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Solvent extraction of trivalent rare earth metal ions by organophosphorus extractants proceeds via binding of phosphoric acid headgroups to the metal ion. Water molecules in the tightly bound first hydration shell of the metal ions must be displaced by oxygen atoms from phosphoric acid headgroups. Here, we use classical molecular dynamics simulations to explore the event in which a fully hydrated Er3+ binds to its first phosphoric acid headgroup. Approach of the headgroup into the region between the first and second hydration shells leads to a fast ejection of a water molecule that is accompanied by reordering of the hydration water molecules, including discretization of their angular positions and collective rotation about the metal ion. The water molecule ejected from the first shell is located diametrically opposite from the binding oxygen. Headgroup binding places a headgroup oxygen closer to Er3+ than its first hydration shell and creates a loosely bound water that subsequently exchanges between the first shell and its environment. This second exchange of water also occurs at discrete angular positions. This geometrical aspect of binding may be of relevance to understanding the binding and transport of ion–extractant complexes that are expected to occur at the organic–aqueous liquid–liquid interface used in solvent extraction processes.

Synchrotron x-ray reflectivity is used to study the structure of a monolayer of F(CF2)10(CH2)2OH self-assembled at the liquid–liquid interface from a solution in hexane placed in contact with water. It is demonstrated that this monolayer is in a high density (solid) phase below a transition temperature. This is in contrast to the conventional expectation that soluble surfactants form disordered monolayers at the liquid–liquid interface. Above the transition temperature the monolayer desorbs into the hexane solution, leaving behind an interface with a very low density of surfactants. Hysteresis in the formation of the monolayer occurs when the temperature is scanned through the transition temperature. The success of these measurements relied upon the development of a novel technique to flatten the liquid–liquid interface to the extent required for x-ray reflectivity. The measurements of F(CF2)10(CH2)2OH at the liquid–liquid interface are compared to x-ray surface diffraction measurements of monolayers of t...

Grazing incidence x‐ray scattering (GIXS) and Brewster angle microscopy (BAM) are used to determine the π–T phase diagram of Behenic acid monolayers supported on the surface of water (pH=2.0) over the temperature range of 3 °C to 20.6 °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 °C the phase diagram exhibits similar phases and topology as the published diagrams, although the location of the phases in the π–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 tem...

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.

The interface between bulk water and bulk hexane solutions of n-alkanols (H(CH(2))(m)OH, where m=20, 22, 24, or 30) is studied with x-ray reflectivity, x-ray off-specular diffuse scattering, and interfacial tension measurements. The alkanols adsorb to the interface to form a monolayer. The highest density, lowest temperature monolayers contain alkanol molecules with progressive disordering of the chain from the -CH(2)OH to the -CH(3) group. In the terminal half of the chain that includes the -CH(3) group the chain density is similar to that observed in bulk liquid alkanes just above their freezing temperature. The density in the alkanol headgroup region is 10% greater than either bulk water or the ordered headgroup region found in alkanol monolayers at the water-vapor interface. We conjecture that this higher density is a result of water penetration into the headgroup region of the disordered monolayer. A ratio of 1:3 water to alkanol molecules is consistent with our data. We also place an upper limit of one hexane to five or six alkanol molecules mixed into the alkyl chain region of the monolayer. In contrast, H(CH(2))(30)OH at the water-vapor interface forms a close-packed, ordered phase of nearly rigid rods. Interfacial tension measurements as a function of temperature reveal a phase transition at the water-hexane interface with a significant change in interfacial excess entropy. This transition is between a low temperature interface that is nearly fully covered with alkanols to a higher temperature interface with a much lower density of alkanols. The transition for the shorter alkanols appears to be first order whereas the transition for the longer alkanols appears to be weakly first order or second order. The x-ray data are consistent with the presence of monolayer domains at the interface and determine the domain coverage (fraction of interface covered by alkanol domains) as a function of temperature. This temperature dependence is consistent with a theoretical model for a second order phase transition that accounts for the domain stabilization as a balance between line tension and long range dipole forces. Several aspects of our measurements indicate that the presence of domains represents the appearance of a spatially inhomogeneous phase rather than the coexistence of two homogeneous phases.

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 nonmonotonic ion-specific potential of mean force are in good agreement with the data.