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Food Chem microstructure > Interfaces > Flashcards

Interfaces Flashcards

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1
Q

interface formed by

Ex: (4)

A

Formed by two phases that are mutually insoluble (immiscible)
G/G S/L
L/L G/S

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2
Q

Gas/liquid interface formed while _____ existed

A

surface tension
–> liquid interface with gas (e.g. air and water) = surface tension

= unbalanced forces at the surface of molecules
attracted inwards

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3
Q

surface tension or interfacial tension

  • -> lead to ____?
  • -> require ____ to overcome?
  • -> Units?
A

Unbalanced forces at the surface lead to a net inward force of attraction on surface molecules that tends to minimise the area of the surface

Contraction of the surface therefore leads to a minimum free energy state and work is required to increase the surface area

Free energy and surface tension are numerically equal with units N m-1 (usually quoted as mN m-1)

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4
Q

What is require to increase surface area

Equation?

A

Work / Energy

ΔE = γΔA

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5
Q

Continuous phase define?

can also called ?

A

That phase in a two or more phase mixture which is continuously interconnected

dispersing medium or external phase

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6
Q

surface free energy is

Units?

A

the ‘work’ required to increase the surface area by 1 m^2

Free energy and surface tension are numerically equal with units N m-1 (usually quoted as mN m^-1)

liquid interface with a solid surface (e.g. water droplet on glass) = surface free energy

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7
Q

Energy (ΔE) required to increase surface area (ΔA) of

liquid interface with gas (e.g. air and water)

A

surface tension

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8
Q

Energy (ΔE) required to increase surface area (ΔA) of liquid interface with a solid surface (e.g. water droplet on glass)

A

= surface free energy

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9
Q

Energy (ΔE) required to increase surface area (ΔA) of interface between immiscible liquids (e.g. oil and water)

A

interfacial tension

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10
Q

Colloid aka ?

define

A

colloidal system - have a dispersed phase and a continuous phase (X/Y)

state of subdivision such that molecules or particles dispersed in a medium have at least one dimension between 1 nm and 1000 nm (1 μm)

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11
Q

Three types of solutions

differences?

A

true solution = dispersed molecules or particles < 1nm

Colloidal solution = molecules or particles dispersed in a medium have at least one dimension between 1 nm and 1000 nm (1 μm)

Suspensions = dispersed molecules or particles > 1um

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12
Q

colloidal can be seen by optical microscopy

T/F ?

A

False

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13
Q

Colloids can have what effects? why ?

A

Tyndall effect as particles (size) scatter light

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14
Q

Colloids has [Large/ Small?] surface area

important factor ?

A

Large surface area - almost entirely surface rather than bulk

interfacial properties are important

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15
Q

example of phase inversion in colloidal systems

A

dispersed phase and a continuous phase (X/Y)= O/W oil in water

turning cream (o/w) into butter (w/o) described as phase inversion

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16
Q

Dispersed phase- define?

may also be described as ____?

A

the phase in a two-phase system that consists of finely divided particles

discontinuous phase or internal phase

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17
Q

Different Colloidal systems (9)

A 
miscible  (G/G )
foam  (G/L)
Solid foam (G/S) 
Liquid aerosol    (L/G)
Emulsion   (L/L) 
Gel or Solid emulsion (L/S)
Solid aerosol  (S/G)
Suspension or Sol (S/L)
Solid sol (S/S)
18
Q

Food colloids (L/G)

A

Liquid aerosol : fog, mist, spray cooking oil

19
Q

Food colloids (S/G)

A

Solid aerosol : smoke, dust

20
Q

Food colloids (G/L)

A

Foam : whipped cream, beer foam

21
Q

Food colloids (L/L)

A

Emulsion : milk, mayonnaise

22
Q

Food colloids (S/L)

A

Suspension / Sol : paint, ink, detergents, gum, thick sauces (e.g. custard)

23
Q

Food colloids (G/S)

A

Solid foam :marshmallow, honeycomb, “aero” chocolate, meringue, bread

24
Q

Food colloids (L/S)

A

Gel / Solid emulsion: jelly, butter, cheese

25
Q

Food colloids (S/S)

A

Solid Sol : glass, boiled sweets

26
Q

Lyophilic colloids

A

solvent-liking
stable dispersions
form spontaneously

Macromolecular solutions, micellar solutions (association colloids)

27
Q

Lyophobic colloids

A

solvent hating
have apparent kinetic stability
thermodynamically unstable

Emulsions, foams

28
Q

Hydrosols define?
(the most common continuous phase in Hydrosols?)

Dispersed phase can be ?

Can it reform ?

A

Water is the most common continuous phase (solvent) in food systems:
Hydrosol (hydrocolloids)
Colloidal solution = Sol

Dispersed phase can be hydrophilic (i.e. lyophilic) or hydrophobic (i.e. lyophobic)

Reversible colloid may be dried/reformed
e.g. powdered milk
Non-reversible colloids do not reform

29
Q

Hydrophobic colloids

A
  • Low/no hydration
  • Usually stabilised by charge effects and non-reversible
  • Easily flocculated
  • Viscosities similar to continuous phase
  • Surface tension similar to continuous phase
30
Q

Preparing a Lyophilic systems

A

Lyophilic systems are easy (relatively) and stable

  • strong affinity between dispersed phase and continuous phase
  • simply mix and colloidal solution forms through self-assembly : e.g. gelatin, gums, starch, egg, albumin pass readily into water to form a hydrosol
  • can be precipitated and directly converted into colloidal state … reversible
31
Q

Preparing a Lyophobic systems

A

Lyophobic systems are difficult and unstable

energy barriers

32
Q

Calculate Interfacial area

20 cm^3 oil in 1 cm radius droplets
&
20 cm^3 oil in 0.1 cm radius droplets,

A

20 cm^3 oil in 1 cm radius droplets, each has:

  • volume (4/3.π.r3) = 4 cm3
  • surface area (4.π.r2) = 12.5 cm2
  • -> 5 droplets with total area of 62.5 cm2

20 cm3 oil in 0.1 cm radius droplets, each has:

  • volume = 0.004 cm3
  • surface area = 0.125 cm2
  • -> 5000 droplets with total area of 625 cm2
33
Q

More interface =

Matter at an interface has what special properties?

A

More interface = more energy

  • different orientation
  • different molecular volume
  • Surfaces have high free energy, hence are high enthalpy and low entropy (highly ordered)
34
Q

Preparation of lyophobic sols

A

usually involves “special” measures:
- Colloid mill (high shear, small gap)
10,000 rpm with 25 micron gap
- Precipitation of a true solution to give particles of colloidal dimensions

35
Q

Comment on Lyophobic Colloidal stability

A

Colloidal dispersions are thermodynamically unstable

- tendency to aggregate

36
Q

Lyophobic Colloidal Stability determined by?

A

Stability determined by interactions between particles:
~ kinetic stability – large energy barrier to aggregation
~ short range repulsive interactions sufficient to prevent bulk formation – electrostatic or steric

37
Q

Colloidal system stability depends on

A

Stability depends on the forces of interaction between dispersed particles:

~ Electrostatic – electrical double layer repulsion
~ Van der Waals (dispersion) forces
~ Osmotic (steric) forces and solvation forces

38
Q

DLVO theory

Equation

A

Deryaguin, Landau, Verway and Overbeek

Assumes that colloidal stability is as a result of total interaction energy between two particles
–> sum of attractive energy (Van der Waals) and repulsive energy (electric double layer)

Total interaction energy = Attractive interactions + Repulsive interactions:
V(T) = V(A) + V (R) –> see slide 25

39
Q

Interaction energy-distance curve

A

Slide 26

40
Q

Attractive interactions:

A

Van der Waals interactions – a volume force

molecule with bond … electrons influenced by other molecules … water has permanent dipole … other molecules induced dipoles

a) dipole/dipole
b) dipole/induced dipole
c) induced dipole/induced dipole

41
Q

Repulsive interactions

A

(Electrostatic) forces between bound surface ions – a surface force

Food Chem microstructure (8 decks)

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