la7b02201_si_001.pdf (2.42 MB)
Hydration and Counterion Binding of [C12MIM] Micelles
journal contribution
posted on 2017-08-08, 00:00 authored by Sergej Friesen, Thomas Buchecker, Alice Cognigni, Katharina Bica, Richard BuchnerSurface-active
ionic liquids based on imidazolium cations are promising
targets for micellar catalysis in aqueous solution, yielding enhanced
rate constants compared to surfactants based on n-alkyltrimethylammonium cations and exhibiting a pronounced counterion
dependence (Bica Chem. Commun. 2012, 48, 5013−5015; Cognigni Phys. Chem. Chem. Phys. 2016, 18, 13375–13384). Probably most relevant to these effects
is the interplay between headgroup hydration and counterion binding.
To obtain more detailed information on these effects, aqueous solutions
of 1-dodecyl-3-methylimidazolium ([C12MIM]) bromide, iodide,
and triflate (TfO–) were investigated at 45 °C
using broadband dielectric spectroscopy, viscosity measurements, and
small-angle X-ray scattering experiments. Effective hydration numbers
were determined, and information on the locations and mobilities of
the condensed counterions, X–, was derived. It was
found that [C12MIM] halide micelles were less hydrated
than the corresponding n-dodecyltrimethylammonium
([C12TA]X) aggregates. Together with their somewhat weaker
counterion condensation, this difference probably explains their higher
catalytic activity. Whereas [C12MIM]Br micelles remained
roughly spherical in the studied concentration range, rodlike aggregates
were formed at high concentrations of the iodide and, in particular,
the triflate surfactants. It appears that the much lower mobility
of condensed TfO– counterions is the reason for
the very low catalytic activity of [C12MIM]TfO micelles.
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Keywords
rate constantsiodiderodlike aggregatesconcentration rangesolutiontriflate surfactantsalkyltrimethylammonium cationsviscosity measurementsbroadband dielectric spectroscopyimidazolium cationscounterion bindingTfOCounterion BindingEffective hydration numberssmall-angle X-rayMicelles Surface-activeBica Cheminformationhalide micellesPhyTAcounterion dependencemicellar catalysiscounterion condensationC 12 MIMheadgroup hydration
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