Surface of a monolayer

The formal potential, E0/, contains useful information about the ease of oxidation of the redox centers within the supramolecular assembly. For example, a shift in E0/ towards more positive potentials upon surface confinement indicates that oxidation is thermodynamically more difficult, thus suggesting a lower electron density on the redox center. Typically, for redox centers located close to the film/solution interface, e.g. on the external surface of a monolayer, the E0 is within 100 mV of that found for the same molecule in solution. This observation is consistent with the local solvation and dielectric constant being similar to that found for the reactant freely diffusing in solution. The formal potential can shift markedly as the redox center is incorporated within a thicker layer. For example, E0/ shifts in a positive potential direction when buried within the hydrocarbon domain of a alkane thiol self-assembled monolayer (SAM). The direction of the shift is consistent with destabilization of the more highly charged oxidation state. 

Figure 5. Schematic diagram of an apparatus for fluorescence microscopy of monolayers. The exciting light, a mercury or xenon arc or a laser, passes through a narrow-band filter into the microscope optics and is focused on the surface of a monolayer that contains a fluorescent probe. The fluorescence is captured by the microscope and separated from the exciting light by a dichroic mirror. It is reflected into a silicon-intensified-target television camera. The image can be viewed on a monitor or recorded. Some experiments have used an inverted microscope in which the surface is viewed through the subphase.

A characteristic parameter of a porous material is its so-called BET-surface. This is the surface of a monolayer adsorbate of N2-molecules at boiling temperature of N2 at po = 1 atm, namely 77.3 K [1.3]. It is determined by nitrogen adsorption experiments at this temperature for pressures (p/po) < 1 The mass of the monolayer load (mO is determined by fitting data (m ) to the adsorption isotherm equation developed by Bmnauer, Emmett and Teller in 1938, originally designed for multilayer adsorbates [1.1-1.3, 1.57]. 

Most corrosion processes, e.g., metal dissolution, hydrogen or oxygen evolution, and passive film formation, involve at least one adsorption step as a part of the overall reaction. This step can be significantly affected by the presence on the metal surface of a monolayer of nonmetal species. As evidenced by studies described in this chapter, adsorbed species may act by loosening the metal-metal bond or changing the electric field at the metal-electrolyte interface. They can also favor or inhibit the adsorption or the recombination of adsorbed atoms normally involved in the anodic or cathodic reactions. 

The accepted explanation for the minimum is that it represents the point of complete coverage of the surface by a monolayer according to Eq. XVII-37, Sconfig should go to minus infinity at this point, but in real systems an onset of multilayer adsorption occurs, and this provides a countering positive contribution. Some further discussion of the behavior of adsorption entropies in the case of heterogeneous adsorbents is given in Section XVII-14. 

When it is desired to evaluate the specific surfaces of a set of closely related samples of solid, however, only one of the samples needs to be calibrated against nitrogen (or argon), provided that all the isotherms of the alternative adsorptive can be shown to have indentical shape. A simple device for testing this identity, by use of the a,-plot, is described in Section 2.13 by means of the a,-plot it is also possible to proceed directly to calculation of the specific surface without having to assign a value to or to evaluate the BET monolayer capacity, of the alternative adsorptive. 

If the surface of the adsorbent is energetically heterogeneous rather than homogeneous each step of the isotherm will be replaced by an assortment of steps, corresponding to the completion of a monolayer on the different homogeneous patches of the surface. If the steps are sufficiently numerous... 

It follows therefore that the specific surface of a mesoporous solid can be determined by the BET method (or from Point B) in just the same way as that of a non-porous solid. It is interesting, though not really surprising, that monolayer formation occurs by the same mechanism whether the surface is wholly external (Type II isotherm) or is largely located on the walls of mesopores (Type IV isotherm). Since the adsorption field falls off fairly rapidly with distance from the surface, the building up of the monolayer should not be affected by the presence of a neighbouring surface which, as in a mesopore, is situated at a distance large compared with the size of a molecule. 

The strength of dispersion interaction of a solid with a gas molecule is determined not only by the chemical composition of the surface of the solid, but also by the surface density of the force centres. If therefore this surface density can be sufficiently reduced by the pre-adsorption of a suitable substance, the isotherm may be converted from Type II to Type III. An example is rutile, modified by the pre-adsorption of a monolayer of ethanol the isotherm of pentane, which is of Type II on the unmodified rutile (Fig. 5.3, curve A), changes to Type III on the treated sample (cf. Fig. 5.3 curve B). Similar results were found with hexane-l-ol as pre-adsorbate. Another example is the pre-adsorption of amyl alcohol on a quartz powder... 

Figure 8.18 shows an X-ray photoelectron spectrum of gold foil with mercury absorbed onto the surface. Both the gold and mercury doublets result from the removal of a 4/ electron leaving /2 and /2 core states for which L = 3, S = and J = or Less than 0.1 per cent of a monolayer of mercury on a gold surface can be detected in this way. 

CO oxidation catalysis is understood in depth because potential surface contaminants such as carbon or sulfur are burned off under reaction conditions and because the rate of CO oxidation is almost independent of pressure over a wide range. Thus ultrahigh vacuum surface science experiments could be done in conjunction with measurements of reaction kinetics (71). The results show that at very low surface coverages, both reactants are adsorbed randomly on the surface CO is adsorbed intact and O2 is dissociated and adsorbed atomically. When the coverage by CO is more than 1/3 of a monolayer, chemisorption of oxygen is blocked. When CO is adsorbed at somewhat less than a monolayer, oxygen is adsorbed, and the two are present in separate domains. The reaction that forms CO2 on the surface then takes place at the domain boundaries. 

Other applications of REELM include monitoring variations like oxidation, segregation, and hydration in the surface chemistry of polycrystalline materials. Differences of 1 /10 of a monolayer in oxygen coverage due to variations in grain... 

SERS substrates with bare metal surfaces irreversibly adsorb thioorganics (Eig. 4.59) and other compounds and can thus serve for the detection and identification of very low gas or solution concentrations of these substances [4.303]. SERS is especially well suited for the analysis of traces of gases, because it combines measurement of surface concentration with extremely high sensitivity. A monolayer in a typical focus of a laser with a diameter of 10 pm has a mass in the range of 10 femtograms even smaller amounts of substance are easily detectable, because 1% of a monolayer in a region 1-pm in diameter results in SERS of sufficient intensity. 

In other materials synthesis applications, the utilization of the strong bonding of fullerenes to clean silicon surfaces, has led to the application of a monolayer... 

SIMS has superb surface sensitivity since most of the secondary ions originate within a few nanometers of the surface and since high detection efficiency enables as little as 10 " of a monolayer to be detected for most elements. Because of its very high surface sensitivity, SIMS can be used to obtain depth profiles with exceptionally high depth resolution (<5 nm). Since the beam of primary ions can be focused to a small spot, SIMS can be used to characterize the surface of a sample with lateral resolution that is on the order of micrometers. Elements with low atomic numbers, such as H and He, can be detected, isotope analysis can be conducted, and images showing the distribution of chemical species across... 

A number of approaches are available to improve the morphology and homogeneity of electrochemically deposited conducting polymer films. Priming of the electrode surface with a monolayer of adsorbed or covalently bonded monomer leads to more compact deposits of polyaniline,87,88 poly thiophene,80 and polypyrrole.89,90 Electrode rotation has been shown to inhibit the deposition of powdery overlayers during poly(3-methylthiophene) deposition.81... 

AFM is used in the surface analysis. Figure 16 is the AFM topography of the monolayer and the multilayer L-B films. It shows that the monolayer L-B film is well packed and highly ordered on the mica surface. The surface of the monolayer film (shown in Fig. 16(a)) has a higher packing density than that of the four-layer L-B film (shown in Fig. 16(b)). This is because the molecules form the different structures in the monolayer film from those in four-layer... 

The first step in the preparation of an LB film is the successful spreading of a monolayer of the material of interest, which may be molecular, polymeric, or particulate and need not be amphiphilic in the traditional sense. This is accomplished by depositing drops of a dilute solution of the material in an appropriate spreading solvent onto the water surface. The concentration is generally millimolar or less, and the solvent selected should be one that will spread across the surface rapidly and evaporate without remaining at the surface or dissolving into the subphase. Common spreading solvents include chloroform, benzene,... 

---