efficiency(起泡效率)

单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,Chapter 7. Foams,2006.5.8.,1,Introduction,(1) Foams : G/L dispersion system,thermodynamic unstable system,polyhedral air bubble groups,Dispersed phase gas,Disperse medium liquid,Two-sided films lamellar of the,foan,(2) Application,Fire fighting,Polymer foamed insulation,Foam finishing,2,2.Foams formation,(1) Necessary condition contact of gas-liquid,do works stirring(搅拌) , bubbling(鼓泡) , boiling(沸腾) , and blowing(吹气) etc.,(2) Sufficient condition foaming rate breaking rate,Forming rate,v,f, besides contacting rate, foaming efficiency(,起泡效率,),Breaking rate,v,b, besides effects of environment, the stability of films,The life period of foams,t,f,=,v,f,v,b,Pure water:,t,f,= 0.5 sec,Stable foams -,foamable,composition,3,3. Breaking mechanism of foams,Besides the reason in thermodynamics the liquor drainage (排液) is the direct reason.,Liquor drainage of the lamellar films,Liquor drainage by gravity (,重力排液,) ,due to the density difference of G-L phase in foams, the liquor drainage from top to bottom,Liquor drainage by interfacial tension(,表面张力排液,),Plateau border see fig,Liquor drainage,P=2/R,P,A,= P,B,+,P,drainage from A to B,4,(2)Diffusion of gases,P,A,= 4/R,A,and,P,B,= 4/R,B,R,A,R,B, P,A,22.4mN/m, foamability, stability, is only important condition and,not necessary condition,(2) The properties of interfacial films,(a) The viscosity of interfacial films,Surface viscosity the viscosity of,monomolecular film,Viscosity of film , viscosity of lamellar,Lamellar interfacial film and bulk solution,In films 100nm thick, ,fw,/,w,= 2(,fw,viscosity of film),In films 100nm thick, ,fw,/,w,= 5(,w,viscosity of water),The mixed films ,fw,/,w,7,The viscosity,the liquor drainage, stability,(b) The elasticity of interfacial films - high elasticity no brittleness(,不脆,),e.g.,cetanol,(,十六醇,),surface activity, , surface viscosity, rigidity too, brittleness ! elasticity,(3) The healing action of surfactants,Gibbs-,Marangoni,effect elasticity of films,The coefficient of elasticity,E = 2,d,/,dln,A = 2A,d,/,d,A (2-two films,d,0,/,d,A=0),8,Typical surfactants concentrations required to attain maximum,foam height, MFH(Ross-Miles Method, 60,C,9,(c) Effect of concentration on elasticity transfer of surfactant,from lower surface tension area to higher - elasticity,from bulk phase to surface - elasticity,the healing action,c, CMC (to see table),rate of adsorption is lower e.g. cetanol may be a anti-foamer,(4) Surface charge -,10,(5) Permeability(透过性) of gases,Laplace eq.,P = 4/R permeability,Compactness(致密性) of lamellar , permeability,(6) The structure of molecules,Length of chain : C,12,- C,14,n 14 elasticity ,(b) Branching degree , Compactness of lamellar ,(c) Hydrophilic groups,Ionics,hydrophility, hydrate water, viscosity ,static action,Nonionics, films loosen,11,5. Antifoaming and Antifoaming Agents,Antifoaming Theory physical & chemical methods,physical methods - stirring , centrifuging , ultrasonic , heating , and changing the pressure etc,chemical methods:,Spreading coefficient S =,m,- ,int,- ,a,m, surface tension of foaming medium,int, interfacial tension between foaming medium and antifoaming agents,a, surface tension of antifoaming agents,S 0,then the antifoaming agents spreads on films of foam,12,(b) Immersing coefficient E =,m,+ ,int,- ,a,E 0,The antifoaming agents could immerse into films of foam,(c) Ross theory solubility of antifoaming agents in lamellar of foams, antifoaming power,dissolved state, foam stabilizer,insolubilized state S0 & E0 then antifoaming agents,(2) Antifoaming Mechanism,Reduce surface tension of local liquid films;,Break elasticity of liquid films,e.g.,cetanol,(,十六醇,);,Reduce viscosity of liquid films,13,(d) Hydrophobic solid powder adsorbing surfactants on its surface,(3) Antifoaming agents hydrophobic surfactants or oil and fat, HLB = 1-3,Natural oil and fat,Modified natural oil and fat,Nonionics,of polyhydric alcohol(,多元醇,),Polyether (,聚醚,),Siloxane,Poly(ether ,siloxane,),14,