Monolayer : definition

Cell permeability, which is measured byPapp, as used herein refers to the ability of a compound to traverse an intact cell monolayer unchanged in an in vitro system. Note that this definition differs from what many chemists think of as cell permeability, the ability to get inside but not necessarily pass through a cell. This too is a useful definition in the right context, but when discussing results in cell monolayer models like the Caco-2 one described later, the through a cell monolayer definition needs to be used. 

It is useful to define the tenns coverage and monolayer for adsorbed layers, since different conventions are used in the literature. The surface coverage measures the two-dimensional density of adsorbates. The most connnon definition of coverage sets it to be equal to one monolayer (1 ML) when each two-dimensional surface unit cell of the unreconstructed substrate is occupied by one adsorbate (the adsorbate may be an atom or a molecule). Thus, an overlayer with a coverage of 1 ML has as many atoms (or molecules) as does the outennost single atomic layer of the substrate. 

It is thus tempting to define the first saturated layer as being one monolayer, and this often done, causing some confiision. One therefore also often uses tenns like saturated monolayer to indicate such a single adsorbate layer that has reached its maximal two-dimensional density. Sometimes, however, the word saturated is omitted from this definition, resulting m a different notion of monolayer and coverage. One way to reduce possible confiision is to use, for contrast with the saturated monolayer, the tenn fractional monolayer for the tenn that refers to the substrate unit cell rather than the adsorbate size as the criterion for the monolayer density. 

Self-assembled monolayers (SAMs) are molecular layers tliat fonn spontaneously upon adsorjDtion by immersing a substrate into a dilute solution of tire surface-active material in an organic solvent [115]. This is probably tire most comprehensive definition and includes compounds tliat adsorb spontaneously but are neither specifically bonded to tire substrate nor have intennolecular interactions which force tire molecules to organize tliemselves in tire sense tliat a defined orientation is adopted. Some polymers, for example, belong to tliis class. They might be attached to tire substrate via weak van der Waals interactions only. 

With today s technology, the definition of the surface as it effects a material s performance in many cases means the outer one or two monolayers. It is the specific chemistry of these immediate surface molecules that determines many of the chemical and physical properties. Therefore, it is important to have available a tool that is able to characterize the chemistry of these layers. One such method that has met with considerable success is Static Secondary Ion Mass Spectrometry (SIMS). 

Figure 12 Monolayer nucleus with uneven end surface (a) definition of terms (b) definition ofAEe and the activation...

Underpotential deposition of heavy metals on H2 evolving electrodes is a well known problem [133], The existence of a direct correlation between H2 evolution activity and metal work function, makes UPD very likely on high work function electrodes like Pt or Ni. Cathode poisoning for H2 evolution is aggravated by UPD for two reasons. First, deposition potentials of UPD metals are shifted to more anodic values (by definition), and second, UPD favors a monolayer by monolayer growth causing a complete coverage of the cathode [100]. Thus H2 evolution may be poisoned by one monolayer of cadmium for example, the reversible bulk deposition potential of which is cathodic to the H2 evolution potential. 

In whole tissue or cell monolayer experiments, transcellular membrane resistance (Rm = Pm1) lumps mucosal to serosal compartment elements in series with aqueous resistance (R = P ). The operational definition of Lm depends on the experimental procedure for solute transport measurement (see Section VII), but its magnitude can be considered relatively constant within any given experimental system. Since the Kp range dwarfs the range of Dm, solute differences in partition coefficient dominate solute differences in transcellular membrane transport. The lumped precellular resistance and lumped membrane resistance add in series to define an effective resistance to solute transport ... 

Initially, UPD features were thought to correspond to deposition of one mono-layer of an element on a second element, and that there should be a single deposition feature. As more work was performed using single crystal electrodes [84, 86, 88], it became clear that more then one feature is frequently associated with UPD, and that coverages are seldom one monolayer, depending on the definition of a monolayer. 

The term monolayer (ML) must be defined clearly. In the work presented here, two definitions are used for surface studies, one ML indicates one adsorbate for each surface atom. For studies of compound formation, a monolayer is a slice of the compound s crystal structure, composed of one atomic layer of each of the constituent atoms. This does not necessarily mean a one unit-cell thick deposit is formed, as most compounds have larger unit cells from the point of view of crystallography, dependent on the orientation (Figure 8). 

The term atomic layer is used here to indicate, in general, a layer of atoms on the surface, where all the atoms are in contact with the surface. The term atomic layer does not specify a coverage, just that the layer is no more then one atom thick, probably less then a ML, relative to the number of substrate surface atoms. There can be several structures formed at different coverages, all under a ML, but all are one atom thick, and all would correspond to an atomic layer. Thus a statement that an atomic layer was formed suggests only that no bulk atoms were deposited. Where as, the statement that a monolayer was formed suggests a coverage, dependent on the ML definition in use. 

As the analytical, synthetic, and physical characterization techniques of the chemical sciences have advanced, the scale of material control moves to smaller sizes. Nanoscience is the examination of objects—particles, liquid droplets, crystals, fibers—with sizes that are larger than molecules but smaller than structures commonly prepared by photolithographic microfabrication. The definition of nanomaterials is neither sharp nor easy, nor need it be. Single molecules can be considered components of nanosystems (and are considered as such in fields such as molecular electronics and molecular motors). So can objects that have dimensions of >100 nm, even though such objects can be fabricated—albeit with substantial technical difficulty—by photolithography. We will define (somewhat arbitrarily) nanoscience as the study of the preparation, characterization, and use of substances having dimensions in the range of 1 to 100 nm. Many types of chemical systems, such as self-assembled monolayers (with only one dimension small) or carbon nanotubes (buckytubes) (with two dimensions small), are considered nanosystems. 

Fig. 2.4. Definition of structural parameters for a two-sublattice monolayer (a) projections of long molecular axes onto the unit cell (b) side view showing the tilt of the molecular axis.

Definition of a toxicity with culture of the primerely trepsinised chicken fibroblasts (TCF) included the following stages - incubation of chicken embryo within 11 day - tripsinisation and cultivation of a TCF cells monolayer - 2 days - contact of the tested preparation with the TCF cells monolayer an and the count of a toxicity by the citopathogenic action (CPA)-1-5 day. Hence, definition of a toxicity on cell culture borrowed 14-18 days and at the last two stages demanded strict sterility. 

The nature of the material that results after the first few ECALE cycles has been questioned by some—whether the surface is covered with separate domains or islands of Cd and of Te or with a monolayer of CdTe, for instance. The electrochemical results are fairly definitive on this point, in that if there were islands of Cd present on the surface, bulk Cd would be expected to strip from the surface at potentials below -0.7 V, which... 

---