CE70006 - Colloids and Interface Science Flashcards
What is a colloid?
A material which is an intermediate between a single molecule and large objects that are dominated by gravitational forces.
A dispersed material in which one of its dimensions is in the range 1-1000 nm.
Thus we can have 1,2 and 3 dimensional colloids.
“A colloid is a mixture in which one substance consisting of microscopically dispersed insoluble particles is suspended evenly throughout another substance.”
Give examples of 1, 2, and 3 dimensional colloids
1D: plates e.g. clays
2D: Needles e.g. asbestos
3D: Particles e.g. titania or latex
What is the name for the following dispersed phase - dispersion media systems?
- Gas-Liquid
- Liquid-Liquid
- Solid-Liquid
- Gas-Solid
- Liquid-Solid
- Solid-Solid
- Liquid-Gas
- Solid-Gas
- Foam
- Emulsion
- Dispersion
- Solid foam
- Solid emulsion (ice cream)
- Solid dispersion
- Aerosol
- Solid aerosol
How are colloids prepared?
- By the breaking up of large lumps of material via grinding, milling, etc.
This will give an average particle size of no smaller than ~1um. - By the aggregation of small molecules. This is hard to achieve and costly to control, but can obtain better products.
No real limit on particle size.
Do colloids have high or low surface areas?
Very high - it is the property of their surfaces that determines their properties.
Total particle volume represented by Φ.
What is the Van der Waal equation?
(P + an^2/V^2)*(V-nb) = RT
Which considers non-deal gas behaviour
P = pressure
R = universal gas constant
T = absolute temperature
V = molar volume
b = gas constant
a = gas constant
V = molar volume
n = moles
b is subtracted from V to account for the finite size of the molecules.
a/V^2 was added to pressure to account for the attractive intermolecular forces.
What are Van der Waals forces?
Distance-dependent interactions between atoms or molecules.
The force results from a transient shift in electron density.
Describe the following Van der Waals ineractions:
Keesom
Debye
London
Keesom: permant-permanent dipole interaction
Debye: permanent-induced dipole interactions
London: fluctuation-induced dipole (or dispersion) interaction [always present]
Van der Waals forces are long range (0.2 - 10 nm)
What are the (4) main features of dispersion forces?
- They are long ranged (acting over several atomic diameters)
- Net forces may be attractive or repulsive, but only repulsive between different materials in a medium
- Dispersion forces can align molecules
- Dispersion interaction of two bodies is affected by the presence of other bodies nearby
What is the equation to consider the dispersion interactions between atoms?
w(r) = -3a^2I/(4(4piε0)^2r^6)
Where:
a - atomic polarisability
I - ionisation potential
ε0 - permittivity of free space
r - distance between the two atoms
How do Van der Waals interactions behave with macroscopic bodies?
The interactions between molecules is of the form w(r) = -C/r^n.
Here, ‘C’ is a constant and ‘n’ is a parameter determining the nature of the interaction.
Assuming additivity, the interaction of a molecule with a surface is the sum of its interactions with all molecules in the body.
W(D) = - πCρ / (6*D^3)
The equation W(D) = - πCρ / (6*D^3) calculates the interaction energy (W) between a molecule and a surface, where ‘D’ represents the distance from the surface, ‘C’ is a constant from the molecule-surface interaction equation, and ‘ρ’ is the number density of molecules in the body.
This equation assumes that the interaction between a molecule and a surface is the sum of all interactions between that molecule and all the molecules within the body, following an additive principle.
If the thermal (k*T) and Van der Waals energy are the same, what does this mean?
It would be the boiling or condensation point.
Van der Waals forces start to dominate over thermal energy, leading to a change in the physical state of matter.
What is the Van der Waals interaction equation between:
1. A sphere and a plate
2. Two equally sized spheres
3. Parallel surfaces
- W(D) = -AR/(6D)
- W(D) = -AR/(12D)
- W(D) = -A/(12D^2)
What is the problem with the Hamaker approach?
It is not suitable when atoms are further away (e.g. 100 nm)
An alternative approach is the Lifshitz theory, which treats matter as a continuum. It is considered to be more accurate but in effect it is an alternative way to calculate the Hamaker constant. The basic relation remains the same.
What happens if the Hamaker constant is negative?
The Van der Waals force will be positive, i.e. repulsive
What is the affect of solvent on the Hamaker constant, A?
A.effective = A.final - A.initial
Meaning the effective Hamaker constant of a system following separation of a solvent is equal to the difference in the system A before and after separation.
Positive A.eff leads to attractive forces
Negative A.eff leads to repulsive forces
What is the Hamaker constant of a vacuum?
0
Air is also 0
What are the 3 sources of a charged surface?
- Dissolution of soluble ions, mainly metal ions, e.g. in clays
- Adsorption of ions, generally anions, to surfaces e.g. iodide in silver iodide
- Adsorption of surfactants or polyelectrolyte
With a charged surface, for electrical neutrality to be met, there must be ions of opposite charge (counter ions) present somewhere in the solution.
From an enthalpic view, it is favourable for the ions to be as close to the charged surface as possible.
From an entropic view, it is favourable for the ions to be as far as possible.
In reality, something in between forms - an electrical double layer.
What is the Hamaker constant?
The Hamaker constant is a coefficient accounting for the van der Waals interaction between two materials.
It has a strong correlation with various physical phenomena, such as liquid wettability, adhesion, friction, adsorption, colloidal stability, polymer flow, and deformation.
How does the decay of interactions vary between atoms and macroscopic bodies?
w(r) ∝ 1/r^6 for atoms
w(D) ∝ 1/D for macroscopic bodies
So the interaction with 1/D decays much slower than the 1/r^6 dependence of the intermolecular pair potential.
What are the units of the Hamaker constant, A?
Joules
How does Debye length vary with concentration?
As concentration increases, Debye length decreases - rapidly at first and then very gradually (on a nm scale).
3-3 electrolytes (i.e. substances with a 3+ 3- charge like AlCl) start with a shorter Debye length, k, than a 1-1 electrolyte like NaCl.
What are the two origins of repulsive potential between 2 charged surfaces?
- Electrostatic interaction from the emanating electric field, exerting a force, Fel, on the ions.
- Osmotic pressure difference between the double layer and the bulk (where the conc. of ions close to a surface is much larger than that of the bulk).
What is zeta potential?
Zeta potential (ζ) is the electric potential at the shear plane of a particle suspended in a liquid. It is measured in volts (V).
The zeta potential arises due to the presence of charges on the surface of colloidal particles or other solid surfaces in a liquid. These charges may come from dissociated ions, adsorbed ions, or other surface functional groups.
Zeta potential provides information about the stability of colloidal dispersions. Particles with higher zeta potentials tend to repel each other more strongly, leading to increased stability. On the other hand, lower zeta potentials may result in particle aggregation or flocculation.