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Colloidal interactions summary for Food physics (FPH20306)
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Complete summary of all the relevant information needed to understand colloidal interactions in this course.
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Chapter 4: Colloidal InteractionsHydrogen bonding
- Hydrogen atoms are bound to strongly electronegative atoms (N,O,F,Cl)
- NOT a covalent bond
- Much weaker than covalent bonds but stronger than most Van der Waal’s interactions
- Short-range interaction with and effective range less than 0.2nm
- Strength of the hydrogen bond depends strongly on the orientation of the molecules;
therefore, temperature dependent
o When the temperature increases the thermal motion will disturb the preferred
orientation thereby weakening the bond
- Can produce inter- and intramolecular bonds, allows it to form networks or 3D-structures
o Examples: α-helix and β-sheet
- Double helix formed by DNA is stabilized by hydrogen bonding
o Since strength of interaction is temperature dependent, formation and stability of
these structures is also temperature dependent
Hydrophobic interactions
- Closely related to hydrogen bonding
- Effective range around 2nm
- Temperature dependent
o Temperature increases, strength of the interaction INCREASES
- When a non-polar molecule is dispersed in a water phase the network formed by the water
dipoles will be locally disturbed
- The water dipoles surrounding the non-polar molecules cannot form hydrogen bonds with
the non-polar molecule, resulting in a loss of hydrogen bond energy
- To compensate, the water dipoles at the surface of the non-polar molecule reorient
themselves to restore the number of hydrogen bonds
- In doing so they form and cage structure around the non-polar molecule
- The size and shape of the cage are determined by the size of the non-polar molecule
- By forming the cage, the number of hydrogen bonds is restored, and the enthalpy of the
system is lowered
o Dipoles lose some translational and rotational entropy because they are in a state
with a higher degree of order
o Therefore, cage formation in entropically unfavourable
The Gibb’s free energy of the cage formation is given by;
Δ𝐺 = Δ𝐻 − 𝑇Δ𝑆
Change in enthalpy is negative, hydrogen bonds are being restored
Change in entropy is negative, cage has a higher degree of order
o For most non-polar particles; |Δ𝐻| ≤ |TΔ𝑆|
o Therefore; Δ𝐺 ≥ 0 (not spontaneous)
o Explains why non-polar molecules are so poorly soluble in water
, - When two non-polar particles approach each other to a short distance the system can
increase its entropy (less order) by removing parts of the cages between the 2 non-polar
particles, building a new cage containing both particles
- System can lower its Gibb’s free energy by this process
- This leads to an attractive interaction between the non-polar particles, which, when not
balanced by repulsive interactions will lead to aggregation
Electrostatic interactions on the colloidal scale
- Important in the stability of emulsions stabilized by proteins, or production of yoghurt
through acidification of milk
- Colloidal particles can be charged in many ways
o Dissociation/association of acid and base groups at the surface of the particles.
Degree of dissociation/association depends on the pH of the solution
o The charge and therefore the strength of the interaction is dependent on the pH
o Colloidal particles can also be charged by the adsorption of charged particles to the
surface
- When a colloidal particle is charged the charges at the particle surface will interact with the
electrolyte ions in the surrounding medium
o You cannot disperse charged particles in water. You need ions in solution.
- Counter-ions: charge of ions in medium are opposite to the particle charge (+ -)
- Co-ions: charge of ions in medium is the same as particle charge (++ or - - )
- Counter ions will be attracted by the particle and the co-ions will be repelled
- When the attractive interactions between the particle and the counter ions are sufficiently
strong and ordered layer of counter ions will form, bound to the particle surface.
o Stern or Helmholtz layer
- Outside this layer the interactions are weaker
- Balance between thermal motion and electrostatic interactions results in formation of a
diffuse electric double layer
Debye screening length
𝜀𝑟 𝜀𝑜 𝑘𝐵 𝑇
𝑘 −1 = √ [𝑚]
2𝑧 2 𝑒 2 𝑛𝑏
- The characteristic dimension for the thickness of the diffuse double layer
𝐶
The potential at the wall is related to the surface charge density 𝜎 [𝑚2] of the wall
- When the surface potential is sufficiently low ( <25 mV)
𝜎
𝜓𝑠 =
𝜀𝑟 𝜀0 𝜅
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