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CPP: Chemische Physik und Polymerphysik

CPP 15: Poster: Dünne Filme, Monolagen und Oberfl
ächen

CPP 15.2: Poster

Mittwoch, 13. März 2002, 16:30–18:00, B

The effects of divalent ions on DMPA monolayers — •Jens Pittler, Mathias Loesche, and Carsten Selle — University Leipzig, Inst. Exp. Physics I, Linnestr. 5, 04103 Leipzig

We are particularly interested in the interactions of divalent cations, with the charged headgroups of DMPA molecules at the air-water interface by using combined techniques of X-ray reflectometry and grazing-incidence diffraction (GIXD). The properties of charged organic molecules as monolayers at the air-water interface are strongly affected by metal ions. X-ray reflectivity measurements of DMPA on BaCl2 subphases indicated that one cannot simply assume a stoichiometric, 2:1 (DMPA:Ba2+), ratio in describing the surface structure. Instead, the number of ions included in the headgroup was found to be larger at low lateral pressure and approached the stoichiometric (2:1) ratio upon compression of the monolayer to low areas. It has been reported that the position of the bound cations along the surface normal coincides with the position of the phosphate, such that one may model the phosphate fragment and the cations using one distribution function along the z axis in a volume-restricted distribution function (VRDF) approach to data modeling. As a practical consequence, one may directly determine the overall number of water molecules in the headgroup without any assumption on a specific association ratio, water:cation.