A recent study [1] of the dielectric frequency response of a two component composite performed on a single specimen shows that local porosity theory LPT [2] represents a substantial improvement compared with other theories predicting the complex dielectric dispersion [3,4,5]. The purpose of the present work is to extend this investigation to a systematic study on several specimens with different compositions.

We report Canonical (NVTMC), Grand Canonical (GOMC) and Gibbs Ensemble Monte Carlo (GEMC) simulations for adsorption of methane on graphite. Lennard-Jones (LJ) potentials are used for the intermolecular interactions, and both structured and structureless (10–4–3) solid-fluid potentials are considered. Several sets of methane-methane L.I parameters have been used in the literature, and we compare results obtained with these sets throughout our simulations. The adsorption isotherm and isosteric heat curve are obtained at 77.5 K and found in good agreement with experiments. The commensurateincommensurate transition (CIT) of methane on a graphite substrate with periodically varying adsorbate-adsorbent potential at 40.0 K is studied and is in qualitative agreement with experiment. The effect of varying the corrugation of the fluid-wall potential on the commensurate and incommensurate phases is explored. The GEMC simulations have been carried out to study the vapor-liquid equilibrium (VLE) of a two-dimensional (2D) LJ fluid with system sizes up to 3000 particles. The effect of system sizes on the critical behavior is investigated. The GEMC method has also been successfully applied to study the VLE in 2D adsorbed films for the first time.

The interfacial geometry of a disordered porous medium strongly influences Knudsen diffusion of gases and interfacial dipolar energy transfer. Some interesting comparison can be made between these two transport processes, involving statistical properties of chords belonging either to the solid matrix or to the pore network. A quantitative analysis of these two mechanisms is proposed, based on a chord distribution model. In a first part, we discuss how chord distribution functions contribute to the stastistical characterization of a porous medium. We show that a direct connection between imaging techniques and small angle scattering provides, in many cases, a reliable description of theses functions. In a second part, interfacial direct energy transfer is analyzed. This one step excitation transfer strongly relies on the interfacial autocorrelation function φ2s(r). An analytic expression of φ2s(r) is given and theoretical predictions compared with available experiments. In a third part and following the seminal work of Derjaguin on the Knudsen diffusion, we critically examine how the self diffusion coefficient can be related to the two first moments of the pore chord distribution. A direct comparison with experimental results and numerical simulations is presented in the case of a model porous medium: the random packing of hard spheres. Finally, we analyze a trapping reaction where an excited gas molecule, diffusing in the Knudsen regime, relaxes primarily by wall effects. We show how the chord distribution of the pore network permits to compute the survival probability of the tagged molecules. Two situations are more closely analyzed: the strong and the very weak wall quenching efficiency.

The nature of the pair interaction between dipoles embedded in a liquid in the vicinity of a non-metallic interface is investigated in the continuum limit. It is shown that the dipole-dipole interaction can be significantly modified in the presence of a boundary in liquids characterized by a nonlocal dielectric function or in liquids whose dielectric properties change due to geometrical restrictions. Different limits are studied and relationships to experimental observables are discussed.

Fluorescence decay curves from particles composed of poly(vinyl acetate) (PVAc) and naphthalene (N) labelled poly (2-ethylhexyl methacrylate) (PEHMA) were analyzed for Direct Energy Transfer (DET) from N to anthracene by the exponential series method(ESM). An averaged apparent dimension of d=2.0 were observed. The Laplace transformed Klafter-Blumen(KB) equation for the lifetime distribution was used to interpret these dimensions. Additional fluorescence experiments were carried out on Polymer films composed of the same material above. Exposure of the films to a pentane solution of 9-anthrylmethyl pivalete (AMP) transferred the AMP molecules exclusively to the PEHMA phase of the material. ESM analysis together with transformed KB equation showed that PEHMA thread has a unique dimension of 1.3 in PVAc matrix of this polymer film.

Near-field optics techniques make it possible to by-pass the optical diffraction limit (“uncertainty principle) and attain spatial resolution of λ/50 or better. We present near-field scanning optical spectroscopy (NSOS) data on α and ß mixed micro-crystals of perylene and on various aggregates of tetracene doped into PMMA. The spatial resolution is limited by the size of the scanning photon tip and its distance from the sample. We use nanofabricated optical fiber tips (aluminum coated) that are as small as 100 nm. These can be piezoelectrically scanned close to the sample. Fluorescence spectra easily differentiate between adjoining microcrystallites of α and β perylene, giving spectra identical with those of large (>1 cm) single crystals. The apparently homogeneous molecularly doped polymer samples of tetracene/PMMA have regions that fluoresce anywhere between green and red. Thus the spatially resolved spectra are much sharper and more detailed than the broad and featureless bulk spectra. The different emission spectra are attributed to different aggregates of the tetracene guest embedded in the PMMA host

It was demonstrated earlier [1,2] in the framework of equilibrium thermodynamics that the morphological stability of the free boundaries and interfaces in crystals is extremely sensitive to the presence of shear stresses. Relying on that idea we have established the formula H = μσ/τ2 of a critical thickness of solidifying He4 films and of the dislocation-free Stranski-Krastanow growth of epitaxial films (where σ – the coefficient of surface tension, μ - the shear module of the crystal, τ - the external or misfit stress). In this report we present certain facts pertaining to possible patterns of the growing corrugations and introduce the second critical thickness at which a symmetry change in the patterns has to occur.

The positive temperature coefficient of resistivity (PTCR) effect in Ba1-xSrxPb1+yO3+z has been systematically studied. The effect of the PTC resistivity was strongly influenced by the preparation condition. The PTCR effect in metallic-conducting BaPb1+yO3+z. is confirmed around 700 °C, and the temperature where the PTCR effect starts can be successfully shifted to higher temperature range by substituting strontium for the A-site barium. The magnitude of the PTCR effect was increased and the resistibility was reduced by the enhancement of the sintering. In addition to Pb(IV) in the perovskite structure, Pb(II) is detected at the grain boundary in the sintered body.

XAFS spectra at the Mn K-edge were obtained for films of In1-xMnxAs (0.0014 ≤x ≤ 0.12) grown by MBE method at two different substrate temperatures Ts, = 200–210 °C and Ts, = 280–300 ° C. It has been found that Mn-As complexes which consist of a central Mn atom surrounded by six neighboring As most likely arranged in the form of a distorted trigonal antiprism with one or two additional Mn atoms placed on the long axis of the antiprism, can substitute for the In-As tetrahedron in the undistorted zincblende structure. For a composition of x = 0.12 we have found the formation of MnAs clusters with NiAs-like structure in the high-growth-temperature samples. We thus conclude that the magnetic properties of the In1-xMnx As semiconductors are mainly determined by the formation and local structure ol'the Mn-As complexes.

Many electron devices and chemical reactions depend on the escape rate of particles confined by potential wells. When the diffusion coefficient of the particle is small, the carrier continuity or the Smoluchowski equation is used to study the escape rate. This equation includes diffusion and field-aided drift. In this work solutions to the Smoluchowski equation are probed to show how the escape rate depends on the potential well shape and well depth. It is found that the escape rate varies by up to two orders of magnitude when the potential shape differs for a fixed well depth.

Through computer simulation we look at the elementary reaction rate laws of the trapping reaction A+T→T reaction on a one-dimensional lattice. In particular, we calculate the heterogeneity exponent h of the integrated reaction rate law. However, as the reaction probability p is varied, the heterogeneity exponent becomes a function of time. We find a bifurcation of asymptotic values of h: 1/2 for reactions where p ≤ 1.0 and 0 where p=0. In addition, the usual constraints of the trapping problem have been relaxed, and we look at systems where the A's move and the T remains immobile and the reverse case, where the T moves and the A's sit.

The elementary batch reaction A + B = 0 is re-examined via Monte-Carlo simulations on a one-dimensional lattice. The relative mobility of the A and B species is varied in this model, but the initial densities of the A and B are always the same. We calculate the rates, the density profiles, and the particle distribution functions. The rate power law is conserved, i.e., the well-known 1/4 behavior is established for all mobilities. The rate coefficient is the only mobility-dependent quantity. The interparticle distribution functions show that the aggregation depends on the relative mobility but the segregation does not. This subtle difference has no effect on the asymptotic reaction order, which is close to 5.

Monte Carlo simulations of the A2 + B2→ P (with P = AA, AB, and BB) reaction on various monodisperse island systems provided various models for the reaction H2 + D2→ HD on alumina supported platinum catalysts. Anomalous reaction kinetics and reaction orders due to the effects of diffusion, reactant correlation, and ensemble character of the catalyst in low dimensional systems are observed not only at early time but also at steady state. Concentration effects and spillover studies indicate that surface diffusion and hydrogen spillover may be responsible for the anomalous behaviors.

Continuing work on elementary A+B→C reactions in capillaries, we study the reaction front dynamics of xylanol orange with Cr3+ in an effectively one-dimentional system with initially separated reactants. This reaction, in contrast to previously studied systems, is not strictly in the diffusion limited regime. i.e. the probability of reaction between species is not unity. Anamalous behavior not seen in the diffusion-limited case has been observed experimentally for the reaction rate, boundary motion, reaction zone width, and local reaction rate. The observed behavior is consistent with recent theoretical studies.

The reaction A+A→0 is simulated in 1-D and 2-D square lattices with various local reaction probabilities, P. The effective reaction order, X, and the nearest neighbor distance distribution (NNDD), are evaluated in all these reactions. For batch reactions, sharp increases in X with increasing P occur at early times. Classical reaction limited kinetics is obtained at early times only when P→0. At long times, all reactions are in the non-classical, diffusion limited regime, regardless of P. For steady state reactions, our results demonstrate a similar behavior of X with P. The NNDD at steady state in 1-D media at P=1.0, i.e. diffusion limited reaction, follows the previously reported skewed exponential shape. This is no longer true for P<I. Finally, at P→0, as expected, an exponential (Poissonian) distribution is obtained for both reaction conditions.

Nanometer light and exciton sources and probes have been prepared by adding various inorganic and organic crystals and molecularly doped polymers to micropipettes and nanofabricated optical fiber tips. Specifically, a new nanotechnology, near-field photonanofabrication, has been developed, leading to a thousandfold miniaturization of immobilized Fiber Optical Chemical Sensors and to a billionfold decrease in necessary sample volume. The response time has also been shortened by a factor of at least 100. Applications of these subwavelength probes include biological single cell analysis, supertip development, Förster-energy transfer and Kasha quenching phenomena at the interface between the positionally controlled nanocrystal tip and its photoactive environment. Practically, this leads to enhanced sensitivity of optical probes, nano-optical chemical sensors and near-field exciton light sources.

The diffusion of ethane and n-pentane on metallic surfaces is studied by molecular dynamics simulations. On atomically smooth, faced centered cubic (111) crystal surfaces, n-pentane has several conformational states, each of which diffuses at a different rate. Diffusion barriers are distinct from intramolecular conformational isomerization barriers. Weak friction is observed. Hopping diffusion occurs near the diffusion threshold. As the temperature is raised, correlated barrier crossing and recrossing increases. n-Pentane slides, transverse to its axis, without changing its orientation with respect to the surface. Conformational changes occur on a longer time scale than diffusional barrier crossing and are restricted by the surface. Far above the diffusion threshold temperature, partial molecular desorption occurs and conformational isomerization is less restricted. The molecules slide, roll and tumble continuously across the surface. A similar pattern of temperature-dependent change in diffusion mechanisms occurs in ethane but without conformational changes.

The dynamic response of water-in-oil AOT microemulsion droplets to perturbations caused by rapid energy transfer has been interpreted in terms of the surfactant interface bending rigidity. The Iodine Laser Temperature Jump technique was used to rapidly (1 μs) increase the microemulsion temperature and thus disturb the system equilibrium. The dynamic relaxation to the new equilibrium position was monitored by measuring the turbidity of the microemulsion over a I ms time interval immediately after the temperature jump. The rate and direction of the turbidity change depended on the initial temperature, size and salt content of the water droplets. We observed characteristic times of 2 to 10 μs and attributed them to the relaxation of the surfactant interface. A bending modulus of 0.4 kT was derived for the AOT interface, using a model for the shape perturbation of a spherical interface.

The behavior of colloidal suspensions confined between two plates narrowly spaced is examined. When plate separations are small, only one layer of particles can be confined; however, as plate separation is increased, multiple layers will eventually form. These layers can exist in a variety of structures depending on the energetics of packing. We examine the unstable modes which form as a single layer buckles on its way to forming two layers. Three ordered states are found; which one is selected depends on the form of the interparticle and wall potentials.

Dynamics of polystyrene molecules inside controlled pore glasses, a typical confining geometry, was studied by dynamic light scattering over a wide range of concentrations of polystyrene in solutions in equilibrium with the porous glasses. Index-matching of the solvent to the silica glasses effectively facilitates the acquisition of information on the dynamics of polymer chains inside the pore without compromising that information by multiple light scattering. When the concentration outside the pore is much smaller than the overlap concentration v*, the apparent diffusion coefficient Dporc of polymers within the pore shows little dependence on concentration. As the outside concentration increases and approaches v*, Dporc rapidly increases. This tendency is more pronounced for polystyrene samples that have higher molecular weights and are predicted to have a lower concentration inside the pore. With further increases of concentration beyond v*, Dporc approaches the apparent diffusion coefficient outside the pore. Moreover, Dporc becomes almost the same for the three different molecular weights of polystyrene fractions studied and depends primarily on the weight concentration of the solute outside the pore. These features are typical of a semidilute solution regime for flexible polymers. The rapid increase in Dporc, is ascribed to a drastic increase of the polymer concentration inside the pore, which results from an equilibration of the chemical potential of the polymer molecule between the interior of the pore and the exterior. Thus, a rapid increase in the osmotic pressure outside the pore drives the polymers into pore channels even at the expense of reduced entropy. We present a quantitative analysis of this highly nonlinear partitioning of polymer molecules.