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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
Physical Pharmacy 2 2
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
Physical Pharmacy 2 3
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.
Physical Pharmacy 2 4
PIT FACTOR
Cloud point of SAA
Type of oil
Length of oxyethylene
chain
Surfactant system
Salt, acid, alkali,
additives
Physical Pharmacy 2 5
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.
Physical Pharmacy 2 6
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.-
Physical Pharmacy 2 7
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
Physical Pharmacy 2 8
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.
Physical Pharmacy 2 9
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
Physical Pharmacy 2 10
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
Physical Pharmacy 2 11
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.
Physical Pharmacy 2 12
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
Physical Pharmacy 2 13
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.
Physical Pharmacy 2 14
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
Physical Pharmacy 2 15
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
Physical Pharmacy 2 16
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 17
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.
Physical Pharmacy 2
End of lecture 1/2 18
DESTABILISATION OF COLLOIDS
• Emulsions
• Suspensions
• Hydrophilic colloid?
• Creaming• Phase separation• Demulsification• Ostwald ripening• Heterocoagulation• Flocculation• Coalescence• Caking• Aggregation
Physical Pharmacy 2 19
DEMULSIFICATION
• By physico-chemical method - Compression of Electrical Double Layer
• Add polyelectrolytes, multivalent cations.
• Add emulsion/dispersion with particles of opposite charge - HETEROCOAGULATION
Physical Pharmacy 2 20
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).
Physical Pharmacy 2 21
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.
Physical Pharmacy 2 22
MECHANISM OF OSTWALD RIPENING
Physical Pharmacy 2 23
Oil molecule absorbed by big droplet
Oil molecule diffused out of small droplet
OIL DROPLETS IN AQUEOUS MEDIUM
Physical Pharmacy 2 24
coalescence
POLYDISPERSE SAMPLES
DESTABILISATION SCHEME
Physical Pharmacy 2 25
From Florence & Attwood
Rupture of interfacial film
Interfacial film intact
Bridging flocculation
SEPARATION OF PHASES IN O/W EMULSIONS
Physical Pharmacy 2 26
Withouthomogenisation Without
surfactant
With 10% surfactant &Homogenisation for 30 min
BREAKING OF EMULSION
DESTABILISATION OF MULTIPLE EMULSION
Physical Pharmacy 2 27
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
Physical Pharmacy 2 28
Destabilisation of Hydrophilic Sols by Anions
• Hofmeister (or lyotropic series): in decreasing order of precipitating power
Physical Pharmacy 2 29
citratetartratesulfate acetate
chloride nitrate
bromideiodide
DESTABILISATION OF SUSPENSIONS
Physical Pharmacy 2 30
• 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
Physical Pharmacy 2 31
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
Physical Pharmacy 2 32
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.
Physical Pharmacy 2 33
ROLE OF POLYMERS IN THE STABILISATION OF DISPERSIONS
Physical Pharmacy 2 34
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.
Physical Pharmacy 2 35
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.
Physical Pharmacy 2 36
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.
Physical Pharmacy 2 37
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
r.htmlPhysical Pharmacy 2 38
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