Electrostatic interaction between two spherical colloidal particles

Explicit exact analytic expressions are obtained in the form of infinite series for the potential distribution and the potential energy of the electrostatic interaction for the system of two dissimilar spheres in an electrolyte solution on the basis of the linearized Poisson—Boltzmann equation without recourse to Derjaguin's approximation. The leading term of the expression for the interaction energy (the zeroth order approximation) corresponds to the interaction energy that would be obtained if both spheres were ion-penetrable spheres (“soft” spheres). This term is a screened Coulomb interaction due to a simple linear superposition of the unperturbed potentials of the respective spheres, which is proportional to the product of their unperturbed surface potentials. The first-order approximation corresponds to the interaction energy that would be obtained if either sphere were a soft particle (the other being hard). The first-order correction term consists of two sub-terms, each of which is proportional to the square of the unperturbed surface potential of either sphere and does not depend on the unperturbed surface potential of the other sphere, can be interpreted as the interaction between the soft sphere and its image with respect to the hard sphere. This image interaction is attractive if the surface potential of the hard sphere is constant and repulsive if the surface charge density of the sphere is constant. It is shown that Derjaguin's method as well as its extension to the interaction of unequal spheres by Hogg, Healy and Fuerstenau (HHF) is quite a good approximation.     

Bemerkung zur Gleichheit der Aufspaltungen ΔI (zwischen den ersten beiden π-Ionisationspotentialen) und ΔE (zwischen den entsprechenden π* ← π Übergangsenergien) des Spiro[4,4]nonatetraens. Vorläufige Mitteilung

For a given molecule M, the difference ΔI between the first two vertical ionization potentials I_v,2 and I_v,2 (from MOs ψ₁ and ψ₂) and ΔE between the corresponding singlet-singlet excitation energies E₁ and E₂ (transitions ψ?₁ ←₁, ψ?₁ ψ₂) are related by ΔE = ΔI- (J_2,?1?J_1,?1) ?2(K_1,?1 ? K_2,?1), using Koopmans approximation. A simple MO model suggests that under certain conditions of symmetry and quasi-alternancy (e. g. in spiro[4,4]nonatetraene 1 ) the bracketed differences between the Coulomb- and exchange-integrals should vanish to first order, thus leading to the simple (almost) equality ΔE = ΔI. It is shown that the results from a photoelectron- and electron-spectroscopic investigation of 1 support this conclusion i.e. ΔI = 1.23 eV, ΔE = 1.19 to 1.23 eV.     

Photostimulated transformations in nanosized MoO3 films

Irradiation of MoO₃ films (with a thickness d = 5–54 nm) with light (λ = 320 nm, I = (1.5–7) × 10¹⁵ quantum cm^?2 s^?1) led to the formation of an absorption band at λ = 870 nm along with the shift of the edge of the absorption band to the short-wave region of the spectrum. Further irradiation of the samples with light at λ = 870 nm caused diffusion of the long-wave absorption band. The conversion of MoO₃ films increased when the incident light intensity and irradiation time increased and the film thickness decreased under the atmospheric conditions. A mechanism of the photochemical transformation of MoO₃ films was suggested. It involves the generation of electron-hole pairs, recombination of some nonequilibrium charge carriers, formation of [(e(V _a )⁺⁺ e] centers, and isolation of photolysis products.     

Surface Forces in Nanodispersions

The stability of nanodispersions are analyzed on the basis of data on interaction forces between nanoparticles calculated using more precise (than Derjaguin's approximation) approaches. When calculating the dispersion attraction between nanoparticles in dispersions, we use Mitchell and Ninham equations for the case of small (compared to particle radii) interlayer thickness. For the calculation of electrostatic interaction in nanodispersions, the approximation of low potentials of particle surfaces developed by McCartney and Levine are employed. Estimates based on more precise approaches demonstrate that the energy of electrostatic repulsion between particles lowers with a decrease in their sizes (other conditions being equal) that can bring a nanodisperse system closer to the coagulation threshold.     

Tuning the concentration of surface/bulk oxygen vacancies in CeO2 nanorods to promote highly efficient photodegradation of organic dyes

To enhance the photodegradation ability of CeO₂ for organic dyes, an effective strategy is to introduce oxygen vacancies (V_o). In general, the introduced V_o are simultaneously present both on the surface and in the bulk of CeO₂. The surface oxygen vacancies (V_o-s) can decrease the band gap, thus enhancing light absorption to produce more photogenerated e⁻ for photodegradation. However, the bulk oxygen vacancies (V_o-b) will inhibit photocatalytic activity by increasing the recombination of photogenerated e⁻ and V_o-b. Therefore, regulating the concentrations of V_o-s to V_o-b is a breakthrough for achieving the best utilization of photogenerated e⁻ during photodegradation. We used an easy hydrothermal method to achieve tunable concentrations of V_o-s to V_o-b in CeO₂ nanorods. The optimized CeO₂ presents a 70.2% removal of rhodamine B after 120 min of ultraviolet−visible light irradiation, and a superior photodegradation performance of multiple organics. This tuning strategy for V_o also provides guidance for developing other advanced metal-oxide semiconductor photocatalysts for the photodegradation of organic dyes.     

Selenophene‐Flanked Diketopyrrolopyrrole Based Conjugated Polymers for Ambipolar Field‐Effect Transistors

The development of selenophene‐flanked DPP (SeDPP) based copolymers, especially for the ambipolar ones, lags behind other aromatic group flanked DPP‐based polymers. Herein, we report two new ambipolar SeDPP‐based conjugated polymers. One is the alternating polymer PSeDPPFT with normal SeDPP and 3,4‐difluorothiophene units. The other is PSeFDFT , in which the electron acceptor unit is replaced by a new SeDPP derivative, referred as to half‐fused SeDPP. The more planar structure of half‐fused SeDPP endows the backbone of PSeFDFT with good rigidity and planarity. Both polymers exhibit ambipolar transporting properties in air. The PSeFDFT based field‐effect transistors (FETs) display higher and more balanced ambipolar properties with μ_h^ave of 0.27 cm²·V^–1·s^–1, μ_e^ave of 0.18 cm²·V^–1·s^–1, and μ_h^ave/μ_e^ave of 1.5 than those of PSeDPPFT (μ_h^ave = 0.11 cm²·V^–1·s^–1, μ_e^ave = 0.042 cm²·V^–1·s^–1, and μ_h/μ_e = 2.6). This is attributed to the more planar structure, lower LUMO level, higher HOMO level, and better interchain packing orientations of PSeFDFT by comparing with PSeDPPFT . Therefore, a new molecular design strategy to modulate the hole and electron transporting properties is proposed for conjugated D‐A polymers.     

Synthesis of PS and PDMAEMA mixed polymer brushes on the surface of layered silicate and their application in pickering suspension polymerization

An ammonium free radical initiator was ion exchanged onto the surface of clay layers. Polystyrene (PS) and poly(2‐(dimethylamino)ethyl methacrylate) (PDMAEMA) mixed polymer brushes on the surface of clay layers were prepared by in situ free radical polymerization. PS colloid particles armored by clay layers with mixed polymer brushes were prepared by Pickering suspension polymerization. Transmission electron microscopy (TEM), atomic force microscopy (AFM), and scanning electron microscopy (SEM) were used to characterize the structure and morphology of the colloid particles. Clay layers on the surface of PS colloid particles can be observed. Because of the cationic nature of the PDMAEMA brushes the colloid particles have positive zeta potentials at low pH values. X‐ray photoelectron spectroscopy (XPS) was used to analyze the surface of the colloid particles. N_1s binding energy of PDMAEMA chains on the surface of clay layers was detected by XPS. The two peaks of the N_1s binding energy indicate two different nitrogen environments on the surface of clay layers. The peak with a lower binding energy is characteristic of neutral nitrogen on PDMAEMA chains, and the peak with a higher binding energy is attributed to protonated nitrogen on PDMAEMA chains. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5759–5769, 2007     

Low Temperature Thermodynamics. The excess Gibbs function and the volumes of mixing for the system methane + propane at 90.68K

The excess Gibbs function G^e and the excess volumes of mixing V^e have been measured at the methane triple point (90.68 K) for the system methane+propane. They were compared with McGlashan's recent theory for mixtures of molecules of different sizes.     

A spreading model for the oxidation of polypropylene

A model for the heterogeneous oxidation of polypropylene (PP) is proposed in which it is considered that there is a small initial fraction, p_o, of oxidizing centres which have a high local rate of oxidation. Within these zones there is a free radical chain reaction producing secondary oxidation products, volatiles and chemiluminescence (CL) from peroxy radical termination reactions. These zones progressively spread (rate coefficient b/s⁻¹) and the free radical reactions die away within the volume of the original zones, producing a measurable concentration of oxidation products (rate coefficient α/s⁻¹). Analysis of the CL-time curve as representing the instantaneous infectious , fraction, p_i, in the spreading model enables the parameters p_o, b and α to be determined and profiles of the remaining fraction (p_r) and dead or oxidized fraction (p_d) constructed. Analysis of CL curves from 120°C to 150°C gives an activation energy for spreading in PP particles of 96kJ/mol. Both single particles and groups of particles of different types of PP have been examined and evidence is presented of rapid surface spreading of oxidation from particle to particle.     

Solvent effects on electronic absorption spectra of nitrochlorobenzenes,nitrophenols and nitroanilines—lll. Excited state dipole moments and specific solute—solvent interaction energies employing Bakhshiev's approach

A step-wise procedure for employing Bakhshiev's expression for spectral frequency shifts to elucidate the excited state dipole moments (μ_e) and specific solute—solvent interaction energies (E_s) from electronic absorption spectra has been described. The μ_es and E_ss of several substituted benzenes deduced on the basis of this approach and using cavity radii estimated in two different ways are found to be closely comparable with similar results deduced on the basis of the more commonly used McRae's expression, and exhibit similar trends as in the latter approach.     

Properties of water in triton N-42 reverse micelles during the solubilization of HCl solutions according to FT-IR and photon correlation spectroscopy

FT-IR and photon correlation spectroscopy methods are used to study the distribution of free (bulk) and bound (hydration) water in Triton N-42 reverse micelles under the conditions of injection solubilization of hydrochloric acid solutions. The amount of each type of water is calculated depending on the solubilization capacity (V _s/V _o) and HCl concentration in the aqueous pseudophase. According to the IR spectroscopy data, the distribution of water is largely determined by the solubilization capacity of the micellar solution, while the fraction of bulk water exceeds significantly the value calculated by the geometric approach based on the photon correlation spectroscopy data. The difference shows that there is bulk water in the surface layer formed by oxyethyl groups of Triton N-42 molecules in spherocylindrical micelles.     

Exchange energy for two closed shells generated by a bare Coulomb potential energy −<Emphasis Type="Italic">Ze</Emphasis><Superscript>2</Superscript>/<Emphasis Type="Italic">r</Emphasis> in the limit of large <Emphasis Type="Italic">Z</Emphasis>, in two dimensions

Two-dimensional (2D) inhomogeneous electron assemblies are becoming increasingly important in Condensed Matter and associated technologies. Here, therefore, we contribute to the Density Functional Theory of such 2D electronic systems by calculating, analytically, (i) the idempotent Dirac density matrix γ(r, r′) generated by two closed shells for the bare Coulomb potential −Ze ²/r and (ii) the exchange energy density e_x(r){\varepsilon_x({\bf r})} . Some progress is also possible concerning the exchange potential V _x (r), one non-local approximation being the Slater potential 2e_x(r)/n(r){2\varepsilon_x(r)/n(r)} , with n(r) the ground state electron density. However, to complete the theory of V _x (r), the functional derivative of the single-particle kinetic energy per unit area δt(s)/δn(r) is still required.     

Dynamic behavior of polymer surface and the time dependence of contact angle

The increased attention has been focused on the re-searches of soft materials proposed by Pierre-Gilles de Gennes, a Nobel Prize Laureate in Physics. A special issue of “Science” on soft surfaces was published in 2002 to review specific surface properti…     

Bound states of a more general exponential screened Coulomb potential

An alternative approximation scheme has been used in solving the Schr?dinger equation to the more general case of exponential screened Coulomb potential, V(r) = −(a/r)[1 + (1 + br)e ^−2br ]. The bound state energies of the 1s, 2s and 3s-states, together with the ground state wave function are obtained analytically upto the second perturbation term.      

Phenacyl–Thiophene and Quinone Semiconductors Designed for Solution Processability and Air‐Stability in High Mobility n‐Channel Field‐Effect Transistors

Electron‐transporting organic semiconductors (n‐channel) for field‐effect transistors (FETs) that are processable in common organic solvents or exhibit air‐stable operation are rare. This investigation addresses both these challenges through rational molecular design and computational predictions of n‐channel FET air‐stability. A series of seven phenacyl–thiophene‐based materials are reported incorporating systematic variations in molecular structure and reduction potential. These compounds are as follows: 5,5′′′‐bis(perfluorophenylcarbonyl)‐2,2′:5′,‐ 2′′:5′′,2′′′‐quaterthiophene ( 1 ), 5,5′′′‐bis(phenacyl)‐2,2′:5′,2′′: 5′′,2′′′‐quaterthiophene ( 2 ), poly[5,5′′′‐(perfluorophenac‐2‐yl)‐4′,4′′‐dioctyl‐2,2′:5′,2′′:5′′,2′′′‐quaterthiophene) ( 3 ), 5,5′′′‐bis(perfluorophenacyl)‐4,4′′′‐dioctyl‐2,2′:5′,2′′:5′′,2′′′‐quaterthiophene ( 4 ), 2,7‐bis((5‐perfluorophenacyl)thiophen‐2‐yl)‐9,10‐phenanthrenequinone ( 5 ), 2,7‐bis[(5‐phenacyl)thiophen‐2‐yl]‐9,10‐phenanthrenequinone ( 6 ), and 2,7‐bis(thiophen‐2‐yl)‐9,10‐phenanthrenequinone, ( 7 ). Optical and electrochemical data reveal that phenacyl functionalization significantly depresses the LUMO energies, and introduction of the quinone fragment results in even greater LUMO stabilization. FET measurements reveal that the films of materials 1 , 3 , 5 , and 6 exhibit n‐channel activity. Notably, oligomer 1 exhibits one of the highest μ_e (up to ≈0.3 cm² V^?1 s^?1) values reported to date for a solution‐cast organic semiconductor; one of the first n‐channel polymers, 3 , exhibits μ_e≈10^?6 cm² V^?1 s^?1 in spin‐cast films (μ_e=0.02 cm² V^?1 s^?1 for drop‐cast 1 : 3 blend films); and rare air‐stable n‐channel material 5 exhibits n‐channel FET operation with μ_e=0.015 cm² V^?1 s^?1, while maintaining a large I_on:off=10⁶ for a period greater than one year in air. The crystal structures of 1 and 2 reveal close herringbone interplanar π‐stacking distances (3.50 and 3.43 Å, respectively), whereas the structure of the model quinone compound, 7 , exhibits 3.48 Å cofacial π‐stacking in a slipped, donor‐acceptor motif.     

Experimental evaluation of adsorption and electrode field effect on the constants of weak acid dissociation near the mercury electrode surface

Two methods are described for the experimental evaluation of the weak acid dissociation constants in the near electrode surface. One of them, based on the dependence of differential capacity of pH of the solution at constant potentials, gives the pK_A^s values for adsorbed acids; the second one uses the ψ₁-value dependence on pH in the presence of the cationic acids (as pyridinium ions) and comparison of the pH of the buffer solution of the weak acid under investigation with pK_A^s of the cationic acid and gives the pK_A values in the volume just near the electrode.     

Dynamics of partial wetting

Two broad classes of models have been used to describe the motion of a contact line when the contact angle θ deviates from the equilibrium value θ_e : a) an Eyring approach, emphasizing the microscopic jump of a single molecule at the tip. b) a hydrodynamic approach, concentrating on the viscous losses inside the liquid wedge of angle θ. In the present review, we compare the predictions from both models, for two critical experiments: 1) The pull out of a vertical plate from a fluid at rest -showing (for finite θ_e) a critical velocity V_c above which the plate is completely wet. 2) The velocity of growth of a dry patch for a non wettable surface covered by a flat liquid film -which turns out to vary like θ_e³ at small θ_e.The net conclusion is that, at small θ_e and for low velocities V, the hydrodynamic losses dominate, while at large θ_e and large V, the molecular features are probably important.     

Dissociation of symmetry-adapted local modes studied by FFT-propagation of bond-adapted wavefunctions

The time evolutions ψ₊(t) and ψ_?(t) of dihydrides' near degenerate local modes with + and ? symmetry may be evaluated by Fast Fourier Transform (FFT) propagation of a single bond-adapted wavefunction ψ _a = 2^?1/2 (ψ₊?ψ_?). This economic technique is demonstrated by model simulations of fast dissociations of highly excited local stretches of H₂O.     

Experiments on solvated electron generation and deposition in hexamethylphosphoramide

Theory is developed and compared to experiment for the cyclic voltammetric ejection and redeposition of solvated electrons e_s^? in NaClO₄ and LiCl solutions in HMPA. In current reversal chronopotentiometry, e_s^? mass transfer can be observed free from uncompensated resistance effects by cathodic generation of e_s^? for time t_f followed by anodic current redeposition leading to a reverse transition time τ_b. The ratio τ_b/t_f depends on t_f, applied current, and LiCl electrolyte concentration. Comparison of τ_b/t_f to theory detects a highly reactive scavenger at micromolar concentration levels.