Module 4 Flashcards
______ is a separation and purification technique employed to produce a wide variety of materials.
Crystallization
Examples of crystalline
materials:
a. Quartz
b. Salt (Himalayan salt)
c. Acetylsalicylic acid
(Aspirin)
d. Sugar
__________ may be defined as a phase change in which
a crystalline product is obtained from a __________
Crystallization; homogeneous
phase.
A crystal may come from any of the following homogeneous phases:
1. A liquid ______ consisting of a solvent (liquid) and a solute (solid) at the condition of interest.
solution
A crystal may come from any of the following homogeneous phases:
2. A _________ , which is a material that is solid at normal conditions but becomes a molten liquid when heated.
melt
Crystallization may be defined as a phase change in which
a crystalline product is obtained from homogeneous
phase. 3. A gaseous mixture; i.e., a solution of a solid in a ________
supercritical fluid
Crystallization can be regarded as a self-assembly process in
which randomly organized molecules in a fluid come together
to form an _________
ordered three-dimensional molecular array with a
periodic repeating pattern.
Crystallization is vital to many processes: both ________ and
.
natural; man-made
Crystals are solids in which
the atoms, ions, or molecules
are arranged in a ______
repeating pattern that
extends in ________.
periodic; three dimensions
All crystals are ________ but not all
solids are crystals. Materials that have short-range ordering are described as _______ solids.
solids; amorphous
A crystal can be represented as a geometric point lattice –
a set of points arranged so that each point has identical
surroundings.
Unit cell
The lengths and angles in
a lattice point system are
known as ________.
lattice
parameters
A single cell constructed
using the parameters is
called a ___________.
unit cell
The representation of planes
in a lattice makes use of
______________.
Miller indices {hkl}
If a plane is parallel to a
given axis, the Miller index
is __________.
zero
Negative indices are
written with ________ over
them
bars
Step 1:
Identify the __________ on the x-,y-,z- axes
intercepts
Step 2: Specify the intercepts in ________ co-ordinates
fractional
Step 3:
Take the _________ of the fractional intercepts
reciprocals
While it seems possible that a number of different lattice
arrangements and unit cells can be constructed, Bravais in
1848, showed that only ____ possible point lattices can be
constructed.
14
These 14 point lattices can be divided into _____ crystal
systems.
7
Simple/Primitive (P) Crystal systems
Triclinic
Monoclinic
Orthorhombic
Tetragonal
Cubic
Trigonal/rhombohedral
Hexagonal
Body Centered (I) systems
Orthorhombic
Tetragonal
Cubic
Face Centered (F) systems
Orthorhombic
Cubic
Base Centered (C) systems
Monoclinic
Orthorhombic
Cubic (axial relationships)
a = b = c
Cubic (interaxial angles)
alpha = beta = gamma = 90
Hexagonal (axial relationships)
a = b =/= c
Hexagonal (interaxial angles)
alpha = beta = 90, gamma = 120
Tetragonal (axial relationships)
a = b =/= c
Tetragonal (interaxial angles)
alpha = beta = gamma = 90
Rhombohedral (axial relationships)
a = b = c
Rhombohedral (interaxial angles)
alpha = beta = gamma =/= 90
Orthorhombic (axial relationships)
a =/= b =/= c
Orthorhombic (interaxial angles)
alpha = beta = gamma = 90
Monoclinic (axial relationships)
a =/= b =/= c
Monoclinic (interaxial angles)
alpha = gamma = 90 =/= beta
Triclinic (axial relationships)
a =/= b =/= c
Triclinic (interaxial angles)
alpha =/= beta =/= gamma = 90
Amorphous state: atoms or molecules possess no ______, and are organized in essentially a random arrangement.
long-range periodicity
Amorphous state is considered as the ______ of a crystalline material.
antithesis
For amorphous materials,
the diffraction pattern
would exhibit a broad _____
with ______
maximum.
halo; few or a single
Crystalline materials
would have characteristic
__________ peaks that
correspond to the
different ________ planes
within the crystal lattice.
diffraction; symmetry
________ are restricted in a crystalline solid – ______ systems
and less susceptible to chemical degradation.
Molecules; static
The area of research involved in understanding the true
nature of crystals and how they are identified based on
knowledge of crystal’s internal structure.
Crystallography
Law that shows the crystal identification using the XRD where the angle of the incident x-ray is equal to the diffracted x-ray angle.
Bragg’s Law
Constructive interference occurs when n*pi = ______ for Bragg’s Law.
2dsin(theta)
Different crystal
forms of the same
compound will exhibit
_______ XRD spectra
different
The development, design, and control of a crystallization
process involve knowledge of a number of the properties
of the solution.
* As an example, ________ provides the concentration at which the
solid solute and the liquid solution are at equilibrium.
solubility
The solubility data allows calculation of the _______ accompanying a change of state from one concentration to another in which crystals form.
maximum yield of product crystals
Mass balance plays an important part on crystallization
process design, development, and experimentation;
however, it tells us nothing about the ______ at which the
crystals form and the _____ required to obtain this amount
of solid.
rate; time
Crystallization is a rate process, depending on a driving
force called __________ .
supersaturation
The amount of solute required to make a saturated solution at a given condition is called ___________.
solubility
The solubility of materials depends on_______. The information provided by solubility is vital in crystallization processes.
temperature
_________ solubility is seen to have a relatively weak
temperature dependence. _______, by contrast, have really high temperature dependence.
Sodium chloride; Potassium nitrate
The solubility of common salts is important because it will determine the amount of ______ required to yield a given amount of product
and whether ________ will provide a reasonable product yield
cooling
Curve 1 (e.g. KNO3): ↑T, ↑solubility: ________ a solution is preferred for
those with steep slopes
Cooling
Curve 2 (e.g. NaCl) Solubility is almost independent of T; _________ of solution is preferred
Evaporation
Curve 3 (e.g. MnSO4∙H2O) : ↓T, ↑sol.
* _________ solubility curve
Inverted
The solubility of a compound in a particular solvent is part
of the system phase behavior and can be described graphically by a ________.
phase diagram
In phase diagrams of solid-liquid equilibria, the _________ of the solid is usually plotted versus .
mass fraction; temperature
The line abcdef is the saturation line that defines a _______ at a given temperature
saturated solution
Line ab is the solubility line for the
_____ (when a solution in this region is cooled, _____ crystallizes out and is in equilibrium with the solution)
solvent; ice
Point b is the _________ (at this point both ______ will separate as solids)
eutectic point; ice and MgSO4
The curve bcdf represents the solubility curve of ________
MgSO4
A solution in which the
solute concentration
exceeds the equilibrium
(saturation) solute
concentration at a given
temperature is known
as a ________.
supersaturated
solution
It is important to note that a supersaturated solution is considered __________.
metastable
__________ involves cooling to change solubility.
Temperature change
_____________ is useful when solvent is non-aqueous with removal of solvent increasing solute concentration.
Evaporation of solvent
Changing solvent composition happens with addition of an _______ (a specific solvent which causes
solubility to decrease greatly)
- the __________ where the solute is not soluble but miscible to the
system
antisolvent
_______ causes supersaturation by precipitation.
Chemical reaction
Referring to the solubility diagram:
If we start with a solution at point 1
and cool it to point 2 just below
saturation, the solution would be
supersaturated. However, crystals
________ to form.
may take days
Referring to the solubility diagram: If we have a sample cooled further
to point 3, crystallization might
occur in _______.
a matter of hours
Cooling a sample beyond point 3, the solution becomes ________ and
crystallization occurs _______.
unstable; spontaneously
The area between the point
of saturation and the
supersaturation limit is
called the _________ and are generally within this
zone.
metastable zone width
The first step of crystallization is the phase separation, or “birth,” of a new crystal. This step is called ________.
nucleation
The second step of crystallization is the _____ of these crystals to larger
sizes by the addition of solute molecules from the supersaturated solution. This step is called crystal
________.
growth
The initial stage of crystallization involves the ________ of a new phase due to the system being in a
condition.
spontaneous appearance; non-equilibrium
Spontaneous appearance of a new phase is followed by ________ dissolved in
the supersaturated solution, leading to the formation of a
nucleus.
aggregation of molecules
A ____________ can be defined as the minimum amount of a new phase capable of independent existence. However, the exact nature of nuclei is still unknown.
nucleus
The birth of these small nuclei in an initially metastable
phase is called __________.
nucleation
Nucleation may occur spontaneously or be induced
artificially by ____________
seeding, agitation, mechanical shock, electric
and magnetic fields, and other external influences.
___________ is the generation
of nuclei directly from a
previously crystal-free solution
Primary nucleation
_______________ takes
place in the presence of
preexisting parent crystals of the
same solute in the solution, which
act as catalysts for further
nucleation
Secondary nucleation
In pure solutions consisting of only solute and solvent
molecules, nucleation is triggered by the _________ , with the probability of a given
fluctuation occurring being identical over the whole
volume of the system.
fluctuations of
solute concentration
Homogeneous
nucleation rarely
occurs in practice, but
it forms the ________.
basis of nucleation theories
The ____________ describes the
homogeneous nucleation process in terms of Gibbs free
energy and a critical cluster size beyond which growth
occurs instantaneously.
classical nucleation theory (CNT)
According to CNT, ________ give
rise to the appearance of _______ , which continuously form and dissolve in the solution until a
_________ is reached.
fluctuations in a metastable phase ; pre-nucleation clusters; critical size
Mathematically, homogeneous nucleation is described as
the sum of the free energy change for ________ and the free energy change for the _______.
phase transformation (∆Gv); formation of a surface (∆Gs)
Because the solid state is more stable than the liquid, the __________ – decreasing the total
free energy.
second term (∆Gv) becomes negative
By contrast, the ___________
increases the total free energy by an amount proportional to the surface area of the pre-nucleating cluster.
first term (∆Gs)
Nucleation depends on the competition between a decrease in _____ , which favors growth, and an increase in _____ , which favors dissolution.
∆Gv; ∆Gs
Small nucleating clusters dissolve unless a _____ is reached,
wherein growth becomes ________ favorable.
critical size; energetically
At the critical size, rc, d/dr(deltaG) =
zero
The maximum value of the free energy change is _________.
ΔGcrit
The growth of clusters is governed by the _____________.
Gibbs-Thomson equation
We introduce a mathematical term for supersaturation:
ln(c/c) = lnS = 2(sigmanu)/(k*T)
The GT relationship relates
supersaturation with _________.
interfacial diffusion
The rate of primary nucleation may
also be empirically expressed using the theory of __________.
chemical kinetics
Theoretically, the frequency factor A is in the order of_______ nuclei/cm3-s
10^30
For heterogeneous nucleation, A ≈ ______ nuclei/cm3-s
10^25
Given the nature of nucleation, a general theory for the prediction of nucleation rate _________.
does not exist
A model based on a _______ was
found to be satisfactory in explaining most experimental
studies of nucleation kinetics.
power-law expression
Assumptions to the CNT: Clusters are _______ droplets with uniform ________ and sharp ________.
spherical; densities; interfaces
Assumptions to the CNT: The molecular arrangement in the crystals’ _______ is identical to the
____________.
embryo; macroscopic crystal
Assumptions to the CNT: The _________ and ______ of the surface energy is neglected.
curvature; temperature dependence
Assumptions to the CNT: Cluster growth takes place via ________ addition, while _______ of clusters are ignored.
monomer-by-monomer; collisions and break-up
Assumptions to the CNT: There is no ___3 ()______ motion.
translational, vibrational, or rotational
Assumptions to the CNT: Clusters are established _________ at the _______ of supersaturation.
instantaneously; onset
Assumptions to the CNT:
Nucleation rate is _______ and does not depend on _______
of the sample.
time independent; thermal history
The clusters are _________. The vapors that might surround them are ideal at __________.
incompressible; constant pressure
The classical nucleation theory is an _________ of
the nucleation process.
oversimplification
Other nucleation theories include the _________ and the _________.
two-step nucleation
theory; pre-nucleation cluster theoretical pathway
Homogeneous nucleation is uncommon in practice
because the presence of foreign particles, such as dust and
dirt, and surfaces, such as container walls and baffles,
induce _________________.
heterogeneous nucleation
Heterogeneous nucleation occurs because the foreign
particles (or surfaces) decrease the ___________ necessary for nucleation via a reduction of the ________of prenucleation clusters.
activation barrier; surface
excess energy
Heterogeneous nucleation occurs at _________ than that necessary for homogeneous nucleation.
much lower supersaturation
__________ happens when crystallization proceeds in the presence of ________ of the
crystallizing solute in a supersaturated solution.
Secondary nucleation; parent crystals
The parent crystals act as _______ that allow nucleation to occur even at lower supersaturation compared to what is required for primary nucleation.
catalysts
The secondary nuclei may be from the following:
1. A ______ which allows breeding from a parent crystal
seeded in the solution.
seed
The secondary nuclei may be from the following:
2. A _________, which may be anything from solid impurities
such as dust or crystalline fragments. Any _______may
become a site for secondary nucleation.
crystallite
- A ________ formed by fluid shear.
The nucleation occurs along the ______________ between
the crystal and the solution.
solute concentration gradient; boundary layer
Once a nucleus reaches critical size and has stabilized, they
begin to grow into macroscopic crystals. This process is
called _________.
crystal growth
Crystal growth is described by a change in the ________
of the crystal with time.
dimension
Crystal growth theories generally describe crystals growing in a ________ fashion – meaning a molecule in solution must _______ on a crystal surface.
layer-by-layer; desolvate and adsorb
There are three possible
sites where a molecule
could attach itself to the
growing crystal: a
___________.
terrace, a step, or a kink
A _______ is generally a growing crystal surface
terrace
A _______ is a location where a molecule is attached to both a growing surface layer and a growing step
step
A __________ is a location where the molecule is attached to three surfaces.
kink
The general mechanism is described as:
1. Transport of solute molecules from _______ to the crystal surface
2. ______ on the growing crystal surface
3. _______ over the crystal surface
4. Attachment to a ______
5. _____ along a step followed by integration into the growing crystal at a _______ site
6. Diffusion of ______ away from the crystal surface
7. Liberation of ________
bulk; Adsorption; Diffusion; step; Diffusion; kink; solvent shell molecules; heat of crystallization
One way to measure crystal growth rate is through measurement of the ____________.
mass change of the crystal
Despite the complex process, we can also express crystal
growth in terms of _________, based on a __________ – a solute
diffuses through a boundary layer and is then incorporated
into the crystal. The rate of growth is based on the distance
moved per unit time in a direction perpendicular to the
surface.
rate equations; diffusion model
Note that as the solute deposits, two resistances are encountered:
* Resistance to mass transfer from the ________ to crystal surface
* Resistance to the integration of molecules into the ___________
bulk liquid; space lattice
When a crystalline solid with a particular crystal size
distribution (CSD) is in contact with a saturated solution,
however, the CSD can change with time.
Ostwald ripening
Ostwald ripening
The phenomena in which smaller particles in solution dissolve and deposit on larger particles in order
to reach a more thermodynamically stable state wherein the
surface to area ratio is minimized.
_________ refers to the external appearance of a crystal.
Crystal habit
The crystal habit (or morphology) is determined by the
relative growth rate of its various crystallographic faces,
with a general rule being that the faces that appear grow the
_________.
slowest
Many factors influence the overall crystal habit: from the
conditions of crystal growth to possible impurities present
during nucleation, and even the kind of solvent used in the
solution. Generally, crystal habit is strongly affected by:
- Degree of supersaturation
- Agitation intensity
- Density and size of crystals
- Purity of solution
Crystal habit can also vary dramatically with the rate of
______ and _______.
crystal growth; nucleation
In the manufacture of terephthalic acid by _________, the rapid
reaction and high supersaturation
results to a product crystal without
any ________.
air oxidation of p-xylene; crystal faces
Crystallization of terephthalic acid at slower rates yields a _______.
monoclinic
needle
______________ may also
be classified by the degree to
which crystal faces are
developed
External crystal habit
Faces are well- developed due to
uncrowded crystal growth
Euhedral
Crystals with partially developed faces
Subhedral
Crystals with undeveloped faces
Anhedral
Depending on the intended application of the crystalline
material, the solid form is considered a very important
parameter in terms of its performance and _______.
processability
The crystal habit may affect a product performance in terms of:
- Solubility
- Product purity
- Dissolution rate
- Stability
- Bioavailability
- Appearance
The crystal habit may affect industry processability in terms of:
- Morphology
- Flowability
- Density
- Particle shape and texture
The phenomenon of a chemical species to adopt in more
than one crystalline packing arrangement.
Crystal polymorphism
When a material crystallizes into a different polymorph,
the _________ of the species remain identical, but
the _________ of the material can be different.
chemical nature; physical properties
Density, heat capacity, melting point,
optical activity, thermal conductivity, etc.
Physical properties
Powder properties
Compaction and flow
According to McCrone (1965): “the number of polymorphs of
a given molecule is proportional to the _______ spent on that compound.”
time, money, and experiments
A compound with many polymorphs.
5-methyl-2-[(2-nitrophenyl)amino]-3-thiopenecabonitrile (ROY)
_____________ are crystals
in which the structure contains either a non-stoichiometric or
a stoichiometric amount of solvent in the crystal lattice.
Solvates (or hydrates when the solvent is water)
Solvates are considered as _________ because of the
inclusion of the solvent molecules in the lattice.
“pseudo-polymorphs”
Crystallization in polymorphic systems is ruled by a
combination of ___________ and ____________ factors.
thermodynamic; kinetic
____________ will provide information on relative
stabilities of polymorphs.
Thermodynamics
__________ will determine which form will be created and for
how long it will survive.
Kinetics
Glycine form that is stabilized only through confinement (least stable).
beta-glycine
Glycine form that is stabilized with cooling/evaporation at neutral pH (middle stability).
alpha-glycine
Glycine form that is stabilized with crystallization at low pH (high stability).
gamma-glycine
In crystallization processes, two expressions are often
used: crystallization from _________ and crystallization from _________.
solution; melts
Ulrich, Özoğuz, and Stepanski (1988) suggested that crystallization from solution occurs when _______
effects dominate the process, and crystallization from melt occurs when _________ effects dominate the
phase change.
mass transfer; heat transfer
Melt crystallization may be applied to the following systems:
- Isomers with close boiling points
- Azeotropic systems
- Temperature-sensitive substances
- Components that tend to polymerize
- Explosive substances
- Eutectic systems
- Solid-solution systems
