Salt effect on microstructures in cationic gemini surfactant solutions as studied by dynamic light scattering

A cationic gemini surfactant, dodecanediyl-1,12-bis(dodecyldiethylammonium bromide) (C12C12C12(Et)), in aqueous solutions with varying NaBr concentration was studied by dynamic light scattering (DLS). As a comparison, its single-chained counterpart, dodecyl triethylammonium bromide (DTEAB), was also investigated under the same conditions. Similar to the case of a polyelectrolyte, C12C12C12(Et) underwent a typical "ordinary-to-extraordinary (o-e) transition" with decreasing salt concentration to zero. At higher salt concentration, a single relaxation mode, corresponding to the diffusion of regular micelles, was observed. While in the "extraordinary regime", DLS detected two characteristic relaxation modes with the values of the diffusion coefficient being different by at least 2 orders. The fast mode was consistent with the polyion-small ion coupled-mode theories, as well as the direct polyion-polyion repulsion interactions. Because the slow mode disappeared at elevated salt concentrations and generated negligible scattered intensity, we attributed it to multimacroion domains.     

Microheterogeneity Domains in Cu—Pb Melts

The kinematic viscosites of chemically heterogeneous Cu—Pb melts were studied in to determine the temperature and concentration limits of the domains of their existence. Experiments are performed by heating and cooling samples in the temperature interval of 1050–1200°C. A quasi-chemical approximation of the irregular solution theory is used to consider the heterogeneous distribution of Cu and Pb atoms, and the short-range order parameter and characteristic enthalpy of mixing are estimated.     

From the X-ray compact structure to the elongated form of the full-length MMP-2 enzyme in solution: a molecular dynamics study

Current understanding on the collagenolytic activity performed by the MMPs assumes some degree of relative motion between the catalytic and the hemopexin-like domains of the enzyme. However, all the crystal structures available for the full-length enzymes display a compact arrangement of the protein domains. Herein, we employ Molecular Dynamics simulations to investigate the structure of the full-length MMP-2 enzyme in aqueous solution. This simulation, together with previous experimental results that have been obtained very recently for the MMP-9 and MMP-12 enzymes, gives strong support to the hypothesis that the interdomain dynamics of the MMP enzymes in solution can result in a manifold of conformations including some structures with a large interdomain separation. The simulation of MMP-2 provides also a detailed molecular picture of the structures involved in the transition from the compact X-ray arrangement to the extended form in solution. Such information could be helpful in future studies of the regulation and/or the collagenolytic activity of these important enzymes.     

Ammonium ionic liquid as modulator of the critical micelle concentration of ammonium surfactant at aqueous solution: conductimetric and dynamic light scattering (DLS) studies

We report measurements of self aggregation in aqueous solution of an ionic liquid (IL), didecyl-dimethylammonium nitrate ([DDA][NO(3)]) and a surfactant hexadecyl-trimethylammonium bromide (CTAB) and of mixtures of these two salts. The electrical conductivity and dynamic light scattering (DLS) measurements were used for the characterization of the aggregation process. The conductivity measurements were performed at three temperatures. The critical micelle concentration (CMC) was determined at different temperatures and at different ratio of two salts. The effect of IL on the micellization of CTAB has been discussed. Our results suggest that organized structures formed by CTAB and [DDA][NO(3)] self assembly in domains of several hundred nanometers size. The micellar solubility of the salicylic acid in mixed salt aqueous solutions was determined to probe the physical properties of these assemblies. We have observed, that the micellar solubility enhancement was only slightly influenced by the nature of micelles present in aqueous solution. This proves that salicylic acid solubilization is enthalpy driven.     

Metal acetylacetonate domains grown on H-terminated porous silicon at room temperature and their specific I-V behavior

Porous silicon (PS) was incubated in an organic solution of metal acetylacetonates of Mn(acac)(3), Fe(acac)(3), Co(acac)(3), and Ni(acac)(2) (acac = MeCOCHCOMe) at room temperature. Crystal-like domains were found to be spontaneously self-assembled on PS surfaces by atomic force microscopy (AFM). Spectroscopic studies with attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) and X-ray photoelectron spectroscopy (XPS) revealed that the domains were grown from metal acetylacetonates. Current sensing atomic force microscopy (CSAFM) was used to measure the I-V curves of domains in nanoscale and specific step-jump currents on the manganese and cobalt acetylacetonate domains were surprisingly detected.     

A preliminary study on fabrication of nanoscale fibrous chitosan membranes in situ by biospecific degradation

A new approach for in situ fabrication of nanoscale fibrous chitosan membrane by biospecific degradation under physiological situation was studied. The chitosan binary blend membranes were fabricated by solvent casting of chitosan solution containing highly deacetylated chitosan (HDC) and moderately deacetylated chitosan (MDC) with different ratio. The biodegradation process was performed in PBS (pH 7.4) containing lysozyme at the temperature of 37 °C. Experimental results from weight loss, reducing sugar in surrounding media, FT-IR, X-ray diffraction, gel permeation chromatography (GPC) and SEM throughout the study showed that the biospecific degradation by lysozyme had removed MDC component selectively. When the ratio of MDC in the binary blend membranes amounted to 0.5, nanoscale domains of HDC and MDC were obtained, and thus a nanoscale fibrous structure was fabricated after biospecific degradation of MDC. This nanofibrous structure and the biospecific degradation of chitosan membranes can have potential advantages and interesting implications in tissue engineering and drug delivery.     

Formation of ordered self-assembled monolayers by adsorption of octylthiocyanates on Au(111)

Self-assembled monolayers (SAMs) were formed by the spontaneous adsorption of octythiocyanate (OTC) on Au(111) using both solution and ambient-pressure vapor deposition methods at room temperature and 50 degrees C. The surface structures and adsorption characteristics of the OTC SAMs on Au(111) were characterized by scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). The STM observation showed that OTC SAMs formed in solution at room temperature have unique surface structures including the formation of ordered and disordered domains, vacancy islands, and structural defects. Moreover, we revealed for the first time that the adsorption of OTC on Au(111) in solution at 50 degrees C led to the formation of SAMs containing small ordered domains, whereas the SAMs formed by vapor deposition at 50 degrees C had long-range ordered domains, which can be described as (radical3 x 2 radical19)R5 degrees structures. XPS measurements of the peaks in the S 2p and N 1s regions for the OTC SAMs showed that vapor deposition is the more effective method as compared to solution deposition for obtaining high-quality SAMs by adsorption of OTC on gold. The results obtained will be very useful in understanding the SAM formation of organic thiocyanates on gold surfaces.     

Directed assembly and separation of self-assembled monolayers via electrochemical processing

Separated domains of 1-dodecanethiolate were fabricated via solution displacement of preformed 1-adamantanethiolate self-assembled monolayers on Au{111}. Subsequently, the 1-adamantanethiolate domains were desorbed selectively, and the substrate was exposed to a 1-octanethiol solution, creating artificially separated self-assembled monolayers of 1-dodecanethiolate and 1-octanethiolate. The molecular order of each lattice type and the apparent height differences imaged with scanning tunneling microscopy and the two distinct cathodic peaks observed with cyclic voltammetry indicated distinct separated domains of each lattice type in the separated self-assembled monolayers. By manipulating the intermolecular interaction strengths of the patterned molecules, we are able to control the structure and properties of the separated self-assembled monolayers via the exploitation of competitive adsorption and the utilization of electrochemical processing, which can be extended to other self-assembly patterning techniques such as microdisplacement printing.     

Shear-Induced Phase Separation of Entangled Polymer Solutions: Formation of Optically Anisotropic Strings as Precursor Structures of Shish-Kebab

Shear-induced structures were investigated for both ultrahigh molecular weight atactic polystyrene (UHMWaPS) and linear polyethylene (UHMWPE) solutions, which were entangled but homogeneous without shear flow, as a function of shear rate ( ) or time after a step-up shear flow. For the PE solutions, the shear flow was imposed at 124 °C which is higher than the nominal melting temperature T_nm of the solution without shear flow. At sufficiently high shear rates both solutions commonly formed highly optically anisotropic string-like structures which are composed of a series of phase-separated domains interconnected by bundles of stretched chains and aligned along the flow direction. After cessation of the shear flow the string-like structures completely disappeared in the UHMWaPS solution, recovering a homogeneous solution, while the UHMWPE solution exhibited shish-kebab structure. The results reveal a new kinetic pathway for shish-kebab formation for the entangled crystallizable solution sheared at T > T_nm which involves first formation of the phase-separated string-like domains and subsequent crystallization into shish from the bundles of stretched chain and then kebab in the demixed domains composed of essentially random coils.     

Characterization of polymeric vesicles of poly(sodium 11-acrylamidoundecanoate) in water

A polysoap poly(sodium 11-acrylamidoundecanoate) was synthesized from sodium 11-acrylamidoundecanoate in water. The molecular weight of the polymer was determined by gel permeation chromatography and static light scattering techniques. Fluorescence probe studies in water have suggested the formation of hydrophobic domains within the same polymer chain. The microenvironment of the hydrophobic domains is highly ordered. The packing of the hydrocarbon chains in the hydrophobic domains formed by intra-chain association increases upon decrease of pH. The transmission electron micrograph revealed large vesicular aggregates in dilute aqueous solution. Temperature-dependent fluorescence anisotropy of the 1,6-diphenyl-1,3,5-hexatriene probe demonstrated stability of the vesicles.     

Dilute Solution Properties and Aggregation Behavior of Alternate Hetero-arm Copolymer Brushes

Alternate hetero-arm copolymer brushes were synthesized by free-radical copolymerizations of vinylbenzyl-terminated polystyrene macromonomers (PS-VB) with a methacryloyl-terminated poly(ethylene oxide) macromonomer (PEO-MA)/tin tetrachloride (SnCl(4): Lewis acid) complex. Dilute solution properties of such hetero-arm copolymer brushes were investigated by static and dynamic light scattering. Effective diffusion coefficients D(eff) for copolymer brushes possessing large aspect ratios showed almost constant values in the range of polymer concentrations 0-6x10(-3) g/cm(3). These results mean that copolymer brushes formed unimolecular structures even in a good solvent such as benzene. It was speculated from angular dependence measurements that in solution copolymer brushes take geometrically anisotropic conformations such as cylinders. To determine the phase-separated cylindrical domains of alternate copolymer brushes, we constructed large aggregates of copolymer brushes in water. Consequently, alternate copolymer brushes seemed to lead self-assemblies among phase-separated hydrophobic PS domains. Copyright 2001 Academic Press.     

Phase behavior and structure of liquid crystalline solutions of cellulose derivatives

Summary Structural and thermodynamic characteristics of liquid-crystalline solutions of four cellulose derivatives in a range of solvents were studied. Basic observations were made on these systems using polarized light microscopy, small angle light scattering, dilute solution and concentrated solution viscosities. The polymers studied include hydroxypropyl cellulose (HPC), cellulose acetate butyrate (CAB), ethyl cellulose (EC), and cellulose triacetate (CT). The formation of the liquid crystalline phase was shown to strongly depend on polymer concentration, solvent type and temperature. The critical volume fraction of polymer required to form the liquid crystal phase varied significantly as the solvent changed. The critical volume fraction decreased with increasing solvent acidity and polymer intrinsic viscosity in a given solvent. The breadth of the two phase region seems to decrease with increasing acidity. The liquid crystalline phase was in most cases determined to be cholesteric. In all cases positively birefringent cellulose derivatives form negative spherulitic domains. In one case, the negativity birefringent system (cellulose triacetate) formed positively birefringent spherulitic liquid crystalline domains. This is interpreted to mean the structure organizes itself by a tangential alignment of polymer chains within the domain. SALS measurements appear to detect domains and in some cases cholesteristic pitch.With 5 figures and 4 tables     

Chain polymerization of diacetylene compound multilayer films on the topmost surface initiated by a scanning tunneling microscope tip

Chain polymerizations of diacetylene compound multilayer films on graphite substrates were examined with a scanning tunneling microscope (STM) at the liquid/solid interface of the phenyloctane solution. The first layer grew very quickly into many small domains. This was followed by the slow formation of the piled up layers into much larger domains. Chain polymerization on the topmost surface layer could be initiated by applying a pulsed voltage between the STM tip and the substrate, usually producing a long polymer of submicrometer length. In contrast, polymerizations on the underlying layer were never observed. This can be explained by a conformation model in which the polymer backbone is lifted up.     

Mapping concentration profiles within the diffusion layer of an electrode: application to redox catalysis

Catalytic reductions of some aromatic halides were performed at a millimetric electrode with several redox mediators. The resulting concentration profiles were monitored amperometrically by placing an ultramicroelectrode inside the diffusion layer produced at the former electrode. The features of redox catalysis and the subsequent structuring of the diffusion layer were investigated experimentally under steady-state conditions imposed by the spontaneous convection of the solution. The concentration profiles established from the probe measurements were in agreement with our theoretical predictions, based on fast kinetics of redox catalysis. Under these conditions, very similar to preparative electrosynthesis, the diffusion layer separates into two domains where pure diffusion takes place and the concentration profiles therein are mainly linear. We demonstrate that the limit between these two zones does not depend on kinetics, but is rather fixed by the product of the ratio of the bulk concentrations of each species and the ratio of their diffusion coefficients.     

Time-dependent organization and wettability of decanethiol self-assembled monolayer on Au(111) investigated with STM

A detailed study on the time-dependent organization of a decanethiol self-assembled monolayer (SAM) at a designed solution concentration onto a Au(111) surface has been performed with scanning tunneling microscopy (STM). The SAMs were prepared by immersing Au(111) into an ethanol solution containing 1 microM decanethiol with different immersion times. STM images revealed the formation process and adlayer structure of the SAMs. It was found that the molecules self-organized into adlayers from random separation to a well-defined structure. From 10 s, small domains with ordered molecular organization appeared, although random molecules could be observed on Au(111) at the very initial stage. At 30 s, the SAM consisted of uniform short stripes. Each stripe consisted of sets of decanethiol mainly containing eight molecules. With the immersion time increasing, the length of the stripes increased. At 5 min, the alkyl chains overlapped each other between the adjacent stripes, indicating the start of a stacked process. After immersing Au(111) in decanethiol solution for 3 days, a densely packed adlayer with a (radical 3 x radical 3)R30 degrees structure was observed. The formation process and structure of decanethiol SAMs are well related to sample preparation conditions. The wettability of the decanethiolate SAM-modified Au(111) surface was also investigated.     

Monte Carlo simulations of antibody adsorption and orientation on charged surfaces

Monte Carlo simulations were performed to study the adsorption and orientation of antibodies on charged surfaces based on both colloidal and all-atom models. The colloidal model antibody consists of 12 connected beads representing the 12 domains of an antibody molecule. The structure of the all-atom antibody model was taken from the protein databank. The effects of the surface charge sign and density, the solution pH and ionic strength on the adsorption and orientation of different colloidal model antibodies with different dipole moments were examined. Simulation results show that both the 12-bead and the all-atom models of the antibody, for which the dipole moment points from the Fc to (Fab)2 fragments, tend to have the desired "end-on" orientation on positively charged surfaces and undesired "head-on" orientation on negatively charged surfaces at high surface charge density and low solution ionic strength where electrostatic interactions dominate. At low surface charge density and high solution ionic strength where van der Waals interactions dominate, 12-bead model antibodies tend to have "lying-flat" orientation on surfaces. The orientation of adsorbed antibodies results from the compromise between electrostatic and van der Waals interactions. The dipole moment of an antibody is an important factor for antibody orientation on charged surfaces when electrostatic interactions dominate. This charge-driven protein orientation hypothesis was verified by our simulations results in this work. It was further confirmed by surface plasmon resonance biosensor and time-of-flight secondary ion mass spectrometry experiments reported elsewhere.     

Influence of the bulk and surface morphology on adhesion of polystyrene-inter-poly-cross-2-ethylhexyl-methacrylate films and particles

The adhesion behavior of semi-interpenetrating polymer networks (semi-IPNs) of linear polystyrene (PS) in crosslinked poly-2-ethylhexylmethacrylate (EHMA) was studied by variation of the bulk and surface morphology, i.e., domain size, continuity, and concentration in the domains. Semi-IPNs were prepared by liquid-liquid demixing upon cooling of a homogeneous solution of PS in methacrylate monomer, followed by gelation of the PS-rich phase and UV polymerization of the methacrylate resin. Welding of films allowed the preparation of larger objects provided that (1) the samples were phase separated to a high degree and contained domains with a high PS concentration (>90%) and (2) polystyrene was present at the interface. For semi-IPN films, a linear dependence of the adhesion strength on the (crack healing time)^1/4 was obtained. Based on these considerations, a process was developed to obtain melt-processable semi-IPN particles, by quenching droplets of the polymer solution into a cold liquid. These particles obtained a PS-rich skin layer and showed good adhesion after blending with a thermoplast. © 1996 John Wiley & Sons, Inc.     

Phase separation of ternary self-assembled monolayers into hydrophobic 1-dodecanethiol domains and electrostatically stabilized hydrophilic domains composed of 2-aminoethanethiol and 2-mercaptoethanesulfonic acid on Au(111)

Ternary self-assembled monolayers (SAM) composed of 2-aminoethanethiol (AET), 2-mercaptoethanesulfonic acid (MES), and 1-dodecanethiol (DDeT) form two types of domains as if it were a two-component SAM: DDeT-rich hydrophobic domains and electrostatically stabilized hydrophilic domains composed of MES and AET on Au(111). MES and AET behave virtually as a single surface-active species. Two distinct reductive desorption peaks in cyclic voltammograms (CV) and binarized images of scanning tunneling microscopy clearly show nanometer scale, yet macroscopically distinguishable, phase separation over a wide range of the mixing ratio of DDeT and MES-AET in the bathing solution. X-ray photoelectron spectroscopy measurements indicate that the ratio of MES to AET in the hydrophilic domains is unity and that both terminal groups are in the charged states, that is, the sulfonate group and the ammonium group. With decreasing the total concentration of the thiols, the mole fraction of DDeT in the bathing solution at which the surface coverage of MES-AET domains is equal to that of DDeT domains dramatically decreases. This suggests that the adsorption kinetics plays a crucial role in the formation of the domains structure.     

Polymerization and self‐assembly of thermally responsive in‐chain functionalized double‐hydrophilic macromonomers

Double‐hydrophilic in‐chain functionalized macromonomers consisting of poly(N‐isopropylacrylamide) (PNIPAM) and poly(ethylene oxide) were prepared by a multistep procedure including esterification of PNIPAM monoester of maleic acid with α‐methoxy‐ω‐hydroxypolyoxyethylene or its amidation with α‐methoxy‐ω‐aminopolyoxyethylene. The polymerization of the macromonomers was carried out in aqueous solutions. The temperature was the key parameter controlling the polymerization process that was performed in the organized domains formed by the macromonomers below and above the phase transition temperature (T_tr). Polymacromonomers with higher degrees of polymerization were prepared at temperatures just below the T_tr. Static light scattering measurements on dilute aqueous solutions of thermally‐responsive macromonomers and their polymerization products demonstrated that they formed aggregates below the T_tr. Supramolecular structures with low density cores, formed by the polymacromonomers at room temperature, were imaged by SEM. Morphological tuning was achieved by varying both the composition of the copolymer and the concentration of the aqueous solution. The rheological behavior of the polymacromonomers in 25 wt % aqueous solution was compared to that of the respective macromonomers. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4720–4732, 2007     

Atomistic Simulation Study of Cu0.327Ni0.673 Alloys: from Solid Solution to Phase Segregation

We present a combined Monte‐Carlo/molecular dynamics study of a Cu_0.327Ni_0.673 alloy system. On the basis of nearest‐neighbor coordination number analyses atomic clustering and phase segregation is explored. Along this line, free energy profiles are calculated and separated into entropic and energetic contributions. The competition of both terms was found in accordance to the experimental phase diagrams (phase separation of the solid solution below about 600 Kelvin). Two independent simulation runs were performed. At 1000 Kelvin the observed configurations correspond to solid solutions exhibiting a weak tendency to cluster atoms of identical species. At room temperature the energetic favoring of atomic separation is clearly dominant and leads to the formation of Ni‐rich and Cu‐rich domains. The latter are separated by interfacial regions whose width ranges from 0.5 to 1 nanometers.