Anomalous effects of concentrated salts on the equimolarly mixed cationic–anionic surfactant mixtures

Abstract

and CsCl) on the morphology of the aggregates of decyltriethylammonium bromide and sodium decylsulfonate (C 10 NE-C 10 SO 3 ) mixtures with equimolar ratios were studied by static/dynamic light scattering and viscosity measurements. The average hydrodynamic radii of the aggregates, the apparent aggregation number, and the relative viscosity as functions of both the concentrations of surfactants and added salts were investigated, respectively. It showed that the addition of a small amount of NaX (e.g., 0.1 M) had only small effects on the C 10 NE-C 10 SO 3 vesicles. However, striking anion specificity, which differed anomalously with the anions of salts varying from F À to I À , was observed when large amounts of NaX (e.g., 1 M) were added. The size of the C 10 NE-C 10 SO 3 aggregates was normally increased and then a liquid-liquid phase separation occurred with the addition of NaF; meanwhile, the addition of NaCl had little effect on the C 10 NE-C 10 SO 3 aggregates under experimental conditions; however, the size of the C 10 NE-C 10 SO 3 aggregates was abnormally decreased when high concentrations of NaBr or NaI (up to 2 M) were added, implying a vesicle-to-micelle transition which might be due to a decrease in the intramicellar attraction between oppositely charged headgroups by electrical shielding, and the mixtures still remained homogeneous. A cation specificity of added salts was also observed with the increase of the ionic radii from Na + to Cs + . However, it was noted that the influence of increasing the ionic radii of cations on the aggregate size was not as pronounced as that of anions in the presence of high concentrations of salts. The aggregates of C 10 NE-C 10 SO 3 became a little smaller when concentrated alkali metal halides with bigger cations were added into this homogeneous equimolar cationic-anionic surfactant mixture. Both the observed cation and anion specificities were consistent with the Hofmeister series.

Hofmeister Series: Insights of Ion Specificity from Amphiphilic Assembly and Interface Property

“…I – has higher ability in decreasing the aggregate size than Br – , which is consistent with the HS law. Since Cl – is the intermediate anion in the HS, the effect of NaCl on the C 10 NE-C 10 SO 3 vesicles is small …”

“…(a) Hydrodynamic radius ( R h ) of C 10 NE-C 10 SO 3 aggregates; (b) apparent aggregation number ( N app ) of C 10 NE-C 10 SO 3 aggregates; (c) relative viscosity ( η r ) of C 10 NE-C 10 SO 3 solutions. Reprinted with permission from ref . Copyright 2011 Royal Society of Chemistry.…”

A Unique Ionic Liquid with Amphiphilic Properties That Can Form Reverse Micelles and Spontaneous Unilamellar Vesicles

“…[34] It should be noted that other authors have obtained vesicles with catanionic mixtures, but without using equimolar mixture, and in those cases, the generated vesicles were mul-tilamellar or unilamellar in equilibrium with multilamellar ones. [35,36] Also, the vesicles phases were obtained at low surfactant concentration. We have obtained unilamellar vesicles of AOT-BHD in water without need to add energy to the system, as was required by traditional methods, [7] and the results were the same, independent from the surfactant concentration used.…”

Spontaneous catanionic vesicles formed by the interaction between an anionic β-cyclodextrins derivative and a cationic surfactant

“…1 Among the compounds that form vesicles, we can mention catanionic surfactants which form different kinds of aggregates and have been subject to extensive experimental investigations. [2][3][4][5] It has been demonstrated that they form molecular bilayers 6,7 and that they are good candidates as drug delivery vehicles, 8 as well as in other areas such as detergents, foaming, etc. 9 During the screening of self-assemblies systems suitable for various applications, the combination of catanionic surfactants with macrocycles have been studied.…”