Counterions under a Surface-Adsorbed Cationic Surfactant Monolayer: Structure and Thermodynamics.

The surface adsorption of ionic surfactants is fundamental for many widespread phenomena in life sciences and for a wide range of technological applications. However, direct atomic-resolution structural experimental studies of noncrystalline surface-adsorbed films are scarce. Thus, even the most central physical aspects of these films, such as their charge density, remain uncertain. Consequently, theoretical models based on contradicting assumptions as for the surface films' ionization are widely used for the description and prediction of surface thermodynamics. We employ X-ray reflectivity to obtain the Ångström-scale surface-normal structure of surface-adsorbed films of the cationic surfactant cetyltrimethylammonium bromide (CTAB) in aqueous solutions at several different temperatures and concentrations. In conjunction with published neutron reflectivity data, we determine the surface-normal charge distribution due to the dissociated surfactants' headgroups. The distribution appears to be inconsistent with the Gouy-Chapman model yet consistent with a compact Stern layer model of condensed counterions. The experimental surfactant adsorption thermodynamics conforms well to classical, Langmuir and Kralchevsky, adsorption models. Furthermore, the Kralchevsky model correctly reproduces the observed condensation of counterions, allowing the values of the adsorption parameters to be resolved, based on the combination of the present data and the published surface tension measurements.

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.

Polyhedral Water Droplets: Shape Transitions and Mechanism.

While classical liquid droplets are rounded, transitions have recently been discovered which render polyhedral water-suspended droplets of several oils. Yet, the mechanism of these transitions and the role of the droplets' interfacial curvature in inducing these transitions remain controversial. In particular, one of the two mechanisms suggested mandates a convex interface, in a view from the oil side. Here we show that oil-suspended water droplets can spontaneously assume polyhedral shapes, in spite of their concave interface. These results strongly support the alternative mechanism, where the faceting in both oil and water droplets is driven by the elasticity of a crystalline monolayer, known to self-assemble at the oil-water interface, independent of its curvature. The faceting transitions in the water droplets allow the fundamental elastic properties of two-dimensional matter to be probed, enable new strategies in faceted nanoparticle and nanoshell synthesis, and provide insight into the molecular mechanisms of morphogenesis.

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.

Nucleation and Growth of PbBrF Crystals at the Liquid Mercury-Electrolyte Interface Studied by Operando X-ray Scattering.

Detailed in operando studies of electrochemically induced PbBrF deposition at the liquid mercury/liquid electrolyte interface are presented. The nucleation and growth were monitored using time-resolved X-ray diffraction and reflectivity combined with electrochemical measurements, revealing a complex potential-dependent behavior. PbBrF deposition commences at potentials above -0.7 V with the rapid formation of an ultrathin adlayer of one unit cell thickness, on top of which (001)-oriented three-dimensional crystallites are formed. Two potential regimes are identified. At low overpotentials, slow growth of a low surface density film of large crystals is observed. At high overpotentials, crossover to a potential-independent morphology occurs, consisting of a compact PbBrF deposit with a saturation thickness of 25 nm, which forms within a few minutes. This potential behavior can be rationalized by the increasing supersaturation near the interface, caused by the potential-dependent Pb2+ deamalgamation, which changes from a slow reaction-controlled process to a fast transport-controlled process in this range of overpotentials. In addition, growth on the liquid substrate is found to involve complex micromechanical effects, such as crystal reorientation and film breakup during dissolution.

Nanostructures, Faceting, and Splitting in Nanoliter to Yoctoliter Liquid Droplets.

Contrary to everyday experience, where all liquid droplets assume rounded, near-spherical shapes, the temperature-tuning of liquid droplets to faceted polyhedral shapes and to spontaneous splitting has been recently demonstrated in oil-in-water emulsions. However, the elucidation of the mechanism driving these surprising effects, as well as their many potential applications, ranging from faceted nanoparticle synthesis through new industrial emulsification routes to controlled-release drug delivery within the human body, have been severely hampered by the micron-scale resolution of the light microscopy employed to date in all in situ studies. Thus, the thickness of the interfacially frozen crystalline monolayer, suggested to drive these effects, could not be directly measured, and the low limit on the droplet size still showing these effects remained unknown. In this study, we employ a combination of super-resolution stimulated emission depletion microscopy, cryogenic transmission and freeze-fracture electron microscopy, to study these effects well into the nanometer length scale. We demonstrate the occurrence of the faceting transition in droplets spanning an incredible 12 decades in volume from nanoliters to yoctoliters and directly visualize the interfacially frozen, few nanometer thick, crystalline monolayer suggested to drive these effects. Furthermore, our measurements allow placing an upper-limit estimate on the two-dimensional Young modulus of the interfacial nanometer-thick surface crystal in the smallest droplets, providing insights into the virtually unexplored domain of nanoelasticity.

Precise Self-Positioning of Colloidal Particles on Liquid Emulsion Droplets.

Decorating emulsion droplets by particles stabilizes foodstuff and pharmaceuticals. Interfacial particles also influence aerosol formation, thus impacting atmospheric CO2 exchange. While studies of particles at disordered droplet interfaces abound in the literature, such studies for ubiquitous ordered interfaces are not available. Here, we report such an experimental study, showing that particles residing at crystalline interfaces of liquid droplets spontaneously self-position to specific surface locations, identified as structural topological defects in the crystalline surface monolayer. This monolayer forms at temperature T = Ts, leaving the droplet liquid and driving at Td < Ts a spontaneous shape-change transition of the droplet from spherical to icosahedral. The particle's surface position remains unchanged in the transition, demonstrating these positions to coincide with the vertices of the sphere-inscribed icosahedron. Upon further cooling, droplet shape-changes to other polyhedra occur, with the particles remaining invariably at the polyhedra's vertices. At still lower temperatures, the particles are spontaneously expelled from the droplets. Our results probe the molecular-scale elasticity of quasi-two-dimensional curved crystals, impacting also other fields, such as protein positioning on cell membranes, controlling essential biological functions. Using ligand-decorated particles, and the precise temperature-tunable surface position control found here, may also allow using these droplets for directed supra-droplet self-assembly into larger structures, with a possible post-assembly structure fixation by UV polymerization of the droplet's liquid.

How faceted liquid droplets grow tails.

Liquid droplets, widely encountered in everyday life, have no flat facets. Here we show that water-dispersed oil droplets can be reversibly temperature-tuned to icosahedral and other faceted shapes, hitherto unreported for liquid droplets. These shape changes are shown to originate in the interplay between interfacial tension and the elasticity of the droplet's 2-nm-thick interfacial monolayer, which crystallizes at some T = Ts above the oil's melting point, with the droplet's bulk remaining liquid. Strikingly, at still-lower temperatures, this interfacial freezing (IF) effect also causes droplets to deform, split, and grow tails. Our findings provide deep insights into molecular-scale elasticity and allow formation of emulsions of tunable stability for directed self-assembly of complex-shaped particles and other future technologies.

Temperature-Tuned Faceting and Shape Changes in Liquid Alkane Droplets.

Recent extensive studies reveal that surfactant-stabilized spherical alkane emulsion droplets spontaneously adopt polyhedral shapes upon cooling below a temperature Td while remaining liquid. Further cooling induces the growth of tails and spontaneous droplet splitting. Two mechanisms were offered to account for these intriguing effects. One assigns the effects to the formation of an intradroplet frame of tubules consisting of crystalline rotator phases with cylindrically curved lattice planes. The second assigns the sphere-to-polyhedron transition to the buckling of defects in a crystalline interfacial monolayer, known to form in these systems at some Ts > Td. The buckling reduces the extensional energy of the crystalline monolayer's defects, unavoidably formed when wrapping a spherical droplet by a hexagonally packed interfacial monolayer. The tail growth, shape changes, and droplet splitting were assigned to the decrease and vanishing of surface tension, γ. Here we present temperature-dependent γ(T), optical microscopy measurements, and interfacial entropy determinations for several alkane/surfactant combinations. We demonstrate the advantages and accuracy of the in situ γ(T) measurements made simultaneously with the microscopy measurements on the same droplet. The in situ and coinciding ex situ Wilhelmy plate γ(T) measurements confirm the low interfacial tension, ≲0.1 mN/m, observed at Td. Our results provide strong quantitative support validating the crystalline monolayer buckling mechanism.

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.

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.

Surface layering and melting in an ionic liquid studied by resonant soft X-ray reflectivity.

The molecular-scale structure of the ionic liquid [C18mim](+)[FAP](-) near its free surface was studied by complementary methods. X-ray absorption spectroscopy and resonant soft X-ray reflectivity revealed a depth-decaying near-surface layering. Element-specific interfacial profiles were extracted with submolecular resolution from energy-dependent soft X-ray reflectivity data. Temperature-dependent hard X-ray reflectivity, small- and wide-angle X-ray scattering, and infrared spectroscopy uncovered an intriguing melting mechanism for the layered region, where alkyl chain melting drove a negative thermal expansion of the surface layer spacing.

In situ X-ray studies of adlayer-induced crystal nucleation at the liquid-liquid interface.

Crystal nucleation and growth at a liquid-liquid interface is studied on the atomic scale by in situ Å-resolution X-ray scattering methods for the case of liquid Hg and an electrochemical dilute electrolyte containing Pb(2+), F(-), and Br(-) ions. In the regime negative of the Pb amalgamation potential Φ(rp) = -0.70 V, no change is observed from the surface-layered structure of pure Hg. Upon potential-induced release of Pb(2+) from the Hg bulk at Φ > Φ(rp), the formation of an intriguing interface structure is observed, comprising a well-defined 7.6-Å-thick adlayer, decorated with structurally related 3D crystallites. Both are identified by their diffraction peaks as PbFBr, preferentially aligned with their axis along the interface normal. X-ray reflectivity shows the adlayer to consist of a stack of five ionic layers, forming a single-unit-cell-thick crystalline PbFBr precursor film, which acts as a template for the subsequent quasiepitaxial 3D crystal growth. This growth behavior is assigned to the combined action of electrostatic and short-range chemical interactions.

Surfactant-induced phases in water-supported alkane monolayers: I. Thermodynamics.

Alkanes longer than n = 6 carbons do not spread on the water surface, but condense in a macroscopic lens. However, adding trimethylammonium-based surfactants, C(m)TAB, in submillimolar concentrations causes the alkanes to spread and form a single Langmuir-Gibbs (LG) monolayer of mixed alkanes and surfactant tails, which coexists with the alkane lenses. Upon cooling, this LG film surface-freezes at a temperature T(s) above the bulk freezing temperature T(b). The thermodynamics of surface freezing (SF) of these LG films is studied by surface tension measurements for a range of alkanes (n = 12-21) and surfactant alkyl lengths (m = 14, 16, 18), at several concentrations c. The surface freezing range T(s)-T(b) observed is up to 25 °C, an order of magnitude larger than the temperature range of SF monolayers on the surface of pure alkane melts. The measured (n,T) surface phase diagram is accounted for well by a model based on mixtures' theory, which includes an interchange energy term ω. ω is found to be negative, implying attraction between unlike species, rather than the repulsion found for SF of binary alkane mixtures. Thus, the surfactant/alkane mixing is a necessary condition for the occurrence of SF in these LG films. The X-ray derived structure of the films is presented in an accompanying paper.

Surfactant-induced phases in water-supported alkane monolayers: II. Structure.

The structure of the Langmuir-Gibbs films of normal alkanes C(n) of length n = 12-21 formed at the surface of aqueous solutions of C(m)TAB surfactants, m = 14, 16, and 18, was studied by surface-specific synchrotron X-ray methods. At high temperatures, a laterally disordered monolayer of mixed alkane molecules and surface-adsorbed surfactant tails is found, having thicknesses well below those of the alkanes' and surfactant tails' extended length. The mixed monolayer undergoes a freezing transition at a temperature T(s)(n,m), which forms, for n ≤ m + 1, a crystalline monolayer of mixed alkane molecules and surfactant tails. For n ≥ m + 2, a bilayer forms, consisting of an upper pure-alkane, crystalline monolayer and a lower liquidlike monolayer. The crystalline monolayer in both cases consists of hexagonally packed extended, surface-normal-aligned chains. The hexagonal lattice constant is found to decrease with increasing n. The films' structure is discussed in conjunction with their thermodynamic properties presented in an accompanying paper.

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.