Sodium Effect on Self‐Organization of Amphiphilic Carboxylates: Formation of Structured Micelles and Superlattices

Abstract

Not only the self-aggregation of dendritic polycarboxylates into structurally persistent micelles, but also that of the micelles themselves into superlattices is controlled by alkali-metal counterions and shows a pronounced sodium effect. Our combined experimental and computational work has revealed the formation of superlattices for the first time. The behavior of a variety of amphiphilic carboxylates and the different effects of the alkali cations Li(+), Na(+), and K(+) have been investigated by conductivity measurements, cryogenic transmission electron microscopy (cryo-TEM), and molecular-dynamics (MD) simulations. Together, these show that sodium salts of the amphiphiles give the most stable micelles, followed by lithium and potassium. Our results suggest that ion multiplets in bridging positions, rather than contact ion pairs, are responsible for the enhanced stability and the formation of hexagonally ordered superlattices with sodium counterions. Potassium ions do not form such ion multiplets and cannot therefore induce aggregation of the micelles. This sodium effect has far-reaching consequences for a large number of biological and technical systems and sheds new light on the origin of specific-ion effects.

Dendritic Architectures with Positively Charged Cores and Negatively Charged Shells

“…This was exemplified by the synthesis of the amphiphile 17 (Scheme 5), which exhibits a dipolar head group and represents the first member of a new family of charged surfactants. [26,27] For the synthesis of 17, the pyridine core of 4 was directly quaternized with 1-iodohexadecane (15), which was assisted by silver nitrate. Deprotection of the tert-butyl protecting groups was accomplished by treatment with formic acid.…”

“…[27,31,32] All other chemicals were purchased from Acros Organics and Sigma-Aldrich. For purification by flash chromatography, silica gel 60 M (grain size 0.04-0.063 mm, less active) from Macherey-Nagel was used.…”

“…In this convergent approach the synthesis started with the easily accessible dendritic amphiphile 21, which contains three terminal carboxylic groups. [27] Scheme 7. Synthesis of 23. v) 5, DCC, HOBt in DMF, 0°C to room temp., 72 h; iv) HCOOH in CHCl 3 , room temp., 24 h.…”

The Potential of Perylene Bisimide Derivatives for the Solubilization of Carbon Nanotubes and Graphene

“…It has recently been shown that sodium as counterion compared to lithium and potassium has a higher affi nity to the carboxylate ions of amphiphilic molecules similar to structure 8 . [ 103 ] This is manifested in changes of the supramolecular self-assembly in aqueous solution. In analogy, the critical micelle concentration of the bolaamphiphilic perylene derivative 1 is dependent on the counterion decreasing in the order K + > Li + > Na + .…”

Predictive Modeling of Molecular Properties: Can We Go beyond Interpretation?

“…Recent simulations on structurally persistent micelles [69][70][71] have demonstrated that counterion effects sometimes dominate. Nucleic-acid force fields must therefore be considered as a combination of nucleic acid, ion, and water force fields [72].…”