Ordering and Nonideality of Air-Ionic Liquid Interfaces in Surface Second Harmonic Generation

Chain-length dependent organisation in mixtures of hydrogenous and fluorous ionic liquids

As part of an ongoing study of the structure and properties of mixtures of ionic liquids in which one component has a hydrocarbon chain and the other a semiperfluorocarbon chain, we now report a study of the mixtures [C8MIM]1-x[C10MIM-F17]x[Tf2N], [C10MIM]1-x[C8MIM-F13]x[Tf2N] and [C10MIM]1-x[C10MIM-F17]x[Tf2N], where [C8MIM][Tf2N] is 1-methyl-3-octylimidazolium bis(trifluoromethylsulfonyl)imide, [C10MIM][Tf2N] is 1-decyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [C8MIM-F13][Tf2N] is 1-(1H,1H,2H,2H-perfluorooctyl)-3-methylimidizolium bis(trifluoromethylsulfonyl)imide and [C10MIM-F17][Tf2N] is 1-(1H,1H,2H,2H-perfluorodecyl)-3-methylimidizolium bis(trifluoromethylsulfonyl)imide. The mixtures were investigated using small-angle X-ray (SAXS) and neutron (SANS) scattering complemented by molecular dynamics simulations (with viscosity and surface tension measurements also possible for the mixtures [C10MIM]1-x[C8MIM-F13]x[Tf2N]). Unlike previous studies of [C8MIM]1-x[C8MIM-F13]x[Tf2N], where no strong evidence of alkyl/fluoroalkyl chain segregation or triphilic behaviour was seen (Elstone et al., J. Phys. Chem. B, 2023, 127, 7394-7407), these new mixtures show the formation of small aggregates of varying sizes of each component, even though all were co-miscible across the full range of compositions. Thus, while a clear polar non-polar peak (PNPP) was observed at large or small values of x, at intermediate compositions the small-angle neutron scattering at low q was dominated by scattering from these small aggregates, while at other compositions, there was little or no evidence of the PNPP. The origins of this behaviour are discussed in terms of inter-chain interactions.

Disentangling the molecular polarizability and first hyperpolarizability of methanol-air interfaces

Liquid-air interfaces have extensive implications in different areas of interest because the dynamical processes at the interface can be different from those in bulk. Thus, its characterization, understanding, and control may be pivotal in advancing discoveries. However, characterizing the interface requires special and selective tools to avoid signals from the bulk region. This surface specificity and versatility is achieved by using the second harmonic generation (SHG) responses. This study adopts multiscale simulation methods to evaluate the surface SHG responses of methanol-air interfaces with submonolayer resolution tackled by sequentially using classical molecular dynamics simulations under different temperatures and then employing quantum chemistry methods to compute the molecular first hyperpolarizabilities (β). This approach ensures the configurational diversity required to evaluate the average β values. The main achievements are (i) a quasi-absence of surface sensitivity of the mean polarizability 〈α〉 with values about 2% larger than those obtained in bulk, (ii) conversely, smooth variations on the polarizability anisotropy Δα are observed up to the fourth molecular layer at around 20 Å from the interface, and (iii) narrow interfacial effects on the SHG responses, β(-2ω;ω,ω), which are limited to the first molecular layer (∼3.0 Å) and characterized by a high contrast in the βZZZ(-2ω;ω,ω) tensor component between the first and the subsequent layers. Similar trends are obtained at different temperatures or when increasing the number of methanol molecules treated at the quantum chemistry level, indicating the robustness of the approach for describing the dipolar molecular responses of air-liquid interfaces.

Liquid-Liquid Extraction of the Eu(III) Cation by BTP Ligands into Ionic Liquids: Interfacial Features and Extraction Mechanisms Investigated by MD Simulations

What happens at the ionic-liquid (IL)/water interface when the Eu³⁺ cation is complexed and extracted by bis(dimethyltriazinyl) pyridine "BTP" ligands has been investigated by molecular dynamics and potential of mean force simulations on the interface crossing by key species: neutral BTP, its protonated BTPH⁺ form, Eu³⁺, and the Eu(BTP)₃³⁺ complex. At both the [BMI][Tf₂N]/water and [OMI][Tf₂N]/water interfaces, neither BTP nor Eu(BTP)₃³⁺ are found to adsorb. The distribution of Eu(BTP)₂³⁺ and Eu(BTP)³⁺ precursors of Eu(BTP)₃³⁺, and of their nitrate adducts, implies the occurrence of a stepwise complexation process in the interfacial domain, however. The analysis of the ionic content of the bulk phases and of their interface before and after extraction highlights the role of charge buffering by interfacial IL cations and anions, by different amounts depending on the IL. Comparison of ILs with octanol as the oil phase reveals striking differences regarding the extraction efficiency, the affinity of Eu(BTP)₃³⁺ for the interface, the effects of added nitric acid and of counterions (NO₃^- vs Tf₂N^-), charge neutralization mechanisms, and the extent of "oil" heterogeneity. Extraction into octanol is suggested to proceed via adsorption at the surface of water pools, nanoemulsions, or droplets, with marked counterion effects.

Probing Liquid Interfaces with Room-Temperature Ionic Liquids Using the Excited-State Dynamics of a Cationic Dye

Interfaces with room-temperature ionic liquids (ILs) play key roles in many applications of these solvents, but our understanding of their properties is still limited. We investigate how the addition of ILs in the aqueous subphase affects the adsorption of the cationic dye malachite green at the dodecane/water interface using stationary and time-resolved surface second harmonic generation. We find that the interfacial concentration of malachite green depends crucially on the nature of both anionic and cationic constituents. This concentration reports on the overall charge of the interface, which itself depends on the relative interfacial affinity of the ions. Our results reveal that the addition of ILs to the aqueous subphase has similar effects to the addition of conventional salts. However, the IL cations have a significantly higher propensity to adsorb than small inorganic cations. Furthermore, the IL constituents show a synergistic effect, as the interfacial concentration of each of them also depends on the interfacial affinity of the other.