1. Chapter 8
Surface phenomena and dispersion system
8.1 Surface tension

2. 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.

3. ? A nickel floating on water surface Buoyancy
Gravity
Once immersed in water, the nickel would sink at once.

4. 2. Micro-mechanism of surface tension:
Unbalanced force and surface tension
A net attractive force exists for molecules at the surface.

5. Surface area enlargement
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.

6. 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

7. -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.

8. Why are free water droplets always in ball shape.

9. 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.

10. Wilhelmy slide method
Slide l
Soap film

11. maximum bubble pressure

12. 6 Influential Factors on surface tension
1) temperature
temperature dependence of surface tension
T / oC
H2O
C6H6
CH3OH
C2H5OH
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.

13. Surface tension decreases with increasing temperature.
At critical temperature (Tc), surface tension equals 0.
vapor
Interface
Liquid
Supercritical fluid

14. 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.

15. As temperature increases, surface tension decreases.
is the heat adsorbed by unit area enlargement.
Adiabatic expansion of surface area will result in temperature decrease.

16. Antonow law 2) co-existing phase dependence of surface tension vapor
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

17. 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

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