Reactions at the air-water interface

Sitzmann E V and Eisenthal K B 1988 Picosecond dynamics of a chemical-reaction at the air-water interface studied by surface second-harmonic generation J. Phys. Chem. 92 4579-80... 

There are some intriguing observations, however, with regard to the possibility of reactions at the air-water interface that have unique kinetics and perhaps mechanisms (see also discussion in the following... 

As discussed elsewhere in this book, there is increasing evidence for reactions at the air-water interface in the atmosphere. Pankow (1997) has treated partitioning of gases to the interface as well and predicts that as for adsorption on a solid and absorption into a liquid, there should be a linear relationship between log Kp and In pL with a slope of approximately 1. 

Organic photochemical reactions in monolayer organi-zates are strongly influenced by the restricted molecular mobility in these systems. Reactions at the air-water interface where molecular relaxation is possible, can be followed by measuring the enhanced light reflection in the spectral range of the absorption band of the involved species. In monolayer systems, photoinduced electron transfer processes have been studied by fluorescence techniques. 

E. V. Sitzmann and K. B. Eisenthal,/. Phys. Chem., 92,4579 (1988). Picosecond Dynamics of a Chemical Reaction at the Air-Water Interface Studied by Surface Second Harmonic Generation. 

In tbe first attempt to prepare a two-dimensional crystalline polymer (45), Co y-radiation was used to initiate polymerization in monolayers of vinyl stearate (7). Polymerization at the air—water interface was possible but gave a rigid film. The monomeric monolayer was deposited to give X-type layers that could be polymerized in situ This polymerization reaction, quenched by oxygen, proceeds via a free-radical mechanism. 

Photosystem I is a membrane pigment-protein complex in green plants, algae as well as cyanobacteria, and undergoes redox reactions by using the electrons transferred from photosystem II (PS II) [1], These membrane proteins are considered to be especially interesting in the study of monomolecular assemblies, because their structure contains hydrophilic area that can interact with the subphase as well as hydrophobic domains that can interact either with each other or with detergent and lipids [2], Moreover, studies with such proteins directly at the air-water interface are expected to be a valuable approach for their two-dimensional crystallization. 

The same way with PPV LB films was not applicable to MOPPV, because its polyion complex was unstable in solid state. The elimination reaction of a sulfbnium leaving group in the polyion complex rapidly progressed in solid state even at room temperature and the complex consequently became insoluble in the conventional organic solvents. Then, there is no way to form the polyion complex monolayer at the air/water interface. 

At the air-water interface, water molecules are constantly evaporating and condensing in a closed container. In an open container, water molecules at the surface will desorb and diffuse into the gas phase. It is therefore important to determine the effect of a monomolecular film of amphiphiles at the interface. The measurement of the evaporation of water through monolayer films was found to be of considerable interest in the study of methods for controlling evaporation from great lakes. Many important atmospheric reactions involve interfacial interactions of gas molecules (oxygen and different pollutants) with aqueous droplets of clouds and fog as well as ocean surfaces. The presence of monolayer films would thus have an appreciable effect on such mass transfer reactions. 

As discussed in Chapter 7.B.3, N02 undergoes a surface reaction with water, which is perhaps enhanced at the air-water interface, forming HONO ... 

As discussed in Chapter 8, enhanced reactions of S02 at the interface have also been observed (Jayne et al., 1990). Surface second harmonic generation (SHG) experiments (Donaldson et al., 1995) subsequently identified a unique adsorbed S02 species at the air-water interface that may be involved in this enhanced reaction. Such SHG work on the uptake and reaction of N02 on water would clearly also be of value in understanding the kinetic anomalies. In addition, the use of sum frequency generation (SFG) spectroscopy, which in effect allows one to obtain the infrared spectrum of species present at interfaces, may shed some light on such reactions. 

Another example of reactions at interfaces that is only now being recognized, due to the lack of suitable experimental techniques in the past, is that of species such as SOz and NOz at liquid interfaces. As discussed in Chapters 7 and 8, there is increasing evidence that the reactions of such species at the air-water interface can be fast relative to that in the bulk and may have unique reaction mechanisms compared to those in the bulk or gas phases. Given the paucity of data on such processes at the present time, they are generally not included in present models of aerosol growth. How-... 

Figure 20.12 Air-water exchange of an aldehyde A converting to a diol D by a hydration/dehydration reaction. Since the diol D cannot leave the water, the slope of its concentration at the air/water interface is zero. For simplicity, the scales of A and D are chosen such that the equilibrium constant of hydration, K and the Henry s law constant of the aldehyde, KAa/vl, are 1. The dashed straight line marked [A] onre>ct,ve he>Ps t0 picture the modification due to the reactivity of A.

Photochemical processes in monolayers at the air-water interface can be controlled externally by variation of the various parameters like matrix composition, subphase composition, temperature and surface pressure. When the product of the reactions has a different area per molecule, the surface pressure may change at constant monolayer area. An interfacial shock wave has been generated in this way. This technique permits the investigation of the kinetics of reorganization processes and the transmission of mechanical signals in monolayers. 

Various comprehensive studies on the polymerization of enantiopure and racemic esters of a-amino acids performed at the air/water interface to yield peptides have been reported over the years [189,190]. Recent reinvestigations of the products of these reactions by MALDI-TOF MS have demonstrated, however, that they are not longer than dipeptides [191]. For this reason, such esters cannot be regarded as realistic prebiotic model systems for the formation of long oligopeptides. On the other hand, amphiphilic Na-carboxyanhydrides [192] and thio-esters [193] of a-amino acids yield longer oligopeptides. 

Fig. 4.8, a monolayer of a barbituric acid derivative forms a hydrogen bonding network with aqueous triaminopyridine. Interestingly, cleavage of carbon-carbon double bonds was detected in this system. Upon the formation of the hydrogen bonding network, a few water molecules were trapped in a hydrophobic environment at the air-water interface. This enzyme-Uke behavior was explained by the enhanced nucleophilicity of the trapped water in this environment and the geometry associated with the reaction. 

Often a rather slow adsorption at the air-water interface has been observed. Whether this is due to electrical potential barriers, or whether a particular orientation is required of the arriving molecule before it can enter the monolayer has not yet been clearly demonstrated. For small ions taking part in reactions at interfaces, such as hydroxyl and permanganate, the latter effect has never been observed, although Alexander (29) claims that ion exchange below monolayers of amines is a slow process. The present author considers that this may be explained in terms of a slow desorption of one ionic species rather than as a slow approach of the other. A gradual change in the structure of the amine film is also a possibility. 

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