Chapter 7. Foams

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Chapter 7. Foams

• 2006.5.8.

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• 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

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• 2.Foams formation
• (1) Necessary condition contact of gas-liquid
• do works stirring(??) , bubbling(??) ,
  boiling(??) , and blowing(??) etc.
• (2) Sufficient condition foaming rate gt breaking
  rate
• Forming rate vf besides contacting rate, foaming
  efficiency(????)
• 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

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• 3. Breaking mechanism of foams
• Besides the reason in thermodynamics the liquor
  drainage (??) is the direct reason.
• Liquor drainage of the lamellar films
• Liquor drainage by gravity (????) due to the
  density difference of G-L phase in foams, the
  liquor drainage from top to bottom
• Liquor drainage by interfacial tension(??????)
• Plateau border see fig
• Liquor drainage
• ?P2?/R
• PA PB ?P
• drainage from A to B

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• (2)Diffusion of gases
• ?PA 4?/RA and ?PB 4?/RB
• RAgtRB, PAltPB
• R ?, ?P ?
• (3)Foamability and Foam stability
• of surfactants
• (a)Ross-Miles Method
• VS200ml, VS50ml
• Determine the highness of foams
• immediate foamability
• after 5 min foam stability

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• (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 ?

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• Alcohol ?22.4mN/m(20C), foamability?, unstable
• Glutin (??) ?gt22.4mN/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 ?

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• The viscosity?, the liquor drainage ?, stability?
• (b) The elasticity of interfacial films - high
  elasticity no brittleness(??)
• e.g. cetanol (???)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/dA0)

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

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• (5) Permeability(???) of gases
• Laplace eq. ?P 4?/R? permeability ?
• Compactness(???) of lamellar ?, permeability?
• (6) The structure of molecules
• Length of chain C12 - C14
• n lt 12 , viscosity?, n gt 14 elasticity ?
• (b) Branching degree ?, Compactness of lamellar ?
• (c) Hydrophilic groups
• Ionics hydrophility?, hydrate water?, viscosity
  ?
• static action
• Nonionics films loosen

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• 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 gt 0
• then the antifoaming agents spreads on films of
  foam

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• (b) Immersing coefficient E ?m ?int - ?a
• E gt 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 Sgt0 Egt0 then antifoaming
  agents
• (2) Antifoaming Mechanism
• Reduce surface tension of local liquid films
• Break elasticity of liquid films,e.g. cetanol
  (???)
• Reduce viscosity of liquid films

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• (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(???)
• Polyether (??)
• Siloxane
• Poly(ether siloxane)