Mark L. Schlossman

LIQUID-LIQUID INTERFACES: STUDIED BY X-RAY AND NEUTRON SCATTERING

Abstract Major recent advances include the development of new experimental techniques that enabled the first precise measurements of interfacial widths at water–oil interfaces and of the ordering of surfactants adsorbed to these interfaces, studies of phase transitions and domain formation in surfactant monolayers, and studies of interfacial fluctuations confined by and coupled across thin liquid films.

Liquid surfaces and interfaces : synchrotron X-ray methods

1. Introduction 2. Instrumentation 3. Theory of x-ray scattering from liquid surfaces 4. Experiments on liquid surfaces and interfaces.

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 homogeneou...

A synchrotron x-ray liquid surface spectrometer

The design of a synchrotron x-ray liquid surface spectrometer at beamline X19C at the National Synchrotron Light Source is described. This spectrometer is capable of performing the full range of x-ray surface scattering techniques. A few examples of measurements made using this spectrometer are presented, including studies of organic monolayers on the surface of water and of the structure of strongly fluctuating oil–microemulsion interfaces. The measurements discussed illustrate the accuracy, resolution, and capabilities of the spectrometer.

X-ray scattering from monolayers of F(CF2)10(CH2)2OH at the water–(hexane solution) and water–vapor interfaces

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...

Tuning ion correlations at an electrified soft interface

Ion distributions play a central role in various settings—from biology, where they mediate the electrostatic interactions between charged biomolecules in solution, to energy storage devices, where they influence the charging properties of supercapacitors. These distributions are determined by interactions dictated by the chemical properties of the ions and their environment as well as the long-range nature of the electrostatic force. Recent theoretical and computational studies have explored the role of correlations between ions, which have been suggested to underlie a number of counterintuitive results, such as like-charge attraction. However, the interdependency between ion correlations and other interactions that ions experience in solution complicates the connection between physical models of ion correlations and the experimental investigation of ion distributions. We exploit the properties of the liquid/liquid interface to vary the coupling strength of ion–ion correlations from weak to strong while monitoring their influence on ion distributions at the nanometer scale with X-ray reflectivity and the macroscopic scale with interfacial tension measurements. These data are in agreement with the predictions of a parameter-free density functional theory that includes ion–ion correlations and ion–solvent interactions over the entire range of experimentally tunable correlation coupling strengths (from 0.8 to 3.7). This study provides evidence for a sharply defined electrical double layer for large coupling strengths in contrast to the diffuse distributions predicted by mean field theory, thereby confirming a common prediction of many ion correlation models. The reported findings represent a significant advance in elucidating the nature and role of ion correlations in charged soft matter.

The liquid surface/interface spectrometer at ChemMatCARS synchrotron facility at the Advanced Photon Source

We discuss results from the first experiments on a new liquid surface/interface X-ray spectrometer recently commissioned by ChemMatCARS at Sector 15 of the Advanced Photon Source. These experiments include studies of liquid/liquid interfaces, monolayers supported on the water surface, and liquid metal surfaces.

MOLECULAR ORDERING AND PHASE TRANSITIONS IN ALKANOL MONOLAYERS AT THE WATER-HEXANE INTERFACE

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 studies of polymer/gold nanocomposites

We demonstrate that x-ray reflection standing wave fluorescence spectroscopy is a highly sensitive technique for the determination of the distribution of metal nanoparticles inside polymer thin films. We investigate both the depth profile and the in-plane spatial correlation of gold nanoparticles in an asymmetric polystyrene-b-poly(2-vinylpyridine) block copolymer film. These copolymer films self-assemble into alternating, nanometer-sized domains that are, upon annealing, selectively decorated by thermally evaporated metal nanoparticles to form metal/polymer composites.

Interfacial localization and voltage-tunable arrays of charged nanoparticles.

Experiments and computer simulations provide a new perspective that strong correlations of counterions with charged nanoparticles can influence the localization of nanoparticles at liquid-liquid interfaces and support the formation of voltage-tunable nanoparticle arrays. We show that ion condensation onto charged nanoparticles facilitates their transport from the aqueous-side of an interface between two immiscible electrolyte solutions to the organic-side, but contiguous to the interface. Counterion condensation onto the highly charged nanoparticles overcomes the electrostatic barrier presented by the low permittivity organic material, thus providing a mechanism to transport charged nanoparticles into organic phases with implications for the distribution of nanoparticles throughout the environment and within living organisms. After transport, the nanoparticles assemble into a two-dimensional (2D) nearly close-packed array on the organic side of the interface. Voltage-tunable counterion-mediated interactions between the nanoparticles are used to control the lattice spacing of the 2D array. Tunable nanoparticle arrays self-assembled at liquid interfaces are applicable to the development of electro-variable optical devices and active elements that control the physical and chemical properties of liquid interfaces on the nanoscale.