Disordered porous networks are important examples of confining geometries. A challenging problem is to couple the morphology and the topology of such disordered systems with the diffusion and the reactive properties of embedded fluids (gases or liquid) inside the porous medium. Looking at the properties of the self-diffusion propagator, we first discuss how the geometric confinement influences the molecular diffusion and how the coupling between interfacial geometry and transport evolves in space and time. In the long time regime, two specific situations are presented. First, we focus on transport properties inside a membrane-like disordered matrix, the sponge phase (symmetric or asymmetric). Second, we discuss some basic properties of the Knudsen diffusion. The particular coupling with the pore network geometry allows to analyse this transport process in term of the continuous time random walk formalism (C.T.R.W.). An interesting consequence for some specific disordered porous media or “low dimension” geometries is a transition from a Gaussian diffusion to a Levy walk. Finally, excitation and relaxation kinetics are discussed. More specifically, NMR relaxation of water inside a Vycor glass is investigated and a comparison with recent experimental results is presented.

We have measured the time resolved phosphorescence of different probe molecules in glassforming solvents under the condition of geometrical confinement in porous glasses. This solvation dynamics technique probes the local dielectric relaxation in the case of a dipolar chromophore in polar liquids. In the absence of dipolar interactions, the observed Stokes shifts reflect the local density or mechanical responses. Therefore, both orientational and translational modes of molecular motions can be measured for liquids imbibed in porous silica glasses. The effect of confinement on the relaxations of supercooled liquids is strongly dependent on the surface chemistry and can be rationalized on the basis of the cooperativity concept. As in the bulk case, we find that the relaxations in nano-confined liquids display heterogeneous dynamics. The density relaxation turns out to be more sensitive to the thermal history relative to the orientational features of molecular motion. By selectively positioning the chromophores at the liquid/solid interface, we observe also that the structural relaxation of the liquid in the immediate vicinity of the glass surface is slowed down but not entirely blocked.

Liquid crystals (LCs)—pentylcyanobiphenyl (5CB) and octylcyanobiphenyl (8CB)—were confined in random porous media with narrow pores of mean size equal to 100 Å and investigated by means of broad band dielectric spectroscopy in deeply supercooled state. In liquid crystalline phases, bulk 5CB and 8CB have two dielectrically active modes. The main mode with the relaxation time τ ∼ 10”8s is due to the rotation of molecules about their short axis, and the secondary mode is due to the tumbling motion of molecules with the relaxation time ツ ∼ 10”10s. Bulk 5CB and 8CB are nonglass formers and they crystallize at cooling. The confinement strongly influences the dynamical behavior of LCs and is resulted in qualitative changes in their properties. Deep supercooling of LCs in pores up to ∼ 150 degrees below the bulk crystallization temperature was observed. The relaxation rate of the process due to the molecular rotation in deeply supercooled state is slower than at the temperatures corresponding to nematic phase by many orders of magnitude. This slowing down is accompanied by anomalous broadening of the dielectric spectra.

Other new properties observed in confined LCs are two low frequency relaxation processes absent in bulk LCs. One of these processes is due to the molecular relaxation in the surface layers at liquid crystal-solid pore wall interface. The second process is probably a collective mode due to the relaxation of the surface induced polarization. The collective process due to surface polarization and the surface molecular mode show features typical for glass formers.

The orientational behavior of liquids confined in nanoporous glasses has been studied using ultrafast Optical Kerr Effect spectroscopy. We discuss the detailed microscopic behavior of a weakly wetting liquid (methyl iodide) and a strongly wetting liquid (acetonitrile) confined in these glasses. We demonstrate that the behavior of methyl iodide is dominated by geometrical effects while the behavior of acetonitrile is dominated by surface interactions. We provide evidence that molecular exchange into the bulk is an important channel for the reorientation of strongly wetting liquids at interfaces.

We undertook comparative differential scanning calorimeter (DSC) measurements on cyclohexane (C6H12) and cyclohexanone (C6H10O), physically confined in porous silica of pore radius 4, 7.5, 15, 30, and 62.5 nim, with a view to ascertain how guest fluid-surface host interactions affected the thermodynamic properties of the confined fluids. Our results can be summarized as follows: (a) No distinct signature of freezing or melting transition was observed for the physically confined cyclohexanone, irrespective of whether the bulk was present outside the pores. However, this was not the case for cyclohexane. (b) The solid-to-solid transition temperature of cyclohexane and cyclohexanone inversely scaled with the pore radius of the host porous silica. (c) The cubic-to-orthorhombic transition of cyclohexanone was strongly influenced by whether the bulk fluid was present outside the pores. In the absence of the bulk, the transition temperature was considerably suppressed relative to the bulk transition temperature. However, in the presence of the bulk, the confined and the bulk transitions occurred at the same temperature.

Lateral Force Microscopy offers the possibility of exploring tribological properties of interfaces atthe nanoscale. Our research focused on some crucial conditions that must be fuffilled to obtainquantitative and reliable LFM friction measurements. We have characterized the mechanical andvibrational properties of the cantilever. Precise force calibration were made based on ourknowledge of the intrinsic coupling modes of the cantilever. We report measurements of the slidingfriction between two silica surfaces. The load dependence of the friction force was analyzedassuming different models for the contact, from Hertzian to Amontons law.

We review the results of two studies [1,2] aimed at clarifying the surface forces of con fined fluids. In the first study [1], molecular-dynamics simulations are used to study the influence of chain branching on the molecular configurations of alkane films physically adsorbed on a solid surface. The symmetric n-decane molecules exhibit strong layering, while t-butyl-hexane films have a novel pillared-layered structure, in which a few randomly distributed molecules orient themselves with the t-butyl end near the surface and the alkyl tail perpendicular to the surface. These molecules are surrounded by parallel, layered molecules. In the second study [2], we outline the development of a new NP‖AT ensemble method, with advantages for simulating confined fluids. For confined Lennard-Jones particles simulated with the new method, clear oscillatory solvation-force profiles and step-like dependencies of the number of confined molecules on surface separation were observed. As the parallel pressure increases, the oscillations in solvation forces are enhanced and tend to become repulsive.

We investigate the response of a confined harmonic chain to an external harmonic driving force. A model is introduced which extends the one-dimensional Tomlinson model to include motion in the normal direction. This model with lateral-normal coupling mimics recent measurements on friction, using surface forces apparatus (SFA). The model predicts a critical driving amplitude below which the response is linear. For higher amplitudes the system exhibits a nonlinear behavior and an apparent shear thinning. We discuss the effects of shear induced dilatancy, which results from the lateral-normal coupling, on the energy dissipation, and therefore on the frictional properties. It is demonstrated that measurements of response in the normal direction provide additional information on the mechanisms of friction.

To understand antiwear phenomena in motor engines at the atomic level and provide evidence inselecting future ashless wear inhibitors, we studied the thermal stability of the self-assembled monolayer(SAM) model for dithiophosphate (DTP) and dithiocarbamate (DTC) molecules on the iron oxidesurface using molecular dynamics. The interactions for DTP, DTC and Fe2O3 are evaluated based on aforce field derived from fitting to ab initio quantum chemical calculations of dimethyl DTP (and DTC)and Fe(OH)2(H2O)2-DTP (DTC) clusters. MD simulations at constant-NPT are conducted to assesrelative thermal stabilities of the DTP and DTC with different pendant groups (n-propyl, i-propyl, npentyl.and i-pentyl). To investigate frictional process, we employ a steady state MD method, in whichone of the Fe2O3 slabs maintained at a constant linear velocity. We obtain the time averaged normaland frictional forces from the interatomic forces. Then, we calculated the friction coefficient at theinterface between SAMs of DTP and the confined lubricant, hexadecane, to assess the shear stability ofDTPs with different pendant groups.

We report detailed nucleation studies on the liquid -to -solid transition of hexadecane using nearly monodisperse hexadecane -in -water emulsions. A careful consideration of the kinetics of isothermal and nonisothermal freezing show deviations from predictions of classical nucleation theory, if one assumes that the emulsion droplet population is homogeneous. Similar deviations have been observed previously [3]. As an explanation, we propose a novel argument based on the dynamic generation of droplet heterogeneity mediated by mobile impurities. This proposal is in excellent agreement with existing data.

Proton nuclear magnetic relaxation dispersion experiments show remarkable differences between water and aprotic liquids in contact with microporous glass surfaces containing trace paramagnetic impurities. Proton surface diffusion permits long range effectively 1-dimensional exploration along the pores for water. This gives a direct measurement of the translational diffusion coefficient at the pore surface.

We study the undulations and the transverse diffusion of a tagged membrane point in both physical (passive) membranes and active biomembranes. In physical membranes thermal undulations generate a transverse subdiffusive motion, 〈r2〉 ∼ t2/3. Active biomembranes include active sites that use chemical energy to pump ions or molecules from one side to the other. In this case we find a few regimes which show a strongly enhanced diffusion, 〈r2〉 ∼ tα with 1 < α > 2.

Broadband dielectric spectroscopy (10−2 Hz - 109Hz) is employed to study the molecular dynamics of low-molecular-weight glassforming liquids being confined to nanopores. For the H-bond forming liquid propylene glycol being confined to (uncoated and silanized) nanopores (pore size: 2.5 nm, 5.0 nm and 7.5 nm) a molecular dynamics is observed which is comparable to that of the bulk liquid. Due to surface effects in uncoated nanopores the relaxation time distribution is broadened on the long term side and the mean relaxation rate is decreased by about half a decade. This effect can be counterbalanced by lubricating the inner surfaces of the pores resulting in a relaxation rate which is slightly faster compared to the bulk liquid. For the H-bonded liquid ethylene glycol (EG) embedded in zeolites of different pore size and topology one observes a sharp transition from a single-molecule dynamics to that of a liquid depending on the coordination number of the confined molecules. While EG in silicalite (showing a single molecule relaxation) has four neighboring molecules, EG in zeolite beta or AIPO4-5 has a coordination number of five and behaves like a bulk liquid.

The local segmental dynamics of polymers confined within the 15–20 Å interlayer spacing of nanocomposites consisting of poly(methyl phenyl siloxane) intercalated within organically modified silicates, has been investigated utilizing Dielectric Relaxation Spectroscopy. The effect of confinement on the local reorientational dynamics is evident by the observation of a relaxation mode, which is much faster than the segmental α-relaxation of the bulk polymer and exhibits much weaker temperature dependence. This is attributed to the restrictions placed by the interlayer spacing on the cooperative volume required for the ax-relaxation.

Polymer layered silicates form an important class of nanocomposite materials. During their formation by melt intercalation, polymer molecules from a bulk fluid move into the galleries of layered silicates. An essential feature of this process is the flow of macromolecules from a bulk fluid to a confined environment. To model this phenomenon, we have performed molecular dynamics simulations of the flow of polymer molecules from a bulk melt into a rectangular slit. The simulations are consistent with a diffusive description of the transport, and show qualitative agreement with time-dependent x-ray diffraction measurements of intercalation kinetics in layered nanocomposites.

The interface between liquid hexadecane and the (010) surface of silicalite was studied by molecular dynamics. The structure of molecules in the interfacial region is influenced by the presence of pore mouths on the silicalite surface. For this surface, whose pores are the entrances to straight channels, the concentration profile for partially absorbed molecules is peaked around 10 monomers inside the zeolite. No preference to enter or exit the zeolite based on absorption length is observed except for very small or very large absorption lengths. We also found no preferential conformation of the unabsorbed tails for partially absorbed molecules.

Molecular dynamics simulations of C1 through C14 n-alkanes have been used to elucidate diffusion mechanisms in siliceous faujasite zeolites. Additional simulations of the bulk liquids were conducted to compare the liquid and adsorbed phases. Macroscopic quantities, such as heats of adsorption, diffusivities, and activation energies, were calculated and compare well with experimental values. In addition, the simulations provide detailed information about the mechanisms of alkane diffusion in the confined pores of faujasite.

We report on the results for the diffusion coefficient (D*) of polystyrene (PS) chains near the PS/Silicon interface. The present study employs secondary ion mass spectrometry (SIMS) to examine diffusion from a deuterated marker layer in thin PS films on silicon. The observed SIMS depth profiles are fit to numerical simulations of the diffusion process. The best fit is obtained for a super-linear dependence of D* vs. distance from the silicon wall. A non-trivial time dependence extending over tens of hours is observed for all the models tested.

This paper reports results of a Monte Carlo simulation for a simplified lattice modelof a supercooled polymer film. The film geometry is realized by two opposite hard walls.The distance between the walls is varied. The chains exhibit a strong tendency to orientparallel to the walls and are flattened when being very close to them. This deviation of thepolymer structure with respect to the bulk is accompanied by an acceleration of local densityfluctuations. On the other hand, the diffusion coefficient of a chain remains unaffected.

The thermal expansion of thin deuterated polystyrene (dPS) films supported on energetically repulsive, fluorinated polyimide (PI) substrates (PI/dPS bilayer) was measured via neutron reflectometry as a function of initial dPS film thickness. Film thickness was measured before and after capping with a top layer of the same repulsive, high glass transition polyimide that comprised the substrate layer (PI/dPS/PI trilayer) in an attempt to observe any effects of the dPS free surface in the bilayer geometry. Bulk thermal expansion behavior, characterized by a discontinuous change in coefficient of thermal expansion (CTE) at the glass transition (Tg), is observed in films with thickness > 70 rim. For thicknesses between 70 nm and 40 nm a transition is seen from bulk behavior in bilayer films to glassy thermal behavior in the trilayer films persisting up to 20 °C above the bulk Tg. In films with thickness < 40 nm the bulk glassy CTE persists well above the bulk Tg for both bilayer and trilayer films