Surface phenomena and dispersion system

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Chapter 8 Surface phenomena and
dispersion system
8.1 Surface tension
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Two important concepts
Surface and Interface
Surface The boundary between non-gaseous phases
and vacuum. The boundary between liquid or solid
phase and their vapors can be approximated as
Surface. In other cases, the boundary is
interface.
So far, we have only considered bulk properties
of materials. In this section, the surface /
interface properties of pure liquid/vapor,
solution surface and solid / gas interface will
be treated.
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A nickel floating on water surface
?
Once immersed in water, the nickel would sink at
once.
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2. Micro-mechanism of surface tension
Unbalanced force and surface
tension
A net attractive force exists for molecules at
the surface.
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The energy of a molecule at the surface is
higher than that in the interior
Surface area enlargement
To move molecules toward the surface, i.e.,
enlarge the surface area, energy must be
expended.
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3. Theoretical definition of the surface tension
For reversible process
? is the excess energy per m2 possessed by the
surface. specific surface energy. Its unit is J
m-2 N m-1, which can be taken as the force
exerted on unit length. surface tension
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-?W ?dA is another kind of useful energy.
At constant temperature and pressure
The energy of the surface increases with the
enlargement of the surface area.
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Why are free water droplets always in ball shape.
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5 Measurement of surface tension
1. capillary rise 2. maximum bubble pressure
3. ring method 4. drop
weight method 5. Wilhelmy slide
method 6. the shape of drops or bubbles
7. flow method 8. capillary waves method, etc.
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Wilhelmy slide method
Slide l
Soap film
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maximum bubble pressure
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6 Influential Factors on surface tension
1) temperature
temperature dependence of surface tension
T / oC H2O C6H6 CH3OH C2H5OH
0 75.64 31.6 29.5 24.0
25 71.94 28.2 27.1 21.8
50 67.91 25.0 24.6 19.8
70 63.5 21.9 22.0
Surface tension decreases with the increase of
temperature.
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Surface tension decreases with increasing
temperature. At critical temperature (Tc),
surface tension equals 0.
vapor
Interface
Liquid
Supercritical fluid
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The empirical equation proposed by Eotvos and
modified by Ramsay and Shields
Vm is the molar volume of the liquid, k is a
proportional factor, for non-polar liquid, equals
2.2 ? 10-7 J K-1.
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As temperature increases, surface tension
decreases.
is the heat adsorbed by unit area enlargement.
Adiabatic expansion of surface area will result
in temperature decrease.
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2) co-existing phase dependence of surface
tension
Surface tension of water at 20 oC contacting with
different liquids.
vapor iso-pentane benzene butanol
72.8 49.6 32.6 6.8
Antonow law
Saturated with benzene
Saturated with water
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3) nature
Because the intermolecular interaction
between liquid molecules is small, the interface
tension is usually less than 80 mN m-1.
Substances ? Substance ?
Hg / 20 oC 486.5 NaCl 400
Fe / m. p. 1880 MgO 1200
NaNO3/308 116.6 mica 2400-5400
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discussion

1. Can you compare the surface tension of two
   liquids by examining the shape of a drop /
   bubble?
2. For the charged surface, will the surface tension
   increase or decrease?