Thermal and Volumetric Properties of Four Aqueous Aroma

May 14, 2012 - The dissolution of all of ... infinite dilution partial molar excess enthalpy H̅1 ... partial molar excess enthalpy and heat capacity ...
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Thermal and Volumetric Properties of Four Aqueous Aroma Compounds at Infinite Dilution Vladimír Dohnal* and Karel Ř ehák Department of Physical Chemistry, Institute of Chemical Technology, 166 28 Prague 6, Czech Republic ABSTRACT: Mixing enthalpies and densities of highly dilute aqueous solutions of four aroma compounds (propyl acetate, methyl butyrate, 2,3butanedione, and cis-3-hexen-1-ol) were measured as a function of solution composition at several temperatures in the range from (288.15 to 318.15) K using a recently described tandem flow arrangement of isothermal mixing microcalorimeter and vibrating-tube densimeter. The dissolution of all of the aroma compounds in water is strongly exothermic and accompanied with volume contraction. Based on the measured data, reliable values of infinite dilution partial molar excess enthalpy H̅ E,∞ 1 , partial molar volume E,∞ V̅ ∞ of the studied solutes at infinite dilution in water were determined. A 1 , and partial molar excess volume V̅ 1 comparison to literature data is possible only for some of our results for esters and indicates excellent agreement. Thanks to the high precision of our measurements, the temperature derivative properties, that is, infinite dilution partial molar excess heat capacity and expansivity, could be evaluated also with a good accuracy. In addition, whenever necessary pure component data were available, enthalpies and heat capacities of hydration for the aroma compounds were calculated, too. The observed thermodynamic behavior for aqueous aroma solutes was briefly discussed in terms of molecular interactions.

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INTRODUCTION Aroma compounds present in food and beverages coexist in these products with water that forms more or less their bulk. Also, the isolation of aroma compounds from natural materials or their synthesis is often accompanied with a surplus of water. Since the aroma−water interaction greatly affects the phase distribution and the organoleptic perception of the aroma, for understanding and control of these processes the thermodynamic characterization of highly dilute aqueous aroma solutions is needed. The present work deals with the following four important aroma compounds: propyl acetate, methyl butyrate, 2,3butanedione (diacetyl), and cis-3-hexen-1-ol. The esters provide fruity scents and occur naturally in many plant products. Propyl acetate is known by its characteristic odor of pears and methyl butyrate by that of apples or pineapples. These esters are commonly used in the preparation of artificial flavors or fragrances.1−3 They also serve as special solvents: methyl butyrate, in particular, has been newly used in Li-ion battery electrolyte formulations to improve their performance at low temperatures.4 2,3-Butanedione bears the aroma of butter and occurs naturally also in aromas of many fruits and alcoholic beverages.5 cis-3-Hexen-1-ol (often called leaf alcohol) has a characteristic odor of freshly cut grass. It occurs naturally in leaves of many plants, for example, in green tea. This very important aroma compound is used to obtain natural green notes in perfumes and flavors.6 In this work, we report precise heat-of-mixing and density data for highly dilute aqueous solutions of the four aroma compounds obtained by tandem flow mixing calorimetry and vibrating-tube densimetry using a highly automated apparatus. © 2012 American Chemical Society

The measurements carried out at several temperatures cover the biologically and environmentally relevant range from (288.15 to 318.15) K. On the basis of the primary measured data we have evaluated various infinite dilution thermodynamic properties of the four aroma compounds in water, namely, partial molar excess enthalpy and heat capacity (dissolution enthalpy and heat capacity), hydration enthalpy and heat capacity, partial molar volume and expansivity, and partial molar excess volume and expansivity. The thermodynamic properties for aqueous 2,3-butanedione and cis-3-hexen-1-ol are not available and have been studied here for the first time. Some results obtained here for the esters can be compared to existing literature data.7−11

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EXPERIMENTAL SECTION Materials. Propyl acetate (mass fraction w = 0.995) and methyl butyrate (w = 0.99) were obtained from Sigma-Aldrich, 2,3-butanedione (w = 0.99) from Acros Organics, and cis-3hexen-1-ol (w = 0.98) from Merck. The samples were dried and stored with 4 A molecular sieves. Their declared purity was verified by gas chromatography using a DB-WAX capillary column. Water contents as determined by Fischer titration did not exceed 0.0002 mass fraction. Water was distilled and subsequently treated by a Milli-Q Water Purification System (Millipore, USA). Before measurements, all liquids were partially degassed by vacuum filtration and sonification. Received: March 2, 2012 Accepted: April 28, 2012 Published: May 14, 2012 1822

dx.doi.org/10.1021/je300280s | J. Chem. Eng. Data 2012, 57, 1822−1828

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Table 1. Experimental Densitiesa of Pure Solutes ρ and Their Comparison with Literature Values

Apparatus and Procedure. The measurements in this work were performed using a calo/densimeter described in detail previously.12 This combined instrument integrating an isothermal mixing microcalorimeter13 and a vibrating-tube densimeter into a tandem flow assembly with a fully automatic control of the entire experimental sequence was used to measure simultaneously mixing enthalpies HE and densities of highly dilute solutions as a function of solute mole fraction. The setup consists of a modified model 4400 isothermal differential heat conduction microcalorimeter (CSC, Provo, USA) equipped with flow mixing cells and a highly asymmetric pumping system capable of delivering accurately very small mass flow rates of one component. The calorimeter is characterized by a thermal power sensitivity of 0.1 μW and a time constant of 300 s. The pumping system is composed of two high-quality syringe pumps, a model HPP 5001 of 400 mL capacity from Laboratorní Přístroje (Praha, Czech Republic), and a model PHD 2000 from Harvard Apparatus (Holliston, MA, USA) equipped with a 8 mL stainless steel syringe. The vibrating-tube densimeter Anton Paar model DMA 5000 is characterized by a repeatability of 1·10−3 kg·m−3. The uncertainty of absolute setting of experimental temperature of the calorimeter and the densimeter is estimated to 10 mK. The experimental procedure was the same as described previously.12 Measurements were conducted at the solvent flow rate of 0.1 mL·min−1 and solute flow rates selected typically from the range (3 to 0.4) μL·min−1 so that sufficiently low concentrations allowing reliable extrapolation of data to infinite dilution were attained. The signals from the calorimeter and the densimeter were recorded at each composition for 5 h. The baseline was recorded for 3 h, both as the initial and the final step of the experimental run. A typical run examining six compositions in the range of solute mole fraction x1 from (0.007 down to 0.001) thus lasted 36 h. Signals corresponding to mixtures were averaged over integral multiples of the rotation period of the solute pump screw to compensate for mechanical imperfections of the pump. The calorimetric signal was calibrated using a Joule effect produced by a built-in calibration heater (6000 μW) on the pure solvent (water). The calibration of the densimeter was performed using water and decane whose densities were taken from the literature.14,15 In addition, ad hoc single-fluid calibrations of the densimeter were carried out at each run using a baseline signal corresponding to the pure solvent (water).

ρ/(kg·m−3) substance propyl acetateb

methyl butyratec

2,3-butanedioned

cis-3-hexen-1-ole

T/K

this work

288.15 298.15

893.20 882.05

308.15 318.15 288.15 293.15 298.15

870.87 859.52 903.34 897.88 892.38

303.15 308.15 313.15 318.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15 288.15 293.15 298.15 308.15 318.15

886.86 881.30 875.74 870.14 998.09 991.78 985.50 979.18 972.83 966.48 960.08 856.22 852.44 848.64 840.99 833.27

lit.

ref

882.1 882.4

19 20

897.87 892.35 892.4 892.49 886.84

21 21 22 23 21

875.71 873.7

21 22

a

The standard uncertainty of temperature is u(T) = 10 mK, and the combined expanded uncertainty of pure solute density is UC(ρ) = 0.4 kg·m−3 with a 0.95 level of confidence. bThe equation ρ/(kg·m−3) = 1216.61 − 1.1222T/K fits the present data with the standard deviation of 8·10−2 kg·m−3. cThe equation ρ/(kg·m−3) = 1222.30 − 1.1067T/K fits the present data with the standard deviation of 5·10−2 kg·m−3. d The equation ρ/(kg·m−3) = 1363.02 − 1.2663T/K fits the present data with the standard deviation of 5·10−2 kg·m−3. eThe equation ρ/(kg·m−3) = 1076.71 − 0.7651T/K fits the present data with the standard deviation of 5·10−2 kg·m−3.

perature were mostly collected from two experimental runs. On the basis of statistical analysis of replicated measurements, the standard uncertainty for density determinations was estimated to be u(ρ) = 5·10−3 kg·m−3 and that for excess enthalpy determinations from u(HE) = 0.005HE (for 2,3-butanedione) to u(HE) = 0.015HE (for cis-3-hexen-1-ol). Suitable extrapolations of primary experimental data given in Table 2 provide reliable values of respective partial molar properties at infinite dilution. The infinite dilution partial molar excess enthalpy H̅ E,∞ and 1 volume V̅ E,∞ were determined by linear extrapolation of the 1 quantity HE/(x1x2) and V E/(x1x2), respectively, as functions of x1 to infinite dilution. The excess volumes needed for the latter case were evaluated using pure solute densities determined in this work (Table 1) and recommended water density values.14 The corresponding extrapolation plots are exemplified in Figures 1 and 2. The infinite dilution value of the partial molar volume V̅ ∞ 1 (also denoted as the standard partial molar volume V̅ °1 ) was in turn obtained by adding the molar volume V̅ •1 to V̅ E,∞ 1 . The infinite dilution partial molar properties of studied solutes in water are listed in Table 3. Each reported value typically corresponds to the average of two replicated

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RESULTS AND DISCUSSION Densities of pure solutes at 288.15 K, 298.15 K, 308.15 K, and 318.15 K were measured first to provide reliable data for the determination of the composition of pumped streams and evaluation of excess volumes. For 2,3-butanedione and cis-3hexen-1-ol, the density was also determined at some additional temperatures in the range from (288.15 to 318.15) K, since no reliable information on density of these liquids could be found in the literature. The combined standard uncertainty of pure solute densities determined in this work is estimated to 0.2 kg·m−3. The measured densities of pure solutes and their comparison to literature values are given in Table 1. Good agreement observed for esters supports the claimed purity of their samples. Primary experimental excess enthalpy and density data for dilute aqueous solutions of the aroma compounds studied in this work are summarized in Table 2. The data at each tem1823

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Table 2. Primary Experimental Excess Enthalpy HE and Density ρ Dataa for Dilute Aqueous Solutions of Four Aroma Compounds at Several Near-Ambient Temperatures ρ

HE x1

−1

J·mol

kg·m

ρ

HE −3

x1

J·mol

−1

kg·m

ρ

HE −3

x1

J·mol

−1

−3

kg·m

Propyl Acetate 0.002494 0.002491 0.002182 0.002180 0.001871 0.001869 0.001560 0.001558 0.001248 0.001246 0.000935 0.002495 0.002494 0.002492 0.002183 0.002183 0.002181 0.001872 0.001871 0.001869 0.001560 0.001560 0.001558 0.001249 0.001248 0.001247 0.000937 0.000936 0.000935

0.001884 0.001882 0.001570 0.001569 0.001257 0.001255 0.000943 0.000941 0.000628 0.000628 0.002195 0.002197 0.001882 0.001883 0.001569 0.001570 0.001256 0.001256 0.000942 0.000942 0.000628 0.000628

T = 288.15 K −29.63 −29.40 −25.77 −25.92 −22.34 −22.40 −18.67 −18.90 −15.17 −15.23 −11.42 T = 298.15 K

−22.54 −19.51 −19.75 −19.76 −16.94 −17.03 −16.91 −14.33 −14.36 −14.51 −11.62 −11.57 −11.46 −8.87 −8.79 −8.63 T = 288.15 K −20.17 −19.81 −16.60 −16.85 −13.44 −13.35 −10.20 −10.35 −6.84 −6.88 T = 298.15 K −16.83 −16.89 −15.06 −14.67 −12.38 −12.55 −10.06 −10.08 −7.71 −7.64 −5.10 −5.15

998.882 998.876 998.906 998.909 998.936 998.934 998.962 998.959 998.988 998.986 999.019 996.655 996.669 996.669 996.703 996.709 996.714 996.748 996.758 996.762 996.803 996.805 996.807 996.848 996.852 996.856

0.002492 0.002492 0.002181 0.002181 0.001870 0.001870 0.001559 0.001559 0.001247 0.001247 0.000936 0.000936 0.002496 0.002493 0.002184 0.002182 0.001873 0.001870 0.001561 0.001559 0.001248 0.001247 0.000937 0.000936

996.774 996.773 996.800 996.810 996.850 996.838 996.885 996.884 996.917 996.921 996.968 996.965

0.002194 0.002191 0.001880 0.001878 0.001567 0.001565 0.001254 0.001253 0.000941 0.000939 0.000627 0.000626 0.002192 0.002195 0.001879 0.001882 0.001567 0.001569 0.001254 0.001255 0.000941 0.000942

ρ

J·mol−1

kg·m−3

2,3-Butanedione T = 308.15 K −15.49 993.500 −15.51 993.503 −13.89 993.559 −13.98 993.557 −11.96 993.628 −12.20 993.621 −10.27 993.691 −10.20 993.692 −8.21 993.761 −8.23 993.761 −6.18 993.831 −6.17 993.832 T = 318.15 K −9.24 989.532 −9.23 989.536 −8.30 989.612 −8.42 989.605 −7.21 989.698 −7.31 989.693 −6.18 989.779 −6.19 989.782 −5.12 989.860 −4.99 989.872 −3.90 989.951 −3.81 989.957

0.006153 0.006157 0.005132 0.005135 0.004111 0.004113 0.003086 0.003089 0.002059 0.002062 0.001030 0.001032 0.006158 0.005136 0.004114 0.003088 0.002060 0.001031 0.006160 0.006160 0.005137 0.005137 0.004114 0.004113 0.003089 0.003089 0.002062 0.002061 0.001032 0.001032

996.900 996.903 Methyl Butyrate 998.981 998.985 999.001 998.999 999.019 999.019 999.038 999.037 999.058 999.058

x1

HE

T = 308.15 K −11.37 993.630 −11.33 993.626 −9.91 993.682 −10.07 993.673 −8.33 993.742 −8.31 993.741 −6.75 993.798 −6.92 993.793 −5.18 993.855 −5.17 993.855 −3.51 993.914 −3.50 993.913 T = 318.15 K −5.89 989.694 −5.97 989.679 −5.26 989.762 −5.19 989.758 −4.42 989.837 −4.44 989.832 −3.62 989.911 −3.65 989.908 −2.76 989.988 −2.80 989.983

0.006159 0.005137 0.004114 0.003089 0.002061 0.001032

0.002135 0.001831 0.001526 0.001221 0.000916 0.000611 0.002132 0.001828 0.001524 0.001220 0.000915 0.000610 0.001835 0.001831 1824

T = 288.18 −121.13 −120.70 −101.00 −100.91 −80.90 −81.13 −61.00 −61.00 −40.83 −40.83 −20.37 −20.41 T = 293.15 −113.93 −95.12 −76.34 −57.80 −38.74 −19.32 T = 298.15 −109.21 −109.12 −91.19 −91.13 −73.01 −73.11 −55.12 −55.05 −36.92 −36.85 −18.55 −18.47 T = 303.15 −101.65 −84.92 −68.14 −51.29 −34.34 −17.20

K 1004.871 1004.865 1003.913 1003.912 1002.950 1002.963 1001.997 1002.000 1001.045 1001.038

0.006152 0.006160 0.005131 0.005137 0.004109 0.004114 0.003086 0.003088 0.002060 0.002060 0.001031 0.001031

1003.731 1002.818 1001.902 1000.996 1000.074 999.136

0.006161 0.005139 0.004116 0.003091 0.002062 0.001032

1002.315 1002.298 1001.445 1001.435 1000.562 1000.566 999.697 999.692 998.823 998.816 997.937 997.929

0.006164 0.006159 0.005140 0.005137 0.004116 0.004114 0.003090 0.003088 0.002062 0.002060 0.001032 0.001031

K

K

T = 308.15 K −97.25 998.821 −97.04 998.816 −81.21 998.031 −80.97 998.026 −64.77 997.222 −64.99 997.237 −48.74 996.432 −48.90 996.443 −32.50 995.637 −32.55 995.637 −16.16 994.828 −16.19 994.831 T = 313.15 K −89.14 996.763 −74.29 996.015 −59.38 995.251 −44.73 994.499 −29.69 993.745 −14.71 992.976 T = 318.15 K −84.58 994.503 −84.09 994.517 −70.35 993.799 −69.83 993.801 −55.76 993.069 −55.81 993.082 −41.67 992.361 −41.84 992.362 −27.65 991.650 −27.57 991.645 −13.61 990.928 −13.56 990.924

K

T = 288.15 K −19.77 −17.17 −14.39 −11.64 −8.79 −5.93 T = 298.15 K −13.21 −11.50 −9.67 −7.83 −5.97 −4.03 T = 308.15 K −6.10

1000.662 999.840 999.012 998.182 997.341 996.490 cis-3-Hexen-1-ol T = 318.15 K 997.950 998.106 998.270 998.432 998.596 998.763 995.788 995.963 996.139 996.319 996.493 996.680

0.002137 0.002137 0.001832 0.001833 0.001527 0.001528 0.001222 0.001223 0.000917 0.000917 0.000611 0.000612

−1.29 −1.30 −1.17 −1.19 −1.01 −1.00 −0.83 −0.81 −0.59 −0.58

988.803 988.815 988.988 988.984 989.183 989.184 989.380 989.388 989.578 989.591 989.794 989.796

992.883 992.912

dx.doi.org/10.1021/je300280s | J. Chem. Eng. Data 2012, 57, 1822−1828

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Table 2. continued ρ

HE x1 0.001529 0.001526 0.001224 0.001221 0.000918

−1

J·mol

T = 308.15 K −5.28 −5.22 −4.28 −4.28 −3.30

kg·m

ρ

HE −3

x1

J·mol

−1

kg·m

ρ

HE −3

x1

J·mol

−1

ρ

HE −3

kg·m

x1

−1

J·mol

kg·m−3

T = 308.15 K 993.059 993.064 993.253 993.253 993.441

0.000916

−3.29

993.442

0.000612

−2.23

993.637

0.000611

−2.21

993.642

0.000306

−1.17

993.834

a Standard uncertainties are u(x1) = 0.005x1, u(T) = 10 mK, and the combined expanded uncertainties are UC(HE) = 0.01HE for 2,3-butanedione, UC(HE) = 0.02HE for propyl acetate and methyl butyrate, UC(HE) = 0.03HE for cis-3-hexen-1-ol, and UC(ρ) = 1·10−2 kg·m−3 with a 0.95 level of confidence.

been previously studied. Our values of partial molar volumes of propyl acetate at infinite dilution in water are in excellent agreement with those of Sakurai et al.10 over the entire temperature range examined, as well as with a single value of Segatin and Klofutar11 at 298.15 K. The value of infinite dilution dissolution enthalpy of propyl acetate in water at 298.5 K reported by Huyskens and Vanderheyden9 is slightly less exothermic than the value of ours, but the difference is fully within a somewhat larger uncertainty of the literature value. A comparison of our values of H̅ E,∞ for methyl butyrate to 1 existing literature data in Table 3 shows perfect agreement with those of Nilsson and Wadsö 8 over the entire examined temperature range, but the value at 298.15 K reported by Della Gatta et al.7 is about 4 % less exothermic than the present value, the difference exceeding the combined uncertainty limits. Additional infinite dilution thermodynamic properties of four aqueous aroma solutes at T = 298.15 K, as derived from the present measurements, are given in Table 4. Values of the partial molar excess heat capacity C̅ E,∞ p,1 were obtained from fitting the linear temperature dependence to the H̅ E,∞ data which provides their adequate representation. 1 The hydration enthalpies ΔhydH1 were obtained from the present H̅ E,∞ data and the values of standard vaporiza1 tion enthalpies ΔvapHo1 of pure solutes from the literature according to

Figure 1. Extrapolation of measured excess enthalpy data to infinite dilution: propyl acetate (1) in water (2), 288.15 K; ■, measured = −12.50 kJ·mol−1. Error values; , linear fit; extrapolated value H̅ E,∞ 1 bars represent the standard uncertainty.

Δhyd H = H1̅ E, ∞ − Δ vapH1o

The hydration heat capacities Δ hydCp,1 were evaluated combining the present C̅ E,∞ p,1 values with the pure solute G,o heat capacities in the liquid C̅ L,• p,1 and the ideal gas state C̅ p,1 , respectively Δhyd Cp ,1 = C̅pE,,1∞ + CpL,,1• − C pG,o ,1

Figure 2. Extrapolation of measured excess volume data to infinite dilution: 2,3-butanedione (1) in water (2), 318.15 K, ●, measured = −15.45 cm3·mol−1. values; , linear fit; extrapolated value V̅ E,∞ 1 Error bars represent the standard uncertainty.

∞ Values of the partial molar expansivity E̅ ∞ 1 = (∂V̅ 1 /∂T) p and E,∞ ∞ the excess partial molar expansivity E̅ 1 = (∂V̅ E,1/∂T)p were calculated on the basis of linear temperature dependences of E,∞ V̅ ∞ 1 and V̅ 1 , respectively, which adequately fit the measured data. The temperature dependence of partial molar excess enthalpies and volumes is displayed in Figures 3 and 4, respectively, and parameters of all temperature dependence fits are collected in Table 5.

measurements. The combined standard uncertainties estimated using the error propagation law comprise contributions from all possible sources of error. As seen from Table 3, only some of our results for esters can be compared to data reported in the literature; no comparison is however possible for the other two solutes as they have not 1825

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∞ Table 3. Experimental Limiting Partial Molar Excess Enthalpies H̅ E,∞ 1 , Limiting Partial Molar Volumes V̅ 1 , and Limiting Partial Molar Excess Volumes V̅ E,∞ of Four Aroma Compounds at Infinite Dilution in Water along with Their Standard Uncertainties 1 and Comparison with Literature Data (in Parentheses)

H̅ E,∞ 1 solute propyl acetate

methyl butyrate

kJ·mol

kJ·mol

288.15

−12.50

0.10

298.15

−9.59 (−9.30)b

0.08 (0.75)b

308.15

−6.88

0.06

318.15

−4.37

0.05

288.15

−11.10 (−10.96)d −8.39 (−8.33)d (−8.04)e −5.75 (−5.67)d −3.14 (−3.13)d −19.84 −18.86 −17.98 −16.71 −15.73 −14.29 −13.16 −9.87 −6.76 −3.86 −1.10

0.09 (0.05)d 0.06 (0.05)d (0.11)e 0.04 (0.03)d 0.02 (0.04)d 0.05 0.08 0.05 0.05 0.05 0.08 0.05 0.05 0.04 0.03 0.01

308.15 318.15 2,3-butanedione

cis-3-hexen-1-ol

a

−1

T/K

298.15

288.15 293.15 298.15 303.15 308.15 313.15 318.15 288.15 298.15 308.15 318.15

V̅ ∞ 1

u(H̅ E,∞ 1 )

−1

V̅ E,∞ 1 −1

kJ·mol

cm ·mol 3

u(V̅ ) −1

cm3·mol−1

103.85 (103.84)a 105.25 (105.24)a (105.12)c 106.71 (106.69)a 108.24 (108.14)a 103.41

−10.49

−9.64

0.03 (0.05)a 0.05 (0.05)a (0.20)c 0.05 (0.05)a 0.06 (0.05)a 0.05

104.79

−9.66

0.08

106.18

−9.71

0.05

107.58

−9.79

0.10

69.16 69.86 70.76 71.56 72.47 73.25 74.21 110.37 111.52 112.72 113.95

−17.09 −16.94 −16.60 −16.36 −16.03 −15.83 −15.45 −6.60 −6.51 −6.38 −6.25

0.03 0.05 0.03 0.05 0.03 0.04 0.03 0.05 0.06 0.09 0.08

−10.54

−10.58 −10.58

Reference 10. bReference 9. cReference 11. dReference 8. eReference 7.

Table 4. Additional Infinite Dilution Thermodynamic Propertiesa of Four Aqueous Aroma Solutes at T = 298.15 K: Partial ∞ Molar Excess Heat Capacity C̅ E,∞ p,1 , Hydration Enthalpy ΔhydH1, Hydration Heat Capacity ΔhydCp,1, Partial Molar Expansivity E̅ 1 , E,∞ and Excess Partial Molar Expansivity E̅ 1 solute propyl acetate methyl butyrate 2,3-butanedione cis-hexen-1-ol

C̅ E,∞ p,1

ΔhydH1

ΔhydCp,1

E̅ ∞ 1

E̅E,∞ 1

J·K−1·mol−1

kJ·mol−1

J·K−1·mol−1

cm3·mol−1·K−1

cm3·mol−1·K−1

271 265 225 292

−49.4 −47.7b −58.7c −68.4d

0.146 0.139 0.169 0.119

−0.003 −0.005 0.055 0.012

b

e

331 330e n.a.f n.a.f

−1 −1 −1 Estimated standard uncertainties are as follows: u(C̅ E,∞ for esters, and u(ΔhydH1) = 1 kJ·mol−1 for p,1 ) = 6 J·K ·mol , u(ΔhydH1) = 0.4 kJ·mol ∞ E,∞ o −1 −1 3 −1 −1 b other solutes, u(ΔhydCp,1) = 15 J·K ·mol , u(E̅ 1 ) = u(E̅ 1 ) = 0.002 cm ·mol ·K . ΔvapH1 taken from CDATA database.24 cΔvapHo1 calculated G,o from vapor pressure data reported in ref 25. dΔvapHo1 approximated by that for 1-hexanol taken from the CDATA database.24 eCL,• p,1 and Cp,1 taken 24 f from the CDATA database. No data available on pure solute heat capacities. a

complex formation. The complexes are formed by hydrogen bonding of the solvent water proton to the electron lone pairs of the solute oxygen atoms. In the case of cis-3-hexen-1-ol, an additional hydrogen bond results from the interaction of its hydroxyl hydrogen with the lone electron pairs on the oxygen atom of water molecule. Because of the amphiphilic character of the solutes, the thermodynamic behavior of their solutions is strongly modulated by hydrophobic effects. This fact can be well-documented on the values of dissolution heat capacity given in Table 4. This quantity acts

As seen from Table 3, the dissolution of all studied solutes in water is strongly exothermic and accompanied with volume contraction. The effects correlate well with the oxygento-carbon atom ratio in the molecules of the solutes, the most negative H̅ E,∞ and V̅ E,∞ values being exhibited by 2,31 1 butanedione (O/C ratio = 0.50), while the least negative ones were exhibited by cis-3-hexen-1-ol (O/C ratio = 0.166). The values of H̅ E,∞ and V̅ E,∞ for the isomeric esters (O/C ratio = 1 1 0.2), lying in between, are closely similar. This observation convincingly indicates an appreciable effect of solute−solvent 1826

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Table 5. Parameters a and b of Linear Temperature Dependence Fits Y = a + b(T/K) of Experimental Infinite ∞ E,∞ Dilution Partial Molar Properties H̅ E,∞ 1 , V̅ 1 , and V̅ 1 Together with Respective Standard Deviations s of Fits and Coefficients of Determination r2 property Y

a

−1 H̅ E,∞ 1 /(kJ·mol ) 3 −1 V̅ ∞ /(cm ·mol ) 1 3 −1 V̅ E,∞ /(cm ·mol ) 1 −1 H̅ E,∞ 1 /(kJ·mol ) 3 −1 V̅ ∞ /(cm ·mol ) 1 3 −1 V̅ E,∞ /(cm ·mol ) 1 −1 H̅ E,∞ 1 /(kJ·mol ) 3 −1 V̅ ∞ /(cm ·mol ) 1 3 −1 V̅ E,∞ /(cm ·mol ) 1

Figure 3. Temperature dependence of infinite dilution partial of studied aroma compounds: points molar excess enthalpy H̅ E,∞ 1 are experimental data for ■, propyl acetate; ●, methyl butyrate; ▲, 2,3-butanedione; ▼, cis-3-hexen-1-ol; lines are respective linear fits.

−1 H̅ E,∞ 1 /(kJ·mol ) 3 −1 V̅ ∞ /(cm ·mol ) 1 3 −1 V̅ E,∞ 1 /(cm ·mol )

b

Propyl Acetate −90.49 0.2710 61.66 0.1463 −9.61 −0.0031 Methyl Butyrate −87.49 0.2652 63.35 0.1390 −8.18 −0.0050 2,3-Butanedione −84.71 0.2245 20.42 0.1689 −33.02 0.0551 cis-3-Hexen-1-ol −93.95 0.2921 75.94 0.1194 −10.01 0.0118

s

r2

0.14 0.05 0.02

0.9989 0.9996 0.8776

0.04 0.01 0.02

0.9999 0.9999 0.9328

0.17 0.06 0.05

0.9958 0.9989 0.9930

0.12 0.03 0.02

0.9993 0.9998 0.9932

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CONCLUSION Precise heat-of-mixing and density measurements performed in this work on highly dilute solutions of four important aroma compounds (propyl acetate, methyl butyrate, 2,3-butanedione, and cis-3-hexen-1-ol) in water as functions of composition and temperature enabled us to obtain accurate values of various infinite dilution thermal and volumetric properties of these aqueous solutes. The reported enthalpies and heat capacities of dissolution and hydration are crucial pieces of information for establishing reliable temperature dependences of corresponding phase equilibria (aqueous solubility and air−water partitioning, respectively, of these aroma substances) and hence are of essential practical importance. For example, the only air−water partitioning study on 2,3-butanedione carried out in a narrow temperature range by Strekowski and George18 yielded, in contrast to our value ΔhydH1 = (−58.7 ± 1) kJ·mol−1, only a very uncertain result (−53 ± 20) kJ·mol−1. Data on the thermal and volumetric infinite dilution properties presented in this work can be also helpful to improve predictive methods and useful in theoretical calculations to parametrize molecular solution models. Currently, further experimental work is under way in our laboratory focusing on liquid−liquid and vapor− liquid equilibria in solutions of the four aroma compounds in water with the goal to complete the picture of thermodynamic behavior of these aqueous solutes.

Figure 4. Temperature dependence of infinite dilution partial of studied aroma compounds: points are molar excess volume V̅ E,∞ 1 experimental data for ■, propyl acetate; ●, methyl butyrate; ▲, 2,3-butanedione; ▼, cis-3-hexen-1-ol; lines are respective linear fits.

as a structure indicatorthe higher the C̅ E,∞ p,1 value, the more structure in the solution compared to that in pure components. Note that the highest value of C̅ E,∞ p,1 is observed for cis-3-hexen-1-ol, while the lowest one for 2,3-butanedione. Apparently, the structuring of water molecules in the hydration shell around the hydrophobic solute is stronger than that caused by the solute−water hydrogen bonding. Data on partial molar expansivities are quite rare in the literature, despite the fact that they can be useful for discussing solute− water interactions, too.16 The analysis of expansivity data, owing to their fragmentary character and complexity of interpretation, is however beyond the scope of this work. Just note that values of close to zero we determined for propyl acetate and methyl E̅E,∞ 1 butyrate are in accord with an almost negligible temperature found by Sakurai et al.10,17 for other esters dependence of V̅ E,∞ 1 and alkylbenzenes at near-ambient temperatures.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +420 2 2044 4297. E-mail: [email protected]. Funding

This work was supported from Ministry of Education of the Czech Republic (Grant No. MSM 604 613 7307). Notes

The authors declare no competing financial interest.

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REFERENCES

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