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Conductivity percolation in loosely compacted microcrystalline cellulose: An in situ study by dielectric spectroscopy during densification
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Nanotechnology and Functional Materials.
Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Pharmacy, Department of Pharmacy.
Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Pharmacy, Department of Pharmacy.
Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Pharmacy, Department of Pharmacy.
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2006 (English)In: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 110, no 41, 20502-20506 p.Article in journal (Refereed) Published
Abstract [en]

The present study aims at contributing to a complete understanding of the water-induced ionic charge transport in cellulose. The behavior of this transport in loosely compacted microcrystalline cellulose (MCC) powder was investigated as a function of density utilizing a new type of measurement setup, allowing for dielectric spectroscopy measurement in situ during compaction. The ionic conductivity in MCC was found to increase with increasing density until a leveling-out was observed for densities above similar to 0.7 g/cm(3). Further, it was shown that the ionic conductivity vs density followed a percolation type behavior signifying the percolation of conductive paths in a 3D conducting network. The density percolation threshold was found to be between similar to 0.2 and 0.4 g/cm(3), depending strongly on the cellulose moisture content. The observed percolation behavior was attributed to the forming of interparticulate bonds in the MCC and the percolation threshold dependence on moisture was linked to the moisture dependence of particle rearrangement and plastic deformation in MCC during compaction. The obtained results add to the understanding of the density-dependent water-induced ionic transport in cellulose showing that, at given moisture content, the two major parameters determining the magnitude of the conductivity are the connectedness of the interparticluate bonds and the connectedness of pores with a diameter in the 5-20 nm size range. At densities between similar to 0.7 and 1.2 g/cm(3) both the bond and the pore networks have percolated, facilitating charge transport through the MCC compact.

Place, publisher, year, edition, pages
2006. Vol. 110, no 41, 20502-20506 p.
National Category
Pharmaceutical Sciences Engineering and Technology
URN: urn:nbn:se:uu:diva-94366DOI: 10.1021/jp063835qISI: 000241192200064PubMedID: 17034236OAI: oai:DiVA.org:uu-94366DiVA: diva2:168192
Available from: 2006-04-21 Created: 2006-04-21 Last updated: 2016-06-22Bibliographically 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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Frenning, GöranGråsjö, JohanAlderborn, GöranStrömme, Maria
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Nanotechnology and Functional MaterialsDepartment of Pharmacy
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