TOPIC 9B Colloidal System Stability Flashcards
change in entropy
extent of disorder of system
in emulsion, it is extent of size reduction of dispersed phase / increase in number of droplets
change in enthalpy
energy required to achieve droplet size
= work required to expand interfacial area to achieve droplet size
why is emulsion formation non-spontaneous and thermodynamically unstable (emulsion theory) (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
kinetic stabiility vs thermodynamic stability
kinetic stability: rate of reaction
thermodynamic stability: spontaneity of reaction
what happens (thermodynamics) when emulsion forms
● surface area of internal phase increases
● entropy increases
● enthalpy increases
● Gibbs free energy increases
why are emulsions prone to coalescence ? (3)
● 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
emulsion theory: how to slow down coalescence of emulsion (main idea)
make emulsion kinetically stable over time period by introducing repulsive forces!
three (3) types of forces in emulsions and their impact on emulsions
● van der waals attraction (destabilises emulsion)
● electrostatic repulsion (stabilises emulsion)
● steric hindrance / repulsion (stabilises emulsion)
what affects van der waals forces and hence stability of emulsion ? how ?
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
electrostatic repulsion only stabilises what type of emulsions ?
O/W emulsions that contain ionic surfactants
how does electrostatic repulsion stabilise emulsions
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
electrical double layer consists of (3)
● core: made of hydrophobic tails and oil-phase
● stern layer: hydrophilic head groups + some counterions
● Gouy-Chapman layer: diffuse layer containing FREE-MOVING counterions
thickness of electrical double layer depends on
ionic strength of solution (greatly compressed in presence of electrolyte)
requirement for electrostatic repulsion to occur in O/W emulsion
overlap of double layer ie. particle separation becomes less than twice the double layer extension
electric potential within electrical double layer is maximum at
micelle surface
what is Zeta potential ?
electric potential at slipping / shear plane (boundary of electrical double layer)
how does absolute value of Zeta potential reflect emulsion stability
larger absolute value (ie. larger than 30mV, max. 100mV) → better dispersability of emulsion droplets → stronger repulsion → more stable emulsion
at isoelectric point, emulsion is
UNSTABLE
Zeta potential is between -30mV and +30mV
how does addition of electrolytes affect Zeta potential (4)
● 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
how does steric repulsion occur (3)
● presence of adsorbed surfactant or polymer layers
● produced by using nonionic surfactants / polymers
● heavily hydrated which prevents droplets from getting too close
emulsion destabilisation mechanisms (6)
no change in droplet size / size distribution (reversible)
● flocculation
● creaming
● sedimentation
change in droplet size / size distribution
● ostwald ripening
● coalescence
● phase inversion
flocculation occurs when
droplets aggregate into larger units due to van der waals attractions
why is flocculation reversible
interfacial film is not broken down (they are just stuck together) so re-dispersion is possible
creaming occurs when (2)
in O/W emulsion,
● external forces (usu. gravitational or centrifugal) causes resultant upward velocity
● oil-rich region pulled towards top surface of emulsion
sedimentation occurs when (2)
in W/O emulsion,
● external forces (usu. gravitational or centrifugal) causes resultant downwards velocity
● water-rich region is pulled towards bottom of emulsion
eventually, creaming and sedimentation result in
coalescence (which is irreversible)
coalescence occurs when (2)
● 2 or more droplets / particles fuse into larger ones
● film-thinning and disruption of interfacial film reduces interfacial area
criteria (2) for Ostwald ripening to occur
- 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)
- emulsion droplets are polydispersed ie. different droplet sizes
why does Ostwald ripening occur
● 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
why do cosmetics rarely have Ostwald ripening
emollient system usually consists of a mixture of emollients with different solubility, which decreases solubility of emollient system in continuous phase
Ostwald ripening eventually results in
creaming / sedimentation due to gravitational force
(which eventually results in coalescence)
how to improve stability of emulsions (Stokes equation) (3)
● 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
how to prevent Ostwald ripening (2)
● use mixture of emollients with different solubilities
● homogenise at high speed or pressure
how do co-emulsifiers help stabilise emulsion
● helps stabilise oil/water interface
● adds body (viscosity) and helps prevent coalescence
