Methyltriphenylphosphonium carboxylate ionic liquids: Green synthesis, adsorption and aggregation behavior in aqueous solution

In this paper, a series of methyltriphenylphosphonium carboxylate room temperature ionic liquids [Ph3PCH3][CnH2n+1COO] with different alkyl chain length (n = 7, 9, 11, 13, 15, 17) have been synthesized by quaternization of triphenylphosphine (TPP) with dimethyl carbonate (DMC) and followed anion exchange with different carboxylic acids. The synthesis route is not only simple, but also can avoid problems such as toxicity, environmental pollution and equipment corrosion. The structures of these compounds were characterized by FT-IR and 1H-NMR. Meanwhile, their surface adsorption and aggregation behavior in aqueous solution have been investigated by equilibrium surface tension, electrical conductivity and transmission electron microscopy (TEM) measurements. Furthermore, the impact of alkyl chain length on their surface adsorption and aggregation behavior were also investigated. TEM results indicated that methyltriphenylphosphonium carboxylate ionic liquid surfactants could self-assemble into regular spherical aggregates in aqueous solution. The diameter of the spherical aggregates is about 50 ∼ 100 nanometers and does not change much with the change of alkyl chain length.