Investigation On The Aggregation Behavior And Mechanism Of Surfactant SDS Micelle

Surfactant is a ubiquitous substance in aquatic environment owing to its popular use. It has the characteristic of enwrapping insoluble substances and forming colloids, and so is the carrier of insoluble and environmental hazardous substances to exist and transfer in water body. In the disposal process of surfactant wastewater, flocculation technology is usually used as a pretreatment method. The flocculation process of this kind of wastewater is very complicated because of its complicated components and unconstant characteristics.This study took anionic surfactant sodium dodecyl sulphate (SDS) micellar solution as model system, on the basis of surfactant micellar theory and existed water treatment flocculation studies, investigated, from experimental and theoretical aspects, the interaction behaviors and mechanisms between SDS and several typical inorganic flocculants (Al₂(SO₄)₃, FeCl₃, PAC) and hardness ions (Ca²⁺, Mg²⁺) commonly encountered in natural water body. According to the different phenomenon observed in each system with the increasement of the metal ion concentration, and taking into consideration the specific dominant species of the different metal ions, the possible process in each system was investigated from the perspective of solution chemistry, and the main interaction mechanism between SDS micelle and each metal ion was deduced. SEM photographs and IR spectrums of the precipitates verified the corresponding conclusion from structural aspect.Results suggest that, firstly, when inorganic flocculant (Al₂(SO₄)₃, FeCl₃ or PAC) is added into surfactant micellar solution, surfactant is flocculated in micellar form. In SDS-Al₂(SO₄)₃ system, Al³⁺ is the dominant species of aluminium in solution, and chemical complexation reaction exists between it and SDS, which makes it strongly adsorbed to SDS micellar surface. When added Al concentration reaches 1/3 of the initial SDS concentration, every three anionic head groups of surfactant on micellar surface are bound together by one Al³⁺ ion, charge neutralization condition is met and the optimum flocculation region begins to present. That is to say, the specific adsorption-charge neutralization effect of Al³⁺ ion on micellar surface is the main mechanism of micellar flocculation in this system. When the excessive aluminium concentration reaches certain extent, SDS and aluminum can form aqueous complex, which results in the redissolution of the precipitates and finally the solution becomes clear again. In SDS-FeCl³ system, Fe(OH)²⁺ ion is the dominant species of iron in solution. When it is adsorbed on micellar surface, charge neutralization effect and hydroxide group's bridging effect cooperate and result in the flocculation of the micelles. In SDS-PAC system, the hydroxide group's bridging effect of the polymolecular hydroxide complex ion (Al_b) is the main mechanism of surfactant flocculation. In SDS-FeCl₃ and SDS-PAC systems, no special chemical complexation reaction exists between the dominant species of metal ions and SDS, so no redissolufion phenomenon can be observed at high flocculant concentrations in these two systems. Secondly, when calcium or magnesium salt, which is very ubiquitous in natural water body, is added into surfactant micellar solution, surfactant can not be flocculated in micellar form. In SDS-CaCl₂ system, the adsorption of Ca²⁺ ions on micellar surface can not result in the charge neutralization and there is no hydroxide group bridging effect in this system, so the surfaetant can not be flocculated in form of micelles. At high CaCl₂ concentration, when the product of the free Ca²⁺ ion concentration in solution and the monomer surfactant concentration (that is to say, its CMC) exceeds the solubility product of Ca(DS)₂ salt, Ca(DS)₂ precipitates can be salted out in this system. That is to say, in SDS-CaCl₂ system, the crystallization phenomenon resulted from the solubility product of Ca(DS)₂ salt, is the main mechanism of surfaetant precipitation. In SDS-MgCl₂ system, no turbidity and precipitation phenomenon can be observed in the whole experimental range. It can be assumed that, the Mg²⁺ ions remain as hydrated ion forms when adsorbed onto micellar surface because of its strong hydration effect, so it can not neutralize the micellar surface charge effectively, make it destabilized and flocculated, neither can it form low-solubility salt with SDS as in SDS-CaCl₂ system.The conclusion of this study can not only promote the development of surfaetant wastewater flocculation technology, but also provide important references for the investigations on the flocculation technologies or theories of other nano or micro size of contaminants.