Chapter 8

Surface phenomena and dispersion system

8.1 Surface tension

Liquid

vapor

Interface

Supercritical fluid

Capillary action

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.

1. Some surface phenomena

Nickel-floating on Shuyu Spring, Jinan

1) Nickel-floating

A nickel floating on water surface

Once immersed in water, the nickel would sink at once.

?

Buoyancy

Gravity

Why do the bubbles formed due to turbulence/torrent disappear immediately?

2. Micro-mechanism of surface tension: Unbalanced force and surface tension

A net attractive force exists for molecules at the surface.

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 '

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

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.

Why are free water droplets always in ball shape.

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:

Wilhelmy slide method

Slide l

Soap film

ldxA 2

ldxAW 2

FdxW

l

F

2

maximum bubble pressure

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

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.

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.

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

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

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?