Abstract
We study the addition of electrolytes to surfactant-free microemulsions in the domain where polydisperse pre-Ouzo aggregates are present. As in previous studies, the microemulsion is the ternary system water/ethanol/1-octanol, where ethanol acts as co-solvent. Addition of electrolytes modifies the static X-ray and neutron scattering, and dynamic light scattering patterns, as well as the position of the miscibility gap, where spontaneous emulsification occurs upon dilution with water. All observations can be rationalized considering that electrolytes are either "salting out" the ethanol, which is the main component of the interface stabilizing the aggregates, or producing charge separation via the antagonistic ion effect discovered by Onuki et al. Amphiphilic electrolytes, such as sodium dodecylsulfate or sodium dietheylhexylphosphate, induce a gradual transition towards monodisperse ionic micelles with their characteristic broad scattering "peak". In these micelles the ethanol plays then the role of a cosurfactant. Dynamic light scattering can only be understood by combination of fluctuations of aggregate concentration due to the vicinity of a critical point and in-out fluctuations of ethanol.
Highlights
The presence of a nano-ordering was observed by Klossek et al with dynamic (DLS) combined to static light scattering (SLS) in the system water/ethanol/1-octanol, which fulfills the conditions of the Ouzo effect, as explained before.[2]
In order to investigate the influence of anions on the structures of the surfactant-free microemulsion, Small-angle X-ray scattering (SAXS) were performed with increasing amounts of sodium salts
Pre-Ouzo aggregates are a general type of weak aggregation that is very sensitive to addition of electrolytes
Summary
The presence of a nano-ordering was observed by Klossek et al with dynamic (DLS) combined to static light scattering (SLS) in the system water/ethanol/1-octanol, which fulfills the conditions of the Ouzo effect, as explained before.[2]. Similar results were found for the systems water/benzyl alcohol/ethanol or ethyl lactate or g-valerolactone as oil phase.[7] It is worth pointing out that surfactant-free microemulsions seem to occur always in the pre-Ouzo region as long as the studied system does exhibit an Ouzo region. This pre-Ouzo effect was investigated in more details. The authors showed that ethanol is distributed over both pseudo-phases
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