Translational motion in aqueous sodium n-octanoate solutions

  • Lindman B
  • Brun B
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Abstract

The translational self-diffusion coefficients of water molecules, sodium ions and octanoate ions in aqueous sodium octanoate solutions have been measured at 25°C with the open-ended capillary tube method using radioactive labeling. Below the critical micelle concentration (CMC), the self-diffusion coefficients of the ions vary only moderately with concentration. Above the CMC, however, a rapid decrease in translational mobility with increasing concentration is observed. The variation of water diffusion with sodium octanoate concentration is stronger before the CMC than at higher concentrations. Assuming that the ions can be found in three different environments, i.e., nonassociated, small complexes, and micelles, the self-diffusion coefficients are used to derive information on premicellar aggregation, CMC, counterion binding, and micelle hydration. From sodium ion and octanoate ion self-diffusion, the CMC is determined to 0.39 moles kg-1. The degree of counterion binding is about 60% for the first formed micelles and increases at higher concentrations. The first formed micelles contain about 8.7 water molecules/octanoate ion. A change in hydration is indicated at roughly 1.1 moles kg-1. These results are found to be in good agreement with conclusions based on studies employing other experimental techniques. © 1973.

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Authors

  • Björn Lindman

  • Bernard Brun

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