Layering critical point

H. S. Nham, G. B. Hess, Layer critical points of multilayer ethane adsorbed on graphite, Phys. Rev. B 38 (1988) 5166-5169. 

Many pairs of partially miscible liquids possess neither a lower nor an upper C.S.T. for reasons outlined in the previous paragraph. Thus consider the two liquid phases from the two components water and diethyl ether. Upon cooling the system at constant pressure, a point will be reached when a third phase, ice, will form, thus rendering the production of a lower C.S.T. impossible, likewise, if the temperature of the two layers is raised, the critical point for the ether rich layer will be reached while the two liquid phases have different compositions. Above the critical point the ether-rich layer will be converted into vapour, and hence the system will be convert into a water rich liquid and an ether rich vapour the upper C.S.T. cannot therefore be attained. 

In the vicinity of the critical point (10) is no longer true, and we must admit the possibility of a special heat capacity of the surface layer. 

As mentioned earlier, the physical properties of a liquid mixture near a UCST have many similarities to those of a (liquid + gas) mixture at the critical point. For example, the coefficient of expansion and the compressibility of the mixture become infinite at the UCST. If one has a solution with a composition near that of the UCEP, at a temperature above the UCST, and cools it, critical opalescence occurs. This is followed, upon further cooling, by a cloudy mixture that does not settle into two phases because the densities of the two liquids are the same at the UCEP. Further cooling results in a density difference and separation into two phases occurs. Examples are known of systems in which the densities of the two phases change in such a way that at a temperature well below the UCST. the solutions connected by the tie-line again have the same density.bb When this occurs, one of the phases separates into a shapeless mass or blob that remains suspended in the second phase. The tie-lines connecting these phases have been called isopycnics (constant density). Isopycnics usually occur only at a specific temperature. Either heating or cooling the mixture results in density differences between the two equilibrium phases, and separation into layers occurs. 

The smectic phases Ai, A2 and A have the same macroscopic symmetry, differing from each other in the wavelength of spacing. Hence it is possible to go from Ai to Aa or from Aa to A2 by varying only the layer periodicity in a continuous or discontinuous way(with the jump in the layer spacing d). Smectic-smectic transition lines of first order may terminate at a critical point, where the differences between the periodicities of the smectic A phases vanish, providing a continuous evolution from an Aa to bilayer A2 phase [12]. 

Under potential deposition is a much-studied phenomenon in electrochemistry and is the electrochemical reduction of a metal cation to form a monolayer or submonolayer of the corresponding metal at the surface of an electrode. The critical point is that deposition occurs at a potential higher than that dictated by the reversible potential of the metal/metal cation couple, suggesting that such a upd layer is energetically quite different from the bulk metal. However, subsequent deposition on a upd monolayer occurs at the expected potential, and the resulting surface is typical of the bulk metal. 

The sharpness of Prussian blue/Prussian white redox peaks in cyclic voltammograms can be used as an indicator of the quality of Prussian blue layers. To achieve a regular structure of Prussian blue, two main factors have to be considered the deposition potentials and the pH of initial growing solution. As mentioned, the potential of the working electrode should not be lower then 0.2 V, where ferricyanide ions are intensively reduced. The solution pH is a critical point, because ferric ions are known to be hydrolyzed easily, and the hydroxyl ions (OH-) cannot be substituted in their... 

The theory of polymer adsorption is complicated for most situations, because in general the free energy of adsorption is determined by contributions from each layer i where the segment density is different from that in the bulk solution. However, at the critical point the situation is much simpler since the segment density profile is essentially flat. Only the layer immedia-... 

For the calculation of AhP we must count contacts within the surface layer, and between the surface layer and the neighbouring layer (which has bulk composition). This calculation is simplified very much if we assume that at the critical point the surface is almost saturated with displacer (i.e. g 1), since not only the polymer, but also the still more weakly adsorbing solvent will have been almost completely displaced. Before exchange, we have a displacer molecule at the surface, and a segment in the solution, giving contributions to the mixing energy h = X Jx 0 and ... 

The boundary layer thickness gradually increases until a critical point is reached at which there is a sudden thickening of the boundary layer this reflects the transition from a laminar boundary layer to a turbulent boundary layer. For both types, the flow outside the boundary layer is completely turbulent. In that part of the boundary layer near the leading edge of the plate the flow is laminar and consequently this is known as a... 

A modified superheat theory was proposed by Shick to explain molten salt (smelt)-water thermal explosions in the paper industry (see Section IV). (Smelt temperatures are also above the critical point of water.) In Shick s concept, at the interface, salt difiuses into water and water into the salt to form a continuous concentration gradient between the salt and water phases. In addition, it was hypothesized that the salt solution on the water side had a significantly higher superheat-limit temperature and pressure than pure water. Thicker, hotter saltwater films could then be formed before the layer underwent homogeneous nucleation to form vapor. 

Fig. 27. Phase diagram of an adsorbed film in- the simple cubic lattice from mean-fleld calculations (full curves - flrst-order transitions, broken curves -second-order transitions) and from a Monte Carlo calculation (dash-dotted curve - only the transition of the first layer is shown). Phases shown are the lattice gas (G), the ordered (2x1) phase in the first layer, lattice fluid in the first layer F(l) and in the bulk F(a>). For the sake of clarity, layering transitions in layers higher than the second layer (which nearly coincide with the layering of the second layer and merge at 7 (2), are not shown. The chemical potential at gas-liquid coexistence is denoted as ttg, and 7 / is the mean-field bulk critical temperature. While the layering transition of the second layer ends in a critical point Tj(2), mean-field theory predicts two tricritical points 7 (1), 7 (1) in the first layer. Parameters of this calculation are R = —0.75, e = 2.5p, 112 = Mi/ = d/2, D = 20, and L varied from 6 to 24. (From Wagner and Binder .)...

The immunoreplica technique (14) is used when it is necessary to detect antigenic sites on the plasma membrane of cultured cells. The cells are cultured on coverslips, and are fixed as described above depending on the antibody in question, and immunolabeled in situ as described in Section 3.1.1.2., steps 3-9. After immunolabeling (Section 3.1.1.2., step 9), they are further fixed with 1% osmium tetroxide and are dehydrated in a graded series of ethanol (70, 90, 100%), critically point-dried, and replicated with a layer of carbon and platinum, The replicas are cleaned with sodium hypochlorite and chronic acid before examination with the transmission electron microscope. Large areas of the replicated plasma membrane remain intact for observation. Colloidal gold probes are probably the only probes of sufficient density that can be detected on these surfaces. 

Flint and Suslick (1991) and Seghal and Wang (1989) clearly demonstrated that temperature and pressure within a collapsing cavitation bubble exceed the critical point of water, on the basis of previously estimated temperatures within a collapsed bubble and a smaller layer of surrounding liquid. However, no experimental data are available for the density of nuclei or actual cavitation bubbles in water during ultrasonic irradiation or SCW accelerated chemical reactions. 

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