Small polymer particles are of interest for many reasons, ranging from their use as coatings, effect — Physical Chemistry — Thermodynamics Chemistry Question
Microemulsions
Small polymer particles are of interest for many reasons, ranging from their use as coatings, effective support for catalysts due to their large surface area, to more ”smart” applications such as biomedical carriers. Well-defined spherical polymer particles within the size range from 10 nm to 200 nm can be synthesized by the method of microemulsion polymerization: a microemulsion consists of small oil droplets having surfactant layers on their surfaces and being dispersed in water. The system is in thermodynamic equilibrium. By using a monomer as an oil phase polymerization takes place resulting in small polymer particles in the volume of the initial oil droplet. The size of the droplets is controlled by the ratio of surfactant to oil.
[VISUAL]
A: microemulsion droplet with liquid monomer inside
B: polymerized microemulsion: polymer particle covered with surfactant
You would like to synthesize small polystyrene spheres, using a mixture of styrene (vinylbenzene) and p-divinylbenzene (mass ratio 10:1) as a monomer and cetyltrimethylammoniumbromide as a surfactant. A hydrophobic starter is added so that a radical polymerization takes place within the droplets.
Density of monomer, polymer and surfactant: 1 g·cm-3
Length of surfactant b = 2 nm.
The surfactant is assumed to be a dense layer on the oil surface where hydrocarbon tails do not penetrate the oil phase.
What is the function of p-divinylbenzene?
Model Answer
It's a crosslinker. The resulting particle is a small spherical polymer network.
Calculate the mass ratio of surfactant to monomer you have to use in order to produce polymer particles with sizes of d = 20 nm, d = 40 nm and d = 120 nm (d = diameter of the particle without surfactant).
Model Answer
The geometric conditions can be described as follows:
When r is the radius of the total microemulsion droplet, b the length of the surfactant molecule and r – b the radius of the polymer particle, you obtain:
V(monomer) = 4/3 * pi * (r - b)^3
and V(surfac.) = 4/3 * pi * r^3 - 4/3 * pi * (r - b)^3
S = m(surfac.) / m(monomer) = V(surfac.) / V(monomer) = r^3 / (r - b)^3 - 1
to obtain particles with diameter d and r = 0.5 * d + 2 nm:
- d = 20 nm, r = 12 nm: S = 0.73
- d = 40 nm, r = 22 nm: S = 0.33
- d = 120 nm, r = 62 nm: S = 0.10
Calculate the total surface area of 1 g of polystyrene particles (after removal of the surfactant) for the three samples.
Model Answer
The surface of a spherical particle is:
A(particle) = A(monomer droplet) = 4 * pi * (r – b)^2
For 1 g of polystyrene, i. e. 1 cm^3 of polymer, the particle number is:
N = 1 cm^3 / V(particle) = 3 * 1 cm^3 / (4 * pi * (r - b)^3)
and the total surface of 1 g of polystyrene particles is:
A(1 g polystyrene) = N * A(monomer droplet) = 3 * 1 cm^3 / (r - b)
For the three samples:
- d = 20 nm: A(1 g of polystyrene) = 3.0×10^20 nm^2 = 300 m^2
- d = 40 nm: A(1 g of polystyrene) = 1.5×10^20 nm^2 = 150 m^2
- d = 120 nm: A(1 g of polystyrene) = 5.0×10^19 nm^2 = 50 m^2
Which kind of enzyme would you choose for this purpose?
true false
a) a hydrophilic enzyme
b) a hydrophobic enzyme
c) an amphiphilic enzyme with the active center in the hydrophilic part
d) an amphiphilic enzyme with the active center in the hydrophobic part
Model Answer
c) is true, the others are false.
An amphiphilic enzyme should be included into the interface of the microemulsion particle. With the active centre in the hydrophilic part, it will be located towards the hydrophilic, aqueous phase and the enzyme may remain active.