Enthalpies of Dilution of Penicillamines in N,N-Dimethylformamide +

Article pubs.acs.org/jced

Enthalpies of Dilution of Penicillamines in N,N‑Dimethylformamide + Water Mixtures at 298.15 K Wei-Na Cheng, Xin-Gen Hu,* Zhao-Peng Jia, Zheng Guo, Hong-Yu Liang, and Guoyong Fang College of Chemistry and Materials Engineering, Wenzhou University, Zhejiang Wenzhou 325035, China S Supporting Information *

ABSTRACT: Dilution enthalpies of two penicillamine enantiomers, namely, D-penicillamine, L-penicillamine, and their racemate D,L-penicillamine, in N,N-dimethylformamide (DMF) + water mixtures of various compositions have been determined at 298.15 K respectively, using an isothermal titration calorimeter (MicroCal ITC200). According to the McMillan−Mayer theory, homochiral enthalpic pairwise interaction coefficients (hXX) of the three penicillamines in DMF + water mixtures of different mass fractions (wDMF = 0 to 0.3) have been calculated regressively. It is found that hXX coefficients of each enantiomer and their racemate change increasingly with wDMF to the maxima at wDMF = 0.20 and then decrease relatively rapidly, which follows the order hLL > hDD ≈ hRR > 0 (R represents the racemate mixture of penicillamine). The results are discussed from the point of view of competitive equilibrium between hydrophobic and hydrophilic interactions in solutions.

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kind of nonstandard α-amino acids bearing a sulfydryl group, it is a metabolite of penicillin, although it has no antibiotic properties. Here, on the basis of the McMillan−Mayer theory,21 dilution enthalpies of its enantiomers and racemate in various DMF + H2O mixtures have been determined at 298.15 K respectively, from which homotactic enthalpic pairwise interaction coefficients (hXX) have been calculated to discuss energetic discrimination effects between different pairs of these optical isomers.

INTRODUCTION Chiral discrimination is of great importance in many fields such as biology, chemistry, material science, and pharmaceutics, since enantiomers often show different physiological activities depending on their absolute configurations.1−8 Enantioselective interactions can be studied by various experimental methods, such as UV, IR, fluorescence spectrometry, circular dichroism (CD), NMR, mass spectrometry (MS), and so forth.9−16 In recent years, microcalorimetric techniques, in particular isothermal titration calorimetry (ITC), have become available. Through microcalorimetry, Takagi et al.17 found that there exists a very weak mixed enthalpy between two liquid chiral molecules (R and S). ́ Using a microcalorimeter, Ramirez-Gualito et al.18 determined the interaction energy between benzene and the different carbohydrates, to distinguish different configurations among sugars. Thermodynamic parameters for inclusion complexation of 43 enantiomeric pairs of chiral guests with β-cyclodextrin have been determined microcalorimetrically by Rekharsky and Inoue,19 to reveal the thermodynamic origin of enantioselectivity and the enthalpy− entropy compensation effect. In our previous work,20 dilution enthalpies of enantiomers of some β-amino alcohols in dimethyl sulfoxide (DMSO) + water mixtures of various mass fractions have been determined by ITC. As a continuation of our interests in energetic effects in molecular pairwise interaction and chiral discrimination mediated by water and polar aprotic cosolvent, D-penicillamine, L-penicillamine, and D,L-penicillamine were chosen as research objects in this work. Penicillamine is well-known as a pharmaceutical of the chelator class, and its pharmaceutical form is D-configuration, while the 8 L-configuration is toxic (it inhibits the action of pyridoxine). As a © 2012 American Chemical Society

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EXPERIMENTAL SECTION Information about chemical samples is shown in Table 1 and the structures of two enantiomers of penicillamines in Figure 1. Table 1. Experimental Sample Information chemical name

source

D-penicillamine

Sigma Sigma D,L-penicillamine TCI N,N-dimethylformamide Sigma water Millipore Elix5/Milli-Q Academic system L-penicillamine

initial mass purification fraction purity method >0.99 0.99 0.98 0.998

none none none none

All of the solutions were prepared by mass using a Sartorius balance with precision of 0.00001 g. Dilution enthalpies were determined at 298.15 K by an isothermal titration calorimeter Received: July 14, 2012 Accepted: November 23, 2012 Published: December 5, 2012 55

dx.doi.org/10.1021/je300783e | J. Chem. Eng. Data 2013, 58, 55−63

Journal of Chemical & Engineering Data

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Article

RESULTS AND DISCUSSION

In our previous work, to check the accuracy of apparatus and method in use, the values23−25 of hXX were listed as a table in ref 20. The experimental systematic errors are considered to arise from either the principles of calorimetry or the different types of microcalorimeters. As an example, the typical titration curve of D-penicillamine in water at 298.15 K can be seen in Figure 2, and the experimental Figure 1. Structures of the two enantiomers of penicillamines.

(MicroCal ITC200), which requires only a 40 μL sample (onesixth of that needed by VP-ITC, the early ITC type of MicroCal). All of the solutions were degassed before use. The sample cell was loaded with 200 μL of H2O (or aqueous mixed solvent), and the 40 μL syringe was filled with penicillamine solution, which was prepared by the same solvent. A titration run consisted of consecutive injections of 2 μL volume and 5 s duration each, with an interval of 2 min between them. The heat effect ΔH(mN−1,mN) per injection was determined by automatic peak integration of thermal power (P) vs time (t) curve recorded. Thermal effects relating to the friction from each injection were considered to be negligible in all experiments. It is reasonable to ignore the changes of volume, but it is necessary for us to pay attention to the small deviation concomitantly. Every sample was determined for three times in the same condition, and the average value of ΔH (mN−1→mN) was calculated according to eq 1.21 ΔH(mN − 1 → mN ) = ΔH(mN − 1 , mN )/nP

Figure 2. Typical titration curve of D-penicillamine in water at 298.15 K.

values of ΔH(mN−1→mN) of D-penicillamine in water plotted as a function of the injection number N are given in Figure 3. The

(1)

where N indicates the number of injections, and nP is the moles of solute in each injection volume (Vinj = 2 μL), which can be calculated as follows: nP = 109Vinjρsol b0 ≈ 109VinjρH O m0

(2)

2

−3

in which ρsol and m0 are the density (kg·m ) and the concentration of the initial solution in the syringe, respectively, and m0 is the well-known molality, mol of solute/kg of solution. If the solutions used are all at lower concentrations, the densities of them can be assumed to be that of pure water. From the McMillan−Mayer framework,22 the thermodynamic formula commonly used to deal with the excess enthalpy of a binary or ternary solution containing solute X and solvent Y (Y = pure water or aqueous mixed solvent) can be expressed as follows, HE(mX ) = hXX mX + hXXX mX2 + ...

(3)

in which, hXX, hXXX, and so forth are known as pairwise, triplet, and higher order enthalpic interaction coefficients, respectively. To evaluate these coefficients, dilution enthalpies of a binary solution (X+Y) are needed. The method of measuring dilution enthalpy by ITC was described elsewhere by us and others (see also Supporting Information).21 The equation necessary for the regression analysis is as follows,

Figure 3. Experimental values of ΔH(mN−1→mN) of D-penicillamine in water as a function of injection number N at 298.15 K (R2 = 0.9990, SD = 0.62). Notes: R2 is the square of correlation coefficient, and SD is the standard deviation of regression.

experimental values of ΔH(mN−1→mN) as a function of N from the dilution experiments of D-penicillamine, L-penicillamine, and D,L-penicillamine in DMF + water mixtures of different mass fractions (wDMF) are listed in Tables 2 to 5. The values of hXX were calculated from the slopes of linear regression of the experimental data based on eq4, all with the squares of correlation coefficients R2 > 0.95. The trends of hXX with wDMF are illustrated in Figure 4, and the corresponding values are reported in Table 6.

ΔH(mN − 1 → mN ) = 2hXX m1N − [hXX (m1 + m0) + hXXX m02] + ... (4)

where mN (N = 1, 2, 3, ...) represents the molality of the solution containing solute X and solvent Y in the measuring cell of ITC, and m0 (mN−1, N = 1) is the initial molality of titrant solution in the syringe. 56



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Table 2. Experimental Dilution Enthalpies of D-Penicillamine, L-Penicillamine, and D,L-Penicillamine in Pure Water at T = 298.15 K and P = 0.1 MPaa N

mN−1

mN

ΔH(mN−1→mN)

mol·kg−1

mol·kg−1

J·mol−1

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

D-Penicillamine (m0 = 0.3187 mol·kg 0.00158 0.00473 0.00473 0.00784 0.00784 0.01092 0.01092 0.01396 0.01396 0.01698 0.01698 0.01996 0.01996 0.02292 0.02292 0.02584 0.02584 0.02873 0.02873 0.03158 0.03158 0.03441 0.03441 0.03721 0.03721 0.03997 0.03997 0.04270 0.04270 0.04540 0.04540 0.04807 0.04807 0.05070 0.05070 0.05331 0.05331 0.05588

2 3 4 5 6 7 8 9 10 11

0.00159 0.00473 0.00785 0.01094 0.01399 0.01701 0.02000 0.02296 0.02589 0.02878

0.00473 0.00785 0.01094 0.01399 0.01701 0.02000 0.02296 0.02589 0.02878 0.03165

N

−1

mN−1

mN

mol·kg−1

mol·kg−1

) L-Penicillamine (m0 = 0.3177 mol·kg −235.74 (0.11) 2 0.00158 0.00471 −232.99 (0.04) 3 0.00471 0.00781 −228.69 (0.09) 4 0.00781 0.01088 −226.59 (−0.02) 5 0.01088 0.01392 −220.84 (−0.14) 6 0.01392 0.01693 −217.71 (0.08) 7 0.01693 0.01990 −213.78 (0.48) 8 0.01990 0.02285 −209.97 (0.04) 9 0.02285 0.02576 −205.75 (−0.05) 10 0.02576 0.02864 −203.16 (−0.20) 11 0.02864 0.03149 −197.50 (0.23) 12 0.03149 0.03431 −194.77 (−0.14) 13 0.03431 0.03709 −191.33 (0.04) 14 0.03709 0.03985 −186.62 (−0.01) 15 0.03985 0.04257 −183.92 (0.03) 16 0.04257 0.04526 −179.75 (−0.14) 17 0.04526 0.04792 −177.43 (0.04) 18 0.04792 0.05055 −174.06 (0.15) 19 0.05055 0.05315 −169.87 (0.15) 20 0.05315 0.05571 −1 D,L-Penicillamine (m0 = 0.3193 mol·kg ) −240.49 (0.27) 12 0.03165 0.03448 −237.39 (1.96) 13 0.03448 0.03728 −236.80 (0.88) 14 0.03728 0.04005 −232.52 (0.65) 15 0.04005 0.04278 −228.33 (0.87) 16 0.04278 0.04549 −224.47 (1.09) 17 0.04549 0.04816 −220.82 (0.81) 18 0.04816 0.05080 −217.09 (0.86) 19 0.05080 0.05341 −212.81 (1.03) 20 0.05341 0.05599 −209.41 (1.02)

ΔH(mN−1→mN) J·mol−1 −1

) −247.51 (0.25) −245.00 (0.13) −241.93 (−0.33) −237.58 (−0.05) −232.86 (−0.89) −229.26 (−1.27) −225.72 (−1.50) −222.34 (−0.81) −217.58 (−0.67) −213.00 (−1.15) −209.15 (−0.84) −205.74 (−0.65) −202.56 (−0.83) −198.37 (−0.75) −194.58 (−0.99) −190.37 (−0.98) −187.27 (−1.19) −183.19 (−1.17) −180.28 (−0.80) −205.68 (0.79) −202.15 (0.93) −197.81 (0.91) −194.20 (0.92) −190.54 (0.97) −186.75 (0.87) −184.21 (0.91) −181.17 (0.88) −177.11 (0.79)

a The values in parentheses are the evaluated uncertainties, 100[ΔH(exp) − ΔH(cal)]/ΔH(cal), in which ΔH(exp) represents the experimental value of ΔH(mN−1→mN) and ΔH(cal) represents the calculated value of ΔH(mN−1→mN). Standard uncertainties u are u(T) = 0.01 K, u(m0) = 0.0001 mol·kg−1, u(w) = 1·10−8 kg, u(p) = 2.09·10−6 w, and the combined expanded uncertainties u are u(ΔH) = 0.26 J·mol−1.

First, it can be found that hXX values of the two enantiomers and the racemate of penicillamine are all positive (Table 6), which are corresponding to negative values of dilution enthalpies, that is, ΔH(mN−1→mN) < 0 (Tables 2 to 5) and that there is a certain difference in the magnitude of hXX between D- and L-penicillamines as well as D,L-penicillamine. More interestingly, the values of hXX for D-enantiomer are generally less than those of L-enantiomer across the whole studied composition range of the mixed solvent (Figure 4). The results imply that the absorbed energy of pairwise interaction of D-enantiomer in solutions must be less than that of L-enantiomer, as the former releases less heat than the latter in the process of dilution. As for D,L-penicillamine, which is a racemic mixture, its values of hXX are slightly lower than those of D-penicillamines over the same range of mixed solvents. Therefore, we have the order of hXX for the three penicillamines, that is hLL > hDD ≈ hRR > 0, in which R represents D,L-penicillamine. To explain the interactions in 1 M racemate, we should use the parameters for the D- and L-forms and assume the solution is 0.5 M in each of the D- and L-forms, as a result of hRR = 1/2hDD + 1/2hLL + hDL, from Table 6, so the values of hDL can be calculated, hDL < 0. As have been found previously,20,24 microcalorimetric measurement does provide a convenient and nonspecific method unlike spectroscopy for the chiral discrimination

between different optical isomers. In the two enantiomer solutions, there exist either D/D or L/L interactions, but in the racemate solution, there exist complex interactions including D/D, L/L, and D/L interactions. Consistently, the results of hXX can be explained from a competitive equilibrium between hydrophobic and hydrophilic interactions in the different solution. As shown in the experimental data of Figure 4, the difference of hXX between the chiral molecules is enough to distinguish them. That the hXX of racemate is close to that of the D-enantiomer is a comprehensive result of complex interactions, which remains to be of further research. Second, from Figure 4, it can be seen that the values of hXX for the two penicillamine enantiomers and their racemate exhibit nearly linear growth with wDMF up to the maxima at wDMF ≈ 0.20 and then decrease relatively rapidly with the rising of wDMF when wDMF ≈ 0.20. The concave down trends of hXX can be interpreted from the point of view of competitive equilibrium between hydrophobic and hydrophilic interactions in aqueous ternary solutions under study. As a highly polar aprotic solvent, DMF is well-known as its planar nitrogen bond configuration arising from the resonance structures between the carbonyl group and the nitrogen lone pair of electrons.26 Due to the contribution of the two possible 57



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resonance structures, the bond order of the CO bond is reduced, while that of the C−N bond is increased, resulting in its partial double bond character. Therefore, the negative pole on

the oxygen atom that juts out from the rest of the DMF molecule, while the positive pole on the nitrogen atom is buried within the molecule by the two methyl groups. Through the unshared pair

Table 3. Experimental Dilution Enthalpies of D-Penicillamine in DMF + Water Mixtures (Mass Fraction w) at T = 298.15 K and P = 0.1 MPaa N

3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 3 4 5 6 7 8 9 10 11 12 13 14 15 16

mN−1

mN

ΔH(mN−1→mN)

mol·kg−1

mol·kg−1

J·mol−1

N

D-Penicillamine in DMF + Water w = 0.0500 (m0 = 0.3179 mol·kg−1) 0.00471 0.00782 −242.87 (−0.04) 3 0.00782 0.01089 −240.77 (0.50) 4 0.01089 0.01393 −236.80 (0.20) 5 0.01393 0.01694 −233.33 (0.36) 6 0.01694 0.01992 −228.27 (0.70) 7 0.01992 0.02286 −224.76 (0.19) 8 0.02286 0.02578 −219.34 (1.04) 9 0.02578 0.02866 −215.40 (0.80) 10 0.02866 0.03151 −211.07 (0.72) 11 0.03151 0.03433 −207.36 (0.58) 12 0.03433 0.03712 −204.13 (0.19) 13 0.03712 0.03987 −200.45 (0.34) 14 0.03987 0.04260 −195.71 (0.42) 15 0.04260 0.04529 −192.51 (0.56) 16 0.04529 0.04795 −187.61 (0.72) 17 0.04795 0.05059 −184.45 (0.89) 18 0.05059 0.05318 −180.56 (0.40) 19 0.05318 0.05575 −177.58 (0.86) 20 w = 0.1499 (m0 = 0.3184 mol·kg−1) 0.00472 0.00783 −242.84 (0.18) 3 0.00783 0.01091 −242.63 (0.10) 4 0.01091 0.01395 −238.18 (0.34) 5 0.01395 0.01696 −233.89 (−0.54) 6 0.01696 0.01995 −229.03 (−1.56) 7 0.01995 0.02290 −226.07 (0.24) 8 0.02290 0.02581 −220.83 (−0.93) 9 0.02581 0.02870 −217.11 (0.38) 10 0.02870 0.03156 −212.80 (−0.02) 11 0.03156 0.03438 −208.72 (−0.04) 12 0.03438 0.03717 −205.07 (−0.30) 13 0.03717 0.03993 −201.22 (0.10) 14 0.03993 0.04266 −196.85 (0.13) 15 0.04266 0.04536 −192.91 (−0.09) 16 0.04536 0.04802 −188.62 (0.11) 17 0.04802 0.05066 −185.28 (0.07) 18 0.05066 0.05326 −182.08 (−0.07) 19 0.05326 0.05583 −177.74 (0.06) 20 w = 0.2516 (m0 = 0.3170 mol·kg−1) 0.00470 0.00779 −200.19 (−0.70) 3 0.00779 0.01086 −199.03 (−0.34) 4 0.01086 0.01389 −195.71 (−0.41) 5 0.01389 0.01689 −192.34 (−0.67) 6 0.01689 0.01986 −189.43 (−0.46) 7 0.01986 0.02279 −186.30 (−0.12) 8 0.02279 0.02570 −183.65 (−0.68) 9 0.02570 0.02857 −179.71 (−0.35) 10 0.02857 0.03142 −176.05 (−0.60) 11 0.03142 0.03423 −172.15 (−0.76) 12 0.03423 0.03701 −169.37 (−0.97) 13 0.03701 0.03975 −167.03 (0.15) 14 0.03975 0.04247 −162.17 (−0.83) 15 0.04247 0.04516 −159.33 (−0.76) 16

58

mN−1

mN

ΔH(mN−1→mN)

mol·kg−1

mol·kg−1

J·mol−1

w = 0.1000 (m0 = 0.4053 mol·kg−1) 0.00601 0.00997 −323.54 (0.73) 0.00997 0.01388 −319.47 (0.73) 0.01388 0.01776 −313.61 (0.80) 0.01776 0.02159 −308.87 (0.75) 0.02159 0.02539 −303.44 (0.77) 0.02539 0.02915 −297.60 (0.76) 0.02915 0.03286 −292.71 (0.73) 0.03286 0.03654 −287.05 (0.69) 0.03654 0.04017 −282.77 (0.47) 0.04017 0.04376 −276.42 (0.56) 0.04376 0.04732 −271.74 (0.35) 0.04732 0.05083 −267.09 (0.40) 0.05083 0.05431 −262.16 (0.58) 0.05431 0.05774 −256.74 (0.74) 0.05774 0.06113 −252.14 (0.86) 0.06113 0.06449 −247.63 (0.71) 0.06449 0.06780 −242.62 (0.86) 0.06780 0.07107 −236.70 (−0.05) w = 0.2000 (m0 = 0.3173 mol·kg−1) 0.00471 0.00780 −260.10 (−0.94) 0.00780 0.01087 −259.85 (0.01) 0.01087 0.01390 −255.38 (−1.55) 0.01390 0.01691 −251.11 (−1.61) 0.01691 0.01988 −247.38 (−0.70) 0.01988 0.02282 −238.86 (−1.22) 0.02282 0.02573 −236.58 (−1.62) 0.02573 0.02860 −230.95 (−1.85) 0.02860 0.03145 −228.08 (−0.65) 0.03145 0.03426 −221.72 (−2.04) 0.03426 0.03705 −219.21 (−1.62) 0.03705 0.03980 −214.48 (−0.88) 0.03980 0.04252 −209.21 (−1.58) 0.04252 0.04520 −205.10 (−1.78) 0.04520 0.04786 −201.07 (−1.25) 0.04786 0.05049 −196.06 (−2.50) 0.05049 0.05308 −192.44 (−2.06) 0.05308 0.05564 −190.03 (0.89) w = 0.3008 (m0 = 0.3167 mol·kg−1) 0.00470 0.00779 −137.98 (−1.06) 0.00779 0.01085 −136.49 (−1.41) 0.01085 0.01388 −134.30 (−1.36) 0.01388 0.01687 −132.47 (−1.26) 0.01687 0.01984 −130.52 (−1.30) 0.01984 0.02278 −127.84 (−1.48) 0.02278 0.02568 −126.02 (−0.98) 0.02568 0.02855 −123.67 (−1.55) 0.02855 0.03139 −120.87 (−1.56) 0.03139 0.03420 −118.56 (−1.71) 0.03420 0.03698 −116.60 (−1.68) 0.03698 0.03972 −114.47 (−1.65) 0.03972 0.04244 −112.08 (−1.63) 0.04244 0.04512 −110.03 (−1.61) dx.doi.org/10.1021/je300783e | J. Chem. Eng. Data 2013, 58, 55−63


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Table 3. continued mN−1 N

mol·kg

−1

ΔH(mN−1→mN)

mN mol·kg

−1

J·mol

mN−1

−1

N

−1

17 18 19 20

mol·kg

ΔH(mN−1→mN)

mN

−1

mol·kg

−1

J·mol−1 −1

w = 0.2516 (m0 = 0.3170 mol·kg ) 0.04516 0.04781 −155.97 (−0.51) 0.04781 0.05043 −153.17 (−0.12) 0.05043 0.05302 −149.73 (−0.08) 0.05302 0.05558 −146.71 (0.01)

17 18 19 20

w = 0.3008 (m0 = 0.3167 mol·kg ) 0.04512 0.04777 −107.98 (−1.61) 0.04777 0.05039 −106.13 (−1.45) 0.05039 0.05298 −103.82 (−1.62) 0.05298 0.05554 −101.70 (−1.53)

a The values in parentheses are the evaluated uncertainties 100[ΔH(exp) − ΔH(cal)]/ΔH(cal), in which ΔH(exp) represents the experimental value of ΔH(mN−1→mN) and ΔH(cal) represents the calculated value of ΔH(mN−1→mN). Standard uncertainties u are u(T) = 0.01 K, u(m0) = 0.0001 mol·kg−1, u(w) = 1·10−8 kg, u(p) = 2.09·10−6 w, and the combined expanded uncertainties u are u(ΔH) = 0.26 J·mol−1.

Table 4. Experimental Dilution Enthalpies of L-Penicillamine in DMF + Water Mixtures (Mass Fraction w) at T = 298.15 K and P = 0.1 MPaa N

3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 3 4 5 6 7 8

mN−1

mN

ΔH(mN−1→mN)

mol·kg−1

mol·kg−1

J·mol−1

N

L-Penicillamine in DMF + Water w = 0.0500 (m0 = 0.4024 mol·kg−1) 0.00597 0.00989 −319.25 (−0.05) 3 0.00989 0.01378 −314.17 (0.09) 4 0.01378 0.01763 −308.93 (−0.11) 5 0.01763 0.02144 −303.91 (−0.04) 6 0.02144 0.02521 −298.26 (−0.04) 7 0.02521 0.02893 −285.92 (−1.46) 8 0.02893 0.03262 −286.78 (0.16) 9 0.03262 0.03627 −280.87 (0.02) 10 0.03627 0.03988 −275.25 (−0.10) 11 0.03988 0.04345 −268.65 (−0.06) 12 0.04345 0.04698 −264.45 (−0.02) 13 0.04698 0.05046 −259.32 (−0.13) 14 0.05046 0.05391 −253.96 (−0.31) 15 0.05391 0.05732 −249.19 (−0.32) 16 0.05732 0.06069 −244.66 (−0.20) 17 0.06069 0.06402 −240.88 (0.03) 18 0.06402 0.06731 −236.48 (0.02) 19 0.06731 0.07056 −231.92 (0.07) 20 w = 0.1499 (m0 = 0.3177 mol·kg−1) 0.00471 0.00781 −270.00 (0.31) 3 0.00781 0.01088 −264.25 (0.72) 4 0.01088 0.01392 −257.14 (1.20) 5 0.01392 0.01693 −251.92 (0.68) 6 0.01693 0.01991 −245.43 (0.93) 7 0.01991 0.02285 −239.17 (1.29) 8 0.02285 0.02576 −230.93 (2.51) 9 0.02576 0.02864 −229.61 (0.76) 10 0.02864 0.03149 −224.54 (0.83) 11 0.03149 0.03431 −219.31 (1.05) 12 0.03431 0.03710 −215.04 (1.04) 13 0.03710 0.03985 −210.63 (1.00) 14 0.03985 0.04258 −206.03 (1.09) 15 0.04258 0.04527 −201.63 (0.97) 16 0.04527 0.04793 −197.31 (1.44) 17 0.04793 0.05056 −192.84 (1.31) 18 0.05056 0.05316 −188.35 (1.40) 19 0.05316 0.05572 −183.65 (1.36) 20 w = 0.2516 (m0 = 0.3169 mol·kg−1) 0.00470 0.00779 −221.59 (−0.69) 3 0.00779 0.01086 −217.11 (−0.91) 4 0.01086 0.01389 −213.40 (−0.44) 5 0.01389 0.01689 −208.20 (−0.75) 6 0.01689 0.01985 −203.63 (−0.92) 7 0.01985 0.02279 −199.39 (−0.83) 8

59

mN−1

mN

ΔH(mN−1→mN)

mol·kg−1

mol·kg−1

J·mol−1

w = 0.1000 (m0 = 0.3178 mol·kg−1) 0.00471 0.00782 −262.29 (0.19) 0.00782 0.01089 −258.62 (0.23) 0.01089 0.01393 −251.65 (−0.47) 0.01393 0.01693 −244.64 (−0.15) 0.01693 0.01991 −238.67 (−0.20) 0.01991 0.02286 −232.95 (−0.15) 0.02286 0.02577 −227.96 (−0.23) 0.02577 0.02865 −222.25 (0.07) 0.02865 0.03150 −218.48 (−0.09) 0.03150 0.03432 −212.44 (0.47) 0.03432 0.03711 −208.98 (0.18) 0.03711 0.03986 −205.08 (−0.03) 0.03986 0.04259 −200.71 (−0.11) 0.04259 0.04528 −196.68 (0.09) 0.04528 0.04794 −192.61 (−0.02) 0.04794 0.05057 −189.69 (0.01) 0.05057 0.05317 −185.93 (0.01) 0.05317 0.05574 −182.23 (−0.11) w = 0.2000 (m0 = 0.3153 mol·kg−1) 0.00468 0.00775 −273.61 (−1.01) 0.00775 0.01080 −270.05 (−0.93) 0.01080 0.01381 −262.39 (−1.79) 0.01381 0.01680 −255.99 (−1.73) 0.01680 0.01975 −250.10 (−1.55) 0.01975 0.02267 −243.40 (−1.56) 0.02267 0.02556 −238.46 (−1.16) 0.02556 0.02842 −232.49 (−1.82) 0.02842 0.03125 −226.65 (−1.47) 0.03125 0.03404 −221.81 (−1.11) 0.03404 0.03681 −215.98 (−0.08) 0.03681 0.03954 −213.46 (−1.13) 0.03954 0.04224 −208.40 (−1.11) 0.04224 0.04492 −203.82 (−1.71) 0.04492 0.04756 −199.38 (−1.61) 0.04756 0.05016 −195.19 (−1.70) 0.05016 0.05274 −190.64 (−1.51) 0.05274 0.05529 −186.48 (−1.57) w = 0.3008 (m0 = 0.3159 mol·kg−1) 0.00468 0.00777 −159.50 (−0.23) 0.00777 0.01082 −156.55 (−0.47) 0.01082 0.01384 −154.28 (−0.47) 0.01384 0.01683 −151.10 (−0.81) 0.01683 0.01979 −148.77 (−0.58) 0.01979 0.02272 −145.71 (−0.71) dx.doi.org/10.1021/je300783e | J. Chem. Eng. Data 2013, 58, 55−63


Article


mol·kg

−1

ΔH(mN−1→mN)

mN mol·kg

−1

J·mol

mN−1

−1

N

−1

9 10 11 12 13 14 15 16 17 18 19 20

mol·kg

ΔH(mN−1→mN)

mN

−1

mol·kg

−1

J·mol−1 −1

w = 0.2516 (m0 = 0.3169 mol·kg ) 0.02279 0.02570 −195.41 (−0.88) 0.02570 0.02857 −191.49 (−0.84) 0.02857 0.03141 −187.69 (−0.94) 0.03141 0.03422 −183.67 (−0.82) 0.03422 0.03700 −180.24 (−0.85) 0.03700 0.03975 −176.99 (−0.71) 0.03975 0.04247 −173.23 (−0.94) 0.04247 0.04515 −169.24 (−0.54) 0.04515 0.04780 −166.01 (−0.62) 0.04780 0.05043 −163.18 (−0.41) 0.05043 0.05302 −159.41 (−0.28) 0.05302 0.05558 −156.41 (−0.36)

9 10 11 12 13 14 15 16 17 18 19 20

w = 0.3008 (m0 = 0.3159 mol·kg ) 0.02272 0.02561 −143.49 (−0.38) 0.02561 0.02848 −140.46 (−0.64) 0.02848 0.03131 −137.59 (−0.62) 0.03131 0.03411 −135.08 (−0.41) 0.03411 0.03688 −132.91 (−0.29) 0.03688 0.03962 −130.56 (−0.55) 0.03962 0.04233 −127.11 (−0.01) 0.04233 0.04500 −125.38 (−0.65) 0.04500 0.04765 −122.98 (−0.76) 0.04765 0.05026 −120.86 (−0.47) 0.05026 0.05285 −118.16 (−0.62) 0.05285 0.05540 −115.40 (−0.30)


Table 5. Experimental Dilution Enthalpies of D,L-Penicillamine in DMF + Water Mixtures (Mass Fraction w) at T = 298.15 K and P = 0.1 MPaa N

3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

mN−1

mN

ΔH(mN−1→mN)

mol·kg−1

mol·kg−1

J·mol−1

N

D,L-Penicillamine in DMF + Water w = 0.0500 (m0 = 0.3185 mol·kg−1) 0.00472 0.00783 −230.24 (−0.08) 3 0.00783 0.01091 −228.78 (−1.23) 4 0.01091 0.01395 −226.98 (−1.65) 5 0.01395 0.01697 −224.09 (−1.22) 6 0.01697 0.01995 −220.08 (−1.43) 7 0.01995 0.02290 −216.17 (−1.27) 8 0.02290 0.02582 −212.57 (−1.40) 9 0.02582 0.02871 −208.12 (−1.65) 10 0.02871 0.03157 −203.85 (−1.17) 11 0.03157 0.03439 −199.48 (−0.81) 12 0.03439 0.03718 −195.63 (−1.29) 13 0.03718 0.03994 −192.60 (−1.09) 14 0.03994 0.04267 −188.55 (−0.98) 15 0.04267 0.04537 −184.37 (−1.08) 16 0.04537 0.04804 −178.87 (−2.52) 17 0.04804 0.05067 −177.68 (−1.31) 18 0.05067 0.05328 −173.47 (−1.51) 19 0.05328 0.05585 −169.59 (−1.89) 20 w = 0.1499 (m0 = 0.3173 mol·kg−1) 0.00470 0.00780 −246.80 (0.61) 3 0.00780 0.01087 −244.26 (0.83) 4 0.01087 0.01390 −240.09 (0.68) 5 0.01390 0.01690 −236.29 (0.85) 6 0.01690 0.01988 −231.69 (0.79) 7 0.01988 0.02282 −227.49 (0.82) 8 0.02282 0.02572 −223.53 (0.89) 9 0.02572 0.02860 −219.10 (0.91) 10 0.02860 0.03145 −215.07 (0.97) 11 0.03145 0.03426 −210.43 (1.03) 12 0.03426 0.03704 −206.62 (1.04) 13 0.03704 0.03979 −202.98 (0.99) 14 0.03979 0.04251 −198.61 (1.11) 15 0.04251 0.04520 −194.60 (1.03) 16 0.04520 0.04786 −190.89 (1.11) 17 0.04786 0.05048 −187.39 (1.10) 18 0.05048 0.05308 −183.93 (1.05) 19

60

mN−1

mN

ΔH(mN−1→mN)

mol·kg−1

mol·kg−1

J·mol−1

w = 0.1000 (m0 = 0.3176 mol·kg−1) 0.00471 0.00781 −237.05 0.00781 0.01088 −234.66 0.01088 0.01392 −230.84 0.01392 0.01693 −227.29 0.01693 0.01990 −223.36 0.01990 0.02284 −218.90 0.02284 0.02576 −214.28 0.02576 0.02864 −210.40 0.02864 0.03148 −206.74 0.03148 0.03430 −201.93 0.03430 0.03709 −198.43 0.03709 0.03984 −194.49 0.03984 0.04256 −190.39 0.04256 0.04526 −186.63 0.04526 0.04792 −182.84 0.04792 0.05054 −179.76 0.05054 0.05314 −176.00 0.05314 0.05571 −171.98 w = 0.2000 (m0 = 0.2436 mol·kg−1) 0.00361 0.00599 −201.73 0.00599 0.00834 −199.97 0.00834 0.01067 −198.72 0.01067 0.01298 −195.78 0.01298 0.01526 −193.26 0.01526 0.01752 −189.48 0.01752 0.01975 −185.82 0.01975 0.02196 −182.83 0.02196 0.02414 −179.17 0.02414 0.02630 −174.93 0.02630 0.02844 −171.89 0.02844 0.03055 −168.51 0.03055 0.03264 −164.58 0.03264 0.03470 −161.79 0.03470 0.03674 −157.72 0.03674 0.03875 −154.22 0.03875 0.04075 −151.37

(−0.06) (0.01) (−0.20) (−0.16) (−0.23) (−0.15) (−0.11) (−0.08) (−0.11) (−0.19) (−0.21) (−0.11) (−0.11) (−0.08) (−0.19) (0.10) (0.01) (−0.02) (−0.81) (−1.20) (−1.98) (−1.69) (−1.44) (−1.84) (−1.86) (−1.27) (−1.36) (−1.39) (−1.34) (−1.59) (−1.89) (−1.66) (−1.63) (−1.46) (−1.10)



Article


mol·kg

−1

ΔH(mN−1→mN)

mN mol·kg

−1

J·mol

mN−1

−1

N

−1

20 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

w = 0.1499 (m0 = 0.3173 mol·kg ) 0.05308 0.05564 −180.10 w = 0.2516 (m0 = 0.2300 mol·kg−1) 0.00341 0.00566 −150.96 0.00566 0.00788 −149.12 0.00788 0.01008 −148.03 0.01008 0.01226 −145.76 0.01226 0.01441 −144.03 0.01441 0.01654 −141.77 0.01654 0.01865 −139.99 0.01865 0.02074 −136.63 0.02074 0.02280 −134.06 0.02280 0.02484 −131.15 0.02484 0.02686 −129.44 0.02686 0.02885 −126.99 0.02885 0.03083 −124.33 0.03083 0.03278 −121.89 0.03278 0.03470 −119.42 0.03470 0.03660 −117.26 0.03660 0.03849 −113.27 0.03849 0.04034 −110.95

mol·kg

ΔH(mN−1→mN)

mN

−1

mol·kg

−1

J·mol−1 −1

(1.14)

20

(1.40) (2.08) (1.16) (1.14) (1.55) (0.75) (1.19) (0.78) (0.03) (0.14) (0.93) (0.93) (0.83) (1.04) (1.15) (1.43) (−1.35) (−0.34)

3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

w = 0.2000 (m0 = 0.2436 mol·kg ) 0.04075 0.04271 −147.94 w = 0.3008 (m0 = 0.2020 mol·kg−1) 0.00300 0.00497 −95.25 0.00497 0.00692 −94.80 0.00692 0.00885 −94.06 0.00885 0.01076 −93.32 0.01076 0.01266 −92.48 0.01266 0.01453 −91.64 0.01453 0.01638 −90.08 0.01638 0.01821 −88.52 0.01821 0.02002 −86.74 0.02002 0.02181 −85.41 0.02181 0.02359 −84.02 0.02359 0.02534 −82.95 0.02534 0.02707 −81.52 0.02707 0.02878 −79.39 0.02878 0.03047 −77.66 0.03047 0.03214 −75.80 0.03214 0.03380 −75.13 0.03380 0.03543 −74.35

(−1.34) (−0.51) (−0.46) (−0.64) (−0.58) (−0.41) (0.00) (0.63) (1.03) (0.85) (0.51) (0.38) (0.20) (0.52) (0.29) (0.20) (−0.55) (−0.28) (−0.03)


Table 6. Homochiral Enthalpic Pairwise Interaction Coefficients of D-Penicillamine, L-Penicillamine, and D,L-Penicillamine in DMF + Water Mixtures (Mass Fraction w) at T = 298.15 K and P = 0.1 MPaa hXX/J·kg·mol−2 wDMF

D-penicillamine

L-penicillamine

D,L-penicillamine

0 0.0500 0.1000 0.1499 0.2000 0.2516 0.3008

584.51 (± 4.26) 623.45 (± 3.18) 631.42 (± 8.33) 637.48 (± 3.14) 700.37 (± 0.32) 544.59 (± 12.88) 345.09 (± 0.35)

613.19 (± 3.19) 650.33 (± 2.48) 742.82 (± 0.83) 782.72 (± 6.04) 819.94 (± 27.32) 604.57 (± 15.87) 408.36 (± 2.68)

583.65 (± 5.75) 604.60 (± 12.87) 622.64 (± 3.60) 634.99 (± 4.21) 674.17 (± 2.59) 522.95 (± 8.41) 331.83 (± 10.77)

a The values in parentheses are the evaluated error ± [∑3i=1|hXX(i) − h̅XX|]/3, in which hXX(i) represents the value of hXX in a single determination of ITC and h̅XX represents the average value of hXX for three determinations of ITC, h̅XX = [∑3i=1|hXX(i)]/3.

Figure 4. Enthalpic pairwise interaction coefficients (hXX) of D -penicillamine, L -penicillamine, and D , L -penicillamine in DMF + H2O mixtures as a function of mass fraction (wDMF) at 298.15 K (■, D-penicillamine; ●, L -penicillamine; △, D,L-penicillamine).

formation of hydrogen bonds for its free electron being protected by two methyl groups (see Figure 5).29 The ratios between the

of electrons, the negatively charged, well-exposed oxygen atom of DMF can be solvated strongly in solution. The addition of DMF to water diminishes the dielectric constant and results in strengthened electrostatic interaction arising from the formation of hydrogen bonds between DMF and water molecules.27 Due to the resonance structure of DMF molecule, hydrogen bonds between water and the carbonyl oxygen of DMF are stronger than those between water molecules themselves.28 In diluted aqueous solutions, the DMF molecule is assumed to be bound by two water molecules with its amide oxygen in the first resonance structure, while the nitrogen atom cannot take part in the

Figure 5. Hydrogen bonding structure of DMF with water.

number of hydrogen bonds to one and two water molecules increase with wDMF. Therefore, the dimer DMF·H2O seems to be 61



■

more stable than the trimer DMF·(H2O)2. Some average numbers of hydrogen bonds are relatively large in diluted aqueous solutions, and the water structure breaks more quickly in high concentrations of DMF. In the DMF-rich region, water molecules are apt to form one or two hydrogen bonds with DMF molecules and form fewer hydrogen bonds with water molecules.30 As these electrostatic interactions occur, the structure of water is broken. The overall sign and magnitude of hXX are considered to be the result of the competitive balance of all interaction factors in the system. On one hand, when the hydrated DMF molecules interact with the hydrated carboxyl (−COOH) and amino (−NH2) groups of penicillamine molecules, there is a relaxation of water molecules from the hydration shell around the polar groups of solutes to the bulky water, which makes a negative contribution to the value of hXX. This tendency becomes even more obvious as the increasing of DMF concentration in the mixture, since DMF molecules are considered to be structure breakers toward ordered water in this DMF-rich region.30 As shown in Figure 4, at approximatively wDMF = 0.20, the value of hXX arrives at a maximum point and then decreases quickly. On the other hand, both hydrophobic−hydrophobic interactions between nonpolar methyls of penicillamine and hydrophobic−hydrophilic interactions of the methyls of penicillamine with H2O and the polar CO group of DMF predominate over all other interactions in the studied system and make a determinant positive contribution to hXX. In general, the possible interactions between a pair of homochiral enantiomer of penicillamine, as well as a pair of their racemate, which are mediated strongly by DMF and H2O in solution, can be classified as follows: (a) Hydrophobic−hydrophobic interactions of CH3-residues of one penicillamine enantiomer with those of another of the same chirality, coupled with similar interactions with the CH3-group of DMF molecules, all of which make larger positive contributions on hXX. In addition, the formation of possible intramolecular hydrogen bonds in penicillamine molecule strengthens such interactions greatly with the help of preferential configuration. (b) Hydrophobic−hydrophilic interactions of CH3-residues of one penicillamine enantiomer with −OH, −SH, −COOH, and −NH2 residues of another of the same chirality, along with similar interactions with CO of DMF and −OH of H2O, all of which usually make positive contributions to hXX. (c) Hydrophilic−hydrophilic interactions of −OH, −SH, −COOH, and −NH2 residues of one penicillamine enantiomer with those of another of the same chirality, together with similar interactions with CO of DMF and −OH of H2O, all of which are mainly in the form of intermolecular hydrogen bond and bring about the broken structure of water and make negative contributions on hXX. In conclusion, considering that the values of hXX obtained are all positive for the three penicillamines across the whole studied composition range of the mixed solvent, we believe that both interactions a and b play a dominant role in pairwise interactions and are strengthened gradually with wDMF in water-rich region of the mixed solvent (wDMF < 0.20); however, when the content of DMF is added up to the value wDMF > 0.20, interaction c is strengthened greatly and leads to the rapid decrease of hXX in this DMF-rich region.

Article

ASSOCIATED CONTENT

S Supporting Information *

Discussion of the value of dilution enthalpy measurement. This material is available free of charge via the Internet at http://pubs. acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +86-0577-86596022. Funding

This work was financially supported by the National Natural Science Foundation of China (No. 21073132). Notes

The authors declare no competing financial interest.

■

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Enthalpies of Dilution of Penicillamines in N,N-Dimethylformamide +

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