Adsorbed layer structure monolayers

While data from many surfactant adsorption experiments would support the assumption that monolayer adsorption occurs, direct measurements of the adsorbed layer structure were not reported until recently. Herder [15] used a combination of contact angle measurements and surface force measurements to show that this is the situation for the adsorption of phosphine oxide onto a mica surface... 

In this section we review several studies of phase transitions in adsorbed layers. Phase transitions in adsorbed (2D) fluids and in adsorbed layers of molecules are studied with a combination of path integral Monte Carlo, Gibbs ensemble Monte Carlo (GEMC), and finite size scaling techniques. Phase diagrams of fluids with internal quantum states are analyzed. Adsorbed layers of H2 molecules at a full monolayer coverage in the /3 X /3 structure have a higher transition temperature to the disordered phase compared to the system with the heavier D2 molecules this effect is... 

Makovicky Hyde (1981) have reviewed incommensurate misfit structures in graphite intercalation compounds, brucite-type compounds, sulphides and related layered systems. A simple two-dimensional incommensurate system is provided by graphite with adsorbed rare gas monolayers. At low densities and high temperatures. 

Globular proteins form close-packed monolayers at fluid interfaces. Hence a large contribution to the adsorbed layer viscoelasticity arises from short-range repulsive interactions between hard-sphere particles. In addition to, or instead of, this glass-like5 structure from hard spheres densely packed in two dimensions, many adsorbed proteins can exhibit attractive interactions leading to a more gel-like5 network structure. Hence the mechanical properties of an adsorbed layer depend on many... 

Calculations from SCF theory of the mixed layer structure, and of the interaction potential for a pair of mixed layers as a function of interlayer separation, suggest that the mixed layer has a heterogeneous morphology perpendicular to die interface (Parkinson et al., 2005). This localized segregation arises from the excluded volume interaction between spaced-out casein chains and the dense brush-like layer that was invoked in the simple SCF model to represent the p-lactoglobulin adsorbed monolayer. 

One of the most exciting observations of LEED studies of adsorbed monolayers on low Miller index crystal surfaces is the predominance of ordering within these layers (18). These studies have detected a large number of surface structures formed upon adsorption of different atoms and molecules on a variety of solid surfaces. Conditions range from low temperature, inert gas physisorption to the chemisorption of reactive diatomic gas molecules and hydrocarbons at room temperature and above. A listing of over 200 adsorbed surface structures, mostly of small molecules, adsorbed on low Miller index surfaces can be found in a recent review (/). 

Displacement of the protein from the adsorbed layer in o/w thin films shows very different behavior from its a/w counterpart. Although displacement of protein from the o/w interfaces initiates at approximately the same solution composition (i.e., R = 0.1), there is little evidence for the stepwise displacement observed in the a/w thin films. This observation is further confirmation of the monolayer versus multilayer structure at the o/w and a/w thin films. The displacement of /3-lg has also been investigated in oil-in-water emulsions of n-tetradecane [46,47], In these reports it was shown that the protein was not completely displaced until R = 10, which was considerably higher than R = 1 - 2 in Figure 22. This will be discussed further below. 

Figure 7 shows adsorption isotherms for this protein on the different sorbents. The adsorption plateau-values at PS-(EO)8, approximately 2.5 mg m 2, is compatible with a complete monolayer of side-on adsorbed a-chymotrypsin molecules. Adsorption saturation at the PS and, even more so, the Teflon surfaces, is beyond monolayer coverage suggesting that on these hydrophobic surfaces the protein molecules are severely perturbed as to accommodate more protein mass in the adsorbed layers and/or adsorption of a second layer of protein molecules (possibly triggered by structurally altered molecules in the... 

The more recent neutron reflectivity studies have established that flattened surface micelle or fragmented bilayer structure in more detail and with more certainty, using contrast variation in the surfactant and the solvent [24, 31]. However, the extent of the lateral dimension (in the plane of the surface) and the detailed structure in that direction is less certain. From those neutron reflectivity measurements [24, 31] and related SANS data on the adsorption of surfactants onto colloidal particles [5], it is known that the lateral dimension is small compared with the neutron coherence length, such that averaging in the plane is adequate to describe the data. The advent of the AFM technique and its application to surfactant adsorption [15] has provided data that suggest that there is more structure and ordering in the lateral direction than implied from other measurements. This will be discussed in more detail in a later section of the chapter. At the hydrophobic interface, although the thickness of the adsorbed layer is now consistent with a monolayer, the same uncertainties about lateral structure exist. 

Solid surfaces may accommodate and orient molecules at distances close to molecular bonds and reaction rates are influenced by physical order in the adsorbed layer. Deposition of small molecules on a crystal surface under appropriate temperature and pressure conditions produces ordered molecular monolayers and multilayers. These structures result from the balance of the forces causing adsorption imposed by the surface and the forces between neighboring adsorbed species. Under such conditions, certain reactant... 

The highly distinctive form of a Type VI isotherm is due to a stepwise layer-by-layer adsorption process. Such isotherms are given by the adsorption of simple non-polar molecules (e.g. argon, krypton and xenon) on uniform surfaces (e.g. the basal plane of graphite). The steps become less sharp as the temperature is increased. The vertical risers can be regarded as the adsorbed layer boundaries and the centres of the treads (inflection points) as the layer capacities. When present, sub-steps are associated with two-dimensional phase changes in the monolayer. Useful information concerning the surface uniformity and adsorbate structure can be obtained from the relative layer capacities and the presence of sub-steps. 

The specific surface area of a ceramic powder can be measured by gas adsorption. Gas adsorption processes may be classified as physical or chemical, depending on the nature of atomic forces involved. Chemical adsorption (e.g., H2O and AI2O3) is caused by chemical reaction at the surface. Physical adsorption (e.g., N2 on AI2O3) is caused by molecular interaction forces and is important only at a temperature below the critical temperature of the gas. With physical adsorption the heat erf adsorption is on the same order of magnitude as that for liquefaction of the gas. Because the adsorption forces are weak and similar to liquefaction, the capillarity of the pore structure effects the adsorbed amount. The quantity of gas adsorbed in the monolayer allows the calculation of the specific surface area. The monolayer capacity (V ,) must be determined when a second layer is forming before the first layer is complete. Theories to describe the adsorption process are based on simplified models of gas adsorption and of the solid surface and pore structure. 

An even more striking comparison can be made between the wild-type signal peptide s conformation when adsorbed to the monolayer and its conformation in aqueous solution. In both of these environments, the peptide should be solvated by water, but its conformations are very different. The peptide is 100% )3 structure when adsorbed to the mono-layer, while it is 80% random in aqueous buffer. Thus, it appears that contact with the lipid surface induces substantial amounts of secondary structure in a molecule that takes on little structure in an aqueous environment. This finding implies that the initial binding of a signal sequence to a membrane may induce a particular structure, which may be important to the mechanism of signal-sequence function. 

Photoinduced electron-transfer reactions have also been observed in monolayer assemblies (23, 24). Such systems can be made essentially free of molecular diffusion and thus most closely resemble the solid state. They can be fabricated with precise geometry and therefore provide well-defined structures often lacking in amorphous solids or adsorbed layers. However, until now no evidence for enhancement of the yield of reaction (1) by added supersensitizers had been obtained, although the concept had been discussed (25). 

At higher surface coverages (more than Vi but less than 1), anions can be entirely displaced by copper adatoms from the surface or both form a two-layer structure in which anions are adsorbed on both the platinum and the copper sites. The final step is the total filling of the copper monolayer to form a bilayer phase with a disordered anion ad-layer on the topmost of Cu-Pt(lll) [106] or an ordered (2 x 2) bilayer of copper-halide structure on Pt(100) [104], The same physical models can be used in the case of bromide and chloride with little differences between the anion distances with a surface structure like that of a honeycomb ad-layer. The situation accounting for iodine adsorption is very different because of its large atomic radius and specific adsorption on noble metals. 

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