Biomedical subjects

E Sloutskin

Publications and source records attributed to E Sloutskin.

11 recordsLinked to original sources

PubMed2006

The surface structure of ionic liquids: comparing simulations with x-ray measurements.

The surface-normal electron density profile of an ionic liquid, [bmim][PF6], derived from x-ray reflectivity measurements, is compared with two independent molecular-dynamics simulations. It is shown that a meaningful comparison requires a detailed accounting for both thermal and nonthermal surface roughening effects. The former is due to thermally excited capillary waves, and the latter is due to the molecular zero-point motion and form. These quantities influence very significantly, but differently, the simulated and measured density profiles. Stripping off these effects from both types of profiles yields the intrinsic structure factor of the surface. The simulated intrinsic structure factors are found to deviate considerably from the measured one. The introduction of additional ad hoc surface roughness to the simulated profiles greatly reduces the deviation, however, no physical origin for this effect can be identified. The method employed in this study should prove useful for simulation-experiment comparisons of other liquid surfaces, provided they obey capillary-wave theory, as do almost all liquid surfaces studied to date by x-ray reflectivity.

Journal Article↗

PubMed2006

Surface freezing in binary alkane-alcohol mixtures.

Surface freezing was detected and studied in mixtures of alcohol and alkane molecules, using surface tensiometry and surface-specific x-ray scattering methods. Considering that surface freezing in pure alkanes forms an ordered monolayer and in alcohols it forms an ordered bilayer, the length mismatch repulsion was minimized by varying the carbon number of the alkane component around 2n, where n is the carbon number of the alcohol molecule. A solutionlike behavior was found for all mixtures, where the ideal liquid mixture phase-separates upon freezing both in the bulk and the surface. The solid exhibits a herringbone crystalline phase below an alkane mole fraction phi(t) approximately 0.8 and a rotator phase above it. The surface frozen film below phi(t) is an alkane monolayer exhibiting a next-nearest neighbor molecular tilt of a composition-dependent magnitude. Above phi(t), no diffraction peaks were observed. This could be explained by the intrinsically shorter-range order of the rotator phase and a possible proliferation of defects.

Journal Article↗

PubMed2005

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.

Journal Article↗

PubMed2004

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.

Adsorption↗

PubMed2004

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.

Fatty Acids↗

PubMed2004

Surface and bulk interchange energy in binary mixtures of chain molecules.

The interchange (interaction) parameter, controlling the phase behaviour of a binary mixture, is determined for the bulk and the surface of binary mixtures of different types of chain molecules, using surface tensiometry and a mean-field theory. For all mixtures and concentrations studied an identical behaviour is observed at the surface, depending only on the square of the reduced chain length mismatch delta n/n, where delta n and dealta n are the difference in and average of the number of carbons of the two components.

Alcohols↗

PubMed2003

Surface freezing in binary mixtures of chain molecules. I. Alkane mixtures.

X-ray surface scattering and surface tension measurements are used to study surface freezing in molten mixtures of alkanes. These binary mixtures consist of protonated and deuterated alkanes, as well as of alkanes of different lengths. As for pure alkanes, a crystalline monolayer is formed at the surface a few degrees above the bulk freezing temperature. The structure of the monolayer has been determined on an angstrom scale. A simple theoretical approach is used to account for the thermodynamical observations at the surface and in the bulk. The model is based on a competition between entropic mixing and a repulsive interaction due to chain-length mismatch. The surface and bulk liquid phases are treated as ideal mixtures, while the solid phases are treated as regular mixtures. The theory is found to account well for all the mixtures studied, both hydrogenated-hydrogenated and hydrogenated-deuterated. The repulsive interaction and its dependence on the chain lengths of the components are determined from fits to the measured data.

Journal Article↗

PubMed2003

Surface freezing in binary mixtures of chain molecules. II. Dry and hydrated alcohol mixtures.

Surface freezing is studied in dry and hydrated alcohol mixtures by surface x-ray scattering and surface tension measurements. A crystalline bilayer is formed at the surface a few degrees above the bulk freezing temperature. The packing is hexagonal, with molecules aligned along the surface normal in all cases. The in-plane lattice constant reveals a qualitatively different behavior with composition for hydrated and dry mixtures. The simple theoretical approach used successfully for alkane and deuterated alkane mixtures accounts well also for the alcohol mixtures. The repulsive length-mismatch term opposing the mixing entropy term in the free energy of the mixtures is shown to have a universal behavior for all mixtures studied: protonated alkanes, deuterated alkanes, and dry and wet alcohols. This universality is somewhat counterintuitive in view of the different interactions (e.g., hydrogen bonding in alcohols) in the different mixtures.

Journal Article↗

PubMed2002

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.

Journal Article↗

PubMed2002

Demixing transition in a quasi-two-dimensional surface-frozen layer.

A thin/thick transition was observed by x-ray reflectivity in a surface-frozen crystalline bilayer on the surface of a molten binary mixture of long alcohols. This rare example of a solid-solid phase transition in a quasi-2D system is shown to result from an abrupt temperature-driven change in the layer's composition, kinetically enabled by the layer's ability to exchange molecules with the underlying 3D liquid bulk. Mean-field thermodynamics yields a Gibbs-adsorption-like expression which accounts very well for the transition.

Journal Article↗

PubMed2001

Surface freezing in n-alkane solutions: the relation to bulk phases.

Surface freezing (SF) was investigated in tricosane-dodecane alkane solutions as a function of temperature (T) and molar concentration of tricosane (phi), using surface tension and synchrotron x-ray surface diffraction techniques. A crystalline SF monolayer, having a rotator R(II) structure, was found to exist for 35 degrees C</=T</=50 degrees C and 0.3</=phi</=1. The extended temperature range allowed to determine the linear-expansion coefficient of the SF monolayer, (dd/dT)/d=6.5 x 10(-4) degrees C-1. A simple thermodynamical model based on the theory of ideal solutions is shown to account well for the phi dependence of the SF temperature T(s)(phi). The study shows that the temperature range of existence of the surface frozen layer at each phi, the phi range over which SF is observed, and the bulk solidification behavior, are intimately related. All are determined by the rotator-liquid dissolution line T(dR)(phi).

Journal Article↗