Introduction To Liquid Crystals

PHM2213 Physical Pharmacy 2, 2006/7PHM2213 Physical Pharmacy 2, 2006/7 11

SurfactantsIntroduction to Liquid Crystals

Kausar AhmadKausar AhmadKulliyyah of PharmacyKulliyyah of Pharmacy


ContentsContents

Properties of liquid Properties of liquid crystalscrystals

Types of liquid crystalsTypes of liquid crystals

ThermotropicThermotropic nematicnematic

smecticsmectic

cholestericcholesteric

LyotropicLyotropic structuresstructures

applicationapplication

NIOSOMESNIOSOMES


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


STATES OF MATTER

Common states: solidliquidgas

Matter can exist in other states


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


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


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


Typical chemical structures cholesterol ester phenyl benzoates surfactants such as

polyethylene-oxides, alkali soaps, ammonium salts, lecithin

paraffins glycolipids cellulose derivatives


Typical applications LCD displays dyes (cholesterics) advanced materials (Kevlar) membranes temperature measurement (by changing colours) solvents for GC, NMR, reactions, etc.

Drug delivery


Types of liquid crystalsThermotropic Phase transition depends on temperature

Nematic Smectic Cholesteric

Lyotropic Phase transition depends on temperature &

concentration


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.


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


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


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


- 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


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


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


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


Structure formation in surfactant solution

micelle rod hexagonalmonolayer

bilayerReverse micelleFormation of MICROEMULSION

REVERSEHEXAGONAL

Oil/alcohol


Effect of temperature and concentration on the structure of lyotropic liquid crystals


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


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.


Application of Liposomes can encapsulate:

drugs,

proteins,

enzymes

administered intravenously, orally or intramuscularly

decrease toxicity

increase specificity of drug uptake

enable slow release


Problems with phospholipids

phospholipids undergo oxidative degradation

handling & storage must be under nitrogenexpensive


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


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


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)


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

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/

http://www.gla.ac.uk/~jmb17n/conacher.pdf

http://plc.cwru.edu/tutorial/enhanced/files/lc/phase/phase.htm

http://plc.cwru.edu/tutorial/enhanced/files/lc/phase/phase.htm


http://plc.cwru.edu

http://plc.cwru.edu/

Health & Medicine

Introduction To Liquid Crystals