Aqueous Phase Fluorescence Quenching Technique for Measuring Naphthalene Partition Coefficients in Nonionic Surfactant Micelles

Abstract

A methodology is presented for determining the partition coefficients for polycyclic aromatic hydrocarbons (PAHs) distributed between the aqueous and the micellar pseudophases of nonionic surfactant solutions. The methodology entails measuring total PAH fluorescence and then using varying copper sulfate concentrations to quench the fluorescence contribution from aqueous pseudophase PAH. Additionally, front-surface sample analysis and absorption corrections to counteract inner-filter effects were used to extend the PAH concentration range over which quenching can be used. The methodology was tested by determining the partitioning of a model PAH (naphthalene) when the nonionic surfactant, octaethylene glycol mono-n-dodecyl ether (C 12 E 8 ), was present. Because the aqueous pseudophase is probed, both saturated and unsaturated systems were examined. Thus, the partition measurements are less restricted than those provided by frequently used analyses that must assume the aqueous pseudophase is saturated in order to close the solubilizate mass balance. The value obtained at saturation with naphthalene was in reasonable agreement with the partition coefficient (log K m ) for naphthalene in a similar nonionic surfactant. Moreover, log K m increased from 4.47 at saturation (0.64 mM naphthalene in 1.88 mM surfactant) to 5.03 at 0.057 mM naphthalene (also in 1.88 mM surfactant). In practice, the methodology may prove useful for characterizing partitioning behavior under the varied conditions found in the field, designing surfactant-enhanced biodegradation experiments, and ascertaining the effects of surfactant solubilization on PAH biodegradation rate