Surfactants and Emulsions Quiz

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Questions and Answers

What characteristic feature is common to all surfactants?

They always form a rigid film at interfaces.

They consist of both polar and non-polar portions. (correct)

They have a single hydrophobic portion.

They are exclusively polar molecules.

How do ionic surfactants primarily stabilize oil-in-water emulsions?

By creating rigid films that prevent coalescence.

By forming complex structures with the oil droplets.

By absorbing energy from the oil.

Through electrical repulsion between charged particles. (correct)

What role does the hydrophilic portion of a surfactant play?

It absorbs UV light to protect the emulsion.

It increases the viscosity of the solution.

It interacts with non-polar substances.

It facilitates interaction with water. (correct)

Which of the following surfactants is likely to stabilize oil-in-water creams?

Sodium lauryl sulphate Signup and view all the answers

What happens to an emulsion when the surfactant film is disturbed?

It destabilizes, leading to separation of phases. Signup and view all the answers

Which factor primarily determines the quality of the emulsion produced by surfactants?

The relative size of polar and non-polar portions of the surfactant. Signup and view all the answers

What type of emulsion do charged surfactants generally promote?

Oil-in-water (o/w) emulsions. Signup and view all the answers

What is a problem associated with using charged surfactants in oral emulsions?

They can have a laxative effect. Signup and view all the answers

What is the primary role of finely divided solids in emulsion stabilization?

To provide a physical barrier around dispersed droplets Signup and view all the answers

According to the preferential wetting theory, which type of emulsion is formed by particles that are preferentially wetted by water?

Oil-in-water (o/w) emulsion Signup and view all the answers

According to the Bancroft rule, what determines the type of emulsion formed?

The relative solubility of the emulsifying agent in the two phases Signup and view all the answers

Which of the following is a characteristic of hydrophilic colloids used as emulgents?

They form a strong multi-molecular film around globules Signup and view all the answers

What effect do macromolecules like proteins and gelatin have on emulsions?

They increase the viscosity of the external phase Signup and view all the answers

Which statement correctly describes emulsifying agents with a high HLB value?

They are more hydrophilic than lipophilic Signup and view all the answers

What is a potential problem when using hydrophilic colloids in emulsions?

They can introduce variability due to natural occurrence Signup and view all the answers

In terms of emulsion types, what is the defining factor for finely divided solids that result in water-in-oil emulsions?

Strong preference for the oil phase Signup and view all the answers

What is the primary function of emulsifying agents in emulsions?

Stabilize the dispersed system Signup and view all the answers

Which property is essential for an emulsifying agent to effectively stabilize an emulsion?

Ability to reduce interfacial tension Signup and view all the answers

What type of system is indicated when oil is the continuous phase and water is dispersed?

Water in oil (W/O) Signup and view all the answers

Which of the following can be used to classify emulsifying agents based on their source?

Source or origin Signup and view all the answers

What type of film is produced at the interface of particles in emulsions using hydrophilic colloids?

Multi-molecular film Signup and view all the answers

What effect does adding a third component have on emulsions?

Helps in emulsification Signup and view all the answers

Which of the following is NOT a desired property of an effective emulsifying agent?

High toxicity Signup and view all the answers

When selecting emulsifiers, which factor related to cost must be considered?

They should be non-interacting and inexpensive Signup and view all the answers

Which system is indicated when emulsions are capable of conducting electric current, especially with ionic surfactants?

Oil-in-water (O/W) Signup and view all the answers

What phenomenon occurs when droplets in an emulsion aggregate or combine, leading to instability?

Coalescence Signup and view all the answers

Study Notes

Emulsions - Pharmaceutical Liquid-Liquid Systems

Emulsions are liquid-liquid systems where one liquid is dispersed as droplets within another liquid
Two immiscible liquid phases are involved; internal phase (dispersed), external phase (dispersion medium)
Oil-in-water (o/w) systems have oil droplets dispersed in water
Water-in-oil (w/o) systems have water droplets dispersed in oil

Learning Objectives

Students should be able to explain liquid-liquid system formulation principles and their use in pharmaceutical products
They should be able to apply the HLB concept to formulate oil-in-water and water-in-oil emulsions
Students should be able to discuss various mechanisms of emulsion instability and strategies for improving stability

Pharmaceutical Use of LL Systems (Topical)

Topical products include creams and lotions, both medicated and non-medicated
These products can be anti-inflammatory, antibacterial, antifungal, moisturizing, and/or cosmetics
Available as aqueous-based, oily, semi-solid, and liquid formulations.

Injectable LLS Products

Intravenous (IV) lipid emulsions (o/w) are used as a nutritional energy source, providing essential fatty acids and triglycerides
Intramuscular and subcutaneous injections can use w/o or o/w emulsions for delayed/controlled release
Sterility is crucial for injectable products
Particle size must be small (e.g., 100-200 nm) for optimal delivery.

Oral LLS Products

Emulsions are used for poorly water-soluble drugs, such as cyclosporine, griseofulvin, and steroids
Dissolving the drug in the oil phase prevents the rate-limiting step of solid-phase dissolution, and enhances absorption
Emulsifiers present changes in biochemical and physical barriers
Larger volumes, poor taste, and patient compliance can be issues. Ideal systems can be placed in capsules.

Liquid-Liquid Systems - Definitions

Immiscible liquids form a dispersed system when one liquid is dispersed as globules in the other
The dispersed phase is the internal phase
The dispersion medium is the external phase
Quiz: Blue is water, red is oil, what LLS type does the diagram show? How is this checked?

Identifying w/o or o/w systems

Diluting with water helps identify the external phase
Conductivity is checked with a meter to evaluate ionic surfactants (important for o/w systems); o/w systems conduct electricity better
Color tests use water-soluble dyes; if dye dissolves, water is the continuous phase. (o/w).
Feel: Creamy/greasy texture points to w/o systems; o/w emulsions have a different feel

Emulsification

Emulsions are unstable due to immiscible phases, large interfacial areas, and high interfacial free energy; collisions lead to flocculation, coalescence, cracking, and separation of phases
Emulsification is stabilized by the addition of a third component - an emulsifying agent
Emulsifiers improve stability by reducing the interactions between the two phases

Desired Properties of an Emulsifier

Quickly adsorb to the internal phase
Form a stable, coherent film
Provide a mechanical barrier.
Reduce interfacial tension to prevent coalescence
Have electrical potential on droplet surface
Improve viscosity
Work effectively at low concentrations
Be non-toxic
Be non-irritating
Be non-interacting (with other components).
Be inexpensive

Classification of Emulsifiers

Traditionally classified by source (animal, vegetable, mineral, synthetic).
Examples include wool fat, acacia, calcium hydroxide, bentonite, and surfactants.

Type of Film Produced

Finely divided solids form particulate films
Hydrophilic colloids form multi-molecular films; these require a film to break for drug release
Surface active agents (SAAs) form mono-molecular films
SAAs are necessary for the stability of the film

Preferential Wetting Theory

Finely divided solids are wetted by both oil and water to some degree
Particles preferentially residing in the liquid form the external phase
Water wetted particles define o/w; oil wetted particles define w/o emulsions

Bancroft Rule

The phase where the emulsifier is more soluble is the external phase
High HLB values favor o/w emulsions (hydrophilic); low HLB values favor w/o (lipophilic).

Hydrophilic Colloids and Macromolecules as Emulsifiers

These form strong, multi-molecular films, preventing coalescence
Drug release necessitates film breakdown, potentially causing problems for absorption/bioavailability.
Example molecules include methylcellulose, modified starches, pectin, alginates, proteins, gelatin, egg yolk, lecithin, and gums.

Surface Active Agents (SAAs)

SAAs decrease surface tension by adsorbing at the oil-water interface
SAAs form complex, flexible, and condensed films at the interfaces, preventing disruption and facilitating rapid reformation of the film after stress.
SAAs can reside at the interface due to their amphiphilic nature; one polar part and one non-polar part.

Characteristic feature of ALL surfactants – Amphiphilic

Surfactants have one polar (hydrophilic) portion and another nonpolar (hydrophobic) portion
The relative size and characteristic nature of polar and nonpolar portions determines the emulsion type and quality

SAA Examples

Examples of SAA include sodium lauryl sulfate, cetrimide, cetostearyl alcohol, polyoxyethylene sorbitan mono-oleate, and dodecylamine.
Hydrophilic groups involve sulfate groups, quaternary ammonium compounds, and polyoxyethylene chains.
Hydrophobic groups involve alkyl acids, alcohols, amines, and hydrocarbon-based fatty acid chains.

How Do SAAs Stabilize an Emulsion?

Electrical repulsions occur via ionic surfactants; cationic and anionic groups.
Steric repulsions occur through non-ionic surfactants.

Ionic Surfactants

Tend to promote o/w emulsions because water is the preferred medium for ions
Produce o/w creams and lotions
Typically not used in oral emulsions (laxative effect may occur)

Anionic Surfactants

Formed from fatty acid salts (soaps) through reaction with univalent or divalent alkali
Aqueous emulsions stabilize with sodium oleate
Oily emulsions stabilize with calcium oleate
In acidic pHs, can precipitate out as free fatty acid
More stable in alkaline medium; incompatible with cationic drugs.

Cationic Surfactants

Formed through reaction of fatty amines with salts, as in cetrimide
Used in antiseptic creams, due to its antimicrobial properties.
More irritating than anionic or non-ionic surfactants. Often used in hair conditioners and fabric softeners.

Non-Ionic Surfactants

Non-charged/negligible charge, e.g. cetomacrogol
Wide range of compatibility
Generally non-toxic & non-irritating
Less sensitive to temperature/electrolytes/pH changes
Used in foods, drinks, pharmaceuticals, and skin-care products; suitable for parenteral products
Hydrophobic moieties involve fatty acids/alcohols (C12 - C18)
Hydrophilic moieties consist of alcohol/ethylene oxide chains

Sorbitan Esters (Spans®) and Polysorbates (Tweens®)

Produced by esterification of the hydroxyl groups of sorbitan with fatty acid
Spans are lipophilic and create w/o emulsions
Tweens are hydrophilic and create o/w emulsions. Often paired.

Other Specialized Surfactants - Such as Lecithin

Contain both hydrophilic and non-hydrophilic properties.
The charge is affected by pH levels (cationic at low pH, anionic at high pH)
Examples include lecithin, and betaine.

Emulgent System

Combinations of emulsifying agents for stability, often pairing a hydrophilic tween and a hydrophobic span.

Emulsion Stability

The stability of a dispersed system is affected by the input of energy, and factors like large interfacial area
Methods for decreasing particle size include mortars, pestles, homogenizers, and ultrasonicators; IV emulsions tend to use smaller particle sizes.

Emulsion Stability – Energy Barrier

Emulsions are fundamentally high-energy systèmes; there is a constant desire to reduce the free energy state, and thus the interface
The energy barrier protects the formulation stability
Energy must be put into the system to prevent phase separation

Emulsion Instability – Summary

Emulsions are highly unstable, and thus stability needs to be promoted and maintained.
Instability involves various changes such as flocculation, creaming, coalescence, cracking, and phase inversion (which can affect the overall emulsion type from o/w to w/o)
Factors include phase volume ratio, droplet size, chemical and physical incompatibilities, and storage temperatures

Mechanisms of Emulsion Instability

Flocculation - aggregation of droplets without losing individual identity. Weak associations.
Creaming - separation by density differences, with the higher-density component migrating to the top or bottom of the emulsion. Reversible with shaking
Coalescence - merging of droplets and is an irreversible process
Cracking - Complete phase separation; not reversible
Phase inversion - change in emulsion type; for example from o/w to w/o

Creaming vs. Sedimentation

Creaming is the upward movement of internal phase droplets
Sedimentation is the downward movement of internal phases (a "negative" creaming)

Coalescence and Cracking

Coalescence is the merging of droplets to form larger droplets.
Cracking leads to complete separation of the two liquids.

Phase Inversion

A change in the emulsion type (o/w to w/o); it is often seen when more internal phase or other factors are added to the system.
The nature of the interfacial film can change due to the addition of salts/chemicals
Temperature effects can also cause phase inversion

Temperature Effects

The HLB values of SAA are sensitive to temperature. Higher temperatures mean more likely a lower effective HLB value; this impacts the emulsification type
Phase inversion temperature (PIT) is characteristic for particular SAAs; this helps with the heating/cooling processes
Emulgent concentrations can also affect the emulsion type, as higher values typically favor o/w

Preservatives in L-L Systems

The preservative needs to penetrate bacterial membranes and must exist in an un-ionized state
Partitioning of preservatives between oil and aqueous phases occurs and must be considered
The preservative concentration should suffice to maximize its effect in the aqueous phase; the microbial growth often happens in aqueous solutions

Preservative Effectiveness

pH affects preservative behavior: certain preservatives are only effective at a particular pH
Some SAAs can hinder preservative effectiveness
Factors such as emulsion type, nutraceutical qualities, and aeration conditions have to be taken into consideration.

Microemulsions

Microemulsions are emulsions with very small particle sizes (colloidal).
Typically homogenous, translucent, and low viscosity systems are favored
The particle size of the dispersed phase impacts its physical characteristics, and the SAA needed changes depending upon the droplet sizes
Increased surfactant concentrations are needed to stabilize the emulsion with very small droplets.

Useful as Drug Delivery Systems

Microemulsions/emulsions/SEDDS can be used to increase drug bioavailability by delivering poorly water-soluble drugs in the internal phase
This technology is now applied to transdermal, ocular, IV, and oral systems
Currently used in cosmetics, food, and industrial processes.

SEDDS and SMEDDS

Self-emulsifying drug delivery systems rely on isotropic oil/surfactant/co-solvent mixtures; they need dilution in aqueous GI fluids to form stable emulsions
Enhance oral bioavailability of lipophilic drugs (e.g., cyclosporine, vitamins, ibuprofen).
Uniform drug absorption and plasma levels are seen, and these systems protect against drug degradation and gastric irritation.
Potential problems exist due to high surfactant concentrations (irritation, toxicity, tissue damage)

Multiple Emulsions (w/o/w etc)

Multiple emulsions contain complex structures, with one phase encased within another phase (w/o or o/w)
The presence of oil layers between phases acts as a rate-controlling membrane between aqueous phases.
Lower viscosity and easier injection results. Examples include delayed-action drug systems.

Practice Questions

Liquid-liquid systems (LLS) are inherently unstable - True (they require continual effort to maintain their status). Discuss the reasons.
Three mechanisms of instability include: flocculation, creaming, and coalescence - these and other techniques/approaches can help to minimize the instability
The choice of emulsifiers and the manufacturing method can impact stability. The manufacturing method choice affects droplet size, viscosity, and other factors.
Creams/lotions are often labelled for storage below a certain temperature to minimize instability; temperature affects HLB values for example.

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Description

Test your knowledge on the properties and functions of surfactants, particularly their role in stabilizing emulsions. This quiz covers key principles such as the hydrophilic portion of surfactants, the types of emulsions they form, and the effects of various factors on emulsion quality. Perfect for students in chemistry or related fields.