I have summarized chapter 4 (colloidal interactions) of the reader Food Physics (). Together with the other summaries of the chapter that I have made, this will be a great preparation for the exam. I hope this helps you!
Attractive colloidal interactions:
1. Hydrogen bonding.
o It’s a dipole-dipole interaction. Its origin is a permanent dipole-
permanent dipole interaction.
o Strength: 10-40 kJ/mol.
o On a colloidal scale it’s quite a strong interaction, but
between 2 molecules it’s a weak interaction.
o Range: <0.2 nm.
o It’s temperature dependent, as the strength decreases when the
temperature decreases.
It explains protein denaturation, as hydrogen bonds can be found I the secondary and
tertiary structures of proteins.
It is responsible for helix structures in polysaccharides.
It is responsible for the double helix structure of DNA.
It is responsible for the triple helix in gelatin gels.
2. Hydrophobic interaction.
o H2O molecules in close proximity of a non-polar particle, cannot make as
many hydrogen bonds with neighboring molecules, as molecules in the bulk
can.
o Origin: apolar particles cannot form hydrogen bonds with water molecules.
o The fact that there can no hydrogen bonds be formed between apolar
particles and water molecules, is energetically unfavorable for those surrounding H 2O-
molecules.
o The water molecules are not in their ideal energetic state, so they
lose some translational and rotational freedom/entropy.
o The H2O-molecules will reorient themselves around the apolar particles a
dynamic ‘’cage’’ of water molecules is formed around the apolar particles
o When 2 apolar ‘’caged’’ particles come in close proximity, they can remove
some cage molecules and join together. This Is an attractive interaction.
The removal of some cage molecules increases the
entropy of the H2O-molecules. ∆ G = negative, so this happens
spontaneously.
3. Van der Waals interactions.
o Source: dipole-dipole interactions.
o All materials either have a permanent dipole moment or
they can have an induced dipole moment.
1. Sphere with radius R of species A, interacting with a wall
of species B.
VAB = (-AABR)/6D
AAB = Hamaker constant. It’s unique for each material combination.
Example: adsorption of proteins at the walls of a heat exchanger (fouling).
2. Two spheres of radius R1 and R2.
VAB = ((-AABR)/6D) * ((R1R2)/(R1 + R2)).
, Example: attraction between fat droplets in an emulsion.
Example: attraction between 2 spherical protein aggregates in gelling of protein solutions.
4. Depletion interaction.
o Smaller particles are excluded from the regions between the larger
particles.
o At some point, the larger particles are so close to each other, that the
smaller particles don’t fit between them. There’s now an osmotic
pressure difference between regions A and B.
o The system will try to compensate for this osmotic pressure difference
by extracting water from region A and transporting it to region B.
o This dilutes the region in B. As a result of this, the two larger particles
move towards each other attractive interaction.
o This interaction occurs between many types of particles.
The electrostatic repulsion finds its origin in charges on the surface of particles.
A particle can be charged by:
o Dissociation of surface groups.
o Absorption of, for example, the surface of charged proteins.
When a particle is charged, it’ll emit an electromagnetic field that starts to
interact with ions present in the solution. It’ll attract the oppositely charged
ions (counterions) and it’ll repel ions with the same charge (co-ions).
o This is an ordering effect.
There’s also thermal energy that randomizes ions around the particles.
o There’s competition between the ordering effect of the
electromagnetic field and the ordering effect by thermal
energy.
o As a result there’s a layer formed around the particle
with a diffusive nature = diffuse double layer.
- Diffuse double layer: a dynamic layer where ions are continuously
moving in and out.
o For an increasing r (greater distance between particle), both curves will reach
the bulk concentration of ions, nb (equals counter + co).
In the picture on the right you can see a positively charged wall and
a profile of counterions as a function of the distance to particle (x).
o This profile creates a potential field: Ψ (x).
If you want to measure the extent of the potential field, you choose
a certain point in the field (K-1). This is the Debye length.
- Debye length: point where the potential has decreased to the wall
potential, Ψ (s), divided by the exponential number e. Ψ =Ψ s /e.
K B T ε0 ε R
−1
K =
√ 2 2
2 z e nb
eo: dielectric constant of vacuum.
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