Molecular dynamics compressed monolayers

The interpretation of the Langmuir experiments with the carbosilane den-drimers is supported by the results of molecular dynamics simulation. Figure 14 shows snapshots of a dumbbell-like conformation of carbosilane dendximers observed during lateral compression of a dendrimer monolayer on a polar sub-... 

Figure 15. Out-of-plane tilt angle as a function of temperature for N2 on graphite. Circles nuclear resonance photon scattering of a compressed monolayer of coverage 1.05 0.02 [241]. Crosses molecular dynamics simulations of a complete monolayer [341]. (Adapted from Fig. 2 of Ref. 241.)...

In a model put forward by Indenbom, the ITIES is considered as an elastic film in which forces of interfacial tension counteract the forces of an external electric field [64]. This causes a potential-dependent compressed grooved structure of the interface at which ion transfer takes place when the compression reaches a critical state. The validity of this picture will rely on detailed molecular dynamics studies of the microscopic features of ion transfer across ITIES. In this connectionit is relevant to mention the work of Cunnane and coworkers in which the kinetics of ion transfer across a monolayer adsorbed at a liquidjliquid interface is discussed in terms of the energy required to form a pore of a size similar to that of the transferring ion [56]. 

Abstract The structure and surface pressure of compressed monolayers consisting of silica nanoparticles at the water-air interface have been studied by means of molecular dynamics computer simulations. The simple hexagonal array of mono-disperse particles model overestimates the range of the repulsive interparticle potentials between the nanoparticles in a monolayer. On the basis of the results of the simulation we proposed a method to assess the error of the estimation. We also investigated the relevance of... 

Motivated by this recent interest in monolayer lubricants, molecular dynamics (MD) simulations have been used to examine monolayers of w-alkanes that are chemically bound or anchored to diamond substrates. A new empirical-potential energy function, which is capable of modeling chemical reactions in hydrocarbons of all phases, has been developed for this work (15). A single-wall, capped armchair nanotube is used to indent these hydrocarbon monolayers and to investigate friction. The effects of tip flexibility and tip speed on indentation and friction are examined. Particular attention will be paid to the formation of defects and bond rupture (and formation) during the course of the simulations. Previous MD simulations have examined the structure (16-18) and compression of -alkanethiols on Au (19,20). The major difference between those studies and the work discussed here is that irreversible chemical changes (or changes in hybridization associated with bond rupture and formation) are possible in these studies. 

The nature of molecular dynamics simulations allows for the quantification of the number of defects formed and the determination of their exact location. These simulations show that gauche defects are formed as a result of the indentation process, as predicted by Salmeron and coworkers (4). Due to the geometry of the nanotube, these defects are localized to the region below and surrounding the tube. In addition, due to the small contact area of the nanotube used in these simulations, the number of defects formed is a function of penetration depth into the monolayer (39). Because the flexible nanotube compresses slightly as it interacts with the C13 monolayer, it does not penetrate the monolayer as deeply as a rigid nanotube. As a result, the flexible tube generates fewer defects as it indents the monolayer. 

Lagutchev A, Patterson JE, Huang W, Dlott DD. 2005. Ultrafast dynamics of self-assembled monolayers under shock compression Effects of molecular and substrate structiue. J Phys ChemB 109 5033-5044. 

Some amphiphilic molecules such as oleic acid and hexadecyl alcohol containing an alkyl chain and a polar head group form monolayers on the surface of water. The polar head groups of these molecules are attracted to and are in contact with water while their hydrocarbon tails protrude above it (Figure 15). The term monolayer implies the presence of a uniform mono-molecular film on the surface of water. Monolayer films can be classified as gaseous, liquid, or solid depending upon the degree of compression and the effective area per molecule. Clearly the liquid phase of a monolayer film and, more so, the solid represent constrained environments for individual molecules of amphiphiles. Monolayers, just like micelles, are dynamic species. 

It needs to be mentioned here that many other experimental techniques are available for studying monolayers at the air-water interface. Most frequently, surface potential is measured to evaluate the molecular orientation of amphiphiles at the interface. This method is, however, better suited to the study of small molecules. Polymeric amphiphiles, due to their conformational dynamics, are difficult to analyze and simple dielectric layer models do not apply, or produce large errors. Grazing incidence X-ray diffraction provides information on molecular packing, and spectroscopic methods are used to study molecular interactions and the structural changes of molecules upon compression. Fluorescence microscopy is useful for studying two-dimensional organization of small molecular mass amphiphiles however, it is not applied to polymer monolayers. For a more comprehensive overview of experimental methods used to study monolayers at the air-water interface, the reader is referred to more specialized articles, e.g. [18]. 

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