The structure and properties of medium-chain surfactant solutions: a case study of sodium octanoate

In the paper the most characteristic properties of binary sodium octanoate solutions of variable ion strengths are discussed. Both pre- and postmicellar association is investigated with reference to available scattering, spectroscopic and thermodynamic data. The focus is then placed on references dealing with, or referring to, results including sodium octanoate. A picture is offered illustrating the sodium octanoate micelle as a rather unstable entity including micelle sizes from oligomeric pre-associates up to some 20 monomers. Due to this circumstance the molecular features obtained with different scattering and spectroscopic techniques are rather divergent. However, the structure emerging describes the micelle as consisting of a small "dry" hydro-carbon core with a radius shorter than the fully extended heptyle chain of the octanoate. Instead a "rough shell" encompassing the hydrated polar head groups, some "wet" methylene/methyle groups and the hydrated counterions separates the micelle from the surrounding solvent. On purely geometric grounds this hydrated shell represent, however, more than 50% of the micellar volume. The numerous distinct properties of medium-chain surfactants should thence not, as is frequently done, be related to those of long-chain surfactants! At high concentrations of octanoate or salt, or upon solubilization, the average degree of water contact seem to diminish due to an increased aggregation number or a lack of available "free" hydration water. The latter effect is suggested to produce observable effects at the second critical concentration as well as close to the phase boundary. The influence of solubilizates are dealth with only when the results provide significant information on the properties of the binary system. © 1992.