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Amorphous Solid

The term amorphous comes from the Greek word amorphous or without form. These type of solids do not have the repetitive interlocking structures that are seen in crystals but are more like randomly arranged, jumbled formations seen in liquids or anything that flows.

These could be attributed to the fact that the atoms in amorphous solids are spaced apart at equilibrium distance but with no definite periodicity as far atom location is concerned. These are sometimes described as super cooled liquids as the molecules are mostly arranged in variated form or manner which are common in flowing materials.

 

Amorphous Solid Definition

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Solids which do not have long range order or something that does not exhibit regular arrangements of atoms as the molecules of such forms shows randomness with no definite spacing or density of atom arrangement. These solids do not exhibit sharp change in phase from solid to liquid at any particular temperature but instead they soften out in a gradual manner when heated.

The physical properties of such solids show identical set of features in all directions and along all axis and hence have isotropic properties.
These are also considered as counterpart to the crystalline state of solids as crystalline solids have regular arrangement of atoms which is not observed in case of amorphous solids.

Whatever solids we see around us, most of these are crystalline nature as they are composed of atoms, ions or molecules arranged in a regular manner and repeated in a three dimensional settings. These long range orderly arrangement is a unique feature of crystals which is not seen in amorphous solids where the observing feature is disorder.

The amorphous solids exhibit short range order arrangement in their structure. For example Boron tri oxide can occur in both crystalline as well as amorphous forms. Both the forms of solids have a central Boron atom surrounded by three oxygen atoms representing the short range order.  $B_{2}O_{3}$ arrangement is same and spaced equally in case of crystalline form whereas the same arrangement in amorphous solids is in patches. The bond strength in amorphous solids may vary in strength due to the absence of long range order. Whenever a amorphous solid is heated the weakest bond in the structure breaks first and hence shows cracks at points at lower temperature, which is neither uniform nor follow any definite pattern.

The uneven bond breaking makes the melting process gradual in amorphous solids which gives an opportunity to mould these materials into newer shapes. In case of crystalline solids, the bond breaking is same all across and hence the process is fast and abrupt.

Amorphous Solid Examples

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For any solids which shows amorphous property the uneven breaking of bond makes them unique amongst all solids. The best examples for amorphous solids are Glass, rubber, fused silica and plastics. All these materials exhibit the same uneven atomic arrangement and hence could be moulded gradually as the melting process is also gradual for these materials.

Crystalline and Amorphous Solids

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There are several manner in which an amorphous solid could be made. When a flowing matter is cooled rapidly in order to crystal formation then the solid thus formed is amorphous.

Glass could be made into amorphous from its liquid state in this manner but is formed well under the transition temperature of glass. The amorphous solids could also be produced by ion implantation in between the silicates. The amorphous solids do not follow any particular geometrical pattern. Amorphous solids do not exhibit sharp melting points like that of a crystalline solids. The arrangement in amorphous forms continue only for few angstrom units. When these are hammered into they break in irregular manner. These are also termed as isotropic crystals. In comparison the crystals have regular arrangement, have specific melting points, break in unison, do not flow like amorphous solids and the mechanical, electrical, and optical properties are not dependent upon the direction along which they are measured and hence are also known as anisotropic crystals.

These solids have non-molecular three dimensional structure formed from atoms or ions which are combined together primarily by bonding extending in one, two, or even in three dimensions.

 Crystalline solids   Amorphous solids 
 These are characterised by unit cell which exhibit regular repeating structure   These solids have no definite unit cell and exhibit irregular repeating structure
 These solids have high degree of atomic scale symmetry  These solids show very low degree of atomic scale symmetry
 The arrangement is of long range order  The arrangement is of short range order
 These solids have sharp melting point  These solids have long melting point range
 The diffraction pattern is very sharp  The diffraction pattern in these solids have diffused form
 At cooling curve the breaks are noticeable  No such breaks are noticeable at cooling curves
 The break is observed along crystallographic planes  The break is irregular as there is no crystal planes

Amorphous Solid Dispersions

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The amorphous solid dispersions or ASD as popularly termed as are nothing but enabled oral formulations that are increasingly getting used in both animal and human drug testing. The amorphous solid dispersions are dispersions of amorphous drugs which are transferred in a carrier matrix like polymer.

They form the super saturated solutions after the dosing which helps in increasing the flux across the membrane of intestine. With proper choice of polymer the sustenance of super saturation duration could be extended for several hours which helps in overcoming the absorption limitations caused by low equilibrium solubility. The polymers which are used as carrier matrix and other excipients are made use of to make amorphous solid dispersions have better acceptability as far daily intake is concerned as compared to excipients used for making simpler formulations.

The amorphous solid dispersions are successfully utilised for oral dosages in toxic studies as well as clinical studies. These have resulted in significant improvements as far plasma exposure is concerned and decrease in the variability as compared to conventional formulations with crystalline base. With the advent of amorphous solid dispersion the pharmaceutical industry has benefitted in finding a method of retaining the effect of dosage for longer period of time and improve the drug application.

What Happens When an Amorphous Solid Breaks?

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Amorphous solids with irregular arrangement of atoms in patches also breaks unevenly whenever force is applied or is broken. These solids do not break in any regular pattern as is evident in crystals but rather break at random as the weakest bond at places is not evenly placed within the amorphous solid structure. The application of force breaks the weak bond first and hence the breakage is found to be uneven as well.
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