Classification of liquid crystals

Classification of liquid crystals

• Liquid crystals may be broadly classified into twoThermotropic liquid crystals: are those that exhibit liquid crystalline state on change of temperature alone. Example: para azoxyanisole.
•  Lyotropic liquid crystals: these exhibit liquid crystalline state in mixtureand when the concentration of one of the constituents is varied. Example: soap.

 Nematic liquid crystals:

• These have thread like texture. ( Greek nematos = thread).
• These are formed from optically inactive compounds.
• The molecules do not have positional order but the molecules are arranged parallel to oneanother and hence have orientational order.
• Less Viscous as they are less closely arranged
• Fluidity is more
• p- azoxy anisole is an example for nematic liquid crystals. 

Chiral nematic or twisted nematic or cholesteric liquid crystals.

• Cholesteric liquid crystals exhibit finger print texture.
• These are formed from optically active compounds.
• A group of molecules is oriented at an angle to the adjacent group of molecules such that the director takes a helical path as it travels through the liquid crystal just as a nut is moved on to a screw.
• The distance through which a director travels as it completes one full rotation is called the pitch of the liquid crystal.
• Less Viscous as they are less closely arranged
• Fluidity is more
• Eg for chiral nematic liquid crystal is cholesteryl benzoate.

Smectic liquid crystals:

• Greek smectos = soap
• Smectic liquid crystals exhibit focal conic (broken fan )texture
• Molecules are arranged in layers. At any instant of time the number of molecules within a layer is much more than the number of molecules between the layers.
• Highly Viscous as they are closely arranged
• Fluidity is less
• Depending of the orientation of the molecules( inclination to the director) smectic liquid crystals are called smectic A,.B,C etc.

Discotic liquid crystals:

•  Here the molecules are disk like.
• These may be columnar liquid crystals or discotic nematic liquid crystals.
• In columnar liquid crystals, the molecules are stacked one above the other forming a column. In discotic nematic liquid crystals the molecules have coin like shape and have no positional order but possess orientational order.The columns form definite shape such as hexagon.
• Highly Viscous as they are closely arranged
• Fluidity is less
• Eg: Benzene-hexa –n- alkanoates