Adsorbed monolayer responses

The Langmuir isotherm describes equilibrium adsorption when there are no lateral interactions between the adsorbed molecules, the limiting surface coverage is dictated simply by the size of the adsorbate, all adsorption sites on the surface are equivalent and adsorption is fully reversible. As discussed in Section 5.5.1 below, spontaneously adsorbed monolayers that fulfill these criteria will exhibit ideal voltammetric responses at slow scan rates, thus including a linear dependence of the peak current, zp, on the scan rate, v, an FWHM of 90.6In mV, where n is the number of electrons transferred, and a formal potential that is independent of the surface coverage. 

Spontaneously adsorbed monolayers of the dimeric complex (Figure 5.11) [(pOp) Os(bpy)2 (4-tet) Os(bpy)2 Cl]3+, where pOp is 4,4 -bipyridyl, bpy is 2,2/-bipyridyl and 4-tet is 3,6-bis(4-pyridyl)-l,2,4,5-tetrazine, have been assembled on platinum microelectrodes in an attempt to address these issues [33]. Significantly, as illustrated in Figure 5.11, the voltammetric response associated with the Osn/m reaction is unusually ideal for both metal centers. Studies using mononuclear model compounds reveal that the redox responses centered at approximately 0.620 and 0.300 V correspond to the inner [(pOp) Os(bpy)2 (4-tet)]2+ and outer [(4-tet) Os(bpy)2 Cl]+ moieties, respectively. The observation of two well-defined voltammetric waves indicates that electron transfer can occur across the [(pOp) Os(bpy)2 (4-tet)]2+ bridge to the outer [Os(bpy)2 Cl]+ moiety, i.e. charge trapping does not occur. 

Atomically thin films constitute layers whose thicknesses are comparable to one or a few atomic layers. An adsorbed monolayer of gas or impurity atoms on a surface is an example of an atomically thin layer. Here the mechanical response of the thin layer is likely to be more influenced by interatomic potentials and surface energy than by macroscopic mechanical properties or by micromechanisms of deformation. 

In another study, QCM-D provides insight into how a protein interacts with a hydrogel film, e.g., monolayer or multilayer adsorption onto the surface versus penetration into the material. Alf et al. (2011a) show how bovine serum albumin penetrates an expanded thermo-responsive copolymer but forms an adsorbed monolayer when the material is collapsed above the phase transition temperature (Figure 6.11). 

A quantity of interest in many studies of surfaees and interfaees is tire eoneentration of adsorbed atomie or moleeular speeies. The SHG/SFG teelmique has been found to be a usefid probe of adsorbate density for a wide range of interfaees. The surfaee sensitivity afforded by the method is illustrated by the results of figure Bl.5.9 [72]. These data show the dramatie ehange in SH response from a elean surfaee of silieon upon adsorption of a fraetion of a monolayer of atomie hydrogen. 

If we consider the optical response of a molecular monolayer of increasing surface density, the fomi of equation B 1.5.43 is justified in the limit of relatively low density where local-field interactions between the adsorbed species may be neglected. It is difficult to produce any rule for the range of validity of this approximation, as it depends strongly on the system under study, as well as on the desired level of accuracy for the measurement. The relevant corrections, which may be viewed as analogous to the Clausius-Mossotti corrections in linear optics, have been the... 

---