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

Abstract

NMR quadrupole splittings (Δ- 23Na) have been measured for sodium ions in the hexagonal phase formed by dodecyl/tetradecyl( 60 40 )dimethyl amine oxide (C 12–14AO) water( 2H 2O) 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/Na 2CO 3, NaSCN/NaCl, Na 2CO 3 NaSCN ). The salting-out electrolyte (Na 2CO 3) 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.