reduce surface tension of water in aqueous solutions >30mN/m
Surfactants reduce surface tension in in non-aqueous media, just not as significantly
Surface Active Agent Composition
Head group - hydrophilic portion of compound
Tail - hydrophobic portion of compound
Head group and tail are segregated from each other
Detergent
A synthetic surfactant such as a fatty sulfate, sulfonate, or long chain quaternary ammonium salts
Anionic Surfactants
- Ionized salts
- The anion possesses the long hydrophobic chain
- The hydrophilic head groups are exposed to the water, increasing the hydrophilicity of the solid surface.
-Tend to shun dense absorption onto solid surfaces from water because these surfaces are often negatively charged
Hydrophobic Bonding
The non-specific desire of the hydrophobic portions of molecules to “get out” of the water
Cationic Surfactants
- Ionized salts
- The cation (+) is the surfactant species
- Good bacterial properties
- Head-down, tail-out absorption
- Most are nitrogen-containing compounds
Anionic Surfactant Examples
Sodium Stearate
Sodium Dodecyl Sulfate (SDS)
Sodium Dodecyl Benzene Sulfonate
Cationic Surfactant Examples/Uses
- Waterproofers and anti-stats
- Quaternary ammonium compounds (synthesized through the reaction of ammonia with fatty alcohols)
Nonionic Surfactants
- Various alkyl poly(ethylene oxide)’s
- Derived from the condensation of ethylene oxide with fatty acids or alcohols
- CnEm (n and m are subscripts)
- As the m/n ratio increases, the surfactants become more hydrophilic
- resistant to hard water
- generally low foamers
- may be soluble in organic solvents
- polydisperse
Nonionic Surfactant Examples/Uses
- Formulating injet printing inks
- Emulsifiers
- Wetting agents and dispersants
Amphoteric Surfactants
- Anionic or Cationic (pH dependent)
- Proteins and natural surfactants
Zwitterionic Surfactants
-ionic head group contains (+) and (-) charges in close proximity
Gemini Surfactants
Two single-tail surfanctants whose heads are connected by a hydrophilic or hydrophobic spacer chain
Telechelic Surfactants
Two hydrophobic groups are connected by a hydrophilic chain
Boloform Surfactants
Hydrophobic chains with hydrophilic groups on each end
function-changing Surfactants
Amphoteric or nonionic surfactants with PEO hydrophilic head groups
-Surfactant characteristics can change with changes in pH or Temperature
Cleavable Surfactants
Intentionally designed with weak linkage
-susceptible to cleavage under low/high pH
Cleavability is motivated in part to:
- biodegradability
- aid to removal of surfactants from a system
Gibbs Monolayer
When the surface of the solution is saturated with vertically oriented molecules
-The slope will be a constant
Traube’s Rule
The longer the hydrocarbon chain length, the more efficient the surfactant
-valid for nonionic and some ionic surfactants
