Aggregation, aggregate composition, and dynamics in aqueous sodium cholate solutions

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Abstract

The translational mobility of aggregates formed in aqueous sodium cholate solutions was obtained from the self-diffusion coefficient of solubilized decanol, which was determined using the capillary tube method with radioactive labeling. Combining these results with the self-diffusion coefficients of cholate ions, sodium ions, and water molecules the following quantities were determined as a function of cholate concentration: the fraction of cholate aggregated, the ratio of counterions and cholate ions in the aggregates, and the hydration number of the aggregates. All the quantities considered differ in their concentration dependences from those of typical micelle forming long chain ionic surfactants. There seems to be no distinctly favored aggregate size but rather the aggregate radius and aggregation number increase progressively with increasing concentration. The aggregation number is much smaller than for typical surfactant micelles. The deduced fraction of aggregated cholate shows that association progresses continuously over a wide concentration range without a well-defined critical micelle concentration. Sodium ion binding to cholate aggregates is highly variable with a rapid increase taking place above 0.1 mole/kg, a conclusion which may also be drawn from 23Na chemical shift and relaxation data. The counterion binding is discussed in relation to the so-called ion condensation model, found to apply with good approximation for simple surfactant aggregates, and in relation to the progressive cholate aggregation. The aggregates are rather strongly hydrated and the hydration number shows a peculiar concentration dependence. Information on the orientation of cholate within the aggregates is deduced by combining deuterium NMR relaxation data of deuterated cholate with the diffusion results. The order parameter is rather insensitive to concentration and points to a packing of cholate in the aggregates which does not change appreciably with concentration and which corresponds to a marked mobility of cholate within the aggregates.

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Authors
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External organisations
  • University of Montpellier
Research areas and keywords

Subject classification (UKÄ) – MANDATORY

  • Physical Chemistry
Original languageEnglish
Pages (from-to)556-565
Number of pages10
JournalJournal of Colloid and Interface Science
Volume73
Issue number2
Publication statusPublished - 1980 Jan 1
Publication categoryResearch
Peer-reviewedYes