ANALYTICAL METHODS:

2.1.1 GRAVIMETRIC METHODS (DYNAMIC VAPOUR SORPTION (DVS)):

This method of solid sample analysis involves the use of a humidity and temperature- controlled system. The system used in this study was a DVS from Surface Measurement Systems, U.K. {Figure 2.1). The ultrasensitive (Cahn) microbalance is capable of measuring changes in sample mass lower than 1 part in 10 million. The microbalance mechanism is sensitive to sorption and desorption which necessitates purging of the balance head with a constant dry flow of nitrogen gas. This will give the best performance of baseline stability. Accurate and desired relative humidities are obtained by mixing both dry and saturated vapour gas flows in the correct proportions through the mass flow controllers. Sample and reference sides are contained in the assembly whereby the sample is placed in a quartz pan held in the sample side and an empty quartz pan is kept in the reference side.

Temperature Controlled InculMtor

Vapour Humidifier Mass Flow Controller 1 Reference * Holder Sample Holder Regulated Dry Gas Flow Mass Flow Controller 2

Figure 2.1 Schematic diagram showing the Dynamic Vapour Sorption System (reproducedfrom the DVS User Guide manual).

Humidity and temperature probes are situated just below the sample and the reference holders to give independent verification of the system performance. Different cycles of sorption and desorption can be chosen and performed. Equally, RH values can be varied for different time intervals in the same experiment giving diversity in methods of manipulating and studying the solid sample under investigation. The quartz pans must be extremely clean. Washing is done with distilled water as a first step followed by

rinsing with ethanol or industrial methylated spirit (IMS). The pan is then left to dry at 0% RH. If any static is generated resulting in high level o f noise in the balance reading, the RH is raised up to 90-95% for around 5 minutes followed by drying at 0% RH until constant weight is achieved. The experiment is then started.

Both weight and humidity calibration must be performed periodically. Weight calibration is done whenever the operating temperature is changed. It is performed using a 100 mg weight in the sample pan and the value is adjusted if deviation is more than + 50 micrograms

2.1.2 ISOTHERMAL MICROCALORIMETRY:

This is a thermal method o f analysis whereby both endothermie (heat-absorbing) and exothermic (heat-producing) processes are observed and quantified. It can be used for a variety o f purposes where heat change is involved including chemical, physical and biological changes. Information can be obtained concerning the rate and extent o f chemical reactions, changes in phase, changes in structure as well as metabolism o f living systems.

The equipment used in the current study was the LKB 2277 Thermal Activity Monitor, Sweden. It is an extremely sensitive thermal technique, which gives it a large advantage making it able to detect temperature differences o f less than lO'^ °C. This high level o f sensitivity may, however, be problematic as minute changes may be misinterpreted. The sensitivity and high level o f precision is largely due to the stability o f the heat sink, which surrounds the measuring cylinders. The heat sink is made up o f a closed 25 litre thermostated water bath maintained to ± 2 x 10^ °C within the measurement range o f 5- 80 ®C {Figure 2.2).

Water is circulated by being pumped upwards into the cylindrical, stainless steel tank where it overflows into a larger outer tank. The water is then re-circulated fi*om the outer tank to the inner one. The thermal activity monitor utilises the heat flow principle where heat produced in a thermally defined vessel flows away in an effort to establish thermal equilibrium with its surroundings {Figure 2.3). Measurement takes place in the measuring vessel (cup), which is kept between a pair of sensitive heat sensors (Peltier elements). The system is designed in such a way that any flow o f heat to or from the measuring vessel has to take place through those sensors.

digital voltmeter (DVM)

.hinged cover

.am poule lifter

hea t exchanger

m easuring cylinder

- w ater th erm o stat

w ater p um p

tem p eratu re regulator unit

connections to external water circulator

Figure 2.2 Schematic representation o f the isothermal Microcalorimeter System (reproducedfrom the LKB 2277 Thermal Activity Monitor Instruction manual).

sample vessel heat flow heat flow heat sink heat sink thermopile thermopile

Figure 2.3 Diagram showing the heat flow principle in the Thermal Activity Monitor (reproducedfrom the LKB 2277 Thermal Activity Monitor Instruction manual).

up

ampoule lifter

«ample How

heat exchange coil

nealed ampoule at équilibration poeition

meaaurmg cup

Peltier elements m etal heat sink electrical

calibration resistor

Figure 2.4 Diagram showing the Microcalorimetric unit with both the sample and the reference sides (reproduced from the LKB 2277 Thermal Activity Monitor Instruction manual).

Calibration of the measurement unit in the TAM must be performed when the instrument is used for the first time, after a change in any experimental conditions such as a change in operating temperature or level of power output as well as after any power-cut conditions.

Peltier elem ent,

.glase am poule

m etal h eat sink

Figure 2.5 Diagram showing the glass ampoule connected to the ampoule-lifting loop and inserted in the measuring site (reproduced from the LKB 2277 Thermal Activity Monitor Instruction manual).

2.1.3 DIFFERENTIAL SCANNING CALORIMETRY (DSC):

The DSC is perhaps the most commonly used method of thermal analysis. Thermal methods of analysis generally have the following advantages over some other analytical methods (Hatakeyama and Quinn, 1999):

1. Samples can be studied over a wide temperature range using different temperature programs.