Groundwater remediation by anionic surfactant micelles - an innovative double layer model applied to Na⁺ and Mg²⁺ association with dodecylsulfate micelles

The association reactions involving counterions, Na⁺ and Mg²⁺, and micelles composed of the anionic surfactant, dodecylsulfate (DS⁻), were investigated in ultrafiltration experiments. To access the data, an innovative model was developed that considered specific counterion binding within a Stern layer, with binding constant dependent upon the electrical potential as derived by the Poisson-Boltzmann equation and with calculation of the cmc as a function of counterion binding (or association). The experimental and model results both show that magnitude of counterion binding is greater for divalent species, Mg²⁺, than that for the monovalent species, Na²⁺. However, high concentration of Na⁺ compete for surface area diminishing the ability of the DS⁻ to bind either divalent species. At experimental conditions from 0 to 100 mM NaCl addition, the binding ratio (BR) varied only from 0.58 to 0.63. The optimum binding constants, K_Mg and K_Na, were determined to be 0.4 and 1.0 L mol⁻¹, respectively, for the model. The experimental data and model calculated results were generally in good agreement.