Two-dimensional assembly of gold nanoparticles grafted with charged-end-group polymers

HYPOTHESIS

Introducing charged terminal groups to polymers that graft nanoparticles enable Coulombic control over their assembly by tuning the pH and salinity of their aqueous suspensions.

EXPERIMENTS

Gold nanoparticles (AuNPs) are grafted with poly (ethylene glycol) (PEG) terminated with (charge-neutral), (negatively charged) or groups (positively charged), and characterized with dynamic light scattering, ζ-potential, and thermal gravimetric analysis. Liquid surface X-ray reflectivity (XR) and grazing incidence small-angle X-ray scattering (GISAXS) are used to determine the density profile and in-plane structure of the AuNPs assembly at the aqueous surface.

FINDINGS

Assembly of PEG-AuNPs at the liquid/vapor interface is tunable by adjusting pH or salinity for COOH but less for terminals. The distinct assembly behaviors are attributed to the overall charge of PEG-AuNPs as well as PEG conformation. COOH-PEG corona is more compact than those of the other terminal groups, leading to a crystalline structure with a smaller superlattice. The net charge per particle depends not only on the PEG terminal groups but also on the cation sequestration of PEG and the intrinsic negative charge of the AuNP surface. [1] The closeness to overall charge neutrality, and hydrogen bonding in play, brought by -PEG, drive -PEG-AuNPs to assembly and crystallinity without additives to the suspensions.

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.

Compact Peptoid Molecular Brushes for Nanoparticle Stabilization

Controlling the interfaces and interactions of colloidal nanoparticles (NPs) via tethered molecular moieties is crucial for NP applications in engineered nanomaterials, optics, catalysis, and nanomedicine. Despite a broad range of molecular types explored, there is a need for a flexible approach to rationally vary the chemistry and structure of these interfacial molecules for controlling NP stability in diverse environments, while maintaining a small size of the NP molecular shell. Here, we demonstrate that low-molecular-weight, bifunctional comb-shaped, and sequence-defined peptoids can effectively stabilize gold NPs (AuNPs). The generality of this robust functionalization strategy was also demonstrated by coating of silver, platinum, and iron oxide NPs with designed peptoids. Each peptoid (PE) is designed with varied arrangements of a multivalent AuNP-binding domain and a solvation domain consisting of oligo-ethylene glycol (EG) branches. Among designs, a peptoid (PE5) with a diblock structure is demonstrated to provide a superior nanocolloidal stability in diverse aqueous solutions while forming a compact shell (∼1.5 nm) on the AuNP surface. We demonstrate by experiments and molecular dynamics simulations that PE5-coated AuNPs (PE5/AuNPs) are stable in select organic solvents owing to the strong PE5 (amine)-Au binding and solubility of the oligo-EG motifs. At the vapor-aqueous interface, we show that PE5/AuNPs remain stable and can self-assemble into ordered 2D lattices. The NP films exhibit strong near-field plasmonic coupling when transferred to solid substrates.

Binary Superlattices of Gold Nanoparticles in Two Dimensions

We have created two-dimensional (2D) binary superlattices by cocrystallizing gold nanoparticles (AuNPs) of two distinct sizes into √3 × √3 and 2 × 2 complex binary superlattices, derived from the hexagonal structures of the single components. The building blocks of these binary systems are AuNPs that are functionalized with different chain lengths of poly(ethylene glycol) (PEG). The assembly of these functionalized NPs at the air-water interface is driven by the presence of salt, causing PEG-AuNPs to migrate to the aqueous surface and assemble into a crystalline lattice. We have used liquid surface X-ray reflectivity (XR) and grazing incidence small-angle X-ray scattering (GISAXS) to examine the assembly and crystallization at the liquid interface.

Effect of Polymer Chain Length on the Superlattice Assembly of Functionalized Gold Nanoparticles

We report on the assembly of gold nanoparticle (AuNPs) superlattices at the liquid/vapor interface and in the bulk of their suspensions. Interparticle distances in the assemblies are achieved on multiple length scales by varying chain lengths of surface grafted AuNPs by polyethylene glycol (PEG) with molecular weights in the range 2000-40,000 Da. Crystal structures and lattice constants in both 2D and 3D assemblies are determined by synchrotron-based surface-sensitive and small-angle X-ray scattering. Assuming knowledge of grafting density, we show that experimentally determined interparticle distances are adequately modeled by spherical brushes close to the θ point (Flory-Huggins parameter, χ≈12) for 2D superlattices at a liquid interface and a nonsolvent (χ = ∞) for the 3D dry superlattices.

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.

Nanoscale viscosity of confined polyethylene oxide

Complex fluids near interfaces or confined within nanoscale volumes can exhibit substantial shifts in physical properties compared to bulk, including glass transition temperature, phase separation, and crystallization. Because studies of these effects typically use thin film samples with one dimension of confinement, it is generally unclear how more extreme spatial confinement may influence these properties. In this work, we used x-ray photon correlation spectroscopy and gold nanoprobes to characterize polyethylene oxide confined by nanostructured gratings (<100nm width) and measured the viscosity in this nanoconfinement regime to be ∼500 times the bulk viscosity. This enhanced viscosity occurs even when the scale of confinement is several times the polymer's radius of gyration, consistent with previous reports of polymer viscosity near flat interfaces.

Molecular Structure of Aromatic Reverse Osmosis Polyamide Barrier Layers

The molecular structures of polyamide barrier layers in reverse osmosis membranes, made by interfacial polymerization of m-phenylenediamine and trimesoyl chloride under different reaction and post-treatment conditions, were characterized by grazing incidence wide-angle X-ray scattering (GIWAXS). The molecular backbone packing is consistent with two different aromatic molecular packing motifs (parallel and perpendicular) with preferential surface-induced orientation. The results suggest that the perpendicular, T-shaped, packing motif (5 Å spacing) might be associated with optimal membrane permeance, compared with the parallel packings (3.5-4.0 Å spacings).

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.

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 T_d 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 T_s > T_d. 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 T_d. Our results provide strong quantitative support validating the crystalline monolayer buckling mechanism.

Nanoscale Structure of the Oil-Water Interface

X-ray reflectivity (XR) and atomistic molecular dynamics (MD) simulations, carried out to determine the structure of the oil-water interface, provide new insight into the simplest liquid-liquid interface. For several oils (hexane, dodecane, and hexadecane) the XR shows very good agreement with a monotonic interface-normal electron density profile (EDP) broadened only by capillary waves. Similar agreement is also found for an EDP including a sub-Å thick electron depletion layer separating the oil and the water. The XR and MD derived depletions are much smaller than reported for the interface between solid-supported hydrophobic monolayers and water.

Structure of n-Alkyltrichlorosilane Monolayers on Si(100)/SiO2

The structure of n-alkyltrichlorosilane self-assembled monolayers (SAMs) of alkyl chain lengths n = 12, 14, 18, and 22 formed on the amorphous native oxide of silicon (100) has been investigated via angstrom-resolution surface X-ray scattering techniques, with particular focus on the proliferation of lateral order along the molecules' long axis. Grazing incidence diffraction shows that the monolayer is composed of hexagonally packed crystalline-like domains for n = 14, 18, and 22 with a lateral size of about 60 Å. However, Bragg rod analysis shows that ∼12 of the CH2 units are not included in the crystalline-like domains. We assign this, and the limited lateral crystallites' size, to strain induced by the size mismatch between the optimal chain-chain and headgroup-headgroup spacings. Analysis of X-ray reflectivity profiles for n = 12, 14, and 22 shows that the density profile used to successfully model n = 18 provides an excellent fit where the analysis-derived parameters provide complementary structural information to the grazing incidence results.

Nanoscale structure of Si/SiO2/organics interfaces

X-ray reflectivity measurements of increasingly more complex interfaces involving silicon (001) substrates reveal the existence of a thin low-density layer intruding between the single-crystalline silicon and the amorphous native SiO2 terminating it. The importance of accounting for this layer in modeling silicon/liquid interfaces and silicon-supported monolayers is demonstrated by comparing fits of the measured reflectivity curves by models including and excluding this layer. The inclusion of this layer, with 6-8 missing electrons per silicon unit cell area, consistent with one missing oxygen atom whose bonds remain hydrogen passivated, is found to be particularly important for an accurate and high-resolution determination of the surface normal density profile from reflectivities spanning extended momentum transfer ranges, now measurable at modern third-generation synchrotron sources.

Pseudorotational epitaxy of self-assembled octadecyltrichlorosilane monolayers on sapphire (0001)

The structure of octadecyltrichlorosilane self-assembled monolayers (SAMs) on sapphire (0001) was studied by Å-resolution surface-specific x-ray scattering methods. The monolayer was found to consist of three sublayers where the outermost layer corresponds to vertically oriented, closely packed alkyl tails. Laterally, the monolayer is hexagonally packed and exhibits pseudorotational epitaxy to the sapphire, manifested by a broad scattering peak at zero relative azimuthal rotation, with long powderlike tails. The lattice mismatch of ∼ 1%-3% to the sapphire's and the different length scale introduced by the lateral Si-O-Si bonding prohibit positional epitaxy. However, the substrate induces an intriguing increase in the crystalline coherence length of the SAM's powderlike crystallites when rotationally aligned with the sapphire's lattice. The increase correlates well with the rotational dependence of the separation of corresponding substrate-monolayer lattice sites.

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.

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.

Nanostructured surfaces frustrate polymer semiconductor molecular orientation

Nanostructured grating surfaces with groove widths less than 200 nm impose boundary conditions that frustrate the natural molecular orientational ordering within thin films of blended polymer semiconductor poly(3-hexlythiophene) and phenyl-C61-butyric acid methyl ester, as revealed by grazing incidence X-ray scattering measurements. Polymer interactions with the grating sidewall strongly inhibit the polymer lamellar alignment parallel to the substrate typically found in planar films, in favor of alignment perpendicular to this orientation, resulting in a preferred equilibrium molecular configuration difficult to achieve by other means. Grating surfaces reduce the relative population of the parallel orientation from 30% to less than 5% in a 400 nm thick film. Analysis of in-plane X-ray scattering with respect to grating orientation shows polymer backbones highly oriented to within 10 degrees of parallel to the groove direction.

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.

Directed assembly of P3HT:PCBM blend films using a chemical template with sub-300 nm features

Surface energy has been demonstrated as a means to direct interfacial-layer composition in polymer:fullerene blends utilized as active layers in organic photovoltaic devices. Combined with recent materials advances in the preparation of nanoscale chemical patterns, surface energy control of nanophase separation presents an opportunity to employ patterned surface energy templates to control the 3D blend morphology of polymer:fullerene blends. This report details the directed assembly of poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM) blends atop linear grating patterns with domains of alternating high and low surface energy of 50 to 600 nm in width prepared by nanoscale oxidative lithography of alkyl-terminated self-assembled monolayers on SiO2 and SiH surfaces. Tapping-, contact-, and current-sensing AFM studies demonstrated that chemical patterns were effective at directing the 3D morphology of P3HT:PCBM blends at dimensions of >200 nm. As the dimensionality of domains approached 100 nm, the chemical patterns were no longer able to direct phase segregation, evidence that a directed spinodal decomposition mechanism was responsible for the observed morphology. Surprisingly, the low surface energy component (P3HT) was found to be atop the high surface energy domains of the template, in conflict with current understanding of the role of surface energy directed assembly in polymer blends. These results suggest that the directed spinodal decomposition mechanism applies to conjugated polymer:fullerene blends, but that additional parameters unique to these types of systems will require refinement of the theory to adequately describe and predict the behavior of these scientifically and industrially interesting materials.

Grazing-incidence transmission X-ray scattering: surface scattering in the Born approximation

Determination of the three-dimensional order in thin nanostructured films remains challenging. Real-space imaging methods, including electron microscopies and scanning-probe methods, have difficulty reconstructing the depth of a film and suffer from limited statistical sampling. X-ray and neutron scattering have emerged as powerful complementary techniques but have substantial data collection and analysis challenges. This article describes a new method, grazing-incidence transmission small-angle X-ray scattering, which allows for fast scattering measurements that are not burdened by the refraction and reflection effects that have to date plagued grazing-incidence X-ray scattering. In particular, by arranging a sample/beam geometry wherein the scattering exits through the edge of the substrate, it is possible to record scattering images that are well described by straightforward (Born approximation) scattering models.

Water-vapor-assisted nanoimprinting of PEDOT:PSS thin films

PEDOT:PSS thin films are successfully patterned using water-vapor-assisted nanoimprinting, a process compatible with organic electronic devices. The imprinted patterns are characterized via grazing-incident small-angle X-ray scattering and scanning electron microscopy. Superior pattern transfer quality of water-vapor assisted nanoimprinting over conventional thermal nanoimprinting is demonstrated.

Two-dimensional order in mercury-supported langmuir films of fatty diacids

The structure of mercury-supported Langmuir films of dicarboxylic acid molecules with 13 ≤ n ≤ 22 carbons is studied by X-ray methods and surface tensiometry. The molecules lie surface-parallel, forming mono-, bi-, or trilayers, depending on coverage. All films exhibit a full 2D order of the same single-molecule oblique unit cell. In particular, the distinct odd-even structure difference of 3D crystals of the same molecules is not observed. The unit cell's width and angle show a small systematic decrease with n, while the length increases commensurately with the molecular length. These results show the films to consist of closely packed, extended, polymer-like chains of diacid molecules, bound by their carboxyl end groups. Evidence is presented for the inclusion of a single mercury atom in the carboxyl-carboxyl bond. The possible conformation of this bond and implications of the parity-independent structure are discussed.

Stability of thin wetting films on chemically nanostructured surfaces

The morphology and stability of thin volatile wetting films on model chemically patterned surfaces composed of periodic arrays of alternating completely and partially wettable nanostripes are investigated. The equilibrium film morphology is recorded as a function of undersaturation using noncontact atomic force microscopy. Films spanning the entire pattern are found to be stable only for thicknesses in excess of a critical value, h(c), whereas thinner films spontaneously dewet the partially wettable regions of the substrate. The critical thickness h(c) increases linearly with the width of the partially wettable stripes in good agreement with an interface displacement model derived from microscopic density functional theory. These results provide detailed insights into the dewetting of thin films driven by competing intermolecular forces.

One-volt operation of high-current vertical channel polymer semiconductor field-effect transistors

We realize a vertical channel polymer semiconductor field effect transistor architecture by confining the organic material within gratings of interdigitated trenches. The geometric space savings of a perpendicular channel orientation results in devices sourcing areal current densities in excess of 40 mA/cm(2), using a one-volt supply voltage, and maintaining near-ideal device operating characteristics. Vertical channel transistors have a similar electronic mobility to that of planar devices using the same polymer semiconductor, consistent with a molecular reorientation within confining trenches we understand through X-ray scattering measurements.

Bilayer order in a polycarbazole-conjugated polymer

One of the best performing semiconducting polymers used in bulk heterojunction devices is PCDTBT, a polycarbazole derivative with solar-conversion efficiencies as high as 7.2%. Here we report the formation of bilayer ordering in PCDTBT, and postulate that this structural motif is a direct consequence of the polymer's molecular design. This bilayer motif is composed of a pair of backbones arranged side-to-side where the alkyl tails are on the outer side. This is in stark contrast to the monolayer ordering found in other conjugated polymers. The crystalline bilayer phase forms at elevated temperatures and persists after cooling to room temperature. The existence of bilayer ordering, along with its high-packing fraction of conjugated moieties, may guide the synthesis of new materials with improved optoelectronic properties.

Nanoimprint-induced molecular orientation in semiconducting polymer nanostructures

The morphology and orientation of thin films of the polymer poly-3(hexylthiophene)-important parameters influencing electronic and photovoltaic device performance-have been significantly altered through nanoimprinting with 100 nm spaced grooves. Grazing-incidence small-angle X-ray scattering studies demonstrate the excellent fidelity of the pattern transfer, while wide-angle scattering convincingly shows an imprinting-induced π-π reorientation and polymer backbone alignment along the imprinted grooves. Surprisingly, temperature-dependent scattering measurements indicate that the imprinted induced orientation and alignment remain intact even at temperatures where the imprinted topographical features nearly vanish.

Checkerboard self-patterning of an ionic liquid film on mercury

Å-resolution studies of room temperature ionic liquid (RTIL) interfaces are scarce, in spite of their long-recognized importance for the science and many applications of RTILs. We present an Å-resolution x-ray study of a Langmuir film of an RTIL on mercury. At low (high) coverage [90 (50) Å2/molecule] a mono-(bi)layer of surface-parallel molecules is found. The molecules self-assemble in a lateral ionic checkerboard pattern, unlike the uniform-charge, alternate-ion layers of this RTIL at its bulk-solid interface. A 2D-smectic order is found, with molecules packed in parallel stripes, forming long-range order normal to, but none along, the stripes.

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.

Unifying interfacial self-assembly and surface freezing

X-ray investigations reveal that the monolayers formed at the bulk alkanol-sapphire interface are densely packed with the surface-normal molecules hydrogen bound to the sapphire. About 30-35 °C above the bulk, these monolayers both melt reversibly and partially desorb. This system exhibits balanced intermolecular and molecule-substrate interactions which are intermediate between self-assembled and surface-frozen monolayers, each dominated by one interaction. The phase behavior is rationalized within a thermodynamic model comprising interfacial interactions, elasticity, and entropic effects. Separating the substrate from the melt leaves the monolayer structurally intact.

Morphology of air nanobubbles trapped at hydrophobic nanopatterned surfaces

The details of air nanobubble trapping at the interface between water and a nanostructured hydrophobic silicon surface are investigated using X-ray scattering and contact angle measurements. Large-area silicon surfaces containing hexagonally packed, 20 nm wide hydrophobic cavities provide ideal model surfaces for studying the morphology of air nanobubbles trapped inside cavities and its dependence on the cavity depth. Transmission small-angle X-ray scattering measurements show stable trapping of air inside the cavities with a partial water penetration of 5-10 nm into the pores, independent of their large depth variation. This behavior is explained by consideration of capillary effects and the cavity geometry. For parabolic cavities, the liquid can reach a thermodynamically stable configuration-a nearly planar nanobubble meniscus-by partially penetrating into the pores. This microscopic information correlates very well with the macroscopic surface wetting behavior.

Surface layering at the mercury-electrolyte interface

X-ray reflectometry reveals atomic layering at a liquid-liquid interface--mercury in a 0.01 M NaF solution. The interface width exceeds capillary wave theory predictions and displays an anomalous dependence on the voltage applied across it, displaying a minimum positive of the potential of zero charge. The latter is explained by electrocapillary effects and an additional intrinsic broadening of the interface profile, tentatively assigned to polarization of the conduction electrons due to the electric field of the electrochemical double layer at the interface.

Wetting of nanopatterned grooved surfaces

The wetting by perfluoromethylcyclohexane of a well-defined silicon grating with a channel width of 16 nm has been studied using transmission small angle x-ray scattering. Prefilling, capillary filling, and postfilling wetting regimes have been identified. A detailed comparison of the data with theory reveals the importance of long-ranged substrate-fluid and fluid-fluid interactions for determining the wetting behavior on these length scales, especially at the onset of capillary condensation and in the prefilling regime.

Equilibrating nanoparticle monolayers using wetting films

Monolayers of bimodal gold nanoparticles on silicon are investigated by a combination of microscopy (dry monolayers) and x-ray diffraction (dry and wet monolayers). In the presence of an excess of small particles, the nanoscale packing structure closely resembles the small-particle-rich scenario of the structural crossover transition that has been predicted and also observed with micron-scale hard-sphere colloids. Structural morphology is monitored in situ during monolayer dissolution and reassembly within the thin liquid wetting film. This approach allows investigation of size and solvent effects on nanoparticles in quasi-two-dimensional confinement.

Molecular layering of fluorinated ionic liquids at a charged sapphire (0001) surface

Room-temperature ionic liquids (RTILs) are promising candidates for a broad range of "green" applications, for which their interaction with solid surfaces plays a crucial role. In this high-energy x-ray reflectivity study, the temperature-dependent structures of three ionic liquids with the tris(pentafluoroethyl)trifluorophosphate anion in contact with a charged sapphire substrate were investigated with submolecular resolution. All three RTILs show strong interfacial layering, starting with a cation layer at the substrate and decaying exponentially into the bulk liquid. The observed decay length and layering period point to an interfacial ordering mechanism, akin to the charge inversion effect, which is suggested to originate from strong correlations between the unscreened ions. The observed layering is expected to be a generic feature of RTILs at charged interfaces.

Dynamics and critical damping of capillary waves in an ionic liquid

The dynamics of thermal capillary waves (CWs) on an ionic liquid's surface are studied at the transition from propagating to overdamped CWs by x-ray photon correlation spectroscopy. The analysis considers both homodyne and heterodyne contributions, and yields excellent full line-shape experiment-theory agreement for the structure factor. The CWs' Brillouin scattering becomes extinct at a critical temperature Tc JK approximately 10 K above Tc LL , the propagating modes' hydrodynamic limit, in agreement with linear response theory. Surprisingly, the same power law applies at both Tc. The results rule out the presence of a suggested surface dipole layer.

Morphology and phase behavior of ethanol nanodrops condensed on chemically patterned surfaces

Equilibrium wetting of ethanol onto chemically patterned nanostripes has been investigated using environmental atomic force microscopy (AFM) in noncontact mode. The chemical patterns are composed of COOH-terminated "wetting" regions and CH3-terminated "nonwetting" regions. A specially designed environmental AFM chamber allowed for accurate measurements of droplet height as a function of the temperature offset between the substrate and a macroscopic ethanol reservoir. At saturation, the height dependence scales with droplet width according to w1/2, in excellent agreement with the augmented Young equation (AYE) modeled with dispersive, nonretarded surface potentials. At small under- and oversaturations, the AYE model accurately fits the data if an effective DeltaT is used as a fitting parameter. There is a systematic difference between the measured DeltaT and the values extracted from the fits to the data. In addition to static measurements, we present time-resolved measurements of the droplet height which enable the study of condensation-evaporation dynamics of nanometer-scale drops.

Wetting of liquid-crystal surfaces and induced smectic layering at a nematic-liquid interface: an x-ray reflectivity study

We report the results of a synchrotron x-ray reflectivity study of bulk liquid-crystal surfaces that are coated by thin wetting films of an immiscible liquid. The liquid-crystal subphase consisted of the nematic or isotropic phase of 4-octyl- 4;{'} -cyanobiphenyl (8CB), and the wetting film was formed by the fluorocarbon perfluoromethylcyclohexane (PFMC), a volatile liquid. The thickness of the wetting film was controlled by the temperature difference DeltaT(micro) between the sample and a reservoir of bulk PFMC, contained within the sealed sample cell. Phase information on the interfacial electron density profiles has been extracted from the interference between the scattering from the PFMC-vapor interface and the surface-induced smectic order of the 8CB subphase. The liquid-crystal side of the nematic-liquid (8CB-PFMC) interface is characterized by a density oscillation whose period corresponds to the smectic layer spacing and whose amplitude decays exponentially toward the underlying nematic subphase. The decay length xi of the smectic amplitude is independent of the PFMC film thickness but increases as the nematic-smectic- A transition temperature T(NA) is approached, in agreement with the longitudinal correlation length xi(parallel) proportional, variant(T-T(NA))(-0.7} for the smectic fluctuations in the bulk nematic. The results indicate that the homeotropic orientation of the 8CB molecules is preferred at the 8CB-PFMC interface and that the observed temperature dependence of the smectic layer growth is consistent with the critical adsorption mechanism. The observed DeltaT(micro) dependence of the PFMC film thickness, L proportional, variant(DeltaT(micro))(-1/3) , implies that PFMC completely wets the 8CB surface and is dominated by the nonretarded dispersion interactions between hydro- and fluorocarbons. The complete wetting behavior of PFMC is nearly independent of the degree of interfacial smectic order in the subphase.

Postassembly chemical modification of a highly ordered organosilane multilayer: new insights into the structure, bonding, and dynamics of self-assembling silane monolayers

Experimental evidence derived from a comprehensive study of a self-assembled organosilane multilayer film system undergoing a process of postassembly chemical modification that affects interlayer-located polar groups of the constituent molecules while preserving its overall molecular architecture allows a quantitative evaluation of both the degree of intralayer polymerization and that of interlayer covalent bonding of the silane headgroups in a highly ordered layer assembly of this type. The investigated system consists of a layer-by-layer assembled multilayer of a bifunctional n-alkyl silane with terminal alcohol group that is in situ converted, via a wet chemical oxidation process conducted on the entire multilayer, to the corresponding carboxylic acid function. A combined chemical-structural analysis of data furnished by four different techniques, Fourier transform infrared spectroscopy (FTIR), synchrotron X-ray scattering, X-ray photoelectron spectroscopy (XPS), and contact angle measurements, demonstrates that the highly ordered 3D molecular arrangement of the initial alcohol-silane multilayer stack is well preserved upon virtually quantitative conversion of the alcohol to carboxylic acid and the concomitant irreversible cleavage of interlayer covalent bonds. Thus, the correlation of quantitative chemical and structural data obtained from such unreacted and fully reacted film samples offers an unprecedented experimental framework within which it becomes possible to differentiate between intralayer and interlayer covalent bonding. In addition, the use of a sufficiently thick multilayer effectively eliminates the interfering contributions of the underlying silicon oxide substrate to both the X-ray scattering and XPS data. The present findings contribute a firm experimental basis to the elucidation of the self-assembly mechanism, the molecular organization, and the modes and dynamics of intra- and interlayer bonding prevailing in highly ordered organosilane films; with further implications for the rational exploitation of some of the unique options such supramolecular surface entities can offer in the advancement of a chemical nanofabrication methodology.

Wetting, mixing, and phase transitions in Langmuir-Gibbs films

Millimolar bulk concentrations of the surfactant cetyltrimethylammonium bromide (CTAB) induce spreading of alkanes, H(CH(2))(n)H (denoted C(n)) 12< or =n< or =21, on the water surface, which is not otherwise wet by these alkanes. The novel Langmuir-Gibbs film (LGF) formed is a liquidlike monolayer comprising both alkanes and CTAB tails. Upon cooling, an ordering transition occurs, yielding a hexagonally packed, quasi-2D crystal. For 11< or =n< or =17 this surface-frozen LGF is a crystalline monolayer. For 18< or =n< or =21 the LGF is a bilayer with a crystalline, pure-alkane, upper monolayer, and a liquidlike lower monolayer. The phase diagram and film structure were determined by x-ray, ellipsometry, and surface tension measurements. A thermodynamic theory accounts quantitatively for the observations.