Various mechanisms of glassy transformations involving computational annealing were investigated by Molecular Dynamics simulations. Large clusters of silicon dioxide ranging from sub to nanometer size regime were considered. Silica is both a prototype ceramics and glassy material. Silica particles are fabricated in flow and flame reactors to design novel granular materials which depend strongly on the heating and cooling processes. During these processes extensive thermally driven relaxation in growing clusters allow for configurational changes from a liquid-like cluster to a glassy cluster. Crystal-like structures were investigated as well. Cooling rates comparable to experimental rates were achieved in these simulations. We find that the glass transition temperature decreases with decreasing cluster size. Calculations were performed by implementing a massively parallel particle decomposition schema of Molecular Dynamics with an excellent speedup and a significant decrease of complexity.

We report a real-time, two-dimensional light scattering study of the evolution of structure of a two component nonionic micelle system undergoing phase separation. The micelles act like molecular slug-a-beds whose domain growth is pathetically lathargic (i.e. slower than the cube root of time prediction for simple binary fluids). In fact, the growth kinetics can be empirically described as a stretched exponential approach to a pinned domain size. Although the kinetics are not yet understood, this anomalous behavior may be due to the ability of the spherical micelles to reorganize into more complex structures.

We have used ellipsometry to measure the glass transition temperature (Tg) of ultrathin films of polystyrene (PS) (less than 10 nm thick) obtained by grafting PS-COOH on the native oxide of Si. We find that Tg in these ultra-thin films is depressed from the bulk value by as much as, 35 K. This is in qualitative accord with our earlier results on thicker non-grafted films of PS.

The molecular dynamics of fluids in porous media has been studied using field-cycling NMR relaxometry and NMR field-gradient diffusometry. The frequency dependences of the 1H and 2H spin-lattice relaxation times T1 of various liquids in porous glass reveal weak and strong adsorption behaviour depending on the polarity of the adsorbates. Correlation times eight orders of magnitude longer than in bulk have been observed. The T1 dispersion moreover reflects geometrical details of the matrix in a length scale three orders of magnitude longer than the adsorbate molecules. The mean-square displacements of adsorbate molecules on the surface are only one order of magnitude less than in bulk. The global diffusivity is reduced by tortuosity and porosity effects. The observed phenomena may be explained by bulk-mediated surface diffusion, i.e., Lévy walks. The dynamics of polymer chains much longer than the pore size is characteristicly different from that in bulk melts. There is evidence that the reptation mechanism explains at least a part of the phenomena observed for the porous matrix in contrast to findings with bulk polymer melts.

In brine-saturated porous media the existence of a space-charge layer at the solid/liquid interface leads to a coupling between fluid and electric currents. This coupling is seen as a streaming potential (STP) across a porous plug when the fluid flows through it, or conversely, as electroosmosis (ELO) of the fluid when an electric field is applied. The magnitude of these electrokinetic effects depends on the thickness of the space-charge layer relative to the pore size, which in turn depends upon the salinity of the brine. From electrokinetic measurements one can obtain an effective hydraulic radius Reff and an effective zeta-potential at the slipplane ζeff. We have developed a high-sensitivity AC technique that can detect these small coupling effects, and have measured them in a suite of natural and artificial rock samples. We find that Reff and ζeff depend on salinity in ways which may be attributed to the roughness of the pore surfaces.

NMR measurements of sorbate mobility in zeolites are especially attractive because of their capability of measuring multicomponent and anisotropic self-diffusion. We have recently reported the application of the pulsed field gradient NMR technique using very large zeolite crystals to study how easily methane can diffuse when we attempt to slow its migration by crowding the pore space. Here we analyze the implications of these PFG NMR experiments involving (i) ethylene blocking of methane in zeolite NaY; and (ii) methane molecules trying to pass one another in the molecular sieve A1PO4-5.

Time-resolved transient grating optical Kerr effect experiments on a nanosecond timescale were used to investigate the reorientational dynamics of pentylcyanobiphenyl (5CB) confined in the nanometer lengthscale pores of silica glasses. Pore size dependent studies (25 Å ≤ d ≤ 100 Å) over a large temperature range from the bulk phase transition temperature Tni, = 35.2 ° C up to 115 ° C demonstrate a drastic influence of geometrical restriction on the cooperative reorientational motion in the nematogenic liquid. At temperatures far above the phase transition (T ≥ 70 °C) a bulk phase and a distinct surface layer with different dynamic properties are observed. With pore size reduction to 25 Å, a decrease of the surface layer thickness and an increase of the surface layer relaxation time are found. This is associated with the modification of the surface layer structure by geometrical factors, e.g., the pore curvature. Measurements at lower temperature in the range Tni ≤ T ≤ Tni = 30 ° C show a drastic decrease of the mean relaxation time τ of the order parameter fluctuations compared to the bulk. The relaxation dynamics exhibit a pronounced pore size dependence which can be explained by a crude correlation length cutoff model as well as a modified LandaudeGennes ansatz. However, these approaches cannot reproduce the nonexponentiality of the observed decays suggesting the importance of interporous interactions.

In this work a newly developed dual control volume grand canonical molecular dynamics technique simulates the diffusion of gas in a cylindrical pore. This allows spatial variation of chemical potential and hence an accurate simulation of steady state pressure driven diffusion. The molecular sieving nature of imicroporous imogolite models and the Knudsen effect are discussed and compared with experimental data.

We have employed a quartz microbalance technique to record adsorption isotherms on silver and gold surfaces whose roughness has also been characterized by x-ray reflectivity or scanning tunneling microscopy. We observe strikingly different behavior for two different liquid adsorbates (oxygen and nitrogen), and attribute this to a difference in their surface tension. Our results demonstrate the impact that capillary condensation phenomena can have on the interpretation of adsorption data, particularly with regards to the fractal nature of the substrate. Valuable information on surface morphology can nonetheless be obtained from adsorption isotherms, if combined with alternate experimental techniques.

We report measurements of the hysteretic capillary condensation of liquid 4He into the porous material Nuclepore. Results are presented for global and hysteretic behavior for 300 Å diameter pores. These observations deviate from the predictions of the Preisach model of independent hysteresis domains. By modifying the Preisach model so as to restrict the ability of some of the pores to drain normally we are able to model many features of the experimental data.

A new model for adsorption on rough surfaces is derived. The model predicts behavior regarding surface smoothing that agrees well with experimental observations. Computer simulations show the model capable of matching a wide variety of isotherm patterns with reasonable physical constants. These simulations also show that a significant error results in applying the standard BET equation to adsorption data obtained on rough surfaces.

The Thermodynamic properties of TNT physically confined in 5, 10, and 20 nm pores were investigated by Differential Scanning Calorimetry (DSC). Depending upon the pore size and the sample condition, different aspects of the thermodynamic properties of the confined TNT were illustrated. The freezing transition of the confined TNT in 10, 20 nm pores can be triggered by excess bulk TNT on the outer surface. The sharp freezing transition of the TNT in 20 nm pores without the presence of the bulk suggests that the confined TNT maintains its interconnectivity during the transition. The confined TNT in 10 nm pores without bulk on surface failed to freeze during the cooling run up to 100 K with cooling rates ranging from 10 K/min. - 1 K/min. However, a sharp crystallization peak was observed upon heating. When the TNT is confined in 5 nm pores, the confined TNT is incapable of freezing.

Broadband dielectric spectroscopy (10−2 Hz – 109 Hz) is employed to study the molecular dynamics of the glassforming liquid salol (phenyl salicylate) being contained in (dielectric inactive) porous glasses with pore sizes of about 2.5 nm, 5.0 nm and 7.5 nm. Besides the bulk relaxation (I) of salol, two further dielectric loss processes are observed due to the geometrical constraint of the inner surfaces of the pores: the “interfacial relaxation” (II) and a Maxwell-Wagner polarization (III). The “interfacial relaxation” is assigned to the restricted dynamics of the molecules close to the inner surface of the pores. It shows a strong pore-size dependence for pores < 5 nm both in relaxation rate and dielectric strength.

The effect of steric hindrance on reorientational molecular dynamics has been studied both theoretically and experimentally in well-defined geometries. We show that, even for a nonwetting fluid, confinement quenches reorientation. This is unambiguously probed by the nuclear relaxation of 33S and 13C in a set of calibrated porous glasses saturated with CS2.

We study steady-state diffusion to an irregular membrane or catalyst surface which “annihilates” arriving particles by transfer across the membrane or chemical reaction at the surface. For diffusion in two dimensions and an arbitrary given surface, we present a simple algorithm to compute the total flux across the surface (annihilation rate) when the permeability of the membrane or reaction probability at the surface is small. The resulting flux increases with increasing surfaceirregularity and depends nonlinearly on the transport parameters and on the surface area. It predicts an optimal temperature, dependent on the surface irregularity, at which the flux is maximal. We illustrate this for self-similar surfaces, in which case the flux is a power law of the transport parameters and is governed by the fractal dimension of the surface.

We have used low-temperature optical spectroscopy to characterize the guest/host interaction of thiazine and oxazine dyes encapsulated in the three-dimensional pore structure of faujasite cages. The system thionine in dehydrated zeolite-Y exhibits a thermal and optical equilibrium between two spectroscopically distinguishable species. Temperature-dependent measurements allow the determination of the energy difference and barrier between these two forms as 170 cm−1 and 120 cm−1, respectively. The two forms are associated with two different locations/conformations of the chromophore within the faujasite pore structure. The degree of freedom responsible for the interconversion of the two forms is extremely sensitive to the relationship between molecular size and form of the void.

Raman and electronic spectra and results from differential scanning calorimetry (DSC) are reported for mercuric iodide (HgI2) confined in four different pore-sized glass hosts. The Raman spectra reveal peaks at 39 and 141 cm−1 that indicate the confined HgI2 is stabilized at 300 K in a modified orthorhombic (yellow) β-phase which is observed at 400 K for the bulk material. An additional band begins to appear at 145 cm−1 for pore radii smaller than 3.75 nm suggesting the presence of new phase of confined HgI2. The DSC measurements show the melting and freezing transitions of confined β-HgI2 are depressed from values reported for the bulk material and the depression increases as the pore size decreases. No transition is observed in the DSC measurement which could be attributed to the α→β transition for the confined material. Evidence supporting the new phase of HgI2 is observed in the electronic spectra of the confined HgI2 by the appearance of an energy gap transitions located at 2.7 eV.

Temperature dependent Raman measurements were conducted on bulk and the confined 2,4,6-trinitrotoluene (TNT) in 2.5, 5, 10, and 20 nm porous silica. Two bands at 23 and 190 cm−1 were chosen to evaluate the structure and the melting and freezing transitions of the confined TNT in pores. The results show that the solid phase TNT confined in larger pores (dp > 5 nm) forms conventional solid TNT structure, while the TNT restricted in small pores has no freezing and melting transition characteristics. The results also suggest that the freezing transition of the confined TNT starts at the pore center and the confined TNT maintains its interconnectivity during the freezing transition.

We undertook differential scanning calorimetry (DSC) measurements at 170 K < T < 300 K on n-decane, physically confined in 8 nm (= diameter D) porous silica derivatized with various functional groups, to understand how surface structure of the confining media affects the freezing or melting transition of the n-decane. Though we observed a typical depression (ΔT) in the freezing or melting transition temperature of the physically confined decane, our results failed to manifest usual linear dependence of ΔT on D−1 when the expected contraction in D, due to the presence of aminopropyl-, hexyl-, phenyl- or trimethyl-groups on the silica surface, was taken into account. However, it is worth noting that a linear behavior was observed between ΔT and D−1 if only alkane-chain derivatized hosts were considered. Our results also indicate that a large fraction of physically confined n-decane (35 to 70 %), depending on the host silica, does not participate either in the melting or freezing transition. The most interesting behavior observed in the present study is the occurrence of the unusual two peaks associated with the freezing transition of physically confined decane. This bimodal behavior is strongly dependent on the chemistry of the confining silica host's surface.

We consider entropic effects of vesicles and passages confined between lamellas in fluid membrane lamellar (Lα) phases. We show that their presence induces attractive interactions between lamellas which strongly compete with steric repulsion in the vicinity of phase transitions from the lamellar to isotropic membrane phases. An interplay between steric repulsion and entropic interactions mediated by passages and vesicles determines the inter-lamellar distance reached at these transitions. This distance crucially depends on the membrane Gaussian bending rigidity constant.