FORCES OF ATTRACTION AND STATES OF MATTER
Physical Pharmacy (0510219)Unit 1FORCES OF ATTRACTIONANDSTATES OF MATTER1
FORCES OF ATTRACTION Intramolecular Forces Typesforces of attraction within the moleculeIonic Bonds Transfer of electrons between a non metal & ametal Observed in formation of salts , example NaClCovalent Bond Sharing of electrons between two non metals Observed in organic compounds , example CH42
Intermolecular ForcesForces of attraction between molecules TypesBinding & Attractive Forces Binding ForcesCohesion – similar moleculesAdhesion – different moleculesRepulsive – prevent molecules from annihilating each other Attractive ForcesVan der Waals3
Binding Forces BetweenMolecules For molecules to exist as aggregates in gases,liquids, and solids, intermolecular forces mustexist.Intermolecular forcesCohesion, or the attraction of like molecules,Adhesion, or the attraction of unlike molecules,Repulsion is a reaction between two moleculesthat forces them apart.For molecules to interact, these forces must bebalanced in an energetically favored arrangement.4
Repulsive and AttractiveForces When molecules interact, both repulsive andattractive forces operate.When the molecules are brought so close thatthe outer charge clouds touch, they repel each.Thus, attractive forces are necessary for molecules tocohere, whereas repulsive forces act to prevent themolecules from interpenetrating and annihilatingeach other.5
Repulsive Forces Repulsion is due to the interpenetration of the electronic cloudsof molecules and increases exponentially with a decrease indistance between the molecules. At a certain equilibrium distance, about (3-4) 10-8 cm(3–4 Å ), the repulsive and attractive forces are equal. At this position, the potential energy of the two molecules is aminimum and the system is most stable . The effect of repulsion on the intermolecular three-dimensionalstructure of a molecule is well illustrated in considering theconformation of the two terminal methyl groups in butene, wherethey are energetically favored in the trans - conformationbecause of a minimization of the repulsive forces.6
Repulsive and attractive energies and net energyas a function of the distance between molecules.Repulsion increasesexponentially with a decrease indistance between the molecules(3–4 Å ), the repulsiveand attractive forces areequal a minimum occurs in the net energy (potential energy)because of the different character of the attraction andrepulsion curves.7
Attractive Forces Van der Waals ForcesOrbital OverlapIon–Dipole and Ion-Induced DipoleForcesIon–Ion InteractionsHydrogen Bonds8
Attractive Forces :1. Van der Waals Forces Van der Waals forces relate to nonionicinteractions between molecules, yet they involvecharge – charge interactions.partial charges can be permanent or be inducedby neighboring groupsTypes of Van der Waals Forces: Keesom forcesDebye forcesLondon forces9
Van der Waal interactions are weak forces thatinvolve the dispersion of charge across a moleculecalled a Permanent dipole.Permanent dipoleIn a permanent dipole, as illustrated by the peptidebond, the electronegative oxygen draws the pair ofelectrons in the carbon–oxygen double bond closerto the oxygen nucleus.10
The bond then becomes polarized due to the fact that theoxygen atom is strongly pulling the nitrogen lone pair ofelectrons toward the carbon atom, thus creating a partialdouble bond.the nucleus of the nitrogen atom pulls the electron pairinvolved in the nitrogen–hydrogen bond closer to itself andcreates a partial positive charge on the hydrogen.11
Keesom forces: dipole–dipole interaction.the permanent dipoles interact with oneanother in an ion-like fashion.the negative pole of one molecule pointstoward the positive pole of the next.because the charges are partial, thestrength of bonding is much weaker.12
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Debye forces: dipole-induced dipoleinteractionsshow the ability of apermanent dipole topolarize charge in aneighboring nonpolareasily polarizablemolecule.14
London forces: induced dipole-induced dipole interaction.Two neighboring nonpolar molecules, for example, Lipid corealiphatic hydrocarbons chains of a biological membrane ,interior neighboring chainsinduce partial chargedistributions (polarity) .Helps to hold the interior interactgive rise to the fluidity andcohesiveness of the membrane.15
Types of van der Waals forces:16
In all three types of van der Waals forces, the potentialenergy of attraction varies inversely with the distance ofseparation, r, raised to the sixth power, r6.The potential energy of repulsion changes more rapidlywith distance.This accounts for the potential energy minimum and theresultant equilibrium distance of separation, re.17
Attractive Forces :2. Orbital overlap dipole–dipole force.the interaction between pi-electron orbitals in systems.For example, aromatic–aromatic interactions can occur whenthe double-bonded pi-orbitals from the two rings overlap18
3. Ion–Dipole and Ion-Induced DipoleForces occur between polar or nonpolar molecules and ions.account in part for the solubility of ionic crystallinesubstances in water; the cation, for example, attracts therelatively negative oxygen atom of water and the anionattracts the hydrogen atoms of the dipolar watermolecules.Ion-induced dipole forces are involved in the formation ofthe iodide complex,19
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4. Ion–Ion Interactions An ionic, electrovalent bond between two counter ions isthe strongest bonding interaction and can persist overthe longest distance.weaker ion–ion interactions, in salt formations,exist and influence pharmaceutical systems.A cation on one compound will interact with an anion onanother compound, giving rise to an intermolecularassociation.Ion–ion interactions can also be repulsive when two ionsof like charge are brought closely together.21
5. Hydrogen Bonds The interaction between a molecule containing ahydrogen atom and a strongly electronegative atomsuch as fluorine, oxygen, or nitrogen.it accounts for many of the unusual properties ofwater including its high dielectric constant,abnormally low vapor pressure, and high boilingpoint.Roughly one sixth of the hydrogen bonds of ice arebroken when water passes into the liquid state, andessentially all the bridges are destroyed when itvaporizes.Hydrogen bonds can also exist between alcoholmolecules, carboxylic acids, aldehydes, esters, andpolypeptides22
hydrogen-bonded structures23
Intermolecular Forces and Valence Bonds24
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1 SolidE2 Liquid E3 GasESolids with vapour pressure e.g. iodine & camphor4 Liquid crystal27
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Gases The intermolecular forces of attraction in gases are virtuallynonexistent at room temperature; gases display little or noordering. Molecules in the gaseous state move in all directions, at highvelocities until collisions occur with other molecules andboundaries of the system (e.g. the walls of a containerholding the gas). This process is responsible for the pressure exhibited withinthe system.29
Gases Hence, they exert a pressure —a force per unitarea, expressed in dynes/cm2 or in atmospheresor in millimeters of mercury (mmHg). Another important characteristic of a gas,its volume, is usually expressed in liters or cubiccentimeters (1 cm3 1 mL). The temperature involved in the gas equations isgiven according the absolute or Kelvin scale.Zero degrees on the centigrade scale is equal to273.15 Kelvin (K).30
Pharmaceutical gases include:Anesthetic gases, such as nitrous oxide andhalothane.Compressed gases include oxygen for therapy;nitrogen, CO2Sterilization gases e.g. ethylene oxideAerosols:31
The Ideal Gas Law: 1.2.3.refers to an ideal situation where:no intermolecular interactions existcollisions are perfectly elastic, andthusno energy is exchanged upon collision.32
The Ideal Gas Law:33
Example:In the assay of ethyl nitrite spirit, the nitric oxidegas that is liberated from a definite quantity ofspirit and collected in a gas burette occupies avolume of 30.0 mL at a temperature of 20 C and apressure of 740 mm Hg. Assuming the gas is ideal,what is the volume at 0 C and 760 mm Hg?34
PV/T under one set of conditions is equal to PV/T under the ratio PV/Tmathematically asanother set, and so on.isconstantandcanbeexpressed in which R is the constant value for the PV/T ratio of an idealgas.This equation is correct only for 1 mole of gas. for n moles it becomes known as the general ideal gas law35
The general Gas Law:The volume (V), the pressure (P) and theabsolute temperature (T) of a given gas areinterrelated by thegeneral ideal gas law:PV nRTWhere n is the number of moles of gas and R- is themolar gas constant (0.082051 L . atm/mole. K )36
R is the molar gas constantIf 1 mole of an ideal gas is chosen, its volume understandard conditions of temperature and pressure (i.e., at0 C and 760 mm Hg) has been found by experiment to be 22.414 liters. Substituting this value in the equation37
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Example : What is the volume of 2 moles of an ideal gasat 25 C and 780 mm Hg?PV nRT39
The approximate molecular weight of a gas canbe determined by use of the ideal gas lawg is the number of grams of gas and M is themolecular weight.40
Example :If 0.30 g of ethyl alcohol in the vapor stateoccupies 200 mL at a pressure of 1 atm anda temperature of 100 C, what is themolecular weight of ethyl alcohol? Assumethat the vapor behaves as an ideal gas.41
Aerosols: Gases can be liquefied under high pressures in a closedchamber as long as the chamber is maintained below thecritical temperature.When the pressure is reduced, the molecules expand and theliquid reverts to a gas.In pharmaceutical aerosols, a drug is dissolved or suspendedin a propellant, a material that is liquid under the pressureconditions existing inside the container but that forms a gasunder normal atmospheric conditions.The container is so designed that, by depressinga valve, some of the drug–propellant mixture isexpelled owing to the excess pressure inside thecontainer.42
Liquefaction of Gases: When a gas is cooled, it loses some of its kinetic energy in theform of heat, and the velocity of the molecules decreases.If pressure is applied to the gas, the molecules are broughtwithin the sphere of the van der Waals interaction forces andpass into the liquid state.Because of these forces, liquids are considerably denser thangases and occupy a definite volume.The transitions from a gas to a liquid and from a liquid to asolid depend not only on the temperature but also on thepressure to which the substance is subjected.43
If the temperature is elevated sufficiently, avalue is reached above which it is impossibleto liquefy a gas irrespective of the pressureapplied.This temperature, above which a liquid can nolonger exist, is known as the criticaltemperature.The pressure required to liquefy a gas at itscritical temperature is the critical pressure.The critical temperature of water is 374 C,or 647 K, and its critical pressure is 218 atm.44
The Critical temperature of water is 374 C, or647 K, and its critical pressure is 218 atm.45
The Supercritical Fluid State46
The Supercritical Fluid State A supercritical fluid is a mesophase from of the gaseousstate where the gas is held under a combination oftemperatures and pressures that exceed the criticalpoint of a substanceSupercritical fluids can be formed by many normal gasessuch as carbon dioxide.Supercritical fluids have properties that are intermediatebetween those of liquids and gases, having better ability topermeate solid substances (gas like) and having highdensities that can be regulated by pressure (liquid like).47
The Supercritical Fluid State Briefly, a gas that is brought above its criticaltemperature Tc will still behave as a gas irrespectiveof the applied pressure; As the pressure is raised higher, the density of thegas can increase without a significant increase in theviscosity while the ability of the supercritical fluid todissolve compounds also increases. A gas that may have little to no ability to dissolve acompound under ambient conditions can completelydissolve the compound under high pressure in thesupercritical range.48
Common uses for supercritical fluids Extraction.Crystallization.Preparation of formulations (polymer mixtures, theformation of micro- and nanoparticles).Decaffeination of coffee.Crystallization49
Carbon dioxide Super critical fluidThe critical temperature of Co2 is 31.1 C, andits critical pressure is 73.8 atm.Used in decaffeination and crystallization to givetotally pure crystals without solvent residue50
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LiquidsThe intermolecular force of attraction in liquids (Van derWaals forces) are sufficient to impose some ordering orregular arrangement among the molecules. Hydrogen bonding, when it occurs, increases thecohesion in liquids and further affects their physicochemical behavior. However, these forces are much weaker than covalentor ionic forces. Thus, liquids, in general, tend to displayshort-range rather than long-range order.52
Although molecules of a liquid would tend toaggregate in localized clusters, no definedstructuring would be evident.Forces of attractionbetween liquid moleculesMovement of liquid molecules53
Vapor Pressure of Liquids Translational energy of motion (kinetic energy) is notdistributed evenly among liquid molecules; some of themolecules have more energy and hence higher velocities thanothers at any moment. When a liquid is placed in an evacuated container at aconstant temperature, the molecules with the highest energiesbreak away from the surface of the liquid and pass into thegaseous state, and some of the molecules subsequently returnto the liquid state, or condense.vaporizationcondensation54
When the rate of condensation equals therate of vaporization at a definite temperature,the vapor becomes saturated and a dynamicequilibrium is established.The pressure of the saturated vapor abovethe liquid is known as the equilibrium vaporpressure.55
As the temperature of the liquid is elevated, moremolecules approach the velocity necessary forescape and pass into the gaseous state. As a result, the vapor pressure increases withrising temperature. if the temperature of the liquid is increased whilethe pressure is held constant, or if the pressure is decreasedtemperature is held constant,whiletheall the liquid will pass into the vapor state.56
The variation of the vapor pressure of someliquids with temperature.57
Boiling Point If a liquid is placed in an open container and heated untilthe vapor pressure equals the atmospheric pressure, thevapor will form bubbles that rise rapidly through the liquidand escape into the gaseous state. The temperature at which the vapor pressure of the liquidequals the external or atmospheric pressure is known asthe boiling point.58
Boiling Point All the absorbed heat is used to change the liquidto vapor, and the temperature does not rise untilthe liquid is completely vaporized. The atmospheric pressure at sea level isapproximately 760 mm Hg; at higher elevations,the atmospheric pressure decreases and theboiling point is lowered. At a pressure of 700 mm Hg, water boils at97.7 C; at 17.5 mm Hg, it boils at 20 C.59
Latent Heats of Vaporization: the heat that is absorbed when watervaporizes at the normal boiling pointthe heat of vaporization for water at 100 Cis 539 cal/g or about 9720 cal/mole.For benzene, the heat of vaporization is91.4 cal/g at the normal boiling point of80.2 C.60
The boiling point of a compound, and the heat of vaporization andthe vapor pressure at a definite temperature, provides a roughindication of the magnitude of the attractive forces. Nonpolar substances, have low boiling points and low heats ofvaporization (the molecules are held together predominantly by theLondon force ) . Polar molecules, such as ethyl alcohol and water, which areassociated through hydrogen bonds, exhibit high boiling points andhigh heats of vaporization. The boiling points of normal hydrocarbons, simple alcohols, andcarboxylic acids increase with molecular weight because theattractive van der Waals forces become greater with increasingnumbers of atoms. Branching of the chain produces a less compact molecule withreduced intermolecular attraction, and a decrease in the boilingpoint results. The boiling points of straight-chain primary alcohols and carboxylicacids increase about 18 C for each additional methylene group.61
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Solids and the CrystallineState The structural units of crystalline solids, suchas ice, sodium chloride, and menthol, arearranged in fixed geometric patterns orlattices.Crystalline solids have definite shapes and anorderly arrangement of units.Crystalline solids show definite melting points,passing from the solid to the liquid state64
The various crystal forms are divided into sixdistinct crystal systems based on symmetry: cubic (sodium chloride),tetragonal (urea),hexagonal (iodoform),rhombic (iodine),monoclinic (sucrose),triclinic (boric acid).65
The units that constitute the crystal structurecan be atoms, molecules, or ionsThe morphology of a crystalline form is oftenreferred to as its habit,the crystal habit is defined as having thesame structure but different outwardappearance (or alternately, the collection offaces and their area ratios comprising thecrystal).66
The sodium chloride crystal consists of a cubic lattice ofsodium ions interpenetrated by a lattice of chloride ions, thebinding force of the crystal being the electrostatic attractionof the oppositely charged ions. In diamond and graphite, the lattice units consist of atomsheld together by covalent bonds. Solid carbon dioxide, hydrogen chloride, and naphthaleneform crystals composed of molecules as the building units. ionic and atomic crystals in general are hard and non brittleand have high melting points molecular crystals are soft and have relatively low meltingpoints.67
Characterization of Crystalline Materials:a. X-Ray Diffraction (birefringent property )b. Melting Point and Heat of Fusionc. Polymorphism68
X-Ray Diffraction( birefringent property ) X-rays are diffracted by the electrons surroundingthe individual atoms in the molecules of thecrystals. By such a method the structure of a crystal maybe investigated. Two polymorphs will provide two distinct powderx-ray diffraction patterns. The presence of a solvate will also influence thepowder x-ray diffraction pattern because the solvatewill have its own unique crystal structure.69
Powder x-ray diffraction patterns for spiraprilhydrochloride70
b. Melting Point and Heat of FusionThe freezing point or melting point of a pure crystalline solid is strictlydefined as the temperature at which the pure liquid and solid exist inequilibrium.In practice, it is taken as the temperature of the equilibrium mixture atan external pressure of 1 atm.heat of fusion The heat absorbed when a gram of a solid melts or the heatliberated when it freezes is known as the latent heat of fusion. The heat of fusion may be considered as the heat required inincreasing the interatomic or intermolecular distances in crystals,thus allowing melting to occur.A crystal that is bound together by weak forces has a low heat offusion and a low melting point and vice versa.71
The latent heat of fusion The heat (energy) absorbed when 1 g of a solidmelts or the heat liberated when it freezes isknown as the latent heat of fusion, and for waterat 0 C it is about 80 cal/g (1436 cal/mole).Why The Water Latent heat of vaporization(539 cal/g ) greater than Its Latent heat of fusion(80 cal/g )72
different intermolecular forces are involvedin holding the crystalline solid together andthat the addition of heat to melt the crystalis actually the addition of energy.73
Changes of the freezing or melting point withpressure can be obtained by using a form of theClapeyron equation: VL and Vsare the molar volumes of the liquid and solid,respectively. Molar volume (volume in units of cm3/mole) iscomputed by dividing the gram molecular weight by the densityof the compound. ΔHf is the molar heat of fusion: ΔT isthe amount of heatabsorbed when 1 mole of the solid changes into 1 mole ofliquid.the change of melting point brought about by a pressurechange of ΔP74
Example:What is the effect of an increase of pressure of 1atm on the freezing point of water (melting pointof ice)?At 0 C, T 273.16 KThe density of ice is 0.9167 g/cm3ΔHf For water ( 6025x107 dynes cm/mole )VL the molar volume of water is 18.018 cm3/moleVS the molar volume of ice is 19.651 cm3/mole75
Melting Point and Intermolecular Forces A crystal that is bound together by weak forcesgenerally has a low heat of fusion and a lowmelting point, whereas one bound together bystrong forces has a high heat of fusion and ahigh melting point.Because polymorphic forms represent differentmolecular arrangements leading to differentcrystalline forms of the same compound, it isobvious that different intermolecular forceswill account for these different forms.76
Then consider polymorph A, which is heldtogether by higher attractive forces than ispolymorph B.It is obvious that more heat will be requiredto break down the attractive forces inpolymorph A, and thus its meltingtemperature will be higher than that ofpolymorph B.77
The melting points of normal saturatedhydrocarbons increase with molecularweight because the van der Waals forcesbetween the molecules of the crystalbecome greater with an increasing numberof carbon atoms.The melting points of the alkanes with aneven number of carbon atoms are higherthan those of the hydrocarbons with an oddnumber of carbon atoms, because alkaneswith an odd number of carbon atoms arepacked in the crystal less efficiently.78
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Polymorphism 1.2.3.4.Some substances, such as carbon or sulfur, mayexist in more than one crystalline form( polymorphs) .Polymorphs are chemically identical.Polymorphs have different:stabilities and may spontaneously convert fromthe metastable form at a temperature to thestable form.melting points.x-ray crystal and diffraction patternssolubility81
Polymorphism 1.2.The formation of polymorphs of a compoundmay depend upon several variablespertaining to the crystallization process,including:solvent differences (the packing of a crystalmight be different from a polar versus anonpolar solvent).impurities that may favor a metastablepolymorph because of specific inhibition ofgrowth patterns;82
Polymorphism3. the level of supersaturation from which thematerial is crystallized (generally the higherthe concentration above the solubility, themore chance a metastable form is seen).4. the temperature at which the crystallizationis carried out.83
Examples of polymorphism Diamond ((metastable) and graphite.all long-chain organic compounds exhibitpolymorphism as fatty acids.Spiperone.haloperidol.sulfonamidesTheobroma oil, or cacao butter, is apolymorphous natural fat.84
Nearly all long-chain organic compounds exhibitpolymorphism. In fatty acids, this results fromdifferent types of attachment between the carboxylgroups of adjacent molecules. Theobroma oil, or cacao butter, is a polymorphousnatural fat. it consists mainly of a single glyceride.Theobroma oil is capable of existing in fourpolymorphic forms:1.the unstable gamma form, melting at 18 C.2.the alpha form, melting at 22 C.3.the beta prime form, melting at 28 C.4.the stable beta form, melting at 34.5 C. 85
If theobroma oil is heated to the point atwhich it is completely liquefied (about 35 C),the nuclei of the stable beta crystals aredestroyed and the mass does not crystallizeuntil it is supercooled to about 15 C.The crystals that form are the metastablegamma, alpha, and beta prime forms, andthe suppositories melt at 23 C to 24 C or atordinary room temperature.86
The proper method of preparation involvesmelting cacao butter at the lowest possibletemperature, about 33 C.The mass is sufficiently fluid to pour, yet thecrystal nuclei of the stable beta form are notlost.When the mass is chilled in the mold, a stablesuppository, consisting of beta crystals andmelting at 34.5 C, is produced.87
Polymorphism and solubility In the case of slightly soluble drugs, this mayaffect the rate of dissolution.As a result, one polymorph may be moreactive therapeutically than another polymorphof the same drug.the polymorphic state of chloramphenicolpalmitate has a significant influence on thebiologic availability of the drug.88
Polymorphism has achieved significance in recent years due tofact that different polymorphs exhibit different solubilities. Asresult, one polymorph may be more active therapeutically thananother polymorph of the same drug.EXAMPLEPlasmaconcentrationThe plasma concentration of 2 polymorphic forms of tetracyclinTim89
Polymorphism can also be a factor insuspension technology.Cortisone acetate exists in at least fivedifferent forms, four of which are unstable inthe presence of water and change toa stable form.Because this transformation is usuallyaccompanied by appreciable caking of thecrystals, these should all be in the form ofthe stable polymorph before the suspensionis prepared.90
Solvates Because many pharmaceutical solids areoften synthesized by standard organicchemical methods, purified, and thencrystallized out of different solvents,residual solvents can be trapped in thecrystalline lattice.This creates a co-crystal termed a solvate.91
Amorphous Solids Amorphous (without shape) solid may be consideredas supercooled liquid in which the molecules arearranged in a random manner somewhat as in theliquid state. Substances, such as glass and manysynthetic plastics, are amorphous solids.They differ from crystalline solid in that: they flow when subjected to sufficient pressurefor some time they do not have definite melting points.92
Amorphous substances, as well as cubiccrystals, are usually isotropic, that is, theyexhibit similar properties in all directions.Crystals other than cubic are anisotropic,showing different characteristics , rate of solubility) in variousdirections along the crystal.93
It is not always possible to determine by allineor Beeswax and paraffin, although they appear to beamorphous, assume crystalline arrangements whenheated then cooled slowly. Petrolatum contains both crystalline and amorphousconstituents. Some amorphous materials, such as glass, maycrystallize after long standing. Being amorphous or crystalline has been shown to affectthe therapeutic activity of the drug. Thus, the crystallineform of the antibiotic novobiocin acid is poorly absorbedand had no activity, whereas the amorphous form is readilyabsorbed and therapeutically active.94
The Liquid Crystalline StateA fourth state of matter is the liquid crystalline state ormesophase. Materials in this state are in many waysintermediate between the liquid and solid states.Materials in liquid crystalline state are intermediatebetween the liquid and solid states. The two main types of liquid crystals are:1. Smectic state (soap like or grease like):molecules are mobile in two directions and can rotate aboutone axis2. Nematic state (threadlike):the molecules rotate only about one axis but are mobile inthree dimensions95
1.2.3.4.Thesmecticmesophaseisofmostpharmaceutical significance because it is thisphase that usually forms in ternary mixturescontaining a surfactant, water, and a weaklyamphiphilic or nonpolar additive.In general, molecules that form mesophases:are organic,are elongated and rectilinear in shape,are rigid, andpossess strong dipoles and easily polarizablegroups.96
The liquid crystalline state may result either from:1. The heating of solids (thermotropic liquidcrystals)2. The action of certain solvents on solids(lyotropic liquid crystals).97
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Properties and Significance of Liquid CrystalsProperties and Significance of Liquid CrystalsLiquid crystals have some of the properties of liquids and someof solids. Liquid crystals are mobile and thus can be considered to havethe flow properties of liquids. At the same time they possess the property of beingbirefringent, a property associated with crystals.99
Significance of Liquid Crystals Some liquid crystals show consistent color changes withtemperature Some liquid crystals may be sensitive to electric fields, aproperty used in developing display systems. The smectic mesophase has applicationSolublization of water-insoluble materials. intheIt also appears that liquid crystalline phases of this typeare frequently present in emulsions and may beresponsible for enhanced physical stability owing to theirhighly viscous nature.100
Thermal Analysis 1.2.3.4.Methods for characterizing physical andchemical changes alterations upon heatingor cooling a sample of the material.Types of thermal analysis :Differential Scanning Calorimetry (DSC)Differential thermal analysis (DTA).Thermogravimetric analysis (TGA).Thermomechanical analysis (TMA).101
Differential Scanning Calorimetry102
Differential Scanning Calorimetry heat flows and temperatures are measured that relateto thermal transitions in materials.a sample and a reference material are placed in separatepans and the temperature of each pan is increased ordecreased at a predetermined rate.When the sample, for example, benzoic acid, reaches itsmelting point, in this case 122.4 C, it remains at thistemperature until all the material has passed into theliquid state because of the endothermic process ofmelting.103
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1.2.3.endothermic reaction on
4 Binding Forces Between Molecules For molecules to exist as aggregates in gases, liquids, and solids, intermolecular forces must exist. Intermolecular forces Cohesion, or the attraction of like molecules, Adhesion, or the attraction of unlike molecules, Repulsion is a reaction between two molecules that forces th
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