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    Aggregation & Sedimentation

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    reyza
    This document discusses aggregation and sedimentation in dispersions. It explains that aggregation occurs when van der Waals attraction or depletion interaction causes particles to stick together, increasing particle size. Sedimentation happens when particles settle due to differences in buoyancy, either downward or upward. Both aggregation and sedimentation result in demixing and an increase in particle size over time. The document provides factors that influence aggregation and sedimentation rates and methods to prevent or retard them.

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    Aggregation & Sedimentation

    Uploaded by

    reyza
    31 views11 pages
    This document discusses aggregation and sedimentation in dispersions. It explains that aggregation occurs when van der Waals attraction or depletion interaction causes particles to stick together, increasing particle size. Sedimentation happens when particles settle due to differences in buoyancy, either downward or upward. Both aggregation and sedimentation result in demixing and an increase in particle size over time. The document provides factors that influence aggregation and sedimentation rates and methods to prevent or retard them.

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    This document discusses aggregation and sedimentation in dispersions. It explains that aggregation occurs when van der Waals attraction or depletion interaction causes particles to stick together, increasing particle size. Sedimentation happens when particles settle due to differences in buoyancy, either downward or upward. Both aggregation and sedimentation result in demixing and an increase in particle size over time. The document provides factors that influence aggregation and sedimentation rates and methods to prevent or retard them.

    Copyright:

    © All Rights Reserved
    Download as PDF, TXT or read online from Scribd
    Download as pdf or txt
    31 views11 pages

    Aggregation & Sedimentation

    Uploaded by

    reyza
    This document discusses aggregation and sedimentation in dispersions. It explains that aggregation occurs when van der Waals attraction or depletion interaction causes particles to stick together, increasing particle size. Sedimentation happens when particles settle due to differences in buoyancy, either downward or upward. Both aggregation and sedimentation result in demixing and an increase in particle size over time. The document provides factors that influence aggregation and sedimentation rates and methods to prevent or retard them.

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    © All Rights Reserved
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    AGGREGATION AND

    SEDIMENTATION
    Physical Chemistry of Food

    Fithri Choirun Nisa


    Instability in Dispersions
    Lyophobic dispersion:
    thermodynamically unstable --- tend to change
    --- minimize interfacial free energy
    àincrease particle size
    Lyophilic dispersion unstable --- pH, ionic
    strength change

    Instability à changes
    External forces (gravitational and
    hydrodynamic force) à changes
    Physical Chemistry of Food
    FCN
    Physical Chemistry of Food
    FCN
    Aggregation:
    - Main cause: van der Waals attraction
    - Another cause: depletion interaction
     driving force: increase in mixing entropy of
    polymer molecules / small species in
    solution

    Physical Chemistry of Food


    FCN
    Sedimentation
    - Settling (downward sedimentation)
    - Creaming (upward sedimentation)
     driving force: difference in buoyancy
    (potential energy)

    Physical Chemistry of Food


    FCN
    Resulting changes:
    - Aggregation: particle size, particle shape
    àspace filling network
    - Sedimentation: position of the particles
    become less random à demixing

    Physical Chemistry of Food


    FCN
    - One kind of change à the rate / occurrence of
    the other
    - All changes leading to an increase in particle
    size (a-e) à sedimentation rate
    - Aggregation Sedimentation

    Physical Chemistry of Food


    FCN
    Aggregation
    Particles stay together for a much longer than
    they would do in the absence of colloidal
    interaction forces

    - Flocculation: weak (reversible)


    - Coagulation: strong (irreversible)

    Attractive free energy acting between them

    Physical Chemistry of Food


    FCN
    Types of aggregation:
    - Perikinetic aggregation
     Brownian motion or diffusion

    - Orthokinetic aggregation
     velocity gradients in the liquid

    - Fractal aggregation
    perikinetic aggregation occurs and particles
    bonded stay in the same relative position

    Physical Chemistry of Food


    FCN
    Sedimentation
    Sedimentation rate = D2.g.
    18 c
    D = particle size
    g = acceleration due to gravity
     = density
    c = viscosity of continuous phase
    Centrifugal sedimentation
    g à R2

    Physical Chemistry of Food


    FCN
    Preventing / retarding:
    - Decreasing particle size -- homogenization
    - Lowering the density difference
    - Increasing viscosity – adding stabilizer
    - Giving the liquid a yield stress
    - Mildly agitating the liquid
    - Preventing aggregation of the particles

    Physical Chemistry of Food


    FCN

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