Chapter 8 Surface phenomena and dispersion system 8.1 Surface tension

Transcription

1 Chapter 8 Surface phenomena and dispersion system Levine p he interphase region

2 1.1 Some important concepts and phenomena (1) Surface and Interface Surface: he boundary between non-gaseous phases and vacuum. he 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 (2) Some important phenomena relevant to liquid surface

4 (2) Some important phenomena relevant to liquid surface Damo crossed Changjiang river on a reed. People in Zunyi city cross Xiangjiang river on a piece of bamboo.

5 (2) Some important phenomena relevant to liquid surface water strider 水黾

6 (2) Some important phenomena relevant to liquid surface A nickel floating on water surface? Buoyancy Gravity Once immersed in water the nickel would sink at once.

7 1.2 Micro-mechanism of surface tension Unbalanced force and surface tension A net attractive force exists for molecules at the surface.

8 1.2 Micro-mechanism of surface tension he energy of a molecule at the surface is higher than that in the interior Surface area enlargement o move molecules toward the surface i.e. enlarge the surface area energy must be expended. W' da W' da

9 1. 3 heoretical definition of the surface tension W' da For reversible process: W ' da W ' dg p G A p is the excess energy per m 2 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

10 1. 3 heoretical definition of the surface tension -W = da is another kind of useful energy. dg Sd VdP da B B dn B At constant temperature and pressure: dg da he energy of the surface increases with the enlargement of the surface area.

11 1.4 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.

12 1.4 Measurement of surface tension: Wilhelmy slide method Slide l A 2ldx Soap film W A 2ldx W Fdx F 2l

13 1.4 Measurement of surface tension: maximum bubble pressure

14 1.5 Influential Factors on surface tension (1) temperature temperature dependence of surface tension / o C H 2 O C 6 H 6 CH 3 OH C 2 H 5 OH A P n B 0 Surface tension decreases with increasing the temperature.

15 B B dn B da VdP Sd dg B n B P A n P A S B n B P A n P A S 0 n B P A S is the heat adsorbed by unit area enlargement. Adiabatic expansion of surface area will result in temperature decrease. 0 n B P A 0 n B P A As temperature increases surface tension decreases. 1.5 Influential Factors on surface tension

16 1.5 Influential Factors on surface tension Surface tension decreases with increasing temperature. At critical temperature ( c ) surface tension equals 0. vapor vapor Interface Liquid Supercritical fluid

17 1.5 Influential Factors on surface tension he empirical equation proposed by Eotvos and modified by Ramsay and Shields : V 2 / 3 m k( c 6.0) V m is the molar volume of the liquid k is a proportional factor for non-polar liquid equals J K -1.

18 1.5 Influential Factors on surface tension (2) co-existing phase dependence of surface tension Surface tension of water at 20 o C contacting with different liquids. vapor iso-pentane benzene butanol Antonow law 12 1 ' 2 ' ' W B W B ' Saturated with benzene Saturated with water

19 1.5 Influential Factors on surface tension (3) Chemical 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 o C NaCl 400 Fe / m. p MgO 1200 NaNO 3 / mica

20 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?