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Ion Binding to Bilayers of the Iridescent Phase in Aqueous Tetradecyldimethylaminoxid Studied by Mass Spectrometry

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Abstract

The behaviour of a tetradecyldimethylaminoxid (C14DMAO) iridescent solution with a strict composition has been investigated in the presence of NaCl, KCl, NaF, KF, NaClO4, or NH4ClO4 at a constant ionic strength. Specific ion effects (the Hofmeister effects), arising when changing ions of the same valency, have been studied. The interactions of C14DMAO molecules with the added ions have been investigated by electrospray ionisation mass spectrometry (ESI-MS). The ESI-MS results indicate that C14DMAO bilayer interactions are stronger with cations than with anions. Among the cations studied here, potassium ions are more favourably bound at the C14DMAO bilayers than sodium ions. The aminoxide groups of the C14DMAO molecules do not bind NH4+ions. The positive charge of the electric double layers on the C14DMAO bilayers is neutralised by anions of the added electrolytes. Consequently, the separation distance between the lamellar layers decreases and the ordering in the studied systems is broken. Among the studied counterions, chloride ions are most strongly bound to the interface causing the greatest breakdown of the ordering in the iridescent solution.

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