STABILITY OF COLLOIDS

Kausar Ahmadhttp://staff.iium.edu.my/akausar

akausar@iium.edu.my

Physical Pharmacy 2 1

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

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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

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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.

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PIT FACTOR

Cloud point of SAA

Type of oil

Length of oxyethylene

chain

Surfactant system

Salt, acid, alkali,

additives

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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.

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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.-

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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

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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.

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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

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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

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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.

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ELECTRICAL THEORIES OF EMULSION STABILITYCh

arge

s ca

n ar

ise

from

:

Ionisation

AdsorptionThe electrical charge on a droplet arises from the adsorbed surfactant at

the interface.

Frictional contact

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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.

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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

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STABILISATION OF EMULSIONS BY SOLIDSEmulsion Type 0/W W/OPetroleum(Pickering)

Hydrated sulfates of iron, copper, nickel, zinc &

aluminumKerosene/benzene (Briggs)

ferric hydroxide, arsenic sulfide &

silica Oil phase? (Briggs)

carbon black & lanolin

Oil phase? (Weston)

Clay Clay

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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!)

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STABILISATION BY LIQUID CRYSTALLINE PHASES

• Emulsion stability increases as a result of:– Protection given by the multilayer liquid crystalline phase

against coalescence (coalescence due to Van der Waals forces of attraction).• The multilayer adsorbed at the interface prevents

thinning of the interfacial films of approaching droplets.

• These are achieved due to the high viscosity of the liquid crystalline phases compared to that of the continuous phase.

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DESTABILISATION OF COLLOIDS

• Emulsions

• Suspensions

• Hydrophilic colloid?

• Creaming• Phase separation• Demulsification• Ostwald ripening• Heterocoagulation• Flocculation• Coalescence• Caking• Aggregation

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DEMULSIFICATION

• By physico-chemical method - Compression of Electrical Double Layer

• Add polyelectrolytes, multivalent cations.

• Add emulsion/dispersion with particles of opposite charge - HETEROCOAGULATION

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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).

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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.

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MECHANISM OF OSTWALD RIPENING

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Oil molecule absorbed by big droplet

Oil molecule diffused out of small droplet

OIL DROPLETS IN AQUEOUS MEDIUM

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coalescence

POLYDISPERSE SAMPLES

DESTABILISATION SCHEME

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From Florence & Attwood

Rupture of interfacial film

Interfacial film intact

Bridging flocculation

SEPARATION OF PHASES IN O/W EMULSIONS

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Withouthomogenisation Without

surfactant

With 10% surfactant &Homogenisation for 30 min

BREAKING OF EMULSION

DESTABILISATION OF MULTIPLE EMULSION

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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 mainly to depletion of water molecules

– when the colloid is contaminated with alcohol

– Evaporation of water

– Addition of anion

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Destabilisation of Hydrophilic Sols by Anions

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

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citratetartratesulfate acetate

chloride nitrate

bromideiodide

DESTABILISATION OF SUSPENSIONS

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

Caking

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

Flocculation

• through dissolution and crystallisation.

Particle growth

MINIMISING CREAMING/SEDIMENTATION/CAKING

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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 VISCOSITYStoke’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

Force acting on particles

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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.

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ROLE OF POLYMERS IN THE STABILISATION OF DISPERSIONS

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Addition of polymeric surfactant

adsorption of the polymer onto the particle surface

provides steric stabilization.

increase viscosity of medium

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.

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FLOCCULATING AGENT

E.g. Polyacrylamide (30% hydrolysed) • an anionic polymer

E.g. 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.

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GEL FORMATION

• When 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.

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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_laye

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