Chemical reactions in monolayers

Chemical reactions in monolayers constitute a field of study in its own right. Several types of reactions can be distinguished, including reactions between mono-... 

The study of surface films of the Langmuir type covers an extremely diverse group of phenomena. Measurements of film viscosity, diffusion on the surface, diffusion through the surface film, surface potentials, the spreading of monolayers, and chemical reactions in monolayers are just a few of the topics that have been studied. One interesting application is the use of long-chain alcohols to retard evaporation from reservoirs and thus conserve water. The phrase to pour oil on the troubled waters reflects the ability of a mono-molecular film to damp out ripples, apparently by distributing the force of the wind more evenly. There are also several different types of surface films only the simplest was discussed in this section. 

Our target is to ultimately fabricate reactive micro- and nanopatterns for the area-selective immobilization of biologically relevant molecules via covalent coupling. In addition to full control of reactivity and pattern sizes, biocompatibility and minimized NSA are important for rendering these systems useful as generic platforms. In this context we review in this contribution our recent efforts in this area. We focus in particular on (1) the elucidation of structure-reactivity relationships, (2) the in situ compositional analysis of wet chemical reactions in monolayer-based systems down to nanometer length scales, and on (3) the application and refinement of various micro- and... 

Chen, B., M. Lu, A. K. Platt, F. Maya, and 1. M. Tour. Chemical reactions in monolayer aromatic films on silicon surfaces. Chem. Mater. 20, 2008 61-64. 

The physical chemist is very interested in kinetics—in the mechanisms of chemical reactions, the rates of adsorption, dissolution or evaporation, and generally, in time as a variable. As may be imagined, there is a wide spectrum of rate phenomena and in the sophistication achieved in dealing wifli them. In some cases changes in area or in amounts of phases are involved, as in rates of evaporation, condensation, dissolution, precipitation, flocculation, and adsorption and desorption. In other cases surface composition is changing as with reaction in monolayers. The field of catalysis is focused largely on the study of surface reaction mechanisms. Thus, throughout this book, the kinetic aspects of interfacial phenomena are discussed in concert with the associated thermodynamic properties. 

Reactions in monolayers (38b) also provide direct chemical evidence for the dependence of molecular orientation on the surface area. The rates of oxidation and halogenation of double bonds midway along the carbon chain of oleic acid, the rate of lactoniza-... 

Figures 8 and 9 ow two specially designed multicompartment Langmuir troughs that permit the treatment of a monolayer with a series of different subphase reagent solutions, thus producing a known series of chemical reactions in the film. Following these treatments, the films may be transferred to a solid support or subjected to quantitative analysis to determine the outcome of the reactions.

Water activity has a profound effect on the rate of many chemical reactions in foods and on the rate of microbial growth (Labuza 1980). This information is summarized in Table 1-9. Enzyme activity is virtually nonexistent in the monolayer water (aw between 0 and 0.2). Not surprisingly, growth of microorganisms at this level of aw is also virtually zero. Molds and yeasts start to grow at aw between 0.7 and 0.8, the upper limit of capillary water. Bacterial growth takes place when aw reaches 0.8, the limit of loosely... 

NEB and almost all chemical reactions in foods are thought to cease in systems that are below a of 0.23 to 0.43, the typical range of the moisture monolayer value (Labuza et al., 1969 Rockland and Nishi, 1980). In Figure 20.5b, this trend appeared as expected for the control formulation, which contained no humectant. However, upon the addition of hydrophilic glycols as humectants (which were liquids at the test temperature and thus miscible in water), the at which the maximum reaction rate occurred shifted to a much lower value. Propylene glycol showed a maximum rate at a of about 0.2, with a rate equal to the maximum rate of the control at a a of 0.8. 

To summarize, chemical reactivity in monolayers is often suppressed due either to blocked access of the reagents to the reaction centre, or to steric congestion during reaction. In rare cases where enforced orientation or juxtaposition of the functional groups in... 

A microemulsion is a thermodynamically stable three-component system two immiscible components (generally water and oil) and a surfactant molecule that lowers the interfacial tension between water and oil resulting in the formation of a transparent solution. Water-in-oil microemulsions involve dispersion of the aqueous phase as nanosized droplets (5-25 nm in diameter) surrounded by a monolayer of surfactant molecules in the continuous hydrocarbon phase. These micellar droplets exhibit a dynamic exchange of their contents, which further facilitates the reactions between reactants dissolved in different droplets. One can synthesize size-controlled crystallites by carrying out a wide variety of chemical reactions in nanodroplets using this micellar exchange. Different types of microemulsions are... 

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. 

Surface phase transformations and surfrice chemical reactions are followed by studying the time evolution of superlattice beams originating from monolayer or submonolayer films. See, for example. Chapters 8-10 in Low-EnergyYaj Hove et al. op cit.). 

The principal applications of REELS are thin-film growth studies and gas-surface reactions in the few-monolayer regime when chemical state information is required. In its high spatial resolution mode it has been used to detect submicron metal hydride phases and to characterize surface segregation and difRision as a function of grain boundary orientation. REELS is not nearly as commonly used as AES orXPS. 

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