Alkylbenzoate Binary Mixtures by

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Preferential Solvation in Alkan-1-ol/Alkylbenzoate Binary Mixtures by Solvatochromic Probes Ana M. Navarro, Bego~na García, Francisco J. Hoyuelos, Indalecio A. Pe~nacoba, and Jose M. Leal* Departamento de Química, Universidad de Burgos, 09001 Burgos, Spain

bS Supporting Information ABSTRACT: The binary mixtures of methanol with (C1C4) alkylbenzoates and of (C1, C3, C5, C7, C9, C11) alkan-1-ols with methylbenzoate were used as solvents to look into the preferential solvation and intermolecular interactions of the solvatochromic indicators 2-nitroanisole, 4-nitroaniline, 4-nitrophenol, and Reichardt’s dye by UVvis measurements. The experimental data at 298.15 K have served to deduce the corresponding Reichardt and KamletTaft parameters of the mixed solvents. The solvation effects exerted on the solvatochromic probes by the solvents used, either pure or binary mixed, were analyzed by means of the preferential solvation model. Likewise, the 1H NMR, 13C NMR, and IR spectroscopic parameters measured for the mixed solvents corroborate the structural effects. The sets of experimental data gathered shed abundant light on the underlying solutesolvent and solventsolvent interactions. The alkanol/methylbenzoate mixtures display stronger solvation ability than the pure solvents.

’ INTRODUCTION Evaluation of the polarity of mixed solvents is a key issue in many settings. Polarity can be assessed by the overall solvation capability or solvation power, 1 a solvent property that relies on all specific and nonspecific solutesolvent interactions; it accounts for the effects on the solute prompted by the shell of surrounding solvent molecules.2 Although dipole moment, relative permittivity, and cohesive pressure can be tuned by switching the solvent nature, none of these properties describe in full the solvent polarity; instead, this is empirically better established by solvent-dependent reference processes. The polarity defined this way underscores the complexity of solvation.3 Solvatochromic effects account for the observed change in the UVvis spectra of a probe due to the changes in the solvation power of the medium; shape, intensity, and wavelength of the bands can be unlike in solvents of different polarity due to the different solvation of the ground and excited states.4 Specific interactions may give rise to preferential solvation of the probe by one solvent constituent, the parameters drawn serving as useful scales;5 among these, those by Reichardt1 and KamletTaft6 are most used.7 Moreover, formation of homo and hetero solvent aggregates often result in complex interactions that could influence the preferential solvation effect; comparison with the bulk solvent may unveil whether the probe is preferentially solvated.8 On the other hand, NMR and IR spectra provide meaningful and useful parameters to predict solvent properties, as the signal location and the line width become affected by virtue of the surrounding r 2011 American Chemical Society

molecules.9 Likewise, the IR spectra reflect the dynamics of the processes coupled to vibrational modes and can serve to infer the nature and extent of the interactions; solvent-induced shifts are related to solutesolvent interaction and chemical bonding.10 Alcohols are polar solvents associated by H-bond; depending on temperature, chain length, and the OH site, H-bonding may influence the alcohol structure. Pure methanol may yield cyclic dimers, propan-1-ol cyclic dimer/trimer mixtures, pentan-1-ol 1:1 monomer/cyclic dimer mixtures, and hexan-1-ol to decan-1ol 1:1 monomer/open dimer mixtures.11 On the other hand, the polarizable π-electron system, the dipolar and hydrophobic nature, and the selective ability of alkylbenzoates to separate polar from nonpolar compounds turn them into suitable tools in many settings.12,13 In this work, the solvatochromic indicators 2-nitroanisole, sensitive to solvent dipolarity and polarizability; 4-nitroaniline and 4-nitrophenol, both sensitive to the solvent H-bonding acceptor ability; and Reichardt’s betaine dye or 2,6-diphenyl4-(2,4,6-triphenylpyridinio)phenolate, mostly sensitive to the H-bond donor solvent ability14 (Scheme 1) were tested in methanol/(C1C4) alkylbenzoates and (C1, C3, C5, C7, C9, C11) alkan-1-ols/methylbenzoate binary solvents. Also IR and NMR measurements have served to assess the interactions between the solvent components. Received: March 2, 2011 Revised: July 22, 2011 Published: August 08, 2011 10259

dx.doi.org/10.1021/jp202019x | J. Phys. Chem. B 2011, 115, 10259–10269

The Journal of Physical Chemistry B Scheme 1. Molecular Structure of 2-Nitroanisole, 4-Nitroaniline, 4-Nitrophenol, and Reichardt’s Dye

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high sensitivity. The spectra were collected placing the sample onto the crystal. The concentration effect on the typical vibration band was analyzed for the different liquid mixtures.

’ RESULTS AND DISCUSSION

’ EXPERIMENTAL SECTION Reagents. Ultrapure water (Milli-Q, Millipore, 18.2 mΩ cm resistivity) and pure solvents, of the highest purity commercially available, were used without further purification. The liquids were degassed with ultrasound and kept out of the light over Fluka Union Carbide 0.3 nm molecular sieves. The purity of the solvents was assessed with a Perkin-Elmer 990 GC and also by comparing densities, speeds of sound (Anton Paar DSA 5000 density and sound meter), viscosities (Anton Paar AMV 200 viscometer), and refractive indices (Leica AR 600 refractometer) with literature values.13 The solvatochromic indicators (A) 2-nitroanisole (Riedel-Ha€en, 99%), (B) 4-nitroaniline (Riedel-Ha€en, 99%), (C) 4-nitrophenol (Riedel-Ha€en, 99%), and (D) Reichardt’s dye (Sigma-Aldrich, 95%) were used as received. Instruments and Procedures. Fully miscible binary mixtures were prepared over the whole composition range by mass using a Mettler AT 261 Delta Range balance ((105 g). The mixture composition was always expressed as the alcohol mole fraction. To prevent the samples from preferential evaporation, the mixtures were placed into suitably stoppered bottles. For the experimental evaluation of solvatochromic parameters, the solutions were prepared by addition of the proper amount of the indicator (1 104 M). The UVvis spectral curves were recorded with a HP 8453A diode array spectrophotometer (accuracy (2 nm, reproducibility (0.5 nm). The spectra were collected at 298.15 K using blank solvent samples, and then recording the spectra of samples of same composition containing the indicator. The wavelengths of highest absorption were obtained by mathematical smoothing of the absorption data using a Gaussian curve-fitting equation.15 The temperature of the 1 cm quartz cell ((0.2 K) was controlled using a HP 89090A Peltier. The solvent effect on the NMR chemical shift was analyzed identifying the H and C signals. The NMR spectra were recorded with a 400 MHz Varian Unity Inova NMR spectrometer at 9.4 T magnetic field operating at 399.941 and 100.574 MHz for 1H and 13 C, respectively; it yields the spectra of nuclei within the 1 H/15N-31P interval and 80 to +120 °C temperature range with a resolution of 0.1 °C. The IR spectra were collected with a Nicolet 8700 Research FT-IR-spectrometer, fitted out with a smart thermal ARK accessory ((1.0 °C) including a thermal plate with ZnSe 45° crystal that provides superior performance and scanning capabilities, including step-scan spectroscopy. It provides advanced velocity for rapid and slow scanning and an extended 25.00020 cm1 spectral range. The multireflection crystal is ideally suited to analyze low-concentration samples with

UVVis Measurements. The variations in the spectral curves of indicators in different solvents can be of help to infer the composition of the solvation shell. The parameters by Kamlet Taft (π*, β)16 and Reichardt (R, ET(30) and ET)17 for the binary solvents were deduced from the wavenumbers using the equations of Appendix A (Supporting Information).18 The parameter π* depends only on the wavenumber of 2-nitroanisole; βB and βC depend on the wavenumber of 4-nitroaniline and 4-nitrophenol, respectively; the parameter π* obtained with 2-nitroanisole and the parameters R and ET(30) depend on the wavenumber of Reichardt’s dye. The mixing solvatochromic parameters were evaluated according to

ΔP ¼ P x1 P1 x2 P2

ð1Þ

where x1 and x2 stand for the alcohol and ester mole fraction, respectively, and P, P1, and P2 for the π, β, R, ET parameters for the mixture, alcohol, and ester, respectively. Table 1S (Supporting Information) summarizes the wavenumbers, ν, of 2-nitroanisole, 4-nitroaniline, 4-nitrophenol, and Reichardt’s dye in pure solvents, and Tables 2S4S summarize those in alkan-1-ol/methylbenzoate and methanol/alkylbenzoate mixtures. The linear variation of the wavenumbers and solvatochromic parameters with solvent composition reflects ideal behavior and absence of preferential solvation; i.e., the solvation sphere of the probe and bulk are the same composition.19 Nonlinear plots can, in turn, be rationalized in terms of nonspecific and specific solvent-probe and solvent solvent interactions.20,21 Figures 14 show the variation with the alcohol content of the measured data as well as the calculated ΔπA* , ΔβB, ΔβC, and ΔEN T values. Wavenumbers for pure alcohols with 2-nitroanisole, 4-nitroaniline, and 4-nitrophenol (Figure 1, a, b, and c) increased with the rise in alcohol length, except for methanol with 4-nitroaniline (Figure 1b). For Reichardt’s dye, wavenumbers decreased when the alcohol size was raised, the difference rising the smaller the alcohol size (Figure 1d). Figure 2 shows the parallel trend of the ester-chain effect. For methanol/alkylbenzoates with 2-nitroanisole and 4-nitroaniline, the wavenumber of pure esters (x1 = 0) increased as the alkyl chain was raised (Figure 2a,b), while with Reichardt’s dye they slightly decreased with an increase in the ester chain (Figure 2d). For methanol/alkylbenzoates, the effect with 4-nitrophenol was not monitored because these mixtures gave no solvatochromic effect, hence pentan-1-ol served to analyze the ester-chain effect. For pentan-1-ol/akylbenzoates with 4-nitrophenol wavenumbers differed appreciably only between methyl/ethyl and propyl/butyl (Figure 2c). Figures 3 and 4 show the solvatochromic parameters Δπ*A, ΔβB, ΔβC, and ΔEN T calculated with eq 1. Large Δπ*A values reveal strong solvent polarity-polarizability. The dipolar 2-nitroanisole is prone to electronic transition from the donor (OMe) to the acceptor (NO2) moiety through the aromatic ring; it shows positive solvatochromism due to the more dipolar first-excited state relative to the ground state, and should be insensitive to H-bonding.22 For alkan-1-ol/methylbenzoate mixtures, the ΔπA* values decreased when the alcohol length was raised (Figure 3a) and also for methanol/alkylbenzoate 10260

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The Journal of Physical Chemistry B

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Figure 1. Experimental wavenumber, ν 103 (cm1) of indicators (a) 2-nitroanisole, (b) 4-nitroaniline, (c) 4-nitrophenol, and (d) Reichardt’s dye, in (x1) alkan-1-ol + (1 x1) methylbenzoate binary solvents at 298.15 K: (b) methanol, (2) propan-1-ol, (+) pentan-1-ol, (9) heptan-1-ol, (1) nonan1-ol, and (f) undecan-1-ol. Continuous lines obtained from the preferential solvation model, eq 6.

mixtures when the ester chain was raised (Figure 4a). The highest polarity attained at x1 ≈ 0.6 implies that the solvation sphere is largest for 2:1 alcohol:ester heteroaggregates, the maximum corresponding to small molecules endowed with high polarizability. Positive β values reveal preferential solvation by the most polar cosolvent;23 this parameter is sensitive to H-bonding and was deduced from the πA* values of 2-nitroanisole. The ΔβB values obtained showed minima at high alcohol content, shifting to higher content the larger the alcohol size (Figure 3b). The ΔβB values for methanol were negative with all four esters (Figure 4b); the minima at x1 ≈ 0.66 reveal formation of 2:1 alcohol:methylbenzoate aggregates with less solvation ability than the pure constituents. The behavior is rather more complex with methanol, a feature explained by the H-bonding ability of alkylbenzoates and the high self-associating power of methanol. 4-Nitrophenol also exhibits positive solvatochromism. The ΔβC parameter reflects sensitivity to solvents with H-bonding ability. This parameter remained essentially unchanged for all alcohols (Figure 3c) and esters (Figure 4c); the maxima at x1 = 0.250.30 reveal the higher solvation ability of the mixture compared to pure constituents. The different figures quoted for the same parameter account for the different sensitivity of the

indicator toward the solvent, insofar as the solventindicator interaction is influenced by the solventsolvent interaction. Reichardt’s dye displayed negative solvatochromism due to solvation of the dipolar ground state. The high dipole moment favors dipoledipole and dipoleinduced dipole interactions; the negative charge at the phenolate O site turns this probe into a strongly basic electron-pair donor prone to H-bonding and Lewis acid interaction. In addition, the delocalized positive charge of the pyridinium moiety is sterically shielded. Thus, Reichardt’s dye is solvated by solvents with H-bond and electron-pair acceptor ability.1 The pronounced negative solvatochromism has served to settle the ET(30) and EN T (30) empirical parameters. ET(30) accounts for the electronic transition energy of the probe24 and is largely influenced by dipole moment, polarizability and H-bonding.25 Its variation with the solvent composition provides valuable information; positive and negative curves indicate preferential solvation of the probe by the least and the most polar constituent, respectively.19 Normalized EN T (30) values reflect either interaction of the most polar constituent with Reichardt’s dye or change in the solvent structure. The positive ΔEN T values yielded by the two sets of solvents (Figures 3d and 4d) indicate preferential solvation of Reichardt’s dye by the most polar component. The maxima for rich 10261



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Figure 2. Experimental wavenumber, ν 103 (cm1), of indicators (a) 2-nitroanisole, (b) 4-nitroaniline, and (d) Reichardt’s dye, in (x1) methanol + (1 x1) alkylbenzoate solvents and (c) 4-nitrophenol, in (x1) pentan-1-ol + (1 x1) alkylbenzoate binary solvents at 298.15 K: (b) methylbenzoate, (+) ethylbenzoate, (2) propylbenzoate, and (9) butylbenzoate). Continuous lines obtained from the preferential solvation model, eq 6.

methylbenzoate content (Figure 3d) decreased with a rise in alcohol length, reverting at x1 = 0.6; however, for methanol/ alkylbenzoate systems (Figure 4d) no variation is observed with the ester chain. The ΔEN T trend for the two sets of solvents parallels that of ΔβC in the alkylbenzoate-rich zone, since both parameters reflect solvation of the probe by the most polar constituent. For alcohol-rich mixtures only modest negative values appeared due to the alcohol H-bonding ability. Pure alcohols are both proton donor and acceptor and self-associate. An interesting synergistic effect is observed for low alcohol content, because more polar H-bonded species are formed relative to pure solvents; synergistic polarity has also been found in alcohol mixtures with CO-containing organic solvents.26 The preferential solvation model has served to fit the sets of experimental data shown in Figures 1 and 2 and to quantify the solvation ability of the mixture and the pure constituents. Preferential Solvation Model. This model has been successfully applied to a wide range of solvent systems, including alcohol/water binary14 and ternary27 mixtures and, more recently, ionic liquids with a number of organic solvents such as N,N-dimethylformamide, dimethylamine, dimethyl sulfoxide,28 alcohol, dichloromethane, and water,29,30 thus proving

it viable to obtain reliable information on solutesolvent and solventsolvent interactions. The interpretation of physicochemical properties that rely on solutesolvent interactions is expected to face (i) preferential solvation by a solvent constituent, and (ii) influence of solventsolvent interactions on the solutesolvent interactions.31 Preferential solvation entails selective enhancement of a solvent constituent in the solvation shell of the solute. Due to extensive intermolecular interaction between the mixed constituents, the solvent may adopt distinct structures with further complexity of solvation.32,33 A linkage between probe behavior (accounted for by ν and the mixing solvatocromic parameters), solvent structure, and solutesolvent interaction can be established using available methods of dealing with solvation effects such as the preferential solvation model,34,35 which was originally developed to interpret the pKa values of weak acids in water propan-2-ol mixtures.36 It was assumed that the two mixed constituents (S1 and S2) interact to form a common structure S1229 (the S12 structure attributed to formation of intersolvent complexes or associates by H-bond interaction with particular properties). For the sake of simplicity, it is considered that the two solvents interact in 1:1 ratio; that is, the mixed structure formed contains the same number 10262



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Figure 3. Solvatochromic parameters (a) ΔπA* , 2-nitroanisole; (b) ΔβB, 4-nitroaniline; (c) ΔβC, 4-nitrophenol; and (d) ΔEN T , Reichardt’s dye, in (x1) alkan-1-ol + (1 x1) methylbenzoate binary solvents at 298.15 K: (b) methanol, (2) propan-1-ol, (+) pentan-1-ol, (9) heptan-1-ol, (1) nonan-1-ol, and (f) undecan-1-ol.

of molecules of solvents 1 and 2. To a good approximation from a practical standpoint, mixed solvents with stoichiometry structures other than 1:1 can be regarded as combination of two structures: the 1:1 mixed structure and the solvent-inexcess structure.36 Therefore, the equilibrium for the formation of the “mixed solvent” can be written as

indicator from solvents 1 and 2. The f2/1 and f12/1 parameters were evaluated as

S1 þ S2 a 2S12 where the coefficient 2 of the right-hand side serves to keep the number of solvent molecules constant. This process should be regarded as an equilibrium between solvent structures rather than between individual solvent molecules. According to this, if two different solvent-exchange processes are involved, then the model used fulfils the two-step pattern by Skwierczynski and Connors:37 IðS12 Þ þ 2S2 a IðS22 Þ þ 2S1

xS2 =xS1 ðx02 =x01 Þ2

ð2Þ

f12=1 ¼

xS12 =xS1 ðx02 =x01 Þ

ð3Þ

where xS1 , xS2, and xS12 (mole fractions of S1, S2, and S12 in the solvation sphere) add up to unity,14 x01 and x02 being the bulk mole fraction of the two constituents. The f2/1 and f12/1 parameters quantify the solvating ability of S2 and S12 relative to S1; such a quantification must be understood in terms of free energy, not in terms of composition of the solvation sphere of the indicator. The f12/2 parameter for the 12/2 exchange is defined as

IðS12 Þ þ S2 a IðS122 Þ þ S1 where I stands for the indicator, S1 and S2 stand for the pure constituents, and S12 stands for the mixed solvent. I(S1) represents the indicator solvated by S1, I(S2) solvated by S2, and I(S12) solvated by S12. In this model, it is assumed that the solvent 12 is formed in the solvation microsphere of the

f2=1 ¼

f12=2 ¼

f12=1 f2=1

ð4Þ

The mole fraction of each constituent in the solvation sphere is easily accessible from bulk composition, x02, and preferential solvation parameters.14 The solvatochromic property of the mixture Y can be deduced from the Y1, Y2, and Y12 contributions 10263



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Figure 4. Solvatochromic parameters (a) Δπ*A, 2-nitroanisole; (b) ΔβB, 4-nitroaniline; (d) ΔEN T , Reichardt’s dye in (x1) methanol + (1 x1) alkylbenzoate binary solvents; and (c) ΔβC, 4-nitrophenol, in (x1) pentan-1-ol + (1 x1) alkylbenzoate binary solvents at 298.15 K, for each indicator: (b) methylbenzoate, (+) ethylbenzoate, (2) propylbenzoate, and (9) butylbenzoate.

of pure (S1, S2) and mixed (S12) solvents in the solvation sphere (see ref 36). Substitution into eq 5, according to eqs 2 and 3, leads to the general eq 6: Y ¼ xS1 Y1 þ xS2 Y2 þ xS12 Y12 , ΔY ¼ kxS2 ðxS1 þ xS12 =2Þ Y ¼

Y1 ð1 x02 Þ2 þ Y2 f2=1 ðx02 Þ2 þ Y12 f12=1 ð1 x02 Þx02 ð1 x02 Þ2 þ f2=1 ðx02 Þ2 þ f12=1 ð1 x02 Þx02

ð5Þ þ ΔY

ð6Þ ΔY being:

14

ΔY ¼

kf2=1 ðx2 0 Þ2 ½ð1 x2 0 Þ2 þ f12=1 ð1 x2 0 Þx2 0 =2 ½ð1 x2 0 Þ2 þ f2=1 ðx2 0 Þ2 þ f12=1 ð1 x2 0 Þx2 0 2 ð7Þ

The correction term ΔY was originally introduced in alcoholwater mixtures to take into account the enhancement of the water structure caused by the alcohol molecules at low alcohol mole fraction. Since the enhancement of the water structure depends on the presence of alcohol molecules and water clusters

already structured, a very simple continuous model is to assume that the modification ΔY is proportional to the product of the corresponding mole fractions. In the case of alcoholwater clusters, these contain equal parts of water and alcohol and the number of alcohol molecules in one cluster is half the number of alcohol molecules in one alcohol cluster of the same molecular size.14,15 This approximation has recently been applied successfully to ionic liquid28,29 and cyclic amide18 mixtures. The above equations correlate satisfactorily the wavenumbers for each solvent/indicator pair (Figures 1 and 2, solid lines). A global minimization procedure with a simulated annealing algorithm without restrictions was used to obtain reliable meaningful parameters.38 Deviations from the ideal (linear) behavior of the wavenumbers with bulk composition reveal preferential solvation and solutesolvent interaction, mainly H-bonding.39 The reasonably good behavior of the model has enabled us to determine the k, f2/1, f12/1, and f12/2 parameters (Tables 5S and 6S, Supporting Information), which reflect the relative propensity of the indicator to be solvated by pure ester (S2) and alcohol/ester mixtures (S12) relative to pure alcohol (S1) and pure ester (S2), respectively. The sequence f12/2 > f12/1 . f2/1 obtained for 4-nitrophenol and Reichardt’s dye suggests that the solvation ability of alkanol/methylbenzoate mixtures relative to ester is higher than 10264



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Figure 5. 1H NMR chemical shift, δ1H (ppm), in (x1) alkan-1-ol + (1 x1) methylbenzoate binary solvents at 298.15 K for (a) OH, alkan-1-ol, (b) CH3, methylbenzoate, and (c) o-CH, methylbenzoate: (b) methanol, (2) propan-1-ol, + pentan-1-ol, (9) heptan-1-ol, (1) nonan-1-ol, (f) undecan-1-ol.

Figure 6. 1H NMR chemical shift, δ1H (ppm), in (x1) methanol + (1 x1) alkylbenzoate binary solvents at 298.15 K for the groups: (a) OH methanol, (b) CH2/CH3, alkylbenzoate, and (c) o-CH, alkylbenzoate: (b) methylbenzoate, (+) ethylbenzoate, (2) propylbenzoate, (9) butylbenzoate.

relative to alcohol and even higher compared to pure constituents (Table 5S). The values for methanol and pentan-1-ol/alkylbenzoate mixtures (Table 6S) point to the same conclusion. For Reichardt’s dye, the propan-1-ol/methylbenzoate systems and those with methanol display highest solvation and H-bonding ability for short-chain alcohols. By contrast, the solvation ability of the mixture is affected very little by the ester chain length. For 2-nitroanisole the f12/2 values were slightly greater than f12/1 and f2/1 in alkan-1-ol/metylbenzoate mixtures (Table 5S) except with methanol, for which it turns out f2/1 > f12/1 (Table 6S), indicating that 2-nitroanisole is more sensitive to the alkylbenzoate polarity/polarizability compared to methanol/ alkylbenzoate. For 4-nitroaniline, f2/1 was always lower than f12/1 in alkan-1-ol/alkylbenzoate. For 4-nitrophenol the f2/1 values decreased for the same ester when the alcohol length was raised, resulting 1.5 units lower than f12/1. Therefore, 4-nitroaniline and 4-nitrophenol are more sensitive to alkan-1ol/alkylbenzoate relative to any pure constituent. By and large, for most of the systems studied it can be concluded that the alkanol/ester mixtures display stronger solvating power than the pure constituents. As for the ΔY values, the data collected (Tables 5S and 6S, Supporting Information) gave ΔY ≈ 0 with k = 0.00 for 38% of

the studied systems. For the remaining systems, the ΔY values were much lower than for ethanol/water and the same order (k ≈ 1) as those for ionic liquids. The very small or even null ΔY correction deduced for numerous alcohol/ester systems reveals that the number of molecules of each cosolvent in the solvation sphere differs from that in alcohol/water mixtures.14 1 H and 13C NMR Study. Further conclusions on solvent effects can be drawn from 1H and 13C NMR spectra; NMR measurements can shed abundant light into the behavior and structure of solvent mixtures40 and solventsolvent interactions.41 Solvent effects on chemical shifts, first observed by Bothner-By and Glick42 and Reeves and Schneider43 have been extensively studied, namely: (i) difference in magnetic susceptibility of solutes and solvents and (ii) nonspecific (dispersion, dipole dipole) and specific solutesolvent interactions in protic and aromatic solvents. 1H NMR is sensitive to H-bonding and molecular association. H-bonding causes the resonance signal to shift downfield, especially with electron-pair donor solvents.44,45 Tables 7S12S in the Supporting Information list the 1H and 13C NMR chemical shifts for pure alkanols, alkylbenzoates, and alkanol/alkylbenzoate mixtures. Figures 5 and 6 plot the 1H chemical shifts for x1 alkan-1-ol/(1 x1) methylbenzoate and x1 methanol/(1 x1) alkylbenzoate, respectively, and Figures 8 10265



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Figure 7. (a) Deviation of 1H NMR chemical shift Δδ(CH3(methylbenzoate)OH(alkan-1-ol)) with alcohol length. (b) Deviation of 1H NMR chemical shift Δδ(CH3(methanol)OH(methanol)) with ester chain length. (c) Deviation of 13C NMR chemical shift Δδ(CH3(methylbenzoate)COO(methylbenzoate)) with alcohol length. (d) Deviation of 13C NMR chemical shift Δδ(CH3(methanol)COO(methylbenzoate)) with ester chain length (b, methanol; 2, propan-1-ol; +, pentan-1-ol; 9, heptan-1-ol; 1, nonan-1-ol; f, undecan-1-ol). (b, methylbenzoate; +, ethylbenzoate; 2, propylbenzoate; 9, butylbenzoate).

and 9 the 13C shifts for x1 alkan-1-ol/(1 x1) methylbenzoate and x1 methanol/(1 x1) alkylbenzoate, respectively. The deviations Δδ relative to pure constituents were obtained as ∞ Δδ ¼ δmix ðx1 δ∞ 1 þ x2 δ2 Þ

ð8Þ δ∞ 1

δ∞ 2

where δmix stands for the mixing values and and for the chemical shifts for the lowest and highest mole fraction extrapolated from the curves.40,46 Figure 7 shows the alcohol effect on the Δδ values. Chemical shift δOH in alcohols as a function of temperature is sensitive to solvent composition and informative of self-association.47 Figures 5a and 6a show the δOH values for alkan-1-ol/ methylbenzoate and methanol/alkylbenzoate, respectively, at 298.15 K over the whole composition range. While the ester exerts no effect on δOH (Figure 6a), the δOH curves overlap in the x < 0.5 region; for methanol the curve differed from the remaining ones (Figure 5a). Except for methanol, the δOH values increased in the alcohol-rich region with a decrease in alcohol chain, revealing that, due to the methylbenzoate polarization, the self-associating OH effect is most effective with short alcohols. The considerable difference in chemical shift (2.53.0 ppm) observed for x1 = 0.051.0 (Figures 5a and 6a) reveals less

alcohol polarization in the ester-rich region relative to pure alcohol. The observed rising trend of δOH for alcohols suggests that H-bonding is more effective when the alcohol content increases, self-association being highest for pure alcohols;48 self-association diminishes as the alcohol polarity drops or the size rises, indicating weakening of H-bonding.49 The 1H NMR δCH3 and δo-CH methylbenzoate chemical shifts behave similarly with x1, increasing with alcohol size up to x1 < 0.7 and the opposite trend in the alcohol-rich stretch (Figures 5b and 5c); self-association diminishes with decreasing alcohol polarity or rising bulk alcohol, indicating weakening of H-bonding.49 The pronounced shift of the H-bonded proton is not fully accounted for by simple electrostatic effects; unshielding and anisotropic effect by an adjacent group can be of importance here. When the proton binds the center of the aromatic π-electron cloud, the ring current causes a noticeable upfield shift that prevails over the unshielding. H-bonding to ring π-electrons causes upfield rather than downfield shift. The H-bonding effect on the chemical shift of the acceptor proton gives way to larger displacement.1 Figure 6, b and c, shows the effect of methanol on the CH2/ CH3 and o-CH ester groups; a notable difference appears between 10266



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Figure 8. 13C NMR chemical shift, δ1C (ppm), in (x1) alkan-1-ol + (1 x1) methylbenzoate binary solvents at 298.15 K for (a) CH3, methylbenzoate, (b) COO, methylbenzoate (in the inset also is shown the COO chemical shift at infinite dilution as a function of the alcohol C atoms), and (c) C, methylbenzoate: (b) methanol, (2) propan-1-ol, (+) pentan-1-ol, (9) heptan-1-ol, (1) nonan-1-ol, (f) undecan-1-ol.

Figure 9. 13C NMR chemical shift, δ1C (ppm), in (x1) methanol + (1 x1) alkylbenzoate binary solvents at 298.15 K for the groups: (a) CH3, methanol, (b) COO, alkylbenzoate, and (c) C, alkylbenzoate: (b) methylbenzoate, (+) ethylbenzoate, (2) propylbenzoate, (9) butylbenzoate.

the CH3 chemical shift for methylbenzoate and the other esters. At low alcohol content, the δ1CH2/CH3 ester (Figure 6b) became larger with an increase in alkyl chain. For alcohol-rich mixtures no appreciable variation was observed with the ester content in the δo-CH, ring chemical shift (Figure 6c). For x < 0.8, δo-CH increased with the ester chain. Possible H-bonding in alkylbenzoate 3 3 3 HOR complexes strengthens the alcohol/ester interaction.49 The positive deviation from ideal behavior observed over the whole composition range for all mixtures entails hydrophobic aggregation. The Δδ(CH3(methylbenzoate)OH(alkan-1-ol)) values decreased with the alcohol length (Figure 7a), displaying a maximum around x1 = 0.3 for undecan-1-ol that shifted to larger alcohol content for shorter alcohols. The Δδ(CH3(methanol) OH(methanol)) values also dropped with a decrease in the ester chain (Figure 7b), the maximum denoting the composition where H-bonding prevails in methanol. 13 C NMR spectra serve to enlighten structural effects on the CH3 and CO groups. Figure 7c,d shows the alcohol content effect on the 13C shift. The Δδ(CH3(methanol)COO(methylbenzoate)) values were positive for low x1, turned to negative with added alcohol, and displayed a minimum in the alcohol-rich

region. The deviations butylbenzoate > propylbenzoate > ethylbenzoate > methylbenzoate give away a strengthened interaction with the ester size (Figure 7d). The Δδ[CH3(methylbenzoate) COO(methylbenzoate)] values, however, were positive for larger alcohols (Figure 7c); for ethanol and methanol it was positive only in dilute regions and negative for higher alcohol content. The minimum for methanol at x1 = 0.7 bears out strong H-bonding. Figure 8a shows the composition effect on the 13C chemical shift of the CH3 ester group in alcohol/methylbenzoate mixtures. For low alcohol content, the shifts increased slightly with the alcohol size; the rise in alcohol content caused the opposite effect, for methanol being highest. The δCH3 (methylbenzoate) values decreased in the ester-rich region with the alcohol size due to an increase in electron density of the CH3 group;48 further addition shifted the signal downfield. The δCOO curves (Figure 8b) were similar in shape to those of δp-CH and δm-CH ester ring (not shown). For low x1 only a modest change in chemical shift was observed with the alcohol length; however, as the alcohol content was raised, the gap increased sharply for shorter alcohols. The inset shows the δCOO chemical shift for alkan-1-ol/methylbenzoate at infinite dilution as a function of the number of C atoms; chemical shifts decreased 10267



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sensitive to electronic interaction. The νOH band obtained reflects the change of perturbation of the stretching frequency induced by H-bonding. The pronounced shift in the OH stretching of alkan-1-ol, brought about by ester addition, reveals weakening of H-bonding.51 As an example, the IR spectra recorded for methanol/methylbenzoate show that the OH stretching frequency shifts to lower values with an increase in alcohol content, indicating a decrease in the electron density of the CH bond.53 Similar effects were observed for the other systems. The OH region was not selected for a separate analysis due to the coincidence in the deconvolution with other bands, as well as the CO group absorption region.

Figure 10. Experimental FTIR-ATR spectra collected in the frequency region (3901.42601.5 cm1) for the methanol + methylbenzoate binary system over the whole composition range at 298.15 K showing the OH stretching vibration band νOH (cm1).

with an increase in alcohol length. The COO group displays H-bonding acceptor ability. In methanol the chemical shift was highest for the COO site; in fact, self-association in alcohols decreases with an increase in alcohol size, denoting a weakening of H-bonding.49 The δC trend (Figure 8c) for methylbenzoate was similar to that of δo-CH of the methylbenzoate ring; except for methanol, the shifts increased in both sets when the alcohol length was raised. Figure 9 shows the 13C shifts for methanol/alkylbenzoate over the composition range. For methylbenzoate mixtures, the values of the COO group in the p-C atom and the substituted C ring moved downfield relative to the other esters (Figure 9b,c), a feature ascribed to the influence of the bulk ester. For methanol (Figure 9a), no change in δCH3 was observed with the ester chain. The variation of the 13C chemical shift in the methanol CH3 group was only slightly dependent on the ester size. IR Measurements. A useful way to analyze H-bonding stems from FT-IR spectra, a powerful tool to study inter- and intramolecular association based on the location and bandwidth of the OH band and the intensity of the first overtone.50 H-bonding in alcohols exerts a considerable effect on the stretching vibration νOH in the 31003700 cm1 range, in which the spectra are informative of alcohol association. Typical variation of the IR spectra with increasing alcohol content has been observed.51 The acidity, steric, and self-association effects have significance on the OH stretching band.52 The frequency range is affected by the bond strength and masses involved as well. IR spectra for methanol/alkylbenzoate, pentan-1-ol/methylbenzoate, and undecan-1-ol/methylbenzoate mixtures were recorded over the whole composition range. Figure 10 plots the spectra collected in the (3901.42601.5 cm1) region for methanol/methylbenzoate, showing the OH stretching vibration band and the variation with composition. The maxima shifted to lower frequency; stepwise band tightening appeared as the alcohol content was raised. H-bonding alters the force constant of the groups involved, and the stretching and the bending vibration should change. Intermolecular H-bonding causes like or unlike association, i.e., dimerization/polymerization.50 The test was focused on the absorption of the OH H-bonding, particularly

’ CONCLUSIONS Spectroscopic data provide valuable information on the structure and interaction of alcohol/alkylbenzoate systems. The solvatochromic parameters and preferential solvation deduced give away complex interactions between the mixture constituents. The results point to strong alcohol/ester H-bonding that depends on the particular ester and alcohol self-association ability. The alkanol/methylbenzoate mixtures display stronger solvation ability than the pure solvents. The NMR measurements nicely adapted to study H-bonding; for alkan-1ol/alkylbenzoates, the chemical shifts of the proton signals unveiled noticeable hetero and mainly self-association. The pronounced chemical shift of the carboxylic C signal reveals ester/alcohol heteroassociation, a valuable clue to infer the solvent structure. The IR measurements bear out polarization and H-bonding. The data measured have disclosed three main features that govern the alcohol/ester structure: (i) disruption of alkan-1-ol H-bonds; (ii) formation of alkan-1-ol/ester H-bonded aggregates; and (iii) the geometry features of the components. The aggregates become weakened with an increase in the size of both ester and alcohol. ’ ASSOCIATED CONTENT

bS

Supporting Information. Appendix A and Tables 1S to 12S. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT The financial support by Junta de Castilla y Leon, Projects BU013A-09 and GR257, and Universidad de Burgos with funding by Caja de Burgos, Spain, is gratefully acknowledged. ’ REFERENCES (1) (a) Reichardt, C. Chem. Rev. 1994, 94, 2319–2358. (b) Reichardt, C. Solvents and Solvents Effects in Organic Chemistry; Wiley-VCH: Weinheim, Germany, 2003. (2) (a) El Seoud, O. A. Pure Appl. Chem. 2007, 79, 1135–1151. (b) El Seoud, O. A. Pure Appl. Chem. 2009, 81, 697–707. (3) Silva, P. L.; Trassi, M. A. S.; Martins, C. T.; El Seoud, O. A. J. Phys. Chem. B 2009, 113, 9512–9519. (4) Moita, M. L.; Teodoro, R. A.; Pinheiro, L. M. J. Mol. Liq. 2007, 136, 15–21. 10268


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Alkylbenzoate Binary Mixtures by

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