Re-entrant Appearance of Phases in a Relaxed Langmuir Monolayer of Tetracosanoic Acid as Determined by X-Ray Scattering

The structure of the fully relaxed phases of a Langmuir monolayer of tetracosanoic acid is determined by x‐ray diffraction and reflection along an isotherm at ∼20.5 °C. Isotherms taken by allowing the surface pressure to stabilize between incremental compressions are seen to be qualitatively different from the constant‐rate nonrelaxed isotherms typically seen in the literature. At low densities the monolayer consists of an inhomogeneous film of islands of a crystalline (or hexatic) phase with molecular tilt ordering that is analogous to that of the smectic I liquid crystal. Small amounts of impurities (∼0.5% of the monolayer) account for the change in surface pressure with area in this region. Upon compression to the point that the free space between islands becomes negligible the film appears homogeneous. On further compression the time required for full relaxation becomes long (i.e., ∼ hours), the tilt angle of the molecular axis decreases and the x‐ray unit cell is compressed. Including this homogeneous I phase the phase sequence observed by diffraction upon compression is I‐U‐I‐U, where U refers to an untilted orthorhombic phase. The outer two phases of this sequence are pure phases which form homogeneous monolayers, but the inner two are inhomogeneous phases each coexisting with an amorphous phase that does not have an observable diffraction signal. At the boundaries demarcating the I and U phases, a phase whose tilt ordering is analogous to that of a smectic F phase is seen to coexist. The preceding phase sequence is sensitive to the degree of relaxation permitted the monolayer after an incremental compression. In particular, if the monolayer is not allowed to relax completely after each compression, the untilted U phase may never appear. The U↔I transition is shown to be reversible for a relaxed monolayer.Engineering and Applied Science

A Determination of the Phase Diagram of Relaxed Langmuir Monolayers of Behenic Acid

Grazing incidence x‐ray scattering (GIXS) and Brewster angle microscopy (BAM) are used to determine the $\pi–T$ phase diagram of Behenic acid monolayers supported on the surface of water (pH=2.0) over the temperature range of 3$^\circ$C to 20.6$^\circ$C. The phase diagram is constructed from measurements taken during isothermal compressions in which the surface pressure relaxed to a stable value at each surface density, and during temperature scans at fixed average surface density. The phase diagram is different than those previously reported for Behenic acid primarily because of the surface pressure relaxation. For temperatures less than 12$^\circ$C the phase diagram exhibits similar phases and topology as the published diagrams, although the location of the phases in the $\pi–T$ plane is different. Temperature scans combined with the isotherms, and the Clausius–Clapeyron relation are used to determine three coexistence lines that meet in a triple point. Changes in entropy across the phase boundaries are determined. Near room temperature (20.6$^\circ$C) only one phase is measured over the range of surface pressure from 0 dynes/cm to the collapse pressure in contrast to reported measurements on monolayers out of equilibrium (i.e., when the surface pressure is not allowed to relax) that exhibit several phases near room temperature at pressures higher than the collapse pressure. Discrepancies are observed between the average area per molecule ($A_T$) and the area per molecule determined from measurements of the unit cell ($A_X$) in the close packed regions of the phase diagram. It is conjectured that the $\pi–A_X$ plane is a better representation of the ordered equilibrium monolayer phases than the $\pi–A_T$ plane. Isotherms plotted in the $\pi–A_X$ plane are used to determine the compressibility of the ordered phases and the nature of the phase transitions.Engineering and Applied Science

Monovalent Ion Condensation at the Electrified Liquid/Liquid Interface

X-ray reflectivity studies demonstrate the condensation of a monovalent ion at the electrified interface between electrolyte solutions of water and 1,2-dichloroethane. Predictions of the ion distributions by standard Poisson-Boltzmann (Gouy-Chapman) theory are inconsistent with these data at higher applied interfacial electric potentials. Calculations from a Poisson-Boltzmann equation that incorporates a non-monotonic ion-specific potential of mean force are in good agreement with the data.Comment: 4 pages, 4 figure

Vaporization and Layering of Alkanols at the Oil/Water Interface

This study of adsorption of normal alkanols at the oil/water interface with x-ray reflectivity and tensiometry demonstrates that the liquid to gas monolayer phase transition at the hexane/water interface is thermodynamically favorable only for long-chain alkanols. As the alkanol chain length is decreased, the change in excess interfacial entropy per area decreases to zero. Systems with small values of excess interfacial entropy form multi-molecular layers at the interface instead of the monolayer formed by systems with much larger excess interfacial entropy. Substitution of n-hexane by n-hexadecane significantly alters the interfacial structure for a given alkanol surfactant, but this substitution does not change fundamentally the phase transition behavior of the monolayers. These data show that the critical alkanol carbon number, at which the change in excess interfacial entropy per area decreases to zero, is approximately six carbons larger than the number of carbons in the alkane solvent molecules.Comment: 27 pages, 10 figures, to be published in J. Phys. Cond. Ma

The Medical Research Council Myeloma IX trial: the impact on treatment paradigms*

Osteolytic bone disease is a hallmark of symptomatic multiple myeloma. Bisphosphonates have been the mainstay of treatment to preserve skeletal integrity and prevent skeletal-related events in patients with myeloma-related bone disease. Recently, the MRC Myeloma IX trial demonstrated for the first time improved survival and delayed disease progression with the use of an intravenous amino-bisphosphonate, zoledronic acid, vs. an oral agent, clodronate, with intensive and non-intensive anti-myeloma treatment regimens in patients with newly diagnosed multiple myeloma. These results validate a large body of preclinical, translational and other clinical data suggesting anti-myeloma effects of amino-bisphosphonates. In addition, this trial also provided the first head-to-head evidence for superiority of one bisphosphonate over another (zoledronic acid vs. clodronate) for reducing skeletal morbidity in patients with multiple myeloma, as well as a prospective comparison of toxicities. Despite the use of non-bortezomib containing anti-myeloma treatment regimens in the MRC Myeloma IX trial, these results are encouraging and provide an impetus to continue to evaluate current treatment guidelines for myeloma-associated bone disease

Relaxation and the Reentrant Appearance of Phases in a Molecular Monolayer

The structure of monolayers of $CH_{3}(CH_{2})_{22}COOH$ on water, along $\sim$21$^\circ$C isotherms between 18 and 35 Å$^2$/molecule, has been studied with glancing-angle x-ray diffraction and specular reflection. Following an incremental area change, in some regions of the isotherm, the diffraction pattern evolves until the surface pressure $\pi$ is relaxing $\lesssim$ 0.01 dyn/cm h. Reentrant behavior between tilted and nontilted phases is observed. Phase coexistence is indicated by differences between the deposited area/molecule and the area of the x-ray-determined unit cell.Engineering and Applied Science