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Chapter 7. Foams

Chapter 7. Foams . 2006.5.8. 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

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Chapter 7. Foams

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  1. Chapter 7. Foams 2006.5.8.

  2. 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

  3. 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 vf– besides contacting rate, foaming efficiency(起泡效率) • Breaking rate vb– besides effects of environment, the stability of films • The life period of foams tf = vf – vb Pure water: tf = 0.5 sec Stable foams - foamable composition

  4. 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 PA= PB+ P drainage from A to B

  5. B A (2)Diffusion of gases PA= 4/RA andPB= 4/RB RA>RB, PA<PB R , P  (3)Foamability and Foam stability of surfactants (a)Ross-Miles Method VS=200ml, VS’=50ml Determine the highness of foams immediate – foamability after 5 min – foam stability

  6. (b) Foamability of surfactants Surface work WS= A WS constant, if  , then A , foamability Reduce of surface tension, foamability (c) Foam stability of surfactants stability and properties of film 4. Factors effecting stability of foams • Reduce of surface tension Laplace eq. P = 4/R , P, the liquor drainage at Plateau border 

  7. Alcohol =22.4mN/m(20°C), foamability, unstable; Glutin (明胶) >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(fwviscosity of film) • In films 100nm thick, fw/w= 5(wviscosity of water) • The mixed films fw/w 

  8. 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 = 2d/dlnA = 2Ad/dA (2-two films, d0/dA=0)

  9. Typical surfactants concentrations required to attain maximum foam height, MFH(Ross-Miles Method, 60°C

  10. (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 -

  11. (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 : C12 - C14 n < 12 , viscosity, n > 14 elasticity  (b) Branching degree , Compactness of lamellar  (c) Hydrophilic groups • Ionics – hydrophility, hydrate water, viscosity  static action • Nonionics – films loosen

  12. 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

  13. (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 ~ S>0 & E>0 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

  14. (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)

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