Part One
Questions
1)Starting from the master equation for thermodynamics derive the Gibbs Adsorption Isotherms.
dg = -SGidmi
Discuss the strengths and limitations of the Gibbs theory of interfaces.
2)For solutions of n-butanol in water at 298K the following values of the surface tension have been obtained as a function of activity.
|
Activity a /10-3 mol dm3 |
3.28 |
6.54 |
13.04 |
25.8 |
51.8 |
98.9 |
192.8 |
379.6 |
711.9 |
|
g /mNm-1 |
72.80 |
72.26 |
70.82 |
68.00 |
63.14 |
56.31 |
48.08 |
38.87 |
29.87 |
Fort solutions of 0.1 and 0.5 activity, estimate the mean area of surface per butanol molecule.
3)the surface pressure, , due to an insoluble film of stearic acid, C15H35CO2H, and tri-p-cresyl phosphate (CH3C6H4O)3PO on water was measured for various film areas with a Langmuir Trough. Results are given below. Each set corresponds to a fixed amount of 1mol adsorbate.
Stearic acid
|
Area /m2 |
0.102 |
0.108 |
0.12 |
0.132 |
0.168 |
|
P /mNm-1 |
0.4 |
0.25 |
0.1 |
0.04 |
0.02 |
Tri-para-cresyl phosphate
|
Area /m2 |
0.26 |
0.3 |
0.36 |
0.42 |
0.48 |
|
P /mNm-1 |
0.1 |
0.085 |
0.07 |
0.05 |
0.03 |
Account for these observations. What other experiments might be performed to improve understanding of these results?
Hint: The second set of data does not follow the Volmer Equation very closely. You will need to make an intelligent extrapolation.
4. Describe an optical spectroscopic technique of your choice may be used to investigate the structure and dynamics of interfaces (<600 words please).
Refs:
IR and Raman
Dluhy et al, Spectrochimica, 51A, (1995) 1413.
Molecular orientation and surface concentrations
Y-S Tung et al, Appl. Spectroscopy, 47, (1993) 1643.
Spectroscopy for surface science,
Adv. In Spectroscopy. 26 (1998) eds R. J. E. Clark, R. E. Hester.
Sum frequency spectroscopy
Casson et al, Faraday Disc. 104, (1996) 209.
5) (Part III 1999)
a)derive from first principles the Gibbs adsorption equation
dg = -SGidmI
Define carefully any terms you introduce.
b)Aqueous solutions of phenol of molality m are found to have a surface tension, at 298K which closely follows the following relations
(i) go-g = am for m<0.05 where a = 0.113 Kg Nm-1 mol-1.
(ii) go-g= b ln(m)+c, for 0.1<m<0.3, where b=0.0107 Nm-1 and c 0.0361 Nm-1.
Comment on these facts and deduce as much as you can about the properties of the phenol/water/air interface.
How does this data indicate the limitations of thermodynamic studies of adsorption?
C) Describe how studies using other techniques have improved and extended our understanding of the adsorption of phenol at the surface of water.
Past Papers
June 2000 2(b)
June 2000 24(b)
2001 38(a)
2001 paper 2 38(a)
1999 45a