X-ray and neutron reflectivity from surface monolayers of a lipopolymer A. Wurlitzer, E. Politsch, 1 P. Krüger, M. Weygand, K. Kjaer, 2 G. Cevc 1 and M. Lösche Institute of Experimental Physics I, University of Leipzig, Linnéstr. 5, 04103 Leipzig, Germany 1 Institute of Medical Biophysics, Technical University Munich, Ismaninger Str. 22, 81675 Munich, Germany 2 Condensed Matter Physics and Chemistry Department, Risø National Laboratory, 4000 Roskilde, Denmark A lipopolymer with a polyoxazoline headgroup (1,2-dioctadecanyl- sn-glycero-3-poly(2-methyl-2-oxazo- line), PMO), designed for application as a pharmaceutical drug carrier, has been characterized in monolayers at the air-water interface. Using x-ray and neutron reflectometry, the molecular conformations of the lipo- polymer at the liquid surface were investigated as a function of lateral area per molecule, particularly with respect to the plateau observed in the isotherm. Conceptually, such a plateau may be caused by a first order transition either within the polymer (mushroom-to-brush) [1,2] or within the lipid hydrocarbon chains [3]. In order to warrant compatibility of the data sets for a simultaneous evaluation we have studied the same lipo- polymer – patterned by isotopic (H/D) substitution on the polyoxazoline moiety – both in neutron and x-ray measurements. Reflectivity data were obtained below and above the phase transition (at lateral pressures π = 17.5 mN/m and 30.0 mN/m) at T = 15°C on H 2 O and D 2 O for neutrons and on H 2 O for x-rays. For evalua- tion, a novel approach to data inversion was applied that utilizes an Evolution Strategy (ES) based algorithm [4] acting on ensemble conformations of the polymers. This procedure provides a realistic impression of the (averaged) organization of the molecules at the interface. From the optimized ensemble configurations, the resulting volume density ( Φ) distributions have been deduced (Fig. 1). We observe that the hydrophilic poly- mers are similarly organized on both sides of the phase transition, and even at high π, the molecules are not completely elongated. The alkyl chains, by contrast, change their average organization at the interface quali- tatively: The model suggests they are confined to the hydrophilic/hydrophobic interface at the lower pres- sure, whereas they may protrude more deeply into the subphase above the phase transition, similar to the situation observed with PEG lipopolymers [5]. From these result we conclude, that the plateau in the investigated pressure range should not be assigned to a mushroom-to-brush transition of the hydrophilic polymer but must be attributed to a transition of the hydrocarbon chains. This work was supported by the DFG (SFB 294, TP C9; SFB 266, TP C8) and the EU-TMR programmes at Risø and HASYLAB. We acknowledge beamtime at the BW1 liquid surface diffractometer of HASYLAB. -50 0 50 100 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 (a) π = 17.5 mN/m lipopolymer volume density, Φ distance from air/water interface, z [Å] -50 0 50 100 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 (b) π = 30.0 mN/m distance from air/water interface, z [Å] lipopolymer volume density, Φ Figure 1: Volume density distribution, Φ = Φ(z), of the lipopolymer across the interface at two lateral pres- sures, (a) π = 17.5 mN/m and (b) π = 30.0 mN/m. Shown are partial volumes of the hydrophilic headgroups (dash-dotted lines) and of the alkyl chains (dotted lines) as well as total molecular volumes of the lipopoly- mer (continuous lines). z > 0 indicates the aqueous subphase, z < 0 corresponds to the air half-space. References [1] S. Alexander, J. Phys. 38, 983 (1977). [2] P.G. de Gennes, Adv. Colloid Interf. Sci. 27, 189 (1987). [3] T. R. Baekmark et al., Langmuir 13, 5521 (1997). [4] H.-P. Schwefel, Evolution and Optimum Seeking, J. Wiley & Sons, New York (1995). [5] T. L. Kuhl et al., J. Am. Chem. Soc. 121, 7682 (1999). View publication stats View publication stats