Weeks 5,6 CHEM 4200
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This 6 page Class Notes was uploaded by Juliana Herran on Monday October 3, 2016. The Class Notes belongs to CHEM 4200 at University of Northern Iowa taught by Dr. Weeks in Fall 2016. Since its upload, it has received 20 views. For similar materials see Into Nano Fall 2016 in Chemistry and Biochemistry at University of Northern Iowa.
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Date Created: 10/03/16
Optical Size Analysis - Rayleigh Scattering o Elastic scattering of light from particles: No transfer of energy (wavelength unchanged). → In elastic collisions you can have energy transferred but momentum is conserved. o Particle size much less than wavelength of light → For larger particles size is approximately λ ~ Phenomenon is more complicated for smaller particles (different equations). n= c o Depends on index of refraction, V where c is the speed of light in vacuum and V is the speed of light in medium; n is the measure of how much light slows down in medium (depends with how much light interacts with stuff in the medium). - Rayleigh Scattering Equations 2 4 2 2 6 I=I 1+cos θ 2( ) (n −1 ) ( ) o 0 2R λ n +1 2 …Scattering of non-polarized incident light (with polarized equation it gets more complicated). o Scattering is proportional to d^6, increases greatly as particle size increases (d is much smaller than λ). o There is less scattering through larger angles. o Scattering is proportional to 1/λ^4, l=the longer the wavelength the less the scattering. o n ratio of particle relative to surroundings. If the particle and surroundings have the same refractive index, there is no scattering. ni n= n . 0 - Light is an electromagnetic field (magnetic is weak compared to electrc) o Light interacts with stuff through polarization. Rayleigh Scattering → Polarizability. Smaller n = “soft and easier to polarize” ...small n = 1 [1≤n]. o Electric field is much stronger than magnetic by approx c. o Polarizability of the medium determines how much interaction happens. - Molecules can be polarized as well 4 2 I=I 8π α (1+cos θ) o Scattering from molecules equation: 0 λ R 2 . o Same λ^-4 dependence. o α is the polarizability of the molecule, “volume or plain polarizability” units are c m J .1 α 3 o Volume polarizability= m where E is the permittivity of a 4πε 0 0 vacuum. - As there are more particles, there is more scattering as well. In the less direct path there is more reflection → scatter …in a more direct path there is less reflection to get to your eye. o Rayleigh scattering is what gives the atmosphere its blue color. Polar → permanent dipole. o Polarizable → Possible to affect motion of electrons. - When revisiting the polarizing filters to take images of the sky it is possible to see that when the polarizing filter is off, there is more scattering as every possible angle of electric and magnetic field lines can go through. o When the polarizing filter is on, only specific orientations can go through. - Light propagation is on x axis, α of magnetic fields are perpendicular to the direction of light propagation. o Electric field causes electrons in molecules to oscillate. This oscillation is in the xy plane. o This oscillation has both a y and a z component (which makes molecular antennas). o An antenna does not radiate along the direction of its own length. So, the observer who is directly below, on the y-axis sees only scattered light that has an oscillation aligned with the z-axis. Hence, the light is polarized. - Nanoscale scattering o Difficult to use quantitatively since often you do not know the refractive index of the material. Scattering from molecules and nanoparticles. o Can be used for detection –Nanoparticles/Nanopores ~ Blue tinge… shorter avelength gets scattered more (blue glass) and the light behind is approx. orange. o Holes/particles show a difference in refractive index. - Nanoparticle size analysis o Dynamic light scattering measures scattered light from suspended particles. o Determines size distribution of nanoparticles in suspension. o Dynamic: Particles in motion. o Was originally done by detecting the shift in frequency when monochromatic light is noted by moving particles. - Brownian motion o Dynamic light scattering depends on Brownian motion. o It was a Scottish botanist studying pollen that discovered this phenomenon. Pollen suspended in H O m2ving. Plus, dead pollen also moved the same waybut fine inorganic particles moved too (not due to being alive). o Brownian motion is random motion but there is a net motion. o The faster the movement, the further the particles get (this is achieved through higher temperatures, less viscosity and smaller particles). o Examine particle size (with tracks). o Think of elastic collisions → Momentum given to particle by collisiokn with molecules. Given momentum transfer p = mv. o Smaller m is bigger v induced into particle by collision. o Smaller m is smaller size: Smaller things move more. o Random collision of particles with fluid molecules causes Brownian motion. o Viscosity: measure of interaction – More viscosity, more drag on motion. - Motion: Parameters o Random motion but speed dependent on: Temperature, viscosity, particle size. o Experiment, control temperature, viscosity a known quantity from solvent, motion depends on particle mass (size). - Motion: Diffusion o Motion defined by collisions with medium, not with walls/boundaries is diffusion. Diffusion = Random distribution of particles in a medium. o Particle siz can influence how fast they are moving. Speed is a good measure of size in tiny molecules. If we know how fast they are moving, we can measure the size. o Ballistic is motion defined by collisions with boundaries (as seen in solid objects). o Tiny objects hit terminal velocity very quick (ration max velocity of a particle/Force applied to it). D=μk TB o Diffusion constant is what defines diffusive motion, where μ=mobility . o Larger D → Faster diffusive motion. D controls rate of diffusion. μ= V D o Mobility: F , terminal velocity/applied force (by collisions). o Navier-Stokes equations apply principle of conservation of momentum to motion in fluids…” generally unsolvable”? ∂v p +v∙∇v =−∇ p+∇∙T+ f , (∂t ) o For particle in a fluid of much smaller molecules diffusion can be defined by the Stokes-Einstein equation: k T D= B 6π ηr η=viscosity r=particleradius - Dynamic light Scattering: Setup o Motion induced random variations in intensity. o Intensity fluctuations if laser output is fluctuated. - Dynamic Light Scattering: Autocorrelation o Everything is random but there is a trend. o Intensity signal auto correlated with time. o In the auto correlation plot because particles are moving, it is possible that as time increases, particles have moved. Thus, they give different scattering. o The auto correlation tells how quickly particles are diffusing. o Examines ∆t at which same intensity occurs. Same intensity, same particle size and orientation. Then ∆t gives idea of how particles diffuse. - Autocorrelation Plot and the Diffusion Constant o Decay constant proportional to diffusion constant (particles in the fluid). - Dynamic light scattering o Laser = Very narrow wavelength distribution. Lambda is specific. o Measure of the distribution of particle sizes to “take small particles, put them in fluid (laser cannot go through. Do not dissolve particles, put in the instrument). o Getting sample ready and determine particle size. o Important to characterize particles. Magnetism at the Nanoscale - What is magnetism? o A magnetic field is produced by moving charged particles. - Magnetic moment o μ= τ max Β τ=max torque producedwhenacurrentcarryingloopis placed∈amagnetic field ,Β Β=Vector o Imagine electrons orbiting about in a material. o If it forms a circular loop – would make a magnetic field (B). o The ability to make such a field, called the moment, proportional to size of loop, proportional to the current in the loop. o Current based on number of electrons and speed. - Magnetic moment = Dipole o Field produced by current loop…there are no isolated north poles and south poles. - Ferromagnets o Typical permanent magnets. o Fe, Co, Ni, Gd o Electrons move in Ferromagnets. - Magnetism at the atomic scale e o μ=( 2m )L o e is electron charge, L is orbital angular momentum of the electron and m is the mass of an electron. o μL is the magnetic moment due to orbital angular motion of electrons. o μs is the spin magnetic moment quantum mechanical effect. o The key thing to ferromagnets is the exchange between the atomic magnetic moments so that they align and point in the same direction. This is a quantum mechanical process. - Magnetic domains o Single units. Some are completely lined up, are not composites. o First heat sampe up, then apply magnetic field and ultimately cool in field through magnetic transition. o A domain is an area within ferromagnetic material. o Within a single domain all atomic magnetic moments are aligned. -Ferromagnets have an “easy” axis of magnetization. It depends on crystal structure. o Must polarize to have net magnetization. – Heath to randomize (non- ferromagnetic), -apply magnetic field (strong), - Cool in field through magnetic transition. - Paramagnetism (State of higher entropy – Unpaired e-) o Ferromagnetism is an ordered state. Like a solid as compare to a liquid. o At high temperatures – ordered state melts. o Ferromagnets typically become paramagnetic → Random orientation of atomic magnetic moments. o Some metals are always paramagnetic. This is a much weaker interaction than ferromagnets. o Magnetic moments in material align with applied external magnetic fields. - Antiferromagnetism o Exchange, lining up to atoms magnetic moment. o No net magnetization. o Local magnetic ordering. – Opposite of ferromagnetism – No net magnetization in material. o Strong local interaction. - Ferrimagnetism o Spins aligned in opposite directions, but magnetic moments of different magnitude. o There is a net magnetization different than zero in the material. - Diamagnetism o Diamagnets, local current loops oppose magnetic field. o Exists in all materials. o Weaker than Paramagnetism. – response enhanced by larger fields. o Induced repulsive interaction. o Levitation tricks. o Magnetic field gradient (Levitates about the material). o There are no unpaired electrons. o Induced current set up when a material with charged particles is placed in an external magnetic field. o Direction of current loop is created so that it opposes the field that caused it. o All materials exhibit demagnetization (weak effect). - Magnetism at Nanoscale o Superparamagnetism→ Occurs in magnetic nanoparticles. Particle size is less than magnetic domain size. o Entire particle is a single uniform domain. o In the macro world, each domain always polarized, but different domains cancel out (No net magnetization). - Supermagnets line up with the field fast and strongly. o Ensemble of ferromagnetic nanoparticles that respond to applied magnetic field. Each will physically rotate to align with field. o Ferromagnetic nanoparticles have much greater response – ferromagnetic nanoparticles act like paramagnet, but much stronger. o Particles randomize without magnetic field and line up very fast. - Magnetization of paramagnet vs. ferromagnet. The material follows a non- linear magnetization curve when magnetized from a zero field value. - Randomize direction they are pointing everywhere. - Aggregation o Magnetic nanoparticles in suspension -Van der Waals interactions attractive to large extent. – Magnetic interactions can be very attractive. o Solution: Add surfactant/ Ionic double layer of particles/ Steric effects are too big. Prevents particles from getting together and aggregating/ Ionic repulsion/Works well in aqueous solution. - Applications o Ferrofluid: Liquid containing suspended magnetic nanoparticles. – Strong response to magnetic fields. – Ultra fast switching times. o Can control motion and position of fluid with magnetic field. o Fishing rod reel (NASA). o Versatile and chemically inert materials (to avoid contamination). o Friction reduction (Fluids moving over solid objects. If moving object is a strong magnet, can coat with ferrofluid). o Vacuum seal on a magnetically permeable shaft mounted on the pole pieces of permanent magnet (Holds ferrofluid in position/ Solvents can be organic or aqueous). o Optics: Since each nanoparticle reflects light and shape can be controlled via applied magnetic field. o Astronomy / scopes. o Atmospheric interference: Dynamic correction / Can change orientation. o High contrast agents for MRI (Magnetic resonance imaging). o Cancer treatments (ongoing research). - In magnetite the structure is made of 2 interlocking sublattices [A(Fe ) fits in a tetrahedral hole]. o Ferrimagnetic material, antiparallel but different sides magnetic moment so they do not completely cancel out.
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