Monolayers solid phase

In many experiments, the isosteric heat of adsorption (latent heat) is determined for adsorption into the 2D solid monolayer phase. This value denoted by Qi, can be assnmed to be constant over the coverage range of coexistence between a 2D solid and a 2D gas phase, and before any phase transition or compression of the monolayer solid phase takes place. At low coverage, where only a dilute phase (2D gas) is formed, the isosteric heat may increase or decrease with coverage depending on whether the lateral interactions between the adsorbed noble gas atoms are attractive or repnlsive. 

The three general states of monolayers are illustrated in the pressure-area isotherm in Fig. IV-16. A low-pressure gas phase, G, condenses to a liquid phase termed the /i uid-expanded (LE or L ) phase by Adam [183] and Harkins [9]. One or more of several more dense, liquid-condensed phase (LC) exist at higher pressures and lower temperatures. A solid phase (S) exists at high pressures and densities. We briefly describe these phases and their characteristic features and transitions several useful articles provide a more detailed description [184-187]. 

Fig. XVII-22. Isosteric heats of adsorption for Kr on graphitized carbon black. Solid line calculated from isotherms at 110.14, 114.14, and 117.14 K dashed line calculated from isotherms at 122.02, 125.05, and 129.00 K. Point A reflects the transition from a fluid to an in-registry solid phase points B and C relate to the transition from the in-registry to and out-of-registry solid phase. The normal monolayer point is about 124 mol/g. [Reprinted with permission from T. P. Vo and T. Fort, Jr., J. Phys. Chem., 91, 6638 (1987) (Ref. 131). Copyright 1987, American Chemical Society.]...

The most common two-dimensional phases in monolayers are the gaseous, liquid-expanded, liquid-condensed, and solid phases. A schematic II-A isotherm is shown in Figure 3 for a fatty acid for the phase sequence gas (G) — G -l- liquid-expanded (LE) — LE — ... 

As for the mechanical response of thin lipid films, surface pressure(fl)-surface area(A) characteristics of lipid monolayer at air/water interface have been well studied under quasi-static conditions. It has been established that different phases are observed for the ensemble of lipid molecules in a two-dimensional arrangement, similarly to the gas, liquid, and solid phases and some other intermediate phases as in three-dimensional molecular assemblies. 

Adsorbed monolayers of antibodies (IgG) can be achieved in the range of 130 to 650 ng/cm or from 10 to lO molecules/mm, depending upon the solid phase. Thus, antibody microspots having diameters of 50 to 120 p would be sufficient. For a weaker binding antibody, -10 °, the ambient... 

In fluorescence microscopy (FM) a small amount of a fluorescent dye is added to the mono-layer. To be incorporated into the monolayer the dye must be amphiphilic. The film is illuminated and the lateral distribution of the fluorescent molecules is observed with an optical microscope [589], Depending on the phase condition of the monolayer, the fluorescent molecules distribute unevenly or have a different quantum yield. Usually the dyes are expelled from condensed liquid and solid phases. With this technique the coexistence of different phases in monolayers on water was demonstrated for the first time [590,591],... 

The surface functionalization of solid phases by adsorbing latex-ODN conjugates leads to the formation of an ordered monolayer of colloidal particles. Under these conditions the performance of such system was proved, with an overall increase of sensitivity ranging from 100 to 1000 in the case of hepatitis B compared to the classical method [6,8]. 

In the sub-monolayer range, three distinctive regions were identified and attributed by Thorny and Duval to 2-D gas , liquid and solid phases. These measurements provided the first unambiguous evidence for the existence of sub-steps in the mono-layer region of a stepwise, Type VI, isotherm. 

To illustrate the interpretation of such sub-steps, the monolayer isotherms for the adsorption of Xe on FeCl2 (Larher, 1992) are shown in Figure 4.2. At temperatures below 99.57 K, there is a single vertical step, which corresponds to the transformation of 2-D gas to the solid phase. Very little further compression of the monolayer is possible before its completion at Point B. A smaller sub-step becomes apparent at temperatures above 99.57 K. As a result of the careful studies of Thorny and Duval and Larher, the consensus interpretation is that this small sub-step represents a first-order transition between the 2-D liquid and solid phases. It is evident that, in the case of the Xe/FeCl2 system, 99.57 K is the two-dimensional triple point. 

Surface rheology Viscoelasticity of the monolayer differentiation between fluid and solid phases. Surface elasticity and viscosity in the transversal and longitudinal mode wave damping characteristics. Relaxation processes in monolayers. Mechanical stability of the monolayer. Interpretation often complicated because several molecular processes may be involved and because viscous and elastic components may both contribute. 

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