An Electrostatic Approach to the Association of Sodium and Calcium With Surfactant Micelles

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Abstract
Summary: Association of sodium and calcium with surfactant micelles was measured and electrostatic models were used to interpret the experimental results. It was hypothesized that the association of calcium with micelles having sulfonate or sulfate anionic groups can be described by electrostatics alone—i.e., specific chemical associations were not considered. Interpretation of data obtained with ultrafiltration (UF) membranes that separate free electrolyte solution from micelles supports this hypothesis. Two parameters estimated for the electrostatic model were the lengths of the hydrophilic groups and the specific areas of surfactants on the surfaces of micelles. The Donnan equilibrium model was used to calculate the exchange of sodium and calcium and to make a comparison with mass-action models.