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Langmuir is devoted to reporting new and original experimental and theoretical research of interest to chemists and chemical physicists in the fields of surface and colloid chemistry. Coverage includes such topics as micelles, visicles, emulsions, gels, surfacants, colloids, crystal growth, nucleation, liquid crystals, imaging spectroscopy, electro-chemistry, biological colloids & interfaces, biopolymers, nanostructures, multicomponent systems, and materials. In addition to these structures, Langmuir also carries articles on theory, simulation, modeling and experimental developments. Along with national award-winning lectures, Langmuir regularly features letters, articles, reviews, commentaries, notes, and special issues.

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American Chemical Society

Publications in this journal

• Electrically Conducting Nanopatterns Formed by Chemical e-Beam Lithography via Gold Nanoparticle Seeds

  Authors: Patrick A. Schaal, Astrid Besmehn, Eva Maynicke, Michael Noyong, Bernd Beschoten, Ulrich Simon

  Langmuir. 28(5):2448–2454.

• Silver-assisted colloidal synthesis of gold patchy particles with tunable patchiness

  Authors: Huixin Bao, Benjamin Butz, Zhou Zhou, Erdmann Spiecker, Martin Hartmann, Robin Klupp Taylor

  Langmuir.

  Patchy particles possessing heterogeneous surface composition show great promise as self-organizing building blocks for new classes of hierarchical functional structures. A major hurdle is thePatchy particles possessing heterogeneous surface composition show great promise as self-organizing building blocks for new classes of hierarchical functional structures. A major hurdle is the scalable synthesis of stable patches on nanosized core particles with arbitrarily defined patch number and coverage. So far, few methods have been reported which could be expected to meet these challenges. Recently we described the heterogeneous nucleation and growth of silver patches on silica nanospheres via a template free colloidal route. The patches produced, although tunable in size and number and showing interesting plasmon resonant properties, were rather unstable and degraded rapidly during attempts to process them further. In the present work, therefore, we set out to explore if related approaches can be employed to produce patchy particles involving gold, which is known to be more stable. The differences between typical patch precursors Ag⁺ and [AuCl_x(OH)_4-x]^- and their respective interactions with amorphous silica make this a significant challenge. We show that preformed small silver patches in addition to the presence of a reducing agent are necessary for the formation of gold patches conformal to the silica nanosphere surface. Systematic study of the process parameters and their influence on the patchiness as well as in-depth analytical TEM investigation of the patch composition reveal that patches spread over the silica surface via a cycle of galvanic dissolution and redeposition of silver.The resulting gold patchy particles remain stable during subsequent storage or washing and display tunable plasmon resonances within the visible and near-IR spectrum.

• Enhanced Optical Properties of Graphene Oxide – Au Nanocrystal Composites

  Authors: Y.H. Lee, L. Polavarapu, Nengyue Gao, Peiyan Yuan, Qing-Hua Xu

  Langmuir.

• Proton and calcium-gated ionic mesochannels: phosphate-bearing polymer brushes hosted in mesoporous thin films as biomimetic interfacial architectures

  Authors: A. Brunsen, C. Díaz, L. I. Pietrasanta, B. Yameen, M. Ceolín, G. J.A.A. Soler-Illia, O. Azzaroni

  Langmuir.

  Rational construction of interfaces based on multi-component responsive systems in which molecular transport is mediated by structures of nanoscale dimensions has become a very fertile research areaRational construction of interfaces based on multi-component responsive systems in which molecular transport is mediated by structures of nanoscale dimensions has become a very fertile research area in biomimetic supramolecular chemistry. Herein, we describe the creation of hybrid mesostructured interfaces with reversible gate-like transport properties that can be controlled by chemical inputs, such as protons or calcium ions. This was accomplished by taking advantage of the surface-initiated polymerization of 2-(methacryloyloxy)ethyl phosphate (MEP) monomer units into and onto mesoporous silica thin films. In this way, phosphate-bearing polymer brushes were used as “gatekeepers” located not only on the outer surface of mesoporous thin films but also in the inner environment of the porous scaffold. Pore-confined PMEP brushes respond to the external triggering chemical signals not only by altering their physicochemical properties but also by switching the transport properties of the mesoporous film. The ion-gate response/operation was based on the protonation and/or chelation of phosphate monomer units in which the polymer brush works as an off-on switch in response to the presence of protons or Ca2+ ions. The hybrid meso-architectured interface and their functional features were studied by a combination of experimental techniques including ellipso-porosimetry, cyclic voltammetry, X-ray reflectivity, grazing incidence small-angle X-ray scattering, X-ray photoelectron spectroscopy, and in situ atomic force microscopy. In this context, we believe that the integration of stimuli-responsive polymer brushes into nanoscopic supramolecular architectures would provide new routes toward multifunctional biomimetic nanosystems displaying transport properties similar to those encountered in biological ligand-gated ion channels.

• Controlled Growth of Polyaniline Fractals on HOPG through Potentiodynamic Electropolymerization

  Authors: Dhrubajyoti Bhattacharjyaa, Indrajit Mukhopadhyay

  Langmuir.

  Polyaniline (PANI) in fractal dimension has been electrodeposited reproducibly on highly oriented pyrolytic graphite (HOPG) from 0.2 M aniline in 1 M aqueous HCl solution by potentiodynamic sweepingPolyaniline (PANI) in fractal dimension has been electrodeposited reproducibly on highly oriented pyrolytic graphite (HOPG) from 0.2 M aniline in 1 M aqueous HCl solution by potentiodynamic sweeping in the range of -0.2 to 0.76 V Vs. Ag/AgCl at room temperature. Fractal growth of PANI dendrimers is affected by diffusion limited polymerization (DLP) at a sweep rate of 15 mV/s for 20 cycles. To the best of our knowledge this type of dendrimeric PANI is prepared for the first time on such large area HOPG substrate by electrochemical technique using rather simple cell set-up. The fractal dimension is found to vary from 1.4 to 1.9 with the duration of electropolymerization. The terminal oxidation potential and the diverse surface anisotropy of the HOPG surface are found to be crucial factors in controlling the growth of such PANI fractals.

• Tunable 1, 2 and 3 Dimensional Self-assembles from an Acceptor-Donor Fullerene- N,N-dimethylaminoazobenzene Dyad: Interfacial Geometry and Temporal Evolution

  Authors: K. Senthil Kumar, Archita Patnaik

  Langmuir.

• Nanoscale Water Condensation on Click-Functionalized Self-Assembled Monolayers

  Authors: M. James, S. Ciampi, T. A. Darwish, T. L. Hanley, S. O. Sylvester, J. J. Gooding

  Langmuir. 27:10753.

• Elemental Depth Profiling of Fluoridated Hydroxyapatite: Saving Your Dentition by the Skin of Your Teeth?

  Authors: Frank Müller, Christian Zeitz, Hubert Mantz, Karl-Heinz Ehses, Flavio Soldera, Jörg Schmauch, Matthias Hannig, Stefan Hüfner, Karin Jacobs

  Langmuir. 26(24):18750.

  Structural and chemical changes that arise from fluoridation of hydroxyapatite (Ca5(PO4)3OH or “HAp”), as representing the synthetic counterpart of tooth enamel, are investigated by X-rayStructural and chemical changes that arise from fluoridation of hydroxyapatite (Ca5(PO4)3OH or “HAp”), as representing the synthetic counterpart of tooth enamel, are investigated by X-ray photoelectron spectroscopy (XPS). Elemental depth profiles with a depth resolution on the nanometer scale were determined to reveal the effect of fluoridation in neutral (pH = 6.2) and acidic agents (pH = 4.2). With respect to the chemical composition and the crystal structure, XPS depth profiling reveals different effects of the two treatments. In both cases, however, the fluoridation affects the surface only on the nanometer scale, which is in contrast to recent literature with respect to XPS analysis on dental fluoridation, where depth profiles of F extending to several micrometers were reported. In addition to the elemental depth profiles, as published in various other studies, we also present quantitative depth profiles of the compounds CaF2, Ca(OH)2, and fluorapatite (FAp) that were recently proposed by a three-layer model concerning the fluoridation of HAp in an acidic agent. The analysis of our experimental data exactly reproduces the structural order of this model, however, on a scale that differs by nearly 2 orders of magnitude from previous predictions. The results also reveal that the amount of Ca(OH)2 and FAp is small compared to that of CaF2. Therefore, it has to be asked whether such narrow Ca(OH)2 and FAp layers really can act as protective layers for the enamel.

• Relating interactions of dye molecules with chitosan to adsorption kinetic data

  Authors: George Z. Kyzas, Margaritis Kostoglou, Nikolaos K. Lazaridis

  Langmuir. 26(12):9617-9626.

  The scope of the present work is the study of the adsorption behaviour of two dyes of different nature/class on several chitosan derivatives. The adsorbents used were grafted with differentThe scope of the present work is the study of the adsorption behaviour of two dyes of different nature/class on several chitosan derivatives. The adsorbents used were grafted with different functional groups (carboxyl, amido, sulfonate, N-Vinylimidazole) to increase their adsorption capacity and cross-linked to improve their mechanical resistance. This complete kinetic analysis was realized at 25, 45, and 65 °C to observe the effect of temperature on adsorption rates for each adsorbent-adsorbate system. Activated carbon was also used as an adsorbent for reference/comparison. The experimental equilibrium data were successfully fitted to the Langmuir-Freundlich (L-F) isotherms, presenting high correlation coefficients (R2~0.998). A detailed pore-surface diffusion with local adsorption-desorption model has been developed to describe the adsorption kinetics in chitosan adsorbents. The existence of kinetic data in several temperatures assists in recognizing the diffusion mechanism in the adsorbent particles. The findings on diffusion mechanisms and the corresponding coefficients, from using the model to match the experimental data, are compatible with the expected adsorbent-dye interactions based on their chemical structure.

• In situ formed "weakly ligated/labile ligand" iridium(0) nanoparticles and aggregates as catalysts for the complete hydrogenation of neat benzene at room temperature and mild pressures

  Authors: E. Bayram, M. Zahmakıran, S. Özkar, R. G. Finke

  Langmuir. 26:12455.

  Weakly ligated/labile ligand" nanoparticles, that is nanoparticles where only weakly coordinated ligands plus the desired catalytic reactants are present, are of fundamental interest. DescribedWeakly ligated/labile ligand" nanoparticles, that is nanoparticles where only weakly coordinated ligands plus the desired catalytic reactants are present, are of fundamental interest. Described herein is a catalyst system for benzene hydrogenation to cyclohexane consisting of "weakly ligated/labile ligand" Ir(0) nanoparticles and aggregates plus dry-HCl formed formed in situ from commercially available [(1,5-COD)IrCl](2) plus 40 +/- 1 psig (similar to 2.7 atm) H(2) at 22 +/- 0.1 degrees C. Multiple control and other experiments reveal the following points: (i) that this catalyst system is quite active with a TOF (turnover frequency) of 25 h(-1) and TTO (total turnovers) of 5250; (ii) that the BE(4)(-) and PF(6)(-) iridium salt precursors, [(1,5-COD)Ir(CH(3)CN)(2)]BF(4) and [(1,5-COD)Ir(CH(3)CN)(2)]PF(6), yield inferior catalysts; (iii) that iridium black with or without added, preformed HCl cannot achieve the Tor, of 25 h(-1) of the in situ formed Ir(0)/dry-HCl catalyst. However and importantly, CS(2) poisoning experiments yield the same activity per active iridium atom for both the Ir(0)/dry-HCl and Ir black/no-HCl catalysts (12.5 h(-1) Ir(-1)), but reveal that the Ir(0)/dry-HCl system is 10-fold more dispersed compared to the Ir(0) black catalyst. The simple but important and key result is that "weakly ligated/labile ligand" Ir(0) nanoparticles and aggregates have been made in situ as demonstrated by the fact that they have identical, per exposed Ir(0) activity within experimental error to (40) black and that they have no possible ligands other than those desired for the catalysis (benzene and H(2)) plus the at best poor ligand HCl. As expected, the in situ catalyst is poorly stabilized, exhibiting only 60% of its initial activity in a second run of benzene hydrogenation and resulting in bulk metal precipitation. However, stabilization of the Ir(0) nanoparticles with a ca. 2-fold higher catalytic activity and somewhat longer lifetime for the complete hydrogenation of benzene was accomplished by supporting the Ir(0) nanoparticles onto zeolite-Y (TOE of 47 h(-1) and 8600 TTO under otherwise identical conditions). Also reported is the interesting result that Cl (present as Proton Sponge center dot H+Cl(-)) completely poisons benzene hydrogenation catalysis, but not the easier cyclohexene hydrogenation catalysis under otherwise the some conditions, results that suggest different active sites for these ostensibly related hydrogenation reaction. The results suggest that synthetic routes to "weakly ligated/labile ligand" nanoparticles exhibiting improved catalytic performance nee is an important goal worthy of additional effort.

• Effects of Sample Preparation on Bacterial Colonization of Polymers.

  Authors: Diana N Zeiger, Christopher M Stafford, Yajun Cheng, Stefan D Leigh, Sheng Lin-Gibson, Nancy J Lin

  Langmuir : the ACS journal of surfaces and colloids.

  Characterization of materials developed for medical usage frequently includes studies in which the materials are inoculated with bacteria in order to assess bacterial colonization and biofilmCharacterization of materials developed for medical usage frequently includes studies in which the materials are inoculated with bacteria in order to assess bacterial colonization and biofilm formation. Observed differences in bacterial growth are typically considered to be due to the material or the incubation conditions. To our knowledge, the method used to prepare the materials has generally not been considered with regard to its influence on bacterial colonization. The objective of this study was to determine the effects that various preparation methods exert on bacterial colonization of polymer disks. Polymer disks of the same dimethacrylate composition were photopolymerized: (1) between untreated glass slides, (2) between polyester release film, (3) between glass slides treated with an alkyl silane, (4) between glass slides treated with a perfluorinated silane, or (5) with one free surface in an argon-purged chamber. Surface chemistry was quantified using X-ray photoelectron spectroscopy, hydrophobicity was assessed by water contact angle, and topography was characterized using atomic force microscopy. The disks were inoculated with Streptococcus mutans for 4 h, fixed, and visualized using confocal laser scanning microscopy. Differences among all groups were found with regard to surface chemistry, hydrophobicity, topography, and bacteria morphology, density, and coverage, indicating that the method of sample preparation strongly affects both the surface properties and the initial bacterial colonization. Polymerization on untreated slides was selected as the preferred method of preparation due to minimal material transfer to the polymer and consistent, reproducible bacterial colonization.

• The Differences in Surfactant Adsorption on Carbon Nanotubes and Their Bundles.

  Authors: Panagiotis Angelikopoulos, Henry Bock

  Langmuir : the ACS journal of surfaces and colloids.

  Dissipative particle dynamics simulations of a mesoscale model are performed to investigate the concentration dependence of surfactant adsorption on small-diameter carbon nanotubes and their bundles.Dissipative particle dynamics simulations of a mesoscale model are performed to investigate the concentration dependence of surfactant adsorption on small-diameter carbon nanotubes and their bundles. Adsorption is found to follow fundamentally different mechanisms in the two cases because of the heterogeneity of the bundle surface and the difference in diameter of bundles compared to that of individual tubes. Whereas aggregation dominates adsorption on individual tubes, on bundles it is largely a Langmuir-type process. High adsorption energy sites on the outer surface of bundles, where surfactant molecules can interact with two tubes simultaneously, dominate at low coverage. They also cause adsorption on bundles to become significant well before adsorption on individual tubes starts. The difference in the adsorption mechanisms leads to a crossover point at higher concentrations, where the adsorbed amount per surface area on individual tubes becomes larger than that for the bundles.

• Elastic, Adhesive, and Charge Transport Properties of a Metal-Molecule-Metal Junction: The Role of Molecular Orientation, Order, and Coverage.

  Authors: Frank W Delrio, Kristen L Steffens, Cherno Jaye, Daniel A Fischer, Robert F Cook

  Langmuir : the ACS journal of surfaces and colloids.

  The elastic, adhesive, and charge transport properties of a metal-molecule-metal junction were studied via conducting-probe atomic force microscopy (AFM) and correlated with molecular structure byThe elastic, adhesive, and charge transport properties of a metal-molecule-metal junction were studied via conducting-probe atomic force microscopy (AFM) and correlated with molecular structure by near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The junctions consisted of Co-Cr-coated AFM tips in contact with methyl-terminated alkanethiols (CH(3)(CH(2))(n-1)SH, denoted by C(n), where n is the number of carbons in the molecular chain) on Au substrates. AFM contact data were analyzed with the Derjaguin-Muller-Toporov contact model, modified by a first-order elastic perturbation method to account for substrate effects, and a parabolic tunneling model, appropriate for a metal-insulator-metal junction in which the thickness of the insulator is comparable to the Fermi wavelength of the conducting electrons. NEXAFS carbon K-edge spectra were used to compute the dichroic ratio R(I) for each film, which provided a quantitative measure of the molecular structure as a function of n. As n decreased from 18 to 5, there was a change in the molecular phase from crystalline to amorphous (R(I) --> 0) and loss of surface coverage, and as a result, the work of adhesion w increased from 82.8 mJ m(-2) to 168.3 mJ m(-2), the Young's modulus of the film E(film) decreased from 1.0 to 0.15 GPa, and the tunneling barrier height varphi(0) - E(F) decreased from 2.4 to 2.1 eV. For all n, the barrier thickness t decreased for small applied loads F and remained constant at approximately 2.2 nm for large F. The change in behavior was explained by the presence of two insulating layers: an oxide layer on the Co-Cr tip, and the alkanethiol monolayer on the Au surface. X-ray photoelectron spectroscopy confirmed the presence of an oxide layer on the Co-Cr tip, and by performing high-resolution region scans through the film, the thickness of the oxide layer t(oxide) was found to be between 1.9 and 3.9 nm. Finally, it was shown that varphi(0) - E(F) is strain-dependent, and the strain at which the film is completely displaced from under the tip is -0.17 for all values of n.

• Formation Mechanism and Mechanics of Dip-Pen Nanolithography Using Molecular Dynamics.

  Authors: Cheng-Da Wu, Te-Hua Fang, Jen-Fin Lin

  Langmuir : the ACS journal of surfaces and colloids.

  Molecular dynamics simulations are used to investigate the mechanisms of molecular transference, pattern formation, and mechanical behavior in the dip-pen nanolithography (DPN) process. The effectsMolecular dynamics simulations are used to investigate the mechanisms of molecular transference, pattern formation, and mechanical behavior in the dip-pen nanolithography (DPN) process. The effects of deposition temperature were studied using molecular trajectories, the meniscus characteristic, surface absorbed energy, and pattern formation analysis. At the first transferred stage (at the initial indentation depth), the conformation of SAM molecules lies almost on the substrate surface. The molecules start to stand on the substrate due to the pull and drag forces at the second transferred stage (after the tip is pulled up). According to the absorbed energy behavior, the second transferred stage has larger transferred amounts and the transfer rate is strongly related to temperature. When molecules were deposited at low temperature (e.g., room temperature), the pattern shape was more highly concentrated. The pattern shape at high temperatures expanded and the area increased because of good molecular diffusion.

• Cysteine and pH-Responsive Hydrogel Based on a Saccharide Derivative with an Aldehyde Group.

  Authors: Qun Chen, Yuxia Lv, Deqing Zhang, Guanxin Zhang, Chenyang Liu, Daoben Zhu

  Langmuir : the ACS journal of surfaces and colloids.

  For the development of chemoresponsive hydrogel, a new saccharide-derived hydrogelator 1 [methyl-4,6-O-(4'-aldehydephenylidene)-alpha-d-glucopyranoside] was synthesized and characterized. TheFor the development of chemoresponsive hydrogel, a new saccharide-derived hydrogelator 1 [methyl-4,6-O-(4'-aldehydephenylidene)-alpha-d-glucopyranoside] was synthesized and characterized. The resulting hydrogel not only shows thermal responsiveness but also exhibits cysteine and pH responsiveness due to the presence of aldehyde and acetal groups in 1. The release of guest molecules from the hydrogel of 1 was examined by employing compound 2 (fluorescein disodium) and compound 3 [methyl-4,6-O-(2'-pyrenylidene)-alpha-d-glucopyranoside] as guest molecules. The results indicate that the release of compound 2 from the hydrogel of 1 is controlled by the diffusion process, while the release of compound 3 from the hydrogel of 1 is facilitated by the presence of cysteine. These results demonstrate (1) the proof of principle to design chemoresponsive gels by incorporating reactive groups into the gelators and (2) it is possible to stimulate the release of guest molecules from hydrogels upon addition of reactive chemicals by manipulating the intermolecular interactions among the hydrogelators and guest molecules.

• Multifunctional Superparamagnetic Janus Particles.

  Authors: Kai P Yuet, Dae Kun Hwang, Ramin Haghgooie, Patrick S Doyle

  Langmuir : the ACS journal of surfaces and colloids.

  In this study, we report the microfluidic-based synthesis of a multifunctional Janus hydrogel particle with anisotropic superparamagnetic properties and chemical composition for the bottom-upIn this study, we report the microfluidic-based synthesis of a multifunctional Janus hydrogel particle with anisotropic superparamagnetic properties and chemical composition for the bottom-up assembly of hydrogel superstructures. In a uniform magnetic field, the resulting Janus magnetic particles fabricated in the present method exhibit chainlike or meshlike superstructure forms, the complexity of which can be simply modulated by particle density and composition. This controllable field-driven assembly of the particles can be potentially used as building blocks to construct targeted superstructures for tissue engineering. More importantly, we demonstrated that this method also shows the ability to generate multifunctional Janus particles with great design flexibilities: (a) direct encapsulation and precise spatial distribution of biological substance and (b) selective surface functionalization in a particle. Although these monodisperse particles find immediate use in tissue engineering, their ability to self-assemble with tunable anisotropic configurations makes them an intriguing material for several exciting areas of research such as photonic crystals, novel microelectronic architecture, and sensing.

• Particulate Templates and Ordered Liquid Bridge Networks in Evaporative Lithography.

  Authors: Ivan U Vakarelsk, Jin W Kwek, Xiaosong Tang, Sean J O'Shea, Derek Y C Chan

  Langmuir : the ACS journal of surfaces and colloids.

  We investigate the properties of latex particle templates required to optimize the development of ordered liquid bridge networks in evaporative lithography. These networks are key precursors in theWe investigate the properties of latex particle templates required to optimize the development of ordered liquid bridge networks in evaporative lithography. These networks are key precursors in the assembly of solutions of conducting nanoparticles into large, optically transparent, and conducting microwire networks on substrates (Vakarelski, I. U.; Chan, D. Y. C.; Nonoguchi, T.; Shinto, H.; Higashitani, K. Phys. Rev. Lett., 2009, 102, 058303). An appropriate combination of heat treatment and oxygen plasma etching of a close-packed latex particle monolayer is shown to create open-spaced particle templates which facilitates the formation of ordered fully connected liquid bridge networks that are critical to the formation of ordered microwire networks. Similar results can also be achieved if non-close-packed latex particle templates with square or honeycomb geometries are used. The present results have important implications for the development of the particulate templates to control the morphology of functional microwire networks by evaporative lithography.

• Imbibition in Porous Membranes of Complex Shape: Quasi-stationary Flow in Thin Rectangular Segments.

  Authors: Sergio Mendez, Erin M Fenton, Gil R Gallegos, Dimiter N Petsev, Scott S Sibbett, Howard A Stone, Yi Zhang, Gabriel P López

  Langmuir : the ACS journal of surfaces and colloids.

  The sustained liquid flow of a typical lateral flow assay can be mimicked by two-dimensional shaped, thin porous membranes, specifically rectangular membranes appended to circular sectors. InThe sustained liquid flow of a typical lateral flow assay can be mimicked by two-dimensional shaped, thin porous membranes, specifically rectangular membranes appended to circular sectors. In designing these fan-shaped devices, we have been aided by analytical equations and finite-element simulations. We show both mathematically and experimentally how a continuous increase in unwetted pore volume causes a deviation from traditional imbibition, and leads to quasi-stationary flow in the rectangular element. These results are both theoretically and practically important because they indicate how medical diagnostic test strips may be fabricated without incorporating an absorbent pad.

• Relationship between Microstructure, Dynamics, and Rheology in Polymer-Bridging Colloidal Gels.

  Authors: Katie Pickrahn, Bharath Rajaram, Ali Mohraz

  Langmuir : the ACS journal of surfaces and colloids.

  We investigate the link between the microstructure, dynamics, and rheological properties in dense (varphi = 0.3) mixtures of charge-stabilized colloidal silica and oppositely charged poly(ethyleneWe investigate the link between the microstructure, dynamics, and rheological properties in dense (varphi = 0.3) mixtures of charge-stabilized colloidal silica and oppositely charged poly(ethylene imine) polymer in a mixed DMSO/H(2)O solvent. Over a finite range of polymer concentrations, the addition of polymer results in the formation of sample-spanning, self-supporting gel networks. As the polymer concentration is increased, a reentrant rheological transition is observed where the gel's elastic modulus and yield stress initially increase and subsequently drop. The dynamic and microstructural changes associated with this transition are resolved using quantitative confocal microscopy. Within the initial regime, a biphasic system consisting of a mixture of arrested and diffusive particles is observed. We segregate the particles with high accuracy into mobile and arrested populations based on their dynamics. The addition of polymer in this regime systematically decreases the proportion of free particles, until all the particles are arrested. Concurrent with this transition, the elastic modulus and yield stress go through their corresponding maxima. However, over the range of polymer concentrations studied, the reentrant transition to weak gels is not captured by the particle dynamics but is instead accompanied by subtle changes in the microstructure of the arrested phase. We discuss two possible scenarios for this behavior in view of the strength of interparticle bonds.

• Esterification of Self-Assembled Carboxylic-Acid-Terminated Thiol Monolayers in Acid Environment: A Time-Dependent Study.

  Authors: Annica Myrskog, Henrik Anderson, Teodor Aastrup, Björn Ingemarsson, Bo Liedberg

  Langmuir : the ACS journal of surfaces and colloids.

  This contribution reports on the influence of acids on the quality of carboxylic-acid-terminated self-assembled monolayers (SAMs) on gold prepared from ethanolic solution of HS-(CH(2))(15)-COOH andThis contribution reports on the influence of acids on the quality of carboxylic-acid-terminated self-assembled monolayers (SAMs) on gold prepared from ethanolic solution of HS-(CH(2))(15)-COOH and HS-(CH(2))(11)CONH-(EG)(6)CH(2)-COOH. Null ellipsometry, contact angle goniometry, and infrared reflection-absorption spectroscopy are used to monitor the physical and chemical changes occurring within the SAMs upon acid post treatment; after incubation with acids present in the solution; and after incubation in aged acid containing solutions. The presence of acid has a positive effect on the crystallinity, packing, and orientation of the supporting alkyl and ethylene glycol subunits of the SAM. Our studies also confirm previous findings stating that the carboxylic groups are rapidly converted into ethyl ester groups in the presence of hydrochloric acid in the incubation solution. It is also evident that the conversion occurs in the presence of the weaker acid, acetic acid, although at a much slower rate than that for hydrochloric acid. This is a new observation that has not been reported on before. The physical and chemical characterization is also complemented with a functional bioaffinity study. The functional evaluation revealed that the present model system was surprisingly insensitive to the degree of esterification of the carboxylic acid groups, but that 4 weeks of storage of the two investigated thiols in hydrochloric acid containing ethanol resulted in SAMs that were completely inactive with respect to immobilization and subsequent binding of the antigen. It was encouraging to note that the nonspecific binding of both antigen and antibody was extremely low on the two SAMs, regardless of the relative amount of ethyl esters on the surface.

• Rheological Behavior of Acid-Swellable Cationic Copolymer Latexes.

  Authors: Beng H Tan, Kam C Tam, Damien Dupin, Steven P Armes

  Langmuir : the ACS journal of surfaces and colloids.

  2-Vinylpyridine (2VP) was copolymerized with four different cross-linker densities ranging from 0.05 to 0.31 wt % divinylbenzene (DVB) via aqueous emulsion polymerization to produce a series of2-Vinylpyridine (2VP) was copolymerized with four different cross-linker densities ranging from 0.05 to 0.31 wt % divinylbenzene (DVB) via aqueous emulsion polymerization to produce a series of submicrometer-sized, lightly cross-linked P2VP latexes. Protonation of the 2VP residues leads to a latex-to-microgel transition due to interchain electrostatic repulsion, as confirmed by dynamic light scattering. The DVB content of these pH-responsive copolymer particles strongly affects their rheological behavior. The particle size and viscosity of the swollen cationic microgels exhibit a maximum at approximately 0.11 wt % DVB. Static light scattering results confirm this density as the minimum amount of DVB required to ensure that all P2VP chains are cross-linked (i.e. that there is no soluble fraction), thus allowing optimal swelling of the microgels. Viscosity studies shows that the solution viscosity of a P2VP microgel at low pH follows two models, depending on its concentration. For volume fractions below 0.30, the P2VP microgels behave as hard spheres, as predicted by the Batchelor equation. For more concentrated P2VP microgels (volume fractions above 0.30), the rheological behavior can be predicted using the Krieger-Dougherty model for strong particle-particle interactions; thus, this semiempirical approach provides a useful description of the aqueous solution behavior of microgel.

• Patterned Organosilane Monolayers as Lyophobic-Lyophilic Guiding Templates in Surface Self-Assembly: Monolayer Self-Assembly versus Wetting-Driven Self-Assembly (dagger).

  Authors: Assaf Zeira, Devasish Chowdhury, Stephanie Hoeppener, Shantang Liu, Jonathan Berson, Sidney R Cohen, Rivka Maoz, Jacob Sagiv

  Langmuir : the ACS journal of surfaces and colloids.

  Monolayer self-assembly (MSA) was discovered owing to the spectacular liquid repellency (lyophobicity) characteristic of typical self-assembling monolayers of long tail amphiphiles, which facilitatesMonolayer self-assembly (MSA) was discovered owing to the spectacular liquid repellency (lyophobicity) characteristic of typical self-assembling monolayers of long tail amphiphiles, which facilitates a straightforward visualization of the MSA process without the need of any sophisticated analytical equipment. It is this remarkable property that allows precise control of the self-assembly of discrete, well-defined monolayers, and it was the alternation of lyophobicity and lyophilicity (liquid affinity) in a system of monolayer-forming bifunctional organosilanes that allowed the extension of the principle of MSA to the layer-by-layer self-assembly of planed multilayers. On this basis, the possibility of generating at will patterned monolayer surfaces with lyophobic and lyophilic regions paves the way to the engineering of molecular templates for site-defined deposition of materials on a surface via either precise MSA or wetting-driven self-assembly (WDSA), namely, the selective retention of a liquid repelled by the lyophobic regions of the pattern on its lyophilic sites. Highly ordered organosilane monolayer and thicker layer-by-layer assembled structures are shown to be ideally suited for this purpose. Examples are given of novel WDSA and MSA processes, such as guided deposition by WDSA on lyophobic-lyophilic monolayer and bilayer template patterns at elevated temperatures, from melts and solutions that solidify upon cooling to the ambient temperature, and the possible extension of constructive nanolithography to thicker layer-by-layer assembled films, which paves the way to three-dimensional (3D) template patterns made of readily available monofunctional n-alkyl silanes only. It is further shown how WDSA may contribute to MSA on nanoscale template features as well as how combined MSA and WDSA modes of surface assembly may lead to composite surface architectures exhibiting rather surprising new properties. Finally, a critical evaluation is offered of the scope, advantages, and limitations of MSA and WDSA in the bottom-up fabrication of surface structures on variable length scales from nano to macro.

• Dispersion and Film-Forming Properties of Poly(acrylic acid)-Stabilized Carbon Nanotubes.

  Authors: Karell Saint-Aubin, Philippe Poulin, Hassan Saadaoui, Maryse Maugey, Cécile Zakri

  Langmuir : the ACS journal of surfaces and colloids.

  We present a detailed study of the influence of pH on the dispersion and film-forming properties of poly(acrylic acid)-stabilized carbon nanotubes. Poly(acrylic acid) (PAA) is a weak polyelectrolyte,We present a detailed study of the influence of pH on the dispersion and film-forming properties of poly(acrylic acid)-stabilized carbon nanotubes. Poly(acrylic acid) (PAA) is a weak polyelectrolyte, with a pH-responsive behavior in aqueous solution. We obtain quantitative UV-visible measurements to show that the amount of polyelectrolyte in optimal pH conditions is weak, showing a good efficiency of the polymer as a carbon nanotube dispersing agent. The best dispersion conditions are achieved at pH 5, a value close to the pK(a) of PAA. Apart from this tenuous pH value, the PAA is not efficient at stabilizing nanotubes and atomic force microscopy allows us to explain the delicate balance between the PAA adsorption and the suspension stability. This study finally permits optimal conditions for making homogeneous and conductive composite films to be determined.

• Thermal/Plasma-Driven Reversible Wettability Switching of a Bare Gold Film on a Poly(dimethylsiloxane) Surface by Electroless Plating.

  Authors: Jian Wu, Hai-Jing Bai, Xian-Bo Zhang, Jing-Juan Xu, Hong-Yuan Chen

  Langmuir : the ACS journal of surfaces and colloids.

  We report an approach for fabricating a tunable wettability surface by electroless gold plating on poly(dimethylsiloxane) (PDMS). A two-layer structured gold film with a tight layer and a loose layerWe report an approach for fabricating a tunable wettability surface by electroless gold plating on poly(dimethylsiloxane) (PDMS). A two-layer structured gold film with a tight layer and a loose layer can be obtained on the surface of a PDMS chip when the PDMS chip is immersed in a gold plating solution at 30 degrees C for 4 h. Its wettability can be rapidly switched between superhydrophilicity and superhydrophobicity by plasma and heat treatments without any self-assembled monolayer, and the superhydrophobicity can be even changed from the gecko-foot-hair-like character to the lotus-leaf-like character. Benefiting from the various wettabilities of the prepared gold/PDMS composites, protein patterning is successfully achieved on a patterned superhydrophobic/superhydrophilic gold/PDMS composite; a superhydrophobic needle for transferring supersmall water droplets (1 muL) to a superhydrophobic surface is successfully fabricated.

• Influence of the Vesicular Bilayer Structure on the Sorption of Ethylbenzyl Alcohol.

  Authors: Pieter Saveyn, Joke De Geeter, Davy Sinnaeve, Paul Van der Meeren, José C Martins

  Langmuir : the ACS journal of surfaces and colloids.

  The influence of the physicochemical properties of the vesicular bilayer on the sorption of poorly water soluble compounds was investigated with pulsed field gradient (1)H nuclear magnetic resonanceThe influence of the physicochemical properties of the vesicular bilayer on the sorption of poorly water soluble compounds was investigated with pulsed field gradient (1)H nuclear magnetic resonance (PFG-NMR) for the case of phosphatidylcholine and dioctadecyldimethylammonium bromide (DODAB), using 4-ethylbenzyl alcohol as a model compound. Hereby, the effect of bilayer thickness at a constant physicochemical state was studied using a range of phosphatidylcholines of varying chain lengths, whereas DODAB was preferred to check the influence of the bilayer physicochemical state since this cationic lipid is characterized by three different states within the studied temperature range. When the phospholipid alkyl chain length was changed, no differences were observed in the sorption which was linked to the surface-mediated sorption. On the other hand, when the chemical composition was preserved but the temperature and thus the physical state of the bilayer were changed, the sorption in dioctadecyldimethylammonium bromide (DODAB) vesicles changed dramatically. From those experiments, a strong relationship between the ordering of the surfactant molecules and the sorption can be assumed.

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