difference between crystalline and amorphous solids pdf

Difference Between Crystalline And Amorphous Solids Pdf

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Difference Between Crystalline and Amorphous

Amorphous forms are, by definition, non-crystalline materials which possess no long-range order. The amorphous solids have always been an essential part of pharmaceutical research, but the current interest has been raised by two developments: a growing attention to pharmaceutical solids in general, especially polymorphs and solvates and a revived interest in the science of glasses and the glass transition.

Amorphous substances may be formed both intentionally and unintentionally during normal pharmaceutical manufactoring operations. The properties of amorphous materials can be exploited to improve the performance of pharmaceutical dosage forms, but these properties can also give rise to unwanted effects that need to be understood and managed in order for the systems to perform as required.

The degree of crystallinity, according to the USP, depends on the fraction of crystalline material in the mixture, which is termed the two-state model. Another way of viewing this situation is that the crystallinity has a range from percent for perfect crystals zero entropy to 0 percent non-crystalline or amorphous ; this is known as the onestate model 2.

Amorphous solids exist in many industrially important products, such as polymers, ceramics, metals, optical materials glasses and fibers , foods, and pharmaceuticals. The amorphous solids have always been an essential part of pharmaceutical research, but the current interest 3 , 4 , 5 has been raised by two developments:. The preparation of amorphous solids, for thermodynamic and kinetic reasons, is easy for some materials good glass formers , but difficult for others poor glass formers.

Thermodynamically, glass forming ability originates from a crystalline state that is not substantially more stable than the amorphous state, which may be the case for molecules that pack poorly or contain many internal degrees of freedom. The effect is amplified if the conformers in crystals correspond to high-energy and low-concentration conformers in solution, which implies that the act of crystallization requires the average molecule to undergo a significant conformational change.

The effect is believed to underlie the different crystallization tendencies of two stereoisomers, for example, mannitol easy and sorbitol difficult 13 , In addition to conformational equilibria, configurational equilibria that between carbohydrate anomers should have similar effect on the tendency of crystallization. The effects of these equilibria on the glass-forming ability have not been well studied.

Poor glass formers can be made amorphous by deliberately preventing crystallization. The routes to the amorphous state include molecular quenching of melts, rapid precipitation by antisolvent addition, freeze-drying 15 , spray-drying 16 , 17 and introduction of impurities The impurity effect may cause a poor glass former to exist in the amorphous state in a multi-component formulation.

Also, amorphous solids can result from solid-dispersion, a process used to enhance bioavailability, and solid-state chemical reactions degradation of crystalline precursors. Process conditions can influence the amount of amorphous materials in the end product. In a freeze drying process, rapid freezing favours the formation of an amorphous solute, whereas introducing an annealing step may promote crystallization Processes that introduce mechanical or chemical stress grinding, milling, and wet granulation can render crystalline materials fully or partially amorphous.

Amorphous solids exhibit properties unique to their disordered state relative to their crystalline counterparts such as:. This is a kinetic phenomenon and, eventually, the solute in the supersaturated solution that is formed will begin to crystallize and the equilibrium solubility of the crystalline phase will be attained.

It is found 21 that at temperatures more than 50 K below Tg, an amorphous drug powder formed compacts that were significantly more brittle than those formed from the crystalline form of the drug; the tensile strengths of the compacts were similar. The strategy for characterizing amorphous solids differs from that for crystalline solids.

Molecular level structural elucidation, as is feasible for crystalline solis by diffraction and spectroscopic methods, is less applicable to amorphous solids, and greater emphasis is placed on structural mobility and changes.

It is customary to characterize an amorphous material both below and above the glass transition temperature both as the frozen solid and as the supercooled viscous liquid. The physical characterization of amorphous solids utilizes a wide range of techniques and offers several types of information:.

Structure Amorphous solids are not random at the molecular level, but may possess short-range order, residual crystallinity, polymorphic states, and regions of different density. The structure of an amorphous solid is usually described as possessing crystal-like short-range molecular arrangement, but lacking longrange order. Also, the immediate environment of a molecule in an amorphous solid may not be significandy different from that in a crystal similar number of and distance to the nearest neighbors , but an amorphous solid lacks any long-range translational-orientational symmetry that characterizes a crystal.

Amorphous solids have higher energy, entropy and free energy than the corresponding crystals. The excess properties are parameters in some theoretical models of crystallzation and structural relaxation. Amorphous solids may co-exist with and have the potential to convert to crystalline solids. Techniques for determining the degree of crystallinity include XRD, DSC 23 , solution calorimetry 24 , water sorption 4 , isothermal calorimetry 25 , and thermally stimulated current TSC Amorphous materials will not diffract X-rays in a coherent manner; thus powder X-ray diffraction patterns are broad halos with no or very few characteristic peaks for these materials.

Dielectric studies of secondary relaxation in amorphous solids advanced the view that a glass may have different regions: the glass transition primary relaxation involves cooperative motions in high-density regions, whereas secondary relaxation involves low-density regions lying between high- density regions.

Thermodynamic properties of an amorphous solid are often presented as excess properties relative to the crystalline state. Excess enthalpy, entropy and free energy can be obtained from heat capacities of the crystalline and amorphous phases as a function of temperature. Excess enthalpy also can be obtained from heats of solution by solution calorimetry or crystallization by scanning or isothermal calorimetry. In principle, excess free energy can be calculated from the solubility of crystalline and amorphous phases, provided that the equilibrium solubility of the amorphous solid can be measured without crystallization.

The Tg of an amorphous material is one of its characteristic properties and can be used to assess its likely stability and suitability for use in pharmaceutical dosage forms 4. So, Tg is a useful material descriptor owing to its correlation with structural and thermodynamic properties. If a more stable crystalline state exists, an amorphous material can crystallize when sufficient molecular mobility exists.

Pharmaceutically important examples include crystallization in freeze- and spray- drying, from supercooled melts, and from amorphous materials during storage, especially on exposure to heat and humidity. The aim of stabilization of amorphous solids is multi-faceted, including:. The chemical and physical stability of amorphous pharmaceutical materials is controlled by the same basic factors as for crystalline materials molecular structure, purity, molecular orientation and molecular mobility.

For any sample of a given molecular structure and purity, there will be more possible molecular orientations that occur in an amorphous sample than in, crystalline sample. Thus many more different types of chemical and physical transformations could potentially take place.

At a given temperature, the molecular mobility in an amorphous material will also be significantly higher than in any of the corresponding crystalline forms, and this can give rise to a greater chemical and physical reactivity in the amorphous sample.

In many instances free radical initiated oxidation reactions , the stability of a drug compound is not significantly affected by either its molecular mobility or the orientation of the molecules; thus the amorphous form has comparable stability to the crystalline material.

In some cases insulin , the more ordered structure of the crystalline material can actually increase the likelihood of certain intermolecular contacts and cause the crystalline form to have a lower level of stability.

So, the main factors, which are necessary to keep in mind during the earliest stages of drug development are:.

As with all crystallization processes, there are the normal nucleation and propagation crystal growth stages to consider, and procedures that increase the barrier to nucleation or slow the rate of crystal growth can be used to physically stabilize many amorphous materials. The sorbed water may participate in a chemical reaction hydrolysis , or may simply act as a catalyst for a chemical reaction. Amorphous substances are an important class of pharmaceutical materials that exhibit distinct physical and chemical properties.

They are ubiquitous, and may be formed both intentionally and unintentionally during normal pharmaceutical manufacturing operations. The properties of amorphous materials can be exploited to improve the performance bioavailability and dissolution rate of pharmaceutical dosage forms, but these properties can also give rise to unwanted effects physical instability that need to be understood and managed in order for the systems to perform as required.

National Center for Biotechnology Information , U. Bosn J Basic Med Sci. Author information Copyright and License information Disclaimer. This article has been cited by other articles in PMC. Abstract Amorphous forms are, by definition, non-crystalline materials which possess no long-range order. Keywords: amorphous solids, preparation, characterization, stabilization. Introduction Amorphous forms are, by definition, non-crystalline materials which possess no long-range order.

Preparation of amorphous solids The preparation of amorphous solids, for thermodynamic and kinetic reasons, is easy for some materials good glass formers , but difficult for others poor glass formers.

Properties of amorphous solids Amorphous solids exhibit properties unique to their disordered state relative to their crystalline counterparts such as: - the apparent aqueous solubility of amorphous materials is much higher than that of their crystalline counterparts.

Characterization of amorphous solids The strategy for characterizing amorphous solids differs from that for crystalline solids. The physical characterization of amorphous solids utilizes a wide range of techniques and offers several types of information: a. Structure Amorphous solids may co-exist with and have the potential to convert to crystalline solids. Thermodynamics Thermodynamic properties of an amorphous solid are often presented as excess properties relative to the crystalline state.

Stabilization of amorphous solids The aim of stabilization of amorphous solids is multi-faceted, including: - the stabilization of labile biomolecules proteins and peptides through additives, - the prevention of crystallization of excipients that must remain amorphous for their intended functions, - the specification of appropriate storage temperatures to achieve acceptable shelf life, and the prevention of chemical degradation and microbial growth through anti-oxidant, pH buffer, preservatives So, the main factors, which are necessary to keep in mind during the earliest stages of drug development are: - physical transformations solid-state crystallization are more often directly linked to molecular mobility and orientation than the most common chemical reactions oxidation and hydrolysis ; thus the major stability concern for amorphous materials is with their tendeney to revert to the crystalline state.

Conclusion Amorphous substances are an important class of pharmaceutical materials that exhibit distinct physical and chemical properties. References 1 Elliot S. R, Rao N. R, Thomas J. The chemistry of the non-crystalline state. Preformulation predictions from small amounts of compound as an aid to candidate drug selection. In: Gibson M, editor. Pharmaceutical preformulation and formulation. M, Royall P. G, Kett V. L, Hopton M.

The relevance of the amorphous state to pharmaceutical dosage forms: glassy drugs and freeze dried systems. C, Zografi G. Characteristics and significance of the amorphous state in pharmaceutical systems. Thermal analysis of glassy pharmaceuticals, Themochim.

G, editor. Polymorphism in Pharmaceutical Solids. New York: Marcel Dekker; M, Stephenson G. Physical characterization of polymorphic drugs: an integrated characterization strategy.

Formation of glasses from liquids and biopolymers. D, Angell C. A, Nagel S. Supercooled liquids and glasses. Pharmaceutical applications of polymorphism. H, Weber T.

Amorphous solid

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Crystalline solids have regular ordered arrays of components held together by uniform intermolecular forces, whereas the components of amorphous solids are not arranged in regular arrays. The learning objective of this module is to know the characteristic properties of crystalline and amorphous solids. With few exceptions, the particles that compose a solid material, whether ionic, molecular, covalent, or metallic, are held in place by strong attractive forces between them. When we discuss solids, therefore, we consider the positions of the atoms, molecules, or ions, which are essentially fixed in space, rather than their motions which are more important in liquids and gases. The faces of crystals can intersect at right angles, as in galena PbS and pyrite FeS 2 , or at other angles, as in quartz. Right Cleavage surfaces of an amorphous solid.

As you should remember from the kinetic molecular theory, the molecules in solids are not moving in the same manner as those in liquids or gases. Solid molecules simply vibrate and rotate in place rather than move about. Solids are generally held together by ionic or strong covalent bonding, and the attractive forces between the atoms, ions, or molecules in solids are very strong. In fact, these forces are so strong that particles in a solid are held in fixed positions and have very little freedom of movement. Solids have definite shapes and definite volumes and are not compressible to any extent. There are two main categories of solids—crystalline solids and amorphous solids.

Amorphous vs. Crystalline Polymers

Surface Science pp Cite as. The atomic structure of crystalline silicon surfaces and amorphous silicon films deviates markedly from the tetrahedral symmetry found in the bulk diamond lattice structure. Consequently, strained bonds, distorted bond angles and unsaturated bonds are present which lead to characteristic features in the electronic density of states and relate to specific interactions with hydrogen.

Amorphous solid , any noncrystalline solid in which the atoms and molecules are not organized in a definite lattice pattern. Such solids include glass, plastic , and gel. Solids and liquids are both forms of condensed matter; both are composed of atoms in close proximity to each other.

Polymers are unlike other types of materials because of their high molecular weight. Molecular weight is the value used to express the size of a molecule. Water, for example, has a molecular weight of 18 atomic mass units.

12.1: Crystalline and Amorphous Solids

Amorphous forms are, by definition, non-crystalline materials which possess no long-range order. The amorphous solids have always been an essential part of pharmaceutical research, but the current interest has been raised by two developments: a growing attention to pharmaceutical solids in general, especially polymorphs and solvates and a revived interest in the science of glasses and the glass transition. Amorphous substances may be formed both intentionally and unintentionally during normal pharmaceutical manufactoring operations. The properties of amorphous materials can be exploited to improve the performance of pharmaceutical dosage forms, but these properties can also give rise to unwanted effects that need to be understood and managed in order for the systems to perform as required. The degree of crystallinity, according to the USP, depends on the fraction of crystalline material in the mixture, which is termed the two-state model. Another way of viewing this situation is that the crystallinity has a range from percent for perfect crystals zero entropy to 0 percent non-crystalline or amorphous ; this is known as the onestate model 2. Amorphous solids exist in many industrially important products, such as polymers, ceramics, metals, optical materials glasses and fibers , foods, and pharmaceuticals.

Amorphous and crystalline are two states that describe typical solids in chemistry. Using X-ray diffraction experiments, the structure of solids can be categorized into crystalline or amorphous non-crystalline. Solids are among the three basic states of matter that include liquids and gases. They are characterized by a rigid structure of molecules, ions and atoms arranged in an orderly or non-orderly manner. These orderly or non-orderly arrangements have led to the categorization as amorphous and crystalline and this article unfolds the key differences between the two terms. A crystalline solid is that in which the constituent particles are orderly arranged in a three-dimensional pattern called the crystal lattice with uniform intermolecular forces, and the particles intersect at angles characteristic of the crystal. The internal structure has a distinct geometric shape, and it shows a clear cleavage when cut anywhere in the structure.

A crystal or crystalline solid is a solid material whose constituents such as atoms , molecules , or ions are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. The scientific study of crystals and crystal formation is known as crystallography. The process of crystal formation via mechanisms of crystal growth is called crystallization or solidification. Examples of large crystals include snowflakes , diamonds , and table salt. Most inorganic solids are not crystals but polycrystals , i. Examples of polycrystals include most metals , rocks, ceramics , and ice.

AMORPHOUS PHARMACEUTICAL SOLIDS

Сьюзан заглянула в распечатку через плечо Джаббы. - Выходит, нас атакует всего лишь первый набросок червя Танкадо. - Набросок или отшлифованный до блеска экземпляр, - проворчал Джабба, - но он дал нам под зад коленом. - Не верю, - возразила Сьюзан.

Все равно сейчас ТРАНСТЕКСТ - это всего лишь дырка в земле. Так какая разница. Повисла тишина. Фонтейн, видимо, размышлял. Сьюзан попробовала что-то сказать, но Джабба ее перебил: - Чего вы ждете, директор.

 Отчаянный парень, - пробормотал Хейл себе под нос. Он знал, что задумал Чатрукьян.

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3 Comments

  1. Octave R.

    Edward soja thirdspace pdf download working principle of hall effect sensor pdf

    07.04.2021 at 14:10 Reply
  2. AmГ©lie B.

    possess a definite and regular geometry and consist of both long-range as well as short-range order of its constituent particles. The particles of the constituents.

    11.04.2021 at 06:39 Reply
  3. TabarГ© O.

    The major difference between crystalline and amorphous is crystalline solid is anisotropic where as amorphous solid is isotropic. To know more differences.

    13.04.2021 at 17:11 Reply

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