Lecture 8 - Gels

Question 1

Why do concentrated polymer gel solutions show high viscosity?

Answer 1

Due to 3D interaction of polymer chains with solvent

Question 2

Define gels

Answer 2

Viscoelastic solid-like materials comprised of an elastic cross-linked network and a solvent

Question 3

Can gels with strong chemical bonds dissolve?

Answer 3

NO they are thermally irreversible

Question 4

Can gels with weak non-covalent bonds dissolve?

Answer 4

YES are reversible

Question 5

What are the characteristic features of gels?

Answer 5

Gels retain shape under low stress - deforms under high stress
Appearance of rubber like elasticity
large increase in viscosity above gel point

Question 6

Give examples of wet solid gels derived from polymeric compounds

Answer 6

Soap, shampoo, toothpaste, hair gel and other cosmetics

Question 7

What are gelators?

Answer 7

Are macromolecules and form gels by physical cross-linking usually H-bonding

Question 8

Give examples of gelators

Answer 8

Gelatin, collagen, agar, strach and gellan gum

Question 9

What might gels contain?

Answer 9

Water - hydrogels
Organic liquid - petroleum

Question 10

How are low molecular mass compounds formed?

Answer 10

aggregation of the small gelator molecules to form Self-assembled Fibrillar networks (SAFINs) - formed through non-covalent interactions e.g. H-bonding, p-p stacking etc

3 situations are possible:
- crystals
- amorphous precipitate
- gel

Question 11

What are liposomes made up of?

Answer 11

Phospholipid and water
orintate in water to give the most thermodynamically stable conformation
can be multicellular or unicellular

Question 12

Define multilamellar liposome

Answer 12

These crystals when first formed are usually comprised of several bimolecular lipid lamellae separated by an aqueous layer

Question 13

What may liposomes carry?

Answer 13

Strongly lipophilic drugs
Strongly hydrophilic drugs
Drugs that have intermediate partition between the lipid and aqueous phase

Question 14

At even higher concentrations how are the molecules arranged

Answer 14

Amphiphiles form long cylinders that arrange themselves into a roughly hexagonal lattice known as the ‘middle soap phase’

Question 15

At low amphiphile concentration how are the molecules arranged

Answer 15

Dispersed randomly

Question 16

At slight higher concentrations how are the molecules arranged

Answer 16

Spontaneously assembled into micelles or vesicles
Hide the hydrophobic tail of the amphiphile inside the middle core exposing a hydrophilic (water-soluble surface to an aqueous solution

Question 17

At the HIGHEST concentration how are the molecules arranged

Answer 17

a lamellar phase ‘neat soap phase’ may form, wherein extended sheets of amphiphiles are separated by thin layers of water

Question 18

What does stability depend on?

Answer 18

Lipid composition, storage condition and stabilisers

Question 19

Define conventional liposomes

Answer 19

Composed of phospholipids
Protect encapsulated molecules from degradation
Passively target tissues or organs that have a discontinuous endothelium

Question 20

Define long circulating liposomes

Answer 20

Covalently attach the hydrophilic polymer, PEG to the liposome bilayers
Highly hydrated PEG group create a steric barrier against interactions with molecular and cellular components in the biological environment

Question 21

Define immunoliposomes

Answer 21

Specific antibiotics or antibody fragments on their surface to enhance target site binding
Targeted delivery of anti-cancer agents

Question 22

Define cationic liposomes

Answer 22

The cationic lipid components of the liposomes interact with and neutralise negatively charged DNA - thereby condensing the DNA into more compact structure

Question 23

Define the toxicity of liposome formulations

Answer 23

most liposome formations are well tolerated