Crystals Flashcards
Crystal with all lengths and angles = 90°
Cubic
Crystal with 2 lengths the same and all angles = 90°
Tetragonal
Crystal with all lengths different and all angles = 90°
Orthorhombic
Crystal with all lengths different and 2 angles = 90° and 1 angle not 90°
Rhombohedral
Crystal with 2 lengths the same and 2 angles = 90° and 1 angle = 120°
Hexagonal
Crystal with all lengths different and 2 angles = 90° and one doesn’t equal 90°
Monoclinic
All lengths different and all angles different
Triclinic
How many atoms are in primitive cubic cell?
1 (8 1/8ths)
How many atoms are in body-centred cubic cell?
2 (8 1/8ths + 1)
How many atoms are in face-centred cubic cell?
4 (8 1/8ths + 4 1/2s + 1)
What states does x-ray diffraction occur in?
Solid only
Bragg’s Law
nλ = 2d sinθ
What causes the scatter of x-rays in a crystal?
The electron cloud around the atoms
What does each diffraction spot correspond to?
A single point in the lattice
What is polymorphism?
When molecules are arranged in 2 or more different ways
In what ways can molecules be arranged differently?
- Differences in orientation or conformation of the molecules
How can polymorphism be detected?
- Differences in x-ray diffraction pattern
- Melting Point
- Solubility
What are 2 types of multi-component systems?
Solvates and hydrates
How are different polymorphs prepared?
Manipulating conditions of crystallisation such as solvent, temperature, pressure, cooling rate and stirring rate)
What processing process may cause polymorphic changes?
Milling, drying, compression and time
What pharmaceutical properties are affected by polymorphism?
Density, melting point, hygroscopicity, apparent solubility
What is bioavailability?
The rate and extent to which a substance is absorbed and circulated around the body
How can polymorphic forms be characterised?
X-ray diffraction, microscopy, thermal analysis and spectroscopy
What does DSC stand for and detect?
Differential Scanning Calorimetry can detect different melting points if they exist
How are forms and temperature related?
As temperature increases, Form III changes to Form II which changes to Form I which finally melts
What is Thermal Gravimetric Analysis?
Constant heating rate is used to show changes in weight with temperature or time
What is Hot Stage Microscopy?
Can be used to detect melting point and loss of solvent and can be used to distinguish between solvates and polymorphs
What is birefringence?
The ability to rotate plane polarised light and in Liquid crystals gives rise to 2 refractive indices.
Why are there 2 refractive indices in liquid crystals?
Light has 2 waves at 90° to each other and when they strike the surface, the light splits and one wave takes a longer pathlength. This results in the light exiting the material at 2 places and are out of phase
What does thermotropic mean?
Liquid crystal shows behaviour as a function of temperature
What type of crystal is a nematic crystal?
A thread-like crystal which is thermotropic
How are lyotropic crystals formed?
Formed in solutions
What kind of ordering does smectic crystals have?
Layer ordering
What kind of ordering does nematic crystals have?
Orientational ordering
How does temperature change colour of cholesteric liquid crystals?
Reflected light is equal to pitch and the pitch depends on the temperature
What has an effect on pitch in cholesteric liquid crystals?
Temperature, pressure and electric and magnetic fields
What are the 2 essential structural features of a lyotropic liquid crystal?
Amphiphilic and 2 distinct regions within the molecule
