surface chemistry and colloidal properties

Question 1

Importance of colloidal
characteristics & interaction

Answer 1

-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

Question 2

2 main interacting forces that
affect colloidal systems

Answer 2

van der Waals
attractive force
Electrostatic
repulsive force

Question 3

When total attractive forces > total repulsive force

Answer 3

the emulsion is unstablr

Question 4

When total repulsive forces < total attractive forces

Answer 4

the emulsion is stabl

Question 5

van der Waals

Answer 5

• Originates from the dipole-dipole interactions
• The force between two droplets is always attractive
• The attractive force increases rapidly as the droplets approach

Question 6

General features of van der Waals
interactions

Answer 6

-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

Question 7

electrostatic interactions

Answer 7

• 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

Question 8

General characteristics of electrostatic
interactions

Answer 8

-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

Question 9

Electrostatic forces

Answer 9

Surface charge
Electrical double layer
zeta potential
dlvo theory

Question 10

Source of surface charge

Answer 10

-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

Question 11

Electrical double-layer

Answer 11

the combination of the charged surfave and the unequal distribution of coions and counterions near the surface

Question 12

Source of Electrical double-layer

Answer 12

-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

Question 13

double layer sensitve to

Answer 13

electrolyte and temperature

Question 14

An electric double layer consists of 3 parts:

Answer 14

• 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

Question 15

The electrical potential within the Electric
Double Layer has

Answer 15

the maximum value on the particle surface (Stern layer).

Question 16

The potential drops with the increase of

Answer 16

distance from the surface and reaches 0 at the boundary of the Electric Double Layer.

Question 17

When a colloidal particle moves in the
dispersion medium,

Answer 17

a layer of the surrounding liquid remains attached to the particle. The boundary of this layer is called slipping plane (shear plane).

Question 18

The value of the electric potential at the
slipping plane is called

Answer 18

Zeta potential, which is very important parameter in the theory of interaction of colloidal particles

Question 19

zeta potential used to measure the

Answer 19

stability of emulsions and colloidal systems against aggregation

Question 20

zeta potential is a measure of the

Answer 20

magnitude of the electrostatic or charge repulsion/attraction between particles

Question 21

particles interact according to their magnitude of the zeta potential

Answer 21

not their surface charge

Question 22

for electrostatistically stabilised dispersions,

Answer 22

the higher the value of zeta potential the more stable the dispersion is likely to be

Question 23

particles with ZP more than 30mV

Answer 23

form a stable dispersion

Question 24

if the particles have low ZP values, the repulsion force may not be sufficient

Answer 24

to overcome the van der waals attraction forve between particles promoting droplet aggregation and dispersion unstability

Question 25

DLVO theory

Answer 25

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

Question 26

Steric stabilization of colloids

Answer 26

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

Question 27

General characteristics of steric interactions

Answer 27

-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

Question 28

how surface tension is formed

Answer 28

-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