Monolayers intermediate phase

In the first version with a mobile phase of constant composition and with single developments of the bilayer in both dimensions, a 2-D TLC separation might be achieved which is the opposite of classical 2-D TLC on the same monolayer stationary phase with two mobile phases of different composition. Unfortunately, the use of RP-18 and silica as the bilayer is rather complicated, because the solvent used in the first development modifies the stationary phase, and unless it can be easily and quantitatively removed during the intermediate drying step or, alternatively, the modification can be performed reproducibly, this can result in inadequate reproducibility of the separation system from sample to sample. It is therefore suggested instead that two single plates be used. After the reversed-phase (RP) separation and drying of the plate, the second, normal-phase, plate can be coupled to the first (see Section 8.10 below). 

As for the mechanical response of thin lipid films, surface pressure(fl)-surface area(A) characteristics of lipid monolayer at air/water interface have been well studied under quasi-static conditions. It has been established that different phases are observed for the ensemble of lipid molecules in a two-dimensional arrangement, similarly to the gas, liquid, and solid phases and some other intermediate phases as in three-dimensional molecular assemblies. 

An intermediate phase of tetragonal syiiunetry - the T phase - has also been detected in a number of systems. A rod structure related to a square mesh surface was foimd to agree well with X-ray and NMR data on a perfluorinated surfactant-water mixture forming the T phase [22], [34]. These examples demonstrate that surfactant or lipid monolayers lining mesh surfaces as well and bilayers wrapped onto three-periodic minimal surfaces (IPMS) are indeed found in these self-assembled systems. 

Most of the linker molecular monolayers are stable over broad potential ranges, limited by reductive and oxidative desorption and cleavage of the Au-S bond. The adsorption process can be followed in real time through several intermediate phases, such as reported for cysteamine [38] (and 1-propanethiol [155]). The in situ STM images shown in Figure 2.6 thus offer an overall impression of the microenvironment for immobilized redox proteins in voltammetric action. ... 

Figure 11.6 Evolution of pentacene films on Cu(l 10). After completion of the first monolayer revealing a predominant (6.5 X 2) phase and occasionally a coexisting c(13 X 2) phase (a) an intermediate phase A is formed whereas for thickness above 2 nm the molecules continue in an upright orientation a-dopting the well known thin film phase B (b).

The first evidence that more than one form of smectic A exists came from an observation of Sigaud et of a phase transition that was detected by calorimetry, but could not be observed optically. X-ray studies revealed that this was a transition between two forms of the A phase the higher temperature phase was characterized by a pair of refiexions corresponding to a layer spacing and the lower temperature one by two pairs of reflexions corresponding to fi / and 21 respectively. The former type of smectic A is called the monolayer (A ) phase and the latter the bilayer (Aj) phase. A third type which has a layer spacing intermediate between / and 21, has been identified and is called the partially bilayer (A ) phase. The structures of these phases are represented schematically in fig. 5.6.1. 

Grygolowicz-Pawlak, E., K. Plachecka, Z. Brzozka, and E. Malinowska. 2007. Further studies on the role of redox-active monolayer as intermediate phase of solid-state sensors. Sens. Actuators B 123 480-487. 

FIG. 4 Phase diagram of Langmuir monolayers at low and intermediate surface coverage (schematic). Not shown are the various phases on the condensed side at high surface coverage. 

Models of a second type (Sec. IV) restrict themselves to a few very basic ingredients, e.g., the repulsion between oil and water and the orientation of the amphiphiles. They are less versatile than chain models and have to be specified in view of the particular problem one has in mind. On the other hand, they allow an efficient study of structures on intermediate length and time scales, while still establishing a connection with microscopic properties of the materials. Hence, they bridge between the microscopic approaches and the more phenomenological treatments which will be described below. Various microscopic models of this type have been constructed and used to study phase transitions in the bulk of amphiphihc systems, internal phase transitions in monolayers and bilayers, interfacial properties, and dynamical aspects such as the kinetics of phase separation between water and oil in the presence of amphiphiles. 

FIG. 8 Phase diagram of a Langmuir monolayer in a model of grafted stiff Lennard-Jones chains. LE denotes a disordered expanded phase, LC-U a condensed phase with untilted chains, LC-NN and LC-NNN condensed phases with collective tilt towards nearest neighbors and next-nearest neighbors, respectively, and LC-mod a phase which has a superstructure and an intermediate direction of tilt. (From Stadler and Schmid [151].)... 

The decomposition of this intermediate on both the Ni(llO) and Ni(lOO) surfaces occurred by an autocatalytic mechanism (99) for adsorbate coverages above about one-tenth of a monolayer. In fact, the decomposition rate was observed to accelerate isothermally as the reaction proceeded on both the Ni(l 10) and Ni(lOO) surfaces (98, 99) the rate of acceleration was more pronounced on the (110) surfaces. Furthermore, the intermediates were observed to form islands, as if a two-dimensional phase condensation occurred at about one-tenth monolayer coverage. The formation of this 2D condensed phase was clear indication of attractive interactions among the adsorbed species. 

Study of the addition of gas-phase D atoms to 1-alkene monolayers adsorbed on Cu(100) suggested that addition to the terminal carbon predominates to generate the corresponding secondary alkyl group (half-hydrogenated intermediate).353... 

Modern methods of vibrational analysis have shown themselves to be unexpectedly powerful tools to study two-dimensional monomolecular films at gas/liquid interfaces. In particular, current work with external reflection-absorbance infrared spectroscopy has been able to derive detailed conformational and orientational information concerning the nature of the monolayer film. The LE-LC first order phase transition as seen by IR involves a conformational gauche-trans isomerization of the hydrocarbon chains a second transition in the acyl chains is seen at low molecular areas that may be related to a solid-solid type hydrocarbon phase change. Orientations and tilt angles of the hydrocarbon chains are able to be calculated from the polarized external reflectance spectra. These calculations find that the lipid acyl chains are relatively unoriented (or possibly randomly oriented) at low-to-intermediate surface pressures, while the orientation at high surface pressures is similar to that of the solid (gel phase) bulk lipid. 

Although we confirmed the formation of epoxides in the case of monolayers, we suggested that their formation may be the result of a catalytic effect of silica, rather than that of an interaction between the rigidly oriented neighboring moleclues as explained by Mead s group. Possibly, hydroperoxide intermediates are the major primary products in the adsorbed phase as well, and the acidic nature of silica favors a selective heterolytic cleavage as proposed by Kimoto and Gaddis 0 9). 

There are many cases in which other techniques have been applied to biphasic systems in order to establish the nature of mixing. For example, fluorescence microscopy of DPPC monolayers containing 2% of a fluorescent probe have shown the coexistence of solid and fluid phases of DPPC at intermediate pressures (Weis, 1991). Similar results have been achieved with a variety of other phospholipids using the same technique (Vaz et al., 1989). The recent application of laser light scattering to this area (Street et al., unpublished data) has yet to produce any conclusive evidence, but the future for this particular technique is also promising. It also provides information about the viscoelastic properties of the monolayer and how these are affected by the inclusion of penetration enhancers. 

Figure 4 The modified stalk mechanism of membrane fusion and inverted phase formation, (a) planar lamellar (La) phase bilayers (b) the stalk intermediate the stalk is cylindrically-symmetrical about the dashed vertical axis (c) the TMC (trans monolayer contact) or hemifusion structure the TMC can rupture to form a fusion pore, referred to as interlamellar attachment, ILA (d) (e) If ILAs accumulate in large numbers, they can rearrange to form Qn phases, (f) For systems close to the La/H phase boundary, TMCs can also aggregate to form H precursors and assemble Into H domains. The balance between Qn and H phase formation Is dictated by the value of the Gaussian curvature elastic modulus of the bIlayer (reproduced from (25) with permission of the Biophysical Society) The stalk in (b) is structural unit of the rhombohedral phase (b ) electron density distribution for the stalk fragment of the rhombohedral phase, along with a cartoon of a stalk with two lipid monolayers merged to form a hourglass structure (reproduced from (26) with permission of the Biophysical Society).

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