Interfaces Flashcards
interface formed by
Ex: (4)
Formed by two phases that are mutually insoluble (immiscible)
G/G S/L
L/L G/S
Gas/liquid interface formed while _____ existed
surface tension
–> liquid interface with gas (e.g. air and water) = surface tension
= unbalanced forces at the surface of molecules
attracted inwards
surface tension or interfacial tension
- -> lead to ____?
- -> require ____ to overcome?
- -> Units?
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)
What is require to increase surface area
Equation?
Work / Energy
ΔE = γΔA
Continuous phase define?
can also called ?
That phase in a two or more phase mixture which is continuously interconnected
dispersing medium or external phase
surface free energy is
Units?
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
Energy (ΔE) required to increase surface area (ΔA) of
liquid interface with gas (e.g. air and water)
surface tension
Energy (ΔE) required to increase surface area (ΔA) of liquid interface with a solid surface (e.g. water droplet on glass)
= surface free energy
Energy (ΔE) required to increase surface area (ΔA) of interface between immiscible liquids (e.g. oil and water)
interfacial tension
Colloid aka ?
define
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)
Three types of solutions
differences?
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
colloidal can be seen by optical microscopy
T/F ?
False
Colloids can have what effects? why ?
Tyndall effect as particles (size) scatter light
Colloids has [Large/ Small?] surface area
important factor ?
Large surface area - almost entirely surface rather than bulk
interfacial properties are important
example of phase inversion in colloidal systems
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
Dispersed phase- define?
may also be described as ____?
the phase in a two-phase system that consists of finely divided particles
discontinuous phase or internal phase
Different Colloidal systems (9)
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)
Food colloids (L/G)
Liquid aerosol : fog, mist, spray cooking oil
Food colloids (S/G)
Solid aerosol : smoke, dust
Food colloids (G/L)
Foam : whipped cream, beer foam
Food colloids (L/L)
Emulsion : milk, mayonnaise
Food colloids (S/L)
Suspension / Sol : paint, ink, detergents, gum, thick sauces (e.g. custard)
Food colloids (G/S)
Solid foam :marshmallow, honeycomb, “aero” chocolate, meringue, bread
Food colloids (L/S)
Gel / Solid emulsion: jelly, butter, cheese
Food colloids (S/S)
Solid Sol : glass, boiled sweets
Lyophilic colloids
solvent-liking
stable dispersions
form spontaneously
Macromolecular solutions, micellar solutions (association colloids)
Lyophobic colloids
solvent hating
have apparent kinetic stability
thermodynamically unstable
Emulsions, foams
Hydrosols define?
(the most common continuous phase in Hydrosols?)
Dispersed phase can be ?
Can it reform ?
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
Hydrophobic colloids
- Low/no hydration
- Usually stabilised by charge effects and non-reversible
- Easily flocculated
- Viscosities similar to continuous phase
- Surface tension similar to continuous phase
Preparing a Lyophilic systems
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
Preparing a Lyophobic systems
Lyophobic systems are difficult and unstable
energy barriers
Calculate Interfacial area
20 cm^3 oil in 1 cm radius droplets
&
20 cm^3 oil in 0.1 cm radius droplets,
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
More interface =
Matter at an interface has what special properties?
More interface = more energy
- different orientation
- different molecular volume
- Surfaces have high free energy, hence are high enthalpy and low entropy (highly ordered)
Preparation of lyophobic sols
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
Comment on Lyophobic Colloidal stability
Colloidal dispersions are thermodynamically unstable
- tendency to aggregate
Lyophobic Colloidal Stability determined by?
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
Colloidal system stability depends on
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
DLVO theory
Equation
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
Interaction energy-distance curve
Slide 26
Attractive interactions:
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
Repulsive interactions
(Electrostatic) forces between bound surface ions – a surface force
