Monolayer solid-condensed

As discussed in section 2.2, a mixture of AMP and AA showed two solid condensed phases above and below about 30 mN m- [5,10]. A loosely stacked structure of two porphyrins was proposed for LB films prepared at higher surface pressures than 30 mNmr1, which was caused by squeezing-out of a monomolecular structure formed at lower surface pressure [5,10]. In this section, photoelectric characteristics of LB films containing AMP and AA deposited at two solid condensed phases will be discussed in relation to multilayer structure and the anisotropic intermolecular tunneling rates [87]. Seven monolayers of 1 5 or 1 10 mixture of AMP and AA were deposited at 20 and 50 mN m-1 on an ITO plate at 18 °C to form stable Y-type LB films. Aluminum was vacuum evaporated onto LB films as sandwich-type electrodes at 10-6 Torr. Steady photocurrents were measured in a similar manner as mentioned above. 

The relationship between surface tension and temperature in emulsifiers was observed two decades ago by Lutton et al.43. They explained that this relationship is due to a transition from a liquid-expanded type of monolayer existing at high temperatures (above 40°C) to a solid condensed monolayer existing at a lower temperature (below 20°C). In solid condensed monolayers the molecular packing of the emulsifier molecules is much denser than in the liquid expanded monolayers, and these differences result in lower or higher surface tension, respectively. 

Monomolecular layers and LB-multilavers. Compound 21 exhibits amphiphilic properties. Spreading at the air-water interface leads to the formation of oriented monomolecular layers. The monolayer forms a solid condensed phase at 20 C with a collapse pressure near SOmN/m. It can easily be transferred onto various hydrophobic substrates such as CaF, ZnS, AgCl, Si, Ga or metal surfaces, and Langmuir-Blodgett-type multilayers (21> of variable thickness can be built up. These multilayers also exhibit a rapid reaction if exposed to UV- or y-irradiation. From an infrared spectroscopic study described recently (ll-JA) a 1,4-addition reaction is evident, as it also occurs in microcrystalline powders of 21- The solid state polymerization of 21 is schematically represented by Figure 14. 

Therefore, the inverse gas chromatography method allows us to determine the model of long-chain aliphatic alcohol distribution on the surface of porous silica gel if the amount of alcohol on adsorbent surface is equal or exceeds the monolayer capacity, then the monolayer is composed of alcohol molecules oriented their polar moieties to the adsorbent surface. The monolayer, in a solid-condensed state, is stable up to 81°C. At this temperature the monolayer transfers into a liquid-expanded state. The threedimensional excess of alcohol, because the autophobicity phenomenon, does not wet the monolayer surface. 

To describe the experimental observation [40] of the solid condensed - liquid expanded phase transitions in brush-like monolayers on silica gel a simple lattice model and the theory of orientational effects in adsorbed monolayers were used [36-38]. It was assumed that interaction between the n-octadecanol molecule and the solid could be presented as... 

Besides, it was assumed that only half of the adsorption centers on silica gel surface can be occupied by n-octadecanol molecules and that all these centers are energetically equivalent. Thus, we can consider a surface lattice of sites be composed of two interpenetrating sublattices. At low surface concentration the adsorbed molecules are distributed randomly over the centers belonging to both sublattices. However, when the film density exceeds a certain value of A /Aj (Aj is the surface area occupied by a single molecule) a preferential adsorption on one of the sublattices must begin. This leads to the formation of a highly ordered structure, e.g. the monolayer in two-dimensional solid condensed state. If the interaction between only neighbouring hydrocarbon chains, perpendicularly oriented to the solid surface are taken into account, we can derive the equation of state for the adsorbed film [39]... 

Serpinet, using the inverse gas chromatography method, demonstrated the existence of oriented monolayers of long-chain hydrocarbons on silica gel surface [13], on the other hand Untz [31] showed that hydrocarbons also form solid condensed and liquid expanded monolayers on glycerol but not on the water surface. However, the addition of some amount of amphiphilic molecules to the hydrocarbon provokes the mixed monolayer formation on the water surface. The phase transition in such a monolayer occurs at the temperature higher than the melting point of bulk hydrocarbon. It also appeared that the monolayers characterized by 1 1 ratio of hydrocarbon to alcohol molecules were particularly stable [41]. 

Figure 10. Pressure-area (IT — A) isotherm for a hypothetical amphiphile organized as a monomolecular layer at the water-air interface showing a phase transition from a fluid-expanded to a solid-condensed state upon compression. The insets show cross-sectional diagrams of the Langmuir trough with the compressed monolayer in different phase states.

The lattice-controlled diyne polymerization can occur in lipid bilayers only, if the amphiphiles are present in the solid-condensed state. Several studies dealt with the characterization of the monolayer properties of diyne surfactants. Fatty acids, for example, were investigated concerning influences of a chain length variation and headgroup ionization on the Glm stability (Fig. 9a) sv.eo. 62-64,70) number of... 

Photoreactioirs in monolayers are usually indicated by changes in the I1-A-isodrerms of the amphiphiles. Monolayers of most of the diyite surfactants exhibit a film contraction upon exposure to UV-light. While the amphiphiles are highly reactive in the solid-condensed state, no reaction occurs in the liquid-expanded state... 

Because of their easy handle Langmuir-Blodgett multilayers are best suited for a characterization of the diyne polymerization in lipid layer structures. The built-up layers consist of monolayers successively deposited on substrates, the number of layers being determined by the number of dipping cycles of the substrate >. The method however is restricted to amphiphiles that are able to form highly stable, solid-condensed films at the air-water interface. 

The variation of the transition between form I and form II with temperature is shown in Fig. 5.10. The pressure of the transition plateau increases with temperature, and above a temperature of 42 C no solid condensed monolayer (form II) is formed. This temperature is also in good agreement with the temperature of transition from... 

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]. 

There is always some degree of adsorption of a gas or vapor at the solid-gas interface for vapors at pressures approaching the saturation pressure, the amount of adsorption can be quite large and may approach or exceed the point of monolayer formation. This type of adsorption, that of vapors near their saturation pressure, is called physical adsorption-, the forces responsible for it are similar in nature to those acting in condensation processes in general and may be somewhat loosely termed van der Waals forces, discussed in Chapter VII. The very large volume of literature associated with this subject is covered in some detail in Chapter XVII. 

It is known that even condensed films must have surface diffusional mobility Rideal and Tadayon [64] found that stearic acid films transferred from one surface to another by a process that seemed to involve surface diffusion to the occasional points of contact between the solids. Such transfer, of course, is observed in actual friction experiments in that an uncoated rider quickly acquires a layer of boundary lubricant from the surface over which it is passed [46]. However, there is little quantitative information available about actual surface diffusion coefficients. One value that may be relevant is that of Ross and Good [65] for butane on Spheron 6, which, for a monolayer, was about 5 x 10 cm /sec. If the average junction is about 10 cm in size, this would also be about the average distance that a film molecule would have to migrate, and the time required would be about 10 sec. This rate of Junctions passing each other corresponds to a sliding speed of 100 cm/sec so that the usual speeds of 0.01 cm/sec should not be too fast for pressurized film formation. See Ref. 62 for a study of another mechanism for surface mobility, that of evaporative hopping. 

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 — ... 

FIG. 3 An isotherm is depicted for a Langmuir monolayer of an amphiphUe showing the ft-A variation for the phase sequence gas (G) —> G + liquid-expanded (LE) —> LE —> LE + tilted condensed phase (L2) —> L2 —> vertical condensed phase (LS) —> S (solid). Schematic depictions of the molecular organization in the phases are shown above the isotherm. 

Initially, the compression does not result in surface pressure variations. Molecnles at the air/water interface are rather far from each other and do not interact. This state is referred to as a two-dimensional gas. Farther compression results in an increase in snrface pressure. Molecules begin to interact. This state of the monolayer is referred as two-dimensional liquid. For some compounds it is also possible to distingnish liqnid-expanded and liquid-condensed phases. Continnation of the compression resnlts in the appearance of a two-dimensional solid-state phase, characterized by a sharp increase in snrface pressure, even with small decreases in area per molecule. Dense packing of molecnles in the mono-layer is reached. Further compression results in the collapse of the monolayer. Two-dimensional structure does not exist anymore, and the mnltilayers form themselves in a non-con trollable way. 

Figure 5.19 shows an idealized form of the adsorption isotherm for physisorption on a nonporous or macroporous solid. At low pressures the surface is only partially occupied by the gas, until at higher pressures (point B on the curve) the monolayer is filled and the isotherm reaches a plateau. This part of the isotherm, from zero pressures to the point B, is equivalent to the Langmuir isotherm. At higher pressures a second layer starts to form, followed by unrestricted multilayer formation, which is in fact equivalent to condensation, i.e. formation of a liquid layer. In the jargon of physisorption (approved by lUPAC) this is a Type II adsorption isotherm. If a system contains predominantly micropores, i.e. a zeolite or an ultrahigh surface area carbon (>1000 m g ), multilayer formation is limited by the size of the pores. 

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