Showing papers in "The Journal of Physical Chemistry in 1993"
TL;DR: In this paper, the authors present an approach to generate electrostatic potential (ESP) derived charges for molecules, which optimally reproduce the intermolecular interaction properties of molecules with a simple two-body additive potential, provided that a suitably accurate level of quantum mechanical calculation is used to derive the ESP around the molecule.
Abstract: We present a new approach to generating electrostatic potential (ESP) derived charges for molecules. The major strength of electrostatic potential derived charges is that they optimally reproduce the intermolecular interaction properties of molecules with a simple two-body additive potential, provided, of course, that a suitably accurate level of quantum mechanical calculation is used to derive the ESP around the molecule. Previously, the major weaknesses of these charges have been that they were not easily transferable between common functional groups in related molecules, they have often been conformationally dependent, and the large charges that frequently occur can be problematic for simulating intramolecular interactions
6,266 citations
TL;DR: In this paper, the authors derived crystal orbital Hamilton populations (COHP) diagrams to visualize chemical bonding in solids by means of density-functional electronic structure calculations, where the band structure energy is defined as a sum of orbital pair contributions.
Abstract: After giving a concise overview of the current knowledge in the field of quantum mechanical bonding indicators for molecules and solids, we show how to obtain energy-resolved visualization of chemical bonding in solids by means of density-functional electronic structure calculations. On the basis of a band structure energy partitioning scheme, i.e., rewriting the band structure energy as a sum of orbital pair contributions, we derive what is to be defined as crystal orbital Hamilton populations (COHP). In particular, a COHP(E) diagram indicates bonding, nonbonding, and antibonding energy regions within a specified energy range while an energy integral of a COHP gives access to the contribution of an atom or a chemical bond to the distribution of one-particle energies
2,603 citations
TL;DR: In this paper, the effects of surface modification on colloidal metal particles in aqueous solution were investigated. And the resulting changes in the chemical, photochemical, and optical properties were discussed.
Abstract: The study of nanometer and subnanometer colloidal metal particles in aqueous solution complements the investigation of small particles in molecular beams, frozen solutions, and inorganic clusters. The electronic properties of the metal particles are changed by surface modification, for example by chemisorption of a nucleophilic molecule or by deposition of a second metal. The resulting changes in the chemical, photochemical, and optical properties are discussed. Methods are described which enable one to store excess electrons or positive holes on the particles in a controlled manner and to investigate the accompanying changes in the optical properties. Metal particles carrying excess electrons initiate electrochemical reactions such as the reduction of water or the deposition of metals
1,633 citations
TL;DR: In this paper, the pore size distribution of porous sorbents is determined by fitting the correlation of the experimental adsorption isotherm of the sorbent to the mean field density functional theory.
Abstract: We present an improved method, based upon density functional theory, for the determination of the pore size distribution (PSD) of porous sorbents. The adsorption isotherms predicted by nonlocal mean field theory for individual slit pores are correlated as a function of pressure and pore width. The PSD is then calculated by fitting this correlation to the experimental adsorption isotherm of the sorbent. We apply the analysis method to adsorption data for nitrogen on several porous carbons. In this application, the porous network is modeled as a continuous size distribution of noninteracting slit-shaped graphitic pores. The PSDs obtained by using the Kelvin equation and using the local and nonlocal versions of the mean field density functional theory are compared and evaluated. The results demonstrate that nonlocal theory provides a more accurate interpretation of the PSD than previous methods have achieved, particularly when the analysis is applied to highly microporous sorbents.
1,077 citations
TL;DR: In this article, it is shown that reverse micelles can be used as microreactors to produce either well-defined nanosized crystallites or chemically modified enzymes, and some prospective aspects of the use of such water-in-oil droplets are discussed.
Abstract: In this paper an extensive introduction to reverse micellar properties is given. It is shown that reverse micelles can be used as microreactors to produce either well-defined nanosized crystallites or chemically modified enzymes. Solubilization of macromolecules induces either the gelation of droplets or a percolation process followed by a phase transition. In the latter case this could favor the extraction of product reactions catalyzed by enzymes. Finally, a summary gives some prospective aspects of the use of such water-in-oil droplets
1,012 citations
TL;DR: In this article, the room temperature solubility of pure C[sub 60] has been determined in 47 solvents, and the solubilities cover a wide range, from 0.01 mg/mL in methanol to 50 mg/m in 1-chloronaphthalene.
Abstract: The room temperature solubility of pure C[sub 60] has been determined in 47 solvents. The solubilities cover a wide range, from 0.01 mg/mL in methanol to 50 mg/mL in 1-chloronaphthalene. The solubilities in CS[sub 2], toluene, and hexane, three of the commonly employed solvents, are 7.9, 2.8, and 0.04 mg/mL, respectively. An examination of the solubilities of C[sub 60] as a function of the solvent properties such as index of refraction, dielectric constant, molecular size, Hildebrand solubility parameter, and H-bonding strength reaffirms the century-old principle like dissolves like. No single solvent parameter can uniformly predict the solubility of C[sub 60], but a composite picture of solvents with high solubility for C[sub 60] emerges: large index of refraction, dielectric constant around 4, large molecular volume, Hildebrand solubility parameter equal to 10 cal[sup 1/2] cm[sup [minus]3/2], and tendency to act as a moderate strength nucleophile. 18 refs., 4 figs., 1 tab.
1,003 citations
TL;DR: In this paper, the catalytic activity of single-phase polycrystalline Pt-Ru bulk alloys toward the electrooxidation of methanol in sulfuric acid electrolyte at room temperature was measured for the first time on well-characterized alloy surfaces, in terms of surface composition and true surface area.
Abstract: The catalytic activity of single-phase polycrystalline Pt-Ru bulk alloys toward the electrooxidation of methanol in sulfuric acid electrolyte at room temperature was measured for the first time on well-characterized alloy surfaces, in terms of surface composition and true surface area. Sputter-cleaned and annealed alloys were prepared in ultrahigh vacuum (UHV) and the surface composition of oxygen-free surfaces was assessed via low-energy ion scattering (LEIS). Pt-Ru alloy surfaces annealed in UHV were strongly enriched in Pt, whereas the surface composition of sputter-cleaned alloys was essentially identical to their bulk composition. It was shown that the sputter-cleaned alloy with a Ru surface of [approx equal] 10 atomic % gave the highest catalytic enhancement (over pure Pt) for the steady-state electrooxidation of methanol at 25[degree]C and 0.5 V, viz., by a factor of 12 in 5.0 mM CH[sub 3]OH solution and by a factor of 30 in 0.5 M CH[sub 3]OH. It was found that the balance between the rate of methanol adsorption and the rate of the oxidative removal of dehydrogenated fragments determines the activities of Pt-Ru alloys with different Ru surface compositions. 51 refs., 9 figs., 2 tabs.
875 citations
TL;DR: In this article, Colloidal TiO[sub 2] electrodes were photosensitized with derivatives of chlorophyll and related natural porphyrins resulting in light harvesting and charge separation efficiencies comparable to those in natural photosynthesis.
Abstract: Colloidal TiO[sub 2] electrodes were photosensitized with derivatives of chlorophyll and related natural porphyrins resulting in light harvesting and charge separation efficiencies comparable to those in natural photosynthesis. The photocurrent action spectra of the electrodes correlate well with the absorption spectra of the dyes in solution. Incident photon to current efficiencies up to 83% are reached in the Soret peak at 400 nm with a 12-[mu]m-thick TiO[sub 2] film sensitized by copper mesoporphyrin IX, which corresponds to nearly unity quantum efficiency of charge separation when light reflection losses are taken into account. Photocurrent/voltage curves of TiO[sub 2] solar cells sensitized with copper chlorophyllin show an energy conversion efficiency of 10% for the red peak at 630 nm. Under simulated sunlight illumination, an open circuit photovoltage of 0.52 V and a short circuit current density of 9.4 mA/cm[sup 2] are measured. The overall energy conversion efficiency of the cell is 2.6% under these conditions, in part limited by ohmic losses at such high current densities. The comparison of different chlorophyll derivatives indicates that free carboxyl groups are important for adsorption and sensitization on TiO[sub 2]. However, conjugation of the carboxyl groups with the [pi] electron system of the chromophore is notmore » necessary for efficient electron transfer. Free bases, zinc, and even the nonfluorescent copper complexes of chlorophyllins and mesoporphyrin IX are efficient sensitizers for TiO[sub 2]. Cholanic acids as coadsorbates were found to be unique in improving both photocurrent and voltage of copper chlorophyllin sensitized cells. This effect is discussed by comparison with other coadsorbates. 23 refs., 3 figs., 1 tab.« less
826 citations
TL;DR: In this article, a new density functional method for treatment of chemical processes in solution is presented, which is based on freezing the electron density of the solvent molecules, while solving the eigenvalue problem for the solute Hamiltonian, which includes the effective potential of solvent molecules.
Abstract: A new density functional method for treatment of chemical processes in solution is presented. The method is based on freezing the electron density of the solvent molecules, while solving the eigenvalue problem for the solute Hamiltonian, which includes the effective potential of the solvent molecules. The method is developed and examined in the simple case of one solvent and one solute molecule. The results are encouraging and the deviation between the unfrozen and frozen treatments can be attributed to the kinetic energy functional used. The method can be implemented in ab initio calculations of solvation free energies, following a recent pseudopotential approach [Vaidehi et al., 19921.
735 citations
TL;DR: In this paper, a functionalized surfactant, silver sulfosuccinate (Ag(AOT)), a reactant, was used to obtain a high concentration of monodisperse and stable nanosize particles even in the presence of air.
Abstract: Silver nanosized crystallites have been synthesized in AOT reverse micelles. By using a functionalized surfactant, silver sulfosuccinate (Ag(AOT)), a reactant, a high concentration of monodisperse and stable nanosize particles can be obtained even in the presence of air. This study focused on the various parameters controlling the nanosize of silver particles such as micellar structure and nature of the reducing agent or of the solvent. At low water content, a precursor such as Ag 4 2+ can be stabilized. The changes in the size of the crystallites with the number of initiators per micelle and with the micellar exchange processes can be explained from a kinetic model
653 citations
TL;DR: In this article, an energy-adjustable pseudopotential scheme for element 105 (hahnium, Ha) with corresponding energy-optimized valence basis sets is presented.
Abstract: Nonrelativistic and quasirelativistic energy-adjusted ab initio pseudopotentials are presented for element 105 (hahnium, Ha) together with corresponding energy-optimized valence basis sets. The method of energy adjustment of pseudopotentials is extended to a two-component formalism and to multiconfiguration wave functions. The accuracy of the pseudopotential scheme is demonstrated by a comparison of atomic valence-only results to corresponding all-electron data. Atomic multiconfiguration self-consistent field and multireference configuration interaction calculations for M and M[sup +] (M = Nb, Ta, Ha) are compared with available experimental data. Corresponding molecular calculations, which included spin-orbit coupling, have been performed for the low-lying states of HaO and are compared to the results from corresponding calculations of the lighter homologs NbO and TaO. 41 refs., 1 fig., 8 tabs.
TL;DR: The structures of the three ferroelectric phases of BaTiO[sub 3] have been determined by Rietveld refinement using powder diffraction data collected at a spallation neutron source as discussed by the authors.
Abstract: The structures of the three ferroelectric phases of BaTiO[sub 3] have been determined by Rietveld refinement using powder diffraction data collected at a spallation neutron source. The correlation between refined atomic displacements and thermal parameters, which has hampered previous structure determinations, has been partially alleviated by using data which extend over a wide range of d spacings. Data collected at a large number of sample temperatures provide information about the temperature dependence of the ferroelectric displacements and changes in the oxygen octahedra which surround the Ti ions. The temperature dependence of the thermal parameters gives atomic Debye-Waller temperature that are remarkably similar to those for the cations in the high-[Tc] superconductors. The results are insensitive to predictions of a soft-mode displacive model; however, values of the anisotropic thermal parameters do not support the order-disorder model suggested by Comes et al. Powder extinction and profile coefficients from the structural refinements show pronounced minima and maxima, respectively, near the phase transitions and provide information about the temperature dependence of the mosaic structure and the strain in the different phases. 37 refs., 12 figs., 6 tabs.
Abstract: Adhesion and friction experiments were carried out with a variety of surfactant monolayer-coated surfaces using the surface forces apparatus technique. The equilibrium and time-dependent adhesion and the interfacial or'boundary friction of these surfaces were measured as a function of monolayer density and phase state, load, temperature, contact time, sliding speed, and relative humidity. The aim was to investigate the correlations between the friction and adhesion of these fairly ideal, molecularly smooth, and chemically homogeneous surfaces. The results show that friction and adhesion are not generally related if one only compares the magnitude of the friction force (or coefficient of friction) with the adhesion force (or adhesion energy). However, since friction is an energy-dissipating process, one might expect to find a relation between friction and adhesion hysteresis. This was confirmed by comparing the adhesion energy hysteresis during loading-unloading cycles with the friction forces measured under similar conditions. The authors find that -- in spite of its great complexity and dependence on many parameters -- the boundary friction force varies with temperature, sliding velocity, load, and relative humidity in highly systematic ways, which can be represented by a dynamic friction phase diagram. 53 refs., 17 figs., 1 tab.
TL;DR: In this paper, the photophysical changes upon complexation with alkali and alkaline-earth metal cations can be described in terms of enhancement of the intramolecular charge transfer in the coumarin dye.
Abstract: The properties of fluoroionophores consisting of coumarin 153 linked to monoaza-12-crown-4, monoaza-15-crown-5, and diaza-18-crown-6 are described. The photophysical changes upon complexation with alkali and alkaline-earth metal cations can be described in terms of enhancement of the intramolecular charge transfer in the coumarin dye owing to the direct interaction between the bound cation and the carbonyl group of the coumarin. The magnitude of the photophysical changes is controlled by the charge density of the bound cation. In the case of the diaza-18-crown-6 linked to two coumarins, an interesting specific increase of the fluorescence quantum yield upon binding of potassium and barium ions is observed
TL;DR: In this article, a theoretical approach to the treatment of solute-solvent interactions is described, where the solute is described as a low dielectric cavity immersed in a Dielectric continuum.
Abstract: A theoretical approach to the treatment of solute-solvent interactions is described. The solute is described as a low dielectric cavity immersed in a dielectric continuum. However, the cavity is not assigned a simple geometric form but rather is determined from the van der Waals envelope of the molecule. Real and partial charges are placed on atomic nuclei as in any molecular mechanics force field. Dielectric and ionic strength effects are obtained through numerical solutions to the Poisson-Boltzmann equation, which has proved to be remarkably accurate for a wide range of applications
TL;DR: In this paper, a Monte Carlo technique was used to obtain accurate theoretical mobilities for carbon cluster cations, which indicated that linear structures exist up to C[sub 10[sup +]].
Abstract: Carbon clusters are generated by laser desorption. Mass-selected beams are then pulse injected into an ion chromatography (IC) device. This device temporally and spatially separates the beam into its isomeric components. Arrival time distributions (ATDs) are then measured at the detector. From these distributions, accurate mobilities are obtained for each isomeric component, along with the the fractional abundance of each isomer. Different isomer structures are calculated using quantum chemical methods. A Monte Carlo technique uses these structures to obtain accurate theoretical mobilities. Comparison of theory with experiment allows unambiguous structural assignment of the various families of isomers present in the cluster beam. The results indicate that, for carbon cluster cations, linear structures exist up to C[sub 10[sup +]]. Several families of planar ring systems begin with monocyclic rings (ring I), which first appear at C[sub 7[sup +]], and persist beyond C[sub 40[sup +]]. Bicyclic rings (ring II) are first observed at C[sub 21[sup +]] and persist beyond C[sub 40[sup +]], followed by tricyclic rings (ring III, initiated at C[sub 30[sup +]]) and tetracyclic rings (ring IV, initiated at C[sub 40[sup +]]). A 3-dimensional family we label as 3-D rings begins at C[sub 29[sup +]], whose structure is not yet unambiguouslymore » assigned. This family never exceeds 5% of the ions at any cluster size. Finally, the first fullerene is observed at C[sub 30[sup +]], with this family dominating above C[sub 50[sup +]]. 32 refs., 14 figs., 4 tabs.« less
TL;DR: In this article, the influence of altervalent cation doping of TiO[sub 2] on its performance as a photocatalyst in water cleavage is investigated, and the results do not correlate with observed alterations in the light-absorption capacity of the doped semiconductor.
Abstract: The influence of altervalent cation doping of TiO[sub 2] on its performance as a photocatalyst in water cleavage is investigated. It is shown that under illumination in the near-UV region (250--400 nm), platinized anatase TiO[sub 2] exhibits H[sub 2] production rates which are significantly higher than those of the rutile form. Photocatalytic efficiency is shown to be independent of the specific surface area of the semiconductor Incorporation of cations of valence higher than that of the parent cation (W[sup 6+], Ta[sup 5+], Nb[sup 5+]) into the crystal matrix of TiO[sub 2] results in enhanced rates of water cleavage while the opposite is observed upon doping with cations of lower valence (In[sup 3+], Zn[sup 2+], Li[sup +]). The enhancement or reduction of photocatalytic activity is found to be dependent on the concentration and valence of the doping cations. These results do not correlate with observed alterations in the light-absorption capacity of the doped semiconductor. They are explained in term of alteration of the bulk electronic structure of the semiconductor, which influences its electron-hole generation and separation capacity, under illumination. 28 refs., 7 figs., 2 tabs.
TL;DR: In this article, the authors constructed and simulated a membrane-water system which consists of 200 molecules of 1-palmitoyl 2-oleoyl-sn-glycero-3-phosphatidylcholine and 5483 water molecules covering the head groups on each side of the bilayer.
Abstract: We have constructed and simulated a membrane-water system which consists of 200 molecules of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine forming a rectangular patch of a bilayer and of 5483 water molecules covering the head groups on each side of the bilayer. The total number of atoms is approximately 27 000. The lateral dimensions of the bilayer are 85 A×100 A, and the distance between the bilayer surfaces as given by the average phosphorus to phosphorus distance is 35 A. The thickness of each water layer is up to 15 A. In all, we simulated 263 ps of the dynamics of the system. To prevent system disintegration, atoms within 5 A from the surface were harmonically restrained and treated by Langevin dynamics, forming a stochastic boundary
TL;DR: In this article, a high-temperature aerosol apparatus for the synthesis of 3-8-nm, surface-oxidized Si crystallites is described, made by homogeneous gas-phase nucleation following pyrolysis of dilute disilane in He.
Abstract: We describe a high-temperature aerosol apparatus for the synthesis of 3-8-nm, surface-oxidized Si crystallites. The particles are made by homogeneous gas-phase nucleation following pyrolysis of dilute disilane in He. The particles are collected as a robust ethylene glycol colloid, and characterized by transmission electron microscopy, X-ray powder Brag scattering, IR, high-pressure liquid chromatography, and optical spectroscopy. The particles exhibit a shell structure, with a crystalline Si core, of bulk lattice constant, capped with an approximately 1.2-nm shell of silicon dioxide
TL;DR: A series of common electrolytes were found either to reduce the rate of bubble coalescence in water or to have no effect as mentioned in this paper, and a system of combining rules emerged that characterizes the behavior of all the salts studies.
Abstract: A series of common electrolytes were found either to reduce the rate of bubble coalescence in water or to have no effect. For a range of cations and anions, a system of combining rules emerges that characterizes the behavior of all the salts studies. The specific effects of electrolytes on bubble coalescence may be related to their effect on water structure and hence the hydrophobic interaction. The phenomenon may have applications in various fields, including decompression sickness, hydrophobic chromatography, and possibly evolution
TL;DR: In this article, it was shown that the photocatalytic decomposition of pure water over ZrO[sub 2] powder without any loaded metals under UV irradiation can be achieved without the use of any loaded metal.
Abstract: It was found for the first time that the photocatalytic decomposition of pure water proceeded over ZrO[sub 2] powder without any loaded metals under UV irradiation. The rate of H[sub 2] and O[sub 2] evolution increased upon addition of Na[sub 2]CO[sub 3] and NaHCO[sub 3]. Moreover, the evolution of CO (the photocatalytic reduction product of CO[sub 2]) was observed from NaHCO[sub 3] solutions. The special characteristics of ZrO[sub 2] semiconductor are believed to be associated with its highly negative flat-band potential and wide bandgap. In the case of Cu(1 wt %)-ZrO[sub 2] catalyst suspended in NaHCO[sub 3] aqueous solution, the rates of gas evolutions were 19.5 [mu]mol/h of H[sub 2], 10.8 [mu]mol/h of O[sub 2], and 2.5 [mu]mol/h of CO. These mass balances were indicative of a stoichiometric and catalytic reaction. 26 refs., 1 tab.
TL;DR: A commercially available TiO[sub 2] powder (Degussa P25) has been used to prepare thin particulate films on conducting glass plates, which provide a convenient method for accelerating the photocatalytic reaction by applying an external bias as mentioned in this paper.
Abstract: A commercially available TiO[sub 2] powder (Degussa P25) has been used to prepare thin particulate films on conducting glass plates. These semiconductor particulate films are photoelectrochemically active with properties similar to an n-type semiconductor. The recombination between the photogenerated charge carriers can be suppressed by applying an external anodic bias. Electron scavengers, such as oxygen, affect the photocurrent generation by competing for the photogenerated electrons. These particulate films provide a convenient method for accelerating the photocatalytic reaction by applying an external bias. For example, the rate of photocatalytic degradation of 4-chlorophenol greatly increases when the TiO[sub 2] particulate film electrode is maintained at an external anodic bias (0.6 V vs SCE) during the UV photolysis. 26 refs., 6 figs.
TL;DR: The binding of enantiomers of Ru(bpy) 2 (pp) 2+ (bpy=2,2'-bipyridine; ppz=4,7-phenanthrolino[6,5-b]pyrazine) to calf thymus DNA was investigated using absorption, fluorecence and resonance enhanced Raman spectroscopies.
Abstract: The binding of enantiomers of Ru(bpy) 2 (pp) 2+ (bpy=2,2'-bipyridine; ppz=4,7-phenanthrolino[6,5-b]pyrazine) to calf thymus DNA is investigated using absorption, fluorecence and resonance enhanced Raman spectroscopies. Both isomers show absorption hypochromicity, steady-state fluorescence increase, reduced accessibility to an anionic quencher, and fluorecence lifetime increase associated with binding to B-form DNA, though the effect for the Λ isomer is always less. However, the maximal fluorescence polarization for both isomers is the same, indicating a similarly rigid binding to DNA
TL;DR: In this paper, the conduction band edge potentials of the acid-exchanged titanates were too positive to prevent semiconductor-mediated recombination of photogenerated H[sub 2] and I[sub 3][sup [minus]].
Abstract: Layered alkali-metal titanates (Na[sub 2]Ti[sub 3]O[sub 7] and K[sub 2]Ti[sub 4]O[sub 9]), niobates (KNb[sub 3]O[sub 8] and K[sub 4]Nb[sub 6]O[sub 17]), and titanoniobates (KTiNbO[sub 5] and CsTi[sub 2]NbO[sub 7]) were internally platinized, acid-exchanged, sensitized with ruthenium polypyridyl complexes, and studied as photocatalysts for the production of H[sub 2] and I[sub 3][sup [minus]] from acidic alkali-metal iodide solutions. The titanates were inactive as photocatalysts, whereas the niobates and titanoniobates were active with quantum efficiencies up to 0.3% for HI photolysis with visible light. Calculations based on electronegativity showed that the conduction band edge potentials of the acid-exchanged titanates were too positive to prevent semiconductor-mediated recombination of photogenerated H[sub 2] and I[sub 3][sup [minus]]. Laser flash photolysis/transient diffuse reflectance spectra established that iodide reduces the oxidized sensitizer, forming I[sub 2][sup [sm bullet][minus]], which subsequently decays in a bimolecular reaction to form I[sub 3][sup [minus]]. The inefficiency of HI photolysis can be attributed to charge recombination between I[sub 3][sup [minus]] and conduction band electrons for the niobates and titanoniobates. 29 refs., 16 figs., 1 tab.
TL;DR: In this paper, several molecular dynamics simulations on the interface between liquid decane and liquid water have been performed with the purpose to study the sensitivity of surface properties to the van der Waals parameters.
Abstract: Several molecular dynamics simulations on the interface between liquid decane and liquid water have been performed with the purpose to study the sensitivity of surface properties to the van der Waals parameters. The models used consisted of 50 decane molecules and 389 water molecules in a two-phase system. By changing the van der Waals parameters, i.e., varying the Lennard-Jones parameters e and σ between the united CH2 and CH3 atoms of the decane molecules and the O atom of the water molecules, sharper interfaces were obtained. The excess free energy of water in decane and the surface tension between water and decane are presented for the van der Waals parameters used. From comparison to experimental values for the solubility of water in decane and the surface tension, the best parameters within the tested sets are when using the SPC/E water model: eO-CH3 = 0.849 kJ/mol, eO-CH2 = 0.706 kJ/mol, and σO-C = 0.344 nm. When using the SPC water model, the best parameters are eO-CH3 = 0.637 kJ/mol, eO-CH2 = 0.529 kJ/mol, and σO-C = 0.344 nm. We also evaluated differences between the interfacial and bulk liquids. At the interface, water showed an orientational preference, whereas the decane molecules were more laterally oriented with respect to the interface.
TL;DR: In this paper, a controlled synthesis of quantized colloidal CdTe nanocrystals (in aqueous solutions) with narrow size distributions and stabilized against rapid oxidation was achieved by capping the quantum dot particles with 3-mercapto-1,2-propanediol.
Abstract: The controlled synthesis of quantized colloidal CdTe nanocrystals (in aqueous solutions) with narrow size distributions and stabilized against rapid oxidation was achieved by capping the quantum dot particles with 3-mercapto-1,2-propanediol. Nanocrystals (i.e., quantum dots) with mean diameters of 20, 25, 35, and 40 A were produced. Optical absorption spectra showed strong excitonic peaks at the smallest size; the absorption coefficient was shown to follow an inverse cube dependence on particle diameter, while the extinction coefficient per particle remained constant. The quantum yield for photoluminescence increased with decreasing particle size and reached 20% at 20 A. The valence band edges of the CdTe quantum dots were determined by pulse radiolysis experiments (hole injection from oxidizing radicals); the bandgaps were estimated from pulse radiolysis data (redox potentials of hole and electron injecting radicals) and from the optical spectra. The dependence of the CdTe bandgap on quantum dot size was found to be much weaker than predicted by the effective mass approximation; this result is consistent with recently published theoretical calculations by several groups. 36 refs., 5 figs., 1 tab.
TL;DR: In this article, the flatband potential of polycrystalline TiO[sub 2] electrode was determined using a newly developed spectroscopic technique, using a range of nonaqueous solvents (MeCN, THF, DMF, MeOH, and EtOH).
Abstract: The flatband potential V[sub fb] of a polycrystalline TiO[sub 2] electrode has been determined, using a newly developed spectroscopic technique, in a range of nonaqueous solvents (MeCN, THF, DMF, MeOH, and EtOH). It has been found that V[sub fb] is significantly more positive for water and nonaqueous protic solvents (MeOH and EtOH) than for nonaqueous aprotic solvents (MeCN, DMF, and THF) due to establishment of a proton adsorption-desorption equilibrium. Further, the authors observe that V[sub fb] is independent of the electrolyte used in nonaqueous protic solvents, but in nonaqueous aprotic solvents V[sub fb] may depend on the electrolyte cation. Cations which have been found to be potential determining are Li[sup +], Na[sup +], and Mg[sup +]. Results indicating that both cation adsorption and intercalation are important in determining V[sub fb] are presented. 23 refs., 4 figs., 4 tabs.
TL;DR: In this article, composite monolayer structures comprised of cytochrome c strongly adsorbed to alkanethiolate self-assembled (SA) monolayers on sputter-deposited gold film electrodes, i.e., cyt c/HOOC(CH 2 )nS/Au (n=5, 10, 15), were examined using cyclicvoltammetry (CV), electrochemical impedance spectroscopy (EIS), and X-ray photoelectron spectrograph (XPS).
Abstract: Composite monolayer structures comprised of cytochrome c strongly adsorbed to alkanethiolate self-assembled (SA) monolayers on sputter-deposited gold film electrodes, i.e., cyt c/HOOC(CH 2 )nS/Au (n=5, 10, 15), were examined using cyclicvoltammetry (CV), electrochemical impedance spectroscopy (EIS), and X-ray photoelectron spectroscopy (XPS). Monolayer coverage of cytochrome c in a stable, functional, electroactive state was obtained in neutral phosphate buffer of low ionic strength for the thickest film (n=15). Somewhat lower electroactive coverages were generally observed with the thinner SA monolayers (n=5, 10)
TL;DR: In this article, the adsorption thermodynamics and molecular-level structure were studied for benzene and p-xylene in silicalite with Pnma symmetry (ORTHO) and P2[sub 1]2.2.
Abstract: Adsorption isotherms and isosteric heats of adsorption for benzene and p-xylene in silicalite have been calculated from molecular simulations. The simulations were performed using newly developed grand canonical ensemble Monte Carlo (GCMC) techniques in which insertion attempts are biased toward the most favorable regions of the zeolite pore space. The new techniques result in a substantial improvement in the efficiency of the simulations compared to traditional GCMC. The adsorption thermodynamics and molecular-level structure were studied for benzene and p-xylene in silicalite with Pnma symmetry (ORTHO) and P2[sub 1]2[sub 1]2[sub 1] symmetry (PARA). The subtle differences between ORTHO and PARA silicalite result in qualitatively different sorption behavior. An explanation of the experimentally observed step in the adsorption isotherm is presented, based on the results of the simulations and the ORTHO to PARA framework transformation that is observed experimentally. Predictions of the adsorption isotherms, isosteric heats, and siting locations of the adsorbates are in good agreement with experiment. 59 refs., 14 refs., 4 tabs.
TL;DR: The phase behavior and aggregate morphology of mixtures of the oppositely charged surfactants cetyltrimethylammonium bromide (CTAB) and sodium octyl sulfate (SOS) are explored with cryotransmission electron microscopy, quasielastic light scattering, and surface tensiometry as mentioned in this paper.
Abstract: The phase behavior and aggregate morphology of mixtures of the oppositely charged surfactants cetyltrimethylammonium bromide (CTAB) and sodium octyl sulfate (SOS) are explored with cryotransmission electron microscopy, quasielastic light scattering, and surface tensiometry. Differences in the lengths of the two hydrophobic chains stabilize vesicles relative to other microstructures (e.g., liquid crystalline and precipitate phases), and vesicles form spontaneously over a wide range of compositions in both CTAB-rich and SOS-rich solutions. Bilayer properties of the vesicles depend on the ratio of CTAB to SOS, with CTAB-rich bilayers stiffer than SOS-rich ones. We observe two modes of microstructural transition between micelles and vesicles. The first transition, between rodlike micelles and vesicles, is first order, and so there is macroscopic phase separation. This transition occurs in CTAB-rich solutions and in SOS-rich solutions at higher surfactant concentrations. In the second transition mode, mixtures...