Monolayer parameters, reproducibility

Results of parameter optimization and MD simulations of small model compounds have been published, including alcohols [63], alkanes [63], aromatic [64] and heteroaromatic [209] compounds and liquid amides [65], Studies of ions in aqueous solution were also performed [61, 88] and results from an MD simulation on a DPPC lipid monolayer have been reported (Harder, MacKerell, Roux, submitted). Notable from the monolayer study was the reproduction of the dipole potential across the monolayer, a value that cannot be reproduced using non-polarizable models. This exciting, unforeseen observation points to the types of results that may be obtained from polarizable macromolecular force fields that are not accessible to the present additive models. 

Fig. 6.32 Experimental I-E (a) and Q-E (b) curves obtained in CV and CVC for a mixed ferrocene monolayer (I-cO.SH — CgSH 1 20) adsorbed at a disc gold electrode of radius r = 0.001 cm in an aqueous solution 1 M NaC104 with potassium ferrocyanide lOmM. The values of the scan rate (E(t) — initial) /v (in Vs-1) appear in (b) and for (a) are the following 0.2 (solid lines), 0.4 (dotted lines), 0.6 (long-dashed lines), 0.8 (short-dashed lines), and 1 (dashed-dot-dot lines). (symbols) I-E (a) and Q-E (b) curves calculated from Eqs. (6.215) to (6.216) by using the following parameters Ef = 0.095 V, kc = 250s 1, Qv = 0.437 pC, and T = 298 K. Reproduced with permission from [71]...

By their very nature, heterogeneous assemblies are difficult to characterize. Problems include the exact nature of the substrate surface and the structure of the modifying layer. In this chapter, typical examples are given of how surface assemblies can be prepared in a well-defined manner. This discussion includes the descriptions of various substrate treatment methods which lead to clean, reproducible surfaces. Typical methods for the preparation of thin films of self-assembled monolayers and of polymer films are considered. Methods available for the investigation of the three-dimensional structures of polymer films are also discussed. Finally, it will be shown that by a careful control of the synthetic procedures, polymer film structures can be obtained which have a significant amount of order. It will be illustrated that these structural parameters strongly influence the electrochemical and conducting behavior of such interfacial assemblies and that this behavior can be manipulated by control of the measurement conditions. 

The BET method is not exact for the determination of the surface area of MMS of pore widths less than about 4 nm [20], Given this, pore filling is observed at pressures very close to the pressure range where a monolayer-multilayer formation on the pore walls takes place, which may lead to an important overestimation of the monolayer capacity in case of a BET analysis [20]. However, the BET surface area is extensively taken as a reproducible parameter for the characterization... 

Figure 4.7. These diagrams are designed to emphasize the reproducibility of various physical parameters in monolayer assemblies of different thicknesses a) reciprocal capacitance per unit area versus number of monolayers of cad-mium arachidate on an aluminum substrate (see r erence 38), (Jb) absorption intensity versus number of monolayers for the symmetric carboxylate stretching mode of cadmium arachidate at 1432 cm (see r erence 39), (c) count rate of rays versus number of layers of barium stearate labeled with (see reference 40), and d) X-ray photoelectron signal (XPS) intensity versus number of layers of cadmium dimethylarcwhidate on silver (see reference 28).

With regard to the monolayers from solvent II, particular care must be taken in spreading. Because these isotherms at various surface concentrations have high reproducibility, the difference between the curves obtained with solvent I and these are not attributed to loss of the polymer from the interface to the substrate. Even in this case we derived the two-dimensional state equation which corresponded to the mean isotherm and, therefore, is valid for the three temperatures. From it we deduced the same parameters described earlier for spreading solvent I (Table I). 

The model reproduces the most prominent phase transitions of phospholipid monolayers [78] and bilayers [80]. In particular, it reproduces a main transitirm from a fluid membrane phase (L to a tilted gel phase Lpi) with an intermediate ripple phase Pp ), in agreement with experiments. The elastic parameters have been studied in the fluid phase and are in reasonable agreement with those of saturated DPPC (dipalmitoyl-phosphatidylcholine) bilayers. Recently, the Lenz model has been supplemented with a simple cholesterol model [81]. Cholesterol molecules are taken to be shorter and stiffer than lipids, and they have a slight affinity to lipids. Mixtures of lipids and cholesterol were found to develop nanoscale raft domains... 

Discussions such as these are necessary to emphasize, most strongly, that numbers attached to a surface area parameter are only approximate. In fact, it is impossible to place an accuracy on a surface area, as distinct from a reproducibility. The number provided is an indication, only, of a maximum adsorption capacity under the conditions of the adsorption experiment. Assessments of values of (monolayer capacity) are certainly not without controversy. To... 

By the identification and quantification of the material process parameters, including the surfactant ratio on magnetite particles Xs/m as well as the surfactant concentration in the solvent xs, the liquid-liquid phase transfer could be performed efficiently and with a high reproducibility, with a minimized use of surfactants and high transfer rates and yield. Typical values for Xs/m and jcs were 0.2 g/g and 2 m% for monolayer coverage on the particles. This result was that secondary effects like emulsion and oleate formation as well as water entrapment could be excluded [2,23]. 

Figure 14 Ex situ TEM micrographs of the indium nanoparticies generated in the deposition experiments of the (a) 0.4-nm-, (b) 0.6-nm-, and (c) 0.8-nm-thickfilms, (d) Corresponding caiorimetric curves, with the vertical dashed lines indicating the position in the temperature of each maximum, (e) Radius difference corresponding to the separation between adjacent maxima, as calculated using eqn [2]. The horizontai dashed iines represent the thickness of a monolayer for the two bulk lattice parameters of indium. Reproduced with permission from Efremov, M. Y. Schiettekatte, F. Zhang, M. etal. Phys. Rev. Lett. 2000, 85,3560-3563. ...

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