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1、Interfacial tension表面张力表面张力1.Free surface energy of interface layer 2.Specific surface energy or interfacial tension3.Factors affecting interfacial tension4.Adsorption and relationship between adsorption and surface tensionInterfacial tension1 Free surface energy of interface layer 1)defintion Becau
2、se the molecule at the interface has an unbalanced attractive force,a certain amount of work is required to move a molecule from within the body of the liquid to the interface.this work is referred to as the Free surface energy.A water molecule which is remote from the interface is surrounded by oth
3、er water molecules,thus having a resulting net attractive force on the molecule of zero.AThe resulting force on molecule B is zero,so molecule B can free moveThe resulting force on molecule A isnot zero,it is an inward-directed force B A molecule at the interface has a force acting upon It from the
4、gas lying above the interface and water molecules lying below the interface.There are several types of interfaces including:liquid-gas(L-G),liquid-liquid(L-L),solid-gas(S-G)and solid-liquid(S-L)interfaces.2)The properties of free surface energyA.Only if two immiscible fluids are in contact,the free
5、surface energy exists.B.Free surface energy increases with surface area.C.Free surface energy increases with density difference of two phase materialD.Free surface energy related to the phase state of two phase material.E.The interface layer is a gradually transitional molecular layer2 Specific surf
6、ace energy or interfacial tension1)definition1)definitionspecific surface energy:The interfacial tension is the work required to create a unit area of new surface.The force per unit length required to create a new surface.xLdxFw=dA=w/A the modern standard expression of the units:J/m2,1J=1N.m N/m,mN/
7、mthe earlier expression of the units:ergs/cm2 ,dynes/cm2)The unit of spcific surface energy1dynes/cm=1mN/m=w/AThe physical meaning of these units is shown below:Work=Force x Length=l dxWork=dA where dA=l dx denotes the change in areatwo different conceptsnenergy per unit area force per unit length F
8、ig Equilibrium of forces at a water-oil-gas interface3 Factors affecting interfacial tension (1)the type of substance Table:Commonly Used IFT ValuesWater-air72 mN/mMercury-air450 mN/mHydrocarbons-air20-50 mN/mWater-oil20 mN/mBrine-oil30 mN/mWater/soap-oil0.001-0.1 mN/mFactors affecting interfacial t
9、ension (2)The phase state of two materials.First phaseSecond phaseInterfacial tension(mN/m)Mercuryair486.5Mercuryalcohol364.0Mercurybenzene357.2Waterair72.9Waterbenzene35.0Table:IFT of several substances at normal tempratureFactors affecting interfacial tension (3)The polarities of two materials.(4)
10、The composition of two materials The less polarity difference in two phase materials is,the smaller the interfacial tension between them will be.Surfactants that two materials compose of can reduce the surface tension.(5)The temperature and pressure Interfacial tension of Oil/gas system P P T T Inte
11、rfacial tension of Oil/water system A.Oil/water system that be not saturated gas T and P have no influnce on interfacial tension B.Oil/water system saturated gasFactors affecting interfacial tension4.Adsorption and relationship between adsorption and surface tension1)principle of Adsorbtion 1)princi
12、ple of Adsorbtion Adsorption(吸附吸附):The phenomenon that concentrationof some material at interface changes automatically iscalled the adsorption.Absorbent(吸附剂吸附剂):Material which has adsorbability is called as absorbent(activated carbon).Adsorbate(吸附质吸附质):material which is adsorbsed is called as adsor
13、bate(bromine).2)adsorption on Solid surface2)adsorption on Solid surfaceProperty of adsorption:（1）The adsorbability on Solid surface increases with surface area.（2）The adsorption on solid surface is selective.(3)The adsorbability on Solid surface decreases with tempreture.（4）The adsorption amount is
14、 proportional to concentration of absorbed material.3)3)adsorption of gas/liquid adsorption of gas/liquid interfaceFig.3 3 The effect of solution concentration on surface tensionPorsitive adsorption(正吸附正吸附):The concentration of dissolved material on the interface is higher than that of solution inte
15、rnal.(to reduce interfacial tension).Negative adsorption(负吸附负吸附):The concentration of dissolved material on the interface is less than that of solution internal.A.Some basic conceptsSurface active agent(表面活性剂表面活性剂):The materials which could significantly decrease its surface tension by dissolved a s
16、mall amount into the solutionB.The mechanism that surface active agent reduces interfacial tensionpolarity:A C B，C is materal adsorbed on A/B interfaceT，R absolute temperature,universal gas constant.C.The relationship of adsorption and concentrationGadsorance,also called specific absorption.Cconcentration of solute interfacial tension.Concentration increase