Determination of aqueous solubility and surface adsorption of polycyclic aromatic hydrocarbons by laser multiphoton ionization

Polycyclic aromatic hydrocarbons (PAHs) are of considerable analytical interest due to their environmental effects. On-line monitoring of these compounds based on microdroplet sampling requires accurate characterization of their solubilities and surface adsorption. A new method to determine aqueous solubility and surface adsorption of PAHs is suggested, using a combination of microdroplet sampling and multiphoton ionization-based fast-conductivity (MPI-FC) techniques. Controlled droplet contamination by PAHs was performed by external deposition upon direct contact with renewable water droplets (congruent to 10 mu L). Our approach relies on the finding that, at the onset of aqueous equilibrium solubility, X-E, a sharp increase in the detected photocharges is recorded. We show that this is directly related to enhanced surface adsorption of a particular PAH material. This behavior manifests itself in slope variation of the respective calibration curves, The PAH materials used, which have provided about four decades variation in X-E figures, were anthracene, perylene, pyrene, and phenanthrene. Applicability of the method for studying the surface excess and adsorption is specified. The MPI-FC technique provides an easy analytical tool for measuring low aqueous solubility of organic compounds and characterization of their adsorption, with no need for laborious sample preparation routines.