扩散模型和凝聚模型耦合作用下胶体凝聚动力学的Monte Carlo模拟研究

以扩散模型(Ds(γ)=D0×sγ)和凝聚模型(Pij(σ)=P0×(i×j)σ)为基础,对胶体体系随时间的演变、团簇大小分布及其标度关系、团簇的重均大小S(t)的变化规律以及模型对最终分形维数的影响四个角度进行了比较研究,发现扩散指数γ0和凝聚概率指数σ0对胶体的凝聚动力学过程有相似的影响.本文在较宽的γ和σ取值范围内,对胶体的凝聚动力学进行了模拟研究,对慢速凝聚向快速凝聚的转化机理作了定量分析,并进一步分析了在团簇-团簇凝聚(CCA)模型下,得到类似扩散置限凝聚(DLA)模型的凝聚体的物理意义,结果表明:(1)γ0代表了体系中团簇或单粒做"定向运动"而非无规则的布朗运动的情况.这种"定向运动"的推动力可能来自于大团簇产生的强"长程范德华力"、"电场力"等,或来自于体系边界处的外力场的作用.(2)当σ0时,体系成为先快后慢的慢速凝聚,这可能对应大团簇为一排斥中心,即胶体颗粒存在"排斥力场"的现象.(3)证实了团簇的重均大小在凝聚过程的早期按指数规律增长,而后期按幂函数规律增长的实验现象.模拟研究还表明,胶体体系的凝聚动力学过程,在σ0时是一个存在正反馈机制的非线性动力学过程,而在σ0时则体现出负反馈的特征.     

Photoinitiation. II. Kinetics of the acrylonitrile polymerization photoinitiated by aromatic hydrocarbons

The kinetics of acrylonitrile polymerization photoinitiated by aromatic hydrocarbons have been studied. For the acrylonitrile polymerization photoinitiated by naphthalene the rate of polymerization depends on the square root of incident light intensity, on the square root of naphthalene concentration, and on the 1.5 power of acrylonitrile concentration. In the system acrylonitrile-1-methoxynaphthalene the rate of acrylonitrile polymerization depends on the first power of acrylonitrile concentration. The monoradical character of this polymerization process has been established. For the interpretation of experimental results a reaction mechanism involving the formation of the exciplex between the first singlet or triplet of aromatic hydrocarbon and acrylonitrile in the ground state as a precursor of polymerization reactions is suggested. The photoinitiating efficiency of various aromatic hydrocarbons in acrylonitrile polymerization increases in the order: fluoranthene (zero efficiency) ? pyrene < phenanthrene, fluorene ≈ 2-methoxynaphthalene ≈ biphenyl < anthracene < 2-methylnaphthalene < 1-methoxynaphthalene < 2,3,6-trimethylnaphthalene < 2,3-dimethylnaphthalene ≈ naphthalene < 1-methylnaphthalene < 2,6-dimethylnaphthalene < p-terphenyl < acenaphthene, provided that the systems absorb the same amount of the incident light. The explanation of this result ensues from the study of the effect of concentration on the rate of polymerization and from the quenching of hydrocarbon fluorescence by acrylonitrile. The photoinitiating efficiency of a given aromatic hydrocarbon is mainly determined by the value of the rate constant k_q for the formation of exciplex as well as the self-quenching efficiency of aromatic hydrocarbon. By using the literature data for the lifetime of fluorescence τ the values of k_q were calculated from the Stern-Volmer equation expressing the quenching of hydrocarbon fluorescence by acrylonitrile. The order of aromatic hydrocarbons according to increasing values of k_q is as follows: pyrene < phenanthrene < anthracene ≈ naphthalene < 2-methylnaphthalene ≈ 1-methylnaphthalene ≈ 2,3-dimethylnaphthalene < 2,6-dimethylnaphthalene < acenaphthene < p-terphenyl < 1-methoxynaphthalene. The study of the concentration effect reflecting the self-quenching of aromatic hydrocarbons during polymerization has given the following sequence for decreasing self-quenching efficiency of aromatic hydrocarbons: 2-methoxynaphthalene ≈ pyrene > anthracene > 1-methoxynaphthalene > fluorene > 2,6-dimethylnaphthalene, phenanthrene, acenaphthene > 2,3,6-trimethylnaphthalene > 2,3-dimethylnaphthalene > 1-methylnaphthalene > naphthalene. It has been shown that the photoinitiating efficiency of a given aromatic hydrocarbon in the polymerization of acrylonitrile can be roughly predicted from the position of that aromatic hydrocarbon in the above-mentioned sequences.     

Kinetics of growth of chain aggregates in magnetic suspensions

Theoretical study of the kinetics of the growth of chain aggregates in suspensions of magnetizing non-Brownian particles is performed. An analytical model for calculating the time-dependent distribution function over the number of particles in the chains is developed and computer experiments on the kinetics of the aggregation of these particles are carried out. Results of calculations using an analytical model are in good agreement with the data of computer experiments when the volume concentration of particles is in the range of 1–2%, which corresponds to many modern magnetorheological suspensions. This allows us to recommend the developed mathematical model as a basis for describing kinetic phenomena in magnetorheological suspensions with low or moderate particle concentrations.     

Energy distribution among reaction products. XIV. F + CH4, F + CH3X(X  Cl,Br, I), F + CHnCl4−n(n = 1−3)

The infrared chemiluminescence technique has been used to obtain relative rate constants k(ν′) for HF(ν′) formed in the following reaction:

For reaction (1) the detailed rate constants [k(ν′ = 1) = 0.30;k(ν′ = 2) = 1.00; k(ν′ = 3) = 0.15; mean fraction of the available energy entering vibration <?^′_ν> = 0.56] confirmed, at much lower reagent pressures, results obtained by previous workers. In series I there was a slight increase in fraction of the energy entering vibration as the molecular reagent altered from CH₃Cl to CH₃Br to CH₃I, viz <?^′_ν> = 0.50 (1a), <?^′_ν> = 0.58 (1b), <?^′_ν> = 0.60 (1c). In series 2, by contrast, there was a marked decrease in fractional conversion of the available energy into vibration with increasing chlorination of the molecular reagent; <?^′_ν> = 0.50 (1a), <?^′_ν> = 0.23 (2a), <?^′_ν> = 0.13 (2b). The rate constants into ν′ = 0, k(ν′ = 0), were obtained by extrapolation of surprisal plots; the trends for both series were, however, also evident from k(ν′ > 0). No separate initial rotational distribution was observed for any of these reactions, indicating that the peak of the initial distribution is not far removed from a 300 K thermal distribution. The decrease in <?^′_ν> for the HF products along series 2 was tentatively ascribed to increasing internal excitation in the ejected radicals CH₂Cl, CHCl₂, CCl₃, due to increase in the number of secondary encounters between HF and the departing radical.     

Interaction between small colloidal particles and polymer chains in a semidilute solution: Monte Carlo simulation

 Interaction between flexible-chain polymers and small (nanometric) colloidal particles is studied by Monte Carlo simulation using two-dimensional and three-dimensional lattice models. Spatial distribution of colloidal particles and conformational characteristics of chains in a semidilute solution are considered as a function of the segment adsorption energy, ɛ. When adsorption is sufficiently strong, it induces effective attraction of polymer segments, which results in contraction of macromolecular coils. The strongly adsorbing polymer chains affect the equilibrium spatial distribution of the colloidal particles. The average size of colloidal aggregates <m> exhibits a nontrivial behavior: with ɛ increasing, the value of <m> first decreases and then begins to grow. The adsorption polycomplex formed at strong adsorption exhibits a mesoscopic scale of structural heterogeneity. The results of computer simulations are in a good agreement with predictions of the analytic theory [P.G. Khalatur, L.V. Zherenkova and A.R. Khokhlov (1997) J Phys II (France) 7:543] based on the integral RISM equation technique. Received: 4 August 1997 Accepted: 16 April 1998     

Molecular-weight distribution and structural transformation in water-soluble complexes of poly(acrylic acid) and bovine serum albumin

Interaction of polyacrylic acid (PAA) with bovine serum albumin (BSA) at different pH values and in a wide range of mixing molar ratios, γ = n_BSA/n_PAA, of components was investigated by size-exclusion high performance liquid chromatography with on-line refractive index, UV, light scattering and viscometer detectors. The results revealed the formation of stable water-soluble polymer-protein complexes at pH 5.0. For the soluble complexes thus formed, the number of the bound BSA molecules with one PAA molecule was expressed by a Langmuir-type equation as a function of the amount of excess BSA existing free in the solution. At saturation, one BSA molecule is bound to about 48 acrylic acid residues.The γ-dependencies of molecular properties and structural parameters (molecular weights, molecular-weight distribution, radius of gyration, and the Mark-Houwink equation constants) of aqueous solutions of polycomplex particles have been studied. It has been concluded from these results that the complex molecule is formed by the molecular association-dissociation processes between particles depending on protein molecules in mixtures. We assume that side-by-side association of BSA-PAA complex particles took place at γ ? 5. At γ > 5, dissociation of the aggregates occurred by the including certain protein molecules into composition and by the compactization of polycomplex particles.     

Molecular weight distributions and moments for condensation polymerizations characterized by two rate constants

Molecular weight distributions (MWD) and moments have been computed for condensation polymerizations of ARB-type monomers wherein the reactions involving the monomer are characterized by a rate constant k₁₁ and those involving other species, by k_p. For k₁₁/k_p < unity, the MWD curves are found to split into two, one for even n and one for odd values of n. Substantial amounts of unreacted monomer are found in the reaction mass for this case and so moments and the polydispersity index computed without the monomer are better representations of the distributions. For k₁₁/k_p > unit, conventional MWDs are observed with dispersions different from those representing polymerizations satisfying the equal reactivity hypothesis.     

Atomic one- and two-electron subshell moments in position and momentum spaces

For 357 subshells of the 53 neutral atoms He through Xe in their ground states, the two-electron intracule (relative motion) <u ^k > _nl and extracule (center-of-mass motion) <R ^k > _nl subshell moments in position space are examined as well as their counterparts <v ^k > _nl and <P ^k > _nl in momentum space, where n and l are the principal and azimuthal quantum numbers of the atomic subshell, respectively. It is clarified that between the intracule and extracule moments the “2 ^k -rule” is strictly valid, which means <u ^k > _nl = 2 ^k <R ^k > _nl and <v ^k > _nl = 2 ^k <P ^k > _nl for any nl subshell. Theoretical analysis also proves that for a particular case of k = +2, two relations <u ²> _nl = (N _nl −1)<r ²> _nl and <v ²> _nl = (N _nl −1)<p ²> _nl hold exactly, where N _nl (≥2) is the number of electrons in the subshell nl, and <r ^k > _nl and <p ^k > _nl are the familiar one-electron subshell moments in position and momentum spaces, respectively. The latter equality establishes a new and rigorous relation between the second electron-pair moments in momentum space and the total energy of an atom through the virial theorem. For k=+1, −1, and −2, the numerical Hartree-Fock results for the 357 subshells show that there are approximate but accurate linear relations between <u ^k > _nl and <r ^k > _nl and between <v ^k > _nl and <p ^k > _nl , in which the proportionality constant in each space depends on n,l, and k. Received: 27 April 1998 / Accepted: 29 May 1998 / Published online: 28 August 1998     

Hydrocarbon peroxidation: protective effect of reverse micelles

Pulse radiolysis technique was used to study radical processes in AOT/n-heptane and AOT/n-heptane/water reverse micellar systems. It was found that reverse micelles, especially socalled 'wet' micelles ([H₂O]/[AOT] > 10), significantly diminished the yield of peroxyl radicals formed when the system contained trace amounts of oxygen. The possible mechanism of such protective effect of micellar aggregates is discussed in terms of scavenging of charges, both electrons and cation radicals, by micellar aggregates where they can eventually form radicals. The latter are however separated from the traces of oxygen which remains mainly in the continuous hydrocarbon phase, where it dissolves much better than in the micellar water core.     

Anodic waves of 9,10-phenanthrenequinone in alkaline media

9,10-Phenanthrenequinone yields at pH > 11 anodic waves, which correspond to a formation of a slightly soluble salt of mercury formed in the reaction of Hg₂²⁺ with the product of a reaction of the quinone with two OH⁻ ions. Anodic waves are a linear function of phenanthrenequinone concentration up to about 1 × 10⁻⁴M and are affected by adsorption at higher concentrations. The concentration limit depends on the cosolvent used and decreases in the sequence MeOH > EtOH > n-PrOH. At given concentrations of phenanthrenequinone and of OH⁻ ions the anodic current decreases with increasing alcohol concentration.     

Self-organization processes and synthesis of nanostructured silver microparticles in AgNO<Subscript>3</Subscript> solutions with variable pH values

The morphology of micrometer-sized silver particles obtained by liquid-phase chemical reduction of silver nitrate with ascorbic acid depends appreciably on the solution pH. The synthesis carried out at 100°C for 20 min at pH < 4 or pH > 9 yields anisotropic faceted nanocrystalline particles, while the synthesis at pH = 5–8 results in self-assembly to give microspheres representing close-packed aggregates of a huge number of silver nanoparticles with a cauliflower structure.     

Applications of k 0 NAA at FRM II with high f values

The research reactor FRM II offers different irradiation facilities with highly thermalized neutron flux. 3 facilities for the k ₀ neutron activation analysis (k ₀ NAA) will be introduced shortly. The influence of flux parameter α on the concentration calculation of samples irradiated in a neutron field with very high ratio of thermal to epi-thermal neutron flux f > 1,000 are here investigated. Even for the most k ₀ isotopes with big Q ₀ values, the uncertainty of a concentration calculation without α correction is <3 %, when the f value larger than 3,000. The uncertainty is about 5 % for the isotope ⁹⁶Zr in this case. The k ₀ library of the computer program MULTINAA is updated. A standard reference material IAEA/soil-7 was analyzed to verify the k ₀ NAA at FRM II.     

Effect of surface-modification on hydration kinetics of nitrofurantoin anhydrate

Following a powder dehydration process, the surface-modified anhydrates (EA, PA and BA) of nitrofurantoin were obtained in a specially designed surface-modification apparatus at 60 °C via the adsorption of ethanol, n-propanol and n-butanol, respectively. The intact (IA) nitrofurantoin was used as control. The X-ray diffraction, FT-IR and DSC results suggested that the crystalline characteristics of anhydrate were not affected by surface-modification treatment, such as the adsorption of ethanol, n-propanol or n-butanol. Powder X-ray diffraction and differential scanning calorimetric (DSC) were used to determine the crystalline characteristics of the samples that had been stored at 93% relative humidity (RH) and at 40±1°C. The hydration of surface-modified samples followed a first-order process with an induction period. The hydration rate constants obtained by the X-ray diffraction method were in the order of PA≈BA<EA<IA. The associated induction periods were in the order of BA≥PA>EA>IA. On the other hand, the order of hydration rate constants measured by DSC method was PA≈BA<EA<IA with induction periods of BA≥PA>EA>IA. The values of kinetic parameters obtained by DSC method were different from those obtained by X-ray diffraction method. The results suggested that the surface-modified anhydrates were more stable in high humidity condition than intact, and the surface-modification by n-butanol or n-propanol adsorption was more effective than that by ethanol. The dissolution test for surface-modified samples was performed following the method as stipulated in the Japanese Pharmacopoeia XIII. Briefly, 20 mg of sample powder was introduced into 900 ml of distilled water at 37±0.5 °C with stirring by a paddle at 0 or 100 rpm. The dissolution kinetics was analyzed based on Hixon–Crowell equation. In the dynamic condition (100 rpm), the dissolution profile of surface-modified sample by n-butanol was superimposable to that of intact sample. In contrast, under the static dissolution condition the intact sample showed faster dissolution than the surface-modified initially. The result indicated that wetability of the sample decreased by surface-modification using n-butanol.     

The role of surface oxides in formic acid oxidation on Au

Compared to Pt or Pd electrodes, Au is a poor catalyst for the direct anodic oxidation of HCOOH, but the formation of Au surface oxides in acidic solutions is accompanied by a fast oxidation of HCOOH. This fast reaction is not simply a secondary reaction of Au surface oxides since those oxides are kinetically stable in HCOOH solutions. They do oxidize HCOOH only via a slow and purely electrochemical process which occurs on free Au sites and is “driven” by oxide reduction. The fast HCOOH oxidation is due to a highly reactive intermediate which is able either to form stable Au oxides Au_nO_m or to react with HCOOH. Our results are consistent with the model that by the charge transfer step a reactive non-equilibrium {Au…O> species is formed which converts to stable equilibrium oxides Au_nO_m after migration and rearrangement steps. Pre-equilibrium <Au…O> oxidizes HCOOH and this oxidation is of lower order with respect to <Au…O> compared with the formation of Au_nO_m.     

Ion—polar-molecule reactions: A CRESU study of He+, C+, N+ + H2O,NH3 at 27, 68 and 163 K

The first measurements of ion—polar-molecule reaction rate constants at very low temperatures are presented. They have been obtained using the CRESU (cine_.tique de reactions en ecoulement supersonique uniforme) technique for H⁺.C⁺ and N⁺ ions reacting with H₂O and NH₃ at 27 and 68 K in helium buffer. Additional data have been obtained for N⁺ reactions at 163 K in nitrogen buffer. In the 27–300 K (27–163 K for N⁺ + NH₃) temperature range, all the results yield a power law, k = k₀T⁻ⁿ (0 <n < 1), for the rate coefficient of each reaction, which should be applied in interstellar cloud model in place of the room-temperature values. The results are compared with various theoretical calculations. Rather good agreement is found although no general behavior can be simply drawn from these experiments.     

Kinetics and mechanism of glucose electrooxidation on different electrode-catalysts: Part I. Adsorption and oxidation on platinum

The processes of adsorption and electrooxidation of glucose on a smooth platinum electrode have been investigated in a wide range of pH values. It is found that glucose adsorption are platinum is accompanied by dehydrogenation of adsorbed molecules. The θ_R vs. E_r dependence represents a bell-shaped curve with unequal sides and with a maximum at E_r = 0.2 V at 0 < pH < 12 or at E_r = 0.4 when pH > 12. The kinetics of adsorption is described by the Roginsky-Zel'dovich equation, and the dependence of the steady-state coverage on the glucose bulk concentrations by the Temkin isotherm.It is shown that in the case of glucose adsorption on platinum Q^dehyd._H > Q_H, i.e. when glucose is brought into contact with a platinum electrode, the catalytic decomposition of glucose molecules occurs in addition to the formation of strongly chemisorbed particles. The transient current at E_r < 1.0 V is a current due to the ionization of hydrogen formed during adsorption with dehydrogenation of glucose and its catalytic decomposition. The glucose electrooxidation rate under steady-state conditions at E_r < 0.7 V is determined by the interaction of the chemisorbed carbon-containing particle with OH_ads. The slow step of glucose electrooxidation in the potential range 1.0 < E_r < 1.5 V is the interaction of glucose molecules from the solution bulk with the surface platinum oxide, the latter undergoing a quick electrochemical regeneration thereafter.The basic regularities and mechanism of glucose electrooxidation on platinum are shown to be analogous to those obtained earlier for such elementary organic fuels as formaldehyde and formic acid.     

Correlations: Alkylsilane property — polynomial coefficients of the adjacency matrix of the molecular graph

Based on the characteristic polynomial coefficients (CPCs) of the adjacency matrix A′ of heterogeneous molecular graphs of the molecules containing a tetravalent heteroatom >Si< (>Ge<, >Sn<), etc. in the chain, a 13-constant additive scheme for the calculation of their physicochemical properties is obtained. The structural meaning of CPCs of the adjacency matrix A′ is established. By the formula obtained the formation enthalpies Δ _f H _{gas, 298K} ⁰ of SiC _n H_2n+2 alkylsilanes not studied experimentally are calculated.     

Living polymers and polymer solutions: a misleading analogy

 Experimental results from colloidal suspensions of worm-like micelles are currently interpreted in terms of close analogies between this kind of systems and polymeric solutions. In particular, it was hypothesized that the viscoelastic properties of dense systems of giant flexible cylindrical micelles can be rationalized in terms of an entangled network of worm-like aggregates, very similar to a neutral random polymeric network. Such an idea is strongly supported by theoretical results that, in a mean-field appro ximation, suggests for an unlimited growth process of the micellar contour length with concentration. The mean-field theory indicates for an exponentially shaped length distribution function, with mean <L> depending on concentration, φ, in agreement with a scaling law <L>∝φ^α (α=0.5 in the simpler approach). A number of experimental results seem to be successfully interpretable within this framework. Aim of this work is to show that the agreement between theory and experiment is just an accident, being the mean-field approach, in principle, inadequate in describing systems dense enough to show a concen tration dependence of the mean micellar size. It will be unambiguously shown that there is no way to describe semi-diluted micellar solutions through a mean-field approximation and that there does not exist any scaling law of the kind <L>∝φ^α. Furthermore, it will be shown that the shape of the size distribution function is markedly different from the exponential one. The basis for a more realistic approach for the growth process of micellar aggregates is also presented and some pre liminary indications are success fully compared with experimental results. Received: 24 February 1997 Accepted: 22 April 1997     

Fluorescence.detected X-band epr spectroscopy of the photosynthetic bacterial triplet state

The X-band FDMR spectrum of the bacterial triplet in reduced Rhodopseudomonas sphaeroides at 5 K has been obtained. Pure S-T₀ mixing is sufficient and k_z <k_x,k_y necessary to explain the polarization pattern and intensity ratios. The kinetic fluorescence response is sigmoidal due to the second-order kinetics of antenna-reaction center energy transfer.