Effects of constituent ions of a phosphonium-based ionic liquid on molecular organization of H2O as probed by 1-propanol tetrabutylphosphonium and trifluoroacetate ions

Aqueous solutions of tetrabutylphosphonium trifluoroacetate, [P 4444 ]CF 3 COO, exhibit a liquid–liquid phase transition with a lower critical solution temperature. Herein, we characterized the constituent ions, [P 4444 ] + and CF 3 COO (cid:2) , in terms of their effects on the molecular organization of H 2 O on the basis of 1-propanol probing methodology devised by Koga et al. The resulting characterization of the hydrophobicity/hydrophilicity is displayed on a two-dimensional map together with previous results, for a total of four cations and nine anions of typical ionic liquid (IL) constituents. The results indicate that [P 4444 ] + is the most significant amphiphile with strong hydrophobic and equally strong hydrophilic contributions among the group of constituent cations of ILs studied so far. The hydration number for [P 4444 ] + was evaluated to be n H = 72, which is three times larger than that of a typical imidazolium-based cation, [C 4 mim] + . Self-aggregation of [P 4444 ] + was found to occur in an aqueous solution of [P 4444 ]CF 3 COO above 0.0080 mole fraction of the IL.


Introduction
][3][4][5] ILs consisting entirely of ions have been increasingly proposed as alternatives to conventional organic solvents.][8][9][10][11][12] In particular, thermoresponsive phase behaviour of aqueous solutions of ILs has been extensively investigated in relation to phase transitions with lower critical solution temperatures (LCSTs) 6,12,13 as well as upper critical solution temperatures (UCSTs).13a,c,d,m, 14 A UCST behaviour is understandable; a strong attraction in terms of enthalpy between species would cause phase separation at low temperatures, while at higher temperatures the total entropic effect drives the system to a random mixture.This transition is caused because the mixing entropic contribution in the mixing Gibbs energy generally becomes more dominant as the temperature (T) increases.Since the enthalpy-entropy compensation is operative particularly in aqueous solutions, the temperature could tip the balance of D mix H and TD mix S terms, where D mix H and D mix S are the change of the mixing enthalpy and entropy, respectively.However, an LCST behaviour is not as simple to explain.The present aqueous solution of [P 4444 ]CF 3 COO exhibits LCST-type phase behaviour with the critical point at 29.2 1C and 0.025 mole fraction of the IL. 7The chemical structure of [P 4444 ]CF 3 COO is shown in Fig. 1.
It has been suggested that LCST behaviour depends on the balance of hydrophilicity and hydrophobicity of the IL constituent ions.Kohno et al. 15 reported that LCST-type phase behaviours of aqueous solutions of ILs strongly depend on the ''hydrophilicity'' of each constituent cation and anion.Their analysis was based on a one-dimensional scale with hydrophilicity and hydrophobicity at the extreme ends.The miscibility of imidazolium-based ILs with water was evaluated from estimated partition coefficients in octanol-water systems. 16,17The origin of LCST-and UCST-type phase transitions of mixtures of thiophene with two ILs, [C 4 C 1 mim]SCN and [C 4 C 1 mim][NTf 2 ], was investigated using NMR spectroscopy and molecular dynamics simulations by Batista et al. 18 Koga et al. [19][20][21] devised a method by which an individual ion can be characterized in terms of a two-dimensional (2D) hydrophilicity and hydrophobicity scale.This technique is known as the 1-propanol (1P)-probing methodology.By this method, ''amphiphiles'' with hydrophobic and hydrophilic contributions can be quantitatively assessed.Here, we applied this method to characterize the constituent ions of [P 4444 ]CF 3 COO to seek a deeper insight.
A differential thermodynamic approach to characterize the effects of a solute on the molecular organization of H 2 O was devised by Koga et al. 19 They realized that aqueous solutions consist of three composition regions in each of which the mixing scenario on the molecular level is qualitatively different.The mixing scenario is identified by the term ''mixing scheme'' instead of solution structure, since the word ''structure'' implies a stable, ordered molecular arrangement.The mixing schemes are labelled as I, II or III from the H 2 O-rich regions.In Mixing Scheme I, the hydrogen bonds of H 2 O are bond-percolated throughout the entire bulk of H 2 O.The transition to Mixing Scheme II from I is regarded as a loss of bond percolation when the hydrogen bond probability is reduced to the percolation threshold.The 1P-probing methodology is applicable only in the region of Mixing Scheme I.
Although the 1P-probing methodology was described in detail elsewhere, 19,20 a brief description is given here.This methodology is based on the finding that, within Mixing Scheme I, the effects of separate solutes on H 2 O are additive. 19Thus, we study a ternary system, 1P-S-H 2 O, where S is the test sample whose effect on H 2 O is being examined.In the ternary system of 1P-S-H 2 O, the thermodynamic signature of 1P, H E 1P1P (defined below), is evaluated.Modification to its x 1P -dependence pattern due to the presence of S is used to characterize the effect of S on H 2 O. H E 1P1P is the enthalpic 1P-1P interaction in a complex ternary system and is defined as, where and H E is the total excess enthalpy in the ( p, T, n 1P , n S , n W ) variable system, N = n 1P + n S + n W and x 1P = n 1P /N.The subscripts 1P, S and W signify 1-propanol, test sample S and H 2 O, respectively.We directly measure the excess partial molar enthalpy of 1P, H E 1P .To do so, we perturb the ternary system by increasing n 1P to n 1P + dn 1P , with a fixed initial mole fraction of S, x 0 S = n S /(n S + n W ), and measure the H E response, dH E .We approximate the quotient dH E /dn 1P to the partial derivative in eqn (2), and evaluate H E 1P1P by differentiating the data set (x 1P , H E 1P ) using the far right of eqn (1).Fig. 2 shows a schematic representation of the changes in the H E 1P1P pattern induced by the presence of various classes of solute, S. As shown in Fig. 2(a)-(d), the peak top is named point X, which marks the end of Mixing Scheme I. Upon addition of S, point X shifts depending on the nature of S. The shifts are in general linear to the initial mole fraction of S, x 0 S , within Mixing Scheme I.The slope of the westward shift (i.e. to the negative direction of x 1P ) of point X per unit increase in x 0 S is taken as the hydrophobicity index, while that towards the south (i.e. to the negative direction of the H E 1P1P axis) is the hydrophilicity index.Fig. 2(a) shows a typical change in H E 1P1P for hydration centres such as in Na + and Cl À ion pairs. 22,23Upon addition of the salt, the H E 1P1P pattern is compressed to the west.This compression indicates that, upon addition of the Na + and Cl À ion pair, the available H 2 O molecules for 1P to interact with are reduced.We thus interpret this westward shift to be due to hydration of the ions.From the westward shift of point X as a function of x 0 S , the hydration number, n H , can be evaluated for Na + and Cl À .It must be stressed that at the starting point, x 1P = 0, the value of H E 1P1P remains the same even in the presence of Na + and Cl À .This invariance and the fact that the value of H E 1P1P at point X also remains the same led to the following suggestions: (1) Na + and Cl À ions are hydrated by a number of H 2 O molecules, (2) the hydrating H 2 O molecules are unavailable to interact with 1P and (3) the bulk H 2 O molecules away from the hydration shells are unperturbed by Na + and Cl À even in the presence of the ions.This finding can be utilized to characterize individual ions.For a given test ion, we chose the counter ion Na + or Cl À and applied the 1P probing on the combined salt.Namely, we used tetrabutylphosphonium chloride, [P 4444 ]Cl, and sodium trifluoroacetate, NaCF 3 COO, to characterize [P 4444 ] + and CF 3 COO À , respectively.Fig. 2(b) shows the behaviour of the hydrophobic solute.For a solute almost equally hydrophobic as 1P such as 2-propanol 24 the H E 1P1P pattern shifts parallel to the west, as shown in the figure.This parallel shift is expected when compared to the case in which 1P was added as S to x 0 S (i.e. S = 1P).For a hydrophobe that is stronger (or weaker) than the probing 1P, such as tert-butanol 25 (or ethanol 26 ), the westward shift is greater (or lesser), reflecting a larger (or smaller) n H .This westward shift of point X indicates that a hydrophobe is hydrated by H 2 O molecules, making them unavailable to interact with 1P, as was the case for the hydration centre.In addition, the value of H E 1P1P at point X shifts northward (or southward), reflecting the fact that the ability of tertbutanol (or ethanol) to reduce the hydrogen bond probability of bulk H 2 O away from the hydration shell is greater (or lesser) than 1P, as studied earlier. 19Furthermore, the hydrogen bonding within the hydration shells is more organized than it is in bulk H 2 O. Fig. 2(c) shows the pattern change for the hydrophilic solute.This finding, as well as others, 19 led to the interpretation that hydrophiles form hydrogen bonds directly to the existing hydrogen bond network of H 2 O and maintain the hydrogen bond connectivity of the network; thus, they act as impurities in the network.As such, they break the H donor/acceptor symmetry.Hence, the southward shift apparent in the figure is interpreted as a reduction of the net entropy-volume cross fluctuation.
Fig. 2(d) is for the amphiphilic solute.The effects seem to be a combination of those observed for hydrophobic and hydrophilic moieties.Their westward and southward components show contributions from hydrophobic and hydrophilic moieties, respectively.8][29][30] These results fit into the special properties of ILs.Low melting points for ionic compounds can be related to the strength of hydrophobicity and/or hydrophilicity. 20,30ith this pair of coordinates, the characterization of each species, including individual constituent ions, is displayed on a 2D map of hydrophobicity/hydrophilicity.

Titration calorimetry for 1P probing
Tetrabutylphosphonium chloride, [P 4444 ]Cl, (Aldrich, 498%) was used as the sample for characterization of the cation, [P 4444 ] + .Sodium trifluoroacetate, NaCF 3 COO, (Sigma-Aldrich, 496%) was used as the sample for that of CF 3 COO À .Deionized H 2 O was prepared using a Milli-Q system (Millipore), with a resulting resistivity of 18.2 MO cm.Stock solutions were prepared using the purified H 2 O and the salts from freshly opened bottles.1-Propanol (1P) (Sigma-Aldrich, Chromasolv for HPLC 99.9+%) was used as supplied.Due care was exercised not to contaminate the 1P with moisture from the atmosphere.2][33] A 1 mL stainless steel container was used as the cell.The initial volume of the sample solution was set at 750 mL and the exact amount was determined gravimetrically.The volume and duration of each titration of 1P were set at 10 mL and 10 s, respectively.The injection volume per titration was calibrated to 9.9483 mL and was used to calculate the concentration.The ratio of titrant over titrand was on the order of 10 À2 and was well within the acceptable range for approximation of the differentiation.The interval of injection was set at 30 min.The averaged uncertainty of the titration measurements was estimated to be AE0.03kJ mol À1 .

Electric conductivity
The IL, [P 4444 ]CF 3 COO, was synthesized and used for conductivity measurements.It was prepared by direct neutralization of aqueous tetrabutylphosphonium hydroxide solutions with trifluoroacetic acid.After evaporation, the product was added to a dichloromethane/water biphasic system and the resulting mixture was washed several times with distilled water.The required [P 4444 ]CF 3 COO was dissolved in dichloromethane phase, and was dried in vacuum for 24 h at 60 1C prior to use.The structure and purity were confirmed by 1 H NMR spectroscopy.The procedure of the synthesis has been described elsewhere in detail. 7he electric conductivity of the aqueous solution was measured using an electrical conduction metre with automatic temperature compensation (Hanna instruments, DiST4).The measured concentration range was from 0.001127 to 0.04610 mole fraction of the IL.

Results and discussion
The data of H E 1P are deposited as Table S1 of the ESI, † and are plotted in Fig. 3.A sigmoidal increase in H E 1P with an inflection point is evident as long as x 0 S is appropriate for performing the 1P-probing methodology.As shown in Fig. 3, the x 1Pdependence pattern of H E 1P changes more rapidly for S = [P 4444 ]Cl with increasing x 0 S than it does for S = NaCF 3 COO.The inflection point seems to disappear at x 0 S = 0.011 for S = [P 4444 ]Cl and at x 0 S = 0.055 for S = NaCF 3 COO.To see this behaviour more clearly, we took the derivative of H E 1P with respect to x 1P .

Evaluation of H E
1P1P from H E 1P data With a good set of raw H E 1P data with an averaged uncertainty of AE0.03 kJ mol À1 and small increments in x 1P , the raw H E 1P data themselves were used to calculate the derivative on the far right of eqn (1).In general, a numerical derivative directly using two neighbouring data points causes large noise, especially in applications with raw experimental data.In the present study, we originally utilized a differential method based on eqn (3) for calculation of the partial molar quantities with higher-order derivatives.We approximate the slope of the tangent at the i-th point as the weighted average of the slopes of the two adjacent arcs; one between the (i À 1)-th and i-th data points and another between the i-th and (i + 1)-th data points.
The x 1P interval of x 1P (i) to x 1P (i + 1), dx 1P , was approximately 0.003 in the present measurements.It is found that dx 1P E 0.008 in graphical evaluation normally exercised is appropriate to approximate the derivative of eqn (1) with the quotient dH E 1P /dn 1P 34 In the present calculations, the total dx 1P for three neighbouring data points was approximately 0.006.Certainty in evaluation of the partial molar quantities with higher-order derivatives is of particular importance in the present study.Fig. 4(a)-(c) show comparisons of H E 1P1P determined using eqn (3) and that determined using other evaluation methods.Fig. 4(a) shows comparison between the H E 1P1P pattern obtained using eqn (3) and that evaluated by the graphical method normally exercised.As shown in Fig. 4(b), combination of eqn (3) and the graphical evaluation shows improvement especially in the pre-peak region.The point X evaluated by the H E 1P1P pattern corresponds to each other regardless of the method.Fig. 4(c) shows comparison between eqn (3) and simple derivative using two neighbouring data points.As mentioned above, the calculation using eqn (3) is superior to the simple derivative using two alternating data points with regard to noise reduction, as shown in Fig. 4(c).Therefore, it is concluded that the calculation based on eqn (3) is the most certain for evaluation of partial molar quantities with higher-order derivatives in the present data set.Thus, calculation based on eqn (3) was utilized here for evaluation of H E 1P1P , leading to determination of the hydrophobicity and hydrophilicity indices.S .Diamonds represent H E 1P in dilute solution, as reported by Wadso et al. 35 This journal is © the Owner Societies 2015 at 0.049 of x 1P for binary (1-propanol-H 2 O) mixture. 19The peak top shifts to the west (direction of smaller x 1P ) as x 0 S increases.Point X nearly disappears at x 0 S = 0.0110 for S = [P 4444 ]Cl and x 0 S = 0.0550 for S = NaCF 3 COO, qualitatively indicating that the hydrophobicity of [P 4444 ]Cl is much larger than that of NaCF 3 COO.The starting point of H E 1P1P at x 1P = 0 shifts to the north (direction of larger H E 1P1P ) with increasing x 0 S .This northward shift also corresponds to the strong hydrophobicity of [P 4444 ] + .For NaCF 3 COO, the equivalent shift at x 1P = 0 is lesser.
The degrees of the shifts are plotted in Fig. 7 and 8.The slopes of x 1P loci shown in Fig. 7 1.
The results indicate that [P 4444 ] + is an amphiphile with strong hydrophobic and equally strong hydrophilic contributions.
[P 4444 ] + forms a large hydration shell with a hydration number of n H = 72, which is three times larger than that of typical imidazolium-based cation 1-butyl-3-methylimidazolium, [C 4 mim] + . 27he value of a hydration number for CF 3 COO À was found to be n H = 10.On the basis of earlier findings in 1P-probing studies on a series of carboxylates, 36 one H 2 O molecule out of the 10 H 2 O molecules should be used for hydration of the -COO À side and the remaining 9.0 H 2 O molecules for the fluoroalkyl group, -CF 3 .This hydration number is much larger than that of the alkyl group in CH 3 COO À ; the hydration number for -CH 3 in CH 3 COO À is estimated to be 2.7. 36The fact that the fluoroalkyl   group is a stronger hydrophobe than the alkyl group could be related to this finding; 37 an aqueous solution of the CF 3 COO À salt of [P 4444 ] + shows LCST, and that of the CH 3 COO À salt does not.

Self-aggregation behaviour of [P 4444 ] +
As shown in Fig. 7, the induced changes at x 0 S = 0.010 for S = [P 4444 ]Cl depart from the linear relationship defined in the lower x 0 S region.This departure shifts to the upper side beyond the data error in the present evaluation.This departure hints the self-aggregation of the kind found in imidazolium-based ILs.For [C 4 C 1 mim]Cl 29 and [C 4 mim]Cl, 27 similar plots are seen that show a break in the slopes at x 0 S = 0.006 and 0.014, respectively.For aqueous solutions of [C 4 mim]BF 4 , this similar behaviour was interpreted as self-aggregation of cations through NMR spectroscopy, 38 electric conductivity, [39][40][41] smallangle neutron scattering, 41,42 density, 43 speed of sound 43 and surface tension. 39,41Thus, we suggest that departure of the induced change at x 0 S = 0.010 for the case shown in Fig. 7 could also be caused by self-aggregation of [P 4444 ] + .
For further investigation of this issue, electric conductivity measurements were conducted for the aqueous solutions of [P 4444 ]CF 3 COO.The obtained conductivity was converted to molar conductivity according to the Kohlrausch empirical relation. 44Fig. 9 shows the isotherm of the molar conductivity as a function of the square root of IL concentration.The linear portion represents the usual behaviour of ionic solutions in the dilute region.This linearity seems to start to deviate at ffiffi ffi c p ¼ 19:6 (mmol L À1 ) 1/2 , which corresponds to a mole fraction of 0.0080.The fact that the conductivity shows deviation from the usual behaviour at x IL = 0.0080 suggests that [P 4444 ]CF 3 COO dissociates into its constituent ions in the dilute region and aggregation of [P 4444 ] + begins at a mole fraction of approximately 0.0080, as the case for imidazolium-based ILs.Considering the larger hydrophobicity (hydration number) of [P 4444 ] + compared to those of [C 4 mim] + and [C 4 C 1 mim] + , the breakpoint (or aggregation) of [P 4444 ] + might be expected at a much lower concentration; however, this was not observed.This discrepancy could be related to the difference in the aggregation mechanisms of imidazolium-based and phosphonium-based cations.The imidazolium ions have an ionic head and alkyl tail, while phosphonium-based ions have an ionic centre and four alkyl tails extending in random directions.Wang et al. suggested that the aggregation aspect of [P 4444 ]CF 3 COO in aqueous solution can be described in terms of microemulsion-like aggregation at the mesoscale level in higher concentration ranges near the critical point. 45 map of hydrophobicity/hydrophilicity Fig. 10 shows the 2D map of hydrophobicity/hydrophilicity for [P 4444 ] + and CF 3 COO À .In the map, H 2 O defines the origin (0,0).1-Propanol used as the probe is necessarily plotted at (À1,0).Relative to these points, ions with greater hydrophobic contributions spread to the west (negative horizontal direction)  and those with greater hydrophilic contributions spread to the south (negative vertical direction).A total of four cations and nine anions of typical IL constituents are plotted in Fig. 10.The cations 20,21  In the present 1P-probing methodology, the hydration number of the species in question is evaluated from ''hydrophobicity'' index.The hydrophobicity is defined on the basis of hydration of the solute with H 2 O molecules, namely the formation of the hydration shell in water-rich region.Hydrophile in the 1P probing is defined as a solute that forms hydrogen bonds directly to the existing hydrogen bond network of H 2 O, discussed in detail in Introduction.As shown in Fig. 10, the typical constituent ions are classified as ''amphiphiles'' with strong hydrophobic and equally strong hydrophilic contributions.Thus, the present a Hydration number evaluated from the loci of point X. b Aggregation observed in the region of Mixing Scheme I.
Fig. 9 The isotherm of electric conductivity for aqueous solutions of [P 4444 ]CF 3 COO based on the Kohlrausch empirical relation. 42The break point was estimated to be c 1/2 = 19.6 (mmol L À1 ) 1/2 for [P 4444 ]CF 3 COO, corresponding to a 0.0080 mole fraction of the IL.ii and S E ii , the third derivative of G E in the binary solute(i)-H 2 O system. 19,48lthough their argument was based on a necessary but not sufficient condition, it was applied to the [C 4 mim]BF 4 -H 2 O system and the H E ii and S E ii for i = [C 4 mim]BF 4 was found to be negative, which is appropriate for the UCST at 4 1C and x i = 0.07. 49For the present case, however, since we have not yet determined H E i and S E i for i = [P 4444 ]CF 3 COO, we will postpone the detailed discussions about the occurrence of LCST for aqueous solution of the present IL.As shown in Fig. 10, 2-butoxyethanol (BE) was found to form an analogously large hydration shell around the molecule, n H = 58, 23 and the aqueous solution of BE exhibits the LCST below 50 1C. 50,51

Fig. 1
Fig. 1 Chemical structure of tetrabutylphosphonium trifluoroacetate, [P 4444 ]CF 3 COO.Aqueous solution of the IL exhibits an LCST-type phase transition.In the present study, [P 4444 ]Cl and NaCF 3 COO were used for characterization of the individual constituent ions, [P 4444 ] + and CF 3 COO À , respectively.

Fig. 2
Fig. 2 Induced changes of H E 1P1P pattern in the presence of various types of solute S. [binary] represents the binary 1P-H 2 O system without test solute S. (a) Effects of the hydration centre on the pattern per unit increase in test sample S. Region of Mixing Scheme I is defined as x 1P = 0 to x 1P at point X, as described on the horizontal axis, (b) the effects of hydrophobes, (c) hydrophiles and (d) amphiphiles.

Fig. 3
Fig. 3 Excess partial molar enthalpy, H E 1P , of 1P in 1P-S-H 2 O at 25 1C and given initial salt concentration, x 0S .Diamonds represent H E 1P in dilute solution, as reported by Wadso et al.35 (a) and 8(a) yielded the hydrophobicity indices and hydration numbers, n H .The contribution from counter ions (Na + or Cl À ) was corrected by subtraction in the evaluation.The slopes of the H E 1P1P loci in Fig. 7(b) and 8(b) directly correspond to the hydrophilicity indices in the present evaluation.The hydrophilicity indices of the counter ions of Na + and Cl À used in the present study are equal to zero, since these ions are classified as the hydration centre.As mentioned in Introduction, hydration centres such as Na + and Cl À unperturb the bulk H 2 O molecules away from the hydration shells around the ions.Hydrophobicity/hydrophilicity indices and hydration numbers of [P 4444 ] + and CF 3 COO À are listed in Table

Fig. 4
Fig. 4 Comparison of H E 1P1P pattern for the binary (1-propanol-H 2 O) mixture calculated based on differentiation using eqn (3) (solid circles) with those using other evaluation methods (open circles); (a) typical graphical evaluation normally exercised for 1P-probing studies, 34 (b) combination of calculation using eqn (3) and graphical evaluation, (c) simple derivative using two data points with dx 1P E 0.006.

Fig. 5
Fig. 5 Enthalpic 1P-1P interaction, H E 1P1P , of 1P-[P 4444 ]Cl-H 2 O at 25 1C for given initial salt concentrations, x 0S .The initial concentrations, x 0 S , were set at 0, 0.001249, 0.002499, 0.005005, 0.01000 and 0.01100 from right to left in the figure.Mixing Scheme I is classified from x 1P = 0 to x 1P at point X.The region decreases with increasing x 0 S and nearly disappears at x 0 S E 0.011.Point X is largely compressed in the westward and southward directions.

Fig. 6
Fig. 6 Enthalpic 1P-1P interaction, H E 1P1P , of 1P-NaCF 3 COO-H 2 O at 25 1C for given initial salt concentrations, x 0 S .The initial concentrations, x 0 S , were set at 0, 0.005007, 0.01000, 0.02503, 0.04000 and 0.05496 from right to left in the figure.Mixing Scheme I decreases with increasing x 0 S and disappears around x 0 S E 0.055.Point X is compressed mainly in the westward direction.

Fig. 7
Fig. 7 Loci of point X as a function of the initial salt concentration, x 0 S , for S = [P 4444 ]Cl.(a) The loci of x 1P at 25 1C.The slope gives the hydrophobicity index and hydration number, n H .These parameters were evaluated from the slope following subtraction of the Cl À counterion contribution.(b) The loci of H E 1P1P at 25 1C.The slope corresponds to the hydrophilicity index, as Cl À is classified as the hydration centre.The hydrophilicity index of Cl À is zero.

Fig. 8
Fig. 8 Loci of point X as a function of initial salt concentration, x 0 S , for S = NaCF 3 COO.(a) The loci of x 1P at 25 1C.The slopes give the hydrophobicity index and hydration number, n H .These parameters were evaluated from the slope following subtraction of the Na + counterion contribution.(b) The loci of H E 1P1P at 25 1C.The slope corresponds to the hydrophilicity index, as Na + is classified as the hydration centre.The hydrophilicity index of Na + is zero.
are [C 2 mim] + , [C 4 mim] + , [C 4 C 1 mim] + and the present [P 4444 ] + .The anions20,21,30 are Cl À , CH 3 COO À , Br À , BF 4À , [OTf] À , PF 6 À , [NTf 2 ]À and the present CF 3 COO À .As evident from Fig.10, [P 4444 ] + shows the most significant amphiphilicity, with strong hydrophobic and equally strong hydrophilic contributions, among the constituent cations of the ILs studied.Furthermore, the IL constituent ions are located farther from the origin than are the typical inorganic small ions.Our recent study on normal ions (i.e.non-IL forming ions) shows that anions are located farther from the origin than cations.46Kohno et al.15 took advantage of the phase separation of IL-H 2 O systems and estimated the H 2 O content in IL phase.They then determined the hydration number per mole of IL and defined it as the hydrophilicity of the IL.They concluded that the LCST behaviour strongly depended on the evaluated ''hydrophilicity'' indices.We point out that their hydrophilicity concept is based on the H 2 O content in the IL-rich situation.In the 1P probing, hydrophilicity is only a part of the effects of the ion in question to H 2 O in the water-rich region, the other part being hydrophobicity.Saita et al.47 suggested another onedimensional scale based on salt effects of test ions fixed as [P 4444 ] + or CF 3 COO À counter ion on the LCST of [P 4444 ]CF 3 COOÀH 2 O system.It is concluded that changes in the critical temperatures of IL-H 2 O systems are clearly related to the ''hydrophilicity'' of the target ions.