SurfactantsIntroduction to Liquid Crystals

Kausar Ahmadhttp://staff.iium.edu.my/akausar

akausar@iium.edu.my

CONTENTS

• Properties of liquid crystals

• Types of liquid crystals

– Thermotropic

– Lyotropic

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

STATES OF MATTER

• Common states: solid, liquid, gas

• Matter can exist in other states

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 CRYSTALCharacteristic Crystal Liquid Crystal

Positional order YES/LONG-RANGED NO/SHORT-RANGED

Orientational order YES YES

UPON MELTING ORDER LOST COMPLETELY

ORDERS LOST IN STAGES

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

TYPES OF LCTHERMOTROPIC

LYOTROPIC

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:Cholesteryl_benzoate.png#/media/File:Cholesteryl_benzoate.png

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

CHOLESTERIC• The first LC observed is cholesteryl benzoate.

– Thus, CHOLESTERIC or chiral nematic

• Cholesteric liquid crystals widely used in LCD display

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

REVERSIBLE CHANGES IN PHASES

Cholesteryl myristate

solid Smectic A71C

Cholesteric Isotropic79C 85C

4, 4’-di-heptyloxyazoxybenxene

solid74C 94C 124C

Smectic C Nematic Isotropic

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)

http://plc.cwru.edu/tutorial/enhanced/lab/lab.htm

Click link

THERMOTROPIC VS LYOTROPICTHERMOTROPIC LYOTROPIC

Presence of solvents

NO YESDepends on Temperature,

Concentration, Salt, Alcohol

Molecules Rigid organic molecules e.g. cholesterol ester, phenyl benzoates, paraffins , glycolipids, cellulose derivatives

Surfactant molecules e.g. polyethylene-oxides, alkali soaps, ammonium salts, lecithin

Temperature dependent

YES YES

Structure Smectic , Nematic, Cholesteric Lamellar, Hexagonal etc.

Applications LCD displays, Dyes (cholesterics) Advanced materials (Kevlar),

Temperature measurement (by changing colours)

Biological membranes Drug delivery

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STRUCTURAL CHANGES FOR LYOTROPIC LC

Formation of MICROEMULSION

Tem

pera

ture

Concentration of surfactant

lamellar

micelles

cubic

hexagonal

WATER

Inverse phases:

Inverse cubic

Inverse hexagonal

OIL

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

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.

SURFACTANT VESICLES

• phospholipid/liposomePhospholipids/lecithin + H2O

• tve charge liposome (carriers for DNA)

Liposomes + stearylamine

• - ve charge liposomeLiposomes + dicetyl phosphate

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 nitrogen

expensive

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

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)

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