Showing papers in "Langmuir in 1996"
TL;DR: In this paper, the use of optical measurements to monitor electrochemical changes on the surface of nanosized metal particles is discussed within the Drude model, and the absorption spectrum of a metal sol in water is shown to be strongly affected by cathodic or anodic polarization, chemisorption, metal adatom deposition, and alloying.
Abstract: The use of optical measurements to monitor electrochemical changes on the surface of nanosized metal particles is discussed within the Drude model. The absorption spectrum of a metal sol in water is shown to be strongly affected by cathodic or anodic polarization, chemisorption, metal adatom deposition, and alloying. Anion adsorption leads to strong damping of the free electron absorption. Cathodic polarization leads to anion desorption. Underpotential deposition (upd) of electropositive metal layers results in dramatic blue-shifts of the surface plasmon band of the substrate. The deposition of just 0.1 monolayer can be readily detected by eye. In some cases alloying occurs spontaneously during upd. Alloy formation can be ascertained from the optical absorption spectrum in the case of gold deposition onto silver sols. The underpotential deposition of silver adatoms onto palladium leads to the formation of a homogeneous silver shell, but the mean free path is less than predicted, due to lattice strain in t...
3,454 citations
TL;DR: In this paper, gold colloids have been homogeneously coated with silica using the silane coupling agent (3-aminopropyl trimethoxysilane as a primer to render the gold surface vitreophilic.
Abstract: Gold colloids have been homogeneously coated with silica using the silane coupling agent (3-aminopropyl)trimethoxysilane as a primer to render the gold surface vitreophilic. After the formation of a thin silica layer in aqueous solution, the particles can be transferred into ethanol for further growth using the Stober method. The thickness of the silica layer can be completely controlled, and (after surface modification) the particles can be transferred into practically any solvent. Varying the silica shell thickness and the refractive index of the solvent allows control over the optical properties of the dispersions. The optical spectra of the coated particles are in good agreement with calculations using Mie's theory for core−shell particles.
1,808 citations
TL;DR: In this article, the authors showed that fractal surfaces can be super water repellent (superwettable) when the surfaces are composed of hydrophobic (hydrophilic) materials.
Abstract: Wettability of fractal surfaces has been studied both theoretically and experimentally. The contact angle of a liquid droplet placed on a fractal surface is expressed as a function of the fractal dimension, the range of fractal behavior, and the contacting ratio of the surface. The result shows that fractal surfaces can be super water repellent (superwettable) when the surfaces are composed of hydrophobic (hydrophilic) materials. We also demonstrate a super-water-repellent fractal surface made of alkylketene dimer; a water droplet on this surface has a contact angle as large as 174°.
1,500 citations
TL;DR: The presence of two sulfur species was detected in X-ray photoelectron spectroscopy (XPS) studies of thiol and disulfide molecules adsorbed onto gold surfaces as discussed by the authors.
Abstract: The presence of two sulfur species was detected in X-ray photoelectron spectroscopy (XPS) studies of thiol and disulfide molecules adsorbed onto gold surfaces. These species are assigned to bound thiolate (S2p3/2 binding energy of 162 eV) and unbound thiol/disulfide (S2p3/2 binding energy from 163.5 to 164 eV). These assignments are consistent with XPS data obtained from different thiols (C12, C16, C18, and C22 alkane thiols, a fluorinated thiol, and a cyclic polysiloxane thiol) and different adsorption conditions (solvent type, thiol concentration, temperature, and rinsing). In particular, the use of a poor solvent for thiol adsorption solutions (e.g., ethanol for long chain alkanethiols) and the lack of a rinsing step both resulted in unbound thiol molecules present at the surface of the bound thiolate monolayer. This has implications for recent studies asserting the presence of multiple binding sites for gold−thiolate species in organic monolayers.
1,224 citations
TL;DR: In this article, the synthesis and characterization of 25−70 amine-capped gold nanocrystals were reported, showing that the stability of the particles appears to be largely kinetic, rather than thermodynamic, in nature.
Abstract: We report on the synthesis and characterization of 25−70 A diameter amine-capped gold nanocrystals. In particular, we show how these particles can be prepared by a simple procedure and confirm the particle composition (including the identity of the amine surface passivant) through several materials characterization techniques that include infrared spectroscopy, nuclear magnetic resonance spectroscopy, ultraviolet−visible spectroscopy, mass spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, X-ray diffraction analysis, differential scanning calorimetry, thermogravimetric analysis, and elemental analysis. All physical characterizations are consistent with a charge−neutral amine/gold surface interaction described by a weak covalent bond. The stability of the particles appears to be largely kinetic, rather than thermodynamic, in nature. Comparison of these nanocrystals to amines adsorbed onto bulk Au surfaces indicates that the stability of the nanocrystal/amine system is a finite-size effect.
923 citations
TL;DR: In this article, a method to control the growth of particle arrays on smooth and wettable solid surfaces is presented. But the results show that the higher the particle monodispersity, the lower the particle volume fraction, and the higher environmental humidity, the larger the size of the forming domains.
Abstract: Forming regular textures of an arbitrary size on smooth solid surfaces is the challenge of future technology to produce new types of optical gratings, optical filters, antireflective surface coatings, selective solar absorbers, data storage, and microelectronics. Here we present a novel approach to form such sophisticated textures: controlling the growth of particle arrays on smooth and wettable solid surfaces. The obtained centimeter-size polycrystalline monolayer films consist of closely packed fine particles. Coloring of the monolayer which arises from the light diffraction, interference, and scattering exclusively inherent in textured films shows the size of the differently oriented crystal domains building the film. The results show that the higher the particle monodispersity, the lower the particle volume fraction, and the higher the environmental humidity, the larger the size of the forming domains.
900 citations
TL;DR: In this article, the electrophoretic mobility of oil droplets, dispersed without any surfactant in the aqueous phase, was measured and the results showed that the oil droplet are negatively charged and the magnitude of their ζ-potential strongly depends on pH and the ionic strength of the aiquous phase.
Abstract: The electrophoretic mobility of oil droplets, dispersed without any surfactant in the aqueous phase, was measured. Four different oils were studied: xylene, dodecane, hexadecane, and perfluoromethyldecalin. Special precautions were undertaken to avoid artifacts caused by the presence of surfactant impurities. The results show that the oil droplets are negatively charged and the magnitude of their ζ-potential strongly depends on pH and the ionic strength of the aqueous phase. The electrophoretic mobility is almost independent of the type of specific nonpolar oil. Series of experiments were performed to check different hypotheses about the origin of the spontaneous charging of the oil−water interfaces. The results lead to the conclusion that hydroxyl ions, released by the dissociation−association equilibrium of the water molecules, adsorb at the oil−water interface. The specific adsorption energy was estimated to be 25kT per ion (kT is the thermal energy). The molecular origin and the implications of this ...
736 citations
TL;DR: In this article, the formation of ordered phases of acid-hydrolyzed cellulose suspensions was studied as a function of cellulose crystallite concentration and added electrolyte (HCl, NaCl, and KCl) concentrations.
Abstract: The formation of ordered phases of acid-hydrolyzed cellulose suspensions was studied as a function of cellulose crystallite concentration and added electrolyte (HCl, NaCl, and KCl) concentrations. A chiral nematic phase formed when the suspension concentration was higher than 5.14 × 10-6 nm-3 in water. For biphasic samples, the cellulose concentrations in both isotropic and anisotropic phases increase with the total suspension concentration and with added electrolyte. The experimental results were compared with the predictions of the theory of Stroobants, Lekkerkerker, and Odijk for the phase separation of charged rods. The suspensions were not stable at electrolyte concentrations sufficiently high to allow complete evaluation of the electrostatic contribution to the interparticle interactions, but the general behavior was in line with theoretical predictions. The chiral nematic pitch of the anisotropic phase decreased with increasing crystallite concentration and with added electrolyte concentration. App...
671 citations
TL;DR: In this article, a polymer mold suitable for the formation of titanium dioxide nanotubes was obtained and the tubular structure was formed by electrochemical deposition in the mold, and the as-deposition TiO2 tubes were amorphous, but polycrystalline anatase samples were obtained after heat treatment.
Abstract: Starting from the naturally occurring structure of porous aluminum oxide, a polymer mold suitable for the formation of titanium dioxide nanotubes was obtained. The tubular structure was formed by electrochemical deposition in the mold. After dissolution of the polymer, titanium dioxide nanotubes were obtained and characterized. The “as deposited” TiO2 tubes were amorphous, but polycrystalline anatase samples of the same structure were obtained after heat treatment. The inner diameter of the tubes decreased from about 100 to 70 nm during the crystallization. A mechanism for the electrochemical deposition is proposed. Furthermore, the preparation procedure could also be applied to other semiconducting materials.
652 citations
TL;DR: In this paper, the effects of polyvinylpyrrolidone (PVP) concentration on the particle size, the UV−vis absorption peak, and the rate of the photoreduction process were studied.
Abstract: Silver nanoparticles have been prepared by photoreduction of silver nitrate with 254 nm UV light in the presence of poly(N-vinylpyrrolidone). The effects of PVP concentration on the particle size, the UV−vis absorption peak, and the rate of the photoreduction process were studied. The average particle size ranged from 15.2 to 22.4 nm, with the corresponding UV−vis absorption peak position at 404−418 nm in 1−0.25 wt % PVP. The rate of the photoreduction process was observed to increase with the PVP concentration. X-ray photoelectron spectroscopic studies further revealed that the polymer interacts with silver particles through the oxygen atom in the >CO group. A negative shift of binding energy in the Ag 3d5/2 for silver nanoparticles was observed.
575 citations
TL;DR: In this article, an emulsion-based technique for the assembly of colloid particles into microstructured or multicomponent clusters (supraparticles) is described, where the particles are gathered, assembled, and fixed together in the restricted, colloid-size 2D or 3D space provided by emulsion droplets.
Abstract: We describe an emulsion-based technique that allows the assembly of colloid particles into microstructured or multicomponent clusters (“supraparticles”). The particles are gathered, assembled, and fixed together in the restricted, colloid-size 2D or 3D space provided by emulsion droplets. The process is carried out by multiple modification of the colloid interactions within the particle/droplet system“interaction-tailored colloid assembly”. In the first paper of the series we provide a general description of the method. Then we present the data on the assembly of negatively charged (sulfate) or positively charged (amidine) latexes into ordered hollow spherical supraparticles. The following steps are included in the assembly schemes: (1) modification of latex surface charge and properties, so the microspheres are able to adsorb on the droplet surfaces but without homocoagulation; (2) adsorption and structure formation around the emulsion drops (at this stage the interfacial mobility of the particles is of...
TL;DR: In this article, the relationship between the critical micellization concentration (cmc) of a surfactant and its free energy of mousellization was derived for a general class of ionic surfactants, made up of i polar groups of valency zs bonded to j alkyl chains.
Abstract: The relationship between the critical micellization concentration (cmc) of a surfactant in solution and its free energy of micellization is probably the most widely used one in surfactant science. Until recently, most surfactants discussed in the literature were made up of one polar head group, nonionic or ionic, bonded to a single hydrophobic moiety. Recently, studies have been initiated on new types of ionic surfactants: bolaforms, surfactants with divalent counterions, gemini, or dimeric surfactants, etc. This paper first derives the relationship between the cmc and for a very general type of ionic surfactant, made up of a surfactant ion containing i polar groups of valency zs bonded to j alkyl chains, with counterions of valency zc. The form of this relationship is examined for several types of ionic surfactants. It is shown that many reported studies did not use the correct relationship to evaluate the free energy of micellization from the value of the cmc. In particular, the counterion contribution...
TL;DR: In this article, self-assembled monolayers (SAMs) were formed on gold colloids in 50% aqueous ethanol in the presence of alkanethiols, HS(CH2)nR, where R represents a series of neutral and acidic functional groups.
Abstract: Self-assembled monolayers (SAMs) were formed on gold colloids in 50% aqueous ethanol in the presence of alkanethiols, HS(CH2)nR, where R represents a series of neutral and acidic functional groups. Chemiadsorption of alkanethiols onto the gold colloids significantly changed the rates of flocculation of the gold dispersions; the magnitudes of these pH-dependent changes were a function of chain length, n, and the terminal functionality, R, in a manner consistent with formation of SAMs on the colloid surface. The reduced rate of dissolution of alkanethiol-treated colloids by wet chemical etchants, transmission electron microscopy, and X-ray photoelectron spectroscopy data further support the formation of SAMs.
TL;DR: The infrared spectra of octadecylphosphonic acid (ODPA) monolayers indicate a degree of conformational order comparable to self-assembled mono-layers on planar substrates.
Abstract: Dense, highly ordered monolayers can be prepared by the adsorption of octadecylphosphonic acid (ODPA) onto nonporous ZrO2, TiO2, and zirconated silica powders. ODPA reacts strongly with Al2O3 to form a bulk (aluminoalkyl)phosphonate. The infrared spectra of the ODPA monolayers indicate a degree of conformational order comparable to self-assembled monolayers on planar substrates. From the solid-state 31P and 13C NMR spectra, the strength of the surface interaction and the degree of conformational order decrease in the following order: zirconated silica > ZrO2 > TiO2. Although the inner methylenes are primarily in an all-trans conformation, the chain termina are highly disordered. In the case of zirconated silica, there is evidence for the formation of some bulk zirconium alkylphosphonate. Proton line width measurements from 2D WISE NMR experiments reveal that considerable chain mobility is present in the ODPA monolayers on ZrO2 and TiO2, in contrast to bulk crystalline phases.
TL;DR: In this paper, the structure of alkanethiolate monolayers adsorbed onto nanometer-sized gold clusters has been investigated using infrared spectroscopy, and it has been found that the smaller chain lengths are relatively disordered, with large amounts of gauche defects present, and thus most resemble the free alkanes in the liquid state.
Abstract: Transmission infrared spectroscopy has been used to probe the structure of alkanethiolate monolayers adsorbed onto nanometer-sized gold clusters. The alkyl chain lengths vary between propanethiolate and tetracosanethiolate; specifically the C3, C4, C5, C6, C7, C8, C10, C12, C16, C20, and C24 alkanethiolates have been examined as solid suspensions in KBr pellets. It has been found that the smaller chain lengths (C3, C4, and C5) are relatively disordered, with large amounts of gauche defects present, and thus most resemble the free alkanes in the liquid state. The longer length alkanethiolates are predominantly in the all trans zigzag conformation. There are detectable amounts of near surface gauche defects, the amount of which decreases with increasing chain length, and a reasonably high percentage of end-gauche defects, the relative amount of which increases with increasing chain length. Internal gauche defects cannot be detected. A model is proposed to explain these observations, and the data are compare...
TL;DR: In this article, the general features of tapping mode operation of a scanning force microscope are presented, such as forces, deformation, and contact times, and they can be calculated as fun...
Abstract: The general features of tapping mode operation of a scanning force microscope are presented. Relevant factors of tapping mode such as forces, deformation, and contact times can be calculated as fun...
TL;DR: Henderson et al. as mentioned in this paper used temperature-programmed desorption (TPD) and oxygen isotopic labeling studies to probe the dissociation of water on the (100) and (110) surfaces of TiO2 (rutile).
Abstract: Temperature-programmed desorption (TPD) and oxygen isotopic labeling studies were used to probe the dissociation of water on the (100) and (110) surfaces of TiO2 (rutile). Water TPD spectra from these two surfaces were distinctive. Three monolayer desorption states were observed for the (100) surface (at 205, 250, and 305 K), while only a single desorption state was observed for the (110) surface (at 270 K). TPD experiments on the surfaces enriched with 18O revealed that water desorbing in the 305 K state from TiO2(100) was isotopically scrambled with the lattice oxygen atoms, strongly suggesting that this TPD state resulted from recombination of surface hydroxyl groups. Isotopic scrambling was not observed for any other desorption state on either surface (in the absence of defects). Since very little water desorption occurred from the (110) surface in the temperature range in which exchange was observed on the (100) surface and since previous HREELS work (Henderson, M. A. Surf. Sci. 1996, 355, 151) indic...
TL;DR: Using surface plasmon resonance (SPR) spectroscopy in the Kretschmann configuration, this paper followed the self-assembly of organic ultrathin films which resulted from exposure of gold surfaces to solutions of CH3(CH2)n-1SH (n = 8, 12, 16, and 18) in ethanol and heptane.
Abstract: Using surface plasmon resonance (SPR) spectroscopy in the Kretschmann configuration, we followed the self-assembly of organic ultrathin films which resulted from exposure of gold surfaces to solutions of CH3(CH2)n-1SH (n = 8, 12, 16, and 18) in ethanol and heptane We monitored film growth in situ and continuously for up to 72 h with an overall thickness resolution of <1 A Film dielectric constants, necessary for accurately calculating average film thicknesses from SPR spectra, were determined unambiguously for fully formed films by comparing spectra from organic films in different solvents In addition, we introduce a novel two-color SPR experiment with which we can obtain both film thickness and film dielectric constant without changing solvents We studied the chain length dependent and concentration dependent kinetics of film formation in ethanol and found that there are at least three distinct kinetics steps The kinetics of the first, most rapid, step and the third, slowest, step can be described w
TL;DR: In this article, isosteric heat of adsorption and isotherms have been measured simultaneously in a calorimeter for a series of gases of increasing size and magnitude of quadrupole moment (Ar, O2, N2, CH4, C2H6, SF6, CO2) on adsorbents of varying pore structure and ion type.
Abstract: Isosteric heats of adsorption and adsorption isotherms have been measured simultaneously in a calorimeter for a series of gases of increasing size and magnitude of quadrupole moment (Ar, O2, N2, CH4, C2H6, SF6, CO2) on adsorbents of varying pore structure and ion type (NaX, H-ZSM-5, Na-ZSM-5). Adsorption isotherms have been checked for reversibility by desorption experiments. The average experimental error in loading is ±0.6%; the average uncertainty in the isosteric heat of adsorption is ±0.5 kJ/mol. Heats of adsorption of nonpolar molecules (CH4, C2H6, SF6) increase in the order NaX, silicalite, H-ZSM-5, Na-ZSM-5 at low coverage. Heats of adsorption of nonpolar molecules are almost identical on silicalite, H-ZSM-5, and Na-ZSM-5 at high coverage. Heats of adsorption of the quadrupolar molecule CO2 increase in the order silicalite, H-ZSM-5, Na-ZSM-5, NaX. The electric field adjacent to Na+ sites is 6.2 V/nm, on the basis of the difference between the heat of adsorption of CO2 in Na-ZSM-5 and the heat of a...
TL;DR: In this article, the authors studied the kinetics of coagulation of monodisperse spherical colloids in aqueous suspension at the early stage of pre-coagulation using a fiber-optics-based detection system.
Abstract: In this work we study the kinetics of coagulation of monodisperse spherical colloids in aqueous suspension at the early stage of coagulation. We have performed the measurements on a multiangle static and dynamic light scattering instrument using a fiber-optics-based detection system which permits simultaneous time-resolved measurements at different angles. The absolute coagulation rate constants are determined from the change of the scattering light intensity as well as from the increase of the hydrodynamic radius at different angles. The combined evaluation of static and dynamic light scattering results permits the determination of coagulation rate constants without the explicit use of light scattering form factors for the aggregates. For different electrolytes fast coagulation rate constants were estimated. Stability curves were measured as a function of ionic strength using different particle concentrations.
TL;DR: In this article, a flexible and general approach to formation of macroscopic colloidal Au surfaces that have well-defined nanostructure is presented. But the assembly method described in this paper is compared with previous methods for controlling the na...
Abstract: Covalent attachment of nanometer-scale colloidal Au particles to organosilane-coated substrates is a flexible and general approach to formation of macroscopic Au surfaces that have well-defined nanostructure. Variations in substrate (glass, metal, Al2O3), geometry (planar, cylindrical), functional group (−SH, −P(C6H5)2, −NH2, −CN), and particle diameter (2.5−120 nm) demonstrate that each component of these assemblies can be changed without adverse consequences. Information about particle coverage and interparticle spacing has been obtained using atomic force microscopy, field emission scanning electron microscopy, and quartz crystal microgravimetry. Bulk surface properties have been probed with UV−vis spectroscopy, cyclic voltammetry, and surface enhanced Raman scattering. Successful application of the latter two techniques indicates that these substrates may have value for Raman and electrochemical measurements. The assembly method described herein is compared with previous methods for controlling the na...
TL;DR: In this paper, the Tian−Calvet type calorimeter is applied to the simultaneous determination of adsorption isotherms and the heats of adorption in zeolites.
Abstract: A Tian−Calvet type calorimeter is applied to the simultaneous determination of adsorption isotherms and heats of adsorption This is the first of a series of studies of the effect of adsorbate size and polarity on the energetics of adsorption in zeolites The adsorbate gases used in this study are quadrupolar (N2 and CO2) and nonpolar (Ar, O2, CH4, C2H6, and SF6) The heats of adsorption of both polar and nonpolar gases are either constant or increase with coverage, so silicalite may be classified as a relatively homogeneous adsorbent compared to X type zeolite Reversibility was established by comparing adsorption and desorption isotherms Reproducibility was studied by comparing runs for different samples of the same adsorbent The average experimental error in loading is ±06% The error in the isosteric heat of adsorption is ±2% for heats larger than 20 kJ/mol and ±5% for heats smaller than 20 kJ/mol
TL;DR: In this article, anionic and cationic polyelectrolytes may lead to the formation of multilayers at a solid surface, characterized by a stepwise increase of the adsorbed amount and layer thickness and by alternating highly positive and highly negative values for the ζ-potential.
Abstract: Sequential addition of anionic and cationic polyelectrolytes may lead to the formation of multilayers at a solid surface. The buildup of such multilayers is characterized by a stepwise increase of the adsorbed amount and layer thickness and by alternating highly positive and highly negative values for the ζ-potential. The prime variables which determine the stability of these structures are the polymer charge and the ionic strength. Very stable multilayers are formed when both polymers are highly charged and when the ionic strength is low. For weakly stable multilayers complexation at the surface may first occur, followed by desorption of the complexes. For strongly charged polyelectrolytes the charge stoichiometry, which is not always 1:1, seems to be unique for each pair of polyelectrolytes; no influence of the substrate, of the pH, or of the ionic strength could be observed.
TL;DR: In this paper, the authors show that the frictional properties of alkylsilane monolayers self-assemble on mica in contact with Si3N4 tips depend strongly on the length of the alkyl chains.
Abstract: We show that the frictional properties of alkylsilane monolayers self-assembled on mica in contact with Si3N4 tips depend strongly on the length of the alkyl chains. Friction is particularly high with short chains of less than eight carbons. We attribute this to the large number of dissipative modes in the less ordered short chains. Longer chains, stabilized by van der Waals attractions form more compact and rigid layers and act as much better lubricants. This lubricating action is lost at a certain threshold load, where wear of the molecular layer occurs, leading to much higher friction force values. The results presented here clearly indicate that the chemical identity of the exposed end groups is not sufficient to determine the frictional properties of monolayer films. The increased number of energy dissipation modes facilitated by the presence of molecular disorder (e.g., rotations about a C−C axis), in fact dominates the frictional behavior of monolayers with short chains.
TL;DR: In this article, the adsorption of CO2 up to pressures of 4 MPa has been studied using two series of activated carbon fibers (ACFs) covering a wide range of burn-off.
Abstract: The adsorption of CO2 up to pressures of 4 MPa has been studied using two series of activated carbon fibers (ACFs) covering a wide range of burn-off. The relative fugacities covered in these experiments range from 3 × 10-4 to 0.76. Additionally, N2 adsorption at 77 K and CO2 adsorption at 273 and 298 K at subatmospheric pressures have been carried out. The experiments performed at high pressures allow us to compare both adsorptives at similar ranges of adsorption potential. The results obtained led to the following conclusions: (i) CO2 adsorption at 273 K at subatmospheric pressures is a suitable technique to characterize the narrow microporosity of the ACF. (ii) The use of N2 to characterize the narrow microporosity is not appropriate because its adsorption is limited by the existence of diffusional restrictions in this type of porosity. (iii) CO2 at 273 K (or 298 K) is an adsorptive that behaves quite similarly to N2 at 77 K at comparable relative pressure ranges; thus, CO2 adsorbs in the super-micropo...
TL;DR: In this article, the photocatalytic degradation of 4-chlorophenol (4-CP) in aqueous titanium dioxide suspensions has been studied and compared for two different catalysts: Sachtleben Hombikat UV 100 and Degussa P25.
Abstract: The photocatalytic degradation of 4-chlorophenol (4-CP) in aqueous titanium dioxide suspensions has been studied and compared for two different catalysts: Sachtleben Hombikat UV 100 and Degussa P25. The influence of pH and initial 4-CP concentration on reaction rate and product distribution has been investigated in detail. Regarding the initial 4-CP concentration, the degradation kinetics formally can be discribed by a Langmuir−Hinshelwood expression. While the degradation of 4-CP itself is hardly influenced by the pH, the overall mineralization rate (measured in terms of total organic carbon reduction) changes considerably, decreasing strongly at pH > 7. With P25 used as the photocatalyst the mineralization of 4-CP is slightly faster than that with Hombikat UV 100, but smaller concentrations and numbers of intermediates are detected in the latter case. Most of the intermediates observed during this photocatalytic degradation process are identical with those identified during direct photolysis. In acidic...
TL;DR: In this article, the surface chemical properties of hydrogen and nitrogen-treated samples of an activated carbon were quantified using several complementary techniques, including point of zero charge measurements, and showed that only H2 treatments at high temperature (>1073 K) create basic (hydrophobic) surfaces which are stable after prolonged air exposure.
Abstract: Differences between the surface chemical properties of hydrogen- and nitrogen-treated samples of an activated carbon were quantified using several complementary techniques. Calorimetric studies conducted at 303 K revealed that the sample treated in N2 at 1223 K adsorbs a great deal of oxygen with unusually high differential heats. In fact, both the quantity and the heat of adsorption increased when the treatment temperature was raised from 773 to 1223 K. In contrast, samples treated in H2 adsorbed less and less O2 as the temperature of treatment was raised; after treatment at 1223 K, virtually no O2 adsorption occurred. At the same time the H/C ratio in the H2-treated samples decreased with increasing treatment temperature. Point of zero charge measurements revealed that only H2 treatments at high temperature (>1073 K) create basic (hydrophobic) surfaces which are stable after prolonged air exposure. These findings are consistent with the notion that the removal of oxygen in the form of CO and CO2 during ...
TL;DR: In this article, surface tension measurements of two series of cationic surfactants, each with two hydrophilic and three hydrophobic groups in the molecule (Geminis and Geminis), were used to study micelle formation and premicellar behavior.
Abstract: Micellization and premicellar behavior of the two series of cationic surfactants, each with two hydrophilic and two hydrophobic groups in the molecule (“gemini” surfactants), one series with a rigid, hydrophobic spacer, and second with a flexible, hydrophilic one, have been studied by use of surface tension measurements. The data show the expected regular increase in surface activity with an increase in alkyl chain length for the shorter chain homologs but show increased deviation from the regularity with an increase in chain length when the number of carbon atoms in the alkyl chain exceeds a certain number. This deviation in surface activity appears to be due to the formation of small, non-surface active aggregates. Equilibrium constants calculated for the aggregation reaction show that the conditions facilitating micelle formation also favor formation of these premicelles, such as lower temperature, stronger ionic strength of the solution, and increased alkyl chain length. Geminis with a flexible, hydro...
TL;DR: In this paper, the authors examined the relationship between the equilibrium phase behavior of oil-water−surfactant mixtures and macro-emulsion type and stability. And they showed that both the phase behavior and emulsion stability are dependent on the bending elasticity of the surfactant monolayer at the oil−water interface.
Abstract: The correspondence between the equilibrium phase behavior of oil-water−surfactant mixtures and the macroemulsion type and stability is examined. Both the phase behavior and emulsion stability are argued to be dependent on the bending elasticity of the surfactant monolayer at the oil−water interface. At positive spontaneous curvatures, O/W emulsions are stable; at negative spontaneous curvatures W/O emulsions are stable, whereas, in the balanced state of the surfactant film, an emulsion break usually occurs for a wide variety of systems. To explain the effect of the monolayer bending properties on the macroemulsion stability, the thermally activated rupture of emulsion films is theoretically studied. We consider emulsion films covered by saturated surfactant monolayers, with strong lateral interactions among the adsorbed surfactant molecules. The monolayer at the edge of a nucleation hole in the emulsion film is strongly curved; the bending energy penalty involved leads to a strong dependence of the coales...
TL;DR: In this article, a quasicontinuum method is proposed for the simultaneous treatment of multiple scales, which is based upon a continuum formulation of the problem of interest as a boundary value problem within the confines of the finite element method.
Abstract: The modeling of processes involving multiple length scales is an area of pressing concern, especially in problems such as nanoidentation and crack tip dislocation activity. In these cases, there is more than one characteristic dimension with the nanometer scale arising due to the presence of extended defects such as dislocations and a second length scale at least 2 orders of magnitude larger set by the scale of the indenter or the crack tip itself. To properly model such processes, both scales must be treated explicitly, which is normally beyond the scope of conventional atomistic and continuum analyses alike. This paper describes a quasicontinuum method which seizes upon the strengths of both atomistic and continuum techniques and allows for the simultaneous treatment of multiple scales. The method is based upon a continuum formulation of the problem of interest as a boundary value problem treated within the confines of the finite element method. We part company with traditional approaches by utilizing direct atomistic calculations as the source of the constitutive input used in the finite element analysis. The method is illustrated through application to the case of the structure and energetics of single dislocations. This case is a stringent test as it represents an extreme limit for the model since dislocation core structures are primarily dictated by lattice effects. It is then shown how the method may be applied to problems of tribological concern such as nanoindentation, where it is found that dislocations are initiated beneath the indenter.