Heterogeneous dissolution of benzo(a)pyrene by surfactant solutions

The dissolution of a solid deposit of benzo(a)pyrene (BaP) has been investigated using three surfactants, a cationic (benzyldimethyl dodecylammonium bromide), an anionic (sodium dodecyl sulfate) and a neutral (t-octylphenoxypolyethoxyethanol). Dissolution is a heterogeneous process characterized by the fragmentation of the solid deposit into solid particles. Successive filtrations have defined three granulometric classes: the coarse fraction greater than 0.45 μm, the fine fraction ranging between 0.05 and 0.45 μm and the micellar fraction lower than 0.05 μm. The chemical nature of the surfactant influences the repartition into the different classes. The coarse fraction becomes negligible when the concentration of the cationic surfactant is increased. It still represents more than 80% of the deposit with neutral and anionic surfactants, even in a large excess of surfactant concentration compared to the critical micellar concentration value. The cationic surfactant is also the most efficient towards the solubilization in micellar phase. This efficiency is attributed to the contribution of two mechanisms: a partition process into the hydrophobic core of the micelle and a surface solubilization into the palisade layer due to specific interactions, surface solubilization being the major contribution. Electrophoretic mobility measurements have shown (i) the pronounced negative charge carried by the deposit surface and by BaP particles (ii) the strong affinity of the cationic surfactant towards these surfaces. The break up of the deposit into fine fragments and the solubilisation by the cationic surfactant is correlated to this adsorption.