The Bilayer Model

A wide variety of different models of the pure water/solid interface have been investigated by Molecular Dynamics or Monte Carlo statistical mechanical simulations. The most realistic models are constructed on the basis of semiempirical or ab initio quantum chemical calculations and use an atomic representation of the substrate lattice. Nevertheless, the understanding of the structure of the liquid/metal surface is only at its beginning as (i) the underlying potential energy surfaces are not known very well and (ii) detailed experimental information of the interfacial structure of the solvent is not available at the moment (with the notable exception of the controversial study of the water density oscillations near the silver surface by Toney et al. [140, 176]). 

In this situation computer simulations are especially important, because meaningful results within the framework of classical statistical mechanics can be generated for well-defined models and predictions can be made. A few general results, which are fairly independent of the details of the model assumptions, are summarized below. 

The behavior of simple and molecular ions at the electrolyte/electrode interface is at the core of many electrochemical processes. The substantial understanding of the structure of the electric double layer has been summarized in various reviews and books (e.g., Ref. 2, 81, 177-183). The complexity of the interactions demands the introduction of simplifying assumptions. In the classical double layer models due to Helmholtz [3], Gouy and Chapman [5, 6], and Stern [7], and in most of the studies cited in the reviews the molecular nature of the solvent has been neglected altogether, or it has been described in a very approximate way, e.g., as a simple dipolar fluid. Computer simulations can overcome this restriction and describe the solvent in a more realistic fashion. They are thus able to paint a detailed picture of the microscopic structure near a metal electrode. 

Understanding the structure and dynamics of pure water on a molecular level is only the beginning. Simulations of electrolyte solutions near metallic interface are much more demanding in terms of computer time than those of bulk water, because the relatively small number of ions even in a highly concentrated electrolyte solution mandates the treatment of systems with a much larger total number of particles than in pure water for a longer time span. Furthermore, as was discussed in section 3, much less is known from quantum chemistry about nature and strength of the ion-metal interaction than about the water-metal interactions, so that the interpretation of the results obtained from the simulations is less clear. 

Since specific adsorption is an important phenomenon in electrochemistry, the solution/metal interface has nevertheless been studied in various ways. An ion is considered to be adsorbed specifically in the inner Helmholtz plane when it is partially dehydrated and in direct contact with the metal surface (see, e.g.. Ref. 15). On the other hand, an ion that is adsorbed further away from the electrode with its hydration shell essentially intact is considered to be adsorbed non-specifically in the outer Helmholtz plane . In the classical treatment of contact adsorption, the balance between the energy of hydration of the ion and the strength of the image interactions determines which ions are specifically adsorbed and which ones are not 

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It is possible that quite different molecular architectures may occur in membranes from different sources. Current research may result in a much more dramatic revision or complete rejection of the bilayer model for some membranes, especially in such systems as mitochondria (30) and chloroplasts (2). However, it is also possible that structural differences are only variations on the basic theme of the bilayer, from myelin at one extreme to mitochondria or chloroplasts on the other. One must not readily reject the fundamentals of the Danielli concept, especially in view of the present inadequate knowledge of the properties of phospholipids in water. Clearly the molecular architecture of membranes is speculative, but most aspects of the problem are amenable to direct experimental test by the new physical techniques. A consistent model for biological membranes will emerge quickly. 

Apart from that model we also verified a number of others -- particularly the bilayer model and the three layer model with a constant amount of PAA in the second layer - but none of them gave better results than the model presented. 

The detailed structure of water at the surface is often discussed in terms of the bilayer model proposed by Doering and Madey [58] to explain water adsorption on metal surfaces in the vacuum at low temperatures. As the name suggests, this model proposes two layers of water that are hydrogen-bonded to each other. Obviously, in the liquid state and at ambient temperatures such a bilayer must be strongly disturbed, though a vestige seems to appear in some simulations [59]. Similar, but more icelike, structures have been suggested by Kramer and coworkers [60]. 

Figure 3-5. Anodic polarization curve of an iron electrode in a borate buffer solution of pH 8.4 (Nagayama and Cohen, 1962). The insert shows the bilayer model of the composition of the passive film with an inner layer of Fe304, an outer layer of y-Fe203 and adsorbed hydroxyl groups (MacDougall and Graham, 1995).

On stainless steels and on nickel-based stainless alloys, the passive film can be described by the bilayer model already presented. The concentration of Cr " in the inner oxide layer is much higher than the nominal chromium content of the alloy. The compositions of passive films formed on ferritic (Fe-Cr) and austenitic (Fe-Cr-Ni) stainless steels, and on Alloys 600 and 690... 

In a microemulsion, the inside component (say) of our ensemble of surfaces is water and the outside component oil. The surfactant film is now a monolayer with a definite direction and there is no inside-outside symmetry. However, this symmetry can be approximately restored by (i) tuning parameters such as the concentration of added salt and alcohol, so as to avoid a strong preferred curvature of the monolayer towards one solvent rather than the other and then (ii) using equal amounts of oil and water. Under these conditions, the phase diagram (fig.4) for the bilayer model can be reinterpreted as that of a balanced microemulsion system. 

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