92168.pdf

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
   m""^1 and its enthalpy of vaporisation is 2,25 kJ g"^1.


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

c/KT^4 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

and 0,08 mol dm~^3. 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 02456789 10 12
y/mN m-^1 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.