Probing surface structure via time-of-escape analysis of gas in Knudsen regime

Continuing the study begun in [Feres, R., Yablonsky, G., 2004. Knudsen's cosine law and random billiards. Chemical Engineering Science 59, 1541-1556], we consider transport in the Knudsen regime of inert gases through straight channels and investigate how small-scale surface geometry of a macroscopically flat channel affects the diffusion characteristics of the gas. We show that the diffusivity constant contains information about the surface micro-geometry.Our investigation is carried out partly by analytical means and also through numerical simulation of what we call time-of-escape experiment. This is a type of multi-scattering experiment that measures the mean residence time in channels of varying lengths, allowing for an analysis of the transport process at various stages of development.We focus attention on the smoothness constantξ=D/D₀, defined as the ratio of diffusivity for a given micro-geometry, D, and diffusivity D₀ under “ideal roughness,” i.e., under the assumption that the Knudsen cosine law holds at each collision. The dependence of ξ on small-scale surface morphology is investigated using a variety of parametrized families of micro-geometries. We show that ξ can tell the presence of certain geometric features, such as curvature or sharp angles at the microscopic level. For example, for a simple two-dimensional model of atomically smooth surface made of a linear packing of spheres of radius R probed by gas molecules of radius A (Fig. 6), we obtain the approximate relation ξ∝1+A/R, with a proportionality constant of roughly 1.3. (See Section 2 for the range of parameters considered.) This relates to rapid diffusion phenomena recently observed by [Holt, J.K., Park, H.G., Wang, Y., Stadermann, M., Artyukhin, A.B., Grigoropoulos, C.P., Noy, A., Bakajin, O., 2006. Fast mass transport through sub-2-nanometer carbon nanotubes. Science 312, (1034)]. In the classical collision model of Maxwell-Smoluchowski, with no detailed regard of surface geometry, one always has ξ?1, but we observe that ξ can sometimes be less than 1 (Section 2.5). We obtain exact values for the mean number and duration of collisions as function of the micro-geometry (Section 3). This is needed to determine the relative importance of these quantities on diffusivity. We also extend the analysis of the scattering operator of random billiards of [Feres, R., Yablonsky, G., 2004. Knudsen's cosine law and random billiards. Chemical Engineering Science 59, 1541-1556] by giving a general criterion for the validity of Knudsen's cosine law, and describe some of the spectral properties of this operator.     

Isomorphic crystallization of aliphatic copolyesters derived from 1,6-hexanediol: Effect of the chemical structure of comonomer units on the extent of cocrystallization

The crystallization behavior has been investigated for poly(hexamethylene sebacate-co-hexamethylene suberate)s [P(HSe-co-HSu)] and poly(hexamethylene suberate-co-hexamethylene adipate)s [P(HSu-co-HA)] copolyesters by differential scanning calorimeter (DSC), wide angle X-ray diffraction (WAXD) and Fourier transform infrared (FTIR) spectrometer. Cocrystallization of comonomer units was found in the crystalline phase of both P(HSe-co-HSu) and P(HSu-co-HA) copolymers. Moreover, both P(HSe-co-HSu) and P(HSu-co-HA) copolymers exhibit isodimorphism. The PHSe and the PHSu type crystals were respectively developed in P(HSe-co-HSu)s, while the PHSu and the PHA type crystals were developed in P(HSu-co-HA)s respectively. The inclusion of the comonomer units in the PHSu type crystalline lattice is more favored than that in the PHSe type crystal of the P(HSe-co-HSu)s. Meanwhile, it is easier for the comonomer units to be incorporated into the PHA type crystalline lattice than into the PHSu type crystal of P(HSu-co-HA)s. Moreover, comparison between the isomorphic crystallization of P(HSe-co-HSu), P(HSe-co-HA) and P(HSu-co-HA) series copolyesters indicates that minimization of the difference in chemical structure helps in increasing the extent of cocrystallization of comonomer units.     

Aniline hydrogenolysis on nickel: effects of surface hydrogen and surface structure

Fluorescence yield near-edge spectroscopy (FYNES) above the carbon K edge and temperature programmed reaction spectroscopy (TPRS) have been used as the methods for characterizing the reactivity and structure of adsorbed aniline and aniline derived species on the Ni(100) and Ni(111) surfaces over an extended range of temperatures and hydrogen pressures. The Ni(100) surface shows appreciably higher hydrogenolysis activity towards adsorbed aniline than the Ni(111) surface. Hydrogenolysis of aniline on the Ni(100) surface results in benzene formation at 470 K, both in reactive hydrogen atmospheres and in vacuum. External hydrogen significantly enhances the hydrogenolysis activity for aniline on the Ni(100) surface. Based on spectroscopic evidence, we believe that the dominant aniline hydrogenolysis reaction is preceded by partial hydrogenation of the aromatic ring of aniline in the presence of 0.001 Torr of external hydrogen on the (100) surface. In contrast, very little adsorbed aniline undergoes hydrogen induced C-N bond activation on the Ni(111) surface for hydrogen pressures as high as 10^–7 Torr below 500 K. Thermal dehydrogenation of aniline dominates with increasing temperature on the Ni(111) surface, resulting in the formation of a previously observed polymeric layer which is stable up to 820 K. Aniline is adsorbed at a smaller angle relative to the Ni(111) surface than the Ni(100) surface at temperatures below the hydrogenolysis temperature. We believe that the proximity and strong -interaction between the aromatic ring of the aniline and the surface is one major factor which controls the competition between dehydrogenation and hydrogen addition. In this case the result is a substantial enhancement of aniline dehydrogenation relative to hydrogenation on the Ni(111) surface.     

Poloxamers,poloxamines and polymeric micelles: Definition,structure and therapeutic applications in cancer

Research for new therapies to treat diseases like cancer, it is one of the focus of Health Sciences. Nowadays, the available therapeutic strategies have in many cases limitations and undesired side effects, which shows a need to find new therapies with a higher efficacy. In this regard, over the past few years, poloxamers and poloxamines have been gaining more attention in the pharmaceutical field, mainly due to their advantages as potential nanosystems. Therefore, poloxamers and poloxamines are amphiphilic block of copolymers constituted by PEO units, poly(ethylene oxide), and PPO units, poly(propylene oxide), presenting the capability to self-assembly in micellar structures in aqueous medium forming polymeric micelles, which improve theirs potential as drug and genetic material nanocarriers. Thereby, in order to create new alternative treatments for current pathologies, like cancer, alterations in the poloxamines and poloxamers, the combination of these with other polymers and the conjugation with ligands are being introduced as a viable option to allow the combination of therapies, such as the simultaneous delivery of a gene and a drug in the same system. These intelligent stimuli-sensitive systems, with well-defined structures and functionalities, make possible the development of a safe, specific and effective therapeutic strategy. Thus, this article intends to review several aspects of poloxamines and poloxamers concerning their general properties and their applications in drug and gene delivery. Also, are reviewed the preparation and characterization techniques of polymeric micelles with a focus in micelleplexes which can be used as a simultaneous drug and nucleic acid carrier.     

Nitroxide-mediated radical polymerization in nanoreactors: Factors influencing compartmentalization effects on bimolecular termination

Modeling and simulations of nitroxide-mediated radical polymerization (NMP) in dispersed systems have been performed to elucidate what factors dictate the magnitude of the segregation effect on bimolecular termination between propagating radicals generated from alkoxyamine activation. The reduction in termination rate due to segregation cannot be predicted merely based on the average number of propagating radicals per particle . This is because the magnitude of the segregation effect is also governed by the distribution of propagating radicals between particles, which is influenced by both the termination (k_t) and the deactivation (k_deact) rate coefficients. The results have implications with regards to improvement of livingness (end-functionality) in NMP by exploitation of particle size, and are expected to apply (qualitatively) to other controlled/living systems based on the persistent radical effect (e.g. atom transfer radical polymerization).     

Effect of structure on light transmission in isotactic polypropylene and random propylene-1-butene copolymers

The structure of semi-crystalline isotactic polypropylene and random isotactic copolymers of propylene and 1-butene at the nanometer and micrometer scales is controlled by the pathway of melt-crystallization, and the concentration of chain defects. Rapid cooling of the melt results in formation of mesomorphic nodules, being not organized in a higher order superstructure. Slow melt-crystallization, in contrast, allows formation of lamellae. These lamellae arrange within spherulites of a size which is decreased in copolymers. Analysis of the light transmission reveals a distinctly higher transparency in non-spherulitic preparations. Spherulitic samples exhibit a lower transparency, increasing with decreasing size of spherulites. Annealing of quenched preparations at elevated temperature leads to an increase of the crystallinity and of the dimensions of crystals, without affecting their habit, and their higher order organization. The transparency is only slightly decreased in these specimens. It can be demonstrated that samples of largely different transparency but identical crystallinity can be generated without the use of optical clarifiers/nucleation agents.     

Epoxy‐clay nanocomposites: Influence of the clay surface modification on structure

In this article, the effect of four different clay surface modifiers on the structure of epoxy‐clay nanocomposites was studied. Various organoclays were prepared via cation exchange reaction between inorganic cations naturally occuring in the clay gallery and different alkylammonium ions. Epoxy‐clay nanocomposites were prepared by in situ intercalative polymerization using a hardener of polyoxypropylenediamine type. It was found that various clay surface modifiers exhibit different catalytic effect on curing of epoxy inside the clay gallery as observed by measuring of the gel time with dynamic mechanical analysis. This was confirmed by monitoring the change in the d‐spacing by wide angle X‐ray scattering performed in situ during curing. Morphology of the cured systems was probed by transmission electron microscopy (TEM) and wide angle X‐ray scattering (WAXS). The degree of dispersion observed by TEM and WAXS corresponds with achieved mechanical properties of cured composites. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Discovery of the bi-layered structure in spin-coated polyacrylamide films on the gold surface

The reflection-absorption Fourier transform infrared spectroscopy (RA-FTIR) with the polarized lights was used to characterize polyacrylamide (PAL) films spin-coated on the gold (Au) surfaces. The PAL films spin-coated on the Au surface show the characteristics of a bi-layered structure. The first layer attached to the Au surface with a thickness of around 70 nm shows no chain orientation due to the dewetting effect of the Au surface. The upper layer is composed of the PAL molecules oriented parallel to the Au surface as the film thickness increases, which is believed to be due to the spin-coating effect. The bottom and the top layers are optically isotropic and anisotropic, respectively, as confirmed by the analysis of the RA-FTIR spectra. The thickness-threshold is of the order of the end-to-end distance (R_ee, 67 nm) of PAL molecules (M_w ∼ 10⁶) indicating the dewetting distance through which the Au surface functions.     

Defect engineering in semiconducting oxides: Control of ZnO surface potential via temperature and oxygen pressure

The technological usefulness of a semiconductor often depends on the types, concentrations, charges, spatial distributions, and mobilities of the atomic‐scale defects it contains. For semiconducting metal oxides, defect engineering is relatively new and involves complex transport and reaction networks. Surface‐based methods hold special promise in nanostructures where surface‐to‐volume ratios are high. This work uses photoreflectance augmented by X‐ray photoelectron spectroscopy to show that the surface potential V_S for Zn‐terminated ZnO(0001) can be manipulated over a significant range 54.97–79.08 kJ/mol (0.57–0.82 eV) via temperature and the partial pressure of O₂. A defect transport model implies this variation in V_S should affect the injection rate of oxygen interstitials by a factor of three. Such injection plays an important role in controlling the concentrations of oxygen vacancies deep in the bulk, which often prove troublesome as trapping centers in photocatalysis and photovoltaics and as parasitic emitters in light‐emitting devices. © 2015 American Institute of Chemical Engineers AIChE J, 62: 500–507, 2016     

Self-assembled silver nanoislands formed on glass surface via out-diffusion for multiple usages in SERS applications

We demonstrate that silver nanoisland film self-assembled on the surface of silver-containing glass in the course of thermal processing in hydrogen is capable to detect 10⁻⁷ M concentration of rhodamine 6G in water using surface enhanced Raman spectroscopy (SERS) technique. The film can be multiply restored on the same glass substrate via annealing of the glass in hydrogen. We showed that the film can be self-assembled after as much as ten circles of the substrate cleaning followed by annealing. The proposed technique of the silver nanoisland film formation enables multiple usage of the same glass substrate in SERS experiments.     

Miscibility analysis in LLDPE/LDPE blends via thermorheological analysis: Correlation with branching structure

This article describes correlation between thermorheological properties and the miscibility of LLDPE/LDPE blends. Samples of LLDPE/LDPE with the blending ratio of 5/95, 10/90, 25/75, 50/50, 75/25, and 90/10 were prepared via melt mixing in a twin screw extruder. Both applied polyethylenes are varying in their long‐chain branches. Five methods including the time–temperature superposition (TTS) principle, van Gurp–Palmen plot, Cole–Cole curve, zero‐shear viscosity as a function of concentration, and relaxation spectrum were employed to examine the miscibility behavior of the samples. The results obtained by these methods indicated the immiscibility of the LLDPE/LDPE blends except the one with 10 wt% LLDPE content. Moreover, Scholz and Einstein models used for further checking of miscibility of the blends showed consistent results. Also, by using the Scholz model, the value of α/R, ratio of interfacial tension to droplet radius, for the blend with 95 wt% LLDPE content was estimated as 876 N m^?2 that was comparable with prior values found for LLDPE/LDPE blends. The potential of thermorheological approach as an alternative powerful tool for analyzing LCB and miscibility issues in PE blends could be highlighted. POLYM. ENG. SCI., 54:1081–1088, 2014. © 2013 Society of Plastics Engineers     

Effect of heat treatment on the surface chemical structure of glass: Oxygen speciation from in situ XPS analysis

Compositional changes in unleached and acid‐leached soda‐lime silicate surfaces were tracked with in‐vacuo heating and X‐ray Photoelectron Spectroscopy. Surface oxygen speciation was determined using a stoichiometry‐based algorithm via elemental composition, instead of the typical O 1s peak‐fitting approach. Accurate surface hydroxyl quantification is shown to require dehydration at temperatures near 200°C. On the unleached surface, no change in surface hydroxyl density (~2.5 OH/nm²) is observed in the temperature range of 200°C‐500°C after the initial dehydration. However, repolymerization in the network (non‐bridging oxygen→bridging oxygen) is observed due to volatilization of sodium. The acid‐leached surface undergoes sodium out‐diffusion from the bulk at sub‐T_g temperatures with laterally resolved inhomogeneity and shows a reduction in the concentration of hydroxyls from 4.5 OH/nm² (200°C) to 3.2 OH/nm² (500°C) accompanied by an increase in bridging oxygen. These results suggest that when [OH] > 2.5~3/nm², vicinal OH undergo dehydroxylation with evolution of water, whereas when [OH] < 2.5/nm², most OHs are non‐interacting and isolated (at temperatures below T_g). Furthermore, at temperatures exceeding 300°C, sodium has enough thermal energy to desorb in vacuum and diffuse from the bulk (depending on the abundance & local structure).     

Role of block copolymer on the coarsening of morphology in polymer blend: Effect of micelles

The reactive compatibilization of polystyrene/ethylene‐α‐octene copolymer (PS/POE) blend via Friedel–Crafts alkylation reaction was investigated by rheology and electron microscope. It was found that the graft copolymer formed from interfacial reaction reduced the domain size and decreased the coarsening rate of morphology. The reduction of the interfacial tension is very limited according to the mean field theory even assuming that all block copolymer stays on the interface. With the help of self‐consistent field theory and rheological constitutive models, the distribution of graft copolymer was successfully estimated. It was found that large amount of copolymer had detached from the interfaces and formed micelles in the matrix. Both the block copolymer micelles in matrix and the block copolymers at the interface contribute to the suppression of coarsening in polymer blend, but play their roles at different stages of droplet coalescence. In droplet morphology, the micelles mainly hinder the approaching of droplets. © 2014 American Institute of Chemical Engineers AIChE J, 61: 285–295, 2015     

Hardened portland blast-furnace slag cement pastes: I. Effects of additives on surface area and on pore structure

Portland blast-furnace slag cement pastes were prepared with various water/cement ratios. Specific surface areas and pore structures of the hardened pastes were investigated by nitrogen adsorption. The “accessibility” of the nitrogen molecules to the pore structure is discussed in terms of degree of hydration and total porosities of the pastes. Effect of presence of CaCl₂, a typical steel reinforcement corrosive agent, was also studied, and results indicated that it alters the area and pore structure extensively, to a more “open structure,” thus facilitating its own accessibility. Lime and gypsum addition was also studied in presence and in absence of CaCl₂, and the effect of the Blaine surface area of the unhydrated cement is particularly emphasized in this investigation.     

Relationship of structure to properties in surfactants: II. Efficiency in surface or interfacial tension reduction

The concept of a quantitative measure for the efficiency with which surfactants depress surface or interfacial tension is introduced. It is shown that the quantity, log (1/C)_π=20, where C is the bulk concentration or surfactant required to reduce the surface or interfacial tension by 20 dynes/cm (surface pressure, π=20), is a suitable measure of the efficiency. This quantity is a linear function of the free energy of transfer of the surfactant molecule from the interior of the bulk phase to the interface. The effect upon the efficiency of various structural groupings in both the hydrophobic and hydrophilic portions of the molecule is calculated and the results discussed. From surface and interfacial tension data, it is shown that the free energy decrease involved in the transfer of a −CH₂-group at π=20 to the aqueous solution-air interface is 450–500 cal mole⁻¹ at 25 C; for the aqueous solution-heptane interface, the free energy decrease/−CH₂-group transferred is greater than 427 cal mole⁻¹ at 50 C. Presented at the AOCS Meeting, Philadelphia, September 1974.     

Influence of pore structure and surface chemistry on electric double layer capacitance in non-aqueous electrolyte

The performance as electric double layer capacitors (EDLC) in non-aqueous electrolyte of a series of alkaline agent-activated carbons with high surface area is presented in this work. The results obtained show that, in general, capacitance increases with surface area. However, the results obtained in this study confirm that capacitance not only depends on surface area, but also on two other parameters: pore size distribution and surface chemistry. It has been shown that capacitance is higher for a sample with wider micropore size distribution than for a sample with higher surface area but too narrow micropore size distribution. In addition, it has been observed that the sample with a very high amount of surface groups presents very high capacitance values. In the present study, a KOH-activated carbon with a capacitance as high as 220 F/g was prepared. Finally, the results obtained with a mesoporous sample have shown that the presence of mesopores in activated carbons with very high surface area (e.g. >2000 m²/g), do not seem to be effective for double layer capacitors.     

Modeling the influence of changes in aliphatic structure on char surface area during coal pyrolysis

The influence of changes in aliphatic structure on char surface area during coal pyrolysis was modeled, and the effect was introduced to a previous char surface area model for lignite pyrolysis established based on the chemical percolation devolatilization (CPD) model. The modified model can predict not only the N₂ and CO₂ char surface area during rapid pyrolysis of three lignites but also the CO₂ char surface area of two high-volatile bituminous coals; the agreement of the modified model with experiments is improved at high temperatures. The decrease in aliphatic chain length can reduce adsorption positions around aromatic core, and decrease char surface area. When mass release is more than 55% at about 1,100 K, the predicted N₂ char surface area starts to decrease with further generation of volatiles, and the increase of predicted CO₂ char surface area with increasing generation of volatiles also become slow at the end of mass release.     

Effect of gelling on the surface structure of a porous lead electrode in sulfuric acid

Effect of immobilizing the electrolyte using thixotropic agents such as sodium silicate on the electrochemical behaviour of lead in sulfuric acid is studied using electrochemical methods such as cyclic voltammetry (CV), charging curve technique, Tafel polarization and surface spectroscopic methods including X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). CV studies indicate a 700mV cathodic shift in the hydrogen evolution potential as a result of gelling while the well-known reversible Pb/Pb2+ couple is found to be thermodynamically disfavoured (100mV and 50mV shifts for the Pb dissolution and Pb2+ reduction reactions respectively). In addition, a significant change in the double layer capacitance by 600Fcm–2 is observed at the same potential probably due to the contribution of silicate adsorption at the interfacial region. Exchange current density values for hydrogen evolution reaction calculated from Tafel plots are 1.35×10–6Acm–2 and 1.31×10–8Acm–2 for free and gelled electrolytes, respectively, which reveal the kinetic suppression of HER. However, XPS studies give no evidence for the chemisorption of silicate on the lead surface, although SEM analysis shows appreciable change in the surface morphology.