Monolayer compressing crystallized

Variations on the vertical dipping technique have been utilized to construct films containing divalent metal ions. For example, the quartz crystal microbalance (QCM) has been used to evaluate the horizontal lifting method of CdSt LB Film construction (26). In this method, the QCM quartz plate was touched to monolayers compressed on a subphase and lifted horizontally. Y-type transfer (transfer ratio of 1) was demonstrated with two centrosymmetric monolayers deposited for each cycle. A combination of the vertical and horizontal dipping techniques has been utilized to prepare multilayer films from an amphiphilic porphyrin compound (27). 

The incommensurate, hexagonal monolayers are compressed compared to the bulk metal and they are rotated from the substrate by several degrees. From the results, the monolayer compressibility could be calculated. The restructuring (i.e. surface reconstruction) of top layers of single crystal metal surfaces as a function of solution composition and electrode potential has been studied [73]. The induced charge density was found to be the critical parameter [74]. Structural changes during... 

FIG. 16 Fomation of a Langmuir lipid monolayer at the air/subphase interface and the subsequent crystallization of S-layer protein, (a) Amphiphilic lipid molecules are placed on the air/subphase interface between two barriers. Upon compression between the barriers, increase in surface pressure can be determined by a Wilhelmy plate system, (b) Depending on the final area, a liquid-expanded or liquid-condensed lipid monolayer is formed, (c) S-layer subunits injected in the subphase crystallized into a coherent S-layer lattice beneath the spread lipid monolayer and the adjacent air/subphase interface. 

These results for spread film and equilibrium spreading suggest that films of racemic N-(a-methylbenzy 1) stearamide may be resolved by seeding the racemic film with crystals of either pure enantiomer. Indeed, when a monolayer of racemic jV- (a-methylbenzyl) stearamide is compressed to 45 A2/molecule (27 dyn cm-1), deposition of a crystal of either R( +)- or S( — )-enantiomer results in a decay of surface pressure from the initial 28 dyn cm-1 film pressure to 3.0 dyn cm-1, the ESP of the enantiomeric systems on a pure 10n sulfuric acid subphase (Table 1). When the experiment is repeated with racemic crystals, the system reaches an equilibrium surface pressure of 11 dyn cm-1, nearly the ESP of the racemic crystal on the clean acidic interface. In either case, equilibrium pressure is reached within a two hour time period. 

The Yl/A isotherms of the racemic and enantiomeric forms of DPPC are identical within experimental error under every condition of temperature, humidity, and rate of compression that we have tested. For example, the temperature dependence of the compression/expansion curves for DPPC monolayers spread on pure water are identical for both the racemic mixture and the d- and L-isomers (Fig. 13). Furthermore, the equilibrium spreading pressures of this surfactant are independent of stereochemistry in the same broad temperature range, indicating that both enantiomeric and racemic films of DPPC are at the same energetic state when in equilibrium with their bulk crystals. 

It is, therefore, clearly concluded from Figures 9-11 that in the case of conventional fatty acids such as myristic, palmitic, stearic and so on, the crystalline or amorphous phase of monolayer completely depends on the relative magnitude of Tsp to Tm of the monolayer, being independent of the magnitude of surface pressure. The fatty acid monolayers do not show any pressure-induced crystallization during compression of the monolayer on the water surface. The crystalline and amorphous monolayers are schematically summarized in Figure 12. 

Scheme 3 shows the preparation process for the crystallized monolayer of lithium 10,12-heptacosadiynoate. The amorphous monolayer was prepared on the water surface at Tsp of 303 K above Tm ( ca. 300 K) and then, was compressed to the surface pressure of 12 mN-nr1. 

Several crystals, such as vaterite and calcite forms of CaC03, or a-glycine, have been nucleated (induced oriented crystallization) at the water surface covered with a monolayer film of carboxylic acids or aliphatic alcohols (compressed to "suitable" distances of the hydrophilic groups with a Langmuir balance) (Mann et al., 1988). 

A racemic film was compressed nearly to its collapse point. It was then seeded by sprinkling crystals of pure enantiomeric amide on the surface. A rapid decrease in surface pressure was observed approaching the equilibrium spreading pressure of the enantiomer. A control experiment in which racemic crystals were sprinkled on the compressed racemic film produced a pressure drop that slowly approached, but did not reach, the ESP of the racemic film. The observed behavior was consistent with what would be expected if the enantiomer seed crystals had removed molecules of the same enantiomer from the racemic film, leaving a monolayer composed mainly of molecules of the opposite configuration. 

Stability of lead monolayers [271] and a dependence of the Pb UPD layer structure on surface coverage have also been studied on Au(lOO) face [272]. A phase transition from an expanded Pb overlayer to compressed hep structure has been considered [270]. A coupled process of gold and Pb UPD oxidation process on single crystal Au(llO) has been studied using XPS method [273]. 

Significantly, equilateral triangular PbS crystals have been grown under compressed monolayers in the same orientation (see Fig. 114). 

A way to stretch or compress metal surface atoms in a controlled way is to deposit them on top of a substrate with similar crystal symmetry, yet with different atomic diameter and lattice constant. Such a single monolayer of a metal supported on another is called an overlayer. Metal overlayers strive to approach the lattice constant of their substrate without fully attaining it hence, they are strained compared to their own bulk state [24, 25]. The choice of suitable metal substrates enables tuning of the strain in the overlayer and of the chemisorption energy of adsorbates. A Pt monolayer on a Cu substrate, for instance, was shown to bind adsorbates much weaker than bulk platinum due to compressive strain induced by the lattice mismatch between Pt and Cu, with Cu being smaller [26]. 

RNA, and proteins when compressed between platinum electrodes. Subsequent studies have established that proteins and ss DNA or RNA do not function as molecular wires however, the conductivity of ds DNA remains the subject of debate. Recently there have been several studies of DNA conductivity in fibers, single crystals, aligned films, and monolayer assemblies. Far from resolving the controversy, they seem to have intensified it. 

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