Lecture 8 - Gels Flashcards
Why do concentrated polymer gel solutions show high viscosity?
Due to 3D interaction of polymer chains with solvent
Define gels
Viscoelastic solid-like materials comprised of an elastic cross-linked network and a solvent
Can gels with strong chemical bonds dissolve?
NO they are thermally irreversible
Can gels with weak non-covalent bonds dissolve?
YES are reversible
What are the characteristic features of gels?
Gels retain shape under low stress - deforms under high stress
Appearance of rubber like elasticity
large increase in viscosity above gel point
Give examples of wet solid gels derived from polymeric compounds
Soap, shampoo, toothpaste, hair gel and other cosmetics
What are gelators?
Are macromolecules and form gels by physical cross-linking usually H-bonding
Give examples of gelators
Gelatin, collagen, agar, strach and gellan gum
What might gels contain?
Water - hydrogels
Organic liquid - petroleum
How are low molecular mass compounds formed?
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
What are liposomes made up of?
Phospholipid and water
orintate in water to give the most thermodynamically stable conformation
can be multicellular or unicellular
Define multilamellar liposome
These crystals when first formed are usually comprised of several bimolecular lipid lamellae separated by an aqueous layer
What may liposomes carry?
Strongly lipophilic drugs
Strongly hydrophilic drugs
Drugs that have intermediate partition between the lipid and aqueous phase
At even higher concentrations how are the molecules arranged
Amphiphiles form long cylinders that arrange themselves into a roughly hexagonal lattice known as the ‘middle soap phase’
At low amphiphile concentration how are the molecules arranged
Dispersed randomly
At slight higher concentrations how are the molecules arranged
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
At the HIGHEST concentration how are the molecules arranged
a lamellar phase ‘neat soap phase’ may form, wherein extended sheets of amphiphiles are separated by thin layers of water
What does stability depend on?
Lipid composition, storage condition and stabilisers
Define conventional liposomes
Composed of phospholipids
Protect encapsulated molecules from degradation
Passively target tissues or organs that have a discontinuous endothelium
Define long circulating liposomes
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
Define immunoliposomes
Specific antibiotics or antibody fragments on their surface to enhance target site binding
Targeted delivery of anti-cancer agents
Define cationic liposomes
The cationic lipid components of the liposomes interact with and neutralise negatively charged DNA - thereby condensing the DNA into more compact structure
Define the toxicity of liposome formulations
most liposome formations are well tolerated
