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Chapter 4

CRYSTALLIZATION

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Crystallization of organic solids

Crystallization: a firstorder phase changein which solidproduct is isolated

from a solution,melt, vapor or evenfrom solid phase

Supersaturation:

prerequisite forachievingcrystallization fromsolution

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Crystallization Process: Nucleation

Initiation of a phase change in a small region, such as theformation of a solid crystal from a liquid solution

Consequence of rapid local fluctuations on a molecular

scale in a homogeneous phase that is in a state of

metastable equilibrium This transformation requires traversing a free energy

barrier

Primary nucleation (homogeneous)-spontaneous

Secondary nucleation (heterogenous)-induced by thepresence of foreign particles

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Free energy diagram for nucleation explainingthe existence of critical nucleus

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Type of Crystal Growth1. Continuous (rough) Growth Referring as continuous growth or rough growth, in which several crystalline

materials with high roughness contain enough kink and step sites on thesurface to integrate basically all close to growth units with small complication.

AB

C

Molecules A as the surface only (one attachment)

B as the surface and a growing step (two attachments)

C as a kink site (three attachments) wheres in energetically C is the mostfavourable for successful bonding to the surface followed by B and lastly is A.

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3. Screw Dislocation (Spiral Growth)

In the existence of screw dislocations, crystalfaces grow through the spiral growth

mechanism at supersaturation lower than the

value where 2D nucleation and growth is the

dominant mechanism (Lovette et al. 2008).

In spiral growth, self-generating step process

are take placed as a result of screw

dislocations that terminate on a crystal face

act as continuous sources of edges.

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Figure below shows the configuration of screw

dislocation that possibly occurs within crystals

as result of stresses in crystal growth on seedsand around surface nuclei and inclusions.

Development of a Growth Spiral Starting from a Screw Dislocation: (a) Start, (b) Later, (c) Final Spiral

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Polymorphism

Polymorph: Crystals in which chemical compoundtakes different arrangements of molecules-solidstate phenomenon

Different morphologis, solubilities, color, melting,sublimation temperatures, densities, thermal/elctrical conductivities

Pharmaceutical industry-affect processing

properties such as filtering, drying, flow,tabletting, rate of dissolution, shelf life andbioavailability

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Thermodynamics of Polymorphism

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C

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