Page 290 - Introduction to Colloid and Surface Chemistry

Problems 279

7. If bubbles of air 10~ 7 m in diameter and no other nuclei are
present in water just below the boiling point, by approximately how
much could water be superheated at normal atmospheric pressure
before boiling starts? The surface tension of water at 100°C is 59 rnN
1 1
m"" and its enthalpy of vaporisation is 2,25 kJ g" .

8. The following surface tensions were measured for aqueous
solutions of the non-ionic surfactant CH3(CH 2)9(OCH 2CH 2)5OH at
25°C:

4
c/KT mol dm~ 3 0.1 0.3 1.0 2.0 5.0 8.0 10.0 20.0 30.0
r/mNrrr 1 63.9 56.2 47.2 41.6 34.0 30.3 29.8 29.6 29.5
Determine the critical micelle concentration and calculate the area
occupied by each adsorbed surfactant molecule at the critical micelle
concentration.

9. The following surface tensions were measured for aqueous
solutions of n-pentanol at 20°C:

c/moldm~ 3 0 0.01 0.02 0.03 0.04 0.05 0.06 0.08 0.10
1
- 72.6 64.6 60.0 56.8 54.3 51.9 49.8 46.0 43.0
Calculate surface excess concentrations and the average area occupied
by each adsorbed molecule for bulk concentrations of 0.01 , 0.02, 0.04
3
and 0,08 mol dm~ . Plot a ir-A curve for the adsorbed n-pentanol
monolayer and compare it with the corresponding curve for an ideal
gaseous film.

10. The surface tensions of aqueous solutions of sodium dodecyl
sulphate at 20°C are as follows:

c/mmoIdirT 3 0 2 4 5 6 7 8 9 1 0 1 2
1
y/mN m- 72.0 62.3 52.4 48.5 45.2 42.0 40.0 39.8 39.6 39.5
Calculate the area occupied by each adsorbed dodecyl sulphate ion
when adsorption is a maximum and explain the discontinuity in the
surface tension-concentration relationship.