Regensburg 2010 – scientific programme

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CPP: Fachverband Chemische Physik und Polymerphysik

CPP 14: Poster: Interfaces and Thin Films

CPP 14.4: Poster

Monday, March 22, 2010, 16:30–18:00, Poster C

On the Thermodynamics of the 2-dimensional Lamellar Phase of Poly(styrene sulfonate) Adsorbed onto an Oppositely Charged Lipid Monolayer — •Thomas Ortmann, Heiko Ahrens, Jens-Uwe Günther, and Christiante A. Helm — Institut für Physik, Ernst-Moritz-Arndt Universität, Felix-Hausdorff-Str. 6, D-17489 Greifswald, Germany

Electrostatic and entropic interactions in monolayers of cationic lipids with adsorbed polystyrene sulfonate (PSS) are studied by isotherms, and grazing incidence X-ray diffraction. Dioctadecyldimethylammonium bromide (DODA) is used as lipid, the PSS concentration is 0.01 mM (with respect to the monomer concentration); the PSS chain length is varied. Bragg peaks attributed to 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 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 (contour length 5.5 nm) and decreases the longer the polymer gets, a limiting value is reached at 25 nm. The latent heat of the phase transition shows nonmonotic behaviour with an extremum at 8 nm contour length, and levels off at 25 nm. Presumably, for very short PSS chains and large chain separations, the rotational entropy of the electrostatically stretched chains hinders alignment. Even for aligned short chains, the transition enthalpy is affected strongly.