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

Liquid Crystals

Definition

Liquid Crystal is a substance which flows like a liquid but has some degree of ordering in the arrangement of its molecules.

•  In crystalline state, not only do the molecules occupy specific positions but also tend to orient in a preferred direction. Thus the molecules have both positional and orientational order.

• In liquid state, the molecules neither occupy specific positions nor are oriented in any particular manner. The molecules are free to move at random and collide with one another abruptly changing their positions thus losing both positional and orientational order.

• Intermediate between the solid and the liquid state exists the liquid crystal phase, wherein the molecules are free to move but are oriented in a particular manner. The molecules have no positional order but retain some orientation order.
•  Liquid crystal phase is also called mesophase and the molecules, which can exist in
  mesophase, are called mesogens.
• Director : in liquid crystal state the molecules have no positional order but are oriented
  in a particular direction. The preferred orientation of the molecules of a liquid crystal is
  called the Director.