TOPIC 9B Colloidal System Stability

Question 1

change in entropy

Answer 1

extent of disorder of system
in emulsion, it is extent of size reduction of dispersed phase / increase in number of droplets

Question 2

change in enthalpy

Answer 2

energy required to achieve droplet size
= work required to expand interfacial area to achieve droplet size

Question 3

why is emulsion formation non-spontaneous and thermodynamically unstable (emulsion theory) (3)

Answer 3

● change in Gibbs free energy , ΔG = γΔA - TΔS
● large increase in surface area ΔA from non-dispersed system to droplets
● thus, γΔA much larger than TΔS so ΔG is positive

Question 4

kinetic stabiility vs thermodynamic stability

Answer 4

kinetic stability: rate of reaction
thermodynamic stability: spontaneity of reaction

Question 5

what happens (thermodynamics) when emulsion forms

Answer 5

● surface area of internal phase increases
● entropy increases
● enthalpy increases
● Gibbs free energy increases

Question 6

why are emulsions prone to coalescence ? (3)

Answer 6

● emulsion formation is non-spontaneous, hence emulsions are thermodynamically unstable
● surface area (A) of dispersed phase increases during emulsion formation, but has tendency to decrease again due to surface tension
● ΔA decreases and ΔG = γΔA - TΔS decreases, so emulsion eventually becomes unstable and returns to non-dispersed phase

Question 7

emulsion theory: how to slow down coalescence of emulsion (main idea)

Answer 7

make emulsion kinetically stable over time period by introducing repulsive forces!

Question 8

three (3) types of forces in emulsions and their impact on emulsions

Answer 8

● van der waals attraction (destabilises emulsion)
● electrostatic repulsion (stabilises emulsion)
● steric hindrance / repulsion (stabilises emulsion)

Question 9

what affects van der waals forces and hence stability of emulsion ? how ?

Answer 9

proximity between droplets
G(A) = - AR / 12h
G(A) increases with
● increasing radius (R)
● decreasing separation distance (h): except for below born repulsion distance where it reverts to repulsion

this causes aggregation / clumping and destabilises colloidal system

Question 10

electrostatic repulsion only stabilises what type of emulsions ?

Answer 10

O/W emulsions that contain ionic surfactants

Question 11

how does electrostatic repulsion stabilise emulsions

Answer 11

NOTE: only for O/W that use ionic surfactants

● ionic surfactants impart charged interface, forming electrical double layer
● like charges repel, so droplets repel each other

Question 12

electrical double layer consists of (3)

Answer 12

● core: made of hydrophobic tails and oil-phase
● stern layer: hydrophilic head groups + some counterions
● Gouy-Chapman layer: diffuse layer containing FREE-MOVING counterions

Question 13

thickness of electrical double layer depends on

Answer 13

ionic strength of solution (greatly compressed in presence of electrolyte)

Question 14

requirement for electrostatic repulsion to occur in O/W emulsion

Answer 14

overlap of double layer ie. particle separation becomes less than twice the double layer extension

Question 15

electric potential within electrical double layer is maximum at

Answer 15

micelle surface

Question 16

what is Zeta potential ?

Answer 16

electric potential at slipping / shear plane (boundary of electrical double layer)

Question 17

how does absolute value of Zeta potential reflect emulsion stability

Answer 17

larger absolute value (ie. larger than 30mV, max. 100mV) → better dispersability of emulsion droplets → stronger repulsion → more stable emulsion

Question 18

at isoelectric point, emulsion is

Answer 18

UNSTABLE
Zeta potential is between -30mV and +30mV

Question 19

how does addition of electrolytes affect Zeta potential (4)

Answer 19

● ionic strength of continuous water phase increased
● accumulation of counter-ions near charged surface
● particles charges are screened and EDL becomes thinner
● reduced Zeta potential → reduced electrostatic repulsion between droplets → reduced stability of emulsion

Question 20

how does steric repulsion occur (3)

Answer 20

● presence of adsorbed surfactant or polymer layers
● produced by using nonionic surfactants / polymers
● heavily hydrated which prevents droplets from getting too close

Question 21

emulsion destabilisation mechanisms (6)

Answer 21

no change in droplet size / size distribution (reversible)
● flocculation
● creaming
● sedimentation

change in droplet size / size distribution
● ostwald ripening
● coalescence
● phase inversion

Question 22

flocculation occurs when

Answer 22

droplets aggregate into larger units due to van der waals attractions

Question 23

why is flocculation reversible

Answer 23

interfacial film is not broken down (they are just stuck together) so re-dispersion is possible

Question 24

creaming occurs when (2)

Answer 24

in O/W emulsion,
● external forces (usu. gravitational or centrifugal) causes resultant upward velocity
● oil-rich region pulled towards top surface of emulsion

Question 25

sedimentation occurs when (2)

Answer 25

in W/O emulsion,
● external forces (usu. gravitational or centrifugal) causes resultant downwards velocity
● water-rich region is pulled towards bottom of emulsion

Question 26

eventually, creaming and sedimentation result in

Answer 26

coalescence (which is irreversible)

Question 27

coalescence occurs when (2)

Answer 27

● 2 or more droplets / particles fuse into larger ones
● film-thinning and disruption of interfacial film reduces interfacial area

Question 28

criteria (2) for Ostwald ripening to occur

Answer 28

1. dispersed phase needs to have a certain solubility (can be low but not negligible) in continuous phase (higher solubility results in higher Ostwald ripening, more significant when only one emollient present)
2. emulsion droplets are polydispersed ie. different droplet sizes

Question 29

why does Ostwald ripening occur

Answer 29

● ΔP = 2γ / r: smaller droplets have higher Laplace pressure (ΔP: difference in pressure between inside and outside) compared to larger droplets
● higher Laplace pressure (ΔP) results in higher solubility of smaller droplets in continuous phase
● smaller droplets disappear as they diffuse to the bulk and deposit on larger droplets which have decreasing ΔP

Question 30

why do cosmetics rarely have Ostwald ripening

Answer 30

emollient system usually consists of a mixture of emollients with different solubility, which decreases solubility of emollient system in continuous phase

Question 31

Ostwald ripening eventually results in

Answer 31

creaming / sedimentation due to gravitational force
(which eventually results in coalescence)

Question 32

how to improve stability of emulsions (Stokes equation) (3)

Answer 32

● make dispersed phase particles very small (ie. high pressure / speed homogenisation)
● reduce density difference between dispersed and continuous phase
● raise viscosity of external phase (decreases creaming/sedimentation rate) by adding thickeners for O/W and adding high vis emollients / paste / wax for W/O

Question 33

how to prevent Ostwald ripening (2)

Answer 33

● use mixture of emollients with different solubilities
● homogenise at high speed or pressure

Question 34

how do co-emulsifiers help stabilise emulsion

Answer 34

● helps stabilise oil/water interface
● adds body (viscosity) and helps prevent coalescence