Oceans, films

On the environmental side, it turns out that the surfaces of oceans and lakes are usually coated with natural films, mainly glycoproteins [8]. As they are biological in origin, the extent of such films seems to be seasonal. Pollutant slicks, especially from oil spills, are of increasing importance, and their cleanup can present interesting surface chemical problems. 

The escaping bubbles from the top of a bubble-fractionation column can carry off an appreciable quantity of adsorbed material in an aerosol of very fine film drops [various papers, J. Geophys. Res., Oceans Atmos., 77(27), (1972)]. If the residu solute is thus appre-ciablv depleted, Cj in Eq. (22-57) should be replaced with the average residual concentration. 

Oil spreads on water to form a film about 100 nm thick (two significant figures). How many square kilometers of ocean will be covered by the slick formed when one barrel of oil is spilled (1 barrel = 31.5 U.S. gal) ... 

The mixture of liposomes and macromolecules was first dried under nitrogen the two types of molecules formed a multilamellar film with sandwich structures. Larger liposomes, containing macromolecules (proteins or RNA) were formed on rehydration. This process could have occurred in hot regions of the young Earth with the help of the tidal rhythm of the oceans. 

The highest concentrations of naturally occurring dissolved and particulate organic matter in the oceans are normally found in the surface films. When organic pollutants are present, they too, tend to accumulate in this surface film, particularly if they are either non-polar or surface active. Much of the available information on these surface films is reviewed by Wangersky [8,9]. 

The effect of wind velocity on (a) thin-film thickness and (b) piston velocity. The solid line represents results obtained from measurements made in wind tunnels. In situ measurements were made from distributions of the naturally occurring radioisotopes of carbon and radon. Source From (a) Broecker, W. S., and T.-H. Peng (1982). Tracers in the Sea. Lamont-Doherty Geological Observatory, p. 128, and (b) Bigg, G. R. (1996). The Oceans and Climate. Cambridge University Press, p. 85. 

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. 

Table E8.7.1 Transfer enhancement ratio for a range of liquid film coefficients common on the ocean surface...

The influence of wind is predominant in determining the liquid film coefficient for lakes, reservoirs, oceans, and many estuaries. Wind creates a shear on the water surface and generates turbulence below and on the water surface. Thus, this section deals with the measurement and prediction of the wind influence on liquid film coefficient. 

Batch Technique. As with river reaeration measurements, tracers can also be put into lakes, estuaries, and oceans to measure the influence of wind on liquid film coefficient. If we have a volatile tracer in a lake with a well-established mixed layer, for example, we can apply the same batch reactor equation from Section 6.A, as though we had a well-mixed tank ... 

This is generally seen as an indication that wind is an important driving factor for lakes, estuaries, and oceans, ft has been shown that breaking waves and water surface slicks are important (Asher and Wanninkhof, 1998), and there are other parameters -such as mean square water surface slope - that have been proposed as better indicators (Jahne, 1991). The problem is that our ability to predict these indicators from wind velocity measurements have not been developed and tested for liquid film coefficient. 

In the seventies, the growing interest in global geochemical cycles and in the fate of man-made pollutants in the environment triggered numerous studies of air-water exchange in natural systems, especially between the ocean and the atmosphere. In micrometeorology the study of heat and momentum transfer at water surfaces led to the development of detailed models of the structure of turbulence and momentum transfer close to the interface. The best-known outcome of these efforts, Deacon s (1977) boundary layer model, is similar to Whitman s film model. Yet, Deacon replaced the step-like drop in diffusivity (see Fig. 19.8a) by a continuous profile as shown in Fig. 19.8 b. As a result the transfer velocity loses the simple form of Eq. 19-4. Since the turbulence structure close to the interface also depends on the viscosity of the fluid, the model becomes more complex but also more powerful (see below). 

As reported by Romano (1996), surfactant films may be more common than previously assumed. In the Indian Ocean he found such films in 30% of coastal and 11% of open sea water. At wind speeds w10 > 6 m s-1 these films seem to be destroyed by turbulence, but they are able to form again on a time scale of a few hours. Frost and Upstill-Goddard (1999) give an overview of available information on the composition of surfactant films. 

E.J. Bock and N.M. Frew, Static and dynamic response of natural multicomponent oceanic surface films to compression and dilation laboratory and field observations, J. Geophys. Res. 98 (1993) 14599-14617. 

If the DMS inventory in Salt Pond is at steady state in summer (5), production should approximately balance removal. Tidal removal of DMS to Vineyard Sound is minimal. Outflow from Salt Pond is thought to be primarily surface water, and using a maximum tidal range of 0-0.2 m/d and a mean surface water concentration of 10 nmol/L, we calculate an export rate of less than 2 /imol/m2/d. The water-air flux of DMS may be calculated using the two film model of liss and Slater (22 flux = -ki C, ). With the same surface water DMS concentration (C ) and an estimated mass transfer coefficient (ki) for DMS of 1.5 cm/h, the projected flux of DMS from the pond into the atmosphere would be 4 /unol/m2/d. This compares with the range of estimated emissions from the ocean of 5-12 /imol/m2/d (1). 

The sea-to-air flux of DMS in the study area was calculated to be about 4.4 /imol m 2d 1 over the open ocean (Drake Passage) and about 1.2 /imol m d 1 from the inshore waters of Gerlache and Bransfield Strait (121. The calculations were based on a simple parameterization of the stagnant film model (22). The results are estimated to be uncertain by a factor of 2 (1). The difference between the open ocean and inshore area values can be attributed mainly to differences in wind velocities rather than sea surface temperatures or aqueous DMS concentrations between both regions. 

Some special problems arise at sea. When crude oil is spilled on the ocean, a slick is formed which spreads out from the source with a rate that depends on the oil viscosity. With sufficient energy an O/W emulsion may be formed, which helps disperse oil into the water column and away from sensitive shorelines. Otherwise, the oil may pick up water to form a water-in-oil emulsion, or mousse ( chocolate mousse ). These mousse emulsions can have high water contents and have very high viscosities, with weathering they can become semi-solid and considerably more difficult to handle, very much like the rag-layer emulsions referred to above. The presence of mechanically strong films makes it hard to get demulsifiers into these emulsions, so they are hard to break. See Chapter 9. 

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