Self-consistent interpretation of the 2D structure of the liquid Au82Si18 surface: bending rigidity and the Debye-Waller effect.

The structural and mechanical properties of 2D crystalline surface phases that form at the surface of liquid eutectic Au82Si18 are studied using synchrotron x-ray scattering over a large temperature range. In the vicinity of the eutectic temperature the surface consists of a 2D atomic bilayer crystalline phase that transforms into a 2D monolayer crystalline phase during heating. The latter phase eventually melts into a liquidlike surface on further heating. We demonstrate that the short wavelength capillary wave fluctuations are suppressed due to the bending rigidity of 2D crystalline phases. The corresponding reduction in the Debye-Waller factor allows for measured reflectivity to be explained in terms of an electron density profile that is consistent with the 2D surface crystals.

X-ray scattering from liquid surfaces: effect of resolution.

Quantitative interpretation of X-ray reflectivity measurements from liquid surfaces requires methodical accounting of the effects of diffuse scattering from thermal capillary roughness. In this paper we discuss how this requires careful attention to the shape of the experimental resolution. These considerations, which are essential for measurement of the intrinsic structure factor of a liquid surface, require knowledge of the liquid surface tension. The paper closes with a brief comment on the most sensitive method for extraction of the surface tension from measurement of the off-specular diffuse scattering.

Crystalline phases of alkyl-thiol monolayers on liquid mercury.

The structure of octadecanethiol monolyers on liquid Hg surfaces, measured with subangstrom resolution, evolves with increasing coverage from a laterally disordered phase of surface-parallel molecules to ordered rotator phases of surface-normal molecules. For the latter, an abrupt transition is found at 19 A(2)/molecule from a rectangular packing of molecules tilted by 27 degrees in the nearest-neighbor direction to a hexagonal unit cell of untilted molecules. The unit cell of the tilted phase is centered for the chains and noncentered for the headgroups. The thiol headgroups associate in pairs with a single Hg atom, and the bonds form long-range orientational order. The different order of thiols on Au(111) and on Hg highlights the subphase's role in determining the overlayer's structure.

Temperature dependence of the structure of Langmuir films of normal-alkanes on liquid mercury.

The temperature dependent phase behavior of Langmuir films of n-alkanes [CH3(CH2)(n-2)CH3, denote Cn] on mercury was studied for chain lengths 19< or =n< or =22 and temperatures 15< or =T< or =44 degrees C, using surface tensiometry and surface x-ray diffraction methods. In contrast with Langmuir films on water, where molecules invariably orient roughly surface normal, alkanes on mercury are always oriented surface parallel and show no long-range in-plane order at any surface pressure. A gas and several condensed phases of single, double, and triple layers of lying-down molecules are found, depending on n and T. At high coverages, the alkanes studied here show transitions from a triple to a double to a single layer with increasing temperature. The transition temperature from a double to a single layer is found to be approximately 5 degrees C, lower than the bulk rotator-to-liquid melting temperature, while the transition from a triple to a double layer is about as much below the double-to-single layer transition. Both monolayer and bulk transition temperatures show a linear increase with n with identical slopes of approximately 4.5 degrees C/CH2 within the range of n values addressed here. It is suggested that the film and bulk transitions are both driven by a common cause: the proliferation of gauche defects in the chain with increasing temperature.

Fatty acid Langmuir films on liquid mercury: X-ray and surface tension studies.

The structure and phase behavior of liquid-mercury-supported molecular films of fatty acids (CH3(CH2)n-2COOH, denoted CnOOH) were studied for molecular lengths 7 < or = n < or = 24, by surface tensiometry and X-ray methods. Two qualitatively different film structures were found, depending on coverage. For high coverage, the film consists of a monolayer of roughly surface-normal molecules, showing a pressure-dependent sequence of structures similar, though not identical, to that of the corresponding water-supported Langmuir films. At low coverage, phases consisting of surface-parallel molecules are found, not observed on the aqueous subphases employed to date. In this range, a two-dimensional (2D) gas followed by a single and, for 14 < or = n < or = 24, also by a double layer of surface-parallel molecules is found as coverage is increased. Depending on chain length, the flat-lying phases have a crystalline 2D-ordered, a smectic-like 1D-ordered, or a disordered in-plane structure consisting of molecular dimers. The structure and thermodynamics of the films are discussed.

The structure and phase diagram of Langmuir films of alcohols on mercury.

The coverage-dependent phase behavior of molecular films of alcohols (CH3(CH2)n-2CH2OH, denoted as CnOH) on mercury was studied for chain lengths 8 < or = n < or = 28, using surface tensiometry and surface specific X-ray methods. Phases with surface-normal-oriented molecules are found at high coverage, showing the CS, S, and LS phases found also on water. Phases comprising surface parallel molecules, which do not exist on water, are found here at low coverage. For the lowest coverage a two-dimensional gas phase is found, followed, upon increasing the coverage, by an n-dependent sequence of condensed phases of up to four layers of surface-parallel molecules before converting to the surface-normal phases. In contrast with the surface-normal phases, all of the surface-parallel phases are found to lack long-range order in the surface-parallel direction. Adsorption energies are derived from the phase diagram for the alkyl chain and the alcohol headgroup.

Structure of a Langmuir film on a liquid metal surface.

The structure of organic monolayers on liquid surfaces depends sensitively on the details of the molecular interactions. The structure of a stearic acid film on a mercury surface was measured as a function of coverage with angstrom resolution. Unlike monolayers on water, the molecules were found here to undergo a transition from surface-parallel to surface-normal orientation with increasing coverage. At high coverage, two condensed hexatic phases of standing-up molecules were found. At low coverage, a two-dimensional (2D) gas phase and condensed single- and double-layered phases of flat-lying molecular dimers were revealed, exhibiting a 1D longitudinal positional order. This system should provide a broader tunability range for nanostructure construction than solid-supported self-assembled monolayers.

Confinement-induced order of tethered alkyl chains at the water/vapor interface.

Packing of tethered alkyl chains in Langmuir monolayers of a hairy-rod polypeptide poly[gamma-4-(n-hexadecyloxy)benzyl alpha,L-glutamate] on water has been studied by x-ray scattering measurements at room temperature. The rods lie parallel to the surface while the alkyl side chains segregate toward the vapor. Results indicate that the herringbone order of the alkyl chains is established initially by one-dimensionally confined chains between aligned rods and grows laterally with compression.

Tetra point wetting at the free surface of liquid Ga-Bi.

A continuous surface wetting transition, pinned to a solid-liquid-liquid-vapor tetra coexistence point, is studied by x-ray reflectivity in liquid Ga-Bi binary alloys. The short-range surface potential is determined from the measured temperature evolution of the wetting film. The thermal fluctuations are shown to be insufficient to induce a noticeable breakdown of mean-field behavior, expected in short-range-interacting systems due to their d(u) = 3 upper critical dimensionality.

Pairing interactions and Gibbs adsorption at the liquid bi-in surface: a resonant X-ray reflectivity study.

Resonant x-ray reflectivity measurements from the surface of liquid Bi(22)In(78) find only a modest surface Bi enhancement, with 35 at. % Bi in the first atomic layer. This is in contrast to the Gibbs adsorption in all liquid alloys studied to date, which show surface segregation of a complete monolayer of the low surface tension component. This suggests that surface adsorption in Bi-In is dominated by attractive interactions that increase the number of Bi-In neighbors at the surface. These are the first measurements in which resonant x-ray scattering has been used to quantify compositional changes induced at a liquid alloy surface.

Hydration of single crystals of dipalmitoylphosphatidylcholine.

Optical birefringence and x-ray measurements are reported for single crystals (0.5 mm X 2 mm X 5 micrometers) of dipalmitoylphosphatidylcholine as a function of temperature and relative humidity. Large single crystals grown in CHCl3/acetone at a low water concentration undergo an irreversible phase transformation at 20 degrees C and 70% relative humidity or at 60 degrees C and 65% relative humidity. After this transformation occurs, the crystals can be made to undergo a reversible transformation into and out of the L alpha phase. The x-ray-determined mosaic spread in the hydrated crystal is less than approximately 0.04 degrees both above and below the main transition.

Alignment and defect structures in oriented phosphatidylcholine multilayers.

The alignment of dilauryl-, dimyristoyl-, and dipalmitoylphosphatidylcholine at various water concentrations into large oriented monodomain multilayers by annealing at elevated temperatures (Powers and Clark, 1975, Proc. Natl. Acad. Sci. U.S.A. 72:840; Powers and Pershan. 1977. Biophys. J. 20:137) is accompanied by the formation and subsequent dissolution of various defect structures. Some of these defects appear similar to those observed in thermotropic and other lyotropic liquid crystals, reflecting the lamellar structure of these materials. The formation and evolution of defects during the alignment of the lipids into the defect-free, monodomain, multilamellar geometry is studied using polarized microscopy. A combination of polarized and dark-field microscopy facilitated characterization of the defects; specific structural models are proposed. A new alignment technique involving compression and dilation of the lipid, which effects sample alignment at temperatures that are lower than those required by the Powers technique, is described. Lower temperature alignment avoids thermal decomposition that will sometimes occur if the lipid is maintained at elevated temperatures for prolonged periods. With this technique, samples (80 micrometer thick) of dilaurylphosphatidylcholine with 20% water by weight were aligned at room temperature.

Water and hermal diffusivity in a lipid-water smectic phase.

We report the first application of light scattering to measurement of the hydrodynamic relaxation of inhomogeneities in water concentration within a multilamellar, or smectic A, phospholipid water system (dipalmitoyl) phosphatidyl choline). Although the relaxation process in the multilamellar phase is different from the diffusion process in liquid phases, the relaxation rate can be described in terms of a diffusion coefficient. For diffusion parallel to the lamellae, diffusion coefficients ranging from 8 x 10(-7) to 2 x 10(-5) cm2/s were measured over a range of temperature and water concentrations. We describe a model that expresses the diffusion coefficient in terms of the chemical potential for water inside the multilamellar phase and the effective thickness of a "free water zone." The deduced thickness of this free water zone is in good agreement with estimates from X-ray diffraction results. The activation energy for the diffusion process is also deduced from the data, and is found to decrease monotonically with increasing water concentration. We also found the thermal diffusivity to be about 10(-3) cm2/s with only a weak temperature and water concentration dependence. The experimental technique is a new version of forced Rayleigh scattering. The method uses the phase information of the scattered light to improve the ability to detect weak signals. Experimental details are reported.

Head-group conformation in phospholipids: a phosphorus-31 nuclear magnetic resonance study of oriented monodomain dipalmitoylphosphatidylcholine bilayers.

Angular-dependent 31P NMR spectra of oriented biaxial monodomain DPPC.H2O multilayers are employed to study head-group conformation in this phospholipid. The results indicate that the O-P-O plane of the phosphate, where the O's are the nonesterified oxygens of the phosphodiester, is tilted at 47 +/- 5 degrees with respect to the bilayer normal. This PO4 orientation could result in the choline moiety being extended parallel to the bilayer plane, and it will explain the breadth of the axially symmetric 31P powder spectrum observed for DPPC in excess water. This work is the first direct observation of this conformation for lecithins and it illustrates the utility of high-resolution solid-state NMR in structural studies of disordered systems.

Brillouin light scattering measurement of the elastic properties of aligned multilamella lipid samples.

Brillouin measurements of the elastic properties are presented for aligned multilamella samples of both pure dipalmitoyl phosphatidylcholine at various temperatures and water concentrations and for the same compound containing 33 mol% cholesterol at various temperatures and two water concentrations. For pure dipalmitoyl phosphatidylcholine the elastic moduli change significantly at the gel transition and the modulus for area compression of individual bilayers is inferred to be an order of magnitude larger below the gel transformation than above. The presence of cholesterol is shown to influence the elastic behavior of dipalmitoyl phosphatidylcholine.

Monodomain samples of dipalmitoyl phosphatidylcholine with varying concentrations of water and other ingredients.

Methods are presented for the preparation of large monodomain phospholipid bilayer arrays containing variable amounts of water approaching the two-phase limit. The optical birefringence of these lamellar phases of dipalmitoyl phosphatidylcholine (DDPC) is measured over a range of temperature and water content, and phase transitions are observed. The techniques employed for pure DPPC and water are extended in order to produce macroscopically aligned samples containing varying concentrations of cholesterol, inorganic salts, antibiotics, and chlorophyll a. Polarization studies of the 670-nm band of chlorophyll a indicate macroscopic orientational order in the chromophore under the same conditions.