Figure 1
Simulation snapshots depicting different types of structural/boundary features discussed in this work. All images have been represented in the same scale, the side of each square measuring 3.5 nm. The polar moieties of the IL are represented as a blue and red mesh (cations and anions, respectively), the non-polar alkyl side chains of the cations as grey space-filled atoms, water molecules as a turquoise mesh, air/vacuum phases as a white background and solid substrates (mica, alumina) as space-filled atoms. Details concerning the nature of the different ILs are given in each subsection.
Figure 2
Dual structuration of a pure IL in the bulk phase. (a) MD simulation snapshot of 1-hexyl-3-methylimidazoliumbis-(trifluoromethylsulfonyl)imide, [C6C1im][Ntf2]; (b) pair radial distribution functions, g(r), between selected interaction centers belonging to the charged parts of the [C6C1im]+ and [Ntf2]- ions. Curve 1: g(r) between the centroid of the imidazolium ring (im) and the nitrogen atom of the anion (NBT); 2: NBT-NBT g(r); 3: im-im g(r). The oscillatory and out-of-phase behavior of the different g(r) functions proves the existence of a polar network in the liquid; (c) g(r)s between the terminal carbon atoms of the alkyl side chains of the cations in the [CnC1im][Ntf2] series (2 ≤ n ≤ 10). The first peak trend (constant above [C6C1im][Ntf2]) emphasizes the emergence of a continuous non-polar domain in the liquid; (d) structure factors, S(q), in the same series, depicting the COP and the development of the PNPP.
Figure 3
Structuration of a pure IL as a liquid crystalline phase. (a) MD simulation snapshot of 1-dodecyl-3-methylimidazolium tetrafluoroborate, [C12C1im][BF4]; (b) pair radial distribution functions, g(r), between selected interaction centers belonging to the charged parts of the [C12C1im]+ and [BF4]- ions. Curve 1: g(r) between the centroid of the imidazolium ring (im) and the boron atom of the anion (B); curve 2: B-B g(r); curve 3: im-im g(r). The polar network is present within each polar layer; (c) g(r)s between carbon atoms in the same position of the alkyl side chains of the [C12C1im]+ ion. The higher g(r) first peaks for the C6 and C7 carbon atoms corroborate the interdigitation of the alkyl chains.
Figure 4
IL micelles in aqueous solution. (a) MD simulation snapshot of a micelle of 1-decyl-3-methylimidazolium chloride, [C10C1im]Cl, in aqueous solution at 298 K; (b) enthalpy change as a function of concentration for the titration of an aqueous solution ofN-dodecyl-N-methylpyrrolidinium bromide, [C1C12Pyrr][Br], (150 mmol L-1) into water at 323.15 K. The data were fitted in a sigmoidal curve and CMC was obtained as a zero of the second derivative (dotted curve); (c) global ideal volumetric behavior at 298 K of aqueous solutions of [C8C1im]Cl, [C10C1im]Cl and [C12C1im]Cl. The inset shows a zoom of the diluted regime comparing the ideal density lines with the experimental density data (x-cross) for [C10C1im]Cl. The position of the CMC can be inferred from the trend shifts in the data.
Figure 5
IL strands in aqueous solution. (a) MD simulation of an aqueous solution of 1-butyl-3-methylimidazolium dicyanamide, [C4C1im][N(CN)2] with 0.021 IL mole fraction; (b) volumetric results (excess molar volume data) on aqueous solutions of 1-octyl-3-methylimidazolium chloride, [C8C1im]Cl, from 283 (rhombs) to 313 K (gray crosses). Concentration ranges from pure water to the pure IL. The vertical rectangle corresponds to concentrations where no density measurements are possible due to the formation of gel-like systems (the up-turned vials correspond to concentrations in the 0.2 to 0.4 IL mole fraction range); (c) discrete probability distribution functions of aggregate sizes, P(na), for the broken polar network of 0.021 IL mole fraction solutions of [C4C1im][N(CN)2] in water (cat-an) and for the anion-water network in the same system (an-w).
Figure 6
The IL-vacuum interface. (a) MD simulation snapshot of the free surface of 1,3-didecylimidazoliumbis-(trifluoromethylsulfonyl)imide, [C10C10im][Ntf2]; (b) surface tension as a function of temperature for the [CnC1im][Ntf2] IL series. Experimental points are plotted only for the [C2C1im][Ntf2] and [C14C1im][Ntf2] series. The scatter for the other series is similar. The error bar corresponds to the estimated overall uncertainty of the surface tension measurements; (c) numerical density profiles, ρ, along the direction normal to the surface, z, for three selected atom types in [C10C10im][NTf2]: chain terminal carbon atoms (curve 1); nitrogen atom of the anion (curve 2); imidazolium ring centroid (curve 3). The image on the top right shows the same distribution for [C6C1im][Ntf2]. The simulation snapshot was scaled and positioned in the same z-scale in order to highlight the free surface. The ions are depicted as red (anions - light gray in the print version), blue (charged parts of the cations - dark gray in the print version), and gray (alkyl side chains) space-filled atoms.
Figure 7
The IL-water interface. (a) MD simulation snapshot of the interface between 1-tetradecyl-3-methylimidazoliumbis-(trifluoromethylsulfonyl)imide, [C14C1im][Ntf2]; (b) numerical density profiles, ρ, along the direction normal to the water surface, z, for three selected atom types in [C18C1im][Ntf2]: chain terminal carbon atoms; nitrogen atom of the anion; imidazolium ring centroid. The image on the top right shows the corresponding simulation snapshot. The ions are depicted as red (anions - light gray large spheres in the print version), blue (charged parts of the cations - dark gray in the print version), and light gray space-filled atoms; (c) ternary diagrams of (K3[PO4] + [C4mim]Cl + H2O) at 298 K and a nominal pressure of 0.1 MPa. Comparison of our data (circles) with those of Rogers and co-workers (empty squares)7575 Gutowski, K. E.; Broker, G. A.; Willauer, H. D.; Huddleston, J. G.; Swatloski, R. P.; Holbrey, J. D.; Rogers, R. D.; J. Am. Chem. Soc.2003 , 125, 6632. in the ABS region.
Figure 8
ILs adsorbed on solid substrates. (a) MD simulation snapshot of the interface between 1-methyl-3-octylimidazolium tetrafluoroborate, [C8C1im][BF4], and an alumina, Al2O3, amorphous surface; (b) top view of the IL-glass interface of 1-(2-hydroxyethyl)-3-methylimidazolium tetrafluoroborate adsorbed on amorphous silica. The contour graphs in the top row show the electric fields generated by the glass atoms (graph on the left) and the IL ions (two graphs on the right). Selected carbon atoms from the imidazolium ring of the cation (C2, gray spheres) and boron atoms from the anion (green spheres, light gray in the print version) contained in the 0.5 nm thick layer closest to the glass surface are also presented in the two right graphs. The color scale is in electric field units of 1.44 × 1011 J C-1 m-1. The bottom graph represents number density profiles of the [BF4]- ions (red, right axis - light gray in the print version) and the silanol groups at the glass surface (light blue, right axis - black in the print version) overlaid on top of the force profile of an AFM tip approaching and retracting from a mica surface covered with the [C2C1im][CH3COO] (data points from reference 85, left axis). The vertical lines were drawn around the separation distances between layers given in the same reference. The silanol number density was divided by a factor of three to fit the scale on the right; (c) AFM image of a pure gold substrate treated with an aqueous solution of [C10C1im][Cl] (0.07 mol L-1), along with the corresponding section analysis.
Figure 9
IL Langmuir film at the air/water interface. (a) MD simulation snapshot of a Langmuir film of trihexyl(tetradecyl)phosphoniumbis(trifluoromethylsulfonyl)imide, [P6 6 6 14][Ntf2], at the air -water interface; (b) room-temperature isotherms of the [P6 6 6 14][Ntf2] system at the air water-interface: (top) surface pressure versus mean molecular area (MMA) data, π-A; (bottom) compressional modulus, (1/CS), versus MMA data; (c) (top) MD simulation snapshots of the [P6 6 6 14][Ntf2] systems with a surface MMA value of 1.44 nm2per ion pair. The left image highlights the positions of the anions (red - light gray space-filled atoms in the print version) and the central charged cores of the cation (blue - dark gray spheres in the print version). The right image uses CPK colors (anions in dark gray and non-polar alkyl chains in light gray space-filled atoms in the print version. Bottom numerical and charge density profiles along the direction normal to the water-vacuum surface. Anion density as curve 1; imidazolium ring densitiy as curve 2; hexyl and tetradecyl terminal carbon atom densitiy as curves 3 and 4, respectively; water molecule density as curve 5. Charge density is represented as curve 6.
Figure 10
IL film confined between two solid surfaces. (a) MD simulation snapshot of a film ofN-decyl-N-methylpyrrolidiniumbis(trifluoromethylsulfonyl)imide, [C10C1pyrr][Ntf2], confined between two mica plates ca. 2.4 nm apart; (b) and (c) numerical profiles along the direction normal to the mica surfaces. Color codes (grayscale in the print version) given by the different moieties of the molecules in the graph insets.