surface chemistry and colloidal properties Flashcards
Importance of colloidal
characteristics & interaction
-Responsible for many of the
most important physicochemical
& organoleptic properties of food
emulsion
-Govern whether emulsion droplets, aggregate or remain as separate
entitie
-Determines the characteristics of any
aggregates formed, e.g. their size, shape, porosity and deformability
2 main interacting forces that
affect colloidal systems
van der Waals
attractive force
Electrostatic
repulsive force
When total attractive forces > total repulsive force
the emulsion is unstablr
When total repulsive forces < total attractive forces
the emulsion is stabl
van der Waals
- Originates from the dipole-dipole interactions
- The force between two droplets is always attractive
- The attractive force increases rapidly as the droplets approach
General features of van der Waals
interactions
-The force between two droplets is always
attractive
-The attractive force increases more and more rapidly as the droplets approach
-The strength of the interaction decreases with droplet separation
-The strength of the interaction depends on the thickness and composition of the adsorbed
emulsifier layer
electrostatic interactions
- Due to the presence of electrical charge on the surface of colloidal particles
- All the droplets in an emulsion are usually stabilized by the same type of emulsifier & therefore have the same electrical charge
- Repulsive force between similarly charged droplets
- Prevent droplets from coming close enough together to aggregate
General characteristics of electrostatic
interactions
-Either attractive or repulsive depending on the sign of the charges on the droplets
-Opposite charges - attractive
-Similar charges - repulsive (which is
usually the case)
-The strength of the interaction decreases
with droplet separation
-The strength of the interaction depends on the
electrical characteristics of the droplet surfaces
Electrostatic forces
Surface charge
Electrical double layer
zeta potential
dlvo theory
Source of surface charge
-Adsorption from solution of small ions
-Ionization of acidic and basic groups
-In protein stabilized emulsions, the
predominant ionizing group at the surface are carboxylic acid (CO2- ) and amines (NH3+
-Surface active polysaccharides -Gum arabic :
CO2- group might be ionized
-Ionic emulsifiers – anionic, cationic
Electrical double-layer
the combination of the charged surfave and the unequal distribution of coions and counterions near the surface
Source of Electrical double-layer
-colloidal particles gain negative electric charege when negativley charged ions of the dsipersion medium are adsorbed on the particles surface
-a negatively charged particle attracts the potitive counteerions surrounding the particle
-oppositely charged ions aka counterions, are preferentially attracted towards the surface, and ions of the samecharge tend to be repelled away
double layer sensitve to
electrolyte and temperature
An electric double layer consists of 3 parts:
- Surface charge – charged ions (commonly negative) adsorbed on the particle surface
- Stern layer - counterions (charged opposite to the surface charge) attracted to the particle surface and closely attached to it by the
electrostatic force - Diffuse layer- a film of the dispersion
medium (solvent) adjacent to the particle. Diffuse layer contains free ions with a higher concentration of the counterions. The ions of the diffuse layer are affected by the electrostatic force of the charged particle
The electrical potential within the Electric
Double Layer has
the maximum value on the particle surface (Stern layer).
The potential drops with the increase of
distance from the surface and reaches 0 at the boundary of the Electric Double Layer.
When a colloidal particle moves in the
dispersion medium,
a layer of the surrounding liquid remains attached to the particle. The boundary of this layer is called slipping plane (shear plane).
The value of the electric potential at the
slipping plane is called
Zeta potential, which is very important parameter in the theory of interaction of colloidal particles
zeta potential used to measure the
stability of emulsions and colloidal systems against aggregation
zeta potential is a measure of the
magnitude of the electrostatic or charge repulsion/attraction between particles
particles interact according to their magnitude of the zeta potential
not their surface charge
for electrostatistically stabilised dispersions,
the higher the value of zeta potential the more stable the dispersion is likely to be
particles with ZP more than 30mV
form a stable dispersion
if the particles have low ZP values, the repulsion force may not be sufficient
to overcome the van der waals attraction forve between particles promoting droplet aggregation and dispersion unstability
DLVO theory
The stability of a colloidal system is determined by the combination of the van der Waals attraction force and electrostatic repulsion (electrical double layer repulsive forces) effects existing between the particles as they approach each other by Brownian motio
Steric stabilization of colloids
is achieved by polymer molecules attached to the particle surface and forming a coating, which creates a repulsive force and separates the particle from another particl
General characteristics of steric interactions
-Steric interactions are always strongly
repulsive at short separations
-The range of steric interactions increases with the thickness of the adsorbed layer
-The strength of steric interactions depends on the precise molecular characteristics of the interfacial layer, e.g. packing, flexibility, rheology, molecular interaction
how surface tension is formed
-water molecules on the surface feel a strong force of attraction from all of the molecules under the waters surface
-the molecules at the surface are attracted to each other and hold on tightly
-this sttraction contributes to the strength of the skin
-the stronger the bonds are between the molecules in water the greater the surface tension
