introduction to liquid crystals
TRANSCRIPT
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SurfactantsIntroduction to Liquid Crystals
Kausar AhmadKausar AhmadKulliyyah of PharmacyKulliyyah of Pharmacy
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ContentsContents
Properties of liquid Properties of liquid crystalscrystals
Types of liquid crystalsTypes of liquid crystals
ThermotropicThermotropic nematicnematic
smecticsmectic
cholestericcholesteric
LyotropicLyotropic structuresstructures
applicationapplication
NIOSOMESNIOSOMES
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Synergistic Effects of Surfactants Observed when surfactants having relatively similar
structure or ionic property are mixed Resulted in the formation of liquid crystal structures or
complexes at the interface by intermolecular interactions between surfactants
ExamplesAnionic and nonionic in synthetic latex emulsion
polymerisation, Mixture of a dispersant and a hydrating agent to
increase dispersion stability in agricultural chemicals
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STATES OF MATTER
Common states: solidliquidgas
Matter can exist in other states
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LIQUID CRYSTALSA state that occurs between a solid & a liquidPossess properties characteristics of both
liquids & crystalline solidsAlso possess properties not found in either
liquids or solidsMay response to external perturbations &
some changes colour with temperature
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A crystal is a highly ordered structure which possesses long-range positional & orientational order
For many substances these two types of order are destroyed simultaneously when the crystal melts to form a liquid
For some substances, these orders are destroyed in stages. These are liquid crystals. E.g. Slide 17
Crystals vs Liquid Crystals
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Liquid crystal can flow like a liquid, due to loss of positional order Liquid crystal is optically birefringent, due to its orientational order Transition from crystalline solids to liquid crystals caused by a
change of temperature – gives rise to THERMOTROPIC liquid crystals substances that are most likely to form a liquid crystal phase at
a certain temperature are molecules that are ELONGATED & have some degree of RIGIDITY. Try slide 29
Properties of liquid crystals
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Typical chemical structures cholesterol ester phenyl benzoates surfactants such as
polyethylene-oxides, alkali soaps, ammonium salts, lecithin
paraffins glycolipids cellulose derivatives
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Typical applications LCD displays dyes (cholesterics) advanced materials (Kevlar) membranes temperature measurement (by changing colours) solvents for GC, NMR, reactions, etc.
Drug delivery
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Types of liquid crystalsThermotropic Phase transition depends on temperature
Nematic Smectic Cholesteric
Lyotropic Phase transition depends on temperature &
concentration
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As temperature increases….. The first liquid crystal phase is the smectic A, where there
is layer-like arrangement as well as translational and rotational motion of the molecules.
A further increase in temperature leads to the nematic phase, where the molecules rapidly diffuse out of the initial lattice structure and from the layer-like arrangement as well.
At the highest temperatures, the material becomes an isotropic liquid where the motion of the molecules changes yet again.
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Nematic Simplest form is a
nematic liquid crystal
i.e. long-range orientational order but no positional order
The preferred direction is known as director
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Nematic…Nematic… Despite the high degree of orientational order, nematic phase
as a whole is in disorder i.e. NO MACROSCOPIC ORDER (orientation within a group is similar but not from one group to another)
Structure of nematic phase can be altered in a number of ways. E.g. electric or magnetic field or treatment of surfaces of the sample container
Thus, possible to have microscopic order & macroscopic order
Nematic liquid crystals are widely used in electro-optic display devices
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Cholesteric The first liquid crystal that was observed through a polarising microscope
is cholesteryl benzoate. Thus, CHOLESTERIC liquid crystal OR chiral nematic liquid crystal
E.g. cholesteryl benzoate: LC @ 147C, isotropic @ 186C
Cholesteric liquid crystals have great potential uses as
- sensors
- Thermometer
- fashion fabrics that change colour with temperature
- display devices
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- In CHOLESTERIC phase, there is orientational order & no positional order, BUT, director is in HELICAL ORDER.
The structure of cholesteric depends on the PITCH, the distance over which the director makes one complete turn One pitch - several hundred
nanometers Pitch is affected by:-
Temperature Pressure Electric & magnetic fields
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Smectic SMECTIC phase occurs at temperature below
nematic or cholesteric
Molecules align themselves approx. parallel & tend to arrange in layers
Not all positional order is destroyed when a crystal melts to form a smectic liquid crystal
Chiral smectic C liquid crystals are useful in LCDS
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LIQUID CRYSTAL POLYMERSCan form nematic, cholesteric, smecticWhen liquid crystal polymers solidify, the liquid
crystal structure ‘freeze in’ This results in materials of high tensile strength
& in some cases unusual electro-optical behaviour
E.g. Kevlar aramid fibre – bullet-proof vest & airplane bodies (aromatic polyamide)
Examples of phase changesCholesteryl myristate
solid smectic A71C cholesteric isotropic79C 85C
4, 4’-di-heptyloxyazoxybenxene
solid74C 94C 124C
smectic C nematic isotropic
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Thermotropic vs LyotropicTHERMOTROPIC Absence of solvent Rigid organic molecules Depends on Temperature Structures:
Smectic Nematic Cholesteric
LYOTROPIC In solvent Surfactants Depends on Temperature,
Concentration, salt, alcohol Structures:
Lamellar Hexagonal etc
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Structure formation in surfactant solution
micelle rod hexagonalmonolayer
bilayerReverse micelleFormation of MICROEMULSION
REVERSEHEXAGONAL
Oil/alcohol
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Effect of temperature and concentration on the structure of lyotropic liquid crystals
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SURFACTANT VESICLES[A] Phospholipids (e.g. lecithin) + H2O ---->
phospholipid vesicles or liposomes
[B] Liposomes + (long chain) stearylamine ------->tve charge liposome (carriers for DNA)
[C] Liposomes + dicetyl phosphate ------ ve charge liposome
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Vesicles Bilayers that fold into a 3D structure Vesicles form because they get rid of
the edges of bilayers, protecting the hydrophobic chains from the water, but they still allow for relatively small layers.
Lipids found in biological membranes spontaneously form vesicles in solution.
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Application of Liposomes can encapsulate:
drugs,
proteins,
enzymes
administered intravenously, orally or intramuscularly
decrease toxicity
increase specificity of drug uptake
enable slow release
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Problems with phospholipids
phospholipids undergo oxidative degradation
handling & storage must be under nitrogenexpensive
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Formation of liquid crystals using surfactants[A] Anionic
e.g. alkane sulfonates
[B] Cationic
e.g. hexadecyl trimethyl ammonium bromide
[C] Amphoteric
e.g. alkyl betaines
Due to toxicity of ionic surfactants, the vesicles are not used for drug delivery
[D] Non-ionice.g. alcohol ethoxylates R-O-(CH2CH2O) m H m: 2-20, R : mixed;
alkyl group C8C18
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Non-ionic + cholesterol -> NIOSOMES These vesicles prolong the circulation of entrapped
drug Properties depend on
1. Composition of bilayer
2. Method of productione.g. cholesterol & single alkyl-chain non-ionic surfactant with a glyceril head group
Niosomes
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Examples of niosome applications Ketoconazole niosomes were prepared by using surfactant
(Tween 40 or 80), cholesterol and drugSatturwar PM; Fulzele SV; Nande VS; Khandare, JN Indian Journal of Pharmaceutical Sciences. 2002 Mar-Apr; 64(2): 155-8
Use for topical immunisation - Bovine serum albumin (BSA)loaded niosomes composed of sorbitan monostearate/sorbitan trioleate (Span 60/Span 85), cholesterol and stearylamine as constitutive lipidsSanyog Jain1, S. P. Vyas2, Journal of Pharmacy and Pharmacology Vol. 57, No. 9, pages 1177 (2005)
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References1. http://www.lci.kent.edu/lc.html#Description
2. http://liqcryst.chemie.uni-hamburg.de/lc_lc.php
3. http://www.glycoprojects.kimia.um.edu.my/website/Glyco/ (carbohydrate liquid crystal)
4. http://www.gla.ac.uk/~jmb17n/conacher.pdf (niosomes)
5. http://plc.cwru.edu/tutorial/enhanced/files/lc/phase/phase.htm
6. J Kreuter (ed.), Colloidal Drug Delivery Systems, Marcel Dekker, New York (1994) Chapter 3&4
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http://plc.cwru.edu