Formation in Monolayers

Schulman and Rideal (74) discovered that if, underneath a mono-layer, a solution of another molecular species were injected, a variety of phenomena might occur. The simplest form of interaction between the two types of molecules is mainly electrostatic in character, leading to 

Complex formation was found by Schulman and Rideal (74) and by Schulman and Stenhagen (77) to be a function of the polarity of the head groups as well as of the lengths and shapes of the hydrocarbon tails. For example, injection of dilute solutions of substances (74) of the general structure CH3-(CH2)nX under monolayers of cholesterol and proteins led to the following order of reactivities for the group X  

It was also found that there is a strong interaction between two long, saturated hydrocarbon chains, which was greatly weakened by the presence of a double bond in one of the chains. 

A stoichiometric complex maybe formed under suitable circumstances. An equimolar (1 1) complex is formed between digitonin and cholesterol or sodium cetyl sulfate and cholesterol, while digitonin and cholesterol are believed to form a 1 2 complex in the interface. 

For discussions of the fundamentals of this important field of interactions at liquid surfaces, the reader is referred to the relevant literature, (24,77,78,79,80,81,82,83). 

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We have conducted preliminary investigations to assess the way LC solvents influence film formation in monolayers of small organic molecules on highly oriented pyrolytic... 

Israelachvili, J. 1994. Self-assembly in two dimensions Surface micelles and domain formation in monolayers. Langmuirl 0 3774—3781. 

FIG. 6 J 2 Schematic representation of hydrogen bond formation in monolayers of urea containing amphiphiles 20 at the air/water interface. 

Besides pol3nnerization another type of polyreaction, the polycondensation, can be used to stabilize membrane systems. Recently, Fukuda et al. (5) described polyamide formation in monolayers. Long chain esters of glycine and alanine were polycondensed to yield non-oriented polyamide films of polyalanine and polyglycine. In analogy to this reaction it is possible to prepare stabilized, oriented membrane systems via polycondensation using long chain a-amino acid esters (6). 

Bain C D and Whitesides G M 1989 Formation of monolayers by the ooadsorption of thiols on gold—variation in the length of the alkyl ohain J. Am. Chem. Soc. Ill 7164-75... 

The suppression of C60 crystallite formation in mixed LB films was attempted by mixing C60 and amphiphilic electron donor compounds [259]. Observation of the C60 LB film transferred horizontally by TEM clearly showed 10-40-nm-size crystallites. The diffraction pattern gave an fee lattice with unit cell length 1.410 nm. Examination of the mixed films with arachidic acid by TEM showed extensive crystallite formation. Mixed LB films of three different amphiphilic derivatives of electron donors with C60 were examined. One particular derivative showed very little formation of C60 crystallites when LB films were formed from monolayers of it mixed with C60 in a 1 2 ratio, while two others reduced C60 crystallite formation but did not eliminate it. 

It is important to note that reaction of COad occurs only at sufficiently high coverages, equivalent to a reduced reaction barrier (see the discussion of CO oxidation on Ru(0001) above). The high coverage is maintained by continuous OHad formation, in competition with re-adsorption of CO. The Pt islands help in maintaining the high coverage via (14.8). Finally, additional CO adsorption on the Pt monolayer islands and reaction with OHad on the Ru(0001) areas may be possible as well, and this would further increase the overall reaction rate. At these potentials, however,... 

Recently, the newly developed time-resolved quasielastic laser scattering (QELS) has been applied to follow the changes in the surface tension of the nonpolarized water nitrobenzene interface upon the injection of cetyltrimethylammonium bromide [34] and sodium dodecyl sulfate [35] around or beyond their critical micelle concentrations. As a matter of fact, the method is based on the determination of the frequency of the thermally excited capillary waves at liquid-liquid interfaces. Since the capillary wave frequency is a function of the surface tension, and the change in the surface tension reflects the ion surface concentration, the QELS method allows us to observe the dynamic changes of the ITIES, such as the formation of monolayers of various surfactants [34]. 

All of these characteristics can be under the regulation of the cell and influenced by the cell culture conditions. The age of the cell monolayer in culture can have a profound impact on the quality of the barrier. In monolayers with actively dividing cells, resistance increases with time in culture as tight junctions form (see Fig. 15, Section III.C.4). Resistance reaches a plateau, then decreases as cell viability declines (Section III.C.4). Time in culture may also be a factor in the expression of polarity, which is related to tight junction formation as well as the state of differentiation of the cells (e.g., differential gene expression). 

Bain CD, Evall J, Whitesides GM (1989) Formation of monolayers by the coadsorption of thiols on gold variation in the head group, tail group, and solvent. J Am Chem Soc 111 7155-7164... 

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). 

Fig. 22. Structures proposed to account for the A (y/7 x y/l) and B (3x3) structures observed in the formation of monolayer of CdTe. Adapted from ref. [279],...

Unlike electron and scanning tunneling microscopy, the use of fluorescent dyes in monolayers at the air-water interface allows the use of contrast imaging to view the monolayer in situ during compression and expansion of the film. Under ideal circumstances, one may observe the changes in monolayer phase and the formation of specific aggregate domains as the film is compressed. This technique has been used to visualize phase changes in monolayers of chiral phospholipids (McConnell et al, 1984, 1986 Weis and McConnell, 1984 Keller et al., 1986 McConnell and Moy, 1988) and achiral fatty acids (Moore et al., 1986). 

For a number of the systems, comparisons were made between the effects of enantiomeric composition in the monolayer and corresponding melting-point-composition curves for the crystals. All of the latter gave clear evidence of racemic compound formation in the crystals, and this type of pattern was repeated in the monolayer properties. 

In the first set of measurements the rate of carbon build-up on a Ni(lOO) surface was measured at various temperatures as follows (1) surface cleanliness was established by AES (2) the sample was retracted into the reaction chamber and exposed to several torr of CO for various times at a given temperature (3) after evacuation the sample was transferred to the analysis chamber and (4) the AES spectra of C and Ni were measured. Two features of this study are noteworthy. First, two kinds of carbon forms are evident - a carbidic type which occurs at temperatures < 650 K and a graphite type at temperatures > 650 K. The carbide form saturates at 0.5 monolayers. Second, the carbon formation data from CO disproportionation indicates a rate equivalent to that observed for methane formation in a H2/CO mixture. Therefore, the surface carbon route to product is sufficiently rapid to account for methane production with the assumption that kinetic limitations are not imposed by the hydrogenation of this surface carbon. 

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