Oriented monolayers

The GOx molecule is a rigid prolate ellipsoid with long and short axes of approximately 140 A and 50 A (22). If GOx is assumed to be immobilized to PPNVP/PEUU in a close-packed monolayer, and interstitial spaces are accounted for, then 0.91 (ig and 2.55 pg of GOx will be present on the surface when the long axis is oriented parallel and perpendicular to the surface, respectively. From the initial response of the GOx-PPNVP/PEUU sample, the specific activity of the immobilized GOx was calculated to be 0.85 U/mg and 0.30 U/mg in the respective monolayer orientations (11). Similarly, if immobilization is assumed not to alter the initial activity, then the number of GOx molecules (13.4 nM x 0.3 mL) immobilized in the GOx-PPNVP/PEUU sample is approximately 71% and 25% of the maximum number that would be present on the surface in the parallel and perpendicular orientations. 

First, it elucidates a number of general regularities of catalysis such as the reactions in the adsorbed monolayer, orientation of molecules, the special action of solid phase borderlines, poisoning of the catalyst and the orientation of the poison molecules, activation of the catlayst with small doses of poison, limiting cases of bond deformation on the surface with formation of free atoms and radicals, migration on the surface and the... 

Figure 46. Phase diagram of N2 on graphite based on adiabatic heat capacity data coverage is reported in units of the complete n/S monolayer. Orientationally ordered commensurate phase (CO), orientationally disordered commensurate phase (CD), orientationally ordered uniaxially compressed incommensurate phase (UIO), orientationally disordered uni-axially compressed incommensurate phase (UID), triangular compressed incommensurate phase (TI), fluid phase (F), speculative reentrant fluid phase (RF). The measurement path for the highest coverage in Fig. 45 is sketched by the dashed line. (Adapted from Fig. 10 of Ref. 156.)...

Membrane-active antimicrobial agents can react with monomolecular lipid films (or monolayers) orientated at an air-water interface. When such an agent is introduced beneath the monolayer, the orientation of the lipid molecules at the interface alters, producing measurable changes in surface pressure. 

The key feature of these works is to use, beyond the full sphere and cylinder models proposed in the literature, a series of composite structures composed of monolayers oriented with head groups in contact with the hydrophilic surface and covered with hemispheres, hemicylinders, finite disks, or another monolayer (making the full structure a bilayer), showing that, for several illustrative cases, composite hemicylinders will form at the cac. ... 

Wilhelmy tensiometry was performed on MGDG and PLPC monolayers as detailed elsewhere [1]. Monolayer molarity was well below the CMC which ordinarily would preclude MGDG monolayer orientation. Treatments were as seen in Figure 2. To prevent O2 >02 conversion, the former was applied in a crown-ether caged form of KO2 [1]. Lipoxygenase (LOX) was assayed on 14-day-old plants which were sprayed twice weekly with 10 or 10 M SNAP in 0.01 M phosphate buffer pH 6.5, containing 10 M CaCl2 and 0.01 % Tween 20. Controls were sprayed with buffer alone. LOX assay was that of Pinsky et al. [6]. 

McBain reports the following microtome data for a phenol solution. A solution of 5 g of phenol in 1000 g of water was skimmed the area skimmed was 310 cm and a 3.2-g sample was obtained. An interferometer measurement showed a difference of 1.2 divisions between the bulk and the scooped-up solution, where one division corresponded to 2.1 X 10 g phenol per gram of water concentration difference. Also, for 0.05, 0.127, and 0.268M solutions of phenol at 20°C, the respective surface tensions were 67.7, 60.1, and 51.6 dyn/cm. Calculate the surface excess Fj from (a) the microtome data, (b) for the same concentration but using the surface tension data, and (c) for a horizontally oriented monolayer of phenol (making a reasonable assumption as to its cross-sectional area). 

The preceding evidence for orientation at the interface plus the considerations given in Section III-3 make it clear that the polar end is directed toward the water and the hydrocarbon tails toward the air. On the other hand, the evidence from the study of the Gibbs monolayers (Section III-7) was that the smaller molecules tended to lie flat on the surface. It will be seen that the orientation... 

The external reflection of infrared radiation can be used to characterize the thickness and orientation of adsorbates on metal surfaces. Buontempo and Rice [153-155] have recently extended this technique to molecules at dielectric surfaces, including Langmuir monolayers at the air-water interface. Analysis of the dichroic ratio, the ratio of reflectivity parallel to the plane of incidence (p-polarization) to that perpendicular to it (.r-polarization) allows evaluation of the molecular orientation in terms of a tilt angle and rotation around the backbone [153]. An example of the p-polarized reflection spectrum for stearyl alcohol is shown in Fig. IV-13. Unfortunately, quantitative analysis of the experimental measurements of the antisymmetric CH2 stretch for heneicosanol [153,155] stearly alcohol [154] and tetracosanoic [156] monolayers is made difflcult by the scatter in the IR peak heights. 

Resonance Raman reflection spectroscopy of monolayers is possible, as illustrated in Fig. IV-14 for cetyl orange [157]. The polarized spectra obtained with an Ar ion laser allowed estimates of orientational changes in the cetyl orange molecules with a. 

There has been much activity in the study of monolayer phases via the new optical, microscopic, and diffraction techniques described in the previous section. These experimental methods have elucidated the unit cell structure, bond orientational order and tilt in monolayer phases. Many of the condensed phases have been classified as mesophases having long-range correlational order and short-range translational order. A useful analogy between monolayer mesophases and die smectic mesophases in bulk liquid crystals aids in their characterization (see [182]). 

Grazing incidence excitation of a fluorescent probe in a phospholipid monolayer can also be used to indicate order. The collective tilt of the molecules in a domain inferred from such measurements is indicative of long-range orientational order [222]. 

Photopolymerization reactions of monolayers have become of interest (note Chapter XV). Lando and co-workers have studied the UV polymerization of 16-heptadecenoic acid [311] and vinyl stearate [312] monolayers. Particularly interesting is the UV polymerization of long-chain diacetylenes. As illustrated in Fig. IV-30, a zipperlike process can occur if the molecular orientation in the film is just right (e.g., polymerization does not occur readily in the neat liquid) (see Refs. 313-315). 

The behavior of insoluble monolayers at the hydrocarbon-water interface has been studied to some extent. In general, a values for straight-chain acids and alcohols are greater at a given film pressure than if spread at the water-air interface. This is perhaps to be expected since the nonpolar phase should tend to reduce the cohesion between the hydrocarbon tails. See Ref. 91 for early reviews. Takenaka [92] has reported polarized resonance Raman spectra for an azo dye monolayer at the CCl4-water interface some conclusions as to orientation were possible. A mean-held theory based on Lennard-Jones potentials has been used to model an amphiphile at an oil-water interface one conclusion was that the depth of the interfacial region can be relatively large [93]. 

Because of the charged nature of many Langmuir films, fairly marked effects of changing the pH of the substrate phase are often observed. An obvious case is that of the fatty-acid monolayers these will be ionized on alkaline substrates, and as a result of the repulsion between the charged polar groups, the film reverts to a gaseous or liquid expanded state at a much lower temperature than does the acid form [121]. Also, the surface potential drops since, as illustrated in Fig. XV-13, the presence of nearby counterions introduces a dipole opposite in orientation to that previously present. A similar situation is found with long-chain amines on acid substrates [122]. 

Heinz T F, Tom H W K and Shen Y R 1983 Determination of molecular orientation of monolayer adsorbates by optical second-harmonic generation Phys. Rev. A 28 1883-5... 

For structures with a high curvature (e.g., small micelles) or situations where orientational interactions become important (e.g., the gel phase of a membrane) lattice-based models might be inappropriate. Off-lattice models for amphiphiles, which are quite similar to their counterparts in polymeric systems, have been used to study the self-assembly into micelles [ ], or to explore the phase behaviour of Langmuir monolayers [ ] and bilayers. In those systems, various phases with a nematic ordering of the hydrophobic tails occur. 

Self-assembled monolayers (SAMs) are molecular layers tliat fonn spontaneously upon adsorjDtion by immersing a substrate into a dilute solution of tire surface-active material in an organic solvent [115]. This is probably tire most comprehensive definition and includes compounds tliat adsorb spontaneously but are neither specifically bonded to tire substrate nor have intennolecular interactions which force tire molecules to organize tliemselves in tire sense tliat a defined orientation is adopted. Some polymers, for example, belong to tliis class. They might be attached to tire substrate via weak van der Waals interactions only. 

Finally, in 1985, the results of an extensive investigation in which adsorjDtion took place onto an aluminium oxide layer fonned on a film of aluminium deposited in vacuo onto a silicon wafer was published by Allara and Nuzzo 1127, 1281. Various carboxylic acids were dissolved in high-purity hexadecane and allowed to adsorb from this solution onto the prepared aluminium oxide surface. It was found that for chains with more than 12 carbon atoms, chains are nearly in a vertical orientation and are tightly packed. For shorter chains, however, no stable monolayers were found. The kinetic processes involved in layer fonnation can take up to several days. 

Monolayers of alkanetliiols adsorbed on gold, prepared by immersing tire substrate into solution, have been characterized by a large number of different surface analytical teclmiques. The lateral order in such layers has been investigated using electron [1431, helium [144, 1451 and x-ray [146, 1471 diffraction, as well as witli scanning probe microscopies [122, 1481. Infonnation about tire orientation of tire alkyl chains has been obtained by ellipsometry [149], infrared (IR) spectroscopy [150, 151] and NEXAFS [152]. 

Outka D A, Stdhr J, Rabe J P, Swalen J D and Rothermund FI FI 1987 Orientation of araohidate ohains in Langmuir-Blodgett monolayers on Si(111) Phys. Rev. Lett. 59 1321-4... 

Nakahara H and Fukuda K 1983 Orientation of chromophores in monolayers and multilayers of azobenzene derivatives with long alkyl chains J. Colloid Interface Sol. 93 530-9... 

Brovelli D, Flahner G, Ruiz L, Flofer R, Kraus G, Waldner A, Schidsser J, Orszlan P, Ehrat M and Spencer N D 1999 Flighly oriented alkanephosphate monolayers on tantalum(V)oxide surfaces Langmuir4324-7... 

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