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An investigation of the interactions between electrolytes and a zwitterionic surfactant using 23Na and 2H NMR measurements

1984; Elsevier BV; Volume: 98; Issue: 2 Linguagem: Inglês

10.1016/0021-9797(84)90183-8

1095-7103

Kevin Rendall, G. J. T. Tiddy, Michael A. Trevethan,

Electrostatics and Colloid Interactions

NMR quadrupole splittings (Δ-23Na) have been measured for sodium ions in the hexagonal phase formed by dodecyl/tetradecyl(6040)dimethyl amine oxide (C12–14AO)water(2H2O) mixtures, as a function of added sodium carbonate (“salting-out” electrolyte), sodium chloride (indifferent electrolyte), and sodium thiocyanate (“salting-in” electrolyte). Water quadrupole splittings (Δ-2H) were also measured to give information on changes in the concentration and ordering of bound water. Further measurements were made on samples containing mixed electrolytes (NaCl/Na2CO3, NaSCN/NaCl, Na2CO3NaSCN). The salting-out electrolyte (Na2CO3) causes Δ-23Na to become more negative, while the salting-in electrolyte (NaSCN) gives more positive Δ-23Na values with NaCl giving intermediate results. The magnitude of Δ-2H is increased with added sodium carbonate and decreased by addition of sodium thiocyanate, but higher electrolyte concentrations are required to cause alterations in Δ-2H than are required to change Δ-23Na. High levels of NaSCN causes both Δ-2H and Δ-23Na to change sign. At added electrolyte levels less than ca. 0.05 m values of Δ-23Na and Δ-2H are invariant. Increasing surfactant concentration (40–60%) generally gives larger Δ values. The NMR quadrupole splittings of both sodium and water are consistent with the idea that positive adsorption of anions at the micelle surface occurs when salting-in electrolytes are present, while negative adsorption of anions takes place with salting-out electrolytes.