Límites en cuanto al objeto

1. In Greek smectos means soap

2. In this the molecules are arranged in layers

3. Force of attraction between the molecules in the layer is weak when compared to the lateral force of attraction between the layers

4. Due to above nature the layers are able to move one another relatively easily 5. Hence these mesophase have fluid properties

6. Depending on the orientation of the molecules Smectic liquid crystals are called Smectic A,B,C etc

Columnar liquid crystal

1. Here the molecules are disk like

2. In this molecules are stacked one above the other forming a column .

3. The columns are arranged in a hexagonal shape

4. Benzene-hexa-n-alkanoates exhibits columnar liquid crystal phase Liquid crystalline behavior in homologous series

1. Homologous series: A series of compounds in which the members have the same functional group and the molecular formula of adjacent member differ by CH2 is called homologous series

Or A series of compounds of the same type wherein successive members Differ in their molecular formulae by a –CH2 group is called a

Homologous series

2. Thermal stability of the members of a homologous series varies with change in the number of carbon atoms in a chain length

3. In a homologous series the transition temp between crystalline state and mesophase changes with change in the number of carbon atoms in the flexible ends

4. A similar change is observed in the transition temperature between mesophase and isotropic liquid state

EX: PAA homologous series :( Para azoxy anisole homologous series)

1. The liquid crystal phase of Para azoxy anisole is stable between 118o_{c & 135}o_c

2. PAA has more than 12 homologous which are formed when –CH2 group are added to its side chain

Para azoxy anisole No of Carbon atoms at flexible ends N N O O H3C CH3 O 2 N N O O C2H5 C2H5 O 4 N N O O C3H9 C3H9 O 6 . . . . 12 N N O O C6H13 C6H13 O

A plot of transition temperatures against number of carbon atoms for PAA series is shown in Fig.

a. When the number of C atoms in the flexible end is 1 to 6, The transition is from solid to Nematic to isotropic liquid. b. When the number of carbon atoms in the flexible end is 7 or 8, The transition is from solid to Smectic to Nematic to

Isotropic liquid

c. When the number of carbon atoms in the flexible end is 9 or10, The transition is from solid to Smectic to isotropic

EX: MBBA homologous series (p-methoxybenzylidine-p-n-butylaniline)

1. In MBBA there is a direct linking of the alkyl chain to the benzene ring on one side (C4H9) and the other alkyl (-CH3) is linked to the benzene ring through oxygen 2. The members of MBBA series do not exhibit Smectic phase; all the members show

transition form solid to nematic to isotropic liquid state.

3. In compounds where the flexible end is alkoxy, the liquid crystalline state is stable at higher temperatures than the ones with alkyl chain as flexible end.

4. It can be seen from the Figure that liquid crystals of PAA are stable between 800 and 1300_{C whereas those of PAAB and MBBA series are stable at relatively lower}

140 120 100 80 PAA CH₃O N=N OCH₃ ↓ O Solid T em pe ra tu re 0 C 4 6 8 10 12 Number of C atoms Liquid Smectic C Nematic MBBA CH₃O CH=N C₄H₉ 1 2 3 4 5 Number of C atoms 60 40 20 Liquid T em pe ra tu re 0 C Solid Nematic (c) Fig.(a)

temperatures (i.e., between 200 _{and 60} 0_{C); hence the latter find application in liquid} crystal displays (LCDs).

In General

• For each of the series, the upper curve represents the transition from liquid crystal state to liquid state (isotropic) and the lower curve shows the transition from solid state to the liquid crystal state.

• The region between the two curves gives the range of temperature at which the liquid crystal exists.

• It can be observed that

5. In general, in all the series, in a curve, as the number of carbon atoms increases, the transition temperature varies and for higher homologues, there is no variation in the transition temperature with change in number of carbon atoms.

6. In the series where the flexible end is alkoxy chain (PAA), the molecules with even number of carbon atoms, in general, show higher transition temperatures than those having odd carbon atoms. The trend is reversed in the case of MBBA

Applications

Liquid crystal Displays (LCD)

Liquid crystal displays are used in a number of applications from clock to oscilloscope. 1. In these devises a thin film of liquid crystals is placed between two sheets of glass.

One of which is coated on one side with a thin layer of an electrically conductive material such a tin oxide

2. When no current is passing though the conductive coating, the molecules(liquid crystal molecules) are uniformly oriented and the light can pass though the cell 3. When the current is applied the molecules alignment changes (because the liquid

crystal molecules are polar) and the cell appears opaque or black. Thus number or a letter is displayed in black against a silvery back ground.

Explanation

a. Light passes though liquid crystal when no electrical current is passed

b. Light is scattered when molecules are disordered due to passing of electric current

Light Light

Instrumental Method of Analysis Types of Instrumental Method of Analysis

Instrumental Method of Analysis can be classified into two types

1. Electrical method : It involves the measurement of current ,voltage or resistance in relation to the concentration of a certain specie in solution

EX: Potentiometric method, Conductometric method etc

2. Optical methods: The optical methods are based on how the sample acts towards the electromagnetic radiation

Theory, Instrumentation, and Application of some Instrumental Method of Analysis

Conductometric:

Definition of conductance: Reciprocal of resistance is called conductance i.e. C=1/R It is expressed in oh-_m-1 _{or siemens}

Theory: The theory involved in conductometric titration is that conductance of an electrolytic solution depends upon the nature, number, and mobility of the ions in solution. Larger the concentration of fast mobile ions higher will be the conductance. Vise versa In conductometric titrations, there is a sudden change in the conductance of the solution near the neutralization point. However the change is not sharp and hence the neutralization point is determined graphically by plotting conductivity against titer values. The principle involved in conductometric titrations is the replacement of ions of a particular conductivity by ions of different conductivity during titration

Applications: Estimation of amount of HCl in HCl solution.