Liquid crystals, types, characterization, applications.

1. LIQUID CRYSTALS
Presented by:
Mahewash Sana A. Pathan

2. INTRODUCTION
Definition: “ A fourth state of matter
intermediate between solids and liquids”
 As we know, molecules in the liquid state are
mobile in three directions & can rotate freely
about three axes perpendicular to one another.
On the other hand, solids are immobile &
rotations are not as readily possible. Therefore it
is reasonable to suppose that an intermediate
state of mobility & rotation should exist & these
are called as Liquid crystals.

3. HISTORY
 The first recorded observation of LCs was made
by Friedrich Reinitzer in 1888 when he heated
cholesteryl benzoate.
 Cholesteryl benzoate exist in solid state & on
heating at 179˚C, it is converted to liquid. But
Reinitzer observed that at 145˚C a turbid liquid is
formed i.e. liquid crystals.
 The name of this state as liquid crystals was first
suggested by Otto Lehmann.

4. TYPES
Depending on the arrangement, mobility & rotation:
1. Smectic LCs ( soap like/ grease like)-
 These are more solid like & less liquid like
 The mesogens (molecules) have both positional
order & oriental order.
 These are bidimensional crystals.
 There is no overlap of molecules.
 These are not oriented by magnetic field
 These are of most pharmaceutical significance bcoz
it usually forms in ternary or more complex mixtures
containing a surfactant, water, weakly amphiphilic or
non polar additives.

5. 2. Nematic LCs (Thread like)-
 Molecules have no positional order but they have
long range oriental order.
 Molecules are arranged parallel to each other.
 These are more liquid like.
 Molecules are mobile in 3 directions & can rotate
about one axis.
 There may be overlap of molecules.
 They have relatively low viscosity.
 these are less temperature sensitive.

6. 3. Cholesteric LCs (Chiral nematic)-
 Arrangement is to some extent combination of
smectic & nematic LCs.
 Structure is helical, twisting repetition of nematic
phase that corkscrew like slowly changes to
head direction.
 Much thicker than smectic layer.

7. 4. Columnar LCs-
 These are disc shaped.
 Characterized by stacked columns of molecules.

8. Depending on their formation, LCs are classified as:
1. Thermotropic LCs:
 Formed by heating of solids upto a particular
temperature.
 Eg. Cholesteryl benzoate at 145˚C form LCs.
2. Lyotropic LCs:
 Formed from the action of certain solvents on solids.
 Generally, these are two component system, where
amphiphilic compound dissolved in a solvent .
 These can be lamellar, hexagonal and cubic.
 Eg. Soaps.

9. PROPERTIES OF LCs
 LCs have some of the solid properties as well as liquid
properties.
 LCs are mobile.
 They possess property of birefringent.
 LCs show anisotropy in refractive index, dielectric constant,
magnetic property, viscosity & conductivity.
 LCs are sensitive to electric & magnetic field.
 LCs exhibit electro-optical phenomena.
 Molecules which can form LCs are:
i. Organic ( chiral or assymmetric)
ii. Elongated & rectilinear
iii. Rigid
iv. Possess strong dipoles & easily polarizable groups.

10. APPLICATIONS
 Pharmaceutical drugs & excipients have been characterized to
show LC state which can form the basis for a new class of drug
delivery system.
 LCs have applications in food & dermal products.
 Creams, ointments, gels, colloidal dispersions & transdermal
patches are based on Lyotropic LC based delivery system.
 Some LCs show colour changes with temperature, can be used
to detect areas of increased temperature under the skin.
 Smectic LCs have application in solubilization of water insoluble
materials.
 Smectic LCs are frequently present in emulsion & maybe
responsible for increased physical stability due to highly viscous
nature.
 Cholesteric LCs are microencapsulated into particles of small
dimensions, treated with binding materials that will contract so as
to flatten microcapsules & produce brighter colour.

11. CHARACTERIZATION/ EVALUATION OF
LCs
1. X- ray diffraction (XRD)
2. Nuclear magnetic resonance (NMR)
spectroscopy
3. Polarizing microscopy
4. Transmission electron microscopy (TEM)
5. Differential scanning colourimetry (DSC)
6. Rheology
7. Vesicle size determination

12. EXAMPLES OF LIQUID CRYSTALLINE
DRUGS
 Nafoxidine HCl
 Palmitoyl propranolol HCl
 Itraconazole HCl
 Arsphenamine
 Nafcillin
 Follic acid ( vit B9)
 Methotrexate
Naturally occuring LCs:
 DNA
 Cholesterol
 Biological membranes