Ionic Liquid Films at the Water–Air Interface: Langmuir Isotherms of

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Ionic liquid films at the water-air interface: Langmuir isotherms of tetra-alkylphosphonium-based ionic liquids. Karina Shimizua, José N. Canongia Lopesa,b,*, Amélia M. P. S. Gonçalves da Silvaa,* a

Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, 1049 001 Lisboa, Portugal Instituto de Tecnologia Química e Biológica, Universidade Nova de Lisboa, 2580 157 Oeiras, Portugal

b

*email addresses: [email protected]; [email protected]

Abstract The behavior of ionic liquids trihexyl(tetradecyl)phosphonium bis(trifluoromethylsulfonyl)imide and trihexyl(tetradecyl)phosphonium dicyanamide, [P6 6 6 14][Ntf2] and [P6 6 6 14][N(CN)2], at the water−air interface was investigated using the Langmuir trough technique. The obtained surface pressure versus mean molecular area (MMA) isotherms, π−A, and surface potential versus MMA isotherms, ∆V−A, show distinct interfacial behaviour between the two systems. The results were interpreted at a molecular level using Molecular Dynamics simulations: the different compression regimes along the [P6 6 6 14][Ntf2] isotherm correspond to the self-organization of the ions at the water surface into compact and planar monolayers that coalesce at a MMA value of ca. 1.85 nm2/ion-pair to form an expanded liquid-like layer. Upon further compression the monolayer collapses at around 1.2 nm2/ion-pair to yield a progressively thicker and less organized layer. These transitions are much more subdued in the [P6 6 6 14][N(CN)2] system due to the more hydrophilic nature of the dicyanamide anion. The numerical density profiles obtained from the MD simulation trajectories are also able to emphasize the very unusual packing of the four long alkyl side chains of the cation above and below the ionic layer that forms at the water surface. Such distribution is also different for the two studied systems during the different compression regimes.

1. Introduction Ionic liquids (ILs) are organic molten salts that generally combine bulky asymmetric cations with anions with a delocalized negative charge. These structural characteristics prevent an efficient crystal packing, reduce the intensity of the inter-ionic interactions and, consequently, depress the melting point temperature of this class of substance.

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Most of the distinctive physical and chemical properties of ILs, such as negligible vapour pressure, relatively high ionic conductivity, large chemical and thermal stability ranges, non-flammability, and unusual solvation characteristics can be tailored by careful selection and tuning of the constituent ions. Such versatility makes these materials very important in different areas of the chemical sciences and several industrial applications have been already implemented [1,2,3]. The incorporation of functional groups —by grafting preexisting groups onto the structure of one of the ions— allows these unique salts to act not only as solvents but also as catalysts or reactants in a range of synthesis, separations, electrochemical applications and biomedical processes. For example, some ionic liquids are currently listed as new anti-cancer agents [4,5]. Conversely, several experimental studies have indicated that the toxicity of ionic liquids seems to be directly correlated to the length of their alkyl side chains (generally present in the cations). The increase of those alkyl side chains also increases the hydrophobicity of the corresponding ionic liquids. Ionic liquids with long alkyl side chains are structurally similar to traditional surfactants and the potential amphiphilic behavior of ionic liquids has become an important area of research [6,7]. Davis and co-workers [8] found that ionic liquids comprising imidazolium cations with fluorinated chains can function as emulsion agents. Apart from studies on the self-aggregation behavior of ILs in aqueous solution [9], studies at interfaces have recently attracted much attention. For example, ionic-liquid membranes have been suggested as effective media for the separation of volatile organic compounds at the air–liquid interface. Other recent work in this area also explore the possibility of using hydrophobic ILs in electrochemical applications and metal ion extractions [10,11] The knowledge of the structure and packing of ionic liquids at an interface is therefore of utmost importance for the rational design of such media. A key aspect in the design and preparation of functional materials is the control of intermolecular interactions and molecular packing in the first few monolayers at the surface [7]. The air–liquid interface is an ideal model surface which is easy to prepare in a pure and wellcharacterized state and whose surface coverage can be smoothly adjusted using the Langmuir trough technique.


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In this work we report the air–water interfacial behavior of two ionic liquids constituted by a common trihexyl(tetradecyl)phosphonium cation, [P6 6 6 14]+, combined with either the bis(trifluoromethylsulfonyl)imide, [Ntf2] –, or dicyanamide, [N(CN)2]–, anions (cf. Scheme 1).

Scheme 1. Molecular structures of the three ion types composing the ionic liquids discussed in this work: (a) +

–

trihexyl(tetradecyl)phosphonium cation, [P6 6 6 14] ; (b) dicyanamide anion, [N(CN)2] ; (c) bis(trifluoromethylsulfonyl)imide, [Ntf2]–. The acronyms next to selected atoms follow the nomenclature/color scheme used in Figs. 2 to 4.

2. Experimental 2.1. Materials Ttrihexyl(tetradecyl)phosphonium bis(trifluoromethylsulfonyl)imide [P6 6 6 14][Ntf2] was supplied by IOLITEC with a stated purity greater than 99%. Trihexyl(tetradecyl)phosphonium dicyanamide, [P6 6 6 14][N(CN)2] was prepared at the Centro de Física Molecular (CFM/IST/UL) and at the Institute of Nanosciences and Nanotechnology (IN/IST/UL). The corresponding synthesis, purification and characterization are described elsewhere [12]. The solvents, chloroform from Fluka (puriss. P.a. grade, ≥ 99.8%) and methanol from Aldrich (Chromasolv, ≥ 99.9%), were used as purchased. The ultra-pure water used in the subphase was distilled and purified with the Millipore Milli-Q system (pH 5.6, resistivity ≥ 18.2 M Ω cm). Stock solutions of ILs (0.5-1.0 mM) were prepared in chloroform. 2.2. Surface Pressure−Area and Surface Potential−Area measurements. Surface pressure-area (π−A) isotherms were carried out on a KSV 5000 Langmuir-Blodgett system (KSV instruments, Helsinki) installed in a laminar flow hood. Procedures for π−A measurements and cleaning were described elsewhere [13]. During each measurement, a precise volume of ionic liquid solution was spread on the pure water subphase with a SGE gas-tight micro-syringe. After


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evaporation of the solvent, the floating layer on the subphase was symmetrically compressed by two barriers at constant speed of 5 mm min-1. The temperature of the subphase was maintained by a circulating water bath (±0.1 °C). The π−A isotherm does not change with the concentration of the spreading solutions in the working interval of concentrations. Triplicate measurements were obtained to check reproducibility (cf. Figs S1 and S2 of the Supplementary Information). Results are presented in the usual form of surface pressure per mean molecular area (MMA) values, which in the present case corresponds, obviously, to the surface area occupied by one ionic liquid ion pair. The surface potential was measured in the Langmuir trough using the vibrating plane method (Kelvin probe, non-contact vibrating plate capacitor method). The computer-driven measuring probe was supplied by KSV Instruments. The surface potential of the monolayers was determined relative to the surface potential of the pure water subphase. A constant distance between the vibrating electrode and the subphase of 2.0±0.5mm was maintained. Details of the experimental procedure are given elsewhere [14]. 2.3. Simulation details The experimental data were interpreted from a molecular perspective using simulation results. Molecular dynamics runs at 298 K were carried out using the DL_POLY package [15]. Water and all ionic liquids were modeled using, respectively, the SPC model [16] and a previously described allatom force field (CL&P) [17,18,19], which is based on the OPLS-AA framework [20] but was to a large extent developed specifically to encompass entire ionic liquid families like [P6 6 6 14][Ntf2] and [P6 6 6 14][N(CN)2]. All simulations were performed using non-cubic simulation boxes (4.8 x 4.8 x 16.0 nm dimensions) filled with 2400 water molecules forming an aqueous layer with two water-vacuum interfaces (4.8 x 4.8 x 3.1 nm dimensions). Then, for each type of ionic liquid, 12, 16 or 24 ion pairs were added to each interface using the Packmol software [21]. These correspond to mean molecular area values of 1.92, 1.44 and 0.96 nm2/ion pair. The use of a water layer with two (symmetrical) surfaces is related to the restrictions imposed by the use of boundary conditions and minimum image convention procedures in all directions during the MD simulations. During the subsequent discussion (cf. below) only one of such surfaces is represented/discussed.


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The boxes were equilibrated under isothermal ensemble conditions for 1 ns at 298 K and 1 atm using the Nosé−Hoover thermostat with time constants of 0.5 ps. Several (at least ten) consecutive simulation runs of 1 ns each were used to produce equilibrated systems at the studied temperature. Electrostatic interactions were treated using the Ewald summation method considering six reciprocal-space vectors, and repulsive−dispersive interacƟons were explicitly calculated below a cutoff distance of 1.6 nm. Details concerning this type of simulation can be found elsewhere [17,18,19]. 3. Results and Discussion 3.1 Compression isotherms using the Langmuir trough technique. The solubility in water of the [P6 6 6 14][Ntf2] and [P6 6 6 14][N(CN)2] ionic liquids is extremely low due to the extensive alkyl side chains present in the [P6 6 6 14]+ ion and the nature of the two anions. Therefore, given their ionic character, both ionic liquids have the potential ability to form reproducible monolayers at the air-water interface. Representative surface pressure versus mean molecular area (MMA), π−A, isotherms for [P6 6 6 14][Ntf2] and [P6 6 6 14][N(CN)2] obtained on a pure water subphase at 298 K are shown in Figure 1a. The results show that distinct anions coupled to the same cation induce differentiated interfacial behaviors. The [P6 6 6 14][Ntf2] system isotherm (Fig 1a, blue line) shows an almost constant π value (around 0.5 mN m-1) between the initial MMA values of ca. 2.4 nm2 molecule-1 and MMA values of ca. 1.85 nm2 molecule-1 . At that point there is a lift-off of the surface pressure, with a noticeable increase until a quasi -plateau is reached at 12 mN m-1 and 1.2 nm2 molecule-1. Such increase of the surface pressure —low-compressibility regime— is compatible with the formation of a liquid-like organized monolayer (cf. discussion below). Upon compression beyond 1.2 nm2 molecule-1, the surface pressure slightly increases until MMA values of ca. 0.5 nm2 molecule-1 are reached. This quasi–plateau may be interpreted as the (partial) collapse of the monolayer.


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Fig. 1. Room-temperature isotherms of the [P6 6 6 14][Ntf2] and [P6 6 6 14][N(CN)2] systems at the air water− interface. (a) Surface Pressure versus Mean Molecular Area (MMA) data, π−A; (b1, b2) Compressional Modulus, (1/CS), versus MMA or π data; (c) Surface Potential, ∆V, versus MMA data.

On the other hand, the [P6 6 6 14][N(CN)2] system isotherm (Fig 1a, red line) shows π values around 2 mN m-1 at the MMA starting point of ca. 2.5 nm2 molecule-1. Upon compression, the surface pressure increases continuously up to values higher than 15 mN m-1 for MMA values around 0.5 nm2 molecule-1. This contrasts with the previous system in the sense there is no lift-off at a given MMA value, no low-compressibility region after such lift-off, nor a quasi-plateau marking the end of such region. The absence of such features indicates a low cohesion of the [P6 6 6 14][N(CN)2] ions at the surface and that their packing increases continuously during compression. The inflection point in the π(A) function at ca. 10 mN m-1 may suggest a second order phase transition. This can be more easily quantified in Figs. b1 and b2, which show the compressional modulus (inverse of the compressibility or Young modulus), (1/CS) = –A(∂π /∂A)T, as a function of MMA or π. The compressional modulus, (1/CS) is frequently used to characterize the monolayer state and to detect more clearly any phase transitions [13]. Values below 50 mN m-1 indicate that both ILs form liquid expanded monolayers. However, the maximum compressional modulus is higher for [P6 6 6 14][Ntf2] (32 mN m-1 at 7-8 mN m-1) than for [P6 6 6 14][N(CN)2] (14 mN m-1 at 6-7 mN m-1). For


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[P6 6 6 14][Ntf2], the (first order) phase transition is clearly detected by the abrupt decrease before the local minimum at 12 mN m-1, while for [P6 6 6 14][N(CN)2] a slight decrease occurs in the range 710 mN m-1 followed by a nearly constant compressional modulus at π >10 mN m-1. The surface potential, ∆V−A, isotherms (Fig. 1c) for the [P6 6 6 14][Ntf2] and [P6 6 6 14][N(CN)2] systems (blue and red lines, respectively) are also very different from each other. For the [P6 6 6 14][Ntf2] system, the surface potential continuously increases between 320 and 520 mV over the whole compression range. On the other hand, the [P6 6 6 14][N(CN)2] system isotherm starts at higher values of surface potential (570 mV) and upon compression it increases until a maximum is reached at a MMA of ca. 0.8 nm2 molecule-1. It does not vary significantly between 1.0 and 0.5 nm2 molecule-1. These results show that the surface potential gradually increases during the first order phase transition of the [P6 6 6 14][Ntf2] monolayer, while it does not change significantly beyond the possible second order phase transition of the [P6 6 6 14][N(CN)2] monolayer. As the surface potential is proportional to the perpendicular component of dipole moments (µ⊥) [22], the plateau in the ∆V−A isotherm beyond the second order phase transition of the [P6 6 6 14][N(CN)2] monolayer indicates that further compression does not change the orientation of dipoles. 3.2 Probing interfacial structure using Molecular Dynamics simulation. To understand from a molecular point of view the interfacial behavior of the [P6 6 6 14][Ntf2] and [P6 6 6 14][N(CN)2] systems we have performed a series of Molecular Dynamics simulations under selected MMA conditions. Ionic Liquid domains at the [P6 6 6 14][Ntf2]-water interface Figure 2 shows a series of simulation snapshots of the with MMA values corresponding to 1.92, 1.44 and 0.96 nm2 molecule-1 (Figs. 2a, 2b and 2c, respectively). These MMA values were obtained using 12, 16 and 24 ion pairs at the water−vacuum interface of simulation boxes with a cross section in the direction normal to the surface of 23.04 nm2. In terms of the [P6 6 6 14][Ntf2] system isotherm, these three MMA values correspond to points a) before the surface pressure lift-off; b) in the low compressibility region; and c) in the quasi-plateau.


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Fig. 2. Molecular Dynamics simulation snapshots of [P6 6 6 14][Ntf2] systems with surface MMA values of (a) 1.92; (b) 2

1.44; and (c) 0.96 nm /(ion pair). The two top rows show views from the top of the surfaces; the bottom row shows surface profiles. The top row highlights the positions of the anions (red) and the central charged cores of the cation (blue).

The different trend shifts observed in the [P6 6 6 14][Ntf2] system isotherm can now be semiquantitatively related to structural changes occuring at the surface: i) for the 1.92 nm2 molecule-1 MMA value the ionic liquid ions form well-defined domains at the water surface that do not percolate the entire surface. In other words, ionic liquid monolayer islands are dispersed at the surface of a continuous layer of superficial water molecules. This fact is exemplified by the snapshots of Fig. 2a where a water “channel” completely separates two ionic liquid domains. This means that upon compression one is simply moving these domains closer to each other and the superficial pressure to make such compression is not very different from that of pure water —hence the relatively small values of π (ca. 0.5 mN m-1) before the lift-off;


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ii) at a 1.44 nm2 molecule-1 MMA value, the amount of ion pairs at the surface is already sufficient to produce a continuous ionic liquid monolayer. The few water molecules in direct contact with the vacuum interface now form isolated domains dispersed in a continuous ionic liquid surface layer. Fig. 2b shows examples of small pools of superficial water molecules. The percolation threshold of the ionic liquid layer must correspond to the MMA value of the lift-off observed in the [P6 6 6 14][Ntf2] system isotherm. The low-compressibility regime found between the lift-off MMA value and the beginning of the quasi-plateau corresponds to the compression of this continuous ionic liquid monolayer and the concomitant disappearance of the last superficial water pools; iii) for MMA values in the quasi-plateau (Fig. 2c) one can observe a continuous ionic liquid layer covering the whole liquid-vacuum interface. Such layer is thicker than the one observed in the previous case and suggests the reorganization of the ionic liquid monolayer into other types of structural arrangements (cf. discussion below). Numerical Density Profiles in [P6 6 6 14][Ntf2]-water systems One way to quantify in more detail the present discussion is to calculate the numerical density profiles of the different species present in the system as a function of the distance normal to the interface. Such functions are presented in Fig. 3a-c for the three [P6 6 6 14][Ntf2] systems with MMA values of 1.92, 1.44 and 0.96 nm2 molecule-1. The position of four types of atomic centre in the ionic liquid were selected to calculate the numerical density profiles: the phosphorus atom, P3, representing the charged core of the tetra-alkylphosphonium cation (blue lines); the nitrogen atom of the anion, NBT, (red lines) ; the carbon atom, CTA, at the end of the tetradecyl chain of the cations (black lines); and the three carbon atoms, CT, at the end of the hexyl chains of the cation (gray lines). The density profile of the water molecules (the oxygen atom of water was selected as the probe atom) was also computed (light blue lines).


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Fig. 3. Numerical and charge density profiles along the direction normal to the water-vacuum surface for [P6 6 6 14][Ntf2] systems with surface MMA values of (a) 1.92; (b) 1.44; and (c) 0.96 nm2/(ion pair). Anion (NBT atom) densities as red curves; Cation charged core densities (P3) as blue lines; CT (atom of the hexyl chains of the cation) densities as gray curves; CTA (atom of the tetradecyl chain of the cation) densities as black curves; water oxygen atom densities as light blue lines (water densities are scaled 1/100). Charge densities are represented as orange lines. The shaded area below each red curve corresponds to the integration of the numerical density function, yielding the inverse of the corresponding MMA value.

The profiles show that for two systems with larger MMA, the charged parts of the ions occupy a narrow Gaussian-like band that is compatible with the existence of a charged monolayer occupied by both types of ion. Integration of the numerical density functions yields the inverse of the MMA values (shown in the graphs as the shaded red areas representing the integration of the anion density profile functions). Fig. 3a and 3b show that when the MMA values decrease from 1.92 to 1.44 nm2 molecule-1 the density profile functions of the ions at the surface increase in intensity (due to the lower MMA values) but their trends remain very similar: ionic monolayers exist for both surface density values, covering a more extensive area and percolating the surface in the case of the system with a MMA of 1.44 nm2 molecule-1. The profiles also show another very interesting feature that is not so common in traditional amphiphilic molecules/ions: due to the existence of four long alkyl side chains in the cation, the atoms closer to the water surface do not


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belong to the charged parts of the ions (P3 atom of the cation or NBT atom of the anion) but rather to one (or two) of the hexyl side chains (cf. Fig. 3a and 3b, gray lines). Since we have differentiated the terminal carbon atoms of the three hexyl and the tetradecyl chains, it is possible to see that the cations always manage to point the longer alkyl side chain (tetradecyl) away from the water surface, while “sinking” one or two of the hexyl chains below the ionic layer (cf. Fig. 3a and 3b, black and gray lines). Finally, the profiles also show that even at the z-value of the ionic layer there is still a non-negligible proportion of alkyl side chains. This can be visualized in the snapshots of the top and middle rows of Fig. 2, that show that although the surface is covered in different proportions by the ionic liquid ions (middle row), the charged parts within such ionic liquid layer are far from covering the surface in a complete or homogeneous way (top row). These facts are a consequence of the rather small volume fraction occupied by the polar parts of the [P6 6 6 14][Ntf2] ionic liquid, relative to the volume occupied by its non-polar moieties (the four alkyl side chains). The top row of Fig. 2 also shows that the charged parts of the two ions are always intercalated with each other forming an effective planar polar network of alternating ionscounter-ions at the water surface. The surface pressure liftoff is thus possibly associated with the percolation of the entire surface by such network, even with non-polar “holes” in its midst. For a MMA of 0.96 nm2 molecule-1 the density profile functions of Fig. 3c show a quite distinct trend: the profiles get broader and with more secondary peaks/shoulders. The thickness of the ionic liquid layer at the water interface is also larger. This is compatible with the progressive destabilization of the planar ionic liquid monolayer. The snapshot at the top of Fig. 2c shows that the charged parts of the ions are no longer forming a planar network but are in some cases overlapping each other in the z direction. The rupture of the ionic liquid monolayer and the formation of a thicker ionic liquid layer explains the occurrence of the quasi-plateau. This findings, namely the transformation of the polar film at the surface of the water sub-phase into a thicker and stratified IL layer for systems with lower MMA values, is consistent with the results obtained by Kakiuchi et al. for the free surface of tetra-alkylammonium-based IL systems. [11] Charge density profiles in [P6 6 6 14][Ntf2]-water systems The last piece of information conveyed by Fig. 3 corresponds to the charge density energy profiles along the direction normal to the water surface (orange lines). These were computed only for the


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charges contained in the ionic liquid ions (the charge distribution within the water molecules was not taken into account). The charge density graph in Fig. 3a shows that the charge distribution in the direction normal to the plane of the ionic monolayer is not completely homogeneous: as one follows the surface normal from the water phase to the vacuum, one encounters first a slightly negative layer, followed by a strong positive peak, a equally strong negative counter-peak and finally a slightly positive region. The fact that the overall charge of each succeeding plane normal to the surface does not completely cancel out in terms of the charges contained in the corresponding ionic liquid ions can also be inferred by the slight shifts between the density profiles of the charged parts of the cation (blue lines) and the anions (red lines). It must be stressed that the charge fluctuations are quite small (a few µacu/nm3) but these will nevertheless induce a dipole moment at the surface which in turn can be correlated to the measured surface potential isotherms. The increase of those fluctuations between the systems with 1.92 and 1.44 nm2 molecule-1 MMA values corresponds in fact to the increase observed in the ∆V values (0.38 and 0.44 V, respectively, cf. Fig. 1c). However such correlation seems to break down if one considers the systems with 1.44 and 0.96 nm2 molecule-1 MMA values: although the increase in the ∆V values after reaching the quasi-plateau is less pronounced (∆V values of 0.44 and 0.46 V at MMA values of 1.44 and 0.96 nm2 molecule-1, respectively) this does not seem to correlate with the much smaller and numerous charge-density fluctuations observed for the system at a MMA value of 0.96 nm2 molecule-1 (cf. Fig. 3c, orange line), relative to the fluctuations at the 0.96 nm2 molecule-1 MMA value (cf. Fig. 3b, orange line). In fact, the two pieces of information can be easily reconciled if one takes into account that at the lower MMA values in the quasi-plateau one has no longer a planar ionic liquid monolayer. This means that the smaller charge fluctuations can be just a consequence of overall charge cancellation between ions located at different z-values. In other words, the correlation between charge density profiles and surface potential only holds when the ions are arranged in a more-or-less planar ionic monolayer. The subdued behavior/ structure of the [P6 6 6 14][N(CN)2] systems MD simulations were also conducted in the [P6 6 6 14][N(CN)2] systems with similar MMA values. In this case the differences between the pre-lift-off, low-compressibility and quasi-plateau regimes are much more subdued (cf. Fig 1a and 1b), which means that we have decided to discuss the obtained MD data only after the complete analysis of the corresponding data for the [P6 6 6 14][Ntf2] system in the previous sub-sections. Simulation snapshots and numerical and charge density


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profiles similar to those presented in Figs. 2 and 3 are given as Supplementary Information (Figs. S3 and S4). Figure 4 shows just one representative simulation snapshot and a density profile for the simulated [P6 6 6 14][N(CN)2] system with 1.44 nm2 molecule-1 MMA.

Fig. 4. Molecular Dynamics simulation snapshot and numerical and charge density profiles along the direction normal to the water-vacuum surface for the [P6 6 6 14][N(CN)2] system with a surface MMA value of 1.44 nm2/(ion pair). Color schemes as in Figs. 2 and 3.

The main difference between the [P6 6 6 14][Ntf2] and [P6 6 6 14][N(CN)2] systems lies in the hydrophobicity of the corresponding anions (ionic liquids based on the [Ntf2] anion are generally much more hydrophobic than the corresponding ionic liquids based on the [N(CN)2] anion). Such difference is easily appreciated in the snapshot and density profiles of Fig. 4: the probability of finding a [N(CN)2] anion in the bulk of the water subphase is not negligible (red line in the bottom graph of Fig. 4), a situation that never occurred for the [P6 6 6 14][Ntf2] system. This causes a structural shift in terms of the distribution of the [P6 6 6 14]+ ions at the water−vacuum interface. The bulky cations will never dissolve into the water subphase which means that the absence of part of their counter-ions will cause a less structured polar network and even partial repulsion between cation cores. The result is an ionic liquid layer that is less compact —one can observe


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more superficial water interspersed with the ionic liquid ions, cf. Fig 4. This means that for high MMA values, instead of having compact ionic-liquid monolayer regions dispersed at the water surface (surface pressure at the starting point of the [P6 6 6 14][Ntf2] system is ca. 0.5 mN m-1), one will obtain a less compact film with larger alkyl side chain coverage which will decrease the surface tension (surface pressure at the starting point of the [P6 6 6 14][N(CN)2] system is ca. 2 mN m-1) and contribute to the gradual increase of the surface pressure with decreasing MMA. The ions and water density profiles also point to a less organized and compact polar layer: the two ions distributions are almost superimposed in the [P6 6 6 14][Ntf2]-based systems (Fig. 3) and less so in the [P6 6 6 14][N(CN)2]-based systems, with the anions closer to the water subphase (Fig. 4); the water density profile also shows an increase near the shifted anion peak in the latter case, indicating the possible partial solvation of [N(CN)2] anions by water molecules even when the ions are included in the polar layer at the top of the water sub-phase. It must be stressed that the two systems are not entirely unalike since some of the trend shifts clearly observed in the [P6 6 6 14][Ntf2] system isotherms are still noticeable in the [P6 6 6 14][N(CN)2] isotherm, specially if one takes the area derivative of the surface pressure values (Fig. 1b): the take-off at ca. 1.85 nm2 molecule-1 MMA in the former system is mirrored by a higher surface pressure increase rate starting at ca. 1.8 nm2 molecule-1 MMA in the latter system; likewise, the attainment of the quasi-plateau at ca. 1.2 nm2 molecule-1 MMA in the first system is echoed in the second system by a return of the surface pressure increase rate to lower values around 0.98 nm2 molecule-1. What is missing in the [P6 6 6 14][N(CN)2] system is the formation of well-defined and compact ionic monolayer domains that are able to percolate the surface at a given MMA value (the lift-off MMA) or will undergo a transition to a non-monolayer structure only after attaining relatively high surface pressure values. The charge profiles presented in Fig.4 also suggest a higher charge separation that corroborates the higher ∆V values experimentally observed for the [P6 6 6 14][N(CN)2] surface. The probability of finding [N(CN)2] anions in the water sub-phase for the 0.96 nm2 MMA system is much lower than that observed for the systems with higher MMA. It must be also stressed that the non-negligible density-profile values for [N(CN)2]-in-water correspond to the migration of just a few individual [N(CN)2] anions between the two simulated IL-water surfaces. The more effective sequestration of the [N(CN)2] anions in the polar layer of the 0.96 nm2 MMA system can be easily explained by the fact that in that case one has a fully formed polar network that is already evolving


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to a thick layer. This means that the [N(CN)2] anions are able to interact with more counter-ions (cf. snapshots of the first row of Fig. S3) and are thus less likely to migrate to the water sub-phase. Moreover, even if the IL-water partition of the [N(CN)2] anions was constant for all systems, the increased amount of IL relative to that of water would decrease the amount of [N(CN)2] present in the water sub-phase anyway. This trend shift between the two systems with higher MMA (a few [N(CN)2] anions in the water sub-phase) and that with the lower MMA (a much lower concentration of [N(CN)2] anions in the aqueous sub-phase) may also explain the observed constant value of the surface potential for low MMA values. Mukherjee et al. [6] reported the surface behavior at the air-water interface of a tetraalkylphosphonium-based ionic liquid combined with bis 2,4,4-(trimethylpentyl)phosphinate anion, [P6 6 6 14][P(iC8)2]. The obtained Langmuir isotherms also showed expanded monolayers with appreciable hysteresis. Under static conditions, the monolayers were only stable 3h after the spreading. These results show that the amphiphilic behavior of ILs with long alkyl chains in the cation is strongly dependent on the attached anion. 4. Conclusions In this work we have reported the air−water interfacial behavior of two ionic liquids constituted by a common trihexyl(tetradecyl)phosphonium cation, [P6 6 6 14]+, combined with either the bis(trifluoromethylsulfonyl)imide, [Ntf2] –, or dicyanamide, [N(CN)2]–, anions. The main objectives of this study were to investigate if this particularly long-chained cation would behave similarly to those of traditional cationic surfactants and to evaluate the counter-ion influence on the formation of possible monolayer structures. It was confirmed that the extensive alkyl side chains present in the [P6 6 6 14]+ ion confer to the corresponding ionic liquids extremely low values of solubility in water and the possibility of forming Langmuir monolayers at the air-water interface. The experimental π−A isotherm of [P6 6 6 14][Ntf2] system at 298 K exhibits three distinct regions: i) a plateau at very low surface pressures and large MMA values (the pre-liftoff regime); ii) a lowcompressibility region after the lift-off of the surface pressure; iii) and a quasi-plateau starting at 12 mN m-1. The low pressure plateau is ascribed to the coexistence regime of two phases, i.e., liquid-expanded (LE) islands of ionic planar monolayers dispersed in a continuous aqueous subphase at the air−water interface; the low-compressibility regime corresponds to the compression of a continuous LE ionic planar monolayer; and the quasi–plateau is due to the LE planar


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monolayer collapse, i.e., the formation of a more condensed and thicker film that includes ionic species at different distances from the water surface. The numerical density profiles calculated from Molecular Dynamics simulations trajectories at fixed MMA values of regimes i) and ii) show that the charged parts of the two ions are always intercalated with each other forming an effective planar polar network of alternating ions and counter-ions at the water surface. This suggests that the driving force behind the self-organization of the monolayer is electrostatic in nature. On the other hand, the density profile functions calculated at a MMA value of the iii) regime show that the charged parts of the ions are no longer forming a planar network and that the thickness of the corresponding ionic layer at the water interface is significantly larger than the thickness at the i) and ii) regimes. These findings confirm that a partial collapse of the ionic liquid monolayer occurs when the quasi-plateau is reached. The numerical density profiles also show a particular characteristic of the [P6 6 6 14]+ ions: due to the existence of four long alkyl side chains surrounding the charged center, at least one of the alkyl side chains (sometimes two of them) will be closer to the water molecules at the interface than the planar ionic monolayer formed by the charged parts of the ions. This rare arrangement is probably due to steric hindrance effects that prevent the simultaneous arrangement of all alkyl side chains away from the water surface. Nevertheless, the longer tetradecyl chains always manage to stay away from the water surface, at least during regimes i) and ii). After the collapse of the planar monolayer, the less organized ionic layer exhibits some tetradecyl chains oriented towards the water subphase. The experimental π−A isotherm of [P6 6 6 14][N(CN)2] system at 25°C is quite distinct from that one obtained for the [P6 6 6 14][Ntf2] system. During the compression of the monolayer, the surface pressure increases continuously and the three regimes described in the previous system are not clearly visible. Instead of a quasi-plateau there is an inflection point in the π(A) function (at ca. 10 mN m-1) suggesting a second order phase transition. Such trend indicates a low cohesion system composed of loosely packed [P6 6 6 14][N(CN)2] ions at the water surface and explains the continuously increase of the surface density during compression. The numerical density profiles calculated at the same MMA selected for the previous system confirm that the main differences between the [P6 6 6 14][Ntf2] and [P6 6 6 14][N(CN)2] systems are a consequence of the different hydrophobicity of the corresponding anions. In fact, the probability


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of finding the [N(CN)2]– anion in the bulk of the water subphase is not negligible (a situation that never occurred for the [P6 6 6 14][Ntf2] system). This state of affairs precludes the formation of a compact and planar monolayer and explains the gradual variation of the surface pressure even at large MMA values. The less compact and ordered monolayer also allows that three of the four alkyl chains of the cation orient themselves away of the water subphase, contributing to higher surface pressure values at large MMA values. The [P6 6 6 14][Ntf2] and [P6 6 6 14][N(CN)2] systems can be also distinguished via their charge-density profiles along the direction normal to the water surface and the MD results are compatible with the experimentally determined surface potential data. Overall, the Molecular Dynamics simulations are able to explain at a molecular level the type of organization at the air-water interface that leads to the dissimilar experimental π−A isotherms observed for the two types of studied IL-water systems. Other water-immiscible ionic liquids with distinct morphologies (nature of the cation or anion charged core, number and position of the alkyl side chains) are also possible. Future studies combining experimental techniques and MD modeling will certainly allow further discoveries in this field. Acknowledgements Financial support provided by Fundação para a Ciência e Tecnologia (FCT) through projects FCTANR/CTM-NAN/0135/2012, PTDC/CTM-NAN/121274/2010, UID/QUI/00100/2013, PEstOE/QUI/UI0100/2013 and PTDC/EQU-EPR/103505/2008. K.S. acknowledges postdoctoral grant SFRH/BPD/94291/2013. The authors would also like to acknowledge the generous contribution of a purified sample of [P6 6 6 14][N(CN)2] by Professor J. A. Fareleira. The ionic liquid was synthesized and characterized by Carolina S. Marques and Professor Carlos A. M. Afonso under FCT-funded Project PTDC/QUI/66826/2006.

Supporting Information Available The SI includes: Extra experimental isotherms for the [P6 6 6 14][Ntf2] and [P6 6 6 14][N(CN)2] systems (Figs. S1 and S2, respectively); Molecular Dynamics simulation snapshots for [P6 6 6 14][N(CN)2]


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systems (Fig. S3); and Numerical and charge density profiles for [P6 6 6 14][N(CN)2] systems (Fig. S4). This information is available free of charge via the Internet at http://pubs.acs.org/.

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Ionic Liquid Films at the Water–Air Interface: Langmuir Isotherms of

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