Transcript Liquid Crystals

Surfactants
Introduction to Liquid Crystals
Kausar Ahmad
http://staff.iium.edu.my/akausar
[email protected]
CONTENTS
• Properties of liquid crystals
• Types of liquid crystals
– Thermotropic
– Lyotropic
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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 at the
interface due to intermolecular interactions between
surfactants
• Examples
– Anionic 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: solid, liquid, gas
• Matter can exist in other states
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LIQUID CRYSTALS
A state that occurs between a solid & a liquid
Possess properties characteristics of both liquids & crystalline
solids
Also possess properties not found in either liquids or solids
May response to external perturbations & some changes
colour with temperature
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CRYSTAL VERSUS LIQUID CRYSTAL
Characteristic
Crystal
Liquid Crystal
Positional order
YES/LONG-RANGED
NO/SHORT-RANGED
Orientational order
YES
YES
UPON MELTING
ORDER LOST
COMPLETELY
ORDERS LOST IN
STAGES
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PROPERTIES OF LC
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
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TYPES OF LC
THERMOTROPIC
LYOTROPIC
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THERMOTROPIC LC
The first liquid crystal phase is the SMECTIC A
• layer-like arrangement as well as translational and rotational
motion of the molecules.
A further increase in temperature leads to the NEMATIC
• molecules rapidly diffuse out of the initial lattice structure and
from the layer-like arrangement as well.
At the highest temperatures, the material becomes ISOTROPIC LIQUID
• Motion of the molecules changes all the time.
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PROPERTIES OF LC MOLECULES
• Substances with molecules that are
– ELONGATED
– HAVING SOME DEGREE OF RIGIDITY.
"Cholesteryl benzoate". Licensed under Public
Domain via Commons https://commons.wikimedia.org/wiki/File:Cholester
yl_benzoate.png#/media/File:Cholesteryl_benzoat
e.png
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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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CHOLESTERIC
• The first LC observed is cholesteryl benzoate.
– Thus, CHOLESTERIC or chiral nematic
• Cholesteric liquid crystals widely used in LCD display
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SMECTIC
• SMECTIC phase occurs at temperature below nematic or
cholesteric
• Molecules align themselves approx. parallel & tend to
arrange in layers
• Chiral Smectic C liquid crystals are useful in LCD
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REVERSIBLE CHANGES IN PHASES
Cholesteryl myristate
solid
71C
Smectic A
79C
Cholesteric
85C
Isotropic
4, 4’-di-heptyloxyazoxybenxene
solid
74C
94C
124C
Nematic
Isotropic
Smectic C
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LIQUID CRYSTAL POLYMERS
Can form nematic, cholesteric, smectic
When 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)
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http://plc.cwru.edu/tutorial/enhanced/lab/lab.htm
Click link
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THERMOTROPIC VS LYOTROPIC
Presence of
solvents
Molecules
Temperature
dependent
Structure
Applications
THERMOTROPIC
LYOTROPIC
NO
YES
Depends on Temperature,
Concentration, Salt, Alcohol
Rigid organic molecules e.g. cholesterol
ester, phenyl benzoates, paraffins ,
glycolipids, cellulose derivatives
Surfactant molecules e.g.
polyethylene-oxides, alkali soaps,
ammonium salts, lecithin
YES
YES
Smectic , Nematic, Cholesteric
Lamellar, Hexagonal etc.
LCD displays, Dyes (cholesterics)
Advanced materials (Kevlar),
Temperature measurement (by changing
colours)
Biological membranes
Drug delivery
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Temperature
STRUCTURAL CHANGES FOR LYOTROPIC LC
Formation of MICROEMULSION
WATER
lamellar
hexagonal
Inverse phases:
Inverse cubic
Inverse hexagonal
cubic
OIL
micelles
Concentration of surfactant
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FORMATION OF LIQUID CRYSTALS USING SURFACTANTS
Anionic
• e.g. alkane sulfonates
Cationic
• e.g. hexadecyl trimethyl ammonium bromide
Amphoteric
• e.g. alkyl betaines
Non-ionic
• e.g. alcohol ethoxylates
• R-O-(CH2CH2O) mH; m: 2-20, R: alkyl group C8-C18
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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
• Lipids found in biological membranes spontaneously form
vesicles in solution.
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SURFACTANT VESICLES
Phospholipids/lecithin
+ H 2O
Liposomes +
stearylamine
• phospholipid/liposome
• tve charge liposome
(carriers for DNA)
Liposomes +
dicetyl phosphate
• - ve charge liposome
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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 nitrogen
expensive
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NIOSOMES
Non-ionic + cholesterol -> NIOSOMES
These vesicles prolong the circulation of entrapped drug
Properties depend on
• Composition of bilayer
• Method of production
e.g. cholesterol & single alkyl-chain non-ionic surfactant with a
glyceril head group
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EXAMPLES OF NIOSOME APPLICATIONS
• Ketoconazole niosomes
– prepared using surfactant (Tween 40 or 80), cholesterol
– Satturwar PM; Fulzele SV; Nande VS; Khandare, JN Indian
Journal of Pharmaceutical Sciences. 2002 Mar-Apr; 64(2): 155-8
• Bovine serum albumin (BSA)loaded niosomes - Topical
immunisation
– composed of sorbitan monostearate/sorbitan trioleate (Span
60/Span 85), cholesterol and stearylamine as constitutive lipids
– Sanyog Jain, S. P. Vyas, Journal of Pharmacy and Pharmacology
Vol. 57, No. 9, pages 1177 (2005)
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REFERENCES
http://www.lci.kent.edu/lc.html#Description
http://liqcryst.chemie.uni-hamburg.de/lc_lc.php
http://www.glycoprojects.kimia.um.edu.my/website/Glyco/ (carbohydrate
liquid crystal)
http://www.gla.ac.uk/~jmb17n/conacher.pdf (niosomes)
http://plc.cwru.edu/tutorial/enhanced/files/lc/phase/phase.htm
Kreuter, J. (ed.) (1994). Colloidal Drug Delivery Systems. New York: Marcel
Dekker, Chapter 3 & 4
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