Interfacial mechanism theory

Hamnett A. 1999. Mechanism of methanol oxidation. In Wieckowski A, ed. Interfacial Electrochemistry Theory, Experiments and Applications. New York Marcel Dekker. pp. 843-883. 

The interfacial pair correlation functions are difficult to compute using statistical mechanical theories, and what is usually done is to assume that they are equal to the bulk correlation function times the singlet densities (the Kirkwood superposition approximation). This can be then used to determine the singlet densities (the density and the orientational profile). Molecular dynamics computer simulations can in... 

The second model assumes an interfacial layer of finite and constant thickness T, so that K = T/lj is the volume of the interfacial layer, which has to be defined on the basis of some appropriate model of gas adsorption. For most practical purposes the two models are equivalent. The first model is easier to apply, but most of the authors in the early development of statistical mechanical theories of adsorption have expressed the problem in terms of an interfacial layer. For completeness, the appropriate definitions are given in relation to both formulations. 

These results represent a challenge for the modern statistical-mechanical theories of the interfacial structure, since the values of SC in Eq. (23b) could be... 

Computational techniques have extensively been used to stu(fy the interfacial mechanics and nature of bonding in CNT-polymer composites. The computational studies can be broadly classified as atomistic simulations and continuum methods. The atomistic simulations are primarily based onMD simulations and DFT [105-110], The main focus of these techniques was to understand and stndy the effect of bonding between the polymer and nanotube (covalent, electrostatic or vdW forces) and the effect of friction on the interface. The continuum methods extend the continuum theories of microme-chanics modeling and fiber-reinforced composites (elaborated in the next section) to CNT-polymer composites [111-114] and explain the behavior of the composite from a mechanics point of view. 

Before we proceed to discuss either the experiments that can give information about surface or interfacial tension, or the statistical mechanical theories allowing more precise predictions of these quantities, we should discuss some of the fundamentals of classical interfacial thermodynamics. The interested reader will find a fuller account in the book by Adamson (1990). 

Hamnett A. Mechanism of methanol electro-oxidation. In A.Wieckowski, editor. Interfacial electrochemistry theory, experiment, and applications. New York Marcel Dekker, 1999 843-83. 

One of the possible reasons why the question of rheological vs. interfacial mechanisms of interaction has been so difficult is that another mechanism has been neglected. The process involved in this mechanism is the conversion of work into heat.(i 3) We will examine this process in the fourth section of this chapter. In the main body of the chapter, we will develop an isothermal theory in a way that will enable us to introduce thermal effects in Section 4. 

Contact mechanics, in the classical sense, describes the behavior of solids in contact under the action of an external load. The first studies in the area of contact mechanics date back to the seminal publication "On the contact of elastic solids of Heinrich Hertz in 1882 [ 1 ]. The original Hertz theory was applied to frictionless non-adhering surfaces of perfectly elastic solids. Lee and Radok [2], Graham [3], and Yang [4] developed the theories of contact mechanics of viscoelastic solids. None of these treatments, however, accounted for the role of interfacial adhesive interactions. 

Dutrowski [5] in 1969, and Johnson and coworkers [6] in 1971, independently, observed that relatively small particles, when in contact with each other or with a flat surface, deform, and these deformations are larger than those predicted by the Hertz theory. Johnson and coworkers [6] recognized that the excess deformation was due to the interfacial attractive forces, and modified the original Hertz theory to account for these interfacial forces. This led to the development of a new theory of contact mechanics, widely referred to as the JKR theory. Over the past two decades or so, the contact mechanics principles and the JKR theory have been employed extensively to study the adhesion and friction behavior of a variety of materials. 

The surfaces of all materials interact through van der Waals interactions and other interactions. These interfacial forces, which are attractive in most cases, result in the deformation of the solid bodies in contact. In practice, the radius of the contact zone is higher than the radius predicted by the Hertzian theory (Eq. 7). Johnson et al. [6] modified the Hertzian theory to account for the interfacial interactions, and developed a new theory of contact mechanics, widely known as the JKR theory. In the following section, we discuss the details of the JKR theory. The details of the derivation may be obtained elsewhere [6,20,21]. 

Given the importance of surface and interfacial energies in determining the interfacial adhesion between materials, and the unreliability of the contact angle methods to predict the surface energetics of solids, it has become necessary to develop a new class of theoretical and experimental tools to measure the surface and interfacial energetics of solids. Thia new class of methods is based on the recent developments in the theories of contact mechanics, particularly the JKR theory. 

The study of acid-base interaction is an important branch of interfacial science. These interactions are widely exploited in several practical applications such as adhesion and adsorption processes. Most of the current studies in this area are based on calorimetric studies or wetting measurements or peel test measurements. While these studies have been instrumental in the understanding of these interfacial interactions, to a certain extent the interpretation of the results of these studies has been largely empirical. The recent advances in the theory and experiments of contact mechanics could be potentially employed to better understand and measure the molecular level acid-base interactions. One of the following two experimental procedures could be utilized (1) Polymers with different levels of acidic and basic chemical constitution can be coated on to elastomeric caps, as described in Section 4.2.1, and the adhesion between these layers can be measured using the JKR technique and Eqs. 11 or 30 as appropriate. For example, poly(p-amino styrene) and poly(p-hydroxy carbonyl styrene) can be coated on to PDMS-ox, and be used as acidic and basic surfaces, respectively, to study the acid-base interactions. (2) Another approach is to graft acidic or basic macromers onto a weakly crosslinked polyisoprene or polybutadiene elastomeric networks, and use these elastomeric networks in the JKR studies as described in Section 4.2.1. 

The mechanism of chemical adhesion is probably best studied and demonstrated by the use of silanes as adhesion promoters. However, it must be emphasized that the formation of chemical bonds may not be the sole mechanism leading to adhesion. Details of the chemical bonding theory along with other more complex theories that particularly apply to silanes have been reviewed [48,63]. These are the Deformable Layer Hypothesis where the interfacial region allows stress relaxation to occur, the Restrained Layer Hypothesis in which an interphase of intermediate modulus is required for stress transfer, the Reversible Hydrolytic Bonding mechanism which combines the chemical bonding concept with stress relaxation through reversible hydrolysis and condensation reactions. 

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