Abstract. Single and double phosphocholine (DPPC and DSPC) bilayers adsorbed at the silicon-water interface have been prepared and characterised. The second bilayer, called "free bilayer", is a novel highly hydrated system floating at 20 to 30Å above the first one. Robust and reproducible preparation has been possible thanks to a combination of Langmuir-Blodgett and Langmuir-Schaeffer techniques. Carefully optimised neutron reflectivity measurements have allowed a precise non-destructive characterisation of the structure, hydration and roughness of the layers. This work opens new possibilities for the investigation of the interaction between membrane lipids and soluble proteins, in particular peptides too small to be visible with other techniques.PACS. 68.55.Jk Structure and morphology; thickness -81.15.Lm Liquid phase epitaxy; deposition from liquid phases (melts, solutions, and surface layers on liquids) -61.12.Ha Neutron reflectometry

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The sodium salt of the di-chained anionic surfactant bis-2-ethylhexylsulfosuccinate [Aerosol-OT or Na(AOT)] stabilises essentially monodisperse, spherical water-in-oil microemulsion (w/o) droplets in alkanes over a wide range of pressure, temperature and composition. In order to investigate the effect of change in counterion charge and size on the microemulsion properties, we have replaced the Na+ counterion by doubly charged ions. The surfactant is then M2+(AOT)2 .nH,O: M is from the series Mg2+, Ca2+, Co2+, Ni2+, Cu2+ and Zn2+, and n is the number of water ligands associated with the surfactant molecule. The value of n was determined by FTlR and depends on M2+, but can be between 2 and 8 per molecule of M2+(AOT)2 depending on the nature of M. The ion replacement, assessed by UV-VIS spectrophotometry, is ca. 100% efficient. The effect of temperature on the phase stability of the single-phase M2+(AOT)2 water-in-oil (w/o) microemulsion systems is negligible, in contrast to that observed for the corresponding Na(A0T) system. The structure and properties of the microemulsion are found to be dependent on the counterion identity. Small-angle neutron scattering (SANS) and viscosity measurements provide evidence for the existence of rod-shaped aggregates for Co2+, Ni2+, Cu2+ and Zn2+ at low water constants given by w = [H,O]/[AOT] % 5, whilst for Mg2+ and Ca2+ spherical aggregates are present as for Na'.. On further addition of water at constant surfactant concentration (w > 10) with Co2+, Ni2+, Cu2+ and Zn2+ the aggregates undergo a shape change, and a more spherical structure is favoured. The results may be explained in terms of the interaction of the different counterions with the SOaead group of the surfactant.

Polyethylene glycol (PEG) brushes are used to reduce protein adsorption at surfaces. Their design needs to allow for two leading adsorption modes at the brush-coated surface. One is primary adsorption at the surface itself. The second is ternary adsorption within the brush as a result of weak PEG-protein attraction. We present a scaling theory of the equilibrium adsorption isotherms allowing for concurrent primary and ternary adsorption. The analysis concerns the weak adsorption limit when individual PEG chains do not bind proteins. It also addresses two issues of special relevance to brushes of short PEGs: the consequences of large proteins at the surface protruding out of a shallow brush and the possibility of marginal solvent conditions leading to mean-field behavior. The simple expressions for the adsorption isotherms are in semiquantitative agreement with experiments.

A single lipid molecular bilayer of 17 or 18 carbon chain phosphocholines, floating in water near a flat wall, is prepared in the bilayer gel phase and then heated to the fluid phase. Its structure (electron density profile) and height fluctuations are determined by using x-ray reflectivity and nonspecular scattering. By fitting the offspecular signal to that calculated for a two-dimensional membrane using a Helfrich Hamiltonian, we determine the three main physical quantities that govern the bilayer height fluctuations: The wall attraction potential is unexpectedly low; the surface tension, roughly independent on chain length and temperature, is moderate (Ϸ5 ؋ 10 ؊4 J⅐m ؊2 ) but large enough to dominate the intermediate range of the fluctuation spectrum; and the bending modulus abruptly decreases by an order-of-magnitude from 10 ؊18 J to 10 ؊19 J at the bilayer gel-to-fluid transition. L ipid bilayers (1, 2) have been often studied as models of two-dimensional soft systems (3). They are increasingly used as controlled idealized models of cell membranes for biophysical studies of membrane-membrane and membrane-protein interactions (1, 4). Lipid bilayers can be characterized by their static structure and dynamic, equilibrium thermal fluctuations. Structural measurements yield information on the variation of chemical composition (using neutrons) or electron density (using x-rays) along the z axis normal to the bilayer plane (for a review, see ref. 5). Thermal fluctuations of the bilayer plane are classically described (6) within the harmonic approximation originally proposed by Helfrich (3) with three physical quantities: (i) The intrinsic bilayer bending modulus stabilizes fluctuations with short in-plane (x, y) wavelengths; (ii) the surface tension ␥, if strong enough, dominates the intermediate scales; and (iii) an external potential per unit surface, U, due, for instance, to the attraction by a nearby surface or neighboring bilayers, stabilizes (through its second derivative UЉ ϭ d 2 U͞dz 2 ) large in-plane wavelength fluctuation modes. The cross-over between these regimes is usually [with few exceptions (7)] at submicronic scales and more accessible to x-ray off-specular surface scattering than to optical microscopy measurements.A considerable effort has been devoted to the measurement of the bending rigidity , which controls both the physical properties (bilayer fluctuations and vesicle shape) and biophysical properties (adhesion, invagination, and membrane-protein interactions) of the bilayers (2,4,8). Experiments have been based on indirect effects (9, 10) or on the direct determination (7) of the fluctuation spectrum, usually on vesicles (7, 11) or on multilayer stacks (5, 12). They have yielded results mostly in the fluid phase (7, 9, 11, 13-16) but also in the gel phase (17) and, more recently, as a function of temperature across the melting transition (10,18,19).We report an experimental determination of the structure and fluctuations of a lipid bilayer, in a planar configuration (in the vicinity of an...

A highly hydrated lipid bilayer, floating a few Å above another one adsorbed on a smooth solid substrate, was prepared at room temperature where lipids (DSPC) are in gel phase, then heated at several temperatures including the pretransitional temperature Tp and the chain melting temperature Tm. A precise in situ characterization by neutron reflectivity led to two new results. First, even when in fluid phase the floating bilayer was structured and stable; it thus provides a flexible model system for physical and biophysical studies of membranes. Second, at intermediate temperatures, a spectacular maximum in both inter-bilayer distance and bilayer roughness was simultaneously observed. It might be a direct observation of the balance between energy minimization and entropic repulsion, leading to an estimation of the dimensionless parameter (kBT ) 2 /Aκ, where A is the Hamacker constant and κ the bending modulus.

We have studied the collective short wavelength dynamics in deuterated 1,2-dimyristoyl-sn-glycero-3-phoshatidylcholine (DMPC) bilayers by inelastic neutron scattering. The corresponding dispersion relation variant Planck's over 2pi omega(Q) is presented for the gel and the fluid phase of this model system. The temperature dependence of the inelastic excitations indicates a phase coexistence between the two phases over a broad range and leads to a different assignment of excitations from that reported in a preceding inelastic x-ray scattering study [Phys. Rev. Lett. 86, 740 (2001)]]. As a consequence, we find that the minimum in the dispersion relation is actually deeper in the gel than in the fluid phase. Finally, we can clearly identify an additional nondispersive (optical) mode predicted by molecular dynamics simulations [Phys. Rev. Lett. 87, 238101 (2001)]].