Surfactants Flashcards
What is a surfactant?
- A compound whose structure contains 2 separate regions: hydrophilic and hydrophobic
- Classified according to nature of the polar head group
- Amphiphiles
Examples
SDS, Benzalkonium chloride, Span 60
Types of surfactants
- Anionic - carboxylate, sulphate, and sulfonate are the polar head groups
- Cationic - usually have halid or methyl sulphate counterions
- Non-ionic - uncharged, polyether or polyhyroxyl
- Zwitterionic - both + and - charges
Anionic surfactants
- Used in greater volume than any other class of surfactant due to ease and low cost of manufacture.
- Used in most detergent formulations.
- e.g. sodium laryl sulphate - preop skin cleaner, action against gram+Ve bacteria, medicated shampoo
Cationic surfactant examples
- Quaternary ammonium and pyridinium: bacterial activity against gram+ and gram- bacteria, may be used on skin esp for cleaning wounds
- Cetrimide BP: clean skin wounds, burns, contaminated vessels
- Benzalkonium chloride - preop disinfection of skin
Non ionic surfactants
Least toxic, second largest class
e.g.
- Sorbitan esters: w/o emulsifiers, wetting agents
- Polysorbates: o/w emulsions
- Cetomacrogol 1000 - o/w emulsions, solubilising agent for volatile oils
Enhance durg absorption, and activity of drug
Dual structure of a surfactant at a liquid / air interface
Adsorption at the liquid / air interface.
- Hydrophobic region of the molecules escapes from the hostile aqueous environment
- Surfactant molecule orientates itself to remove it’s hydrophobic part from the water
Dual structure of a surfactant at an oil/water interface
Adsorption at the interface between aqueous and non-aqueous liquids
Hydrophilic group in aqueous phase, hydrophobic group in non-aqueous phase
Dual structure of a surfactant at a liquid / hydrophobic solid interface
**Surfactants adsorb onto the surface, which reduces contact between hydrophobic groups and water
**
Allows attainment of a minimum energy state
Dual structure of a surfactant in micellisation / micelle formation
Molecules are arranged in spheroidal aggregates with the hydrophobic areas shielded from the water by a mantle of hydrophilic groups
How does surface tension work?
(4 steps)
- Molecules at the surface of a liquid are not completely surrounded by other like molecules compared to those in bulk
- Net inward force of attraction exerted on the surface molecule by bulk molecules
- Tendency for surface to contract
- Surface is in a state of tension
Surface tension of a liquid is defined as…
Work required to increase the surface area by 1m²
W = surface tension x change in A
What happens to surface tension when surfactant is added to a liquid?
- Surfactant absorbed at solution / air interface
- Some water molecules replaced by surfactant molecules
- Forces of attraction between bulk water molecules and surfactant molecule is less than water-water attraction
- Contracting power of the surface is reduced
- Surface tension is REDUCED
What happens at the interface between aqueous and non-aqueous phases when a surfactant is added?
When a surfactant is added, the surfactant adsorbs at the interface between aqueous and non-aqeuous liquids
1. At interface between 2 immiscible liquids, the interfacial tension is generally between those of the surface tensions of the 2 liquids, except where there is attraction between them
2. When surfactant is added to a mixture of 2 immiscible liquids, the surfactant migrates to interfacial layer. Consequently, interfacial tension is reduced
Surface / interfacial tension of aliphatic hydrocarbon0water is 40-50. Hydrocarbon-aqueous surfactant solution is 1-10
What happens to surface tension when a surfactant is added to a solid / liquid interface?
Adsorption of surfactants onto hydrophobic solutes (e.g. carbon)
- When surfactant reduces the interfacial tension at a solid / liquid interface
- The solid can be ‘wetted’ by the liquid
- Solid wetted = liquid has spread over its surface
- Wetting is important for drugs to be dissolved in the body
Types of adsorption
Physical - adsorbate is bound through weak VDW forces
Chemical - chemisorption - which involves stronger forces e.g. ion-exchange process
(Adsorbent absorbs, adsorbate is adsorbed_
Factors affecting adsorption
- Solubility of adsorbate: adsorption of solute inversely related to solubility in the solvent i.e. high drug solubility = low adsorption. In a homologous series, adsorption increases as series is ascended and molecules become more hydrophobic
- pH: affects ionisation of a weakly acidic / basic molecule, thus it’s aqueous solubility and adsorption
- Nature of adsorbent: particular adsorbents have particular affinities for particular adsorbates e.g. bentonite and kaolin have cation-exchange sites; these have strong affinities for protonated compounds
- SA of adsorbent: Higher SA = more adsorption
- Temp: Adsorption is exothermic, increase in temp = decrease in adsorption
What is micellisation?
- Hydrophilic headgroup goes into the water (skin)
- Hydrophobic tail escapes into the air (core)
- Takes place when you keep adding more surfactant
How does a micelle form?
- When surfactant molecules aggregate into the bulk phase of a medium
- Driven by the need to shield the hydrophobic part of the molecule from the aqueous medium
- Water molecules are excluded from the micellar core
Micelles in dynamic equilibrium with monomers in solution i.e. they are continuously breaking and forming
Inverted micelles
In a non-polar medium, where the hydrophobic tails can interact with the medium, and the head groups do not, inverted micelles are formed
CMC (critical micelle concentration)
Concentration of surfactant molecules in a medium at which micelles start to form
Micelle size
Radius of the micelle core is close to the extended length of the alkyl chain
Difference between ionic and non-ionic micelles
Ionic micelles have low aggregation number and adopt a spherical or near spherical shape
Non ionic micelles are larger, work is not needed to add a monomer.
○ Certain non-ionic micelles have a shell of oxyethylene chains which is called the palisade layer
Factors affecting CMC and micellar size (5)
- Structure of hydrophobic group
- Nature of hydrophilic group
- Nature of counterion
- Addition of electrolytes
- Solubilisation
How does structure of the hydrophobic group affect CMC and micellar size?
○ Surfactants - hydrophobic groups commonly hydrocarbon chains
○ Increased hydrocarbon length leads to increased micellar size
Increased hydrocarbon length leads to decreased CMC
How does nature of hydrophilic group affect CMC and micellar size?
○ There are pronounced differences between ionic and uncharged surfactants - in general, non-ionic surfactants have lower CMC values and higher aggregation numbers mainly because no electrical work is involved
○ Increased hydrophilic group leads to increased hydrophilicity, this leads to higher CMC
How does nature of counter-ion affect CMC and micellar size?
○ Increasing size of counterion results in increasing micellar size and reduced CMC
○ Decreased ionic field
○ Large counterions are less solvated - therefore they can be more readily adsorbed on the micellar surface
How does addition of electrolytes affect CMC and micellar size?
○ Lowers the CMC and increases micellar size
○ More counterions, which can reduce the repulsion between charged head groups in micelle
○ Less electrical work is needed in the formation of micelles
Solubilisation
Process whereby water-insoluble substances are brought into solution by incorporation into micelles
Does not occur until micelles are formed
How does solubilisation affect CMC and micelle size?
- Above CMC, increase in concentration of surfactants leads to increasing number of micelles which leads to increasing amount of solubilisate (that which is solubilised)
- Location - Solubilisate within core results in increase in micelle size, as the micelle core is enlarged. Aggregation number of the micelle also increases as more surfactant molecules join a micelle.
- Solubilisation in palisade leads to no change in aggregation number, but increase in micelle size due to solubilisate molecules
Pharmaceutical applications of solubilisation
○ Phenolic compounds - e.g. cresol, chlorocresol, solubilised with soap to form clear solutions which are widely used for disinfection
○ Non-ionic surfactants solubilise iodine (iodophors) used for instrument sterilisation
○ Iodophor is also able to penetrate hair follicles of skin which enhances activity of iodine
○ Polysorbates have been employed to prepare aqueous injections of water-insoluble vitamins
