STABILITY AND BEHAVIOR OF THIN

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STABILITY AND BEHAVIOR OF THIN OIL/WATER/OIL
FILMS WITH BETA-LACTOGLOBULIN
Elka S. Basheva, Theodor D. Gurkov, Krastina G.
Krumova, and Bruce Campbell
Laboratory of Chemical Physics
Engineering, Faculty of Chemistry, University of
Sofia, James Bourchier Avenue 1, Sofia 1164,
BULGARIA Kraft Foods, Inc., 801 Waukegan
Road, Glenview, IL 60025, USA E-mail
tg_at_LCPE.uni-sofia.bg Web site
www.lcpe.uni-sofia.bg
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Outline and Motivation

• Study of aqueous films between Soybean Oil
  phases
• ? pressure Pc 10-50 Pa, similar to buoyancy in
  concentrated emulsions and creams
• Film thickness and lifetime role of pH, salt
• Comparison with DLVO theory force/ energy
  barriers
• ?? potentials of droplets are measured surface
  charge is used to obtain ?(h) isotherm,
  stabilizing barrier
• Proposed explanation of why the films rupture
  when ?max gtPc , and d?/dh lt 0
• Stabilization by gel-like network of protein on
  the surfaces at the isoelectric pH

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Results for thickness and stability of emulsion
films
?-Lactoglobulin (BLG) from bovine milk, Sigma
Without added NaCl qualitative agreement with
DLVO (exception at pH5.2)
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?-potential measurements
We want to calculate P(h) and determine the
barrier ?-potential ? surface charge is
needed O/W emulsions with 10 volume fraction of
soybean oil, Rotor-stator homogenizer drop
diameter 40 µm Zetasizer II C (Malvern, UK)
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Disjoining pressure isotherms
Dashed line Pc
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Barrier of repulsive forces

• In equilibrium, Pc?
• Maximum in ? should play a stabilizing role,
  since ?max gtgt Pc20 Pa
• Instability would occur if Pc gt ? (then, the
  film is forced to thin down), or if d?/dh gt 0
  (then, fluctuation waves grow spontaneously)

Fig. 4. Maxima in the ?(h) isotherms
Why does rupture happen easily at pH6, 7, with
thick films? Pc ltlt ?max , d?/dh lt 0
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Barrier of repulsive forces

• Passing over the barrier to reach an unstable
  thickness (?ltPc or d?/dh gt0) requires an energy
  increase of 0.1 erg/cm2
• The smallest piece of surface area capable of
  undergoing fluctuations is 20 nm2. So, the
  energy barrier is 0.5kBT
• It is easy to have a small spot in the film that
  jumps spontaneously into instability

Gel-like network at pH pI
Reflected monochromatic light (? 546 nm),
microscope Jenavert Newton Black Film at pH4
(no NaCl) two layers of molecules in contact, h
5-6 nm
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Gel-like network at pH pI NaCl
Black color of film low reflectivity, very
small h

• pH5 (no NaCl)
• ? high stability at the isoelectric point
• Role of salt
• With 0.15 M NaCl, DLVO cannot explain the film
  behavior
• ? high stability, but no electrostatic barrier
  at pHlt7 (Fig. 4)
• Likely reason pI, and NaCl, favor
• intermolecular linkages
• ? gel-like layers sticking (adhesion)
• of film surfaces aggregation

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Conclusions

• Oil/Water/Oil films with ?-Lactoglobulin
• ? Without added salt, electrostatic and van der
  Waals forces dominate (DLVO)
• Instability is observed in thick films (50 nm),
  at low capillary pressure, Pc ltlt ?max , and at
  d?/dh lt 0
• ? Only a small amount of energy (ltkBT) would be
  sufficient for emergence of an unstable
  fluctuation
• At the isoelectric pH, the film stability
  increases considerably
• ? Layer is reinforced, due to the lack of
  tangential repulsion formation of a skin-like
  layer is favored
• ? Similarly, NaCl brings about increased film
  stability it enhances the ability of the protein
  molecules to entangle and aggregate with their
  neighbors