Week 8 disperse systems surfactants and suspending agents

Question 1

Surface tension

Answer 1

Problematic in pharmaceutics;
-Wetting process hindered if surface tension too high
-Add surfactant molecules to replace some of the water molecules in the surface
-Forces of attraction between surfactant and water molecules are less than those between two water molecules, hence the contraction force is reduced

Question 2

Surfactant

Answer 2

Surfactant – Surface Active Agent
A surfactant molecule is:
-An amphiphilic/ amphipahic molecule that consists of a distinct hydrophobic and hydrophilic region
-A molecule that accumulates at interfaces - can arrange themselves to have hydrophilic heads facing the aqueous region and hydrophobic tails away from (in oil for e.g.,)
-A molecule that aggregates or self-assembles
-If there are enough surfavtants they can arrange themselves into a spherical structure - known as micelle

Question 3

Pharmaceutical applications - surfactants

Answer 3

-Surfactants are present in suspensions and emulsions
-Aggregated surfactants can be used as drug delivery vehicles:
e.g. micelles and vesicles to solubilise insoluble drugs
To stabilise the water within the oil / oil within the oil e.g., creams / ointments
Some drugs need to be stabilised in lipid nanoparticles

Question 4

Arrangements of amphipahtic molecules

Answer 4

Micelle - hydrophilic head points towards aqueous and hydrophobic tail towards the oil
Reverse micelle - Same arrangement but the other way around

Question 5

How surfactants work

Answer 5

Adsorption at the oil-water interface lowers interfacial tension;
-Aids the dispersal of the oil into droplets of a small size
-Maintains the particles in a dispersed state

Question 6

Ionic surfactants

Answer 6

Surfactants that have a charge
-Anionic (dissociate at high pH)
-Cationic (dissociate at low pH)

Question 7

Non-ionic surfactants

Answer 7

Widely used, less toxic and irritant than ionic surfactants

Question 8

Mixtures of surfactants

Answer 8

To make nicer micelles, a combination of ionic and non-ionic surfactants is used
-Produce more stable emulsions

Question 9

Classification of surfactants : Anionic

Answer 9

Negatively charged surfactant, positive counterion
Sodium dodecyl sulfate (SDS) (aka sodium lauryl sulfate, SLS):
-Freely soluble in water
-Forms self-emulsifying bases with fatty alcohols
-Detergent in medicated shampoos
-Skin cleanser in topical applications

Question 10

Classification of surfactants : Cationic

Answer 10

Positively charged surfactant, negative counterion;
Cetylpyridinium chloride (quaternaryammonium/ pyridinium)
-Freely soluble in water
-Has antimicrobial activity – cleaning of wounds, contaminated utensils,
preservative
-> Bacterial cell membrane is made of lipids, the positive charge can disturb the cell membrane of the bacteria so they can no longer survive
-Enhance transdermal drug delivery preparations

Question 11

Classification of surfactants : Non-ionic

Answer 11

Uncharged, still have the hydrophobic hydrocarbon chain and the hydrophilic side that likes water, has lots of oxygen even though uncharged
-Spans = Sorbitan fatty acid
esters
-Tweens/Polysorbates =
Polyoxyethylene sorbitan fatty
acid esters

Question 12

Classification of surfactants : Zwitterionic

Answer 12

Two charges in the same molecule, overall neutral (net charge is 0)

Question 13

Surfactants other forms

Answer 13

-Drugs: Surfactants can be drugs themselves
-Naturally occurring;
-Bile salts -> helps to emulsify fats we ingest
-Lecithin
-Synovial fluid
-Lungs surfactant

Question 14

Co-surfactants

Answer 14

To surround an oil droplet for example we can use a mixturfe on ionic and non-ionic surfacts however, there may still be gaps in between them -> therefore we use co-surfactants
Co-surfactants;
-Accompany the main surfactant
-Usually short chain alcohols or amines ranging from C4 to C10
-Helps in the formation and stabilisation of micelles / microemulsions
-Increases the flexibility and fluidity of the interface

Question 15

Co-surfactants 2

Answer 15

-The surfactant/cosurfactant mixture is able to form an efficiently packed film, covering the whole droplet
-Produces an excellent emulsion
-If you do not have the optimal number of surfactant and co-surfactant molecules, it will produce a poor emulsion as it cannot form a nicer micellar shape that will effectivley en compass the oil

Question 16

Pharmaceutical surfactant and co-surfactant mixtures

Answer 16

Pharmaceutical companies produce mixtures that have the surfactants and co-surfactants in the right amounts for us

Question 17

Micelles - How surfactants form micelles

Answer 17

-Colloidal spherical or near spherical nanoparticles
-Loose aggregates of 50 – 100 surfactant molecules into a single entity
-Size of micelles ~ 5 – 10 nm
-Micelles are dynamic structures - dyanamic process, once they are formed they do not permanently stay formed they can form and break;
->Individual monomers may enter or leave the micelles = rapid process
->Micelles are constantly breaking down and reforming = slow process

Question 18

Surfactant self-assembly

Answer 18

E.g.,
60 randomly positioned molecules of polysorbate 80 first aggregate into small clusters, which then come together to
form a single micelle
-> The micelle then undergoes
restructuring to yield its final equilibrium arrangement

Question 19

Critical packing parameter

Answer 19

Determines when there are enough surfactants in a micelle
Dependent on the;
-Volume of the hydrocarbon chain, V
-Length of the hydrocarbon chain, lc
-Cross sectional area of the hydrocarbon chain, a

Question 20

Micelle formation in water

Answer 20

Water:
-Extensively hydrogen bonded
-“Flickering clusters”
-Highly ordered water molecules form “cages” around the hydrophobic alkyl chains

Question 21

Dispersion of lipids in water

Answer 21

If you have multiple surfactants in the water -> a monolayer
Each lipid molecule forces surrounding water molecules to become highly ordered
Clusters of lipid molecules: only lipid molecules at the edge of the cluster force the ordering of water -> fewer water molecules are ordered, entropy is increased

Question 22

Dispersion of lipids in water 2

Answer 22

With more surfactants they form the micelle,
All hydrophobic groups are sequestered (remove all the water so that only oil remains inside) from water, no highly ordered shell of water molecules is present, and entropy is increased
Core of the micelles will be oil

Question 23

Thermodynamics of micelle formation in water

Answer 23

Thermodynamics
-The second law: The level of disorder (entropy) in the universe is steadily increasing
-Reaction favours lowest free energy state
Micelle formation results in an increase in entropy because water molecules regain their freedom upon micellization - due to loss of “cage” structure
When free energy is negative the reaction will be highly favorable

Question 24

Interaction of water and hydrophilic head groups

Answer 24

Water molecules interact with
surfactant headgroups
Head groups start to repel each
other if brought in too close in
contact -> stops micelles from
growing
Ionic surfactants are smaller as the heads repel eachother stronger than in non-ionic surfactants

Question 25

Critical micelle concentration (CMC)

Answer 25

When micelles are being formed
-The surfactant concentration at and above which micelles are
formed
-Determined by measuring the surface tension at different surfactants

Question 26

CMC

Answer 26

When you add sjurfactants the surface tension is reduced
At the point where the surface tension becomes constant (CMC) we know the micelles have been formed
At CMC they form the monolayer and beyond the CMC miceller structures start to form, the monolayer is still there but miceller structures are forming

Question 27

Osmotic pressure