colloid practicequestions

Question 1

Describe how consideration of the pairwise interaction energies in a liquid can
explain the phenomenon of surface tension

Answer 1

• surface tension occurs due to the surface particles lacking neighbours o n the air side so the bulk pulls the surface particles closer to itself creating surface tension. increasing thr pairwise interaction enrgy which is the enrgy that neighbouring particles experience, the surface particles would be pulled in lcoser to the bulk therefore increasing the surface tension.

this is seen in the equation: ST = Zb Nb W.bb

Question 2

What happens to the surface tension of a surfactant solution as the
concentration of surfactant increases? Explain why these changes occur.

Answer 2

• surfactant is added to the surface and reduces surface tensiondue to disruption strong interactions such as hydrogen bonding!
• further increasing conc of sf continues to decrease the surface tension.
• the cmc is then reached where the surface is saturated with surfactant and the sf starts to form micelles in the bulk.

once micelles form,, the st doesnt decreease asny more!! as sf forms micelles and not adsorb onto surface

Question 3

What are the different thermodynamic contributions to surfactant micellisation?

Answer 3

gibbs is negative aka the reaction is spontaneous.

enthalpy is usually small but positive as ur not making or breaking bonds,,its all about intermolecular interactions.

entropy is negative due to the freeing of water which has to surround the individual surfactant molecules and once micelles form these are released into the bulk increasing its entropy!!! also the hydrophobic tails are able to freely move due to interacting less with water when in micelle form….

Question 4

Explain how the closed association model and the phase seperation models
treat a surfactant solution from two different thermodynamic perspectives. What
common equation is obtained for the free energy of micellisation?

Answer 4

closed association states that there is an equailirbium between micelles and starting surfactant monomer solution!! normally given by nK1 <–> Kn
as there cant be an equilibrium constant for each sf turnig into a micelle.

the phase separation approach talks about how they (micelles and surfactant monomer solution) are separate phases and the micelles only form once the cmc has been reached.

they both say that G =RT ln(k) //cmc

Question 5

Explain why the surface tension of the solution is around
40 mN m-1 whereas the surface tension of pure water is
approximately 72 mN m-1.

Answer 5

surfactant adsorbs onto the interface an changes its composition.

this reduces the h bonding of the pure water surface and adds vdw character,, which are lower in energy than hydrogen bonds therefore reducing the strength of surface tension.

u need to break imf in order to make surface.

Question 6

when were given cmc what do we need to make sure we do

Answer 6

we need to make sure we find the mole fraction

turn mM into M by /1000.

then do y/y+55.5

Question 7

describe what a microemulsion looks like

Answer 7

see through
transparent bc particles are too small to reflect light!!!