Low Frequency Dielectric Investigations on Hydrated Pharmaceutical Powders

Measurements of dielectric properties on hydrated systems at low frequencies, present particular difficulties with regard to the phenomenon of electrode polarisation, which can mask the dielectric properties of the sample. The reproducibility of DRS measurements on powdered systems has also been questioned due to difficulties inherent in the sample electrode interface. Blocking electrodes are used here, as part of a specially developed dielectric measurement system, to facilitate dielectric measurements on hydrated microcrystalline cellulose (MCC) and hydrated lyophilised proteins (papain, ovalbumin, pepsin and lysozyme) in the high millihertz to low megahertz frequency range. These materials are representative of two pharmaceutical technologies, viz. tabletting (including granulation) and the lyophilisation of proteins. A previously poorly characterised low frequency dispersion in MCC is shown to be a bulk process (a separate dispersion due to the presence of the blocking electrodes was observed) that is almost certainly due to the movement of hydroxyl groups via extended chain motions. This relaxation is shown not to be due to proton conduction, or other ionic currents, by the effect of hydration with various solutions (including the effect of /?H, [D2O] and ionic strength). Additionally, particle size was found not to affect the relaxation time. This slow dispersion is extremely sensitive to the level of moisture, especially in the lower hydration ranges investigated here. Transitions in the hydration dependent behaviour of the dielectric relaxation time constant, correlated with specific whole number watermionomer ratios (1:1 and 3:2), indicated the presence of different populations of water. Measurements on hydrated proteins indicated the presence of several relaxation modes, including a relaxation mode with an activation enthalpy of 75 kJ/mol, found only in proteins with a hydrolytic activity.

Name dc:type.qualificationname

PhD

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

De Montfort University

Year dc:date.issued

1999

Author dc:creator

• Pearson, David S.

Licence

• https://dora.dmu.ac.uk/bitstreams/a80747d8-5756-469d-92c7-4743aadc9423/download