Thermal Analysis Flashcards
What is thermal analysis?
Studies the effect of temperature on the behaviour of samples
What does thermal analysis study?
State changes (i.e. melting, boiling)
Degradation profiles
Complex chemical phenomena
List 3 recoverable thermal events
Phase transition
Desolvation
Glass transition
List the 4 phase transitions which are recoverable thermal events
Melting = transition of a solid to a liquid under the application of heat
Vaporisation (evaporation) = transition of a liquid to a gas under the application of heat
Sublimation = transition from a solid to a gas without entering a liquid phase
Deposition = transition from a gas to a solid without entering a liquid phase
Define: Desolvation
The removal of solvent from a material under the application of heat
Dehydration when the solvent is water
Define: Glass transition
Transition of amorphous materials from a hard and brittle state into a soft and flexible state under the application of heat
List 3 non-recoverable thermal events
Decomposition
Denaturation
Oxidation
Define: Decomposition
The breakdown of a product into, usually simpler, chemical products under the application of heat
Define: Denaturation
The structural change of proteins that involves the disruption and possible destruction of both the secondary and tertiary structures
Define: Oxidation
The reaction of a substance with oxygen, under the application of heat
What is hot stage microscopy and what is it used for?
Samples are heated through a pre-set heating programme and visualised through polarises, which assist in contrast
It can determine phase transition temperatures
What is thermogravimetric analysis (TGA)?
Thermal analysis in which the rate of material weight change change upon heating vs temperature is plotted
Temperature is varied in a controlled manner
Which thermal events cause a weight change of the sample according to thermogravimetric analysis (TGA)?
Vaporisation
Sublimation
Desolvation
(Decomposition)
Which thermal events do NOT cause a weight change of the sample according to thermogravimetric (TGA)?
Glass transition (Tg)
Melting (Tm)
(Decomposition)
What is the formula that predicts mass loss by desolvation during thermogravimetric analysis?
FORMULA
List 3 instrumental factors which can affect TGA curves
Heating rate = quicker heating rates give higher degradation temperatures
Furnace atmosphere = inert atmosphere prevents oxidation
Material and geometry of pans = surface area and conductivty
List 5 sample factors which can affect TGA curves
Mass = small mass changes may be difficult to detect
Particle size = reactive surface area increases at smaller particle size
Samples pre-treatment = crystalline vs amorphous
Packing = compressed samples affect thermal conductivity and combustion
Thermal conductivity = transfer of heat
List 3 applications of TGA
Solvent content
Estimates lifetime of a product
Decomposition kinetics of materials
Define: Endothermic
A process that involves taking in heat from its environment
Define: Exothermic
A process that involves giving out heat to its environment
What is the formula to calculate free energy change?
FORMULA
Define the terms of the free energy change formula
(triangle) G = free energy change
(triangle) H = enthalpy change
T = temperature
(triangle) S = entropy change
What is the relevance of the (triangle)H value in the free energy change formula?
If (triangle) H 0 = endothermic
Define: Crystallisation temperature (Tc)
The temperature at which a solid crystalises into an ordered form, under the application of heat
Define: Melting temperature (Tm)
The temperature at which a solid transforms to a liquid, under the application of heat
What is the purpose of differential scanning calorimetry (DSC)?
When a material undergoes a change in physical state or when it reacts chemically, heat is absorbed (endo) or liberated (exo)
The DSC instrument keeps the sample and the inert reference material at the same temperature (triangle T = 0), whilst the sample is heated or cooled
The energy required to maintain (triangle)T = 0 is measured and therefore gives heat measurements directly
How are the temperature changes associated with a thermal event found from DSC?
Take the peak value/centre of the curve
How are heats (or enthalpies) of thermal events found from DSC?
Look at the area under the curve
Explain the method of DSC
A flow of purge gas (N2) is used to provide a dry, inert atmosphere, aid heat flow and carry away gases
Small amounts of sample (1-5mg)
Spread thinly over sample pan to aid heat transfer
Pans normally made of aluminium
