chapter 8 surface phenomena and dispersion system 8.1 surface tension
TRANSCRIPT
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Chapter 8
Surface phenomena and dispersion system
8.1 Surface tension
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Liquid
vapor
Interface
Supercritical fluid
Capillary action
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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.
Two important concepts
Surface and 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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1. Some surface phenomena
Nickel-floating on Shuyu Spring, Jinan
1) Nickel-floating
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A nickel floating on water surface
Once immersed in water, the nickel would sink at once.
?
Buoyancy
Gravity
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Why do the bubbles formed due to turbulence/torrent disappear immediately?
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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.
dAW '
dAW '
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3. Theoretical definition of the surface tension
dAW ' dAW '
For reversible process:
,' T pW dG ,T p
G
A
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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dAdG
The energy of the surface increases with the enlargement of the surface area.
B
BBdndAVdPSdTdG
At constant temperature and pressure:
-W = dA is another kind of useful energy.
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Why are free water droplets always in ball shape.
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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.
5 Measurement of surface tension:
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Wilhelmy slide method
Slide l
Soap film
ldxA 2
ldxAW 2
FdxW
l
F
2
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maximum bubble pressure
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6 Influential Factors on surface tension
0,,
BnPAT
Surface tension decreases with the increase of temperature.
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
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Surface tension decreases with increasing temperature.
At critical temperature (Tc), surface tension equals 0.
The empirical equation proposed by Eotvos and modified by Ramsay and Shields :
)0.6(3/2 TTkV cm
Vm is the molar volume of the liquid, k is a proportional fact
or, for non-polar liquid, equals 2.2 10-7 J K-1.
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B
BBdndAVdPSdTdG
BB nPAnPT TA
S
,,,,
BB nPAnPT TT
A
ST
,,,,
0,,
BnPTA
ST is the heat adsorbed by unit area enlargement.
Adiabatic expansion of surface area will result in temperature decrease.
0,,
BnPA
TT
0
,,
BnPAT
As temperature increases, surface tension decreases.
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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
'' 212,1 '', BWBW
Saturated with benzene
Saturated with water
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Because the intermolecular interaction between liq
uid 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
3) nature
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discussion
1) Can you compare the surface tension of two liquids
by examination of drop / bubble shape?
2) For the charged surface, will the surface tension
increase or decrease?