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PSF > Emulsions > Flashcards

Emulsions Flashcards

1
Q

Define emulsion

A

An emulsion is a disperse system where atleast one liquid is dispersed in another immiscible liquid as droplets

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2
Q

Is an emulsion thermodynamically stable or unstable?

A

It is thermodynamically unstable due to the interfacial energy associated with the huge interfacial area. All emulsions are doomed to break – it is just a matter of time

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3
Q

Typical droplet sizes of emulsions

A

0.1–100μm, though some as small as 0.01μm or as large as 1000 μm

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4
Q

What can emulsions be used for

A

oral, topical and parenteral applications

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5
Q

Submicron emulsions

A

up to 500nm, milky white emulsions. E.g. Intralipid®, Diprivan®.

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6
Q

Nanoemulsions

A

Clear or translucent emulsions, <200nm (0.2μm)

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7
Q

Microemulsions

A

Strictly speaking these are not emulsions. They are single-phase, transparent systems. They represents a intermediate state between solubilised systems and emulsions. There are different understandings on this topic

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8
Q

Emulsion types

A 
Oil-in-water(o/w):
• Water-in-oil(w/o)
• Oil-in-water-in-oil(o/w/o)
• Water-in-oil-in-water(w/o/w)
• For oral use:o/w(viscosity, aqueous feel)
• For external use: o/w or w/o
• For oral application: o/w
•For i.v. application: o/w, average diameter <1μm. No droplets > 5 μm.
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9
Q

which should have a

higher conductivity, o/w or w/o?

A

o/w

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10
Q

Theories of emulsification - by adding surface active agents ..

A

by adding surface active agents, the surface energy and interfacial energy can be reduced and stability is increased

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11
Q

Theories of emulsification - interfacial film strength

A

emulsifiers can form a stable film between the oil droplets and the dispersion medium, which stabilises emulsion

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12
Q

Zeta potential

A

– charge repulsion

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13
Q

Interfacial energy

A

E=γA

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14
Q

What are the 2 types of interfacial films?

A

monolayer and multilayer films

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15
Q

What are monolayer films?

A 
Monolayer films (more important) – surface active agents, e.g. Span 80 and Tween 80
– Surface tension is usually reduced, so is interfacial energy
– A mixture of emulsifiers is usually better than a single emulsifier
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16
Q

What are multilayer films?

A

hydrophilic colloids (some naturally occurring polymers), e.g. acacia gum

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17
Q

Are all emulsifiers surface active agents?

A

Not all emulsifiers are SAA.

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18
Q

What is a common feature of surfactants.

A

must have a hydrophilic section and a lipophilic (hydrophobic) section in the same molecule

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19
Q

What is not easily soluble in water and shows surface activity?

A

chain of carbons is not easily water soluble, the longer the carbon chain, the more lipophilic the molecule is.
CH3(CH2)14OH

20
Q

Define HLB

A

Hydrophile-lipophile balance.

First designated by Griffin to indicate how hydrophilic or lipophilic an surfactant is

21
Q

Low HLB is

A

lipophilic eg. oleic acid

22
Q

High HLB is

A

hydrophilic eg. potassium oleate

23
Q

Which phase is the external phase

A

The phase in which the emulsifiers have higher solubility tends to be the external phase. i.e.
– hydrophilic emulsifiers, o/w emulsions
– Hydrophobic emulsifiers, w/o emulsions

24
Q

Emulsifiers with HLB 4-6

A

w/o emulsions

25
Q

Emulsifiers with HLB 8-16

A

o/w emulsions

more soluble in water

26
Q

Required HLB of an oil phase for producing stable emulsions

A

Different oils require different HLB of emulsifiers for the best stability of emulsions produced
Note: the required HLB of an oil is the HLB of the emulsifiers required by the oil to form a stable emulsion. A typical oil does not have a HLB

27
Q

The HLB of a mixture of non-ionisable surfactants A and B is given by

A

(HLB )m =

xa (HLB )a + xb (HLB )b / xa+xb

28
Q

Where xa is the quantity of surfactant A, and xb is the quantity of surfactant B, (HLB)a and (HLB)b are the HLB values of A and B, (HLB)m is the HLB of the mixture

A

(HLB)m = xa / xa +xb (HLB)a + xb / xa +xb (HLB)b

(HLB)m = A% x (HLB)a + B% x (HLB)b

A% + B% = 100%

29
Q

How to work out A% and B% to achieve a required (HLB)m?

A

A% = (HLB)m − (HLB)b / (HLB)a −(HLB)b
(REMEMBER THIS)
B% = 100% - A%

30
Q

Types of surfactants

A

• Anionicsurfactants
– Soaps, e.g. sodium stearate
– Sulfated and sulphonated compounds, e.g. sodium dodecyl sulfate
• Cationic surfactants
– Cetrimide (cetyl trimethylammonium bromide)
• Non-ionic surfactants
– Glycol and glycerol esters
– Sorbitan esters(Spanseries,Span60,Span80)
– Polysorbates (Tween series, Tween 80= polysorbate 80)
– Fatty alcohol polyglycol ethers
– Poloxamers (Pluronic Series, Pluronic F68= poloxamer 188)
• Amphoteric surfactants,e.g.lecithin
• Naturally occurring polymers, e.g. Acacia
• Semi synthetic polysaccharides, e.g. cellulose derivatives, methyl cellulose

31
Q

Preparation of emulsions

A
  • Mechanical mixing device, e.g. Silverson homogeniser
  • High shear homogeniser – liquid samples passes through narrow gaps under high pressure
  • Microfluidizer – split feed into twin jets and collide at supersonic speed
  • Ultrasonication – for laboratory and small samples only
32
Q

Why is work required in prep of emulsions

A

Work is required due to the increased interfacial energy

33
Q

Define creaming (physical stability)

A

Emulsion droplets moves up to the top of the emulsion and form a droplet-rich layer (cream). When the oil layer forms on the top
– The droplets remain independent (separate)
– Redispersible
– May eventually lead to the breaking of emulsions

34
Q

How to avoid creaming

A
  • Reduction of droplet size. The creaming velocity will be reduced according to Stokes’ law. For the same formulation, use a more powerful homogenisation device generate relatively more stable emulsions
  • Small particles have stronger Brownian motion,which helps the droplets remain dispersed
  • Control viscosity–this is possible for oral emulsions.
  • Screen for best formulations
35
Q

Factors leading to flocculation (physical stability)

A

– Charge reduction by the addition of electrolytes (compression of diffuse layer)
– Charge neutralisation by oppositely charged ions, e.g. Ca2+ or cationic polymers (for negatively charged emulsion droplets)
– Bridge flocculation by added polymers (refer to lecture slides on colloidal dispersions)

36
Q

How to avoid flocculation

A
  • Avoid the addition of charge reducing agents and
    maintain sufficient zeta potential
  • Steric stabilisation
37
Q

What is flocculation

A

Flocculation is the prelude of coalescence

38
Q

Define coalescence and breaking (cracking) - (physical stability)

A
  • Coalescence – refers to the process that two or more droplets merge together to form a bigger droplet.
  • Breaking – coalescence occurred substantially in an emulsion so that a free clear oil layer can be seen. This is an irreversible process (in contrast to creaming which is a reversible process
39
Q

What prevents coalescence

A

A stable/strong interfacial film helps to prevent coalescence

40
Q

How can creaming and coalescence be reversed

A

creaming can be reversed by shaking but coalescence is irreversible

41
Q

Physical Stability – Ostwald ripening

A

Smaller droplets get smaller (have larger solubility) and larger droplets get larger (have smaller solubility)

42
Q

Chemical aspects of emulsions

A

The active pharmaceutical ingredients must be compatible with the excipients in the emulsions
– Most emulsions are negatively charged, so any positively charged materials may disturb the stability of emulsions, e.g. some proteins, Ca2+
– Most electrolytes will destabilise emulsions due to • compression of the diffuse layer
• salting - out effect (on the hydrated layer of emulsifiers)
– The addition of acidic materials to emulsions stabilised by soap-type emulsifiers: destabilise the emulsion.

43
Q

Preservation of emulsions

A

Preservatives are needed to prevent the growth of bacteria for non-sterile products
• Even for sterile products preservatives are sometimes required to minimise the impact of in- use contamination (e.g. Diprivan®)
• Preservatives exert their effect in the aqueous phase where bacteria growth occurs

44
Q

Preservatives –general requirements

A

A wide spectrum of activity against all bacteria
• Tolerability-Non-toxic,non-irritant and free from sensitisation activities
• High aqueous solubility–preservatives work in the aqueous phase as bacteria grow there
• Stability and effectiveness over a wide range of pH and temperature. pH of some emulsions tend to drop during storage
• The preservative activity needs to be tested by appropriate microbiological tests

45
Q

Other additives in emulsion formulations

A

Antioxidants–some oil and emulsifiers are susceptive to oxidation, e.g. lecithin. Vitamin E can be used as antioxidant.
• Flavours
• Colours
• Sweetening agents

46
Q

Assessment of emulsion stability

A

Macroscopic examination
– Creaming–cream ratio against time – Breaking?
– Changing colour?
• Microscopic examination
- Select suitable magnification so that enough number of droplets are examined
– Examine sufficient area under the microscope and avoid selecting rare exotic objects
– Beware of alien particles (not from emulsions)
• Particle size analysis –laser diffraction, Coulter counter, single
particle optical counting etc. • Rheological (viscosity) test–
– A change in size may result in a change of viscosity
• Accelerated stability test – centrifugation, freeze-thaw cycling, temperature cycling

47
Q

Formulations of some emulsions

A 
Diazepam emulsions
– Fractionated soy bean oil
– Diacetylated monoglycerides
– Fractionated egg phospholipids – Glycerol (anhydrous-used for stability) 
– Sodium hydroxide (to pH8)
– Water for injections
• Diprivan® - a propofol emulsion 
– Glycerol
– Purified egg phosphatide – Sodium hydroxide
– Soya-bean oil, refined
– Nitrogen
– Disodium edetate
– Water for injections

PSF (11 decks)

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