Bulk and surface characteristics

In dentistry, silicones are primarily used as dental-impression materials where chemical- and bioinertness are critical, and, thus, thoroughly evaluated.546 The development of a method for the detection of antibodies to silicones has been reviewed,547 as the search for novel silicone biomaterials continues. Thus, aromatic polyamide-silicone resins have been reviewed as a new class of biomaterials.548 In a short review, the comparison of silicones with their major competitor in biomaterials, polyurethanes, has been conducted.549 But silicones are also used in the modification of polyurethanes and other polymers via co-polymerization, formation of IPNs, blending, or functionalization by grafting, affecting both bulk and surface characteristics of the materials, as discussed in the recent reviews.550-552 A number of papers deal specifically with surface modification of silicones for medical applications, as described in a recent reference.555 The role of silicones in biodegradable polyurethane co-polymers,554 and in other hydrolytically degradable co-polymers,555 was recently studied. 

The binary systems were synthesized using two different processes, and the bulk and surface characteristics of suspensions synthesized by these two processes were indistinguishable. One, the mixed suspension, was prepared by mixing an Fe oxide suspension with a suspension of Si02. In the other method, Fe(OH)3 was precipitated from a solution in which amorphous Si02 particles were suspended. 

Table I. Bulk and Surface Characteristics of Reference Oxides and Binary Solid...

Zaki, M.I., Hasan, M.A., Pasupulety, L., Fouad, N.E., and Knozinger, H. Bulk and surface characteristics of pure and alkalized Mn203 TG, IR, XRD, XPS, specific adsorption and redox catalytic studies. NewJ. Chem. 1999,23,875-882. 

The availability of high-intensity, tunable X-rays produced by synchrotron radiation has resulted in the development of new techniques to study both bulk and surface materials properties. XAS methods have been applied both in situ and ex situ to determine electronic and structural characteristics of electrodes and electrode materials [58, 59], XAS combined with electron-yield techniques can be used to distinguish between surface and bulk properties, In the latter procedure X-rays are used to produce high energy Auger electrons [60] which, because of their limited escape depth ( 150-200 A), can provide information regarding near surface composition. 

Due to the difficulties in having rigorous analytical expressions for the flux at any given geometry and flow conditions, in many instances it is convenient to include all the characteristics of the supply in the mass transfer coefficient ms and use expression (50). It must be pointed out that expression (50), stating a linearity between the flux and the difference between bulk and surface concentrations, cannot be - in general valid for nonlinear processes, such as coupled complexation of the species i with any other species (see Chapters 4 and 10 for a more detailed discussion). 

To first order, we consider the molecular structure of the surface layers to be identical to that of the bulk layers. Consequently, all the characteristics corresponding to short-range intralayer interactions (e.g. Davydov splitting, vibrational frequencies, excitonic band structure, vibronic relaxations are similar for bulk and surface layers). In fact, we shall see that even slight changes may be detected. They will be analyzed in Section III.C, devoted to surface reconstruction. Therefore, our crystal model consists of (a,b) monolayers translated in energy relative to the bulk excitation by 206, 10, and 2cm-1 for the first three layers, as indicated in Fig. 3.5. No other changes are considered in this first-order crystal model. 

Bulk and Surface Compositions. The chemical compositions of the molecular sieves used in this study are given in Table I in terms of tetrahedral atom (T-atom) fractions, and are grouped according to structure type. The bulk compositions of AIPO4-5, AlPO -20 and VPI-5 show the ideal 1 1 ratio of A1 and P characteristic of aluminophosphate molecular sieves. The SAPO materials have frameworks consisting of Si, A1 and P T-atoms. 

This chapter focuses on the application of solid-state NMR techniques for the characterization of oxidation catalysts. Initially, a brief introduction to these techniques is provided (Section 5.2), within which methods suitable for the study of both bulk structure (Section 5.2.1) and surface characteristics (Section 5.2.2), are described. Examples of the application of these techniques are then provided in Section 5.3, for bulk oxides, and Section 5.4, for surface properties. Finally, Section 5.5 provides an outlook as to future directions in this area. 

In real situations surface and volume changes are often made with systems that are at equilibrium with their environment, characterized by a set of chemical potentials p, rather than keeping In ] fixed, as in [2.2.7 and 8j. In other words, area changes in open systems are considered. In statistical thermodynamics the conversion from closed to open implies the transition from the canonical to the grand canonical ensemble. The characteristic function of the latter is nothing other than the sum of the bulk and surface mechemical work terms (see [1.3.3.12] and [I.A6.23D which are the quantities of interest ... 

Compared to SRO effects on surface segregation in solid solutions, the role of LRO should be naturally more prominent and common. Its elucidation requires calculations that take into account various factors contributing to the net segregation characteristics in ordered alloys including the temperature dependence the crystal bulk structure and surface orientation, effective bulk and surface interatomic interactions (NN, non-NN) in relation to segregation driving forces, deviation from exact stoichiometry, possible surface relaxation and reconstruction, atomic vibrations, etc. This section attempts to quantify some of these factors and present several possible scenarios of segregation/order interplay. 

The properties of alloy and intermetallic compound surfaces play an important role for the development of new materials. Attention has been stimulated from various topics in microelectronics, magnetism, heterogeneous catalysis and corrosion research. The investigation of binary alloys serves also as a first step in the direction to explore multi-component systems and is of particular regard in material science as a consequence of their widespread use in technical applications. The distribution of two elements in the bulk and at the surface probably results in new characteristics of the alloy or compound as compared to a simple superposition of properties known from the pure constituents. Consequently, surfaces of bulk- and surface- alloys have to be investigated like completely new substances by means of appropriate material research techniques and surface science tools. [1-6]. 

Designation Bulk composition and bulk structure Surface characteristics... 

This volume is composed primarily of the expanded papers from the symposium, The Effects of Hostile Environments on Plastics A Symposium Held in Honor of Plastics Pioneer Raymond B. Seymour. Additional papers were included for better coverage of the overall topic. Each session of the symposium had a special theme introduction and background, bulk properties, and surface characteristics. 

In contrast to liquids, two different volumes of a solid phase can not be merged together upon contact. Since the mobility of molecules within solid phases is low, the differences in the bulk and surface structure of these volumes can not disappear spontaneously. Thus, even at the closest contact possible, the real physical interface having its own characteristic value of the specific surface free energy a is present between the two solid phases. For the two solid crystals, a is referred to as the specific surface energy of the grain boundary, agb. For nonpolar solids -1/2 Umo, (b) is less than the surface energy, a, i.e. -Umo[ (b)= = 2a-a. ... 

Tin oxide-based materials are potent oxidation and isomerization catalysts. Their bulk and surface properties, as well as their presumed mechanism in oxidation catalysis, have been reviewed (53j. Considerable uncertainty remains concerning the phase compositions, solid-solution range, and the redox behavior (Sn / Sn" vs. Sb WSb ) of these materials. Structural investigations have so far concentrated on the use of " Sn and Sb Mossbauer spectroscopy. Surprisingly, no " Sn solid-state NMR studies have appeared to date on this system, although it was recently demonstrated that isotropic " Sn chemical shifts and chemical shift anisotropies give characteristic fingerprints of the various tin coordination environments in Sn(IV) oxide compounds [54]. In situ C NMR has been used to study the double bond shift of 1-butene to t /.s-2-butene, and the subsequent cis-trans isomerization over tin antimony oxide catalysts [55 j. 

Additional effects can be introduced by variation of surfactant characteristics, like the adsorption and desorption rates on the interface, by measuring the chemical kinetics and the partition coefficient, and by measuring the relative bulk and surface concentrations at equilibrium. 

This scenario calls for a deep characterization of Pt based materials in terms of how many alloyed phases and segregated phases are present in the mixture as well as their composition. The more accurately the bulk of the Pt based materials is characterized the more precise the characteristics of surface are because the bulk and surface should not be so dissimilar, above the micrometric dimension, in terms of phases and composition. However, the same should not be strictly expected for nanoscale dimensions. Platinum nanoparticle based materials may not form a true alloy but a surface composition much dissimilar from the core given the equalized quantity of both the bulk and surface free energy. To complicate the picture even more, the electrochemical results often depend on the technique used to evaluate the activity of the catalysts. 

With the development of new polymerization chemistry, catalysis and formulation processes, a great number of polymeric materials with diverse properties can be produced. A detailed characterization of these materials is important to relate their chemical structure and composition to their functions. For example, modification of the end groups of a polymer can significantly alter its characteristics, such as chemical reactivity, solubility, and miscibility with other chemicals. Polymer characterization is not a simple task and often involves the use of multiple analytical techniques, with each generating a piece of useful information that is necessary to provide a comprehensive interrogation of the polymer. A number of analytical techniques, including chromatographic methods, spectroscopy, and mass spectrometry (MS), have been developed and applied to study areas such as polymer structure, polymer composition, molecular mass and molecular mass distribution, bulk and surface properhes and impurity content [1-3]. 

---