Adsorption Properties of Alkylsulfate Ions at the Ionic Liquid/Water Interfaces: Ionic Liquid Cation Dependence

NISHI Naoya, MINAMI Eiji, SAKKA Tetsuo

doi:10.2116/bunsekikagaku.70.521

<p>The adsorption properties of dodecylsulfate and decylsulfate ions have been evaluated using electrocapillary (phase boundary potential dependence of interfacial tension) measurements at the electrochemical liquid/liquid interfaces between water (W) and three highly hydrophobic ionic liquids (ILs). The three ILs investigated are composed of a common anion, bis(nonafluorobutanesulfonyl)amide (C₄C₄N⁻), and three different cations: N-octadecylisoquinolinium (C₁₈Iq⁺), trioctylmethylammonium (TOMA⁺), and trihexyltetradecylphosphonium (THTDP⁺). For all the three ILs, the adsorption activity of the alkylsulfate ions is enhanced when the IL side of the IL/W interface is positively charged, compared with the conventional oil/water interface. This IL peculiarity is likely to originate from the stabilization of surface-active anions at the IL/W interface via the electrostatic interaction with IL cations. The enhancement is the highest at the [C₁₈Iq⁺][C₄C₄N⁻]/W interface among the three ILs, suggesting that the positively charged moiety of C₁₈Iq⁺, which is relatively exposed to neighboring ions, stabilizes more the surface-active anions adsorbed at the IL/W interface. In contrast, TOMA⁺ and THTDP⁺ have positively charged moieties buried in their alkyl chains, markedly for the latter IL, which weakens the interaction with the surface-active anions, although the adsorption activity is still higher than the case with the oil/water interface.</p>

Adsorption Properties of Alkylsulfate Ions at the Ionic Liquid/Water Interfaces: Ionic Liquid Cation Dependence

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