1. Chapter 7. Foams 2006.5.8.

2. 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 (a)Fire fighting (b)Polymer foamed insulation (c)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 (a)Forming rate v f – besides contacting rate, foaming efficiency( 起泡效率 ) (b)Breaking rate v b – besides effects of environment, the stability of films (c)The life period of foams t f = v f – v b Pure water: t f = 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. (1)Liquor drainage of the lamellar films (a)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 (b)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

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

6. (b) Foamability of surfactants Surface work W S =  A W S 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 (1)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(  fw viscosity of film) In films 100nm thick,  fw /  w = 5(  w viscosity 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 (a)Gibbs-Marangoni effect – elasticity of films (b)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 (a)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

12. 5. Antifoaming and Antifoaming Agents (1)Antifoaming Theory – physical & chemical methods physical methods - stirring, centrifuging, ultrasonic, heating, and changing the pressure etc chemical methods: (a)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 (a)Reduce surface tension of local liquid films; (b)Break elasticity of liquid films,e.g. cetanol ( 十六醇 ); (c)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 (a)Natural oil and fat (b)Modified natural oil and fat (c)Nonionics of polyhydric alcohol( 多元醇 ) (d)Polyether ( 聚醚 ) (e)Siloxane (f)Poly(ether – siloxane)