Characterization and solution properties of adamantane-containing quaternary-ammonium-salt-type amphiphilic ionic liquids

Abstract

Quaternary-ammonium-salt-type amphiphilic compounds ([CnAdA][X], where n represents the alkyl chain length (n = 1, 2, 4, 6, 8, or 10), X represents a counterion such as [BF4]−, [PF6]−, trifluoromethanesulfonate ([OTf]−), bis(fluorosulfonyl)amide ([FSA]−), or bis(trifluoromethanesulfonyl)amide ([NTf2]−), and Ad and A of AdA represent the adamantane structure and quaternary ammonium group, respectively) were synthesized. The melting points of the prepared compounds were determined by differential scanning calorimetry, and the derivatives with melting points lower than 100 °C were classified as ionic liquids for subsequent analyses. The amphiphilic ionic liquids, [CnAdA][NTf2] (n = 6 and 8), exhibited the lowest melting points (30.6 and 38.7 °C, respectively). Further, the [CnAdA][NTf2] series of amphiphilic ionic liquids exhibited significantly lower conductivities and higher viscosities than the corresponding ionic liquids without the adamantane moiety, [Cn][NTf2]. The viscosities of [CnAdA][NTf2] (n = 6 and 8) decreased significantly with increasing temperature, and showed a larger temperature dependence than that of the viscosities of the [Cn][X] series. The amphiphilic ionic liquids, [C8AdA][X] readily adsorbed at the air/water interface and oriented themselves but did not show the critical micelle concentration in the concentration range over which they could be dissolved in water. The amphiphilic ionic liquids and compounds tended to form ion pairs or premicelles, such as dimers or trimers, in aqueous solutions.