Heiko Ahrens

Hydrophilic/hydrophobic nanostripes in lipopolymer monolayers.

The self-organization of macromolecules with a complex architecture is studied extensively in the last years, with the aim to nanostructure surfaces. Linear diblock copolymers with mutual antagonistic groups are a simple, much investigated system, since the respective block lengths determine the size of the nanostructure[1]. Yet little is known about the equilibrium structure when the forces from the two interfaces, and the intraand intermolecular interactions are all of comparable magnitude. An active control of the domain morphology as well as the possibility to manipulate the structures in situ would obviously improve our understanding.

Structure of hydroxylated galactocerebrosides from myelin at the air-water interface

Hydroxy-galactocerebrosides (mixed chain length, constituent of myelin membranes) from bovine brain are investigated as monolayers at the air-water interface with isotherms, fluorescence microscopy, x-ray reflectivity and grazing incidence diffraction. With grazing incidence diffraction a monoclinic tilted chain lattice is found in the condensed phase. According to x-ray reflectivity, the longest chains protrude above the chain lattice and roughen the lipid/air interface. On compressing the chain lattice, the correlation length increases by approximately 65%; obviously, the sugar headgroups are flexible enough to allow for lattice deformation. With fluorescence experiments, small coexisting fluid and ordered domains are observed, and there is lipid dissolution into the subphase as well. The dissolved hydroxy-galactocerebroside molecules reenter on monolayer expansion. The electron density profiles derived from x-ray reflectometry (coherent superposition) show that the chain-ordering transition causes the molecules to grow into the subphase.

Two-Dimensional Lamellar Phase of Poly(styrene sulfonate) Adsorbed onto an Oppositely Charged Lipid Monolayer

Polystyrene sulfonate (PSS 77 kDa) adsorbed onto oppositely charged dioctadecyldimethylammonium bromide (DODA) monolayers at the air/water interface is investigated with X-ray reflectivity and grazing incidence diffraction. The alkyl tails of DODA in the condensed phase form an oblique lattice with large tilts and intermediate azimuth angle. On PSS adsorption, the alkyl tail structure is maintained; only the tilt angle changes. Bragg peaks caused by flatly adsorbed, aligned PSS chains are observed, when DODA is in the fluid and also when it is in the condensed phase. The two-dimensional lamellar phase is only found at intermediate PSS bulk concentrations (0.001-1 mmol/L). In this phase, the PSS coverage can be varied by a factor of 3, depending on DODA molecular area and polymer bulk concentration. Charge compensation in the lamellar phase is almost achieved at 1 mmol/L. At larger bulk concentrations, PSS adsorbs flatly yet without chain alignment. Presumably, a necessary condition for a two-dimensional lamellar phase is a pronounced electrostatic force which causes a large persistence length as well as repulsion between the aligned chains.

Lipid monolayers and adsorbed polyelectrolytes with different degrees of polymerization.

Polystyrene sulfonate (PSS) of different molecular weight M(w) is adsorbed to oppositely charged DODAB monolayers from dilute solutions (0.01 mmol/L). PSS adsorbs flatly in a lamellar manner, as is shown by X-ray reflectivity and grazing incidence diffraction (exception: PSS with M(w) below 7 kDa adsorbs flatly disordered to the liquid expanded phase). The surface coverage and the separation of the PSS chains are independent of PSS M(w). On monolayer compression, the surface charge density increases by a factor of 2, and the separation of the PSS chains decreases by the same factor. Isotherms show that on increase of PSS M(w) the transition pressure of the LE/LC (liquid expanded/liquid condensed) phase transition decreases. When the contour length exceeds the persistence length (21 nm), the transition pressure is low and constant. For low-M(w) PSS (<7 kDa) the LE/LC transition of the lipids and the disordered/ordered transition of adsorbed PSS occur simultaneously, leading to a maximum in the contour length dependence of the transition enthalpy. These findings show that lipid monolayers at the air/water interface are a suitable model substrate with adjustable surface charge density to study the equilibrium conformation of adsorbed polyelectrolytes as well as their interactions with a model membrane.

Characterizing the phases of Langmuir monolayers of cylindrical brushes with x-ray reflectivity and diffraction

Langmuir monolayers of polymacromonomers (cylindrical brushes) with poly(vinyl)pyridine side chains of different length, polyPV P20.8 and polyPV P46.7, are studied at the air?water interface by means of x-ray diffraction and reflectivity. The advantages of measurements on an aqueous subphase, which contains NaI, are demonstrated. This subphase does not affect the structure of an uncharged monolayer, although NaI is incorporated into the side chains, it provides an enhanced contrast for the x-ray investigations. A structural transition from aligned single molecules to a homogeneous monolayer is found, which is attributed to intra-?and intermolecular interactions.