Surface-induced layering

Fig. 2.7.3. High resolution specular reflectivity data for 80CB near the peak due to the formation of smectic-like layers near the free surface of the nematic phase. The open circles refer to the scale on the left and the filled circles to the scale on the right. The temperatures T— are (a) 0.10 °C, (Z>) 0.21 C, (c) 0.40 and (

The surface-induced layering of molecules has some interesting consequences, when a liquid is confined between two flat surfaces, and the separation between the surfaces is reduced to molecular dimensions. When the surface separation is changed, an oscillatory force is detected on both surfaces, which is due to layering of molecules near both walls. This can be intuitively understood as a result of periodic match or mismatch between the number of molecular layers and the total surface separation D. As this force originates from the local structure of a liquid, it is usually called the structural (or solvation) force [2]. 

The surface induced layering produces an oscillatory periodic force, with period d = (4.2 0.1)nm, that corresponds to the inter-miceUes distance in the bulk. Compared to smectics (see Fig. 3.18), the force is spatially damped and only nine oscillations are clearly identified. It is therefore a structural force of pre-smectic origin, induced by the surface. The oscillations are not parabolic and the minima lay on a fully attractive baseline. 

By considering a simple spatial dependence of the order parameter, i.e. S z) = exp(—2/ ), the integral in (4.3) can be calculated and the ellip-ticity coefficient for the surface-induced layer of a pre-nematic liquid crystal on an isotropic substrate is... 

Surface-induced layering corresponds to oscillations of the disjoining pressure in the molecular range of thickness (see also chapter 1). Well-known saw-tooth force profiles are obtained with surface forces apparatus (SFA) setups. In the case of microdroplets, where the thickness is free to adjust, an unstable part in the disjoining pressure behaves just like the usual pressure at three dimensions a Maxwell construction leads to a horizontal part in n(f), corresponding to the coexistence of films with different thickness (see chapter 3). 

A remarkable feature is the vertical part on the thickness profiles, whose upper boundary (UB) and lower boundary (LB) are fixed at a given temperature, whatever the drop volume and time. On a given profile, the part above the UB is smooth. Steps are visible below the LB in a narrow range of temperature. This corresponds to a complex shape of the disjoining pressure a Maxwell construction leads to a horizontal part between the LB and the UB, defining a forbidden range of thickness. Below the LB, steps, if any, reveal surface-induced layering and therefore a smectic-like structure. 

An interesting alternative method for formulating f/(jt) was proposed in 1929 by de Boer and Zwikker [80], who suggested that the adsorption of nonpolar molecules be explained by assuming that the polar adsorbent surface induces dipoles in the first adsorbed layer and that these in turn induce dipoles in the next layer, and so on. As shown in Section VI-8, this approach leads to... 

Sorption and Desorption Processes. Sorption is a generalized term that refers to surface-induced removal of the pesticide from solution it is the attraction and accumulation of pesticide at the sod—water or sod—air interface, resulting in molecular layers on the surface of sod particles. Experimentally, sorption is characterized by the loss of pesticide from the sod solution, making it almost impossible to distinguish between sorption in which molecular layers form on sod particle surfaces, precipitation in which either a separate soHd phase forms on soHd surfaces, covalent bonding with the sod particle surface, or absorption into sod particles or organisms. Sorption is generally considered a reversible equdibrium process. 

The importance of surface characterization in molecular architecture chemistry and engineering is obvious. Solid surfaces are becoming essential building blocks for constructing molecular architectures, as demonstrated in self-assembled monolayer formation [6] and alternate layer-by-layer adsorption [7]. Surface-induced structuring of liqnids is also well-known [8,9], which has implications for micro- and nano-technologies (i.e., liqnid crystal displays and micromachines). The virtue of the force measurement has been demonstrated, for example, in our report on novel molecular architectures (alcohol clusters) at solid-liquid interfaces [10]. 

Other forces can arise as a result of elastic strain on the growing film, which can be due to a surface-induced ordering in the first few layers that reverts to the bulk liquid structure at larger distances. This elastic energy is stored in intermolecular distances and orientations that are stretched or compressed from the bulk values by the influence of the substrate at short distances [7]. Similar phenomena are well known to occur in the growth of epitaxial layers in metals and semiconductors. 

Room temperature deposition of silver on Pd(lOO) produces a rather sharp Ag/Pd interface [62]. The interaction with a palladium surface induces a shift of Ag 3d core levels to lower binding energies (up to 0.7 eV) while the Pd 3d level BE, is virtually unchanged. In the same time silver deposition alters the palladium valence band already at small silver coverage. Annealing of the Ag/Pd system at 520 K induces inter-diffusion of Ag and Pd atoms at all silver coverage. In the case when silver multilayer was deposited on the palladium surface, the layered silver transforms into a clustered structure slightly enriched with Pd atoms. A hybridization of the localized Pd 4d level and the silver sp-band produces virtual bound state at 2eV below the Fermi level. 

We should point out that up to now we have considered only polycrystals characterized by an a priori surface area depleted in principal charge carriers. For instance, chemisorption of acceptor particles which is accompanied by transition-free electrons from conductivity band to adsorption induced SS is described in this case in terms of the theory of depleted layer [31]. This model is applicable fairly well to describe properties of zinc oxide which is oxidized in air and is characterized by the content of surface adjacent layers which is close to the stoichiometric one [30]. 

S. Stapf, R. Kimmich 1997, (Translational mobility in surface induced liquid layers investigated by NMR diffusome-try), Chem. Phys. Lett. 275, 261. 

As was pointed out in Chapters 2 and 3, a dipole layer exists at the surface of a metal, which gives rise to a concomitant surface dipole potential x- The magnitude of this potential changes in the presence of an external electric field. A field E directed away from the surface induces an excess charge density, o e0e E, where e is the dielectric constant of the medium outside the metal. The field E pushes the electrons into the metal, producing the required excess charge, and decreasing the dipole potential (see Fig. 3.4). This has consequences for the interfacial capacity. 

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