Dresden 2011 – wissenschaftliches Programm

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CPP: Fachverband Chemische Physik und Polymerphysik

CPP 10: Poster: Charged Soft Matter

CPP 10.11: Poster

Montag, 14. März 2011, 17:30–19:30, P2

The 2-dimensional Lamellar Phase of PSS Adsorbed onto an Oppositely Charged Lipid Monolayer — •Thomas Ortmann, Heiko Ahrens, Jens-Uwe Günther, Frank Lawrenz, and Christiane A. Helm — Inst. für Physik, Uni Greifswald, Felix-Hausdorff-Str. 6, D-17487 Greifswald

Monolayers of cationic lipids with adsorbed polystyrene sulfonate (PSS) are studied by isotherms, grazing incidence X-ray diffraction and X-ray reflectivity. Dioctadecyldimethylammonium bromide (DODA) is used as lipid, the PSS molecular weight is varied. For a PSS concentration of 0.01 mM (with respect to the monomer concentration) in pure water Bragg peaks of flatly adsorbed, aligned PSS chains are observed for PSS both beneath the lipids in the solid (chain separation 2-2.7 nm) and in the fluid phase (chain separation 3-6 nm), the latter only if the PSS contour length Lc exceeds 10 nm. The peak position is independent of the chain length. However the isotherms depend strongly on PSS chain length: the surface pressure of the liquid/condensed phase transition is largest for the shortest PSS used (Lc = 5.5 nm) and decreases the longer the polymer gets until a limiting value is reached (contour length Lc exceeds 25 nm). The latent heat of the phase transition shows nonmonotic behaviour with an extremum at Lc = 8 nm, and levels off at Lc = 25 nm. We suggest an explanation based on the rotational entropy of very short chains, and the confinement of very long chains. First experiments with salt in the subphase show the two-dimensional lamellar phase for low molecular weight PSS and suggest a dependence of the electron density profile on the PSS contour length.