Summary on Summary on intermolecular forcesintermolecular forces

Origin of intermolecular Origin of intermolecular forcesforces

• Intermolecular forces are the forces that Intermolecular forces are the forces that hold the molecules together.hold the molecules together.

• All intermolecular forces come from same All intermolecular forces come from same origin ----- the polarity of the molecules.origin ----- the polarity of the molecules.

Classification of Classification of intermoleculintermolecular forcesar forces

• Two types of intermolecular forces are Two types of intermolecular forces are hydrhydrogen bondogen bond and and Van der Waals' forcesVan der Waals' forces

• Hydrogen bond only exist in the molecules Hydrogen bond only exist in the molecules that containing hydrogen atom bond to a hthat containing hydrogen atom bond to a highly electronegative atom (N,O,F)ighly electronegative atom (N,O,F)

• Van der Waals' forces exits in All molecules, Van der Waals' forces exits in All molecules, because ideal gas does not existbecause ideal gas does not exist

What is Dipole momentWhat is Dipole moment• Ionic and polar covalent bonds have an unequIonic and polar covalent bonds have an unequ

al sharing of electrons between the two atoms.al sharing of electrons between the two atoms. In these cases one end of the bond is more ne In these cases one end of the bond is more negative and the other more positive. If the molegative and the other more positive. If the molecule is a diatomic species then we call the molcule is a diatomic species then we call the molecule “polar”ecule “polar”

• These “bond dipoles” can be added up in mThese “bond dipoles” can be added up in more complicated molecules. Frequently the suore complicated molecules. Frequently the sum of the bond dipoles gives rise to an overall dm of the bond dipoles gives rise to an overall dipole moment in a polyatomic molecule. Howeipole moment in a polyatomic molecule. However, sometimes, due to symmetry or accident, ver, sometimes, due to symmetry or accident, the bond dipoles can cancel to give rise to a nothe bond dipoles can cancel to give rise to a non-polar molecule (even though the individual n-polar molecule (even though the individual bonds are polar).bonds are polar).

Three types of interactions of VThree types of interactions of Van der Waals' forcesan der Waals' forces

1. permanent dipole-permanent dipole interactions1. permanent dipole-permanent dipole interactions

Permanent dipoles: These occur when 2 atoms in a molecule have substantially different electronegativity — one atom attracts electrons more than another becoming more negative, while the other atom becomes more positive.

2. dipole-induced dipole interactions

Induced dipoles: These occur when one molecule with a permanent dipole repels another molecule's electrons, "inducing" a dipole moment in that molecule.

3.3. instantaneous dipole-induced dipole interactionsinstantaneous dipole-induced dipole interactions(Dispersion)(Dispersion)

Instantaneous dipoles: These occur due to chance when electrons happen to be more concentrated in one place than another in a molecule, creating a temporary dipole.

Relative magnitude of Relative magnitude of different type of different type of interaction  interaction  

Type of interaction  Type of interaction   MagnituMagnitudede

Dipole-dipoleDipole-dipole 5-255-25

dipole-induced dipoledipole-induced dipole 2-102-10

instantaneous dipole-induced instantaneous dipole-induced dipoledipole

0.05-500.05-50

Dipole-dipole >dipole-induced dipole >

instantaneous dipole-induced dipole

Factors affecting strength of VaFactors affecting strength of Van der Walls’ Forcen der Walls’ Force

1.1. Size of Electron cloudSize of Electron cloud The polarizability increases with the no. of electroThe polarizability increases with the no. of electro

n in the molecule (i.e. size of electron cloud)n in the molecule (i.e. size of electron cloud) There is more displacement in the electron cloud There is more displacement in the electron cloud

and unequal distribution of chargesand unequal distribution of charges Strength of Van der Walls’ Force increaseStrength of Van der Walls’ Force increase

2.2. Surface area of moleculeSurface area of molecule As the surface area of a molecule increase, the areAs the surface area of a molecule increase, the are

a contact to other molecules increasea contact to other molecules increase Larger dipole can be inducedLarger dipole can be induced Strength of Van der Walls’ Force increaseStrength of Van der Walls’ Force increase

Hydrogen bonds• A H-bond arises from an un

usually strong dipole-dipole force. When H is bonded to a very electronegative element (F, O, N) the bond is polar covalent. H is unusual because with only one electron, it leaves a partially exposed nucleus (H has no other core electrons to shield the nucleus).

• The bond can be thought of as forming between the hydrogen atom and the lone pairs of the F, N, or O.

Only F, O, and N?• The H-bond occurs between H-atom and a lon

e pair. Can other elements with lone pairs form H-bonds? e.g. Cl?

• F, O and N are all 2nd row elements, which conveys certain properties:

• They are the most electronegative elements• They are small (only 2s, 2p in outer shell) • They have lone pairs• Consequently, although, e.g. Cl lone pairs DO c

ontribute to the dipole-dipole force, this force is not unusual in size (examine the previous plot). So we consider H-bonds ONLY to form between bonds involving FH, OH and NH

Drawing H-bonds

• We indicate a H-bond using a dotted line. The H-bond is strongest when the bond angle is 180º.

Intramolecular H-bonds

•All the H-bonds we’ve seen thus far have been between two molecules (intermolecular H-bonds). H-bonds can also occur within the same molecule (intra-molecular H-bond).

Intramolecular H-bonds

How does theory of intermolecular forcHow does theory of intermolecular forces help to explain physical properties of es help to explain physical properties of substances?substances?• Van der Waals' forces are much weaker than covalent Van der Waals' forces are much weaker than covalent

bond, ionic bond & metallic bondbond, ionic bond & metallic bond• simple molecular substances have simple molecular substances have lower mellower mel

ting point & boiling pointting point & boiling point than giant covalent networ than giant covalent networks, ionic compounds & metalsks, ionic compounds & metals

• Note that we cannot consider intermolecular forces as Note that we cannot consider intermolecular forces as the ONLY FACTORS affecting melting point because ththe ONLY FACTORS affecting melting point because the SYMMETRY of the molecule which affects the ESAE Oe SYMMETRY of the molecule which affects the ESAE OF PACKING INTO A SOLID is also an important factor.F PACKING INTO A SOLID is also an important factor.

• Although van der Waals' forces are usually consider as Although van der Waals' forces are usually consider as a type of weak forces, macromolecules with very large a type of weak forces, macromolecules with very large molecular sizes have very strong van der Waals' forces,molecular sizes have very strong van der Waals' forces, e.g. polyvinyl chloride (high b.p.) e.g. polyvinyl chloride (high b.p.)

How does theory of intermolecular forcHow does theory of intermolecular forces help to explain physical properties of es help to explain physical properties of substances?substances?• Since Van der Waals' forces are much Since Van der Waals' forces are much

weaker than covalent bond, ionic bond and weaker than covalent bond, ionic bond and metallic bond, only small amount of energy metallic bond, only small amount of energy is needed to break the intermolecular is needed to break the intermolecular forces of molecular substances.forces of molecular substances.

• molecular crystals / liquids are molecular crystals / liquids are volatilevolatile

• molecular crystals are molecular crystals are softsoft• Molecular crystals are non-conductors since Molecular crystals are non-conductors since

there is no delocalized electrons.there is no delocalized electrons.

How does theory of intermolecular forcHow does theory of intermolecular forces help to explain chemical properties oes help to explain chemical properties of substances?f substances?• DISSOLVING is a relationship between SOLDISSOLVING is a relationship between SOL

UTE and SOLVENT.UTE and SOLVENT.A solute is dissolved in a solvent providing tA solute is dissolved in a solvent providing that hat

• The attractive force between the solute -solThe attractive force between the solute -solvent is greater than that between the solute vent is greater than that between the solute - solute and between the solvent - solvent - solute and between the solvent - solvent (attraction).(attraction).

How does theory of intermolecular forcHow does theory of intermolecular forces help to explain chemical properties oes help to explain chemical properties of substances?f substances?• non-polar solutes dissolve in non-non-polar solutes dissolve in non-

polar solventspolar solventsParaffin wax (C30H62) is a non-polar Paraffin wax (C30H62) is a non-polar solute that will dissolve in non-polar solute that will dissolve in non-polar solvents like oil, hexane (C6H14) or solvents like oil, hexane (C6H14) or carbon tetrachloride (CCl4).carbon tetrachloride (CCl4).Paraffin wax will NOT dissolve in Paraffin wax will NOT dissolve in polar solvents such as water (H2O) or polar solvents such as water (H2O) or ethanol (ethyl alcohol, C2H5OH). ethanol (ethyl alcohol, C2H5OH).

How does theory of intermolecular forcHow does theory of intermolecular forces help to explain chemical properties oes help to explain chemical properties of substances?f substances?• polar solutes dissolve in polar solvents polar solutes dissolve in polar solvents

polar solutes like glucose (C6H12O6) will diss polar solutes like glucose (C6H12O6) will dissolve in polar solvents such as water (H2O) or eolve in polar solvents such as water (H2O) or ethanol ( alcohol, C2H5OH) thanol ( alcohol, C2H5OH)

• As the partially positively charged atom of the As the partially positively charged atom of the solute molecule is attracted to the partially nesolute molecule is attracted to the partially negatively charged atom of the solvent molecule,gatively charged atom of the solvent molecule, and the partially negatively charged atom of t and the partially negatively charged atom of the solute molecule is attracted to the partially he solute molecule is attracted to the partially positively charged atom of the solvent molecupositively charged atom of the solvent molecule.le.

• And that's why glucose will NOT dissolve in noAnd that's why glucose will NOT dissolve in non-polar solvents such as oil, hexane (C6H14) or n-polar solvents such as oil, hexane (C6H14) or carbon tetrachloride (CCl4). carbon tetrachloride (CCl4).

How does theory of intermolecular forcHow does theory of intermolecular forces help to explain chemical properties oes help to explain chemical properties of substances?f substances?• Ionic solutes dissolve in polar solvents Ionic solutes dissolve in polar solvents

sodium chloride (NaCl) will generally dissodium chloride (NaCl) will generally dissolve in polar solvents such as water (H2solve in polar solvents such as water (H2O)but not in non-polar solvents. O)but not in non-polar solvents.

• since the positive ion of the solute is attrsince the positive ion of the solute is attracted the partially negatively charged atacted the partially negatively charged atom in the polar solvent molecule, and thom in the polar solvent molecule, and the negative ion of the solute is attracted te negative ion of the solute is attracted to the partially positively charged atom oo the partially positively charged atom on the solvent molecule n the solvent molecule

A General SummaryA General SummaryIonic Ionic compoundcompoundss

Covalent Covalent compoundscompounds

MetalsMetals Molecular crystalsMolecular crystals

Basic Basic unitunit

Cations and anCations and anionsions

AtomsAtoms AtomsAtoms MoleculesMolecules

BondinBondingg

Ionic bond betIonic bond between cations aween cations and anionsnd anions

Covalent bond Covalent bond between between shared shared electrons and electrons and atomsatoms

Metallic bond betMetallic bond between mobile electween mobile electrons and cation of rons and cation of metalmetal

Intramolecular force Intramolecular force (covalent bond) + (covalent bond) + intermolecular forces intermolecular forces (van der Waals’ forces)(van der Waals’ forces)

StrengtStrength of h of bondbond

StrongStrong StrongStrong StrongStrong Intramolecular force: Intramolecular force: strongstrongintermolecular forces: intermolecular forces: weakweak

M.P & M.P & B.P.B.P.

HighHigh Very highVery high HighHigh LowLow

HardneHardnessss

Hard and Hard and brittlebrittle

Very hardVery hard Hard and Hard and malleablemalleable

SoftSoft

ConducConductivitytivity

Conduct Conduct electricity in electricity in liquid and liquid and aqueous aqueous statestate

Non-conductor Non-conductor (except (except graphite)graphite)

Conduct Conduct electricity in electricity in both solid and both solid and liquid statesliquid states

Non-conductorNon-conductor

SolubiliSolubilityty

Some are Some are soluble in soluble in waterwater

Insoluble in Insoluble in both water and both water and non-polar non-polar solventsolvent

Insoluble in both Insoluble in both water and non-water and non-polar solventpolar solvent

Soluble in non-polar Soluble in non-polar solventsolvent

Summary of IM Forces• Dispersion forces exist between ALL molecules.

The force increase in strength with molecular mass.

• Forces associated with permanent dipoles are found only in substances with overall dipole moments (polar molecules). Their existence adds to the dispersion forces

• When comparing substances of widely different masses, dispersion forces are usually more significant than dipolar forces.

• When comparing substances of similar molecular mass, dipole forces can produce significant differences in molecular properties (e.g. boiling point).

Summary of IM Forces

Examples to show the exisExamples to show the existence of intermolecular fortence of intermolecular forcesces

Examples to show the existencExamples to show the existence of intermolecular forcese of intermolecular forces• ExampleExample 1: Hot pressing hair 1: Hot pressing hair

• Example2: ProteinExample2: Protein

• Example3: DNAExample3: DNA

• Example4: Soap and detergentExample4: Soap and detergent

• Example5: Water and iceExample5: Water and ice

• Example6: LCDExample6: LCD

ExampleExample 1: Hot pressing hair 1: Hot pressing hair• HOW TO MAKE ?HOW TO MAKE ?• First the hair is washed and partially dried witFirst the hair is washed and partially dried wit

h a towel. A small amount of pressing oil is coh a towel. A small amount of pressing oil is combed through the hair. This oil is for lubricatiombed through the hair. This oil is for lubrication to allow the comb to pass through the hair mn to allow the comb to pass through the hair more easily and also to act as a conductor of heaore easily and also to act as a conductor of heat from the comb to the hair. t from the comb to the hair.

• A metal pressing comb is heated to between 3A metal pressing comb is heated to between 300 and 500 degrees Fahrenheit andis passed q00 and 500 degrees Fahrenheit andis passed quickly through the hair. uickly through the hair.

Principle of hot pressing hairPrinciple of hot pressing hair• The high temperature breaks the The high temperature breaks the

biochemical disulfide bonds between and biochemical disulfide bonds between and within the keratin protein sand allows the within the keratin protein sand allows the hair to be straightened through the tension hair to be straightened through the tension applied to the hair during the combing applied to the hair during the combing procedure.procedure.

• After the comb has passed through the hair After the comb has passed through the hair the temperature drops rapidly and this allows the temperature drops rapidly and this allows the broken biochemical bonds in the hair to the broken biochemical bonds in the hair to reconnect and fix their new position. reconnect and fix their new position.

• This reformation of the bonds holds the hair This reformation of the bonds holds the hair in its new, straightened shape. in its new, straightened shape.

Principle of hot pressing hairPrinciple of hot pressing hair• In the helical protein of hair, hydrogen bonds In the helical protein of hair, hydrogen bonds

within individual helices of keratin, and within individual helices of keratin, and disulfide bridges between adjacent helices, disulfide bridges between adjacent helices, impart strength and elasticity to individual impart strength and elasticity to individual hairs. hairs.

• Water can disrupt the hydrogen bonds, Water can disrupt the hydrogen bonds, making the hair limp. When the hair dries, making the hair limp. When the hair dries, new hydrogen bonding allows it to take on the new hydrogen bonding allows it to take on the shape of a curler. Permanent wave solutions shape of a curler. Permanent wave solutions induce new disulfide bridges between the induce new disulfide bridges between the helices .helices .

• Genetically determined, natural curly hair also Genetically determined, natural curly hair also has a different arrangement of disulfide has a different arrangement of disulfide bridges compared with straight hair. bridges compared with straight hair.

Example2: ProteinExample2: Protein• The primary structure of a protein is a polThe primary structure of a protein is a pol

e peptide which is a polymer of amino acie peptide which is a polymer of amino acids .Polypeptide chains form a helical struds .Polypeptide chains form a helical structure owing to the hydrogen bonds formcture owing to the hydrogen bonds formed between the N-H and C=O groups. ed between the N-H and C=O groups.

• This creates the secondary structure of pThis creates the secondary structure of proteins .In many proteins, including thosroteins .In many proteins, including those in hair, wool and nauls , hydrogen bone in hair, wool and nauls , hydrogen bonding causes the polypeptide chains to beding causes the polypeptide chains to become twisted into tightly coiled helices.come twisted into tightly coiled helices.

Polypeptides (proteins)• polypeptides are another biopolymer, and we

shall examine their structure further…

Example2: ProteinExample2: Protein

• ˙̇In the pleated sheet arrangementIn the pleated sheet arrangement        protein chains run parallel or in alternating         protein chains run parallel or in alternating directions. The chains are held together by directions. The chains are held together by hydrogen bonds that join the H of an NH group to hydrogen bonds that join the H of an NH group to the O in a CO group on the neighboring chain. the O in a CO group on the neighboring chain.

Example2: ProteinExample2: Protein

• ˙̇In the a-helical arrangementIn the a-helical arrangement         protein chain is coiled into a helix. Each NH group is          protein chain is coiled into a helix. Each NH group is hydrogen bonded to a CO group one helical turn (3.6 amino hydrogen bonded to a CO group one helical turn (3.6 amino acid units) away in the same chain, giving a fairly rigid acid units) away in the same chain, giving a fairly rigid cylindrical structure with side chains on the outside.cylindrical structure with side chains on the outside.

Polypeptides (or proteins)

Example3: DNAExample3: DNA• DNA is present in the nuclei of living cells and cDNA is present in the nuclei of living cells and c

arries genetic information. The DNA molecule arries genetic information. The DNA molecule consists of two helical nucleic acid chains whicconsists of two helical nucleic acid chains which is very stable. h is very stable.

• Each nucleic acid is made up of 3 components :Each nucleic acid is made up of 3 components : a sugar , a phosphoric acid unit and a nitroge a sugar , a phosphoric acid unit and a nitrogen-containing heterocyclic base : adenine , cyton-containing heterocyclic base : adenine , cytosine, guanine or thymine .The two nucleic acisine, guanine or thymine .The two nucleic acid chains are held together by hydrogen bonds. d chains are held together by hydrogen bonds.

• These hydrogen bonds are formed between spThese hydrogen bonds are formed between specific pairs of bases on the chains. The two strecific pairs of bases on the chains. The two strands coil tightly tound each other . ands coil tightly tound each other .

Hydrogen Bonding in biological molecules

Example3: DNAExample3: DNA

•structure of structure of DNADNA

Example4: Soap and Example4: Soap and detergentdetergent• Molecules liquid state experience Molecules liquid state experience

strong strong intermolecular attractive intermolecular attractive forcesforces. When those forces are . When those forces are between like molecules, they are between like molecules, they are referred to as cohesive forces.referred to as cohesive forces.

• the molecules of a water droplet are the molecules of a water droplet are held together by cohesive forces, and held together by cohesive forces, and the especially strong cohesive forces the especially strong cohesive forces at the surface to form surface tension.at the surface to form surface tension.Surface tension is a type of Surface tension is a type of intermolecular forces.intermolecular forces.

The Role of Surfactants•Surfactants are molecules that act as

a bridge between polar and

•non-polar molecules, thereby considerably increasing solubility.

•They achieve this by having a polar/ionic end, which interacts preferentially with polar molecules, and a non-polar end, which interacts preferentially with non-polar molecules.

Detergent actionDetergent action • A detergent is just one type of surfactant. A detergent is just one type of surfactant.

Surfactant molecules occupy the spaces between Surfactant molecules occupy the spaces between water molecules at the surface, reducing the water molecules at the surface, reducing the force of attraction between the water molecules, force of attraction between the water molecules, lowering the surface tension.lowering the surface tension.

The Role of Surfactants

Example5: Water and iceExample5: Water and ice• Hydrogen bondHydrogen bondinging in in Water or iceWater or ice• A water molecule is composed of two hydrogen atoA water molecule is composed of two hydrogen ato

ms (H) and one oxygen atom (O). The atoms of hydrms (H) and one oxygen atom (O). The atoms of hydrogen and oxygen are bound by sharing their electroogen and oxygen are bound by sharing their electrons with one another. This bond is called a “covalens with one another. This bond is called a “covalent bond”.nt bond”.

• However, since oxygen atoms pull electrons more However, since oxygen atoms pull electrons more strongly than hydrogen atoms, the oxygen atom in strongly than hydrogen atoms, the oxygen atom in a water molecule has a slightly negative charge ana water molecule has a slightly negative charge and the hydrogen atoms have a slightly positive chard the hydrogen atoms have a slightly positive charge. ge.

• So adjacent water molecules So adjacent water molecules are attractedare attracted toto on one another e another through the slightlythrough the slightly negatively charge negatively charged oxygen atoms d oxygen atoms and the slightlyand the slightly positively charg positively charged hydrogen atoms. This interaction is called “hyded hydrogen atoms. This interaction is called “hydrogen bonding”. rogen bonding”.

WaterWater• Structure of waterStructure of water• A water molecule is formed when twA water molecule is formed when tw

o atoms of o atoms of hydrogen bondhydrogen bond covalencovalentlytly with an atom of with an atom of oxygenoxygen. In a cov. In a covalent bond electrons are shared betalent bond electrons are shared between atoms. In water the sharing is ween atoms. In water the sharing is not equal. The oxygen atom attracts not equal. The oxygen atom attracts the electrons more strongly than ththe electrons more strongly than the hydrogen. This gives water an e hydrogen. This gives water an asyasymmetrical distribution of chargemmetrical distribution of charge. .

• Molecules that have ends with partiMolecules that have ends with partial negative and positive charges are al negative and positive charges are known as polar molecules. It is this known as polar molecules. It is this polar property that allows water to polar property that allows water to separate polar solute molecules and separate polar solute molecules and explains why water can dissolve so explains why water can dissolve so many substances. many substances.

The special case of water• Water is perhaps the most unusual liquid. Each water

molecule is H-bonded to FOUR other water molecules (donating 2 H atoms and accepting two H-atoms to the lone pairs), forming a tetrahedral network.

iceice• Structure of iceStructure of ice• In ice, each water forms four hydIn ice, each water forms four hyd

rogen bonds with O---O distancerogen bonds with O---O distances of 2.76 Angstroms to the neares of 2.76 Angstroms to the nearest oxygen neighbor. The O-O-O ast oxygen neighbor. The O-O-O angles are 109 degrees, typical of ngles are 109 degrees, typical of a a tetrahedrally coordinated lattitetrahedrally coordinated lattice structurece structure. The density of ice Ih . The density of ice Ih is 0.931 gm/cubic cm. is 0.931 gm/cubic cm.

• This compare with a density of 1.This compare with a density of 1.00 gm/cubic cm. for water.00 gm/cubic cm. for water.

iceice• Structure of iceStructure of ice• There are lots of diffeThere are lots of diffe

rent ways that the wrent ways that the water molecules can bater molecules can be arranged in ice. The arranged in ice. The one below is knowe one below is known as "cubic ice", or "in as "cubic ice", or "ice Ic". It is based on tce Ic". It is based on the water molecules ahe water molecules arranged in a diamonrranged in a diamond structure.d structure.

Why does ice float on water?Why does ice float on water?• The arrangement of water molecules The arrangement of water molecules

in ice creates an open structure . This in ice creates an open structure . This accounts for the fact that ice is less accounts for the fact that ice is less dense than water at 273K . dense than water at 273K .

• When ice melts, the regular lattice When ice melts, the regular lattice breaks up .the water molecules can breaks up .the water molecules can then pack more closely together ,so its then pack more closely together ,so its liquid form has a higher density. liquid form has a higher density.

• In water , the strong hydrogen bonding In water , the strong hydrogen bonding results in some ordered packing of results in some ordered packing of water molecules over a short range. water molecules over a short range.

Why does ice float on water?Why does ice float on water?• The fact that ice is less dense than The fact that ice is less dense than

water at 273K make ponds and lakes water at 273K make ponds and lakes freeze from the surface downwards. freeze from the surface downwards.

• The layer of ice insulates the water The layer of ice insulates the water below and prevent complete below and prevent complete solidification. That allows solidification. That allows fish ,aquatic plants and other aquatic fish ,aquatic plants and other aquatic organisms to survive.organisms to survive.

Example6: LCDExample6: LCD• A A liquid crystalliquid crystal display, or LCD, is a thin, light display, or LCD, is a thin, light

weight weight display devicedisplay device with no moving parts. with no moving parts. It consists of an electrically-controlled lighIt consists of an electrically-controlled light-polarising liquid trapped in cells between t-polarising liquid trapped in cells between two transparent polarising sheets. two transparent polarising sheets.

• The polarising axes of the two sheets are aliThe polarising axes of the two sheets are aligned perpendicular to each other. Each cell gned perpendicular to each other. Each cell is supplied with electrical contacts that allois supplied with electrical contacts that allow an w an electric fieldelectric field to be applied to the liquid i to be applied to the liquid insidenside

About About liquid crystalliquid crystal • Liquid crystalsLiquid crystals are a class of are a class of moleculemolecules s

that, under some conditions, inhabit a that, under some conditions, inhabit a phasephase in which they exhibit in which they exhibit isotropicisotropic, , fluidfluid-like behavior – that is, with little lo-like behavior – that is, with little long-range ordering – but which under othng-range ordering – but which under other conditions inhabit one or more phaseer conditions inhabit one or more phases with significant anisotropic structure as with significant anisotropic structure and long-range ordering while still having nd long-range ordering while still having an ability to flow.an ability to flow.

Example6: LCDExample6: LCD• Before an electric field is applied, the long, thiBefore an electric field is applied, the long, thi

n molecules in the liquid are in a relaxed state. n molecules in the liquid are in a relaxed state. Ridges in the top and bottom sheet encourage Ridges in the top and bottom sheet encourage polarisation of the molecules parallel to the ligpolarisation of the molecules parallel to the light polarisation direction of the sheets. ht polarisation direction of the sheets.

• Between the sheets, the polarisation of the moBetween the sheets, the polarisation of the molecules twists naturally between the two perpelecules twists naturally between the two perpendicular extremes. Light is polarised by one shndicular extremes. Light is polarised by one sheet, rotated through the smooth twisting of theet, rotated through the smooth twisting of the crystal molecules, then passes through the se crystal molecules, then passes through the second sheet. econd sheet.

• The whole assembly looks nearly transparent. The whole assembly looks nearly transparent. A slight darkening will be evident because of liA slight darkening will be evident because of light losses in the original polarising sheet.ght losses in the original polarising sheet.

Example6: LCDExample6: LCD• Reflective twisted nematic liquid crystal diReflective twisted nematic liquid crystal di

splay. splay. • 1.Vertical filter film to polarize the light as i1.Vertical filter film to polarize the light as i

t enters. t enters. • 2.Glass substrate with ITO electrodes. The 2.Glass substrate with ITO electrodes. The

shapes of these electrodes will determine shapes of these electrodes will determine the dark shapes that will appear when the the dark shapes that will appear when the LCD is turned on. Vertical ridges are etcheLCD is turned on. Vertical ridges are etched on the surface so the liquid crystals are id on the surface so the liquid crystals are in line with the polarized light. n line with the polarized light.

• 3.Twisted nematic liquid crystals. 3.Twisted nematic liquid crystals. • 4.Glass substrate with common electrode f4.Glass substrate with common electrode f

ilm (ITO) with horizontal ridges to line up ilm (ITO) with horizontal ridges to line up with the horizontal filter. with the horizontal filter.

• 5.Horizontal filter film to block/allow thro5.Horizontal filter film to block/allow through light. ugh light.

• 6.Reflective surface to send light back to vi6.Reflective surface to send light back to viewer. ewer.

THE ENDTHE END

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