Published on March 7, 2014
Structure of matter Contents: 1. Introduction. 2. Matter 3. Interatomic bonding a). Primary bonding Ionic Covalent Metallic b). Secondary bonding Hydrogen bonds Vanderwaals forces 4. Interatomic bond distance & Bond energy. 5. Crystalline structure – Lattice types. 6. Diffusion. 7. Adhesion & bonding. 8. Factors influencing adhesion. a). Wetting. b). Surface energy. c). Contact angle. 9. Solubility and sorption. 10. Adhesion to tooth structure. 11. References. Page 1
Structure of matter INTRODUCTION: An object can occupy one of the three different states of matter, such as solid, liquid, gas. In dentistry we make use of all of them although dental materials exist primarily as solids or liquids. The state of a material is a function of temperature. The more energy that is put into a material by increasing its temperature, the more difficult it is to keep the atoms (or) molecules in close proximity to one another. Thus, the atoms (or) molecules tend to move apart and expand as heat is applied. Therefore increasing the energy within a given material through the application of heat can have a destabilizing effect on both its structure and dimensions. The structure of a material can be described on both a microscopic and macroscopic level. On the microscopic level, we experience the material through the arrangement of its atoms and their bonding schemes. On the macroscopic level we see a material as a solid liquid or gas. The principle goal of dentistry is to maintain or improve the oral health of the patient. A wide variety of dental materials are involved in the clinical application. Material should be carefully selected. Through understanding and experimentation it is possible to maximize any one property, but in no application is it possible to select a material for one property above. It is precisely in the balance of one factor against another that the materials are used successfully. Hence it is essential to know, the properties of the dental materials, to be able to understand the properties and reactions of the material and predict the outcome. Page 2
Structure of matter MATTER: It is the substance from which all physical things are made. Matter is any thing that occupies space and has weight. Every thing that exists in the universe (seen or unseen) is matter and it is composed of millions of tiny particles called molecules. Molecules are composed of still smaller units called atoms and atoms themselves are made up of even tinier particles called protons, neutrons and electrons. Properties of matter: 1) All matter has mass and weight. Mass is the measure of the quantity (or) amount of matter, and this remains same always. Weight of an object is the force of attraction exerted on the object by gravitation, and this varies at different places. For Ex: An object weighing one pound on earth would weigh only a few ounces on the moon, which has a weaker gravitational pull, but its mass would not change as in the case of space travelers who float in the space. 2) All matter has inertia : That means, matter has a tendency to remain at rest if at rest, or to continue moving in a straight path with constant speed if in motion. Inertia of matter depends on its mass-greater the mass of an object, greater is its inertia. 3) Density: All matter has density. This is the mass per unit volume of a substance. 4) Matter can conduct heat and electricity. 5) There are three states of matter. Solid, liquid and gas. Page 3
Structure of matter Molecule: It is the smallest particle of a substance, which can exist on its own, and retains the properties of that substance. Molecular weight: is the weight of a molecule relative to that of an atom of carbon 12 taken as 12. This is expressed in gram molecule. Atom: Is the smallest particle of any element that shows the chemical behaviour of that element. Atom cannot be divided as it is the smallest. There are many kinds of atoms as there are elements. An element is made up of only one kind of atoms. Element: Is a basic substance that cannot be separated into different substances. Atom is the basic unit from which molecules and aggregates of molecules, which represent particular matter are built. Atom is made up of a 3 types of fundamental particles like protons, neutrons and electrons. Number of protons and number of neutrons, gives the mass to the nucleus and also the atomic weight. INTERATOMIC BONDING: Atoms do not exist singly, instead are joined with other atoms of the same kind, to form molecules. The number of atoms joining together may be two or in thousands. When such a thing happens there exists a real thing or substance. The mechanism of atoms coming together is through forces of attraction and the atoms going away from each other is through forces of repulsion. Forces of attraction that make the atoms to come together are called interatomic bonds. These bonds are classified as 1) Primary bonds – Chemical in nature, permanent and strong Page 4
Structure of matter a) Ionic bond b) Covalent bond c) Metallic bond 2) Secondary bonds – Physical in nature, weak bonds also called Vanderwalls forces. PRIMARY BOND: Ionic bond: Normally atom is electrically neutral because of balance of positively charged proton and negatively charged electron. An atom can become ion if it loses or gains one of its outermost circulating electron. This can be a positive ion or negative ion. Atom can be a positive ion or a negative ion. Such differently charged two atoms will be attracted to each other because of differing polarity, and a bond takes place between atoms. Ex: Such a bond is seen in sodium chloride molecule. When such 2 atoms come together sodium atom loses its one outermost electron and becomes positive ion. Chlorine atom takes one more electron from sodium and becomes negative ion. Thus +ve Na and –ve Cl attract each other and stay together to form an NaCl molecule. Ex: Such bonding takes place with glass ionomer or polycarboxylate cement and tooth enamel. The bond in this case is between negatively charged atoms in the cement and positively charged atoms in the tooth enamel. This is called as true form of chemical adhesion. Covalent bond: This kind of bond, takes place by sharing of electron between 2 atoms. Ex: Such a bond is seen when 2 hydrogen atoms come together each hydrogen atom has only one electron surrounding it when 2 hydrogen atoms come together one electron of each atom is shared by both atoms. Page 5 H + H = H2
Structure of matter Similarly chlorine atom will join with another chlorine atom and share the outer electrons by covalent bond to form Cl 2 covalent bond is very strong and stable. This kind of bond is seen with carbon-carbon bond that takes place in denture base resin (acrylic) or dental composite resin. Also seen in siliconoxygen bond of dental ceramics. Metallic bond: This is seen in metallic elements. The atoms of the metal are arranged in orderly rows. The atoms lose their outermost valence electrons to form metal ions with a net positive charge called cations . The freed valence electrons roam about together like a gaseous cloud in the interstices formed by the arrangement of solid spheres. This electron cloud acts like a glue that holds together different atoms. This is called metallic bond, which is responsible for the strength, conductivity, of heat and electricity of metals. This bond is strong and stable. INTER ATOMIC SECONDARY BONDS In contrast to primary bonds secondary bonds do not share electrons. Instead, charge variations among molecules or atomic groups include polar forces that attract the molecules. Hydrogen bonding This bond can be understood by studying a water molecule. Attached to the oxygen atom are two hydrogen atoms. These bonds are covalent because the oxygen and hydrogen atoms share electrons. As a result the protons of the hydrogen atoms pointing away from the oxygen atoms are not shielded effectively by the electrons. Thus the proton Page 6
Structure of matter side of the water molecule becomes positively charged. On the opposite side of the water molecule, the electrons that fill the outer orbit of the oxygen provide a negative charge. Thus a permanent dipole exists that represents an asymmetric molecule. H2 bond, associated with the positive charge of hydrogen caused by polarization is an important example of this type of secondary bonding. When a H2O molecule intermingles with other water molecules, the hydrogen (+ve) portion of one molecule is attracted to the oxygen portion of its neighboring molecule, and the hydrogen bridge is are formed. VAN DER WAALS FORCES It is a more a physical than chemical bond. These forces form the bases of a dipole attraction. Eg : in an inert gas, the electron field is constantly fluctuating. Normally the electrons of the atoms are distributed equally round the nucleus and produce an electrostatic field around the atom. However this field may fluctuate so that its charge becomes momentarily positive and negative. A fluctuating dipole is thus created that will attract other similar dipoles. Such interatomic forces are quiet weak. Inter atomic bond distance and bonding energy: Regardless of the type of matter, there is a limiting factor that prevents the atoms or molecules from approaching each other too closely, that is the distances between the center of an atom and that of its neighbor is limited to the diameter of the atoms involved. If the atoms approach too closely, they are repelled from each other by their electron charges. On the other hand, forces of attraction tend to draw the atoms together. The position at which these forces of repulsion and Page 7
Structure of matter attraction become equal in magnitude is the normal or equilibrium position of the atoms. Thermal energy Thermal energy is accounted for by the kinetic energy of the atoms or molecules at a given temperature. The atoms in a crystal at temperatures above absolute zero temperature are in a constant state of vibration and the average amplitude will be dependent on the temperature, the higher the temperature the greater the amplitude, and consequently, the greater the kinetic or internal energy. The overall effect represents the phenomenon known as thermal expansion. If the temperature continues to increase the interatomic spacing will increase, and eventually a change of state will occur. The thermal conductivity depends mainly on the number of free electrons in the material. As metallic structures contain many free electrons and most metals are good conductors of heat as well as electricity, whereas non-metallic materials do not include many free electrons and consequently they are generally poor thermal and electrical conductors. CRYSTALLINE STRUCTURE: Dental materials consist of many millions of atoms or molecules. They are arranged in a particular configuration. In 1665 Robert Hooke simulated the characteristic shapes of crystals by stacking musket balls in piles. The atoms are bonded by either primary or secondary forces. In solid state they combine in the manner that will ensure a minimal internal energy. Page 8
Structure of matter For eg. Sodium and chlorine share one electron as described previously. In the solid state, however they do note simply pair together but rather all of the positively charged sodium ions attract all of the negative chlorine ions, with the result that they form a regularly spaced configuration known as space lattice or crystal, here every atom is spaced equally from every other atom. There are 14 possible lattice types, but many of the metals used in dentistry belong to the cubic system. Non crystalline structure eg. Glass and waxes structures other than the crystalline form that occur in the solid state eg. Glass and waxes. Waxes – solidify as amorphous materials meaning that the molecules are distributed at random. Though there may be a tendency for the arrangement to be regular. Glass is considered to be a noncrystalline solid, yet its atoms tend to forma short – range order lattice instead of the long-range order lattice characteristic of crystalline solids. In other words, the ordered arrangement of the glass is more or less localized with a considerable number of disordered units between them. Such an arrangement is also typical of liquids such solids are sometimes called supercooled liquids. Non crystalline solids do not have a definite melting temperature but rather they gradually softer as the temperature is raised and gradually hardens as they cool. The temperature at which there is an abrupt decrease in the thermal expansion cuff, is called the glass transition temperature or glass temperature. Below Tg a glass loses its fluid characteristics and has significant resistance to deformation. Eg : synthetic dental resins. Page 9
Structure of matter DIFFUSION Diffusion of molecules in gases and liquids is not known. However molecules and atoms diffuse in the solid state as well. At any temperature above absolute zero, the atoms of a solid possess some amount of kinetic energy as previously discussed. However the fact is that all the atoms do not possess the same amount of energy, these energies vary from very small to quiet large. With the average energy related to the absolute temperature. Even at very low temperatures some atoms will have large energies. If the energy of a particular atom exceeds the bonding energy, it can, move to another position is the lattice. Atoms change position in pure solids, even under equilibrium conditions, this is known as self diffusion. Increase temperature greater the rate of diffusion .The diffusion rate will however vary with the atom size, interatomic or intermolecular bonding, lattice. ADHESION AND BONDING Adhesion is a phenomenon involved in many situations in dentistry. Eg. Leakage adjacent to dental restorative material is affected by the adhesion process. The retension of artificial dentures is probably dependent, to some extent on the adhesion between denture and saliva and between Three states of matter: Three states of matter are solids, liquid and gas. Solid: Solid has definite shape and volume. Ex: Block of wood, stone work is necessary to change the shape of solid. They have definite atomic arrangement and are resistant to deformation. The molecules of the solid are Page 10
Structure of matter moving about in a restricted area, atoms of solid have high attraction force between them and low kinetic energy. Crystalline solid: Crystalline solid is one in which molecules are arranged in a definite geometric pattern Ex: metals. Amorphous solid: Amorphous solid is one in which molecular arrangement is irregular ex: Glass, wax, dental resin. Glass if also called as supercooled liquid, because it is basically a liquid, at a low temperature which can be rigid or semirigid. It is highly viscous and has little resistance to deformation. Glass has no fixed melting or solidifying point, it gradually changes to liquid and in reverse gradually solidifies. The temperature at which this change occur is known a glass transition temperature which is designated as Tg. Liquid: Liquid has a definite volume, but not a definite shape. Liquid takes the shape of the container in which it is kept. The molecules in a liquid are free to move about. The molecules are sufficiently close to each other to have mutual attraction on each other and on this depends the liquids fluidity or viscosity. Gas: Gas has molecules which have freedom of movement and are not restricted to a given area. Gas has no definite shape and no definite volume. It expands to fill its container. Page 11
Structure of matter Change of state: When a solid is heated, the molecules acquire more kinetic energy and vibrate more rapidly. They acquire enough energy to breakout of their positions and move about among the other molecules. When this happens solid turns into liquid and the process is called melting. When a liquid is heated, the molecules gain more KE and some gain enough energy to break away from the surface of the liquid and become gas. This is evaporation when bubbles are formed during this change it is called boiling. Change of state is basically controlled by proper temperature and pressure on the mass. Crystal – Crystal is any solid whose atoms are arranged in an orderly and repeated pattern ex: Crystals of quartz metals. Uses of crystals: In jewellary, in watches, hearing aids, microphones mica crystals are used as insulators in electrical equipments. PROPERTIES: Physical Properties: most crystals have sharp melting point ex: Metal many crystals cleave or split along change planes, which are planes of weak bonding that run parallel to one another through out the crystal. This is due to the orderly and repeatition of atomic structure. Crystals conduct heat and electricity in one direction but act as insulator in other direction. Optical properties: light is refracted in crystals. X-ray diffraction – breaking up spread The structure of the crystals can be studied by X-ray diffraction. Classification of crystals: Crystals are classified by the form of the unit cells. Page 12
Structure of matter The unit cell is the basic 3 dimensional repeating structure of which crystals are composed. The unit cell may grow in 7 basic different shapes, determined by the relative lengths of their axis and the angle that these axes make with each other. The seven forms of unit cell define the crystal systems and every crystal is classified by the position of the atoms or ions in these cells. In a simple cubic type, only the corners of the cells are occupied by atoms. In body centered cells, the corners and the centers of the cell are occupied by atoms. In face centered cells, the corners and the centers of the faces are occupied by atoms. There are 14 different combinations of cell structures and atomic arrangement. These combinations are called space lattices rocks, metals and ice are not single crystals, but are composed of many small crystalline material. Substances composed of single crystal are called monocrystalline unit crystal cell is the smallest repeating unit of atoms. Diffusion of atoms in solids: Diffuse = Spread out Atoms have energy particles – means they have internal energy, some atoms have high energy level than others. High energy atom can move or change its place and occupy another position in the material. Rate of diffusion depends on temperature, higher the temperature greater will be the diffusion. Consequences of diffusion of atoms: Page 13
Structure of matter 1) Solid state reactions: Number of solid state reactions occur in a solid substance and it is due to the diffusion of atoms. For ex: In gold-copper alloys, gold and copper atoms may be distributed randomly in the space lattices leading to disordered lattice. Due to heat treatment this pattern may change in which gold atoms occupy the center of the cubic type of space lattice and copper atoms occupy the corner positions in the space lattice. This will change the physical properties of hardness, brittleness strength and conductivity of a material. 2) Diffusion can bring about change in the shape or contour of a material known as warpage or distortion. 1) ADHESION: Adhesion is attraction between unlike molecules. i.e., if two different substances are made to come in contact with each other at their inter surfaces and if the two surfaces are stuck together it is said that adhesion has taken place. For example gum and paper. Gum and paper are two different types of materials having different types of molecules. When gum is applied to the surface of the paper, the attraction between the molecules of gum and molecules of paper takes place and it is adhesion. 2) COHESION: Cohesion is attraction between like molecules. In this case only one materials is involved. For example water. It is mad up of molecules of hydrogen and oxygen. The two together form water molecule such as H2O. There are innumerable numbers of water molecules in water. Water remains as water as long as there is attraction Page 14
Structure of matter between one water molecule and another molecule. This attraction between similar type of molecules is cohesion. Adhesion and cohesion help in the retention of complete dentures as follows. When the denture is placed in the mouth in contact with mucous membrane, adhesion and cohesion both play a role. In this three materials (or) three types of molecules are involved, they are – Denture material Saliva Mucous membrane. Fitting surface of denture has saliva layer, and saliva is in contact with mucous membrane. Thus, there is adhesion between denture surface molecules and saliva molecules. There is cohesion between saliva molecules and saliva molecules. There is adhesion between saliva molecules and mucous membrane surface. In dentistry, adhesion is an important requirement of any restorative materials so that there is a bond between tooth enamel and artificial restorative materials, which helps in retention of the restoration in the tooth. Adhesive – is a material (or) film of material used to produce adhesion. Adherand – is the materials or surface of a material to which adhesive is applied. For ex: Gum is an adhesive, paper on which it is applied is adherand. Attraction between molecules of gum and paper is adhesion. Factors influencing adhesion: These are 1) Wetting 2) Surface energy Page 15
Structure of matter 3) Contact angle. 1) WETTING: It is the ability of a liquid (adhesive) to flow and adapt to the surface of a solid. It is mainly dependent on surface tension. Surface tension is the molecular attraction at the surface of liquids and so the surface of a liquid is actually in a state of tension as if it were being pulled tight. This property makes the surface of a liquid to behave like thinly stretched rubber sheets. Such surfaces tend to become as small as possible. For example: A drop of water is round in shape, mercury also forms a droplet of round shape. But water as such has low surface tension, where as mercury has high surface tension. Therefore a drop of water may be round in shape but spreads immediately and flows on the surface. On the other hand drop of mercury remains same and does not spread out, but will wet the surface only if the surface is clean. It is difficult to force two solid surface to adhere. When placed in apposition only high spots are in contract. Because these areas usually constitute only a small percentage of the total surface, no perceptible adhesion takes place. The attraction is generally neglible when the surface molecules of the attracting substances are separated by distances greater than 0.7 nm. One method of overcoming this difficulty is to use a fluid that flows into these irregularities and thus provides contact over a greater part of the surfaces of the solid. Page 16
Structure of matter To produce adhesion in this manner, the liquid must flow easily over the entire surfaces and adheres to the solid. This characteristic is referred to as wetting. Ability of an adhesive to wet the surface is influenced by number of factors. Cleanliness Eg. Oxide film on metallic surfaces. Some substances have surface energy hence only a few liquids wet their surface. Close packing of the structural organic groups and the presence of halogens may prevent wetting. Metals interact vigorously with liquid adhesive because of increase surface energy. 2) SURFACE ENERGY: For adhesion to exist, the surfaces must be attracted to one another at their interface. The energy at the surface of a solid is greater than in its interior inside the lattice all of the atoms are equally attracted to each other. The interatomic distances are equal and the energy is minimal. At the surface of the lattice the energy is greater because the outermost atoms are not equally attracted in all directions. The increase in energy per unit area of surface is referred to as the surface energy. In liquids the surface energy is known as surface tension. 3) CONTACT ANGLE: The contact angle is the angle formed by the adhesive with the adherend at their interface. The extent to which an adhesive will meet the Page 17
Structure of matter surface of an adherend may be determined by measuring the contact angle, between the adhesive and the adherend. The greater the tendency to wet the surface, the lower the contact angle, until complete wetting occurs at an angle equal to zero. Capillary rise : The penetration of liquids into narrow crevices is known as capillary action. P = 2 cos This equation relates the differential capillary pressure developed when a small tube of radius r is inserted in a liquid of surface tension (usually expressed in dynes / cm) and with a contact angle . If the contact angle of the liquid on the solid is less than 900 P will be negative and the liquid will be depressed. CONTACT ANGLE OF WETTING The extend to which an adhesive wets the surface of an adherand may be determined by measuring the contact angle between the adhesive and adherand. The contact angle is the angle formed by the adhesive with the adherend at their interface. If the molecules of the adhesive are attracted to the molecules of the adherend as much as or more than they are to themselves, the liquid adhesive will spread completely over the surface of the solid, and no angle ( = 0 degrees) will be formed. Thus the forces of adhesion are stronger than the cohesive forces holding the molecules of the adhesive together. Tendency of liquid to spread increases with decrease in contact angle. Therefore contact angle is the indication of spreadability or wettability. Page 18
Structure of matter Thus the smaller the contact angle between an adhesive and an adherend, the better the ability of the adhesive to fill in irregularities on the surface of the adherend. Also the fluidity of the adhesive influences the extent to which these voids or irregularities are fitted. SOLUBILITY AND SORPTION: One of the required of a dental restorative material is that, it should be stable in the oral environment. It should undergo a minimal amount of dimensional change and chemical alteration. All dental materials are soluble to some extent and dissolve in water. The least soluble of dental materials are the porcelains and ceramics. In polymers the unreacted molecules may be readily extracted or dissolved into oral fluids. The loss of small organic molecules from soft tissue conditioners and denture liners is responsible for them hardening in the mouth and becoming irritating. Metallic ions are slowly released from cast restorations and amalgams. Sorption is the uptake of fluids or substances by a material. This process is usually confined to polymeric materials and can also occur at the union between 2 materials, such as porcelains and a metal interface as a PFM restoration. Sorption may lead to subtle discoloration of the porcelain. The result of sorption in a polymeric material is often a swelling or increase in dimension. The uptake of foreign materials can lead to chemical disintegration that occurring in dental cements at the margin between the cast restoration Page 19
Structure of matter and the tooth. This results in loosening of restoration and decay of possible tooth structure. ADHESION TO TOOTH STRUCTURE: Associated principles of adhesion can be readily related to dental situations. For eg. when contact angle measurements are used to study the wettability of enamel and dentin. It is found that the wettability of these surfaces is markedly reduced after the topical appreciation of an aqueous fluoride solution. Thus fluoride treated enamel surface retains less plaque over a given period, presumably because of a decrease in surface energy. Therefore decreases in dental caries. Higher surface energy of many restorative materials compound with that of the tooth, there is great tendency for the surface and margins of the restoration to accumulate debris. Therefore increases marginal caries. Under certain instances, 1) Recurrent caries 2) Pulpal sensitivity 3) Deterioration of the margins of restoration can be associated with a lack of adhesion between restoration. Enamel and dentin of tooth have varying amounts of organic and inorganic components. A material that can adhere to the organic components may not adhere to the inorganic components, and an adhesive that bonds to enamel may not adhere to dentin to the same extent. Page 20
Structure of matter After cavity preparation, tenacious microscopic debris covers the enamel and dentin surfaces. This surface contamination called the smear layer, reduces wetting. REFERENCES 1. Anusavice K.J.-“Phillips’ Science of Dental materials” 11th edition , 2003 2. Combe E.C. – “Notes on Dental Materials”6th edition , 1992 3. Craig’s R.G., Powers J.M. – “Restorative Dental Materials” 11thedition, 2002 4.Gladwin M, Bagby M – “Clinical Aspects of Dental Materials” 2nd edition, 2004 5. Mc Cabe J.F. – “Applied Dental Materials” 7th edition , 1992 6. Phillips R.W.-“Skinner’s Science of Dental Materials”9th edition , 1992 Page 21
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