Physical Pharmacy 21 Stability of Colloids Kausar Ahmad Kulliyyah of Pharmacy, IIUM

Physical Pharmacy 2 1

Stability of Colloids

Kausar Ahmad

Kulliyyah of Pharmacy, IIUM

http://staff.iiu.edu.my/akausar

http://staff.iiu.edu.my/akausar


ContentsLecture 11) Non-ionic SAA and Phase Inversion Temperature 2) Stabilisation factors

Electrical stabilisation Steric stabilisation

Finely divided solids Liquid crystalline phases

Lecture 23) Destabilisation factors

Compression of electrical double layer Addition of electrolytes Addition of oppositely charged particles Addition of anions

4) Effect of viscosity


Phase Inversion Temperature

PIT, or Emulsion Inversion Point (EIP), is a

characteristic property of an emulsion (not surfactant

molecule in isolation).

At PIT, the hydrophile-lipophile property of non-ionic

surfactant just balances.

If temperature >> PIT, emulsion becomes unstable

because the surfactant reaches the cloud point


Cloud Point

Definition - The temperature at which the SAA precipitates.

Common for non-ionic SAA.

As temperature increases, solubility of the POE chain decreases i.e. hydration of the ether linkage is destroyed.

Hydration of POE is most favourable at low temperature.

For the same type of SAA, cloud point depends on length of POE.

PIT Factor – Cloud point

the higher the cloud point in aqueous surfactant solution, the higher the PIT.

This coincides with Bancroft’s rule that the phase in which the emulsifier is more soluble will be the external phase at a definite temperature.



PIT Factor – Type of oil

the more soluble the oil for a non-ionic emulsifier,

the lower the PIT.

e.g. at 20oC, POE nonylphenylether (HLB=9.6) dissolves

well in benzene, but not in hexadecane or liquid paraffin.

The PIT was ca. 110oC compared to only 20oC for

benzene with 10% w/w of the emulsifier.

-


PIT Factor - Length of oxyethylene chain

the longer the chain length, the higher the PIT

e.g. in benzene-in-water emulsions, the PIT increased

as the chain length increased


PIT Factor - Surfactant mixtures

when stabilised by a mixture of surfactants, the

PIT increased compared to the expected PIT from

single surfactant.

e.g. in heptane-in-water emulsion, blending POE

nonylphenyl ether having HLB of 15.8 and 7.4 resulted

in a higher PIT.


PIT Factor - Salts, acids and alkalis

Increase in concentration of salt will decrease PIT

of o/w emulsion.

e.g. PIT of cyclohexane-in-water emulsion

NaCl (N) PIT of o/w (C)

0 75

1.2 50


PIT Factor - Additives in oil

in the presence of fatty acids or alcohols, the PIT

of both o/w & w/o emulsions decreases as the

concentration of these additives increases,

regardless of the chain length of the additives.

e.g. lauric/myristic/palmitic/stearic acids in liquid

paraffin-in-water emulsion

Acid (mol/kg) PIT (C)

0 100

0.25 30


FORCES OF INTERACTION between colloidal particles

Electrostatic forces of repulsion Van der waals forces of attraction

Born forces – short-range, repulsive force

Steric forces – depends on geometry of molecules adsorbed at particle interface

Solvation forces – due to change in quantities of adsorbed solvent for close particles.


Electrical theories of emulsion stability

Charges can arise from:

1. Ionisation

2. Adsorption

The electrical charge on a droplet arises from

the adsorbed surfactant at the interface.

3. Frictional contact


Charges arising from frictional contact

For a charge that arises from frictional contact, the

empirical rule of Coehn states that:

substance having a high dielectric constant (d.c.) is

positively charged when in contact with another

substance having a lower dielectric constant.

E.g. most o/w emulsions stabilised by non-ionic

surfactants are negatively charged – because water

has a higher d.c. than oil droplets. At 25oC and 1 atm,

the d.c. or relative permittivity for water is 78.5; for

benzene ca. 2.5.


Electrical stabilisation

The presence of the charges on the droplets/particle causes mutual repulsion of the charged particles.

This prevents close approach i.e. coalescence, followed by coagulation, which leads to breaking of an emulsion

Aggregation of solids


Stabilisation of emulsions by SOLIDS

The first observations on emulsions stabilised by solids

were made by Pickering.

Basic sulfates of iron, copper, nickel, zinc and

aluminum in moist conditions act as efficient

dispersing agents for the formation of petroleum o/w

emulsion

The DRY calcium carbonate can also promote

emulsification but emulsion not stable.


Emulsion formation with solids

Briggs observed formation of

o/w emulsion with kerosene/benzene and ferric

hydroxide, arsenic sulfide and silica

w/o emulsions were produced with carbon black

and lanolin

Weston produced o/w and w/o emulsions with clay.


Adsorption of solids at interface The ability of solids to concentrate at the

boundary is a result of: wo > sw + so

The most stable emulsions are obtained when the

contact angle with the solid at the interface is near

90o.

A concentration of solids at the interface represents

an interfacial film of considerable strength and

stability (compare with liquid crystal!)

Physical Pharmacy 2

End of lecture 1/2 18

Stabilisation by Liquid Crystalline Phases

Emulsion stability increases as a result of:

1. Protection given by the multilayer against

coalescence due to Van der Waals forces of

attraction.

2. Prevent thinning of the films of approaching

droplets.

These are achieved due to the high viscosity of the

liquid crystalline phases compared to that of the

continuous phase.


Destabilisation of Colloids

Emulsions

Suspensions

Hydrophilic colloid?

CreamingPhase separationDemulsificationOstwald ripeningHeterocoagulationFlocculationCoalescenceCakingAggregation


Demulsification

By physico-chemical method

Compression of double layer

Add polyelectrolytes, multivalent cations.

add emulsion/dispersion with particles of

opposite charge - HETEROCOAGULATION


Effect of polyelectrolyte

Schulze-Hardy Rule states that

The valence of the ions having a charge opposite to

that of the dispersed particles determines the

effectiveness of the electrolytes in coagulating the

colloids: suspensions or emulsions.

Thus, presence of divalent or trivalent ions should be

avoided.

Preparation should use distilled water, double

distilled water, reverse osmosis or ion-exchange

water (soft water).


Ostwald Ripening

If oil droplets have some solubility in water.

The extent of Ostwald ripening depends on the

difference in the size of the oil droplets.

The larger the particle size distribution, the greater

the possibility of Ostwald ripening.


Mechanism of Ostwald Ripening

Oil molecule absorbed by big droplet

Oil molecule diffused out of small droplet


Oil droplets in aqueous medium

coalescence

Polydisperse sample

Non-spherical

spherical


Destabilisation scheme

From Florence & Attwood

Rupture of interfacial film

Interfacial film intact

Bridging flocculation


Separation of phases in o/w emulsions

Withouthomogenisation Without

surfactant

With 10% surfactantHomogenisation for 30 min

BREAKING OF EMULSION


Destabilisation of Multiple Emulsion

For w/o/w: Coalescence of internal water droplets.

Coalescence of oil droplets.

Rupture of oil film separating internal and external aqueous phases.

Diffusion of internal water droplets through the oil phase to the external aqueous phase resulting in shrinkage.


Destabilisation of hydrophilic colloid

Due to mainly

Depletion of water molecules

when the colloid is contaminated with alcohol

Evaporation of water

Addition of anion


Destabilisation of Hydrophilic Sols by Anions

Hofmeister (or lyotropic series): in decreasing order of precipitating power

citratetartratesulfate acetatechloride nitrate bromideiodide.


Destabilisation of suspensions

Caking

• as a result of sedimentation• difficult to re-disperse.

Flocculation

• cluster of particles held together in loose open structure (flocs)• Presence of flocs increases the rate of sedimentation.• BUT re-disperse easily.

Particle growth

• through dissolution and crystallisation.


Minimising Creaming/Sedimentation/Caking

Addition of viscosity modifiers

Carboxymethylcellulose (CMC)Aluminium magnesium silicateSodium alginateSodium starchPolymer

Mechanism of their operation:

1) Adsoption onto the surface of particles2) Increasing the viscosity of medium3) Bridging

Effect of viscosity

Stoke’s Law

The velocity u of sedimentation of spherical particles of radius r

having a density r in a medium of density ro &

a viscosity ho

& influenced by gravity g is

u = 2r2(r – ro)g / 9ho

Forces acting on particles


Gravity

Brownian movement2-5 μm


Viscosity modifier fornon-aqueous suspension

E.g. amorphous silica for ointments

Aerosil at 8-10% to give a paste.

The increase in viscosity resulted from hydrogen

bonding between the silica particles and oils: peanut oil,

isopropyl myristate.


Role of polymers in the stabilisation of dispersions

provides steric stabilization.

minimise sedimentation

Flocculation

Because of the ability to adsorb, polymers are used as

flocculating agent by

promoting inter-particle bridging

BUT, at high concentration of polymers, the polymers will

coat the particles (and increase the stability). No floc!

With agitation the flocs are destroyed.

Thus caking may result.



Flocculating agent Polyacrylamide (30% hydrolysed)

an anionic polymer which can induce flocculation in

numerous system such as silica sols and kaolinite at very low

concentrations.

Application

only 5 ppm of polyacrylamide is required to flocculate 3%

w/w silica sol.

Restabilisation of the colloid occurs when the dosage of

polymer exceeds the requirement.


Definition - Gel Formation

When the particles aggregate to form a continuous

network structure which extends throughout the available

volume and immobilise the dispersion medium, the

resulting semi-solid system is called a gel.

The rigidity of a gel depends on the number and the

strength of the inter-particle links in this continuous

structure.


References

PC Hiemenz & Raj Rajagopalan, Principles of Colloid and Surface

Chemistry, Marcel Dekker, New York (1997)

HA Lieberman, MM Rieger & GS Banker, Pharmaceutical Dosage

Forms: Disperse Systems Volume 1, Marcel Dekker, New York (1996)

F Nielloud & G Marti-Mestres, Pharmaceutical Emulsions and

Suspensions, Marcel Dekker, New York (2000)

J Kreuter (ed.), Colloidal Drug Delivery Systems, Marcel Dekker,

New York (1994)

http://www.chemistry.nmsu.edu/studntres/chem435/Lab14/double_l

ayer.html

http://www.chemistry.nmsu.edu/studntres/chem435/Lab14/double_layer.html

http://www.chemistry.nmsu.edu/studntres/chem435/Lab14/double_layer.html

Documents

Physical Pharmacy 21 Stability of Colloids Kausar Ahmad Kulliyyah of Pharmacy, IIUM