Challenges in teaching ?Colloid and Surface Chemistry? ? A ... - Wiley

Chapter 18. Thermodynamics and Colloid and Surface Chemistry
Problem 1. The dispersion component of the surface tension of water using
the Fowkes equation
Estimate the dispersive and specific components of the surface tension of
water using the Fowkes equation and the experimental value for the liquid-
liquid interfacial tension of water-cyclohexane (50.2 mN/m). The surface
tension of water is 72.8 and of cyclohexane is 25.5, all values are in mN/m
(or dyn/cm) at 20 oC. Repeat the calculation for the water-n-hexane system
(interfacial tension=51.1 mN/m; surface tension of hexane=18.4 mN/m).
Compare the results obtained with the two mixtures. Problem 2. Interfacial Tensions for liquid-liquid systems with the Fowkes
equation
The interfacial tension of mercury with benzene is (at 20 oC) [pic]mN/m.
Using the values given in table 18.12 for the surface tensions of mercury
(Hg), benzene (b) and water (w), estimate using the Fowkes equation for the
liquid-liquid interfacial tension:
i. The dispersion part of the surface tension of mercury at 20 oC ([pic]).
ii. The interfacial tension of the mercury-water system ([pic]). Compare
the result to the experimental value, which is in the range 415-426 mN/m
at 20 oC.
iii. The interfacial tension of water/benzene and compare it to the
experimental value (35 mN/m).
Explain the assumptions required and discuss briefly the results. If the
results using the Fowkes equation are not satisfactory, which other method
would you recommend using for estimating the interfacial tension ? Table 18.12. Values for the surface tensions of mercury, benzene, water, n-
pentane and n-octane.
|Compound |Surface tension (at 20 oC) in mN/m|
|Mercury |485 |
|Benzene |28.9 |
|Water |72.8 |
|n-pentane |16.8 |
|n-octane |21.8 | Problem 3. Liquid-liquid interfacial tensions with the Fowkes equation
i. The structure of monoethylene glycol (MEG) is: OH-CH2-CH2-OH. MEG has a
surface tension equal to 47.7 mN/m. Table 18.13 shows experimental data
for the surface tension of two hydrocarbons and their liquid-liquid
interfacial tensions with MEG. Based on these data, can you conclude
whether Fowkes equation can be applied to glycol-alkane interfaces?
Justify your answer with calculations.
ii. Perform the same exercise as part i. for the CHCl3-water and CCl4-water
interfaces using the data of table 18.14. Use the well-accepted value for
the dispersion surface tension of water (21.8 mN/m).
Table 18.13. Experimental data for the surface tension of two hydrocarbons
and their liquid-liquid interfacial tensions with MEG. |Alkane |Surface Tension (mN/m)|Interfacial tension |
| | |against MEG (mN/m) |
|cyclohexane |25.5 |14 |
|n-hexane |18.4 |16 |
Table 18.14. Experimental data for the surface tension of two components
and their liquid-liquid interfacial tensions with water. |Compound |Surface Tension (mN/m)|Interfacial tension |
| | |against water (mN/m) |
|chloroform |27.1 |28 |
|carbon tetrachloride |26.9 |45 |
Problem 4. Interfacial Tensions for hydrocarbon/water systems with the
Hansen-Beerbower equation
Two sets of the Hansen dispersion, polar and hydrogen bonding solubility
parameters have been reported for water: set 1 (7.6, 8.1, 20.6) and set 2
(10.8, 14.3, 15.6). All values are given in (cal/cm3)1/2. The molar volume
of water is 18.0 cm3/mol.
i. Calculate, using the Hansen-Beerbower model, equation 18.6, the
dispersion and specific (polar and hydrogen bonding) parts of the surface
tension of water based on both sets of water solubility parameters. Which
set of water solubility parameters results in values for the dispersion
and specific surface tensions for water closer to those estimated from
the Fowkes theory?
ii. Estimate, using the Hansen-Beerbower equation, the dispersion, polar
and hydrogen bonding parts of the surface tension of pentane, octane and
benzene. The experimental surface tensions of pure liquids are known
(Table 18.12). Comment on the results.
iii. Estimate, using the Hansen-Beerbower equation for the interfacial
tension, the interfacial tensions for water-octane and water-benzene.
Which water solubility parameter set gives better results (closer to the
experimental values)? The experimental interfacial tensions are 50.8 and
35.0 mN/m, respectively. Data:
The Hansen solubility parameters (dispersion, polar, hydrogen bonding) and
the volume of pentane are: 7.1, 0.0, 0.0 and 116.2, in (cal/cm3)1/2 and
cm3/mol, respectively.
Similarly for octane: 7.6, 0.0, 0.0 and 163.5
And for benzene: 9.0, 0.0, 1.0 and 89.4
Problem 5. Work of adhesion and contact angles from the Hansen/Beerbower
theory
The liquid (l)-solid (s) interfacial tension can be given by the
Hansen/Beerbower expression:
[pic] 18.61
i. Show that, when the spreading pressure is zero, the contact angle for a
solid-liquid interface based on equation 18.61, is given by the equation:
[pic] 18.62
ii. A very simple theory for the interfacial tension which performs
satisfactorily for some systems is a modified form of the Girifalco-Good
equation (using a correction parameter equal to one):
[pic] 18.63
Derive an expression for the work of adhesion as a function of the
surface tensions of the solid and the liquid.
iii. Derive the expression for the solid-liquid work of adhesion from the
Owens-Wendt theory. Problem 6. Adhesion between paint layers based on epoxy and silicone
In a new fouling-release paint produced by a major paint company are
included several layers of which two are based on epoxy and silicone.
Various epoxies have been tried because adhesion problems have been
observed in certain cases. In order to achieve a better understanding of
the surfaces, contact angles have been measured for three liquids on the
various epoxies and the results for three of the epoxies are shown in table
18.15. Table 18.15. Measured contact angles for three liquids on various epoxies. |Epoxy type |Cos (contact |Cos (contact |Cos (contact |
| |angle) |angle) |angle) |
| |of water |of ethylene |of benzaldehyde |
| | |glycol (EG) | |
|45742 |0.511 |0.846 |0.972 |
|45182 |0.536 |0.703 |0.943 |
|45143 |0.442 |0.742 |0.943 | The surface tension of the silicone layer (on top of the epoxy) is 29.5
mN/m. It is moreover expected that the epoxy with the highest surface
tension may yield better adhesion with the silicon layer. Using the van Oss-
Good approach:
i. Estimate the LW, acid/base and the total surface tension of all three
epoxies. The surface tension components for the van Oss et al. Method are
for water and EG (21.8, 25.5, 25.5) and (29, 1.92, 47.0), respectively.
The order in parenthesis is LW, acid and base component. For
benzaldehyde, it can be assumed that only LW contribution exists and the
surface tension is 38.5 mN/m. Comment on the values obtained for the
individual components of the surface tensions for the three epoxies.
ii. Which epoxy surface is expected to be better wetted by silicone and
for which epoxy-silicon system is expected the highest adhesion? Problem 7. Interfacial tension for 'complex' liquid-liquid interfaces with
various models
Many theories have been proposed for the estimation of interfacial
tensions. These theories can be tested against experimental data for liquid-
liquid interfaces but testing is more difficult for solid-liquid interfaces
(where the interfacial tension cannot be measured directly). In this
problem we consider aqueous mixtures with organic compounds like aniline
and alcohols and will select the best among a number of these theories. The
first application is the water-aniline system. The surface tension of
aniline is 42.9 mN/m and the ratio of dispersion to specific surface
tension is, for the same liquid, 1.294.
i. Calculate the water/aniline interfacial tension with the Fowkes equation
and with the following two versions of the harmonic mean equation:
[pic] 18.64
[pic] 18.65
Compare the results to the experimental value which is 5.8 mN/m ?
What do you observe?
ii. Calculate the water/aniline interfacial tension with the Owens-Wendt
expression, which is similar to Fowkes but includes an extra term for the
specific forces:
[pic] 18.66
How does the Girifalco-Good model perform for this system? Which of
the four models compares best with the experimental data?
iii. The second application involves the immiscible water- heavy alcohols
(i.e. heavier than propanol). The surface tensions of butanol, hexanol,
heptanol and octanol are 24.6, 25.8, 25.8 and 27.5 mN/m, respectively.
Which of the above models (consid