Phase partitioning, multiphase aqueous systems

DYNAMICS OF DISTRIBUTION The natural aqueous system is a complex multiphase system which contains dissolved chemicals as well as suspended solids. The metals present in such a system are likely to distribute themselves between the various components of the solid phase and the liquid phase. Such a distribution may attain (a) a true equilibrium or (b) follow a steady state condition. If an element in a system has attained a true equilibrium, the ratio of element concentrations in two phases (solid/liquid), in principle, must remain unchanged at any given temperature. The mathematical relation of metal concentrations in these two phases is governed by the Nernst distribution law (41) commonly called the partition coefficient (1 ) and is defined as = s) /a(l) where a(s) is the activity of metal ions associated with the solid phase and a( ) is the activity of metal ions associated with the liquid phase (dissolved). This behavior of element is a direct consequence of the dynamics of ionic distribution in a multiphase system. For dilute solution, which generally obeys Raoult s law (41) activity (a) of a metal ion can be substituted by its concentration, (c) moles L l or moles Kg i. This ratio (Kd) serves as a comparison for relative affinity of metal ions for various components-exchangeable, carbonate, oxide, organic-of the solid phase. Chemical potential which is a function of several variables controls the numerical values of Kd (41). 

The experiments showed that the p-chloromercuribenzoate (p-CMB) was partially adsorbed into the lipid phase, and the different inhibitions observed were considered to result from the different partitions of the p-CMB between the aqueous phase and the various substrates. Only the jp-CMB dissolved in the lipid was considered effective as an inhibitor. Desnuelle et al. (349) observed similar effects when using p-nitrophenyl phosphate. This compound also inhibited pancreatic lipase far more strongly when dissolved in the fat phase of the emulsion than when dissolved in the aqueous phase. These experiments stress that a new factor, that of adsorption at the interface, must always be considered when studying inhibitions in multiphase systems. The concentration in the aqueous phase may bear no relation to the observed inhibition. 

In a multiphase formulation, such as an oil-in-water emulsion, preservative molecules will distribute themselves in an unstable equilibrium between the bulk aqueous phase and (i) the oil phase by partition, (ii) the surfactant micelles by solubilization, (iii) polymeric suspending agents and other solutes by competitive displacement of water of solvation, (iv) particulate and container surfaces by adsorption and, (v) any microorganisms present. Generally, the overall preservative efficiency can be related to the small proportion of preservative molecules remaining unbound in the bulk aqueous phase, although as this becomes depleted some slow re-equilibration between the components can be anticipated. The loss of neutral molecules into oil and micellar phases may be favoured over ionized species, although considerable variation in distribution is found between different systems. 

Noble gases exist in formation waters as uncharged, nonpolar species. They preferentially partition from aqueous solutions into nonpolar solvents such as crude oil and natural gas (Kharaka and Specht, 1988). The degree of partitioning depends on such factors as temperature, gas atomic radius, and the salinity of the aqueous phase. Distinct variations in Ne/ Ar, Kr/ Ar, and °Xe/ Ar are produced by fractionation in multiphase fluid systems. These variations have been used as a tool in oil exploration and reservoir evaluation. Pinti and Marty (2000) give detailed examples of these applications in the Pannonian Basin, Hungary and in the Paris Basin, France. 

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