Anna Clemens

Water desorption from nanostructured graphite surfaces

Water interaction with nanostructured graphite surfaces is strongly dependent on the surface morphology. In this work, temperature programmed desorption (TPD) in combination with quadrupole mass spectrometry (QMS) has been used to study water ice desorption from a nanostructured graphite surface. This model surface was fabricated by hole-mask colloidal lithography (HCL) along with oxygen plasma etching and consists of a rough carbon surface covered by well defined structures of highly oriented pyrolytic graphite (HOPG). The results are compared with those from pristine HOPG and a rough (oxygen plasma etched) carbon surface without graphite nanostructures. The samples were characterized using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The TPD experiments were conducted for H2O coverages obtained after exposures between 0.2 and 55 langmuir (L) and reveal a complex desorption behaviour. The spectra from the nanostructured surface show additional, coverage dependent desorption peaks. They are assigned to water bound in two-dimensional (2D) and three-dimensional (3D) hydrogen-bonded networks, defect-bound water, and to water intercalated into the graphite structures. The intercalation is more pronounced for the nanostructured graphite surface in comparison to HOPG surfaces because of a higher concentration of intersheet openings. From the TPD spectra, the desorption energies for water bound in 2D and 3D (multilayer) networks were determined to be 0.32 ± 0.06 and 0.41 ± 0.03 eV per molecule, respectively. An upper limit for the desorption energy for defect-bound water was estimated to be 1 eV per molecule.

Lyotropic lipid phases confined in cylindrical pores: structure and permeability.

A model membrane system based on lipid lyotropic phases confined inside the pores of a well-defined scaffold membrane, thereby forming a double-porous membrane structure, is described. The model membrane system is characterized with regard to lipid structure, lipid location, and phase transitions, using small-angle X-ray scattering, differential scanning calorimetry, and confocal microscopy. The system enables studies of transport across oriented lipid bilayers as well as of lipids in confinement. The lipids are shown to be located inside the membrane pores, and the effect of confinement on lipid structure is shown to be small, although dependent on the surface properties of the scaffold membrane. For transport studies, Franz diffusion cells and different types of drugs/dyes are used, and the transport studies are complemented with theoretical modeling. Lipids investigated include monoolein, dioleoyl phosphatidylcholine, dimyristoyl phosphatidylcholine, and E. coli total lipid extract. In the case of monoolein, the lipid structure can be changed from a bicontinuous cubic Ia3d phase to a liquid crystalline lamellar phase, by controlling the osmotic pressure of the surrounding solution through addition of water-soluble polymer. The osmotic pressure can thereby be used as a switch, changing the permeability of the lipid phase up to 100-fold, depending on the properties of the diffusing substance. The large effect of changing the structure implies an alignment of the lamellar phase inside the pores.