X-ray and Neutron Reflectometry of Thin Films at Liquid Interfaces

Illuminating the nanostructure of diffuse interfaces: Recent advances and future directions in reflectometry techniques

Diffuse soft matter interfaces take many forms, from end-tethered polymer brushes or adsorbed surfactants to self-assembled layers of lipids. These interfaces play crucial roles across a multitude of fields, including materials science, biophysics, and nanotechnology. Understanding the nanostructure and properties of these interfaces is fundamental for optimising their performance and designing novel functional materials. In recent years, reflectometry techniques, in particular neutron reflectometry, have emerged as powerful tools for elucidating the intricate nanostructure of soft matter interfaces with remarkable precision and depth. This review provides an overview of selected recent developments in reflectometry and their applications for illuminating the nanostructure of diffuse interfaces. We explore various principles and methods of neutron and X-ray reflectometry, as well as ellipsometry, and discuss advances in their experimental setups and data analysis approaches. Improvements to experimental neutron reflectometry methods have enabled greater time resolution in kinetic measurements and elucidation of diffuse structure under shear or confinement, while innovation in analysis protocols has significantly reduced data processing times, facilitated co-refinement of reflectometry data from multiple instruments and provided greater-than-ever confidence in proposed structural models. Furthermore, we highlight some significant research findings enabled by these techniques, revealing the organisation, dynamics, and interfacial phenomena at the nanoscale. We also discuss future directions and potential advancements in reflectometry techniques. By shedding light on the nanostructure of diffuse interfaces, reflectometry techniques enable the rational design and tailoring of interfaces with enhanced properties and functionalities.

The Role of Sum-Frequency Generation Spectroscopy in Understanding On-Surface Reactions and Dynamics in Atmospheric Model-Systems

Surfaces, both water/air and solid/water, play an important role in mediating a multitude of processes central to atmospheric chemistry, particularly in the aerosol phase. However, the study of both static and dynamic properties of surfaces is highly challenging from an experimental standpoint, leading to a lack of molecular level information about the processes that take place at these systems and how they differ from bulk. One of the few techniques that has been able to capture ultrafast surface phenomena is time-resolved sum-frequency generation (SFG) spectroscopy. Since it is both surface-specific and chemically sensitive, the extension of this spectroscopic technique to the time domain makes it possible to study dynamic processes on the femtosecond time scale. In this Perspective, we will explore recent advances made in the field both in terms of studying energy dissipation as well as chemical reactions and the role the surface geometry plays in these processes.

Structure of graphene oxide-phospholipid monolayers: A grazing incidence X-ray diffraction and neutron and X-ray reflectivity study

HYPOTHESIS

Graphene oxide-based nanotechnology has aroused a great interest due to its applications in the biomedical and optoelectronic fields. The wide use of these materials makes it necessary to study its potential toxicity associated with the inhalation of Graphene Oxide (GO) nanoparticles and its interaction with the lung surfactant. Langmuir monolayers have proven to be an excellent tool for studying the properties of the lung surfactant and the effect of intercalation of nanoparticles on its structure and properties. Therefore, to know the origin of the phospholipids/GO interaction and the structure of the lipid layer with GO, in this work we study the effect of the insertion of GO sheets on a Langmuir film of 1,2-Dipalmitoyl-sn-glycerol-3-phosphocholine (DPPC).

EXPERIMENTS

Surface pressure-area isotherms, Neutron (NR) and X-ray Reflectivity (XRR) and Grazing Incidence X-ray Diffraction (GIXD) measurements of hydrogenated and deuterated DPPC monolayers with and without GO have been carried out.

FINDINGS

The results outline a strong interaction between the GO and the zwitterionic form of DPPC and prove that GO is in three regions of the DPPC monolayer, the aliphatic chains of DPPC, the head groups and water in the subphase. Comparison between results obtained with hydrogenated and deuterated DPPC allows concluding that both, electrostatic attractions, and dispersion forces are responsible of the interaction GO/DPPC. Results also demonstrated that the insertion of GO into the DPPC aliphatic chains does not induce significant changes on unit cell of DPPC.

An electrochemical perspective on the interfacial width between two immiscible liquid phases

Molecular dynamics simulations and vibrational sum-frequency spectroscopy are historically the main techniques applied to the description of the molecular structure and dynamics of the immiscible liquid/liquid interface. A molecular sharpness is estimated for oil/water interfaces, with an interfacial width that extends from hundreds of Å to 1 nm. However, electrochemical studies have elucidated a deeper liquid/liquid interface on the order of several micrometers. The breaking down of single-entity electrochemistry to simpler systems and the combination of high-resolution microscopies is confirming a larger extension of the interface. What can be the role of the electrochemist in clarifying this fundamental question? We try to give a suggestion at the end of a brief historical overview of the liquid/liquid interface studies.

X-ray Reflectivity Study of Polylysine Adsorption on the Surface of DMPS Monolayers

The results of a systematic study on the adsorption of polylysine molecules of different lengths on the surface of a 1,2-dimyristoyl-sn-glycero-3-phospho-L-serine (DMPS) monolayer in the liquid (LE) and condensed (LC) states are presented. A compressibility diagram and the Volta potential were recorded with the Langmuir monolayer technique and further analyzed with the empirical approach. The structure of the monolayer films with adsorbed polypeptides was studied with synchrotron X-ray reflectometry. Two- and three-layer slab models describe the reflectivity data fairly well and reveal both the significant structural changes and the dehydration of the polar groups induced by all polylysines used at the maximal coverage of the monolayer interface in both the LE and LC states. On the one hand, in the LE phase of the monolayer (area per molecule A ≅ 70 Ǻ²), the integrated electron density of the lipid headgroup region is approximately half the density contained in the clean monolayer. This indicates both significant compaction and dehydration in the polar groups of the lipids, caused by the adsorption of polypeptides. On the other hand, in the LC state (A ≅ 40 Ǻ²), the degree of the hydration of the polar region is similar to that for the initial DMPS monolayer. However, both the electron density and the thickness of the head group region differ significantly from the values of these parameters for the clean monolayer in the LC state.

Zwitterionic Poly(carboxybetaine) Brush/Albumin Conjugate Films: Structure and Lubricity

Artificial cartilages build up a highly lubricious system with the harmony of biomacromolecules and water. Bioconjugate thin films composed of a zwitterionic poly(carboxybetaine methacrylate) (PCB) brush platform and bovine serum albumin (BSA) were designed. BSA conjugation to the PCB brush chains was achieved by carbodiimide chemistry to give PCB brush/BSA conjugate films. The PCB brush/BSA conjugate films exhibited adaptable interfacial properties due to the amphiphilic nature of BSA. Neutron reflectivity showed that BSAs were localized at the liquid side of the conjugate films in PBS and the BSA conjugation slightly reduced the water content of the top layer, while the swollen state of the carpet PCB brush layer remained unchanged. The PCB brush/BSA conjugate films showed improved lubricity in the boundary lubrication mode but slightly worse fluid lubrication induction properties. This conjugate film could be a model system for the investigation of zwitterion/protein composite interfaces and is worth developing biomaterials that require lubrication in vivo.

Interaction of Surface-Modified Alumina Nanoparticles and Surfactants at an Oil/Water Interface: A Neutron Reflectometry, Scattering, and Enhanced Oil Recovery Study

The evaluation of the mechanism of nanoparticle (NP)/surfactant complex adsorption at the critical oil/water interface was studied. A sophisticated technique (neutron reflectometry) was used to give a unique insight on NP/oil interactions in oil recovery systems. Herein, the adsorption of two modified alumina NPs with different degrees of hydrophobicity [hydrophilic = 2-[2-(2-methoxyethoxy)ethoxy]acetic acid and hydrophobic = octanoic acid (OCT)] stabilized with two different surfactants were studied at the oil/water interface. A thin layer of deuterated (D) and hydrogenated (H) hexadecane (contrast matching silicon substrate) oil was formed on a silicon block by a spin coating freeze process. The distribution of the NPs across the oil/water interface with the CTAB surfactant is similar between the two systems. NPs coated with CTAB have more affinity toward the oil/water interface, which explains the oil recovery increase by around 5% when flooding the core with the OCT-NP/CTAB system compared to the surfactant flooding alone. These results suggest that the NP/surfactant complexes can have potential usage in EOR recovery applications.

Cononsolvency of Poly[2-(methacryloyloxy)ethyl phosphorylcholine] in Ethanol-Water Mixtures: A Neutron Reflectivity Study

Molecular mechanisms underlying the cononsolvency, a re-entrant coil-to-globule-to-coil conformational transition of polymer chains in mixtures of two good solvents, of poly[2-(methacryloyloxy)ethyl phosphorylcholine] (PMPC) in ethanol-water binary mixtures were complementarily investigated. This was accomplished by following a statistical mechanical model for competitive hydrogen bonding combined with the cooperative solvation concept as well as neutron reflectivity (NR) experiments employing contrast variation in the cononsolvents. The experimental re-entrant aggregation of the PMPC chains in ethanol-water mixed solvents, obtained on the basis of turbidity was accurately reproduced by theoretical calculations. The calculation proved the relatively strong cooperativity of ethanol and the preferential interaction of water, while the total coverage of solvents was the lowest at an ethanol volume fraction (f_ethanol) of 0.90. At this level, the cononsolvency was the most significant, and the collapsed PMPC chains were solvated with more water than the bulk mixed solvent. The ethanol-water cononsolvency for the PMPC brushes on a planar silicon wafer was investigated by NR experiments, and the solvent composition involved in the collapsed PMPC brush was addressed according to the contrast variation study with mixed solvents of water, deuterium oxide, ethanol-d₅, and ethanol-d₆. The collapsed PMPC brushes at f_ethanol = 0.90 contained more water than the bulk solvent. The preferential distribution of water in the collapsed PMPC brush was consistent with the simulation results. Therefore, the molecular mechanism for the cononsolvency of PMPC in ethanol-water mixed solvents based on competitive hydrogen bonding coupled with cooperative solvation was experimentally rationalized.

Lipopolysaccharides at Solid and Liquid Interfaces: Models for Biophysical Studies of the Gram-negative Bacterial Outer Membrane

Lipopolysaccharides (LPSs) are a constitutive element of the cell envelope of Gram-negative bacteria, representing the main lipid in the external leaflet of their outer membrane (OM) lipid bilayer. These unique surface-exposed glycolipids play a central role in the interactions of Gram-negative organisms with their surrounding environment and represent a key element for protection against antimicrobials and the development of antibiotic resistance. The biophysical investigation of a wide range of different types of in vitro model membranes containing reconstituted LPS has revealed functional and structural properties of these peculiar membrane lipids, providing molecular-level details of their interaction with antimicrobial compounds. LPS assemblies reconstituted at interfaces represent a versatile tool to study the properties of the Gram-negative OM by exploiting several surface-sensitive techniques, in particular X-ray and neutron scattering, which can probe the structure of thin films with sub-nanometer resolution. This review provides an overview of different approaches employed to investigate structural and biophysical properties of LPS, focusing on studies on Langmuir monolayers of LPS at the air/liquid interface and a range of supported LPS-containing model membranes reconstituted at solid/liquid interfaces.

Changes to lung surfactant monolayers upon exposure to gas phase ozone observed using X-ray and neutron reflectivity

Lung surfactant at the air water interface reacts with ozone leading to changes in surface tension, film thickness, structure.

In situ determination of the structure and composition of Langmuir monolayers at the air/water interface by neutron and X-ray reflectivity and ellipsometry

This review focuses on the description of the structure and composition of a variety of Langmuir monolayers (LMs) deposited at the air/water interface by using ellipsometry, Brewster Angle microscopy and scattering techniques, mainly neutron and X-ray reflectometry. Since the first experiment done by Angels Pockels with a homemade trough in her home kitchen until today, LMs of different materials have been extensively studied providing not only relevant model systems in biology, physics and chemistry but also precursors of novel materials via their deposition on solid substrates. There is a vast amount of surface-active materials that can form LMs and, therefore, far from a revision of the state-of-the-art, we will emphasize here: (i) some fundamental aspects to understand the physics behind the molecular deposition at the air/water interface; (ii) the advantages in using in situ techniques, such as reflectometry or ellipsometry, to resolve the interfacial architecture and conformation of molecular films; and, finally, (iii) a summary of several systems that have certain interest from the experimental or conceptual point of view. Concretely, we will report here advances in polymers confined to interfaces and surfactants, from fatty acids and phospholipids monolayers to more unconventional ones such as graphene oxide.

Structure and Hydration of Asymmetric Polyelectrolyte Multilayers as Studied by Neutron Reflectometry: Connecting Multilayer Structure to Superior Membrane Performance

Porous membranes coated with so-called asymmetric polyelectrolyte multilayers (PEMs) have recently been shown to outperform commercial membranes for micropollutant removal. They consist of open support layers of poly(styrene sulfonate) (PSS)/poly(allylamine) (PAH) capped by denser and more selective layers of either PAH/poly(acrylic acid) (PAA) or PAH/Nafion. Unfortunately, the structure of these asymmetric PEMs, and thus their superior membrane performance, is poorly understood. In this work, neutron reflectometry (NR) is employed to elucidate the multilayered structure and hydration of these asymmetric PEMs. NR reveals that the multilayers are indeed asymmetric in structure, with distinct bottom and top multilayers when air-dried and when solvated. The low hydration of the top [PAH/Nafion] multilayer, together with the low water permeance of comparable [PAH/Nafion]-capped PEM membranes, demonstrate that it is a reduction in hydration that makes these separation layers denser and more selective. In contrast, the [PAH/PAA] capping multilayers are more hydrated than the support [PSS/PAH] layers, signifying that, here, densification of the separation layer occurs through a decrease in the mesh size (or effective pore size) of the top layer due to the higher charge density of the PAH/PAA couple compared to the PSS/PAH couple. The [PAH/PAA] and [PAH/Nafion] separation layers are extremely thin (∼4.5 and ∼7 nm, respectively), confirming that these asymmetric PEM membranes have some of the thinnest separation layers ever achieved.

Hydration State Variation of Polyzwitterion Brushes through Interplay with Ions

Polyzwitterions have emerged as a new class of antifouling materials alternating poly(ethylene glycol). The exemplary biopassivation and lubrication behaviors are often attributed to the particular chemical structure of zwitterions, which involve a large dipole moment of the charged groups and a neutral net charge, while the hydration state and dynamics also associate with these characteristics. Polymer brushes composed of surface-tethered polyzwitterion chains produced by surface-initiated controlled radical polymerization have been developed as thin films which exhibit excellent antifouling and lubrication properties. In past decades, numerous studies have been devoted to examining the structure and dynamics of polyzwitterion brush chains in aqueous solutions. This feature article provides an overview of recent studies exploring the hydration state of polyzwitterion brushes with specular neutron reflectivity, highlights some newly published work on the nonuniform equilibrium structure, ion concentration dependence, ion specificity, and the effects of charge spacer length in the zwitterions, and discusses future perspective in this field.

Recent Progresses in Nanometer Scale Analysis of Buried Layers and Interfaces in Thin Films by X-rays and Neutrons

In the early 1960s, scientists achieved the breakthroughs in the fields of solid surfaces and artificial layered structures. The advancement of surface science has been supported by the advent of ultra-high vacuum technologies, newly discovered and established scanning probe microscopy with atomic resolution, as well as some other advanced surface-sensitive spectroscopy and microscopy. On the other hand, it has been well recognized that a number of functions are related to the structures of the interfaces, which are the thin planes connecting different materials, most likely by layering thin films. Despite the scientific significance, so far, research on such buried layers and interfaces has been limited, because the probing depth of almost all existing sophisticated analytical methods is limited to the top surface. The present article describes the recent progress in the nanometer scale analysis of buried layers and interfaces, particularly by using X-rays and neutrons. The methods are essentially promising to non-destructively probe such buried structures in thin films. The latest scientific research has been reviewed, and includes applications to bio-chemical, organic, electronic, magnetic, spintronic, self-organizing and complicated systems as well as buried liquid-liquid and solid-liquid interfaces. Some emerging analytical techniques and instruments, which provide new attractive features such as imaging and real time analysis, are also discussed.