Solid interface, characterization

The characterization of surfaces undergoing corrosion phenomena at liquid-solid and gas-solid interfaces remains a challenging task. The use of STM for in situ studies of corrosion reactions will continue to shape the atomic-level understanding of such surface reactions. 

The power of optical spectroscopies is that they are often much better developed than their electron-, ion- and atom-based counterparts, and therefore provide results that are easier to interpret. Furtlienuore, photon-based teclmiques are uniquely poised to help in the characterization of liquid-liquid, liquid-solid and even solid-solid interfaces generally inaccessible by other means. There has certainly been a renewed interest in the use of optical spectroscopies for the study of more realistic systems such as catalysts, adsorbates, emulsions, surfactants, self-assembled layers, etc. 

The determination of adsorption isotherms at liquid-solid interfaces involves a mass balance on the amount of polymer added to the dispersion, which requires the separation of the liquid phase from the particle phase. Centrifugation is often used for this separation, under the assumption that the adsorption-desorption equilibrium does not change during this process. Serum replacement (6) allows the separation of the liquid phase without assumptions as to the configuration of the adsorbed polymer molecules. This method has been used to determine the adsorption isotherms of anionic and nonionic emulsifiers on various types of latex particles (7,8). This paper describes the adsorption of fully and partially hydrolyzed PVA on different-size PS latex particles. PS latex was chosen over polyvinyl acetate (PVAc) latex because of its well-characterized surface PVAc latexes will be studied later. 

New directions have been recently advanced in the use of IR spectroscopy for the characterization of adsorbates, including the investigation of liquid-solid interfaces in situ during catalysis. Both ATR [91,92] and RAIRS [86,93] have been recently implemented for that purpose. RAIRS has also been used for the detection of intermediates on model surfaces in situ during catalytic reactions [94-96], The ability to detect monolayers in situ under catalytic environments on small-area samples promises to advance the fundamental understanding of surface catalytic reactions. 

Surface-enhanced Raman spectroscopy (SERS) has also been employed to characterize metal catalyst surfaces [103], The low sensitivity and severe conditions required for the signal enhancement have limited the use of this technique [104], but some interesting work has been published over the years in this area, including studies on model liquid-solid interfaces [105],... 

Figure 13. Schematic representation of the setup used for the infrared characterization of liquid-solid interfaces [63], The main cell consists of a platinum disk used for adsorption and reaction, a Cap2 prism for guidance of the infrared beam, and a liquid solution trapped between those two elements. The overall arrangement includes gas and liquid sample introduction stages as well as the electronics used for the electrochemical oxidation-reduction cycles needed to preclean the platinum surface.

Metal catalytic activity may be expected to be a function of the solubility of the active species and/or the ease of electron transfer to the catalyst. The results given in Table IV show conclusively that the suggestion that catalysis occurs at a gas-solid interface (13) does not hold in these systems. Preliminary experiments showed that copper ion- and haemin-catalyzed systems oxidized rapidly with no trace of solid precipitation, and that cobalt and nickel catalysis were characterized by the production of colored solutions and precipitates. Filtration experiments showed these precipitates played only a small part in catalysis (Table IV). 

Inspired by these Surface Science studies at the gas-solid interface, the field of electrochemical Surface Science ( Surface Electrochemistry ) has developed similar conceptual and experimental approaches to characterize electrochemical surface processes on the molecular level. Single-crystal electrode surfaces inside liquid electrolytes provide electrochemical interfaces of well-controlled structure and composition [2-9]. In addition, novel in situ surface characterization techniques, such as optical spectroscopies, X-ray scattering, and local probe imaging techniques, have become available and helped to understand electrochemical interfaces at the atomic or molecular level [10-18]. Today, Surface electrochemistry represents an important field of research that has recognized the study of chemical bonding at electrochemical interfaces as the basis for an understanding of structure-reactivity relationships and mechanistic reaction pathways. 

Duplicate and triplicate IFT aging curves were obtained at one or two temperatures for most of the interfaces characterized in this study. The replicate IFT data reported in Figures 1,3,4,7,8 and 10-14 show that many IFT aging curves for citrus oil/aqueous phase interfaces differ by a maximum of 1.7mJ/m2. Replicate curves often differ by less than lmJ/m2. Because each IFT aging experiment involved formation and separation of a new complex coacervate and supernatant phase, replicate IFT aging curves measure the combined effect that several factors have on reproducibility. These factors include variability of the complex coacervation procedure, protocol followed for separation of the coacervate and supernatant phases, and the IFT measurement process itself. The variability in solids content of replicate coacervate and supernatant phases shown in Table 1 could contribute to the observed IFT variability. 

The fluid phase that fills the voids between particles can be multiphase, such as oil-and-water or water-and-air. Molecules at the interface between the two fluids experience asymmetric time-average van der Waals forces. This results in a curved interface that tends to decrease in surface area of the interface. The pressure difference between the two fluids A/j = v, — 11,2 depends on the curvature of the interface characterized by radii r and r-2, and the surface tension, If (Table 2). In fluid-air interfaces, the vapor pressure is affected by the curvature of the air-water interface as expressed in Kelvin s equation. Curvature affects solubility in liquid-liquid interfaces. Unique force equilibrium conditions also develop near the tripartite point where the interface between the two fluids approaches the solid surface of a particle. The resulting contact angle 0 captures this interaction. 

Polymer Adsorption. A review of the theory and measurement of polymer adsorption points out succinctly the distinquishing features of the behavior of macromolecules at solid - liquid interfaces (118). Polymer adsoiption and desorption kinetics are more complex than those of small molecules, mainly because of the lower diffusion rates of polymer chains in solution and the "rearrangement" of adsorbed chains on a solid surface, characterized by slowly formed, multi-point attachments. The latter point is one which is of special interest in protein adsoiption from aqueous solutions. In the case of proteins, initial adsoiption kinetics may be quite rapid. However, the slow rearrangement step may be much more important in terms of the function of the adsorbed layer in natural processes, such as thrombogenesis or biocorrosion / biofouling caused by cell adhesion. 

The present state of the theories of atomic and molecular processes in condensed phases is characterized by great non-uniformity of its development. Matters are much problematic in the theory of the kinetics of processes at a molecular level. The kinetics of surface processes mainly employs models taking no account of the interaction of the adsorbed particles (the law of masses or surface action) [14-16]. This does not reflect the real properties of a gas solid interface. There is also a diversity of models when considering the interaction of the particles because various approximations are used (equilibrium is described with a view to the correlation effects, while kinetics ignores them). The problem of approximations is of a fundamental significance in the theory of condensed systems. Interaction between the particles causes all the particles to be bound to... 

We are specifically interested in the system in which a liquid-solid interface reaction has taken place. An example of this type of reaction is the chemical grafting of a rubber with a monomer at the interface. The function of the grafted rubber as an adhesive has been postulated (II, 29, 46, 64) but has never been proved. Since the grafted rubber is the key to bridging two incompatible polymers together, we devoted a major portion of our experimental work to the characterization of the grafted polymer as an adhesive at the interface. 

Fuel cell researchers deal primarily with interfaces between solid electrolyte materials and solid metallic electrodes. The characterization of electrochemical systems with solid-solid interfaces has become a major issue in the study of fuel cells. It is generally believed that the interface of a solid electrode and solid electrolyte is similar to the electrode/liquid electrolyte interface but more complicated [4],... 

The local transport cell is represented by a straight pore with an inclination to the vertical characterized by angle 0 (see Figure 8). The curvatures of gas-liquid and liquid-solid interfaces are assumed to be negligible. The model also assumes that the laminar liquid flow is motivated only by gravity. 

In this chapter, our aim is to give an introductory account of the methodology and underlying thermodynamic principles of adsorption at the liquid-solid interface. We are mainly, but not exclusively, concerned with the characterization of the liquid-solid interface. In this context, there are two relevant topics ... 

Thermoporometry. Thermoporometry is the calorimetric study of the liquid-solid transformation of a capillary condensate that saturates a porous material such as a membrane. The basic principle involved is the freezing (or melting) point depression as a result of the strong curvature of the liquid-solid interface present in small pores. The thermodynamic basis of this phenomenon has been described by Brun et al. [1973] who introduced thermoporometry as a new pore structure analysis technique. It is capable of characterizing the pore size and shape. Unlike many other methods, this technique gives the actual size of the cavities instead of the size of the openings [Eyraud. 1984]. 

A variety of methods have been used to characterize the solubility-limiting radionuclide solids and the nature of sorbed species at the solid/water interface in experimental studies. Electron microscopy and standard X-ray diffraction techniques can be used to identify some of the solids from precipitation experiments. X-ray absorption spectroscopy (XAS) can be used to obtain structural information on solids and is particularly useful for investigating noncrystalline and polymeric actinide compounds that cannot be characterized by X-ray diffraction analysis (Silva and Nitsche, 1995). X-ray absorption near edge spectroscopy (XANES) can provide information about the oxidation state and local structure of actinides in solution, solids, or at the solution/ solid interface. For example, Bertsch et al. (1994) used this technique to investigate uranium speciation in soils and sediments at uranium processing facilities. Many of the surface spectroscopic techniques have been reviewed recently by Bertsch and Hunter (2001) and Brown et al. (1999). Specihc recent applications of the spectroscopic techniques to radionuclides are described by Runde et al. (2002b). Rai and co-workers have carried out a number of experimental studies of the solubility and speciation of plutonium, neptunium, americium, and uranium that illustrate combinations of various solution and spectroscopic techniques (Rai et al, 1980, 1997, 1998 Felmy et al, 1989, 1990 Xia et al., 2001). 

The use of gas flow techniques are now widely recognized as important for the study of adsorption/desorption - phenomena at solid interfaces. Such studies are particularly important for the characterization of catalytically active sites on the surfaces of solids. One particular example is a study of the adsorption of ammonia onto activated carbon. It can be shown that adsorption of ammonia consists of reversible and irreversible steps and that these steps can be attributed to physisorp-tion for the former and adsorption on particular chemical groups for the latter. The heats of adsorption for various sites can be measured and the data can be used to reveal the existence of a wide distribution of acid sites that are accessible to ammonia to varying degrees. 

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