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A few studies have provided information on longitudinal trends in VOC concentrations and emissions in new houses (Lindstrom et al., 1995, Hodgson et al., 2002). In four manufactured houses, the area specific emission rates of formaldehyde and hexanal were generally similar at the beginning and end of the 7.5-month study period demonstrating that the sources of these compounds were not depleted rapidly (Hodgson et al., 2000). In existing, occupied residential units, seasonal trends in VOC concentrations have been observed in a cross-sectional study in three German cities and in a longitudinal study of ten apartments (Schlink et al.,. | Organic Chemicals in the Environment Where are they found What are the effects do they have Surface effects Membranes Waxes Foams Soil - Humus The air-water interface The surface tension of water is 0.073 N m. Ions can increase this value somewhat to 0.075 N m. However organic compounds Tend to lower the value. The surface tension of organic liquids e.g. octane benzene etc. range from 0.020 to 0.050 N im. There are two types of surface adlayers on water. 1. Organic thin films such as oil spills prior to oxidative weathering . These compounds are both insoluble and less dense than water. 2. Surfactants are amphipathic molecules. There are dry surfactants such as detergents mostly hydrophobic and wet surfactants such as proteins mostly hydrophilic . Surface tension Liquids tend to adopt shapes that minimize their surface area. This places the maximum number of molecules in the bulk. Droplets of liquids tend to be spherical because a sphere is the shape with the smallest surface-to-volume ratio. The work needed to change the area by dơ is dw Ydơ The coefficient Y is called the surface tension. It has dimensions of energy per unit area J m2 . At constant volume and temperature the work of surface formation is equal to the Helmholtz free energy dA Ydơ Since dA 0 is a spontaneous change surfaces tend to .
