Compact monolayer

The experimentally estimated k2 value (= 0s tr[Os(II)]// ru) (36X107 s-1) for the A- Ru(II)/A-Os(II) pair is much larger than that (8.5 X 107 s-1) for the A- Ru(II)/A-Os(II) pair. This result is consistent with the chirality effect on the surface pressure versus molecular area (tt-A) isotherms, in which the A-ruthen-ium(II) complex forms a more compact monolayer than the racemic mixture of the ruthenium(II) complex in other words, the racemic mixture induces some steric repulsion between A- and A-ruthenium(II) complexes. 

The region between 0.05 and 0.25 relative pressures is called the BET region, and it is used for the determination of the so-called monolayer capacity—the amount of nitrogen molecules adsorbed on the sample surface in a compact monolayer fashion. The BET equation represents the dependence of amount of adsorbed nitrogen as a function of the relative equilibrium pressure iplpf) ... 

Molecular resolution images from both STM and AFM on compact monolayers show a wealth of lattice structures, which can conveniently be analyzed in terms of lattice parameters using their two-dimensional Fourier transforms. Using an NSOM the molecular orientation in an LB monolayer was presented in polarized fluoresence images" ). With STM it is not only possible to image the arrangement of... 

K C or occurs, where the Langmuir constant has different dimensions in these two cases. As to the surface concentration, conversion of the surface concentration (in mol m ) into the dimensionless fractional coverage = / r tmax) requires knowledge of the plateau adsorption or, for that matter, of the molecular cross-section in a compact monolayer,. This quantity is not a priori known, but can be obtained by linearization (sec. II. 1.4a). In fact, linearization is a useful exercise anyway, because it tells us whether the Langmuir equation applies at all. Below we shall mainly use the dimensionless sets 0 lx ] and as being the most general. As only one adsorbing component will be... 

Figure 6.9 Possible mechanism for the field-driven ionic binding to thiobis(ethyl aceto-acetate) moieties arranged in a compact monolayer on gold. It includes a field assisted enolization in acid solution followed immediately by a binding of a metal ion. At negative E the first step may include penetration of the ion into the monolayer to form a weak diketone complex. ...

In many physically important cases of localized adsorption, each adatom of the compact monolayer covers effectively n > 1 adsorption sites [3.87-3.89, 3.98, 3.122, 3.191, 3.214, 3.261]. Such a multisite or 1/n adsorption can be caused by a crystallographic Me-S misfit, i.e., the adatom diameter exceeds the distance between two neighboring adsorption sites, and/or by a partial charge of adatoms (A < 1 in eq. (3.2)), i.e., a partly ionic character of the Meads-S bond. The theoretical treatment of a /n adsorption differs from the description of the 1/1 adsorption by a simple Ising model. It implies the so-called hard-core lattice gas models with different approximations [3.214, 3.262-3.266]. Generally, these theoretical approaches can only be applied far away from the critical conditions for a first order phase transition. In addition, Monte Carlo simulations are a reliable tool for obtaining valuable information on both the shape of isotherms and the critical conditions of a 1/n adsorption [3.214, 3.265-3.267]. 

Consequently, the formation of the Pb adlayer in this underpotential range can be considered as an 1/2 localized adsorption on a square lattice. In this case each adatom in the compact monolayer covers effectively two adsorption sites. Thus, domains with an Ag(100)-c(2 x 2) Pb structure located on different substrate sublattices Oike white and black fields of a chessboard) separated by mismatch boundaries are obtained as shown, for example, by Monte Carlo simulation (cf. Section 8.4) of 1/2 adsorption on a square lattice [3.214], The fit of experimental coverage data of the first Pb adsorption step on Ag(lOO) (cf. Fig. 3.9) by Monte Carlo simulation is illustrated in Fig. 3.30. From this fit, a lateral attraction energy between the Pb adatoms of V Pbads-Pbads 2.5 X 10 J (corresponding to 1.5 x 10 J mole ) can be estimated [3.184, 3.190, 3.191, 3.214]. Preferential Me adsorption on surface heterogeneities like monatomic steps was disregarded in the fit procedure. 

Under electrochemical conditions and T, P = constant, adsorption isotherms can be derived using standard statistical considerations to calculate the Gibbs energy of the adsorbate in the interphase and the equilibrium condition for the electrochemical potentials of the adsorbed species i in the electrolyte and in the adsorbed state (eq. (8.15) in Section 8.2). A model for the statistical considerations consists of a 2D lattice of arbitrary geometry with Ns adsorption sites per unit area. In the case of a 1/1 adsorption, each adsorbed particle can occupy only one adsorption site so that the maximal number of adsorbed particles per unit area in the compact monolayer is determined by A ax = Ng. Then, this model corresponds to the simple Ising model. The number of adsorbed particles, A ads< and the number of unoccupied adsorption sites, No, per unit area are given by... 

Ts = 1-42 x 10 6/z4/3molm 2. A conclusion was made that small charged molecules can hardly form a compact monolayer and that the formation of such a monolayer requires that the size of adsorbed molecules must be comparable with the distance between the adsorbed molecules which, however, depends on their charge number. Owing to repulsive interactions in the adsorbed layer, the drop in the surface tension can be large even if the limiting surface concentration is low. 

Churaev et al. (1994) recently published actual results on the relationship between wetting film thickness h and disjoining pressure 11, from which the macroscopic contact angle of the liquid on the substrate can be calculated. Today a compact monolayer is known as a French pancake, a bilayer a Swedish pancake and thick films as an American pancake. To sum up, wetting films and their transitions are combined with dynamic processes which elucidate rather complex and yet unsolved issues. 

Radiostearic acid was adsorbed from n-hex-adecane onto mica and thin vapor-deposited films of iron, gold, and copper that had been exposed to dry and water-saturated helium or air. Adsorption was measured directly and continuously by a recently developed technique. The mica substrate showed essentially zero adsorption. None of the metals adsorbed more than one stable compact monolayer. Iron and gold showed a large difference in adsorption in dry helium or air, but adsorbed about the same amoxmt, 0.2 to 0.5 monolayer, when exposed to either wet helium or air. Copper adsorbed 0.3 to 0.7 monolayer in all atmospheres except wet air, in which it showed a weak adsorption of nine monolayers rinsing with hexaiie removed all but one monolayer. 

The apparatus was calibrated by determining the counting rate of a compact monolayer of the radiostearic acid on the film balance. Such a layer should have the same coimting rate as that of a compact mono-layer adsorbed on a relatively smooth metal film. The smoothness of... 

Adsorption of stearic acid onto iron and gold in the absence of moisture was generally consistent with the relative reactivities of the adsorbents, but when moisture was present, the adsorption properties were nearly the same. In no case was there a stable adsorption of more than one compact monolayer. Copper weakly adsorbed nine mono-layers in wet air all but one were easily removed by rinsing. Apparently the first layer of adsorbed molecules is boxmd more strongly than any succeeding layer its molecules are the only ones that can be chemisorbed or can react chemically with the surface. When less than a compact monolayer is adsorbed, it may be either a loosely packed monolayer in which the molecules are not vertically oriented, or a mixed monolayer of vertically oriented stearic acid and n-hexadecane molecules. 

Hydrous oxides are of major interest in many areas of technology, e.g., corrosion and passivation of metals, formation of decorative, protective, and insulating films, aqueous battery systems, catalysis and electrocatalysis, electrochromic display systems, pH monitoring devices, soil science, colloid chemistry, and various branches of material science. Detailed accounts of some of the nonnoble hydrous metal oxide systems, especially aluminum,1 have appeared recently. In the case of the noble metals such as platinum or gold most of the electrochemical work to date has been concerned with compact monolayer, and submonolayer, oxide growth. 

Concepts of ordering and reactivity in two dimensions can be also addressed in studies with organic, ionic, or metallic (sub)-monolayers at potentiostatically controlled electrode-electrolyte interfaces. This approach offers the advantage, in comparison to a nonelectrochemical environment, that the structural and dynamic properties of the adsorbate and the substrate can be directly tuned through the applied electrode potential. The first electrochemical studies of 2D phase transitions were mostly confined to processes occurring at ideally smooth mercury electrodes. Typical examples are the formation of compact monolayers of organic molecules or salts [15, 16] and so-called... 

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