Worked solutions for:
‘Applied Colloid & Surface Chemistry’
by R.M. Pashley and M.E. Karaman
(J.Wiley, 2004)
Chapter 2
1. Answer:
Use Laplace equation in the form (eqn 2.8):
r
P
θ
γ
cos
2
=
∆
. Gives pressure
of 2 bars. Addition of surfactants will reduce the interfacial tension and hence the
pressure. The use of hydrophobic porous materials to prevent water flow is used in
clothing and in concentrating orange juice.
2. Answer:
the Laplace equation for spherical soap films is
r
P
γ
4
=
∆
. Hence we can
calculate the pressure difference between each bubble and the atmosphere and therefore
between the bubbles. The pressure difference between the two bubbles can then be used
in the Laplace equation to obtain the radius (1.5cm) of the (spherical) soap film formed
between the bubbles.
3. Answer:
Use the capillary rise equation (2.9). Make sure that you convert the
parameters to SI units. The calculated surface tension value is: 0.62Nm
-1
.
4. Answer:
Apply the full Laplace equation to calculate the pressure difference across the
water interface, relative to atmospheric pressure. The difference in these two pressures
must equal the hydrostatic pressure difference in the water (ie h
ρ
g). This gives a height
difference of
4.1mm.
1

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