Films under monolayers

Formation of silver and gold particulate films under monolayers, floating on aqueous solutions, will be highlighted in this section. 

Saliba, R., Mingotaud, C., Argoul, F. and Ravaine, S. (2001) Electroless deposition of gold films under organized monolayers. 

A number of surface diffraction techniques can be employed in the structural study of electrochemical interfaces, depending on the details of the system under study. For bulk materials or thick films (such that the X-ray beam only samples that layer) conventional diffraction experiments can be performed and, in fact, a number of in situ X-ray diffraction studies of this type have been reported.126 129 In the case of thin films or monolayers, two different techniques can be employed and these are the reflection-diffraction technique introduced by Marra and Eisenberger,3 ), 32 and the technique based on surface truncation rods.131 In the first case, the incident X-ray beam impinges on the sample at an angle below... 

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). 

Scientists are currently using LB film assemblies as solutions to problems in diverse areas such as microlithography, solid-state polymerization, light guiding, electron tunneling, and photovoltaic effects. In the case of such films as Mg stearate, if a clean glass slide is dipped through the film, a monolayer is adsorbed on the downstroke. Another layer is adsorbed on the upstroke. Under careful conditions,... 

Importantly, no silver particulate film formation could be observed at the air/liquid interface, either in the chemical or in the electrochemical generation, in the absence of surfactants or under monolayers prepared from positively charged surfactants. 

Monolayers prepared from arachidic acid, bovine-brain phosphatidylserine (16), and n-hexadecyl-4- vinylbenzamido)undecyl hydrogen phosphate (4) Size-quantized CdS, ZnS, PbS, and CuS semi-conductor particulate films generated in situ under monolayers... 

Monolayers prepared from arachidic acid Size-quantified CdS particulate films generated in situ under monolayers... 

Discontinuities are seen in the relationship between increase in film pressure, An, and lipid composition following the injection of globulin under monolayers of lecithin-dihydro-ceramide lactoside and lecithin-cholesterol mixtures. The breaks occur at 80 mole % C 16-dihydrocaramide lactoside and 50 mole % cholesterol. Between 0 and 80 mole % lactoside and between 0 and 50 mole % cholesterol the mixed films behave as pure lecithin. Two possible explanations are the formation of complexes, having molar ratios of lecithin-lactoside 1 to 4 and lecithin-cholesterol 1 to 1 and/or the effect of monolayer configurations (surface micelles). In this model, lecithin is at the periphery of the surface micelle and shields the other lipid from interaction with globulin. 

The PTCDA moiety is a class of molecule that readily exhibits excimer fluorescence. A direct method to attain the enhancement factor for a substrate is to deposit a LB monolayer onto a glass slide that contains an area of a silver island film. By comparing the fluorescence intensity measured on glass and the island film under identical conditions, an excellent assessment of the EF can be attained. 

The degree to which l c(C. ) " penetrates the film is reflected in Ret, thus providing a quantitative measure of the relative DNA surface coverage. The EIS method has been applied to assay hybridization/dehybridization events of both pure DNA films as well as DNA/mercaptohexanol mixed monolayers (Figure 5-2). It has proven to be a simple and reliable technique to report on the composition of DNA films under a wide range of experimental conditions. 

The thicknesses of the films were determined using an ellipsometer SE 400 (Sentech Instruments GmbH) under an incidence angle of 70° at a wavelength of 633 nm. Optical film thicknesses were determined assuming a complex refractive index N=n-ik with a real part n= 1.45 and an imaginary coefficient k = 0. The parameters n and k of the gold substrates were obtained by ellip-sometric measurements of the plasma-cleaned films before monolayer formation. 

To find the primary decomposition products at low temperatures, a method should be used by which a reaction even of a monolayer may be observed and that in a manner characteristic for the molecules in question. A method appropriate for the investigation of the elementary processes of decomposition reactions is to produce the electrically conducting catalyst in form of a transparent film under the evaporation and to study the change of its resistance by influence of the decomposing molecules. Hereby the electric resistance R of the film decreases or increases, the product molecules transferring electrons to the surface of the film or receiving electrons from it, as the case may be (I). On adsorption of hydrogen (J, ), (Fig. 1), water vapor ( , 3) (Fig. 2), or carbon dioxide (3), (Fig. 3), the resistance of the nickel layer decreases. On adsorption of carbon monoxide, however, (2, 3), (Fig. 4), it increases. 

The creation of 2D crystals of both micron sized and nanometre sized particles remains a somewhat empirical process due to the ill-defined role of the substrate or surface on which nucleation takes place. Perrin first observed diffusion and ordering of micron sized gamboge 2D crystals in 1909 under an optical microscope [32]. Several techniques have been proposed for the formation of 2D arrays at either solid-liquid surfaces or at the air-water interface. Pieranski [33], Murray and van Winkle [34] and later Micheletto et al. [14] have simply evaporated latex dispersions. Dimitrov and coworkers used a dip-coating procedure, which can produce continuous 2D arrays [35,36]. The method involves the adsorption of particles from the bulk solution at the tricontact phase line. Evaporation of the thin water film leads to an attractive surface capillary force which aids condensation into an ordered structure. By withdrawing the film at the same rate as deposition is occurring, a continuous film of monolayered particles is created. Since the rate of deposition is measured with a CCD camera, it is not possible to use nanometer sized particles with this method, unless a nonoptical monitor for the deposition process can be found. 

In principle, we can distinguish (for surfactant self-assemblies in general) between a microstructure in which either oil or water forms discrete domains (droplets, micelles) and one in which both form domains that extend over macroscopic distances (Fig. 7a). It appears that there are few techniques that can distinguish between the two principal cases uni- and bicontinuous. The first technique to prove bicontinuity was self-diffusion studies in which oil and water diffusion were monitored over macroscopic distances [35]. It appears that for most surfactant systems, microemulsions can be found where both oil and water diffusion are uninhibited and are only moderately reduced compared to the neat liquids. Quantitative agreement between experimental self-diffusion behavior and Scriven s suggestion of zero mean curvature surfactant monolayers has been demonstrated [36]. Independent experimental proof of bicontinuity has been obtained by cryo-electron microscopy, and neutron diffraction by contrast variation has demonstrated a low mean curvature surfactant film under balanced conditions. The bicontinuous microemulsion structure (Fig. 7b) has attracted considerable interest and has stimulated theoretical work strongly. 

A metal overlayer ATR-FTIR technique which markedly enhances the SNR of monolayer and submonolayer film spectra has been described. The metals investigated were gold and aluminium, with a substrate of polyether-urethane elastomer (375). The use of silver films under ATR geometry has improved the detection limit of the surface layer on polymer films. An enhancement factor of 10 and 2 is reported for polydimethylsiloxane and polycarbonate respectively, compared to the conventional ATR technique (362). 

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