Preferential Solvation of a Highly Medium Responsive

Article pubs.acs.org/JPCB

Preferential Solvation of a Highly Medium Responsive Pentacyanoferrate(II) Complex in Binary Solvent Mixtures: Understanding the Role of Dielectric Enrichment and the Specificity of Solute−Solvent Interactions Raffaello Papadakis* Department of Chemistry − Ångström, Uppsala University, Box 523, 751 20 Uppsala, Sweden S Supporting Information *

ABSTRACT: In this work, the preferential solvation of an intensely solvatochromic ferrocyanide(II) dye involving a 4,4′-bipyridine-based ligand was examined in various binary solvent mixtures. Its solvatochromic behavior was rationalized in terms of specific and nonspecific solute−solvent interactions. An exceptional case of solvatochromic inversion was observed when going from alcohol/water to amide/water mixtures. These effects were quantified using Onsager’s solvent polarity function. Furthermore, the sensitivity of the solvatochromism of the dye was determined using various solvatochromic parameters such as π* expressing the dipolarity/polarizability of solvents and α expressing the hydrogen-bond-donor acidity of solvents. This analysis was useful for the rationalization of the selective solvation phenomena occurring in the three types of alcohol/water and amide/water mixtures studied. Furthermore, two preferential solvation models were employed for the interpretation of the experimental spectral results in binary solvent mixtures, namely, the model of Suppan on dielectric enrichment [J. Chem. Soc. Faraday Trans. 1 1987, 83, 495−509] and the model of Bosch, Rosés, and co-workers [J. Chem. Soc., Perkin Trans. 2, 1995, 8, 1607−1615]. The first model successfully predicted the charge transfer energies of the dye in formamide/water and N-methylformamide/water mixtures, but in the case of MeOH/water mixtures, the prediction was less accurate because of the significant contribution of specific solute−solvent interactions in that case. The second model gave more insights for both specific solute−solvent as well as solvent−solvent interactions in the cybotactic region. The role of dielectric enrichment and specific interactions was discussed based on the findings.

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approaches are the dielectric enrichment model of Suppan,7−9 the preferential solvation model proposed by Bosch and Rosés,10−13 the Bagchi−Chatterjee model,14,15,4 the quasilattice quasi-chemical (QLQC) theory of preferential solvation of Marcus,4,16,17 etc. The first three models make use of spectrochemical data in order to provide quantitative predictions of the local solvent composition. The model of Marcus is one of the most prominent models based on thermodynamic studies.4 In this work, only models based on spectrochemical data, namely the model of Suppan and the model of Bosch and Rosés, are utilized for the rationalization of preferential solvation effects observed in the case of an intensely solvatochromic ferrocyanide(II) compound in various BSMs. The model of Suppan on dielectric enrichment is a widely used model on preferential solvation, which takes into account only nonspecific solute−solvent interactions. The model often fails to predict the local solvent compositions when hydrogenbonding interactions are important, and in those cases

INTRODUCTION The majority of chemical processes in nature as well as many of those utilized in industrial applications take place in solution.1,2 Medium polarity is a key parameter influencing chemical rates, chemical equilibria, as well as the spectra of compounds.1,3 In order to rationalize these effects and furthermore to predict their influence, solvatochromic compounds being able to probe medium polarity are widely used.1−3 One of the most significant questions is what happens when such a probe compound is dissolved in a binary solvent mixture (BSM).4−6 Today, it is well established that when a solute molecule is dissolved in a BSM, its interactions with each of the solvent components are different and thus preferential solvation of the solute molecules by one of the solvents typically occurs.4 Even though there are various theories describing this phenomenon, which in many cases result in very good quantitative approximations of the selective solvation, an intriguing milestone remains the rationalization of the preferential solvation of highly polar solutes in solvent mixtures of dipolar/polarizable solvents, which furthermore show significant aptitude to develop strong specific solvent−solvent as well as solute−solvent interactions.1 Among the most important © XXXX American Chemical Society

Received: June 10, 2016 Revised: August 12, 2016

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DOI: 10.1021/acs.jpcb.6b05868 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B

Figure 1. Limiting resonance structures of the medium responsive dye 1,34 and the structure of the solvatochromic betaine of Reichardt 2.

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modification of the model is required.8 The model of Bosch and Rosés is another model of preferential solvation that is proved to result in very good predictions of the spectral solvatochromic shifts observed in BSMs. It provides predictions of the local compositions of the two solvents of a BSM and furthermore predicts the composition of one-to-one solventcosolvent complexes in the cybotactic region, which can often influence the solvatochromism of medium responsive solutes. The latter model does not have special requirements or limitations.10−13 A general requirement in any case is the use of intensely medium responsive compounds as probes of the cybotactic region. Ferrocyanide(II) complex dyes involving electron withdrawing ligands such as 4,4′-bipyridine constitute a class of highly polar and intensely solvatochromic dyes, often utilized as chemical probes of dielectric and specific solute− solvent effects in neat solvents and BSMs.18−22 The strong HBA character of the −CN groups23−25 of ferrocyanide(II) along with the strong electron withdrawing cationic pyridinium segment utilized in various recent studies (both in free pyridinium-based cations as wells as in monoquat-ligated metal complexes)25−31 give rise to various interactions with the solvent molecules in solution. In this work, a previously synthesized solvatochromic ferrocyanide(II) complex involving a monoquat-based ligand (compound 1, Figure 1) was utilized as a potent chromic probe for the rationalization of the solute− solvent and solvent−solvent interactions in the vicinity of solvation region in aqueous BSMs (also known as cybotactic region). Three types of BSMs were used: MeOH/water, formamide (FA)/water, and N-methylformamide (NMF)/ water mixtures. Previously, the author and co-workers studied the solvatochromism of compound 1 in BSMs.18 BSMs involving water and an amide (formamide or N-methylformamide) are especially important for the understanding of interactions of water molecules with various solute molecules, the structure of which resembles that of peptides (which contain repeating amide units).32,33 These approaches could be used as simple models of the intermolecular interactions in homogeneous solutions of biomolecules such as proteins or even in biophysical heterogeneous systems. In the latter case, perichromic probes such as ferrocyanide(II) complexes could serve as such probe candidates.

MATERIALS AND METHODS Compound 1 was synthesized and isolated according to a published procedure.21 UV−vis spectra were recorded using a Varian CARY 1E UV−Visible spectrophotometer at 25 °C. UV−vis spectra of 1 have been reported in a previous work.18 Linear and nonlinear regressions were obtained using the program QtiPlot ver. 0.9.9.3. Bosch and Rosés nonlinear fittings were performed using a Scaled Levenberg−Marquardt algorithm.

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RESULTS AND DISCUSSION Solvatochromism and Solvatochromic Sensitivity of 1. The study of the electronic spectra of 1 in various neat solvents indicates an intense negative solvatochromism. Bathochromic shifts of up to ∼14 kcal/mol (∼230 nm) can be observed when going from water to EtOH (i.e., lowering the solvent polarity)18 as shown in Figure 2. This solvatochromic effect attributed to the MLCT transitions dp(FeII) → π*(bpy) (where bpy stands for 4,4′bipyridine) was examined previously in neat solvents by the author and co-workers (see, e.g., refs 18−21). The contribution

Figure 2. Normalized vis absorption spectra of compound 1 in various neat solvents. Arrow indicates bathochromism when decreasing solvent polarity, corresponding to negative solvatochromism. Abbreviations of solvent names used: TFE: 2,2,2-trifluoroethanol; FA: formamide; EG: ethylene glycol; NMF: N-methylformamide. B


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The Journal of Physical Chemistry B Table 1. Regression Fits to Solvatochromic Parameters ECT,0b

sb

ab

Pπ*%

Pα%

Rb

MSEc

15.331 −28.112 68.584 47.930

16.426 123.758 24.252 3.313

14.667 −61.985 −51.644 −4.532

56 37 72 84

44 63 28 16

0.995 0.905 0.997 0.990

0.048 0.908 0.019 0.108

type of medium a

neat solvents MeOH/H2O FA/H2O NMF/H2O a

The solvents taken into account for this regression were (in order of decreasing polarity) H2O, 2,2,2-trifluoroethanol (TFE), formamide (FA), ethylene glycol (EG), N-methylformamide (NMF), MeOH, and EtOH. bIn kcal/mol. cThe mean square of the error (MSE) is calculated through 1 i=1 ̂ i − ECT, i)2 , where, Ê CT,i are the predicted through regression values and ECT,i are the experimental (input) values the equation: MSE = n ∑n (ECT, of the charge transfer energies of 1 in neat solvents and various BSMs. For neat solvents it was n = 7, and for all three cases of BSMs n = 11.

of the dipolarity/polarizability as well as of HBD-acidity of neat solvents was quantified using Kamlet Abboud Taft (KAT) equation and the results obtained after regression are shown in Table 1 (first entry).35 The solvatochromism was further studied in three types of aqueous BSMs (with MeOH, FA, and NMF), and the results are shown in Tables 2−4. The observed solvatochromic effects

Table 4. MLCT Energies of 1 Measured in NMF/Water Mixtures along with Polarity Parameters and Functions of These BSMs NMF/water

Table 2. MLCT Energies of 1 Measured in MeOH/Water Mixtures along with Polarity Parameters and Functions of These BSMs MeOH/water xMeOH

ECT (kcal/mol)a

ET(30) (kcal/mol)36

π*36

α36

ε37

φ(ε)

1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

39.890 43.344 44.510 45.036 45.528 46.100 46.763 47.621 48.845 50.389 51.327

55.7 55.8 56.1 56.5 56.8 57.3 57.8 58.5 59.5 61.0 63.1

0.60 0.62 0.65 0.68 0.71 0.75 0.80 0.85 0.91 0.99 1.09

0.98 0.99 1.00 1.01 1.03 1.04 1.06 1.08 1.10 1.13 1.17

34.0 40.5 46.4 51.5 56.1 60.3 64.2 67.9 71.6 75.3 79.2

0.956 0.963 0.968 0.971 0.974 0.975 0.977 0.978 0.979 0.980 0.981

Table 3. MLCT Energies of 1 Measured in FA/Water Mixtures along with Polarity Parameters and Functions of These BSMs FA/water xFA

π*36

α36

ε37

φ(ε)

1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

42.584 43.764 44.394 45.041 45.810 46.483 47.148 47.996 48.993 50.155 51.327

55.8 56.3 56.6 57.0 57.4 57.9 58.5 59.3 60.2 61.4 63.1

1.15 1.16 1.16 1.16 1.16 1.16 1.16 1.16 1.15 1.13 1.09

0.63 0.65 0.68 0.71 0.74 0.77 0.80 0.83 0.89 0.99 1.17

109.1 112.0 110.3 107.2 107.1 100.8 99.8 91.8 87.1 84.8 79.2

0.986 0.987 0.986 0.986 0.986 0.985 0.985 0.984 0.983 0.982 0.981

π*b

αb

ε37

φ(ε)

1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

39.890 41.296 41.720 41.983 42.280 42.795 43.710 45.059 46.638 48.411 51.327

54.1 54.4 54.8 55.2 55.6 56.2 56.8 57.5 58.5 59.8 63.1

0.90 0.91 0.91 0.92 0.93 0.94 0.95 0.97 1.00 1.04 1.09

0.62 0.64 0.66 0.69 0.72 0.75 0.79 0.83 0.88 0.95 1.17

176.6 152.0 133.1 121.5 111.9 106.4 95.6 91.0 86.0 81.8 79.2

0.992 0.990 0.989 0.988 0.987 0.986 0.984 0.984 0.983 0.982 0.981

in these three cases of BSMs differ from the effect in neat solvents as KAT equation implies (Table 1). The most important observation is that the sensitivity to the solvent polarity parameters π* and α is way different in amide/water mixtures compared to MeOH/water mixtures (see also Figure 3). Dipolarity and polarizability play a more important role than HBD-acidity in the case of the amide/water mixtures. However, the opposite applies to the MeOH/water mixtures. Complementary conclusions can be drawn through the correlations of the MLCT energies of 1 with the solvent polarity parameters π* and α separately (see Tables S1 and S2, Supporting Information (SI)). Through these correlations it is not possible to determine the relative contributions of each of the parameters π* and α (which is feasible through the linear solvation energy relationship: eq 1). However, the sensitivity of the MLCT energies of 1 to the dipolarity/polarizability and HBD-acidity of the medium can be determined. As shown in Figure 3 and Tables S1 and S2 (SI) the sensitivity analysis indicates that in MeOH/water mixtures the molecules of dye 1 are more sensitive to HBD-acidity, whereas in FA/water and NMF/water mixtures dye 1 becomes more sensitive to the dipolarity/polarizability of the medium. Through these two approaches it can be concluded that, in MeOH/water mixtures, specific solute−solvent interactions are more important, while in FA/water and NMF/water mixtures nonspecific interactions dominate. For the rationalization of the solvatochromic effects of 1 in BSMs, a simplified version of KAT equation was used (eq 1) as parameter β expressing HBA-basicity of solvents was found to

Absorption maxima MLCT energies of 1 measured in MeOH/water mixtures.18

ET(30) (kcal/mol)38

ET(30) (kcal/mol)38

a MLCT energies of 1 measured in NMF/water mixtures.18 bThese parameters were determined according to a methodology described in the SI.

a

ECT (kcal/mol)a

xNMF

ECT (kcal/mol)a

a

Absorption maxima MLCT energies of 1 measured in FA/water mixtures.18

C


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Figure 4. Experimentally obtained ECT values of 1 versus calculated ECT values through eq 1.

opposite to MeOH/water mixtures where the FeII/cationicligand ground state is expected to be the most stabilized. Similar findings have been reported for other D−π−A (D: an electron donor; π: a conjugated bridge; A: and electron acceptor) dyes recently.39 These results can be rationalized in terms of the unique dielectric behavior of these amides and their mixtures with water. Both NMF and FA are characterized by exceptionally high dielectric constants, even higher than that of water (Tables 3 and 4) as well as by an increased aptitude to form polymeric amide chains stabilized by hydrogen bonds, when neat, but also in their binary mixtures with water. FA, which is the simplest of the family of amides, is capable of both donating and accepting H atoms in order to form hydrogen bonds. It has also a dipole moment as high as 3.37 D, i.e., higher than that of water.40 These special characteristics give rise to formation of highly structured media when FA is neat or when combined with other polar solvents like water.1,40 Additionally, its very high dielectric constant is attributed to the formation of rigid polymeric chains in the liquid state. Moreover, NMF also has significant HBD and HBA aptitude and even higher dipole moment than FA (3.86 D).40 Its strikingly high dielectric constant (even higher than FA) has been attributed to the flexibility of the polymeric chains [(NMF)n] it forms and the low cancellation of the electric field of each molecule exactly because of the high chain-flexibility.40 Because of these unique properties of FA and NMF, the interactions of these amides with water molecules in their aqueous mixtures are way different to those observed in MeOH/water mixtures. Figure 5 depicts the various ways water, amide and the complexes thereof can interact with the solvatochromic solute 1, reflecting the high complexity of the solvation effects in such BSMs. A recent study on NMF/water mixtures showed that NMF can form complexes with water molecules in proportions that depend on the mole fraction of NMF.41 This finding is clearly associated with the special dielectric character of amide/water mixtures. The measured ECT,m values of 1 correlate very well also with Reichardt’s polarity scale ET(30) (more information is found in the SI). In previous works it has been reported that ET(30) is a solvent polarity parameter which successfully correlates with the MLCT energies of ferrocyanide(II) solvatochromic dyes (see the SI). This is anticipated as long as there are similarities between the solvatochromic behavior of ferrocyanide(II)-based complex dyes and Reichardt’s betaine (compound 2, Figure1). Both of them respond vividly to changes of the dipolarity/

Figure 3. Sensitivity of the MLCT energies of 1 on medium polarity expressed by the solvent parameters π* and α, in BSMs of (A) MeOH and water, (B) FA and water, and (C) NMF and water. Error bars in all cases correspond to the regression errors of the slopes of the corresponding lines obtained through linear regression (details are found in the SI).

be statistically insignificant (symbols involved are found in the nomenclature appendix). The success of this linear solvation energy relationship (LSER) in describing the solvatochromism of 1 is revealed through the plot of Figure 4. As shown small discrepancies from the ideal line: ECT,calc = ECT,exp are observed, and the correlation coefficient (R2) was determined to be as high as 0.967. ECT = ECT,0 + sπ * + aα

(1)

Moreover, a remarkable inversion of solvatochromism is revealed when the experimental MLCT energies are plotted against the Onsager polarity function (φ(ε) determined through eq 4). Albeit in MeOH/water mixtures an increase in φ(ε) (expressing the dipolarity of the medium) results in an increase in the MLCT energy of 1 (corresponding to a negative solvatochromic effect1), the reverse occurs in the studied amide/water mixtures (Figure 6). The described inversion of solvatochromism can be rationalized in terms of the efficient stabilization of the excited state: FeIII/radical-cation-ligand system with regard to the ground state, via efficient solvation of these species by amides and water-amide complexes, attributed to the special effects of these amides and their mixtures. This is D


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chromism with sensitivities similar to neat solvents. This inconsistence between the predictions obtained through parameter ET(30) and those obtained through φ(ε) is attributed to the fact that ET(30) is not a neat measure of dipolarity/polarizability of a medium, but instead it represents a mixture of HBD-acidity and dipolarity of solvents.1,3 Generalities on the Selective Solvation of 1 in Binary Solvent Mixtures. The analysis on the solvatochromism of 1 in neat solvents and the three types of BSMs analyzed in the previous section are quite important for the interpretation of the preferential solvation phenomena observed in these BSMs. As shown in Figure 7, the experimental MLCT energies of 1

Figure 5. Illustration of five different types of interactions between water and amide solvent molecules and the anionic part of compound 12‑ (solute). R = 4-OMePh.

polarizability of solvents as well as on the solvent HBD-acidity, because both types of dyes have high ground state dipole moments as well as both of them bear groups which can efficiently form hydrogen bonds with HBD-solvents. In contrast to the φ(ε) correlations (Figure 6) the corresponding correlations with ET(30) indicate in both types of BSMs (MeOH/water and amide/water mixtures) negative solvato-

Figure 7. Plot of excess MLCT energies of 1 in aqueous BSMs as a function of the bulk cosolvent (FA, MeOH, or NMF) mole fraction. Lines correspond to polynomial fits.

deviate much from the ideal. The plot of Figure 7 implies the existence of preferential solvation phenomena of 1 in all three cases of BSMs. FA/water mixtures show a behavior that is closer to ideal solvent mixtures and that is attributed to the similarity of the two solvents (FA and water).4 However, in MeOH/water and NMF/water mixtures, sizable deviations from linearity are observed. The excess MLCT energies of 1 measured in NMF/water mixtures approach −3 kcal/mol and in case of MeOH/water mixtures 2 kcal/mol, implying a large preferential solvation effect in both cases. However, the preferential solvation phenomena as displayed through these plots of the MLCT energies of 1 cannot give direct insights on the solvent composition of the cybotactic region. Interestingly, it has been pointed out by various authors that different kinds of spectroscopic measurements on the same chemical system in given BSMs can lead to different conclusions regarding the preferential solvation phenomena.4,42 Hence, the importance of the choice of the methods/models for the interpretation of the preferential solvation as well as for the quantification of the extent of preferential solvation at various solvent compositions is high. Herein two models are employed, namely, Suppan’s model on dielectric enrichment and the model of Bosch, Rosés, and co-workers (which here is referred to as the Bosch and Rosés model) taking into account both solvent−solvent and solute−solvent interactions. The choice of these two models is based on the spectroscopic analysis of the solvatochromism of compound 1. For the amide/water mixtures studied here, the nonspecific solute−solvent interactions are more important, and thus the model of Suppan is expected to result in more accurate results. In the case of MeOH/water BSMs hydrogen

Figure 6. Dependence of experimental ECT of 1 on Onsager’s polarity function (φ(ε)). (A) Plots obtained for MeOH/water and NMF/ water mixtures and (B) plots obtained for MeOH/water and FA/water mixtures. Blue squares: experimental ECT of 1 in MeOH/water mixtures; red squares: experimental ECT of 1 in amide/water mixtures (in both A and B); solid lines: ideal lines; dashed lines: linear fits. E


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The Journal of Physical Chemistry B bonding seems to contribute largely to the observed solvatochromic/preferential solvation effects. Therefore, the model of Bosch and Rosés should be more appropriate for the study of this type of BSMs than the model of Suppan. Importantly, as the latter model is characterized as “holistic” it is expected to give successful results for all three types of BSMs.

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PREFERENTIAL SOLVATION THROUGH DIELECTRIC ENRICHMENT A well-established model of preferential solvation of solutes in BSMs where the medium is represented by a continuum dielectric, is the model of Suppan and co-workers on dielectric enrichment. The principle here is that when a dipolar solute is dissolved in a BSM, it is selectively solvated by the solvent being most dipolar. This implies that the solvation shell of the solute tends to be filled by molecules of the dipolar solvent, a process that is exothermic yet entropically disfavored. The mole fraction in the cybotactic region is thus enriched in the polar solvent, and that means that the mole fraction of the two solvents shall be different in the bulk area and in the cybotactic region of the solution. Suppan has shown that the local mole fractions of the two solvents in a BSM can be determined via eq 2 through the so-called index of preferential solvation Zps.7 Suppan’s model permits the determination of Zps through spectrochemical data according to eq 3. This methodology has been successfully employed by various authors in the past.6,8,9,15,42,43 In cases were specific solute−solvent interactions (H-bonding) demonstrate no or negligible contribution to the solvatochromism of a solvatochromic dye, plotting 1 versus

xn xp

ΔECT

should lead to straight lines. However, in the case

where H-bonding is an important interaction, the lines obtained are expected to exhibit sizable deviations from linearity.6−8 yn x = n e−Zps yp xp (2) ⎡ ⎤ x 1 2a3 ⎢1 + n e−Zps ⎥ =− 2 xp ⎥⎦ ΔECT μ Δφ(ε)p − n ⎢⎣

Figure 8. (A) Inverse peak shift (measured against the MeOH peak position) of the absorption of 1 versus the solvent bulk composition ratio xn/xp. (B) Experimental and calculated (through eq 3) MLCT energies of 1 as a function of the mole fraction of water (xp). (C) Experimental versus calculated charge transfer energies.

(3)

where φ (ε ) =

2(ε − 1) 2ε + 1

(4)

3 for 0 ≤ xn(MeOH) ≤ 0.6 was attempted for the determination of the preferential solvation index (see Figure 8A). Of course in this case it is anticipated that the calculated values via eq3, and the experimental values of the MLCT energies of 1, would differ for 0.6 ≤ xn(MeOH), and this is indeed the case (Figure 8B). Nevertheless, the model predicts the MLCT energies of 1 with quite high accuracy (R2 = 0.933 for the linear fit of Figure 8C). The results are also summarized in Tables 5 and 6. Conclusively, this analysis for the MeOH/water mixtures, shows undoubtedly selective solvation of 1 by water molecules, and this is consistent with the fact that water is by far a much more dipolar solvent than MeOH (see polarity information on Table 1). The results for the two types of amide/water mixtures studied in this work, are markedly different from those pertaining to MeOH/water mixtures. The effect of dielectric enrichment on the preferential solvation of 1 is expected to be more important in FA/water and NMF/water mixtures, as long

In the case of MeOH/water mixtures, eq 3 resulted in a small but significant deviation from linearity (Figure 8). It is obvious through the solvatochromic sensitivity and the statistical significance analysis of the solvent polarity parameters π* and α, mentioned above, that solvent HBD-acidity demonstrates a noticeably important contribution on the solvatochromism of 1 in MeOH/water mixtures. Therefore, dielectric enrichment cannot adequately describe the selective solvation effect occurring in MeOH/water mixtures. Judging by the excess charge transfer energy of 1 in these BSMs, one can expect a nearly ideal behavior for MeOH mole fractions up to 0.4 (Figure 7). The highest deviation in the plot of Figure 7 appears in the MeOH-richer bulk composition region (with 0.4 ≤ xn(MeOH)) and it is maximized at xn(MeOH) ≈ 0.85. One interpretation of this effect is that in the water rich bulk composition region the contribution of specific solute−solvent interactions is not very high, but it starts to become significant as the MeOH content increases. In light of this, utilization of eq F


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corresponds to the somewhat increased contribution of specific effects close to the FA rich region (when xp(FA) > 0.9). Formamide has very similar properties to water.4 This is why the deviation from linearity in the excess MLCT energy curve for FA/water of Figure 7 is very small. Typically, FA/water mixtures exhibit an almost ideal behavior,4 as both solvents behave as very good hydrogen-bond-donors and hydrogenbond-acceptors and have also quite similar dielectric constants and refractive indices. FA has about 27% higher permittivity than water at 25 °C (Table 2), and the dipolar dye/solute 1, according to the approach of Suppan should be preferentially solvated by FA in a FA/water mixture. This is indeed the prediction of the dielectric enrichment model (Figure 9B). The local mole fractions calculated through eq 2 are listed in Table 6, indicating a clear preferential solvation by FA molecules (see also SI). The preferential solvation of 1 in NMF/water mixtures can also be rationalized in terms of the dielectric enrichment concept. Neat NMF exhibits a noticeably high dipolarity as mentioned above. Its dielectric constant at 25 °C is close to ∼180 rendering it significantly more dipolar than water. Because of the fact that NMF differs a lot from water in terms of various solvent polarity parameters, one would expect excess MLCT energies much different than zero, and this is obvious in Figure 7 were δECT values as high as ∼3 kcal/mol were observed in that case. In this case, Suppan’s model gives excellent results as well (see Figure 10). The calculated charge transfer energies of 1 were in very good agreement with the experimentally obtained values exhibiting only a small discrepancy in the region 0.9 ≤ xp(NMF) ≤ 1. This has to be interpreted the same way as in FA/water mixtures because of the slight sigmoidal curve observed in the bulk mole fraction region which is rich in NMF (Figure 10b). As expected in this case also, selective solvation of 1 by NMF was predicted as long as NMF exhibits higher dipolarity than water. The calculated local mole fractions thorough Suppan’s model are in all cases listed in Table 6. Noteworthy, for both types of amide/water mixtures investigated, the slope of the lines of Figure 9A and 10A were negative, indicating a positive solvatochromic effect whereas for the MeOH/water mixtures positive slopes were obtained as shown in Figure 8A (see also

Table 5. Results Obtained through Suppan’s Model on Dielectric Enrichment more polar solvent

less polar solvent

water FA NMF

intercepta

MeOH water water

a

Intercept = −

2a 3

μ2 Δφ(ε)p − n −1 c

slopeb

0.0951 −0.0804 −0.0279

Zpsc

R2d e

00612 −0.121 −0.0942

0.102 0.412 1.217

in (kcal/mol)−1 bSlope =−

0.945 0.999 0.998

2a 3 μ2 Δφ(ε)p − n

(e−Zps)

in (kcal/mol) Determined through eq 3. dCorrelation coefficient correspond to plots of Figures 8Ae, 9A, and 10A. eThe Zps in this case was calculated on the basis of the line: xn(MeOH) ≤ 0.6.

1 ΔECT

=f

( ) with 0 ≤ xn 1 − xn

Table 6. Bulk and Local Mole Fractions Calculated Using Suppan’s Model for the Three Types of BSMs Studied MeOH/water

water/FA

water/NMF

xp(water)

yp(water)

xp(FA)

yp(FA)

xp(NMF)

yp(NMF)

0.100 0.200 0.300 0.400 0.500 0.600 0.700 0.800 0.900

0.147 0.280 0.400 0.509 0.608 0.700 0.784 0.861 0.933

0.100 0.200 0.300 0.400 0.500 0.600 0.700 0.800 0.900

0.144 0.274 0.393 0.502 0.602 0.694 0.779 0.858 0.931

0.100 0.200 0.300 0.400 0.500 0.600 0.700 0.800 0.900

0.273 0.458 0.591 0.692 0.772 0.835 0.887 0.931 0.968

as in those two cases the nonspecific solute−solvent effects have a notably higher impact/contribution on the solvatochromism of 1 (see Figure 3 and Tables 3 and 4). More specifically, in FA/water mixtures eq 3 resulted in a line with R2 = 0.999 (plot of Figure 9A). The calculated ECT values were in excellent agreement with the spectral data (Figure 9B) reflecting the success of Suppan’s model in this case. A very small deviation when approaching the termination of the prediction close to xp(FA) = 1 is observed, and this is attributed to the slightly sigmoidal nature of the experimental curve (Figure 7). This small sigmoidal part of the curve potentially

Figure 9. (A) Inverse peak shift (measured against the water peak position) of the absorption of 1 versus the solvent bulk composition ratio xn/xp. (B) Experimental and calculated (through eq 3) MLCT energies of 1 as a function of the mole fraction of FA (xp). Inset: experimental versus calculated MLCT energies. G


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Figure 10. (A) Inverse peak shift (measured against the water peak position) of the absorption of 1 versus the solvent bulk composition ratio xn/xp. (B) Experimental and calculated (through eq 3) MLCT energies of 1 as a function of the mole fraction of NMF (xp). Inset: experimental versus calculated MLCT energies.

Table 5). These findings are of course in agreement with the analysis summarized in Figure 6 and represent an interesting case of solvatochromic inversion as already mentioned. It is important to mention here that the Zps values obtained here are not “contaminated” by the dielectric-non ideality of the BSMs studied. Suppan’s model is capable of separating dielectric enrichment from dielectric nonideality effects, and the corresponding analysis for the BSMs studied herein is found in the SI. Study of the Preferential Solvation of 1 Using the Model of Bosch and Rosés. As mentioned in the previous section, the model describing preferential solvation through dielectric enrichment takes into account the solute−solvent nonspecific interactions occurring in a BSM in which a polar compound such 1 has been dissolved. Indeed, when the contribution of hydrogen-bonding interactions is high (higher than that of the nonspecific interactions), the model does not provide accurate solutions. That is the case of MeOH/water mixtures as already analyzed (see Figure 8A). For a complementary study of the preferential solvation of 1, the model developed by Bosch, Rosés,10−13 was employed addressing the role of the specific interactions (solute−solvent as well as solvent−solvent). The basis of this preferential solvation model was initially introduced by Skwierczynski and Connors44 and is based on the two-step solvent exchange model described by the chemical equations below: m m I(S1)m + S2 ⇌ I(S12)m + S1 2 2

be solvated by solvents S2 and S12, respectively, with reference to solvent S1, according to the following equations.

f2/1 =

f12/1 =

y2 /y1 (x 2/x1)m

(5)

y12 /y1 (x 2/x1)m

(6)

Furthermore, the MLCT energy of the solvatochromic dye will be given from the following linear equation: ECT, m = y1ECT,1 + y2 ECT,2 + y12 ECT,12

(7)

where, y1, y2, and y12 are the local mole fractions of solvent S1, S2, and S12, respectively. ECT,m is the MLCT energy of the solvatochromic probe molecule measured in a BSM, ECT,1 and ECT,2 are the MLCT energies measured in neat solvents S1 and S2, respectively, and ECT,12 is the charge transfer energy when the solvatochromic dye is solvated by solvent S12. eqs 5−7 can lead to eq 8. Using this equation and through nonlinear regression (see Materials and Methods section for details), one can determine the preferential solvation parameters f 2/1 and f12/1 as well as the parameter ECT,12. Finally, knowing f 2/1 and f12/1, it is possible to determine the local mole fractions y1, y2, and y12 according to eqs 9−11. ECT, m = ECT,1(1 − x 2)2 + ECT,2f2/1 (x 2)2 + ECT,12f12/1 (1 − x 2)x 2 (1 − x 2)2 + f2/1 (x 2)2 + f12/1 (1 − x 2)x 2

I(S1)m + mS2 ⇌ I(S2)m + mS1

(8)

In these equations, S1 and S2 correspond to the two pure solvents mixed, and S12 symbolizes a one-to-one complex of the two solvents. The solvatochromic indicator, which is solvated by S1, S2, and S12, is represented by I(S1), I(S2), and I(S12), respectively. Finally, m is the number of solvent molecules involved in the cybotactic region.10 The method of Bosch, Rosés uses m = 2, and the model in this form is the most commonly used45−48 and in this form it is also employed here. The two solvent-exchange processes described by the two chemical equations mentioned above can then be defined by two preferential solvation parameters, namely f 2/1 and f12/1, which quantify the tendency of the solvatochromic indicator to

y1 =

y2 =

y12 = H

(1 − x 2)2 (1 − x 2)2 + f2/1 (x 2)2 + f12/1 (1 − x 2)x 2

(9)

f2/1 (x 2)2 (1 − x 2)2 + f2/1 (x 2)2 + f12/1 (1 − x 2)x 2

(10)

f12/1 (1 − x 2)x 2 2

(1 − x 2) + f2/1 (x 2)2 + f12/1 (1 − x 2)x 2

(11)


Article

The Journal of Physical Chemistry B Table 7. Parameters of the Binary Mixtures Studied Using the Model of Bosch and Rosés solvent 1

solvent 2

ECT1 (kcal/mol)

ECT2 (kcal/mol)

ECT12 (kcal/mol)

f 2/1

f12/1

MSEa

water water water

MeOH FA NMF

51.327 51.327 51.327

39.890 42.584 39.890

43.976 44.026 45.000

0.061 0.13 5.9

34 14 1.2

0.046 0.0028 0.19

1 i=1 ̂ i − ECT, i)2 where, Ê CT,i are the predicted through The mean square of the error (MSE) is calculated through the equation: MSE = n ∑n (ECT, regression values and ECT,i are the experimental (input) values of the charge transfer energies of 1 in different BSMs. n = 11 for all three cases of BSMs.

a

the specific interactions (including both solute−solvent and solvent−solvent interactions), the predicted MLCT energies were in very good agreement with the experimental ones (Figure 11A). Moreover, the charge transfer energies of 1 in FA/water mixtures behaving nearly as ideal mixtures, were flawlessly predicted by this model as shown in Figure 11B. The excellent regression in this case is reflected by the very small mean square of the error (see MSE in Table 7). The sigmoidal shape of the line ECT,m = f(xNMF) was also well predicted by the model (Figure 11C). With regard to the obtained regression values of parameters f 2/1 and f12/1 the following conclusions can be drawn: (a) For MeOH/water mixtures the model predicts preferential solvation of the solute (1) by water molecules. The preferential solvation parameter f 2/1 (with solvent 1 being water) is much smaller than unity (0.061) reflecting the higher propensity of water molecules to fill the cybotactic region of 1. This is of course consistent with the findings of the model of Suppan predicting solvation by the most dipolar solvent. Moreover, the prediction of the MLCT energies for all solvent mole fractions studied was excellent (see Figure 11A). With regard to the preferential solvation parameter f12/1 the model of Bosch and Rosés resulted in the value 34, implying that the one-to-one MeOH−water complexes are way more abundant in the cybotactic region than water molecules. This is also anticipated because of the high stability of the hydrogen bonded MeOH−water molecules. Their dissociation demands more energy than the energy gain though the solvation of the solute. This is a common rule of thumb applicable for BSMs of highly polar solvents.8 The calculated local mole fractions of MeOH, water, and the MeOH−water complex at various bulk mole fractions of MeOH are shown in Figure 12A. (b) In case of FA/water mixtures the model of Bosch and Rosés gave a regression value of 0.13 for the parameter f 2/1 (with solvent 1 being water) implying preferential solvation of compound 1 by water molecules. This finding seems to be inconsistent with the results of Suppan’s model, which in this particular case of BSMs was very accurate (see the plot of Figure 9A with R2 = 0.999). However, a more careful analysis of the result of the model of Bosch and Rosés is needed in this case. The preferential solvation parameter f12/1 was determined to be as high as 14. This suggests that the FA−water complex can much more efficiently solvate the solute than solvent 1, i.e., water. However, the dielectric behavior of the FA/water mixtures is essentially also influenced by these intermolecular solvent−solvent interactions or in other words the mixtures of FA and water (containing significant amounts of the FA−water complex) exhibit significantly high dielectric constants.

As long as the solvent species involved in the two step process analyzed above are S1, S2, and S12, eq 12 should be fulfilled in the cybotactic region (this is obvious also by eqs 9−11). y1 + y2 + y12 = 1

(12)

In the case of the solvatochromic dye 1 as a solute in aqueous mixtures of MeOH, FA, and NMF, the application of the model of Bosch and Rosés gave excellent results, as shown in Table 7 and Figure 11. As mentioned, MeOH/water mixtures were not successfully described through the model of dielectric enrichment (especially in the MeOH rich region). However, with the use of the model of Bosch and Rosés, which takes into account

Figure 11. MLCT energies of 1 in BSMs consisting of (A) MeOH and water, (B) FA and water, and NMF and water, as a function of the bulk cosolvent mole fraction. Lines were obtained through eq 8 with the Bosch−Rosés parameters listed in Table 6. I


Article


These results look very similar to those obtained in case of MeOH/water mixtures (Figure 12A). (c) In NMF/water mixtures, there is a clear preferential solvation of the solvatochromic dye 1 by NMF (f 2/1 = f NMF/water = 5.9). This is of course in agreement with the conclusion drawn through the dielectric enrichment model. The MLCT energies of 1 in NMF/water mixtures were predicted quite accurately by the model of Bosch and Rosés, as shown in Figure11C. The consistency between the two models in this case is highly anticipated due to the high permittivity difference between NMF and water and the lower contribution of specific solute− solvent interactions. However, the model of Bosch and Rosés provides additional insights on the existence of dimers of the type NMF−water in the cybotactic region. The value f12/1 = 1.2 indicates that NMF−water species are present in the cybotactic region but not much more abundant than single water molecules ( f12/1 ≈ 1). In other words, NMF single molecules are by far the ones that preferably solvate compound 1 (see Figure 12C). The ECT12, which was determined to be 45 kcal/mol in this case, is closer to the average: (ECT1+ECT2)/2 = 45.609 kcal/mol. Comparison of the calculated cosolvent local mole fractions at various bulk mole fractions of cosolvent in the three types of aqueous mixtures examined through various methods are found in the SI.

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CONCLUSIONS The solvatochromic equation (eq 1) employed in the case of dye 1, showed that the dipolarity and polarizability of the medium is more important in the case of the two types of amide/water mixtures, whereas for MeOH/water mixtures, HBD-acidity has more significant effects on the solvatochromism of 1. Further analysis using Onsager’s polarity function showed an inversion of solvatochromism from negative in MeOH/water to positive in amide/water. The case of MeOH/ water mixtures has likely a solvent composition-dependent contribution of specific effects. Application of the model of Suppan on preferential solvation leads to the conclusion that for low to moderate MeOH compositions, the dielectric enrichment is the main selective solvation mechanism, whereas for elevated MeOH compositions, specific solvent−solvent and solute−solvent effects are more pronounced. The same model gave excellent results in the case of the two amide/water mixtures because nonspecific interactions are much more dominant in these two cases. Furthermore, the model of Bosch and Rosés was successfully applied to all three cases of BSMs. This is an anticipated result as long as the latter model takes into account all types of interactions in the cybotactic region, including solvent−cosolvent interactions, which are very significant in all three cases.

Figure 12. Local mole fractions, y1 (cosolvent: blue squares), y2 (water: red squares) and y12 (complex molecule cosolvent−water: black cycles) around the solute 1 for (A) MeOH−water, (B) FA− water, and (C) NMF−water mixtures at 25 °C.

The results of Suppan’s model and the model of Bosch and Rosés are therefore not in disagreement considering that the dielectric enrichment model does not take into account separately the formation of solvent−cosolvent dimers. Additionally, previous studies of the solvatochromic indicator, compound 2 (Reichardt’s betaine, Figure 1) by Rosés and co-workers reported that also in case of 2 as a solute in a series of FA/water mixtures, selective solvation of 2 by water molecules was predicted and the value of f 2/1 was very close to the one obtained in this work (0.31 for 2 and 0.13 for 1).38 This is anticipated due to the similarities that betaine 2 and the solvatochromic dye 1 exhibit. Both of them are prone to interactions with HBD and dipolar solvents. Besides, the spectra of 2 have been used as a basis of Reichardt’s polarity scale (expressing HBD-acidity and dipolarity/ polarizability of solvents). Another important finding here is that the calculated MLCT energy of 1 corresponding to the complex FA−water is much closer to the value of FA than to the value of water (see Table 7). Plots of the calculated mole fractions of FA, water, and the FA−water complex are depicted in Figure 12B.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.6b05868. Contribution analysis for parameters involved in eq 1; solvent polarity information for NMF/water mixtures; plots of local (obtained through Suppan’s method) versus bulk mole fractions (Figure S1−3); plots of local J


Article


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(through various methods) versus bulk mole fractions (Figures S4−6) (PDF)

AUTHOR INFORMATION

■

Corresponding Author

*Tel: +46728368595; E-mail: [email protected]. Notes

REFERENCES

(1) Reichardt, C.; Welton, T. Solvents and Solvent Effects in Organic Chemistry, 4th ed.; Wiley VCH Verlag GmbH & Co.: Weinheim, Germany, 2011. (2) Gutmann, V.; Resch, G. Lecture Notes on Solution Chemistry; World Scientific Publishing Co.: Singapore, 1995. (3) Reichardt, C. Solvatochromic Dyes as Solvent Polarity Indicators. Chem. Rev. 1994, 94, 2319−2358. (4) Marcus, Y. Solvent Mixtures: Properties and Selective Solvation; Marcel Dekker, Inc.: New York, 2002. (5) Ben-Naim, A. Preferential Solvation in Two- and in ThreeComponent Systems. Pure Appl. Chem. 1990, 62, 25−34. (6) Khajehpour, M.; Welch, C. M.; Kleiner, K. A.; Kauffman, J. F. Separation of Dielectric Nonideality from Preferential Solvation in Binary Solvent Systems: An Experimental Examination of the Relationship between Solvatochromism and Local Solvent Composition Around a Dipolar Solute. J. Phys. Chem. A 2001, 105, 5372−5379. (7) Suppan, P. Local Polarity of Solvent Mixtures in the Field of Electronically Excited Molecules and Exciplexes. J. Chem. Soc., Faraday Trans. 1 1987, 83, 495−509. (8) Lerf, C.; Suppan, P. Hydrogen Bonding and Dielectric Effects in Solvatochromic Shifts. J. Chem. Soc., Faraday Trans. 1992, 88, 963− 969. (9) Henseler, A.; Von Raumer, M.; Suppan, P. Observation of Dielectric Enrichment upon the Formation of Benzophenone Radical Anion in a Binary Solvent Mixture. J. Chem. Soc., Faraday Trans. 1996, 92, 391−393. (10) Rosés, M.; Ráfols, C.; Ortega, J.; Bosch, E. Solute−Solvent and Solvent−Solvent Interactions in Binary Solvent Mixtures. Part 1. A Comparison of Several Preferential Solvation Models for Describing ET(30) Polarity of Bipolar Hydrogen Bond Acceptor-Cosolvent Mixtures. J. Chem. Soc., Perkin Trans. 2 1995, 8, 1607−1615. (11) Bosch, E.; Rosés, M.; Herodes, K.; Koppel, I.; Leito, I.; Koppel, I.; Taal, V. Solute−Solvent and Solvent−Solvent Interactions in Binary Solvent Mixtures. Part 1. A Comparison of Several Preferential Solvation Models for Describing ET(30) Polarity of Bipolar Hydrogen Bond Acceptor-Cosolvent Mixtures. J. Phys. Org. Chem. 1996, 9, 403− 410. (12) Ortega, J.; Ráfols, C.; Bosch, E.; Rosés, M. Solute−Solvent and Solvent−Solvent Interactions in Binary Solvent Mixtures. Part 3. The ET(30) Polarity of Binary Mixtures of Hydroxylic Solvents. J. Chem. Soc., Perkin Trans. 2 1996, 1497−1503. (13) Rosés, M.; Buhvestov, U.; Ráfols, C.; Rived, F.; Bosch, E. Solute−Solvent and Solvent−Solvent Interactions in Binary Solvent Mixtures. Part 6. A Quantitative Measurement of the Enhancement of the Water Structure in 2-Methylpropan-2-ol−Water and Propan-2-ol− Water Mixtures by Solvatochromic Indicators. J. Chem. Soc., Perkin Trans. 2 1997, 1341−1348. (14) Chatterjee, P.; Bagchi, S. Preferential Solvation of a Dipolar Solute in Mixed Binary Solvent: a Study of UV-Visible Spectroscopy. J. Phys. Chem. 1991, 95, 3311−14. (15) Boggetti, H.; Anunziata, J. D.; Cattana, R.; Silber, J. Solvatochromic Study on Nitroanilines. Preferential Solvation Vs Dielectric Enrichment in Binary Solvent Mixtures. Spectrochimica Acta A 1994, 50, 719−726. (16) Marcus, Y. Preferential Solvation of Ions in Mixed Solvents. Part 4.Comparison of the Kirkwood−Buff and Quasi-Lattice QuasiChemical Approaches. J. Chem. Soc., Faraday Trans. 1 1989, 85, 3019. (17) Marcus, Y. A Quasi-Lattice Quasi-Chemical Theory of Preferential Solvation of Ions in Mixed Solvents. Aust. J. Chem. 1983, 36, 1719−1731.

The author declares no competing financial interest.

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ACKNOWLEDGMENTS The author is grateful to Prof. Tsolomitis Athanase (National Technical University of Athens). IKY (Greek State Scholarship Foundation) is gratefully acknowledged for financial support.

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yn:local mole fractions of the less polar solvent component (Suppan’s model7) yp:local mole fractions of the more polar solvent component (Suppan’s model7) Zps:index of preferential solvation (Suppan’s model7)

APPENDIX

Nomenclature:

a:solvatochromic parameter involved in KAT equation49 expressing HBD-acidity α:molecular radius of the solute (Suppan’s model7) a:coefficient of KAT parameter a (Equation1).49 δE CT :excess MLCT energy determined through the equation: δECT = ECT − ECT,id ΔECT:measured MLCT energy relative to the measured one in less polar component of the solvent mixture Δφp‑n:the difference between the Onsager polarity function of the polar and nonpolar components of the binary solvent mixture ECT:recorded absorption maxima MLCT energies of 1 ECT,0:intercept of eq 1: ECT = ECT,0 + sπ* + aα obtained through regression analysis ECT,1:MLCT energies of dye 1 measured in neat solvent 1 ECT,2:MLCT energies of dye 1 measured in neat solvent 2 ECT,12:determined MLCT energies of dye 1 when solvated by the solvent complex S12 (eq 8) ECT,m:MLCT energies of the probe molecule measured in a BSM ECT,id:ideal MLCT energies determined through: ECT,id = x1ECT,1 + x2ECT,2 ET(30):Reichardt’s solvent polarity scale f 2/1:preferential solvation parameter measuring the tendency of the probe to be solvated by solvents S2 (eq 5) f12/1:preferential solvation parameter measuring the tendency of the probe to be solvated by the solvent complex S12 (eq 6) μ:the dipole moment of the solute π*:solvatochromic parameter involved in KAT equation49 expressing dipolarity/polarizability Pπ*:contribution of solvent polarity parameter π* Pα:contribution of solvent polarity parameter a s:coefficient of KAT parameter π* (Equation1) S1:solvent 1 (Bosch-Rosés model10) S2:solvent 2 (Bosch-Rosés model10) S12:complex of solvents 1 and 2 (Bosch-Rosés model10) Si:solvatochromic sensitivity of the probe compound φ(ε):Onsager’s solvent polarity function (eq 4) x:bulk mole fractions xn:bulk mole fractions of the less polar solvent component (Suppan’s model7) xp:bulk mole fractions of the more polar solvent component (Suppan’s model7) y:local mole fractions K


Article

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L


Preferential Solvation of a Highly Medium Responsive

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