POLYMORPHIC STRUCTURES OF AMPHIPHILIC MOLECULES IN AQUEOUS SOLUTION:

MICELLE AND BILAYERS

ABHISHEK.M

AMPHIPHILE:

Derived from Greek words AMPHI = Both and

PHILIOS = FriendThus the word itself means something that likes or rather is friendly to both.

In chemistry it means a molecule that likes both water and oil.

The use of amphiphilic molecules creates a means to dissolve water into oil and vice versa.

This is something necessary for many applications. Some practical examples are milk, margarine and digestion of fats.

(and of course washing of your dirty clothes!!)

What does an amphiphilic molecule look like?

It may be visualized like a tadpole having a water loving HEAD and an oil loving TAIL.

Basically a molecule which has a dual polarity.

That is having both a hydrophilic group(HEAD) and a lypophilic group(TAIL)

Some other terms we generally use to describe these molecules are soap ,detergent, surfactant,

tenside etc

Schematic picture of a surfactant molecule

CRITERIA:

1)The headgroup must be hydrophilic enough.

2) The hydrophobic part must be of right size.

(carbon chain of 8 or carbons but not too long either)

Amphiphilic molecules like lipids spontaneously form a variety of structures when added to aqueous solution.

These structures are formed in ways that minimize contact between the hydrophobic lipid chains and the aqueous milieu.

For e.g.: When small amounts of fatty acids are added to an aq. solution a monolayer is formed at the water-air interface

A MONOLAYER

If we keep on adding fatty acids to this aqueous

solution, the monolayer ceases to exist and a

totally new shape is attained by the fatty acids.

This is called as a micelle

MICELLE

These are formed when an amphipathic lipid in water positions its hydrophobic tail to the centre of lipid aggregation and their polar head groups facing outwards.

The formation of micelles takes place only above a particular temperature called asKraft temperature (Tk )

and also above a particular concentration called asCritical Micelle Concentration (CMC)

Below the cmc individual lipid molecules predominate

.

CMC of some detergents

Sr no.

Name of detergent

Mr CMC

1 TRITON X-100 625 0.24mM

2 Octyl glucoside 292 25mM

3 Dodecyl octaoxyethylene ether

538 0.071mM

MICELLES

BILAYER

Bilayers consists of back to back arrangements of monolayers

The nonpolar portions of the lipids face the middle of the bilayer, with the polar head groups arrayed on the bilayer’s surface

.

Phospholipid bilayers form spontaneously and rapidly when they are added to water and they are stable structures.

As opposed to micelles which are small , self-limiting structures of a few hundred molecules, bilayers may form over large areas.

A BILAYER

.As the exposure of the edges of the bilayer to the solvent is highly unfavorable, extensive bilayers normally wrap around themselves and form closed vesicles.

The nature and integrity of these vesicle structures are dependent on

their lipid composition.

Physiochemical studies with these substances in the laboratory reveals that they may exist

as either

UNILAMELLAR VESICLESOR

MULTILAMELLAR VESICLES

THANKS…….