Micellar solutions of anionic surfactants usually precipitate in the presence of cations, following a mechanism by which initially cations bind themselves to the micellar surface until saturation is achieved. At higher cation concentrations, unbound cations precipitate with surfactant monomers. In a few cases cations, and especially Al 3+ , cause surfactant micelles to flocculate. These flocs have properties as adsorbents of acidic organic compounds, which might be used in water treatment processes. Both α-olefinsulfonates C 14 -C 16 and laurylsulfate micellar solutions are fast flocculation colloidal systems in the presence of Al 3+ . JSD 1, 399-402 (1998).Flocculation of micellar solutions of surfactants is a sparsely researched topic, as its occurrence is avoided by any means by workers in micellar enhanced crossflow filtration (MEUF) as a nuisance, and ignored by workers in tertiary oil recovery and related topics as it seems indistinguishable from precipitation. Micellar flocculation by polyvalent cations seems mostly associated with the Al 3+ cation, and takes place when micellar surface charge is neutralized by the charges of adsorbed cations (1); more often the cations (Ca 2+ , Cu 2+ , etc.) saturate the micellar surface before electrostatically neutralizing the micellar surface charge (2,3). When this happens, any available excess cations precipitate with surfactant monomers (4,5). In this paper we describe the behavior of α-olefin sulfonates C 14 -C 16 (AOS) and lauryl sulfate (SDS) micellar solutions as flocculating colloids. In the case of SDS, z-potentiometry and light scattering are used to show the occurrence of micelle aggregation. The main interest in the flocculation phenomenon is that the flocculated aggregate has sequestering properties for water-soluble acidic organic compounds (6). Therefore, understanding the flocculation process is needed to help explain this sequestering behavior and its applicability limits. Turbidity is also investigated as being important in connection with surfactantbased water treatment processes (6,7), or in prediction of flocculation conditions to prevent membrane fouling in MEUF processes.
EXPERIMENTAL PROCEDURESSDS, AOS, and dodecylbenzene sulfonic acid (DDBS) were kindly supplied by Kao Corporation (Barberá del Vallés, Spain) and used as received. Aluminum sulfate was chemically pure grade from Probus (Badalona, Spain). Stock solutions of aluminum sulfate, SDS, DDBS, and AOS were kept at 25ºC, mixed, and brought to 100 mL with Milli-Q water (Millipore Waters, Milford, MA), also at 25ºC. In a series of experiments the samples were inspected visually to identify when turbidity or consistent flocs appeared. In other experiments, samples were filtered by vacuum with cellulose nitrate membranes with pore size of 45 µm. Filtered solutions were analyzed by double-phase titration (hyamine method) to obtain the surfactant concentration remaining in solution. SDS-Al 3+ solutions were inspected by light scattering with a Malvern Instruments 4700 Argon laser (wavelength 488...