title: Atomistic Insight into Tetraalkylphosphonium-Bis(oxalato)borate Ionic Liquid/Water Mixtures. I. Local Microscopic Structure.
abstract: Atomistic simulations have been performed to investigate the microscopic structural organization of aqueous solutions of trihexyltetradecylphosphonium bis(oxalato)borate ([P6,6,6,14][BOB]) ionic liquid (IL). The evolution of the microscopic liquid structure and the local ionic organization of IL/water mixtures as a function of the water concentration is visualized and systematically analyzed via radial and spatial distribution functions, coordination numbers, hydrogen bond network, and water clustering analysis. The microscopic liquid structure in neat IL is characterized by a connected apolar network composed of the alkyl chains of [P6,6,6,14] cations and isolated polar domains consisting of the central segments of [P6,6,6,14] cations and [BOB] anions, and the corresponding local ionic environment is described by direct contact ion pairs. In IL/water mixtures with lower water mole fractions, the added water molecules are dispersed and embedded in cavities between neighboring ionic species and the local ionic structure is characterized by solvent-shared ion pairs through cation-water-anion triple complexes. With a gradual increase in the water concentration in IL/water mixtures, the added water molecules tend to aggregate and form small clusters, intermediate chain-like structures, large aggregates, and eventually a water network in water concentrated simulation systems. A further progressive dilution of IL/water mixtures leads to the formation of self-organized micelle-like aggregates characterized by a hydrophobic core and hydrophilic shell consisting of the central polar segments in [P6,6,6,14] cations and [BOB] anions in a highly branched water network. The striking structural evolution of the [P6,6,6,14][BOB] IL/water mixtures is rationalized by the competition between favorable hydrogen bonded interactions and strong electrostatic interactions between the polar segments in ionic species and the dispersion interactions between the hydrophobic alkyl chains in [P6,6,6,14] cations.
authors.affiliation: †Department of Materials and Environmental Chemistry, Arrhenius Laboratory, Stockholm University, SE-106 91 Stockholm, Sweden.
authors.forename: Yong-Lei
authors.initials: YL
authors.lastname: Wang
authors.affiliation: †Department of Materials and Environmental Chemistry, Arrhenius Laboratory, Stockholm University, SE-106 91 Stockholm, Sweden.
authors.forename: Sten
authors.initials: S
authors.lastname: Sarman
authors.affiliation: ‡System and Component Design, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden.|§Mechanical Construction and Production, Ghent University, B-9000 Ghent, Belgium.
authors.forename: Sergei
authors.initials: S
authors.lastname: Glavatskih
authors.affiliation: ∥Chemistry of Interfaces, Luleå University of Technology, SE-971 87 Luleå, Sweden.|⊥Department of Physics, Warwick University, CV4 7AL Coventry, United Kingdom.
authors.forename: Oleg N
authors.initials: ON
authors.lastname: Antzutkin
authors.affiliation: #Surface and Corrosion Science, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden.|∇Chemistry, Materials and Surfaces, SP Technical Research Institute of Sweden, SE-114 86 Stockholm, Sweden.
authors.forename: Mark W
authors.initials: MW
authors.lastname: Rutland
authors.affiliation: †Department of Materials and Environmental Chemistry, Arrhenius Laboratory, Stockholm University, SE-106 91 Stockholm, Sweden.|○Stellenbosch Institute of Advanced Study (STIAS), Wallenberg Research Centre, Stellenbosch University, Marais Street, Stellenbosch 7600, South Africa.
authors.forename: Aatto
authors.initials: A
authors.lastname: Laaksonen