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Water-induced charge transport in tablets of microcrystalline cellulose of varying density: Dielectric spectroscopy and transient current measurements
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Pharmacy, Department of Pharmacy.
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
2003 (English)In: Chemical Physics, ISSN 0301-0104, E-ISSN 1873-4421, Vol. 295, no 2, 159-165 p.Article in journal (Refereed) Published
Abstract [en]

Room temperature dielectric frequency response data taken over 13 decades in frequency on microcrystalline cellulose (MCC) tablets of varying density are presented. The frequency response shows on three different processes: the first one is a high-frequency relaxation process whose magnitude increases and reaches a plateau as the tablet density increases. This process is associated with orientational motions of local chain segments via glycosidic bonds. The second relaxation process, related to the presence of water in the MCC matrix, is insensitive to changes in tablet density. At lower frequencies, dc-like imperfect charge transport dominates the dielectric spectrum. The dc conductivity was found to decrease with increasing tablet density and increase exponentially with increasing humidity.

Transient current measurements indicated that two different ionic species, protons and OH ions, lied behind the observed conductivity. At ambient humidity of 22%, only one in a billion of the water molecules present in the tablet matrix participated in long range dc conduction. The diffusion coefficient of the protons and OH ions were found to be of the order of 10−9 cm2/s, which is the same as for small salt building ions in MCC. This shows that ionic drugs leaving a tablet matrix may diffuse in the same manner as the constituent ions of water and, thus, elucidates the necessity to understand the water transport properties of excipient materials to be able to tailor the drug release process from pharmaceutical tablets.

Place, publisher, year, edition, pages
2003. Vol. 295, no 2, 159-165 p.
Keyword [en]
Microcrystalline cellulose; Tablets; Dielectric spectroscopy; Transient currents; Water; Protons; Diffusion
National Category
Nano Technology
URN: urn:nbn:se:uu:diva-94363DOI: 10.1016/j.chemphys.2003.09.001OAI: oai:DiVA.org:uu-94363DiVA: diva2:168189
Available from: 2006-04-21 Created: 2006-04-21 Last updated: 2015-09-11Bibliographically approved
In thesis
1. Water-Induced Charge Transport in Microcrystalline Cellulose
Open this publication in new window or tab >>Water-Induced Charge Transport in Microcrystalline Cellulose
2006 (English)Doctoral thesis, comprehensive summary (Other academic)
Alternative title[sv]
Vatteninducerad laddningstransport i mikrokristallin cellulosa
Abstract [en]

Microcrystalline cellulose (MCC) is the most frequently used excipient for direct compaction of tablets within the pharmaceutical industry. It has earlier been indicated that the interactions between the hydration shell – surrounding the drug molecules in an MCC tablet – and the cellulose regulate the speed of the drug release process. These interactions, and the charge transport governed by moisture, are therefore important to analyze and understand to be able to tailor make new functional drug delivery systems.

In this thesis the physical parameters affecting the water-induced ionic transport have been studied with impedance spectroscopy, transient current measurements, nitrogen adsorption and scanning electron microscopy. Dielectric relaxation processes, pertaining to other processes, have also been assessed and analysed, and a generalized regular singular point model has been shown to be able to describe all features of the dielectric spectrum.

It has been shown that the ionic charge transport mechanism in humid MCC most likely is governed by two parallel processes: One involving water constituent ions diffusing between adjacent lowest energy sites (free OH- groups) in disordered regions of the cellulose and the other caused by impurity ions, such as Na+, and protons or H3O+ ions, jumping between neighboring cellulose OH- groups to which primary water molecules are attached. At relative humidities of ~ 37 % (representing monolayer coverage) and higher, the latter process is totally dominating the charge transport.

At a given moisture content, there are two parameters determining the magnitude of the water-induced ionic conductivity in MCC: The connectedness of the interparticulate bonds and the connectedness of pores with a diameter in the 5-20 nm size range.

The presented findings emphasize the importance of analysing and being able to control the nanostructure of a pharmaceutical cellulose-based system in order to tailor the drug transport properties. The presented results should also be significant for other areas where cellulose-water interactions are of key issue; such as for paper and sanitary product research and for food industries using cellulose-based gels.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2006. 53 p.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 170
Materials science, Materialvetenskap
urn:nbn:se:uu:diva-6815 (URN)91-554-6541-2 (ISBN)
Public defence
2006-05-12, Polhemsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:30
Available from: 2006-04-21 Created: 2006-04-21Bibliographically approved

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Alderborn, GöranStrömme, Maria
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Solid State PhysicsDepartment of Pharmacy
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