170 research outputs found
Microscopic measurement of the linear compressibilities of two-dimensional fatty acid mesophases
The linear compressibility of two-dimensional fatty acid mesophases has
determined by grazing incidence x-ray diffraction. Surface pressure vs
molecular area isotherms were reconstructed from these measurements, and the
linear compressibility (relative distortion along a given direction for
isotropic applied stress) was determined both in the sample plane and in a
plane normal to the aliphatic chain director (transverse plane). The linear
compressibilities range over two orders of magnitude from 0.1 to 10 m/N and are
distributed depending on their magnitude in 4 different sets which we are able
to associate with different molecular mechanisms. The largest compressibilities
(10m/N) are observed in the tilted phases. They are apparently independent of
the chain length and could be related to the reorganization of the headgroup
hydrogen-bounded network, whose role should be revalued. Intermediate
compressibilities are observed in phases with quasi long-range order
(directions normal to the molecular tilt in L_2 or L_2' phases, S phase), and
could be related to the ordering of these phases. The lowest compressibilities
are observed in the solid untilted CS phase and for 1 direction of the S and
L_2'' phases. They are similar to the compressibility of crystalline polymers
and correspond to the interactions between methyl groups in the crystal.
Finally, negative compressibilities are observed in the transverse plane for
L_2' and L_2'' phases and can be traced to subtle reorganizations upon
untilting.Comment: 24 pages, 17 figure
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Relative Rates of Metal-Free Azide-Alkyne Cycloadditions: Tunability over 3 Orders of Magnitude.
The thermal (3 + 2) dipolar azide-alkyne cycloaddition, proceeding without copper or strained alkynes, is an underutilized ligation with potential applications in materials, bioorganic, and synthetic chemistry. Herein, we investigate the effects of alkyne substitution on the rate of this reaction, both experimentally and computationally. Electron-withdrawing groups accelerate the reaction, providing a range of relative rates from 1.0 to 2100 between the slowest and fastest alkynes studied. Unexpectedly, aryl groups conjugated to the alkyne significantly retard the reaction rate. In contrast, a sulfonyl, ester-substituted alkyne is reactive enough that it couples with an azide at room temperature in a few hours. This reactivity scale should provide a guide to those who wish to use this ligation under mild conditions
Binary separation in very thin nematic films: thickness and phase coexistence
The behavior as a function of temperature of very thin films (10 to 200 nm)
of pentylcyanobiphenyl (5CB) on silicon substrates is reported. In the vicinity
of the nematic/isotropic transition we observe a coexistence of two regions of
different thicknesses: thick regions are in the nematic state while thin ones
are in the isotropic state. Moreover, the transition temperature is shifted
downward following a 1/h^2 law (h is the film thickness). Microscope
observations and small angle X-ray scattering allowed us to draw a phase
diagram which is explained in terms of a binary first order phase transition
where thickness plays the role of an order parameter.Comment: 5 pages, 3 figures, submitted to PRL on the 26th of Apri
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Surface Roughness of Water Measured by X-Ray Reflectivity
The roughness of the liquid-vapor interface for pure water was measured by a technique of x-ray reflectivity. With synchrotron radiation , the angular dependence of the x-ray reflectivity was measured from grazing incidence , where the reflectivity was greater than 0.96, to an incident angle of , where the reflectivity was . A fit to the data by a theory with only one adjustable parameter obtains for the root-mean-square roughness of the water surface.Engineering and Applied Science
Surface Structure of Liquid Metals and the Effect of Capillary Waves: X-ray Studies on Liquid Indium
We report x-ray reflectivity (XR) and small angle off-specular diffuse
scattering (DS) measurements from the surface of liquid Indium close to its
melting point of C. From the XR measurements we extract the surface
structure factor convolved with fluctuations in the height of the liquid
surface. We present a model to describe DS that takes into account the surface
structure factor, thermally excited capillary waves and the experimental
resolution. The experimentally determined DS follows this model with no
adjustable parameters, allowing the surface structure factor to be deconvolved
from the thermally excited height fluctuations. The resulting local electron
density profile displays exponentially decaying surface induced layering
similar to that previously reported for Ga and Hg. We compare the details of
the local electron density profiles of liquid In, which is a nearly free
electron metal, and liquid Ga, which is considerably more covalent and shows
directional bonding in the melt. The oscillatory density profiles have
comparable amplitudes in both metals, but surface layering decays over a length
scale of \AA for In and \AA for Ga. Upon controlled
exposure to oxygen, no oxide monolayer is formed on the liquid In surface,
unlike the passivating film formed on liquid Gallium.Comment: 9 pages, 5 figures; submitted to Phys. Rev.
Surface layering of liquids: The role of surface tension
Recent measurements show that the free surfaces of liquid metals and alloys
are always layered, regardless of composition and surface tension; a result
supported by three decades of simulations and theory. Recent theoretical work
claims, however, that at low enough temperatures the free surfaces of all
liquids should become layered, unless preempted by bulk freezing. Using x-ray
reflectivity and diffuse scattering measurements we show that there is no
observable surface-induced layering in water at T=298 K, thus highlighting a
fundamental difference between dielectric and metallic liquids. The
implications of this result for the question in the title are discussed.Comment: 5 pages, 4 figures, to appear in Phys. Rev. B. 69 (2004
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