surfactants Flashcards
what are surfactants
molecules which accumulate at surfaces
- used to prod many medicines, where they’re used as excipients
hydrophilic (polar) and hydrophobic (non-polar) regions/groups interact to make these amphiphilic surfactants
amphiphiles are molecules that contain both hydrophilic and hydrophobic moieties
what are surfactants usually e.g what atoms are in them etc
surfactants are
- usually unsaturated hydrocarbon chains (8-18 carbon atoms), branched or linear, & less commonly heterocyclic or aromatic ring systems
- usually classified according to the nature of the polar head group
TYPES OF SURFACTANTS: (4)
- anionic
- cationic
- non-ionic
- zwitterionic
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
- Na+ counterion usually
- polar head groups include
→ Carboxylate (-COO-)
→ Sulphate (-O-SO3-)
→ Sulfonate (SO3-)
→ Phosphate (-OPO32-)
cationic surfactants:
Eg. Quaternary ammonium and pyridinium: bactericidal activity against Gram+ and Gram- bacteria. May be used on skin esp for cleaning wounds. Aq solution used for cleaning contaminated utensils.
- usually have methyl sulphate, or halide counterions
Non-ionic surfactants:
Enhance drug absorption, enhance activity of drug, and are often in the form of emulsions.
- uncharged
- polyether or polyhydroxyl group as the polar head group
- least toxic of surfactants & second most used
Zwitterionic surfactants
these have low eye and skin irritation so these often used in cosmetics or shampoos.
- possesses both +vely and -vely charged groups over a certain pH range
- +ve charge usually NH4+
- -ve charge may vary, often COO-
How does the dual structure of the surfactant affect their behaviour
- At a liquid/air interface
- adsorption (accumulation) at the liquid/air interface
Hydrophobic region of the molecules ‘escapes’ from the ‘hostile’ aqueous environment.
The surfactant molecule orientates itself to remove its hydrophobic part from the water.
https://www.notion.so/Surfactants-08-10-24-lecture-1-3-13300bb3982d81f6967dfadfa8d006a0?pvs=4#13300bb3982d81438a80de13e9b23f6d
How does the dual structure of the surfactant affect their behaviour 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
https://www.notion.so/Surfactants-08-10-24-lecture-1-3-13300bb3982d81f6967dfadfa8d006a0?pvs=4#13300bb3982d8123a0c3f04f329d0752
https://www.notion.so/Surfactants-08-10-24-lecture-1-3-13300bb3982d81f6967dfadfa8d006a0?pvs=4#13300bb3982d813ba8eedf7ed9ecc32a
How does the dual structure of the surfactant affect their behaviour at a liquid / hydrophobic solid interface
Surfactants adsorb onto the surface, which reduces contact between hydrophobic groups and water;
This allows attainment of a minimum energy state
https://file.notion.so/f/f/f2f8cfd9-49c5-4e46-8b36-c5afee96299f/85c7acdb-773c-44a3-b00f-e451b7e557b9/Screenshot_2024-10-08_at_16.44.51.png?table=block&id=13300bb3-982d-81b5-be34-ca2fb5e1fd5c&spaceId=f2f8cfd9-49c5-4e46-8b36-c5afee96299f&expirationTimestamp=1737244800000&signature=FBpMwxwXk21td9XKBBJooc-Lcd97mLQ0rN33SQeTNH4&downloadName=Screenshot+2024-10-08+at+16.44.51.png
What is micellisation
when molecules form micelles by arranging into spheroidal aggregates with the hydrophobic areas of water shielded by a mantle of hydrophilic groups
what is surface tension?
how 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
does water have a high or low surface tension?
high (72-73) , then alcohols, benzene, then alkanes
surface detention real definition
the work required to increase the surface area by 1m^2
W = y delta A
What happens to surface tension when a surfactant is added to a liquid? (tldr - they reduce the tension is liquids)
- Surfactant adsorbed 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? (tldr: surfactant reduces the interfacial tension at a liquid/liquid interface)
first we need to look at the interface between aq and non aq interfaces like oil-water
https://www.notion.so/Surfactants-08-10-24-lecture-1-3-13300bb3982d81f6967dfadfa8d006a0?pvs=4#13300bb3982d817cb82ddbe71a579657
when a surfactant is added, the surfactant adsorbs at the interface between aq and non aq liquids:
honestly go look at ur lecture notes here for pics
as the surfactant is added, the interfacial tension changes:
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.
When surfactant is added to a mixture of 2 immiscible liquids, the surfactant migrates to interfacial layer. Consequently, interfacial tension is reduced
what about a solid/liquid interface
(surfactant reduces the interfacial tension at a solid/liquid interface)
adsorption (accumulation @ interface / surface effect) of surfactants onto the hydrophobic solutes (e.g. carbon) solid / liquid interface
- as a result, the solid / liquid interfacial tension decreases
don’t confuse adsorption with ABSORPTION (the penetration of one component throughout the body of water)
what is wetting
when a surfactant reduces the interfacial tension at a solid/liquid
→ the solid can be wetted by the liquid
this means the liquid has spread over its surface
- e.g in pharmacy: wetting needed for drugs to be suspended/dissolved when pharmacists are making medicines or for a drug to be dissolved in the body, so that it can be absorbed and act
what are the 2 general types of adsorption
Physical adsorption - adsorbate is bound through weak intermolecular forces
Chemical adsorption - chemisorption - which involves stronger forces
e.g. ion-exchange process
adsorbent vs adsorbate
adsorbent (that which adsorbs e.g. carbon)
adsorbate (that which is adsorbed)
factors affecting adsorption:
- solubility of adsorbate
- pH
- Nature of adsorbatent
- Influence of surface area of adsorbent
- Temperature
how does solubility of adsorbate affect adsorption
- adsorption of a solute is inversely related to its solubility in the solvent from which adsorption occurs, i.e. high drug solubility is equivalent to low adsorption.
In a homologous series, adsorption increases as the series is ascended and as the molecules become more hydrophobic
how does pH affect adsorption
pH affects ionisation of a weakly acidic/basic molecule, thus its aqueous solubility and its adsorption.
how does nature of adsorbent affect adsorption
particular adsorbents have affinities for particular adsorbates. E.g. bentonite and kaolin have cation-exchange sites; these have strong affinities for protonated compounds which they can adsorb by ion- exchange.
how does surface area of adsorbent affect adsorption
Greater surface area of adsorbent will lead to greater adsorption
how does temperature affect adsorption
adsorption is an exothermic process, i.e. heat is released upon adsorption thus, an increase in temperature normally leads to a decrease in adsorption.
what happens when the liquid/air interface is saturated with surfactant molecules as you add more in - Micellisation:
Hydrophilic headgroup goes into the water & hydrophobic tail escapes into the air as it hates water
Micellisation is driven by the need to shield this hydrophobic part of the molecule from the medium.
→ a few surfactant molecules are dissolved in the water, though micelles may also be formed in other polar solvents e.g. formamide
a micelle is formed when surfactant molecules aggregrate into the bulk phase of a medium
what is an Aggregation number
number of surfactant molecules in a micelle
what is Critical Micelle Concentration (CMC)
Concentration of surfactant molecules in a medium, at which micelles start to form
what is the micelle size
The radius of micelle core is close to the extended length of the alkyl chain.
micelles are in dynamic equilibrium with what?
the monomers in solution
where are inverted micelles formed and whats diff abt them
formed in a non polar medium: the hydrophobic tails can interact with the medium, and the headgroups do not
whats a counter ion
an ion of opposite charge to a given ion: e.g. in a crystal of NaCl, the Cl-ions can = counter ions to the sodium ions
-> surfactant head groups are positively charged: the counter ion has to be the neg one
do ionic micelles have a low or high aggregation number?
a low aggregation number
therefore they adopt a spherical / ish shape
how are non-ionic micelles different to ionic
- these are larger compared to ionic micelles:
in charged micelles, work is done to add a monomer to an existing charged micelle
this work is not needed in non-ionic micelles
details on micellar structure
look on ur notes xoxoxo
Certain non-ionic micelles have a shell of oxyethylene chains “the palisade layer”
Palisade layer of micelle = capable of entrapping water molecules.
Thus the micelles are highly hydrated.
from inner to out
1. micelle core
2. palisade layer and oxyethlene chain
3. shear surface
The micelle formed by the surfactant with the longer hydrocarbon chain will be
larger
the surfactant with the longer hydrocarbon chain will have a lower
CMC
The ionic surfactant will have a higher […] and lower […]
higher CMC and lower aggregation number
The surfactant with the larger […] has a higher CMC
head group
higher CMC is more […] soluble
water
what are the 4 factors affecting CMC and micellar size
the structure of the hydrophobic group
nature of the hydrophillic group
how does nature of the counter ion
addition of electrolytes
how does the structure of the hydrophobic group affect cmc and micellar size
- inc hydrocarbon length = higher micellar size & decreased CMC
- whether the compound has a rigid aromatic or heteroaromatic ring structure = stacking of molecules on top = aggregates together = increase continuously in size = no CMCs
how does nature of the hydrophillic group affect cmc and micellar size
- non-ionic surfactants tend to have lower CMC values and
higher aggregation numbers as no electrical work is involved - increased hydrophilic groups = increased hydrophilicity = higher CMC
(High hydrophilicity of surfactant = high solubility = low tendency to form micelles)
how does nature of the counter ion affect cmc and micellar size
such as increasing size of counterion =
- inc micellar size
- reduced CMC
this is as inc ion = inc ionic radius and field
→ large ions are less solvated (surrounded by solvent/water/molecules), so they can be more readily adsorbed on the micellar surface
example:
https://www.notion.so/Surfactants-08-10-24-lecture-1-3-13300bb3982d81f6967dfadfa8d006a0?pvs=4#13300bb3982d81c9a139d7ff73439053
and look avbove this xoxo
how does addition of electrolytes affect cmc and micellar size
- lowers the CMC and increases micellar size
- this means there are more counterions, which can reduce the
repulsion between charged head groups in micelle - Thus less electrical work is needed in the formation of micelles
what do surfactants have to do with solubilisation
- this is enhancing drug solubility using surfactants
water insoluble substances are brought into solution by incorporation into micelles
→ does not occur until micelles are formed
- increase in concentration of surfactants leads to increasing number of micelles which leads to increasing amount of solubilisate (that which is solubilised)
- location of solubilisate (related to it’s chemical nature)
- solubilisate within core = increase in micelle
size, as the micelle core is enlarged.
Aggregation number of the micelle also increases as more surfactant molecules join a micelle.
- solubilisate in palisade leads to no change in aggregation number, but increase in micelle size due to solubilisate molecules.
pharmaceutical applications of solubilisation?
phenolic compounds solubilised with soap to form clear solutions for disinfection
non ionic surfactants used to product clear solutions for steroid use
xtra notes
surfactants have polar and non polar groups in the same molecule
A surface is defined as the outer or top part or layer of something e.g. surface of water, surface of the table, etc.
notes on interfaces
An interface is the boundary between 2 phases. So for example, the surface of the tea in your cup can also be said to be a liquid/air interface, since there is air above it and liquid beneath it.
Similarly, you can have liquid/liquid interface, e.g. if you have oil and water in the same cup or
solid/liquid interface, e.g. at the boundary between the tea and the cup
2 types generally of adsorption
- physical - adsorbate is bound through weak Van der Waals forcces
- chemical - chemisorption involves stronger forces e.g ion exchange process
remember: adsorption onto hydrophobic material occurs in order to remove hydrophobic parts from water/aqueous phases. So, if something ‘likes’ water, there is less need to ‘escape’ from it.
whats an amphoteric surfactant and how do it work
it can have a positive charge, or negative charge or both, depending on the pH of the environment.
In an acidic medium, it can accept H+ ions, and thus becomes positively charged.
In a basic medium, it can give up H+ ions, and thus becomes negatively charged.
At the isoelectric point, it has both + and - charged groups. The net charge is zero.
if the net charge is zero, the material interacts less with water (compared to when there is a charge).
Its water solubility is therefore lowest (compared to when there is a charge).
Lowest solubility will lead to greatest adsorption.
