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ACS701S - APPLIED COLLOID AND SURFACE CHEMISTRY - 2ND OPP - JULY 2023 |
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nAmlBIA unlVERSITY
OF SCIEnCE Ano TECHnOLOGY
FACULTYOF HEALTH,NATURALRESOURCESAND APPLIEDSCIECNCES
SCHOOLOF NATURALAND APPLIEDSCIENCES
DEPARTMENTOF BIOLOGY,CHEMISTRYAND PHYSICS
QUALIFICATION:BACHELOROF SCIENCE
QUALIFICATION CODE: 07BOSC
COURSENAME: APPLIED COLLOIDAND SURFACE
CHEMISTRY
SESSION:JULY 2023
LEVEL:7
COURSECODE:ACS701S
PAPER:THEORY
DURATION: 3 HOURS
MARKS: 100
SUPPLEMENTARY/SECONDOPPORTUNITYEXAMINATION QUESTION PAPER
EXAMINER(S) Prof Habauka M Kwaambwa
MODERATOR: Prof Edet F Archibong
INSTRUCTIONS
1. Answer ALL the FIVE questions
2. Write clearly and neatly
3. Number the answers clearly
4. All written work must be done in bule or black ink
5. No books, notes and other additional aids are allowed
6. Mark all answers clearly with their respective question numbers
PERMISSIBLEMATERIALS
Non-programmable Calculators
ATTACHMENT
List of Useful Constants
THIS QUESTION PAPERCONSISTSOF 5 PAGES(Including this front page and List of Useful
Constants)
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QUESTION 1
[26]
(a) Explain briefly what you understand by the following terms:
Colloid particle, lyophi/ic colloid
(2)
(b) Characterise the following colloids in terms of what the dispersed phase (gas, liquid,
solid) and dispersion medium ((gas, liquid, solid) are:
(i) Smoke, Dust
(ii) Fire-extinguisher foam
(iii) Fog, Liquid sprays
(iv) Milk, Mayonnaise, Emulsion
(c) The surface tensions of the liquids mercury, water and n-hexadecane, in mNm-1, are
485, 72.8 and 27.46, respectively. Explain the differences.
(4)
(d) How does the temperature and presence of impurities affect the surface tension of
liquids?
{3)
(e) For a dilute aqueous solution of surfactant, the Gibbs adsorption isotherm may be
written as follows:
r =-_s_ dy
s RT des
(i)
rs In this equation, what do the symbols and Csstand for?
(2)
(ii) Use the values of the slope from the plot of y versus Ines(linearised form)
below the critical micelle concentration and at 25°C, to calculate the area
per molecule, As, at the interface for both surfactants: (6)
Slope/mNm- 1
C12EO6
- 6.856
C12H25OSO; Na+
- 20.56
(f) Which system of aqueous surfactant solutions in each pairs given below would give
rise to lower critical micelle concentration? Explain your answer briefly.
(5)
(i) dodecyldimethylammonium chloride with no added salt versus
dodecyldimethylammonium chloride in 0.20 M aqueous NaCl?
(ii) dodecyltrimethylammonium
bromide
{C12TAB)
versus
cetyltrimethylammonium bromide {C15TAB)?
QUESTION 2
[18]
{a) The initial spreading coefficient, Sa/Ais reported to be 36.0 mNm-1 for n-heptanol on
water at 20°C. The surface tensions of n-heptanol, ys, and water, YAat 20°C are 26.0
mNm-1 and 72.8 mNm-1, respectively. Calculate the interfacial tension of n-heptanol
and water, YAB-
(3)
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(b) At 2O°C,the surface tension of oleic acid, ye, is 32.5 mNm·1 and the interfacial tension
of oleic acid and water, YABis, 15.5 mNm·1. Solely based on this data and in (a) above,
which liquid between oleic acid and n-heptanol would you recommend for water
evaporation control? Show clearly your reasoning.
(5)
(c) Derive the expression (in terms of the appropriate work of adhesion, Wa, and work of
cohesion, We) for the spreading coefficient for a substance Cat the interface between
two liquids A and B.
(5)
(d) The effect of an impurity on spreading of oil on water depends on whether it is in the
oil or aqueous phase. Explain this statement.
(5)
QUESTION 3
(18]
The linear form BETequation is given by:
;,~J i(
-+I-- (c-()P
v(1-
vmc VmCPO
(a) State what the symbols in the equation represent.
(4)
(b) State any three assumptions involved in the derivation of the BET adsorption
isotherm.
(3)
(c) Show how the equation above may be used to determine Vmand C.
(5)
(d) State any three criticisms of the BETadsorption isotherm model.
(3)
(e) If the monolayer capacity of the nitrogen gas adsorbed on 1 g silica gel was found to
be 121.7 cm3 (stp). Calculate the specific surface area for silica gel, taking the
molecular area of nitrogen as 16.2 x 10-20m2.
(3)
QUESTION 4
(10]
(a) Using combining relations based on the Hamaker constants of pure materials (Ai),
calculate the composite Hamaker constants for the following interacting systems:
(i)
Quartz-Octane-Quartz
(ii) CaFi-Octane-Quartz
Given:
Material
Ai x 10-20 J
Quartz
3.7
n-Octane
4.5
CaF2
7.0
(6)
(b) Discuss the implications in colloid stability of positive and negative composite
Hamaker constants of the type Am and Arn.
(4)
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QUESTION 5
[28]
(a) Electrostatic interactions (due to surface charge) are important in colloid stability of
colloidal dispersions. There are several mechanisms by which colloid particles may
acquire charge. Briefly describe these mechanisms.
(10)
(b) State as briefly as possible the origins of van der Waals attraction, steric and depletion
interaction potentials acting between colloidal particles.
(6)
(c) On the same well-labelled diagram, show schematically the variation of the he
variation of the van der Waals potential (VA),electrostatic potential (VRJand total pair
potential, Vr =VA+ VR,with particle separation, h, for a stable colloidal dispersion. For
such a stable, label clearly the primary minimum, primary maximum and secondary
minimum. On the same schematic diagram of the pair potential, Vr, as function of particle
distance, h, between two spherical colloidal particles, show the influence of electrolyte
concentration (i.e. K) at fixed 'I'd.
(6)
(d) Briefly explain the significance in colloid stability of the primary minimum, primary
maximum and secondary minimum of Vr in (c) above.
(6)
END OF EXAM QUESTIONS
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USEFULCONSTANTS:
Universal Gas constant
Boltzmann's constant,
Planck's constant
Debye-Huckel's constant,
Faraday's constant
Mass of electron
Velocity of light
Avogadro's constant
1 electron volt (eV)
R=
k=
h=
A=
F=
me =
C
=
NA =
=
8.314 J K-1 mo1-1
1.381 x 10-23 J K-1
6.626 X 10-34 J S
0.509 (mol dm-3) 112 or mo1-0·5kg0·5
96485 C mo1-1
9.109 X 10-31 kg
2.998 x 108 m s-1
6.022 X 10 23
1.602 X 10-19 J
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