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Introduction to Dispersed Systems

Introduction to Dispersed Systems. FDSC400 09/28/2001. Goals. Scales and Types of Structure in Food Surface Tension Curved Surfaces Surface Active Materials Charged Surfaces. COLLOIDAL SCALE. Dispersed Systems.

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Introduction to Dispersed Systems

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  1. Introduction to Dispersed Systems FDSC400 09/28/2001

  2. Goals • Scales and Types of Structure in Food • Surface Tension • Curved Surfaces • Surface Active Materials • Charged Surfaces

  3. COLLOIDAL SCALE

  4. Dispersed Systems A kinetically stable mixture of one phase in another largely immiscible phase. Usually at least one length scale is in the colloidal range.

  5. Dispersed Systems Dispersed phase Continuous phase Interface

  6. Continuous phase Dispersed phase

  7. Properties of Dispersed Systems • Too small to see • Affected by both gravitational forces and thermal diffusion • Large interfacial area • SURFACE EFFECTS ARE IMPORTANT

  8. Increased Surface Area The same oil is split into 0.1 cm radius droplets, each has a volume of 0.004 cm3 and a surface area 0.125 cm2. As we need about 5000 droplets we would have a total area of 625 cm2 We have 20 cm3 of oil in 1 cm radius droplets. Each has a volume of (4/3.p.r3) 5.5 cm3 and a surface area of (4.p.r2) 12.5 cm2. As we need about 3.6 droplets we would have a total area of 45.5 cm2

  9. For a Fixed COMPOSITION • Decrease size, increase number of particles • Increase AREA of interfacial contact decrease area

  10. LYOPHOBIC Weak interfacial tension Little to be gained by breaking e.g., gums LYOPHILIC Strong interfacial tension Strong energetic pressure to reduce area e.g., emulsions Tendency to break

  11. Surface Tension-molecular scale-

  12. Surface Tension-bulk scale- Force, g Slope g Interfacial energy Area, A Interfacial area

  13. Curved Surface Highly curved surface Slightly curved surface

  14. Curved Surfaces Molecules at highly deformed surfaces are less well anchored into their phase

  15. Laplace Pressure Surface pressure pulls inwards increasing pressure on dispersed phase pressure Surface tension Increased pressure radius

  16. Curved Surfaces -Consequences- • Dispersed phase structures tend to be round • Small fluid droplets behave as hard spheres • Solubility increases with pressure so… • Large droplets may grow at the expense of small (Ostwald ripening) • Depends on the solubility of the dispersed phase in the continuous

  17. Surface Active Material • Types of surfactant • Surface accumulation • Surface tension lowering

  18. Types of Surfactant-small molecule- Hydrophilic head group (charged or polar) Hydrophobic tail (non-polar)

  19. Types of Surfactant-polymeric- Polymer backbone Sequence of more water soluble subunits Sequence of less water soluble subunits

  20. Surface Binding Equilibrium ENTHALPY COST ENTROPY COST

  21. Surface Binding Isotherm Surface saturation Surface concentration /mg m-2 No binding below a certain concentration ln Bulk concentration

  22. Surface Tension Lowering Bare surface (tension g0) Interface partly “hidden” (tension g) Surface pressure – the ability of a surfactant to lower surface tension p = g-g0

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