Part One
Questions
(1) Colloidal stability
Explain what is meant by colloidal stability?
Why do colloids stick together?
How can charge be used to prevent aggregation?
How do the conditions effect stability?
(2) Explain what is meant by the term ‘Form factor’, P(Q), and ‘Structure Factor’, S(Q), in SANS. Why is the sky blue and what do 'rings' round the moon tell us about the water in atmospheric clouds.
(3)Why did ancient Egyptians add egg proteins to soot to make an ink, to be dispersed in water?
(4) What is the CMC? What factors effect the CMC, both in terms of molecular architechure and conditions?
(5) What is meant by the term ‘colloid’ and what examples can you give?.
(6) Why add aluminium to clean water at the sewage works?
Show that this data follows the Schultz-Hardy rule.
(7) Deduce an equation for the sedimentation velocity of colloidal particles
What is the sedimentation velocity of 1 mm radius polystyrene particles in water?
(8) Show that j= const . exp(-kx) is a solution of the Possion-Boltzmann equation in the Debye-Huckel limit. Eg. d2j/dx2 = k2j.
What is 1/k for 0.1 M NaCl solution? Is this likely to be effective for stabilizing a colloidal dispersion?
(9) Show that V(r) between a single molecule and a flat plate is proportional to 1/H3 where H is the separation between the molecule and the surface.
Where V(R) goes as 1/R6.
Hint: Find an expression relating H, X, R and r and hence find V(R) in terms of V(H, x,r). You may find R6 = (R2)3 useful.
This is the interaction between atoms. To find the total interaction between an atom and the whole surface you will need to integrate this potential. This is the integration shown above.
First let u=(H+x)2+r2), then let w=(H+x)
Past papers
2001 paper 2 38(b)
2001 15(b)
2000 (2) 24(b)