Lipid vesicle pools studied by passive X-ray microrheology.

Vesicle pools can form by attractive interaction in a solution, mediated by proteins or divalent ions such as calcium. The pools, which are alternatively also denoted as vesicle clusters, form by liquid-liquid phase separation (LLPS) from an initially homogeneous solution. Due to the short range liquid-like order of vesicles in the pool or cluster, the vesicle-rich phase can also be regarded as a condensate, and one would like to better understand not only the structure of these systems, but also their dynamics. The diffusion of vesicles, in particular, is expected to change when vesicles are arrested in a pool. Here we investigate whether passive microrheology based on X-ray photon correlation spectroscopy (XPCS) is a suitable tool to study model systems of artificial lipid vesicles exhibiting LLPS, and more generally also other heterogeneous biomolecular fluids. We show that by adding highly scattering tracer particles to the solution, valuable information on the single vesicle as well as collective dynamics can be inferred. While the correlation functions reveal freely diffusing tracer particles in solutions at low CaCl[Formula: see text] concentrations, the relaxation rate [Formula: see text] shows a nonlinear dependence on [Formula: see text] at a higher concentration of around 8 mM CaCl[Formula: see text], characterised by two linear regimes with a broad cross-over. We explain this finding based on arrested diffusion in percolating vesicle clusters.

Comment on "Bi-layering at ionic liquid surfaces: a sum - frequency generation vibrational spectroscopy - and molecular dynamics simulation-based study" by T. Iwahashi, T. Ishiyama, Y. Sakai, A. Morita, D. Kim and Y. Ouchi, Phys. Chem. Chem. Phys., 2020, 22, 12565.

This Comment raises several questions concerning the surface structure concluded in the paper referenced in the title. Specifically, that paper ignores previous experiments and simulations which demonstrate for the same ionic liquids depth-decaying, multilayered surface-normal density profiles rather than the claimed molecular mono- or bi-layers. We demonstrate that the claimed structure does not reproduce the measured X-ray reflectivity, which probes directly the surface-normal density profile. The measured reflectivities are found, however, to be well-reproduced by a multilayered density model. These results, and previous experimental and simulation results, cast severe doubt on the validity of the surface structure claimed in the paper referenced in the title.

Surface structure evolution in a homologous series of ionic liquids.

Interfaces of room temperature ionic liquids (RTILs) are important for both applications and basic science and are therefore intensely studied. However, the evolution of their interface structure with the cation's alkyl chain length [Formula: see text] from Coulomb to van der Waals interaction domination has not yet been studied for even a single broad homologous RTIL series. We present here such a study of the liquid-air interface for [Formula: see text], using angstrom-resolution X-ray methods. For [Formula: see text], a typical "simple liquid" monotonic surface-normal electron density profile [Formula: see text] is obtained, like those of water and organic solvents. For [Formula: see text], increasingly more pronounced nanoscale self-segregation of the molecules' charged moieties and apolar chains yields surface layering with alternating regions of headgroups and chains. The layering decays into the bulk over a few, to a few tens, of nanometers. The layering periods and decay lengths, their linear [Formula: see text] dependence, and slopes are discussed within two models, one with partial-chain interdigitation and the other with liquid-like chains. No surface-parallel long-range order is found within the surface layer. For [Formula: see text], a different surface phase is observed above melting. Our results also impact general liquid-phase issues like supramolecular self-aggregation and bulk-surface structure relations.

Nanoscale Structure in Short-Chain Ionic Liquids.

The temperature (T) and cationic chain length (n) evolution of the nanoscale structure of the sub-layering-threshold members of a model family of room temperature ionic liquids (RTILs) is investigated by x-ray scattering. The measured curves are computer-resolved into individual Teubner-Strey-like lineshapes. The polar-apolar layering is found to start at n = 3 . Opposite n-trends are found at n ≤ 3 for the spacings and correlation lengths associated with the diffraction patterns' two main peaks, and assigned to a shift of balance between the two main interactions, Coulomb and van der Waals, and to increasing packing constraints due to the addition of methylenes. The spacings' thermal expansion coefficients are found to deviate from the macroscopically-measured values, and to anomalously decrease with increasing temperature. Finally, the reduced temperature scale, t = ( T - T m ) / T m , ( T m = melting temperature), is demonstrated to render the observed trends significantly more systematic than those on a conventional T scale.

Insights into the Formation Mechanism of CdSe Nanoplatelets Using in Situ X-ray Scattering.

Two-dimensional ultrathin CdSe nanoplatelets have attracted a large interest due to their optical properties but their formation mechanism is not well understood. Several different mechanisms have been proposed: confined growth in a surfactant mesophase acting as a template, anisotropic ripening of small seeds into 2D nanoplatelets, or continuous anisotropic growth of a limited number of nuclei. However, quantitative in situ data that could validate or disprove these formation scenarios are lacking. We use synchrotron-based small-angle and wide-angle X-ray scattering to probe the formation mechanism of CdSe nanoplatelets synthesized using a heating-up method. We prove the absence of a molecular mesophase in the reactive medium at the onset of nanoplatelet formation ruling out a templating effect. We also show that our data are inconsistent with the anisotropic ripening of small seeds whereas the evolution of the SAXS patterns during the reaction is consistent with the continuous lateral growth of nanoplatelets fed by reactive monomers. Finally, we show that when the final temperature of the synthesis is lowered, nanoplatelets with larger lateral dimensions form. We reveal that they bend in solution during their growth to yield nanoscrolls.

Observation of Ultrathin Precursor Film Formation during Ge-Si Liquid-Phase Epitaxy from an Undersaturated Solution.

Our in situ X-ray study shows that a silicon substrate in contact with an undersaturated In(Ge) solution is wetted by an approximately 1 nm thin germanium film, which does not grow any thicker. The results can be understood by the use of thickness-dependent correlated interfacial energies. This near-equilibrium heterogeneous interface structure marks the initial stage of crystal growth before the formation of bulk material, which can only form under conditions of supersaturation. This finding uncovers a fundamental aspect of the thermodynamics at solid-liquid interfaces relevant for understanding the transition from equilibrium to supersaturation and is of importance for nanoscale solution growth methods.

Molecular scale structure and dynamics at an ionic liquid/electrode interface.

After a century of research, the potential-dependent ion distribution at electrode/electrolyte interfaces is still under debate. In particular for solvent-free electrolytes such as room-temperature ionic liquids, classical theories for the electrical double layer are not applicable. Using a combination of in situ high-energy X-ray reflectivity and impedance spectroscopy measurements, we determined this distribution with sub-molecular resolution. We find oscillatory charge density profiles consisting of alternating anion- and cation-enriched layers at both cathodic and anodic potentials. This structure is shown to arise from the same ion-ion correlations dominating the liquid bulk structure. The relaxation dynamics of the interfacial structure upon charging/discharging were studied by impedance spectroscopy and time resolved X-ray reflectivity experiments with sub-millisecond resolution. The analysis revealed three relaxation processes of vastly different characteristic time scales: a 2 ms scale interface-normal ion transport, a 100 ms scale molecular reorientation, and a minute scale lateral ordering within the first layer.

Self-segregated nanostructure in room temperature ionic liquids.

The nanosegregated bulk structure, and its evolution with the cation's alkyl length n, are studied by X-ray scattering for an unprecedentedly broad homologous series of a model room-temperature ionic liquid, [CnMIM][NTf2] (n = 4-22). A tri-periodic local structure is found, with the lateral periodicities, dII and dIII independent of n, and a longitudinal one, dI, linearly increasing with n. The results are consistent with a local structure comprising alternating layers of polar headgroups and apolar, interdigitated, partly overlapping, cations' alkyl tails, of an average macroscopic mass density close to that of liquid alkanes. A slope decrease in the linear dI(n) suggests a change from a lower to a higher rate of increase with n of chain overlap for n ≥ 12. The order decay lengths of the layering, and of the lateral chain packing, increase with n, as expected from the increasing van der Waals interaction's domination of the structure. The headgroups' lateral packing decay length decreases with n, due to increasing frustration between the longer lateral periodicity preferred by the headgroups, and the shorter lateral periodicity preferred by the chains. A comparison of the bulk and surface structures highlights the surface's ordering effect, which, however, does not induce here a surface phase different from the bulk, as it does in liquid crystals and liquid alkanes.

Correction: Self-segregated nanostructure in room temperature ionic liquids.

Correction for 'Self-segregated nanostructure in room temperature ionic liquids' by Diego Pontoni et al., Soft Matter, 2017, DOI: 10.1039/c7sm01464c.

Real Space Imaging of Nanoparticle Assembly at Liquid-Liquid Interfaces with Nanoscale Resolution.

Bottom up self-assembly of functional materials at liquid-liquid interfaces has recently emerged as method to design and produce novel two-dimensional (2D) nanostructured membranes and devices with tailored properties. Liquid-liquid interfaces can be seen as a "factory floor" for nanoparticle (NP) self-assembly, because NPs are driven there by a reduction of interfacial energy. Such 2D assembly can be characterized by reciprocal space techniques, namely X-ray and neutron scattering or reflectivity. These techniques have drawbacks, however, as the structural information is averaged over the finite size of the radiation beam and nonperiodic isolated assemblies in 3D or defects may not be easily detected. Real-space in situ imaging methods are more appropriate in this context, but they often suffer from limited resolution and underperform or fail when applied to challenging liquid-liquid interfaces. Here, we study the surfactant-induced assembly of SiO2 nanoparticle monolayers at a water-oil interface using in situ atomic force microscopy (AFM) achieving nanoscale resolved imaging capabilities. Hitherto, AFM imaging has been restricted to solid-liquid interfaces because applications to liquid interfaces have been hindered by their softness and intrinsic dynamics, requiring accurate sample preparation methods and nonconventional AFM operational schemes. Comparing both AFM and grazing incidence X-ray small angle scattering data, we unambiguously demonstrate correlation between real and reciprocal space structure determination showing that the average interfacial NP density is found to vary with surfactant concentration. Additionally, the interaction between the tip and the interface can be exploited to locally determine the acting interfacial interactions. This work opens up the way to studying complex nanostructure formation and phase behavior in a range of liquid-liquid and complex liquid interfaces.

Liquid-Mercury-Supported Langmuir Films of Ionic Liquids: Isotherms, Structure, and Time Evolution.

Ionic liquids have been intensively developed for the last few decades and are now used in a wide range of applications, from electrochemistry to catalysis and nanotechnology. Many of these applications involve ionic liquid interfaces with other liquids and solids, the subnanometric experimental study of which is highly demanding, and has been little studied to date. We present here a study of mercury-supported Langmuir films of imidazolium-based ionic liquids by surface tensiometry and X-ray reflectivity. The charge-delocalized ionic liquids studied here exhibit no 2D lateral order but show diffuse surface-normal electron density profiles exhibiting gradual mercury penetration into the ionic liquid film, and surface-normal structure evolution over a period of hours. The effect of increasing the nonpolar alkyl chain length was also investigated. The results obtained provide insights into the interactions between these ionic liquids and liquid mercury and about the time evolution of the structure and composition of their interface.

Real-time in situ probing of high-temperature quantum dots solution synthesis.

Understanding the formation mechanism of colloidal nanocrystals is of paramount importance in order to design new nanostructures and synthesize them in a predictive fashion. However, reliable data on the pathways leading from molecular precursors to nanocrystals are not available yet. We used synchrotron-based time-resolved in situ small and wide-angle X-ray scattering to experimentally monitor the formation of CdSe quantum dots synthesized in solution through the heating up of precursors in octadecene at 240 °C. Our experiment yields a complete movie of the structure of the solution from the self-assembly of the precursors to the formation of the quantum dots. We show that the initial cadmium precursor lamellar structure melts into small micelles at 100 °C and that the first CdSe nuclei appear at 218.7 °C. The size distributions and concentration in nanocrystals are measured in a quantitative fashion as a function of time. We show that a short nucleation burst lasting 30 s is followed by a slow decrease of nanoparticle concentration. The rate-limiting process of the quantum dot formation is found to be the thermal activation of selenium.

Solid-liquid interfaces of ionic liquid solutions--Interfacial layering and bulk correlations.

The influence of the polar, aprotic solvent propylene carbonate on the interfacial structure of the ionic liquid (IL) 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate on sapphire was investigated by high-energy x-ray reflectivity. Experiments at solvent concentrations between 17 mol. % and 83 mol. % bridge the gap between diluted electrolytes described by the classical Gouy-Chapman theory and pure ionic liquids. Analysis of our experimental data revealed interfacial profiles comprised of alternating anion and cation enriched regions decaying gradually into the bulk liquid. With increasing solvent concentration, we observed a decrease in correlation length of the interfacial layering structure. At high ion concentrations, solvent molecules were found to accumulate laterally within the layers. By separating like-charged ions, they reduce their Coulomb repulsion. The results are compared with the bulk structure of IL/solvent blends probed by x-ray scattering and predictions from fundamental fluid theory.

Chemistry-dependent X-ray-induced surface charging.

Materials science in general, and surface/interface science in particular, have greatly benefited from the development of high energy synchrotron radiation facilities. Irradiation with intense ionizing beams can however influence relevant sample properties. Permanent radiation damage and irradiation-induced sample modifications have been investigated in detail during the last decades. Conversely, reversible sample alterations taking place only during irradiation are still lacking comprehensive in situ characterization. Irradiation-induced surface charging phenomena are particularly relevant for a wide range of interface science investigations, in particular those involving surfaces of solid substrates in contact with gaseous or liquid phases. Here, we demonstrate partially reversible radiation-induced surface charging phenomena, which extend far beyond the spatial dimensions of the X-ray beam mainly as a consequence of the interaction between the surface and ionized ambient molecules. The charging magnitude and sign are found to be surface chemistry specific and dependent on the substrates' bulk conductivity and grounding conditions. These results are obtained by combining a scanning Kelvin probe with a synchrotron surface diffractometer to allow simultaneous in situ work function measurements during precisely controlled hard X-ray micro-beam irradiation.

Modification of deeply buried hydrophobic interfaces by ionic surfactants.

Hydrophobicity, the spontaneous segregation of oil and water, can be modified by surfactants. The way this modification occurs is studied at the oil-water interface for a range of alkanes and two ionic surfactants. A liquid interfacial monolayer, consisting of a mixture of alkane molecules and surfactant tails, is found. Upon cooling, it freezes at T(s), well above the alkane's bulk freezing temperature, T(b). The monolayer's phase diagram, derived by surface tensiometry, is accounted for by a mixtures-based theory. The monolayer's structure is measured by high-energy X-ray reflectivity above and below T(s). A solid-solid transition in the frozen monolayer, occurring approximately 3 °C below T(s), is discovered and tentatively suggested to be a rotator-to-crystal transition.

Ordering of ionic liquids at a charged sapphire interface: Evolution with cationic chain length.

HYPOTHESIS: Room Temperature Ionic Liquids (RTILs) bulk's molecular layering dominates their structure also at the RTIL/sapphire interface, increasing the layer spacing with the cationic alkyl chain length n. However, the negatively-charged sapphire surface compresses the layers, increases the layering range, and affects the intra-layer structure in yet unknown ways. EXPERIMENTS: X-ray reflectivity (XR) off the RTIL/sapphire interface, for a broad homologous RTIL series 1-alkyl-3-methylimidazolium bis(trifluoromethansulfonyl)imide, hitherto unavailable for any RTIL. FINDINGS: RTIL layers against the sapphire, exhibit two spacings: da and db. da is n-varying, follows the behavior of the bulk spacing but exhibits a downshift, thus showing significant layer compression, and over twofold polar slab thinning. The latter suggests exclusion of anions from the interfacial region due to the negative sapphire charging by x-ray-released electrons. The layering range is larger than the bulk's. db is short and near n-independent, suggesting polar moieties' layering, the coexistence mode of which with the da-spaced layering is unclear. Comparing the present layering with the bulk's and the RTIL/air interface's provides insight into the Coulomb and dispersion interaction balance dominating the RTIL's structure and the impact thereon of the presence of a charged solid interface.

Macroscopic Biaxial Order in Multilayer Films of Bent-Core Liquid Crystals Deposited by Combined Langmuir-Blodgett/Langmuir-Schaefer Technique.

Bent-core liquid crystals, a class of mesogenic compounds with non-linear molecular structures, are well known for their unconventional mesophases, characterized by complex molecular (and supramolecular) ordering and often featuring biaxial and polar properties. In the nematic phase, their unique behavior is manifested in the formation of nano-sized biaxial clusters of layered molecules (cybotactic groups). While this prompted their consideration in the quest for nematic biaxiality, experimental evidence indicates that the cybotactic order is only short-ranged and that the nematic phase is macroscopically uniaxial. By combining atomic force microscopy, neutron reflectivity and wide-angle grazing-incidence X-ray scattering, here, we demonstrate that multilayer films of a bent-core nematic, deposited on silicon by a combined Langmuir-Blodgett and Langmuir-Schaefer approach, exhibit macroscopic in-plane ordering, with the long molecular axis tilted with respect to the sample surface and the short molecular axis (i.e., the apex bisector) aligned along the film compression direction. We thus propose the use of Langmuir films as an effective way to study and control the complex anchoring properties of bent-core liquid crystals.

An X-ray chamber for in situ structural studies of solvent-mediated nanoparticle self-assembly.

Spontaneous ordering of nanoparticles (NPs) occurring as a consequence of solvent evaporation can yield highly ordered and extended NP superlattices bearing both fundamental scientific interest and potential for technological application. A versatile experimental chamber has been developed allowing (i) controlled in situ deposition of NP solutions on solid substrates, (ii) rate-controlled evaporation of the bulk solvent, and (iii) adsorption/desorption of nano-thick solvent films onto preformed NP assemblies. Within this hermetically sealed chamber all the stages of self-assembly, including macroscopic solution evaporation, NP thin-film formation and its subsequent structural transformation induced by nano-thick solvent films, can be characterized in situ by X-ray scattering techniques. Here, technical design and calibration details are provided, as well as three experimental examples highlighting the chamber's performances and potential. Examples include the controlled adsorption of thin toluene films on flat silicon wafers, the observation of transient accumulation of gold NPs near the toluene-vapour interface, and preliminary data on the structural effects of fast macroscopic solvent evaporation followed by nanoscale solvent adsorption/desorption from a vapour phase. By combining bulk evaporation rate control, fine tuning of the thickness of adsorbed solvent films and in situ X-ray characterization capabilities, this cell enables explorations of both near-to-equilibrium and far-from-equilibrium routes to NP self-assembly.

A scanning Kelvin probe for synchrotron investigations: the in situ detection of radiation-induced potential changes.

A wide range of high-performance X-ray surface/interface characterization techniques are implemented nowadays at every synchrotron radiation source. However, these techniques are not always `non-destructive' because possible beam-induced electronic or structural changes may occur during X-ray irradiation. As these changes may be at least partially reversible, an in situ technique is required for assessing their extent. Here the integration of a scanning Kelvin probe (SKP) set-up with a synchrotron hard X-ray interface scattering instrument for the in situ detection of work function variations resulting from X-ray irradiation is reported. First results, obtained on bare sapphire and sapphire covered by a room-temperature ionic liquid, are presented. In both cases a potential change was detected, which decayed and vanished after switching off the X-ray beam. This demonstrates the usefulness of a SKP for in situ monitoring of surface/interface potentials during X-ray materials characterization experiments.

X-ray reflectivity at polarized liquid-Hg-aqueous-electrolyte interface: challenging macroscopic approaches for ion-specificity issues.

We report Angstrom-resolved x-ray reflectivity analysis of externally polarized liquid-Hg surface in contact with molar LiCl, LiBr, and MgSO4 aqueous electrolytes. Interpretation of reflectivity curves demonstrates a dependence of Hg-surface layering on both applied potential and ion nature. It further highlights how interfacial polarization degree impacts electron density profiles at a molecular scale. These profiles indicate accumulation of anions and cations at the Hg surface. Upon decrease of the potential from the point of zero charge, anions are gradually expelled from the Hg surface. The study challenges traditional thermodynamic approaches for deriving countercharge composition at the Hg-electrolyte-solution interface from macroscopic Hg-surface tension data. It further dismisses the long-standing approximation that assimilates the Hg surface to a smooth, perfect chemically inert conductor with a uniformly smeared-out surface charge density.