Partitioning between compartments

Partition between compartments within the aqueous environment is typically described through the octanol water partition coefficient, Aiow, or the organic carbon-water partitioning coefficient, (see Chapter 5). This approach assumes that partitioning from water into fish or suspended solids within the water is determined by the availability of organic matter whose properties as a solvent can be described by a parallel with octanol for which partitioning data are widely available... 

The fate of chemicals in the environment depends not only on processes taking place within compartments, but also by chemical partitioning between compartments. For example, there may be exchange of chemicals between air and water or soil. Movement from the water or soil into the air is accomplished by volatilization and evaporation of volatile or semivolatile compounds. Movement of chemicals from the air to water or soil is accomplished by deposition or diffusion into the water. Chemicals can also move from water to soil or sediment and vice versa. If a solid chemical in the soil or sediment dissolves into the water, this is called dissolution , while the opposite is called precipitation . If a chemical dissolved in water attaches to a soil or sediment particle, this is called adsorption , while the opposite is called desorption . The fugacity of a chemical, that is, its tendency to remain within a compartment, is affected by the properties of that chemical, as well as the chemical and physical properties of the environments such as temperature, pFF, and amount of oxygen in water and soil. Wind or water currents, wave action, water turbulence, or disturbance of soil or sediment (through the action of air or water currents, animals, or human activities) may also affect partitioning of chemicals. 

To test this question, we constructed a three-compartment wind tunnel ( breeze tunnel would be more descriptive) through which air moved at a rate of about 0.3 m /min (Sachs, 1997). Females could be placed upwind or downwind from males, which were always in the center compartment. Partitions between compartments allowed free passage of air and sounds, but prevented direct contact between rats. 

Lipophilicity has emerged as the key parameter for assessing the potential environmental impact of contaminants. This property constitutes a measure for the preference of a substance for either aqueous or non-aqueous phases. The partitioning between compartments of different polarity determines the rate and the direction of the transport of chemicals in the environment and thus their accumulation in some of its components. Therefore, lipophilicity appears in QSARs in environmental studies in two ways - as an endpoint by itself as well as a chemical descriptor to model further distribution-related parameters. 

The US Environmental Protection Agency s (EPA s) EPISuite software, for example, contains a Level III fugacity model based on Dr. Mackay s EQC model [65]. EPISuite allows the user to estimate how a chemical partitions between compartments and its overall persistence in the environment. The model represents four main compartments air, water, sediment, and soil. The software essentially solves a series of equations that represent advection... 

The data in Table 4.1 indicate that ONCB has limited water solubility and volatility. ONCB does not appear to biodegrade readily. The Level III fugadty model within EpiSuite 4.11 predicts the environmental distribution and half-lives in various media shown in Figures 4.1 and 4.2. (As discussed in more detail in Chapter 2, these model results do not serve as an absolute predictor of the fate and transport of a substance, but rather indicate in general terms the tendency of a substance to partition between compartments and biodegrade.)... 

EpiSuite 4.11 predicts the environmental fate of 1,4-DCB pictured in Figures 4.1 and 4.2. Although these modeling results do not absolutely predict the fate and transport of 1,4-DCB, they do indicate its tendency to partition between compartments and biodegrade. 

As previously noted, the penetration flux will depend on the concentration gradient between the outer and inner compartments and on the ability of the herbicide molecule to partition between compartments. 

Unlike the previous kinetics imposed by the sink condition, steady-state transport kinetics under non-sink conditions will lead to equilibrium partitioning between the aqueous phase of the donor and receiver compartments and the cell mono-layer. In contrast to the sink condition wherein CR 0 at any time, under nonsink conditions CR increases throughout time until equilibrium is attained. As previously stated in Eqs. (1) and (3), the rate of mass disappearing from the donor solution is... 

In its simplest form a partitioning model evaluates the distribution of a chemical between environmental compartments based on the thermodynamics of the system. The chemical will interact with its environment and tend to reach an equilibrium state among compartments. Hamaker(l) first used such an approach in attempting to calculate the percent of a chemical in the soil air in an air, water, solids soil system. The relationships between compartments were chemical equilibrium constants between the water and soil (soil partition coefficient) and between the water and air (Henry s Law constant). This model, as is true with all models of this type, assumes that all compartments are well mixed, at equilibrium, and are homogeneous. At this level the rates of movement between compartments and degradation rates within compartments are not considered. 

Partition coefficients can then be combined to describe the ecosystem, assuming all the compartments are well mixed such that equilibrium is achieved between them. This assumption is generally not true of an environmental system since transfer rates between compartments may be slower than transformation rates within compartments. Therefore, equilibrium is never truly approached, except for perhaps with very stable compounds. However, such simplifications can give an indication into which compartments a chemical will tend to migrate and can provide a mechanism for ranking and comparing chemicals. 

Equilibrium. Equilibrium between compartments can be expressed either as partition coefficients K.. (i.e. concentration ratio at equilibrium) or in the fugacity models as fugacity capacities and Z. such that K.. is Z./Z., the relationships being depicted in Figur 1. Z is dellned as tfte ratio of concentration C (mol/m3) to fugacity f (Pa), definitions being given in Table I. 

The multilamellar bilayer structures that form spontaneously on adding water to solid- or liquid-phase phospholipids can be dispersed to form vesicular structures called liposomes. These are often employed in studies of bilayer properties and may be combined with membrane proteins to reconstitute functional membrane systems. A valuable technique for studying the properties of proteins inserted into bilayers employs a single bilayer lamella, also termed a black lipid membrane, formed across a small aperture in a thin partition between two aqueous compartments. Because pristine lipid bilayers have very low ion conductivities, the modifications of ion-conducting... 

The efficiency of any chromatographic technique depends upon the number of sequential separations or equilibria that take place, which in the case of paper chromatography are due to the large number of compartments of cellulose-bound water. The test solutes are carried up the paper dissolved in the mobile phase and encounter successive compartments of water. At each one, rapid partition between the two phases occurs leaving the mobile phase to carry up the residual solute to the next water compartment and another partitioning effect. The solute, which is dissolved in the water and hence not carried up the paper, is now presented with fresh solvent rising up the paper and again is redistributed between the two phases. 

The basic principle is shared by several methods In chromatographic or electrophoretic systems where the liposomes (vesicles) are immobilized, pseudostationary, or carried by an electroendosmotic flow, migrating amphiphilic drug molecules partition between the water outside the liposomes, the lipid bilayer of the liposome, and the aqueous compartment within the liposome (Fig. 3). In all cases the migration rate basically reflects the par-... 

Environmental Fate. The factors governing the environmental fate of silver are not well characterized. While silver and its compounds are transported in the air, water, and soil, and are partitioned between these media, the mechanisms of transport and partitioning are not well-defined. No partition coefficients or constants have been determined for silver or its compounds. Little information was found in the available literature on transformation of silver in water or soil. Some microorganisms present in these media may be able to transform silver and silver compounds however, silver is not expected to be significantly transformed in the environment because it is toxic to microorganisms. Further information on the size and flux of environmental compartments and the transport and transformations of silver and silver compounds in the environment would be useful in defining pathways for potential human exposure. 

Possible fate in the environment. An industrial chemical that has been released into the environment will exist in differing concentrations in the various environmental compartments. The concentrations of a substance in air, water, soil and other media following release can be modelled using the concept of fugacity.2 At its simplest, this involves only the use of standard physico-chemical data to estimate the partitioning between the various media. 

Environmental fate models make use of chemical properties to describe transfer, partitioning, and degradation (Mackay et al. 1992a Cahill et al. 2003). For organic chemicals, quantitative structure-property relationships (QSPRs) may be used to predict partitioning from physical-chemical properties, such as Kow and Kov Such properties may also allow for a prediction of the transfer of chemicals between compartments. Recently, some successful attempts have also been made to predict persistency of chemicals (Raymond et al. 2001), although this mainly concerns... 

Having defined a model environment, partitioning between the various compartments is predicted on the basis of equilibrium relationships,... 

We know by experience that when two dissimilar gases are kept in two different compartments and if the partition between them is removed, the gases mix spontaneously and tend to make a homogeneous mixture. On the other hand, we never find that from a mixture of gases the molecules segregate to form two volumes of pure gases spontaneously. If this were to happen, entropy of the universe would have decreased hy an amount In TV +Ng In Ng). 

Figure 6. Dynamics of free diffusion of pa-GFP in a live protoplast. Five selected IP-transmission fluorescence images of a tobacco BY-2 protoplast expressing pa-GFP (A) at the onset of the experiment before activation, (B) during 2P-activation of pa-GFP, and (C-E) after 2P-activation were taken at the time points indicated. (A) Before 2P-activation of pa-GFP in the nucleus (dotted line) the average fluorescence intensity is barely detectable. 2P-activation of pa-GFP was initiated with a fs-laser burst of 3 s covering an area of 7 x 8 p,m with 4 parallel laser foci (10 mW at 800 nm per focus). (B) Shortly after photo-activation a strong fluorescence signal was detected and (C-E) the diffusion of photo-activated pa-GFP from the nucleus into the cytoplasm was monitored until equilibrium of partitioning between the two cellular compartments was reached. Fluorescence intensity scales are shown in each panel to the left.

Partition of deposit in each region between compartments ... 

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