Monolayer subphase, transfer

Monolayers can be transferred onto many different substrates. Most LB depositions have been perfonned onto hydrophilic substrates, where monolayers are transferred when pulling tire substrate out from tire subphase. Transparent hydrophilic substrates such as glass [18,19] or quartz [20] allow spectra to be recorded in transmission mode. Examples of otlier hydrophilic substrates are aluminium [21, 22, 23 and 24], cliromium [9, 25] or tin [26], all in their oxidized state. The substrate most often used today is silicon wafer. Gold does not establish an oxide layer and is tlierefore used chiefly for reflection studies. Also used are silver [27], gallium arsenide [27, 28] or cadmium telluride wafer [28] following special treatment. 

Arachidic acid monolayers were prepared from a benzene solution on the water subphase of pH5.8(pure water) and 12.6(adjusted by addition of NaOH) at Tsp of 303 K below Tm(=328 K) of the monolayer [31]. The ionic dissociation state of hydrophilic group was estimated on the basis of the stretching vibrations of carbonyl and carboxylate groups by Fourier transform-infrared attenuated total reflection, FT-IR ATR measurements. 70 arachidic acid monolayers were transferred on germanium ATR prism, resulting in the formation of the multi-layered film. Transfer on the prism was carried out at surface pressures of 25 or 28 mN-nr1. Infrared absorption measurements revealed that almost carboxylic groups of arachidic acid molecules did not dissociate on the water subphase of pH5.8, whereas all carboxylic groups dissociated as carboxylate ions on the water subphase of pH 12.6. 

As an extension of their previous work, the Tachibana group (82, 83) studied the collapse fragments that occur when monolayers of 12-hydroxystearic acid are compressed slowly (18 A /molecule hr) at surface areas of less than 21 A /molecule, the normal cross-sectional area of a hydrocarbon chain. The collapsed monolayers were transferred from the subphase to hydrophilic supports by a horizontal lifting method for electron microscopic observation, which revealed (Fig. 30) flat platelets when the sample was racemic and twisted... 

Transmission electron microscopy (TEM) has been an underutilized yet valuable too in particle size characterization of MC particles in LB films. Monolayer films of trioctylphosphine oxide-capped CdSe (18), spread as a monolayer on an aqueous subphase, were transferred to a TEM grid. A close-packed hexagonal arrangement of 5.3-nm (cr —4%) crystallites was found. TEM images were also obtained for HMP-stabilized CdS incorporated in BeH/octadecylamine films (79) and for CdS formed under an amine-based surfactant monolayer and transferred to a TEM grid (14). In one study, direct viewing of CdS and CdSe particles made from Cd2+-FA films on TEM grids was not possible due to poor phase contrast between the particles and the film (30). Diffraction patterns were observed, however, that were consistent with crystalline (3-CdS or CdSe. Approximately spherical particles of CdSe could... 

The hydrophilic substrate is moved continuously out of the water subphase at constant film pressure. During the upstroke the monolayer is transferred onto the wafer with the headgroups oriented towards the solid substrate and the alkyl chains exposed to the air. This renders the hydrophilic solid surface with a high surface energy of about 50 mN/m (for silicon) to a hydrophobic surface with a relatively low surface energy in the range of 20-30 mN/m. 

Film transfer experiments demonstrate that there is always a sheath of subphase associated with the monolayer. However when the mono-layer is compressed or decompressed in an enclosed area, there will be a certain degree of slippage at the monolayer/subphase interface. When the subphase water molecules are strongly bound to the monolayer, as for a protein monolayer spread on an aqueous substrate at its isoelectric point, little slippage exists, and the phenomenon in Figure 1 is observed. For monolayer systems where there is substantial slippage at the mono-layer/ subphase interface the effect will not be observed during compression or decompression experiments. This effect helps explain why proteins are used with other surfactants to stabilize emulsion and foam formulations 1,2). 

A Millipore Milll-Q purification system was used for subphase preparation, and a constant temperature bath was used to control the subphase temperature. The mixed spreading solutions were dispersed at the air-water Interface and then slowly compressed at speeds of about 5 A2 mol min-" to surface pressures of 10-15 mN/m prior to deposition. Monolayers were transferred onto electron microscope grids for transmission electron microscopy and electron diffraction, using both the horizontal and vertical dipping techniques. Multilayer assemblies were prepared onto platinum-coated substrates using the vertical dipping technique for Near Edge X-Ray Absorption Fine Structure Spectroscopy (NEXAFS). 

Glycolipid 1 and matrix lipid 2 at various concentrations (1 / 2 = 1 / 0, 5 / 1, 9/1,19/1 and 0 / 1) in chloroform (lO MlO M) was spread on the surface of an aqueous subphase. After waiting for 10 min to vaporize the chloroform, the mixed lipids were compressed to the pressure of 30 mN/m at a rate of 0.1 nm /molecule/min to form a monolayer. The monolayer was equilibrated for 10 min under the same surface pressure and polymerized by UV irradiation (254 nm, 8 W) for 10 min. The distance between the lamp and the monolayer surface was 12 cm. The polymerized monolayer was transferred onto the OTS (octadecyltrichlorosilane) coated glass plate or the SPR sensor chip carrying... 

As a rule, the liquid subphase from which the monolayer is transferred is water or, more often, an aqueous electrolyte solution. Indeed, when ionic surfactant molecules are involved low-molecular-weight ions play an important role in stabilizing the interaction between the charged head groups in the F-type film. In particular, divalent (cat)ions serve to stabilize films of monovalent (an)ionic surfactants. 

A nanoparticulate Ti02-stearate (Ti02-St) monolayer was obtained directly using Ti02 hydrosol as the subphase. The surface pressure versus surface area isotherm showed that the monolayer could be compressed to a mean molecular area of 0.25 nm The monolayer was transferred onto hydrophobic and hydrophilic /i-type, p-type Si substrates at a dipping speed of 18 cm/min under a surface pressure of 25 mN/m. The transfer ratio was 1.0 0.1. The transmission electron microscopic (TEM) images of TiOi-stearate monolayers showed relatively densely... 

In the Langmuir-Schaefer method, a flat substrate is placed horizontally onto a compressed monolayer on the liquid-air interface. When the substrate is lifted horizontally and separated from the water subphase, the monolayer is transferred onto the substrate (Figure 4). The method is useful to transfer viscous films as well as monolayers of lipids and proteins. ... 

X-ray diffraction has been applied to spread monolayers as reviewed by Dutta [67] and Als-Nielsen et al. [68], The structure of heneicosanoic acid on Cu and Ca containing subphases as a function of pH has been reported [69], as well as a detailed study of the ordered phases of behenic acid [70], along with many other smdies. Langmuir-Blod-gett films have also been studied by x-ray diffraction. Some recent studies include LB film structure just after transfer [71], variations in the structure of cadmium stearate LB films with temperature [72], and characterization of the structure of cadmium arachidate LB films [73], X-ray [74,75] and neutron reflectivity [76,77] data on LB films can be used to model the density profile normal to the interface and to obtain values of layer thickness and roughness. 

FIG. 9 Silver nanoparticles capped by 4-carboxythiophenol electrostatically adsorbed to positively charged octadecylamine monolayers, (a) Mass uptake versus number of layers at subphase pH 12 and pH 9 the inset shows the contact angle of water versus the number of layers, (b) Absorbance spectra as a function of the number of layers transferred (left), with the inset showing the plasmon absorbance at 460 nm versus the number of layers. Thickness versus number of layers as determined by optical interferometry is shown on the right. (Reprinted with permission from Ref. 103. Copyright 1996 American Chemical Society.)... 

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