Page 294 - Introduction to Colloid and Surface Chemistry

Problems 283
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23. At 20°C the surface tension of benzene is 28.9 mN m" and its
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molar volume is 89.2 cm mol" . Determine the relative pressures at
which condensation of benzene vapour should begin in a cylindrical
capillary of radius 10 nm if the capillary is (a) closed at one end, (b)
open at both ends. Assume zero contact angle and neglect adsorption
on the walls of the capillary.

24. Iron has a body-centred cubic lattice (see Figure 5.16) with a
unit cell side of 286 pm. Calculate the number of iron atoms per cm 2
of surface for each of the Fe(lOO), Fe(llO) and Fe(lll) crystal faces.
Nitrogen adsorbs dissociatively on the Fe(lOO) surface and the LEED
pattern is that of a C(2 x 2) adsorbed layer. Assuming saturation of
this layer, calculate the number of adsorbed nitrogen atoms per cm 2
of surface.

25. The contact angle for water on paraffin wax is 105° at 20°C.
Calculate the work of adhesion and the spreading coefficient. The
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surface tension of water at 20°C is 72.75 mN m" .
26. The pressure required to prevent liquid from entering a plug of
a finely divided solid is twice as great for a liquid of surface tension
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50 mN m" , which completely wets the solid, as it is for a liquid of
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surface tension 70 mN m" , which has a finite contact angle with the
solid. Calculate this contact angle.
27. The following data refer to the adsorption of dodecanol from
solution in toluene by a sample of carbon black, the BET nitrogen
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adsorption area of which is 105 m g" :
Equilibrium concentration/
moldm" 3 0.012 0.035 0.062 0.105 0.148
Amount adsorbed/
jimolg" 1 24.1 50.4 69,8 81.6 90.7

Show that the data fit a Langmuir adsorption isotherm equation and
calculate the area occupied by each adsorbed dodecanol molecule at
limiting adsorption.

28. Calculate the 'thickness' of the diffuse electric double layer for
a negatively charged solid surface in contact with the following