Notes Flashcards
Material Dispersions
Multiple constituent (phase) system in which one phase is contained within a continuous phase.
Short Range Forces
- Electromagnetic by nature
- Vander Waals forces
- ions<1nm (nanoscale)
Long Range Forces
Electrical and Magnetic
1nm-10um in size (microscale)
Gravitational Forces
10um-mm) Macroscale
Colloids
A state of subdivision with a dispersed particle (phase) dispersed with at least 1 direction of a dimension (1nm-1um)
Lyophilic
- Solvent-loving
- Colloids: behave like a single phase solution, which can contain macro-molecules (polymers and proteins) and association colloids (self assembled structures like micelles)
- Causes no true interface - solvent particles all around molecules
- THERMO and KINETICALLY stable
Lyophobic
- solvent-fearing
- Colloid particle (discontinuous phase) well defined by interface
THERMODYNAMICALLY stable, not kinetically
Protected Lyophobic
Stabilized with surface chemistry to increase kinetic stability
Forming Colloid Particles (Top Down)
-Breaking down bulk materials
Comminution
- wet and dry
- Mechanical disintegration
Forming Colloid Particles (Bottom Up)
- Start with solution or gas mixture with rxn precursors
- Induced to nucleate through reaction and precipitate or be stabilized to form our desired particles
- Process
1) Precursors –> controlled precipitation: supersaturated solution in which particle size grows –> insoluble salt
2) Chemical rxn that produces particles
Electrostatic Stabilization
Existence of a layer of charge on particle surface surrounded by a diffuse “cloud” of oppositely charged counterions
*Repulsion of these like charges causes stabilization
Steric Effects
Particles coated with stabilizing or capping agent (surfactant or ligand) causes repulsion of neighboring particles
Ostwald Ripening
Inhomogeneous phases leading to precipitation of larger particles, coming from smaller particles, due to energy factors
-Surface atoms desire proper coordination to minimize cohesive energy
Limiting Ostwald Ripening
1) Arrested Precipitation
2) Stabilizing Agent
6 Prevalent Strategies to Synthesize Colloids
1) Controlled Precipitation
2) Reduction of Metal Complexes
3) Sol-gel Process
- Hydrolysis of a metal salt
- Condensation/evaporation to remove the water
4) Emulsion and Dispersion Polymerization (to form colloidal polymer particles)
5) Template Controlled Growth
Ex: Colloidal Semiconductors (Quantum Dots)
-Formed through arrested precipitation
Three “Rules” of Quantum Confinement
1) Motion of electron inside the well increases in energy in 1 dimension
2) Electrons behave like waves - “particle-wave duality”
3) Standing waves will have an amplitude of 0 at the walls
Size vs. Energy for Nanoparticles
Smaller particles have high energy!
Aggregation
“small” particles clump together, do NOT fuse into a new particle
*No significant surface reduction
Coalescence
-2 or more particles fuse together to form a single larger particle
- Reduced Surface Area
- Ostwald Ripening takes place
- Solid particles: Sintering
Flocculation
- Chains of particles that aggregate together
- A “flock” of particles
Coagulation
Compact aggregate, possibly at the bottom of solution
Aggregate Reversal Examples
- Dispersion
- Deflocculation
- Peptization
Creaming
Concentration of particles move upward in a dispersion to the surface (based on density)
Sedimentation
- Settling of dispersed particles under the action of gravity or a centrifugal field
- Should be done at constant Temperature
RCF
Relative Centrifugal Force
Ultracentrifuge (particles they are useful for)
nanoparticles and lyophilic colloids
Sedimentation Coefficient (Units, Definition)
- Seconds
- Sedimentation velocity per unit of centrifugal acceleration
Diffusion
Tendency to keep things dispersed
Surface Energy
Atoms on the surface are at a lower energy state
- They have stronger lateral bonds, causing surface tension
- Surface energy refers to solids, surface tension refers to liquids
- Good indicator of wettability by liquids
- High SE –> more wettable
- Polymers have low SE, while metal solids and clean glass have high SE
Work of Cohesion
Work required to pull apart a material by overcoming attraction between atoms
γ (lowercase gamma)
- Measure of resistance of a surface to increase its area
- Force operating perpendicular and inward from a surface boundary
Contact Angle
Angle measured in the liquid that is formed at the junction of the 3 phases
Goniometer
- Used to measure the contact angle when a liquid droplet is placed on a solid surface
- A digital camera will record the contact angle with respect to different droplets
Variation in γLV due to what intermolecular forces?
Metallic Bonding
Hydrogen Bonding
Polar Molecules “Keesom”
London Dispersion Forces
γLV = Σγforces
As T is increased, γLV is decreased due to less interaction
Capillary Effect
Rise or fall of liquid in a small diameter tube inserted in liquid
Micelle
Self-assembled ordered structure
CMC
- Critical Micelle Concentration
- Gibb’s Monolayer is formed, number density of molecules at the surface will increase with concentration until the CMC is hit
Surface Excess (Γ)
Difference in solute concentration between the bulk and surface
Aggregation Number (n)
- Number of molecules to form a micelle
- Influenced by headgroup repulsion and hydrophobic tail attraction
Krafft Point
Sharp increase in solubility of surfactant
-The longer the hydrophobic taill, the higher the Krafft Point temperature
