Apparent Molar Volumes and Expansibilities of Thiourea, 1,3

Article pubs.acs.org/jced

Apparent Molar Volumes and Expansibilities of Thiourea, 1,3Dimethylurea, and 1,3-Dimethylthiourea in Water at Temperatures from T = (278.15 to 318.15) K and Atmospheric Pressure Evgeniy V. Ivanov* and Vladimir K. Abrosimov Laboratory of Thermodynamics of Solutions of Non-electrolyte and Biologically Active Substances, G.A. Krestov Institute of Solution Chemistry of Russian Academy of Sciences, 1 Akademicheskaya Street, Ivanovo, Russian Federation 153045 ABSTRACT: Densities of dilute aqueous solutions of thiourea, 1,3-dimethylurea, and 1,3dimethylthiourea were obtained using the Anton Paar DMA 5000 M vibrating-tube densimeter. The apparent molar volumes and expansibilities (down to infinite dilution) were computed from the measured density data. The experiments were performed at five temperatures from (278.15 to 318.15) K and atmospheric pressure. A comparison of volumetric and some other thermodynamic changes caused by the N,N′-methylation in thiourea and urea molecules are discussed.

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INTRODUCTION Volumetric properties for aqueous solutions involving organic molecular compounds are important for process design and knowledge of structure-packing transformations due to intermolecular interactions. Apparent (and partial) molar volumes are among the most often determined characteristics of such binary systems. To know the solute−solute and solute− solvent interaction behavior, the apparent/partial properties of dilute solutions (down to infinite dilution) in a wide temperature range are significant.1−4 In this context, volumetric properties of aqueous urea (U) and some of its N-alkyl-substituted derivatives, as small model bioactive solutes (being predominantly hydrophilic or hydrophobic in nature), have been rather well investigated,5−23 while thiourea (TU) and N-alkylated thioureas have been far less explored. On the basis of the results of heat capacity,24 isopiestic25 and enthalpy-related26 measurements, it was shown that the hydrogen-bonding ability of the functional grouping −N(H)−C(S)−N(H)− can be modulated by the insertion of alkyl substituents. Herewith the “structural topography” of such hydrophobic moieties plays an important role in determining their bonding/hydration as found for U and its N-alkyl-substituted derivatives.13,18,27,28 As a consequence, thioureas are widely utilized in the field of molecular recognition,29,30 as well as for the construction of nanostructured materials31 and pharmaceuticals.32,33 Moreover, taking into account the fact that ureas and thioureas differ in the molecule polarity (dipole moments of the latter were found to be slightly higher),34−36 a comparative analysis of volumetric properties of aqueous U, TU, and their alkyl-substituted derivatives could yield useful information on the interaction of © XXXX American Chemical Society

>CO and >CS moieties with the surrounding water molecules. In this work, we report the results of densimetric measurements and derived from them data on the apparent volumetric molar characteristics of a solute for highly and infinitely dilute aqueous solutions of TU, 1,3-dimethylurea (1,3-DMU), and 1,3-dimethylthiourea (1,3-DMTU) in the temperature range from (278.15 to 318.15) K (with a step of 10 K) at atmospheric pressure. As far as the authors know, the chosen binary liquid systems have been investigated densimetrically either only at T = 298.15 K (in the case of TU) 8,35−37 or at some other temperatures (for 1,3DMU). 12,14,18,19,22,23 But, unlike the (water +U) system,5,6,9−12,15−21 no systematic temperature-dependent density measurements at high dilutions for the aqueous solutions under consideration are presently available.

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EXPERIMENTAL SECTION Materials. The chemical sample descriptions are given in Table 1. For the purpose of this investigation, TU and 1,3DMU were purified by recrystallization from absolute ethanol (Fluka puriss, x ≥ 0.998), while 1,3-DMTU was purified by recrystallization from anhydrous tetrahydrofuran (Fluka puriss, x ≥ 0.999) followed with slow precipitation by the addition of diethyl ether (Fluka, ACS reagent, x > 0.995) to suppress the supercooling effect, according to the recommendations of Della Gatta and Badea et al.26,28 All compounds were then dried to

Received: September 21, 2012 Accepted: March 30, 2013

A

dx.doi.org/10.1021/je301203z | J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

Table 1. Sample Description abbreviated name TU 1,3-DMTU 1,3-DMU a

IUPAC name thiourea 1,3-dimethyl-2thiourea 1,3-dimethylurea

CAS No.

source

initial mole fraction purity

purification metod

initial mole fraction purity

analysis method

62-56-6 534-13-4

Aldrich-Sigma Aldrich-Sigma

0.99 0.99

recrystallization recrystallization

0.996 0.996

IR spectraa IR spectraa

96-31-1

Aldrich

0.99

recrystallization

0.995

IR spectraa

Analyzed using the AVATAR 360 FT-IR high-resolution spectrometer.

constant mass under reduced (down to 10 Pa) pressure at T ≈ 310 K. Used for the solution preparation, the deionized water was triply distilled (initially from a KMnO4 dilute solution) in a quartz still, and its conductance was κ ≈ 1.3·10−5 S·m−1. Densimetric Measurements. The solutions were prepared by weighing (under airtight conditions), from partially degassed components using an analytical balance (LLB200 Model, Russia) with a precision of ± 5·10−5 g. Their compositions were expressed in the form of molality, m, ranging from ca. (0.05 to ca. 1.5) mol·kg−1. The overall uncertainty in the m value was estimated to be less than ± 3·10−5 mol·kg−1. Densities of solutions, ρ, were measured using an Anton Paar DMA 5000 M vibrating-tube high precision densimeter operated under the static mode. The temperature of the measuring cell was kept constant to ± 0.01 K at a temperature chosen. The apparatus was calibrated with dry air and freshly prepared water just prior to each series of ρ measurements. The water densities, ρ1, were assumed to be those of the IAPWS Formulation 1995.38 The analysis of temperature-dependent ρ1 values made additionally with a precise bicapillary Oswald-type pycnometer indicated that the examined sample of water was different from the reference one38−40 no more than by 1·10−5 g·cm−3, as a whole. The filling of the densimeter cell with solution (whose volume did not exceed 1.5 cm3 at the temperature desired) was realized by self-flowing without contact with the atmospheric air. Every solution sample was slightly preheated above the measurement temperature to prevent possible bubble-forming (being visually observed) and then step-by-step cooled before making density serial measurements. Under such conditions, 5fold ρ measurements were reproducible to within ± 8·10−6 g·cm−3. Taking into account the influence of all possible factors, the error of the measured ρ did not exceed statistically ± 1.5·10−5 g·cm−3.

k−1

ρ (m ) =

where k, the number of ai coefficients used in eq 2, has been determined by applying an F-test41 at the 95% confidence level. In our case, k was found to be 3 for all the aqueous systems considered. Table 3 lists the adjustable parameters ai obtained in the regression, together with the standard errors of the fit σ defined by41 ⎡ 1 σ (ρ ) = ⎢ ⎣n − k

⎤1/2

∑ (ρcalcd −ρexptl )2 ⎥⎦

(3)

In eq 3, n represents the number of direct experimental concentration-dependent values at each temperature taken (see in Table 2). To compare the concentration-dependent volumetric characteristics of U and TU with their N,N′-dimethyl-substituted analogues in aqueous solution, we show the Vϕ,2 against m dependences at three reference temperatures in Figure 1panels a to d. One can see that the most significant difference between TU or U and 1,3-DMTU or 1,3-DMU is the change in sign of the Vϕ,2 vs m slope on methylation. In the case of aqueous TU and U, there is a clear positive (∂Vϕ,2/∂m)T,p at least at T < 298 K, although this derivative decreases in magnitude with increasing temperature and may become temperature-independent or slightly negative at higher T (see Figure 1a,b). It is noteworthy also the fact that the given curves (being close to linear) exhibit a weakly pronounced minimum appearing clearly at T = 318 K and shifting probably toward higher molalities as the temperature increases. Figure 1 panels c and d illustrate that (∂Vϕ,2/∂m)T,p for aqueous 1,3-DMTU and 1,3-DMU are negative in sign and visually comparable in magnitude, although these N,N′-dimethyl-substituted derivatives differ from each other by both the interacting (mainly, donor-accepting) capability of thionyl and carbonyl as well as amino groups and their structure-packing properties in the hydrated state.24−28,35,36 In general, this is an expected consequence of the Wen and Saito45,46 model being applied previously by Philip et al.7 to the aqueous urea and its methyl-substituted derivatives. According to inferences,7 the solutes under consideration have been described as both “structure breakers” and “structure makers” with respect to the structure of aqueous surroundings. In the first case (for the TU and U solutions), the positive slope can be interpreted as the “substitutional” solution process (due to filling up the void space in the solvent) resulting in breakdown of the water structure due to the solute−solvent interaction. Alternatively negative slope in the second case (for the 1,3DMTU and 1,3-DMU solutions) is attributed to an “interstitial” solution process, in which the alkyl chains are hidden in a complete or partial solvent cage; conjugated to the solvent induced solute−solute association,46 this results in a

RESULTS AND DISCUSSION Density and Apparent Molar Volume. In Table 2, the measured densities and the apparent molar volumes derived from them, Vϕ,2, for dissolved components at each temperature are presented. The latter quantities were computed using the formula Vϕ ,2

(2)

i=0

■

(ρ − ρ1) ·103 M2 = − ρ mρρ1

∑ aimi

(1)

Here, M2 is the molar mass of a solute (TU, 1,3-DMU, or 1,3DMTU) in g·mol−1. The previously obtained temperaturedependent data15 on ρ and Vϕ,2 for the (water + U) system are included in the table, too. To calculate adequately the smoothed ρ(m) values (for practical and other purposes), for each concentration dependence studied, the density data have been fitted by a leastsquares polynomial regression to B



Article

Table 2. Experimental Densities, ρ/(g·cm−3), and Apparent Molar Volumes, Vφ,2/(cm3·mol−1), for Aqueous Solutions of Thiourea, Urea, 1,3-Dimethylthiourea and 1,3-Dimethylurea at Different Temperatures, T/K, and Molalities, m/(mol·kg−1) and at p ≈ 0.1 MPaa 278.15 K m

ρ

288.15 K Vϕ,2

ρ

0.05326 0.09670 0.21150 0.31509 0.40681 0.49771 0.60709 0.74869 0.91871 0.94601 1.0535 1.2243 1.3577 1.5185

1.00124 1.00227 1.00496 1.00736 1.00945 1.01150 1.01394 1.01703 1.02066 1.02124 1.02348 1.02696 1.02962

52.18 52.19 52.23 52.27 52.30 52.34 52.38 52.44 52.51 52.52 52.57 52.65 52.72

1.00030 1.00127 1.00381 1.00607 1.00804 1.00998 1.01228 1.01521 1.01866 1.01921 1.02134 1.02466 1.02720 1.03019

0.04989 0.05022 0.09992 0.1009 0.1991 0.2013 0.4998 0.5003 0.9984 1.0006 1.4998 1.5013

1.00084 1.00085 1.00171 1.00173 1.00343 1.00346 1.00856 1.00856 1.01670 1.01672 1.02446 1.02451

42.45 42.45 42.46 42.46 42.47 42.47 42.51 42.51 42.58 42.59 42.68 42.68

0.99993 0.99993 1.00076 1.00077 1.00237 1.00240 1.00722 1.00721 1.01493 1.01495 1.02229 1.02235

0.05361 0.09097 0.20178 0.29704 0.40888 0.56180 0.78496 0.89219 1.0165 1.0420 1.2100 1.4656 1.5036

1.00075 1.00129 1.00288 1.00424 1.00582 1.00796 1.01103 1.01249 1.01416 1.01450 1.01672 1.02002 1.02051

89.57 89.54 89.46 89.40 89.33 89.23 89.09 89.03 88.96 88.94 88.85 88.71 88.69

0.99984 1.00034 1.00185 1.00313 1.00462 1.00663 1.00952 1.01089 1.01246 1.01278 1.01486 1.01797 1.01842

0.05811 0.09557 0.20236 0.29879 0.39710 0.48199 0.57345 0.69526 0.80011 0.88735 0.98531 1.1377 1.3082 1.4997

1.00051 1.00087 1.00188 1.00279 1.00372 1.00452 1.00539 1.00654 1.00753 1.00836 1.00929 1.01073 1.01235 1.01417

78.62 78.59 78.51 78.44 78.37 78.30 78.23 78.14 78.06 78.00 77.92 77.81 77.68 77.54

0.99962 0.99995 1.00089 1.00174 1.00260 1.00335 1.00415 1.00522 1.00614 1.00690 1.00775 1.00908 1.01056 1.01221

298.15 K ρ

Vϕ,2

308.15 K Vϕ,2

Water + Thiourea 53.63 0.99819 54.74 53.63 0.99911 54.74 53.65 1.00154 54.75 53.67 1.00370 54.75 53.69 1.00559 54.76 53.71 1.00744 54.77 53.73 1.00965 54.78 53.77 1.01246 54.79 53.81 1.01577 54.81 53.82 1.01630 54.82 53.85 1.01835 54.83 53.90 1.02156 54.86 53.94 1.02401 54.89 54.00 1.02691 54.92 Water + Ureab 43.48 0.99783 44.28 43.48 0.99784 44.28 43.48 0.99861 44.28 43.48 0.99864 44.28 43.48 1.00018 44.28 43.48 1.00019 44.28 43.50 1.00480 44.29 43.50 1.00479 44.29 43.54 1.01218 44.32 43.55 1.01219 44.32 43.60 1.01922 44.37 43.60 1.01928 44.37 Water + 1,3-Dimethylthiourea 90.47 0.99775 91.29 90.44 0.99823 91.26 90.37 0.99967 91.20 90.31 1.00088 91.14 90.25 1.00230 91.08 90.16 1.00421 91.00 90.03 1.00696 90.88 89.97 1.00826 90.82 89.90 1.00975 90.76 89.89 1.01005 90.75 89.80 1.01202 90.67 89.68 1.01497 90.56 89.66 1.01540 90.54 Water + 1,3-Dimethylurea 79.29 0.99753 79.91 79.27 0.99785 79.89 79.20 0.99874 79.83 79.14 0.99954 79.78 79.08 1.00035 79.73 79.02 1.00106 79.68 78.97 1.00181 79.63 78.89 1.00281 79.57 78.83 1.00367 79.52 78.77 1.00438 79.47 78.71 1.00518 79.42 78.62 1.00641 79.34 78.52 1.00779 79.25 78.40 1.00932 79.16 C

318.15 K

ρ

Vϕ,2

ρ

Vϕ,2

0.99513 0.99602 0.99835 1.00042 1.00225 1.00403 1.00616 1.00888 1.01208 1.01259 1.01458 1.01769 1.02007 1.02290

55.71 55.71 55.70 55.70 55.69 55.69 55.69 55.69 55.69 55.69 55.69 55.70 55.71 55.72

0.99128 0.99214 0.99439 0.99641 0.99818 0.99991 1.00197 1.00461 1.00772 1.00822 1.01015 1.01317 1.01549 1.01824

56.50 56.49 56.48 56.47 56.47 56.46 56.45 56.45 56.44 56.44 56.44 56.45 56.45 56.46

0.99479 0.99480 0.99555 0.99557 0.99704 0.99707 1.00152 1.00152 1.00864 1.00866 1.01546 1.01552

44.93 44.93 44.93 44.93 44.93 44.93 44.93 44.93 44.95 44.95 44.98 44.98

0.99095 0.99096 0.99168 0.99171 0.99315 0.99317 0.99749 0.99748 1.00442 1.00444 1.01106 1.01111

45.49 45.49 45.49 45.49 45.48 45.48 45.48 45.48 45.49 45.49 45.51 45.51

0.99471 0.99518 0.99655 0.99771 0.99907 1.00090 1.00353 1.00477 1.00619 1.00647 1.00836 1.01117 1.01158

92.07 92.05 91.99 91.94 91.88 91.80 91.69 91.64 91.59 91.58 91.50 91.40 91.38

0.99086 0.99131 0.99262 0.99374 0.99504 0.99680 0.99931 1.00049 1.00185 1.00213 1.00393 1.00661 1.00700

92.90 92.88 92.82 92.77 92.72 92.65 92.54 92.50 92.45 92.44 92.37 92.27 92.26

0.99450 0.99480 0.99565 0.99641 0.99718 0.99784 0.99856 0.99950 1.00031 1.00098 1.00173 1.00289 1.00418 1.00561

80.53 80.52 80.47 80.42 80.38 80.34 80.30 80.24 80.20 80.16 80.12 80.05 79.98 79.90

0.99066 0.99094 0.99175 0.99248 0.99321 0.99384 0.99452 0.99541 0.99617 0.99680 0.99751 0.99859 0.99980 1.00113

81.17 81.16 81.12 81.08 81.04 81.01 80.98 80.94 80.90 80.87 80.84 80.79 80.73 80.67



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Table 2. continued a Standard uncertainties u are u(ρ) = 1.5 × 10−5 g·cm−3, u(Vϕ,2) = 0.02 cm3·mol−1, and u(T) = 0.01 K (level of confidence = 0.95). bDensity data for (H2O + U), being obtained using a precise sealed densimeter with a magnetic float, were taken from our previous work (see ref 15).

Table 3. Regression Coefficients, ai, and Standard Errors of the Fit, σ, from eq 2 for the Compared Binary Systems at Different Temperatures and Atmospheric Pressurea T/K

a0

278.15 288.15 298.15 308.15 318.15

999.965 999.102 997.048 994.035 990.216

(0.002) (0.002) (0.002) (0.002) (0.002)

278.15 288.15 298.15 308.15 318.15

999.966 999.100 997.047 994.039 990.217

(0.002) (0.002) (0.002) (0.002) (0.001)

a1

a2

Water + TU 23.959 (0.007) −1.559 22.533 (0.006) −1.355 21.481 (0.005) −1.195 20.615 (0.006) −1.058 19.973 (0.005) −1.000 Water + 1,3-DMTU 14.577 (0.008) −0.607 13.743 (0.006) −0.593 13.088 (0.007) −0.587 12.546 (0.007) −0.586 12.039 (0.005) −0.583

103σ

a0

a1

a2

103σ

b

(0.005) (0.004) (0.003) (0.004) (0.004)

3.15 3.00 2.62 2.85 2.79

999.962 999.100 997.042 994.029 990.211

(0.003) (0.004) (0.005) (0.004) (0.003)

(0.006) (0.004) (0.005) (0.005) (0.003)

4.12 3.19 3.63 3.72 2.35

999.965 999.103 997.047 994.038 990.216

(0.001) (0.002) (0.002) (0.002) (0.002)

Water + U 17.607 (0.014) −0.848 (0.009) 16.605 (0.016) −0.759 (0.011) 15.861 (0.021) −0.712 (0.014) 15.301 (0.017) −0.671 (0.011) 14.866 (0.015) −0.641 (0.010) Water + 1,3-DMU 9.445 (0.005) 0.017 (0.003) 8.847 (0.006) −0.070 (0.004) 8.382 (0.005) −0.132 (0.004) 7.981 (0.005) −0.176 (0.003) 7.635 (0.006) −0.239 (0.004)

6.58 7.79 9.92 7.91 6.90 2.39 2.90 2.71 2.54 2.89

Units: a0 and σ, kg·m−3; a1, kg2·m−3·mol−1; a2, kg3·m−3·mol−2. In parentheses, standard fitting deviations at the 95 % confidence level for each of calculated values of ai are given. bComputed using the density data from ref 15. a

Figure 1. Apparent molar volumes of aqueous thiourea (a), urea (b), 1,3-dimethylthiourea (c), and 1,3-dimethylurea (d) as a function of the solution molality m at temperature T: squares, T = 278.15 K; circles, T = 298.15 K; triangles, T = 318.15 K.

structural rearrangement and stabilization of the aqueous clusters surrounding the hydrophobic part of a solute molecule. Both structure-breaking and structure-making effects in the compared aqueous solutions must be stronger at lower

temperatures, and should decrease with increasing concentration.14,19,26 The given circumstances explain partly why the (∂Vϕ,2/∂m)T,p derivative for aqueous TU and U reverses the sign in the range of temperatures close to T = 318.15 K (see in D



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Table 4. Limiting Apparent Molar Volumes, V∞ ϕ,2, and Coefficients bV and cV of eq 4 for (Water + Thiourea, Urea, 1,3Dimethylthiorea, or 1,3-Dimethylurea), with Their Standard Uncertainties (± u), at Different Temperatures, T/K, and at p ∼ 0.1 MPaa T/K

a

V∞ ϕ,2

bV

278.15 288.15 298.15 308.15 318.15

52.16 53.62 54.74 55.71 56.50

± ± ± ± ±

0.02 0.02 0.02 0.02 0.02

278.15 288.15 298.15 308.15 318.15

89.60 90.50 91.32 92.10 92.93

± ± ± ± ±

0.02 0.02 0.02 0.02 0.02

Water + TU 0.31 ± 0.02 0.15 ± 0.03 0.02 ± 0.02 −0.07 ± 0.03 −0.12 ± 0.03 Water + 1,3-DMTU −0.70 ± 0.02 −0.65 ± 0.03 −0.61 ± 0.03 −0.57 ± 0.03 −0.53 ± 0.03

V∞ ϕ,2

cV

bV

0.071 0.067 0.063 0.059 0.056

± ± ± ± ±

0.008 0.010 0.008 0.010 0.010

42.42 43.44 44.25 44.90 45.46

± ± ± ± ±

0.02 0.02 0.02 0.02 0.02

0.062 0.059 0.061 0.059 0.058

± ± ± ± ±

0.009 0.010 0.010 0.009 0.010

78.67 79.33 79.94 80.56 81.19

± ± ± ± ±

0.02 0.02 0.02 0.02 0.02

cV

Water + Ub 0.11 ± 0.02 0.04 ± 0.03 0.01 ± 0.02 −0.01 ± 0.02 −0.03 ± 0.03 Water + 1,3-DMU −0.76 ± 0.02 −0.63 ± 0.03 −0.54 ± 0.03 −0.47 ± 0.03 −0.38 ± 0.03

0.031 0.031 0.030 0.030 0.029

± ± ± ± ±

0.009 0.011 0.008 0.009 0.010

0.007 0.011 0.015 0.018 0.021

± ± ± ± ±

0.008 0.008 0.009 0.009 0.008

Units: V, cm3·mol−1; bV, cm3·kg·mol−2; cV, cm3·kg2·mol−3. bSee footnote b in Table 3.

3 −1 Table 5. Comparison of Measured Values of Standard Molar Volumes, V∞ ϕ,2/(cm ·mol ), of Thiourea, Urea, and 1,3Dimethylurea in Water at T = 298.15 K (Table 4) with the Values for T = 298.15 K and p = 0.1 MPa Taken from the Literature

V∞ ϕ,2, this work

V∞ ϕ,2, lit

a ΔdevV∞ ϕ,2

ref

TU

54.74 ± 0.02

1,3-DMU

79.94 ± 0.02

U

44.25 ± 0.02b

54.79 ± 0.04 54.793 ± 0.01 80.04 ± 0.05 80.03 ± 0.01 80.32 ± 0.03 79.77 ± 0.07c 80.21 ± 0.16d 80.04 ± 0.02 44.23 ± 0.01 44.24 ± 0.05 44.23 ± 0.01 44.20 ± 0.02 44.20 ± 0.10 44.24 ± 0.03 44.02 ± 0.06 44.06 ± 0.01 44.238 ± 0.01 44.22 ± 0.04 44.24 ± 0.04 44.04 ± 0.01

−0.05 ± 0.04 −0.053 ± 0.02 −0.10 ± 0.06 −0.09 ± 0.03 −0.38 ± 0.04 0.17 ± 0.08 −0.27 ± 0.16 −0.10 ± 0.04 0.02 ± 0.04 0.01 ± 0.06 0.02 ± 0.04 0.05 ± 0.05 0.05 ± 0.11 0.01 ± 0.05 0.23 ± 0.07 0.23 ± 0.04 0.012 ± 0.04 0.03 ± 0.06 0.01 ± 0.06 0.21 ± 0.04

Lo Surdo et al.8 Cabani et al.37 Philip et al.7 Lo Surdo et al.8 Mizutani and Yasuda13 Brown et al.19 Bravo-Sánchez et al.23 Cabani et al.37 Stokes and Hamilton5,6 Philip et al.7 Lo Surdo et al.8 Jákli and Van Hook9,10 Bartovská et al.11 Mizutani and Yasuda13 Brown et al.19 Korolev20 Cabani et al.37 Choi and Bonner42 Mathieson and Conway43 Józw ́ iak and Tyczyńska44

solute

Deviation/(cm3·mol−1) between this work and literature value. bSee footnote b in Tables 3 and 4. cp = 0.35 MPa. dComputed by us using the density data from the literature source specified. a

necessary, but not a sufficient, condition for an overall solvent structure enhancement. Furthermore, it will be recalled that concentration-dependent changes in Vϕ,2 are influenced by not only solute−solvent, but also solute−solute interactions. It makes the analysis of “fine” volumetric (structure-packing) effects being caused by the replacement of carbonyl with thionyl in the solute molecules under comparison much more difficult. For this reason, the V∞ ϕ,2 values are considered to be of more fundamental importance because of their usefulness in examining solute−solvent interactions, independent of the composition effect.48 (The hypothetical state corresponding to an infinitely dilute solution suggests that the dissolved component exists as a monomer molecule surrounded by a solvation shell of solvent molecules.) Apparent (Partial) Molar Volume at Infinite Dilution. To obtain apparent or partial molar volumes at infinite dilution, ∞ V∞ ϕ,2 = V̅ 2 (hereafter referred to as a standard molar volume),

Table 2 and in Figure 1a,b). As for the alkyl-substituted ureas and thioureas, there is a strong competition between the substitutional and interstitial solution processes.13,14 This is especially important for aqueous 1,3-DMTU and 1,3-DMU, whose molecules have the balanced polar and apolar group content and capability to act as both a hydrogen bond >N−Hdonor and a hydrogen bond >CS or >CO acceptor. Following the usual terminology, one can say that, although in 1,3-DMTU and 1,3-DMU solutions the structure-making or caging process dominates over the structure-making one (see Figures 1c,d), this does not mean that these solutes are overall structure makers. As is often emphasized,14,19,25−28,47 the overall structural influence of a solute should be deduced from solution/solvation entropy and enthalpy or heat capacity data. For this reason, we believe the observation of a negative slope in the Vϕ,2 versus m curve (depicted in Figure 1c,d) is a E



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the Vϕ,2(m) values were least-squares fitted adequately (applying an F-test) to the second-order polynomial Vϕ ,2(m) = V ϕ∞,2( = V2̅ ∞) + b V m + c Vm2

(4)

The results of fitting are represented by full lines in Figure 1 panels a to d. In eq 4, bV (being the slope of the Vϕ,2 vs m curve) and cV are the adjustable parameters, which are sometimes considered to be the volumetric pairwise and triplewise (solute−solute) interaction coefficients, respectively.49,50 The values of these coefficients obtained for aqueous TU, U, 1,3-DMTU, and 1,3-DMU by using eq 4 with unit weights are included in Table 4 along with the standard molar volumes. The uncertainties in V∞ ϕ,2 given in the table represent the combined expanded errors,51 which involve the scatter due to the approximating procedure with using eq 4, as well as systematic errors being estimated from uncertainties of temperature and the calibration constant. Unfortunately, as it is already noted above (in the Introduction section), we have not found any literature data on volume characteristics of aqueous TU, 1,3-DMU, and 1,3DMTU to verify our values of V∞ ϕ,2 at all the temperatures desired, excepting quantities at T = 298.15 K and some others. Herewith the specified volumetric characteristics for the last of the above-named solutes seem to be lacking. Table 5 includes a qualitative comparison of our data on V∞ ϕ,2 with the corresponding literature values at T = 298.15 K. A survey of the results presented in Table 5 shows that our quantities of V∞ ϕ,2 are in rather good (within the experimental uncertainties) agreement with those reported by other authors except for works13,20,23,44 whose V∞ ϕ,2 values disagree more significantly with ours, being systematically higher for aqueous 1,3-DMU13,23 and lower for aqueous U.20,44 As for aqueous 1,3DMTU and TU (apart from T = 298.15 K), we suppose that the quantities in question (Table 4) are reported here for the first time. As follows from the data of Table 4, the V∞ ϕ,2 values increase monotonically with rising temperature for all the solutes compared, but this increase is found to be more pronounced in the case of both unsubstituted solutes and aqueous thioureas, on the whole. Suffice it to say that if at T = 278.15 K, ∞ ∞ ∞ V∞ ϕ,2(TU)/Vϕ,2(U) = 1.230 and Vϕ,2(1,3-DMTU)/Vϕ,2(1,3DMU) = 1.139, at T = 318.15 K, these ratios reach 1.243 and 1.145, respectively. Herewith for “pair correlations” 1,3DMTU/TU and 1,3-DMU/U, the specified fractional distinctions in V∞ ϕ,2 amount to 1.718 and 1.644 for thioureas and 1.854 and 1.786 for ureas at the same temperatures. The fact of greater growth of V∞ ϕ,2 for aqueous TU and U compared to like quantities for their N,N′-dimethyl-substituted derivatives with increasing temperature may be primarily due to additional disruption of the “heterocomponent” H-bonded local structures (hydration complexes) being formed at the expense of protondonor/acceptor groups of solute molecules.17,47 Our interest is focused on the structure-packing effects induced by the U → TU substitution under influence of increasing temperature and N,N′-methylation process, too. Especially important here are data on the free volume or “excluded space”52 in the solvation complexes compared. Figure 2 illustrates the temperature dependences of packing ∞ density, d = Vw,2/V∞ ϕ,2, which represents the fraction of Vϕ,2 53 occupied by the intrinsic volume of the solute. The latter quantity is expressed in terms of the van der Waals volume occupied by one mole of solute molecules, Vw,2 = vw,2NA, where

Figure 2. Ratios between the van der Waals volume (expressed in cm3·mol−1) and limiting apparent molar volume for thioureas (solid lines) and ureas (dashed lines) in the aqueous solution as a function of temperature. The unsubstituted (TU and U) and N,N′-dimethylsubstituted (1,3-DMTU and 1,3-DMU) solutes are denoted respectively as squares and circles.

NA is the Avogadro number. According to the Bondi method,53,54 a molecule volume vw,2 was calculated as the sum of volumes of atom/group increments, allowing for the corresponding decrements in vw,2 due to intermolecular crowding and related volumetric effects. (Evaluating Vw,2 for TU and 1,3-DMTU, the volume contribution reported by Edward55 for the >CS group was applied, too). Herewith it is postulated that Vw,2, being found to be (in cm3·mol−1) 32.6 ± 0.1 for U, 53.7 ± 0.1 for 1,3-DMU, 37.9 ± 0.2 for TU and 59.0 ± 0.2 for 1,3-DMTU, is independent of T over the specified temperature range. Inspection of results depicted in Figure 2 shows that the d−T function for infinitely dilute aqueous solutions of 1,3-DMTU and 1,3-DMU is close to linear, whereas a like dependence for (H2O + TU) or (H2O + U) changes nonlinearly, becoming decreasingly “negative-tilted” with increasing temperature. Expressed as Vw,2/V∞ ϕ,2, the fractional free volume decreases on going from unsubstituted solutes to their N,N′-dimethylated analogues and in a lesser degree from ureas to thioureas; the specified packing differences decrease distinctly in the former case and increase slightly in the latter case as the temperature is rising. Herewith the introduction of two methyl groups (instead of the hydrogen atoms) on the N,N′-sited (1,3-cis) positions of a TU molecule, to form 1,3-DMTU, results in a relatively denser packing of the hydration complex than does the same substitution in a U molecule, to form 1,3-DMU. One would think so that this should have to do with the distinction in hydration enthalpies, ΔhydrH∞ 2 , between thioureas and ureas studied but, unfortunately, such data have been reported previously26−28 at T = 298.15 K only. The estimated values of ΔhydrH∞ 2 for TU and 1,3-DMTU, being −89.3 ± 2 and −97.8 ± 3 kJ·mol−1, are slightly higher in magnitude than those for U and 1,3-DMU, being −81.8 ± 0.6 and −87.7 ± 0.4 kJ·mol−1, respectively. As CH3 groups substitute the H atoms into a TU molecule, the solute hydration becomes more exothermic than what follows from the replacement of U with 1,3-DMU, a situation not unlike what we have had above by considering the structure-packing effects (see Figure 2). F



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solutes should have negative and positive in sign derivatives (∂E∞ p,ϕ,2/∂T)p, respectively. ∞ ∞ and (∂Ep,ϕ,2 /∂T)p were evaluated The values of Ep,ϕ,2 respectively by 1- and 2-fold differentiating the second-power V∞ ϕ,2 versus (T−T*) function with respect to (T−T*) where T* = 298.15 K. Resulting from such approximation, the E∞ p,ϕ,2/(in cm3·mol−1·K−1) quantities are (0.108 ± 0.005) for TU, (0.075 ± 0.003) for U, (0.083 ± 0.004) for 1,3-DMTU, and (0.063 ± 3 −1 −2 0.002) for 1,3-DMU. For the (∂E∞ p,ϕ,2/∂T)p/(in cm ·mol ·K ) derivative, the following values have been derived: (−2.1 ± 0.2)·10−3 for TU, (−1.5 ± 0.1)·10−3 for U, (−0.3 ± 0.2)·10−3 for 1,3-DMTU, and (−0.07 ± 0.08)·10−3 for 1,3-DMU. Seen in the context of the above-named common classification,59 aqueous TU and U should be the typical structure-breaking ∞ /∂T)p solutes. Herewith for the former solute the (∂Ep,ϕ,2 quantity seems to be more negative, reflecting primarily the higher rate of temperature-induced structure changes in the more structured aqueous surroundings, a fact being in good accordance with the above inferences based on volume- and enthalpy-related hydration characteristics. As regards the (water + 1,3-DMTU) and (water + 1,3-DMU) systems, the (∂E∞ p,ϕ,2/ ∂T)p derivatives are slightly negative in sign, close to zero. It can serve as evidence of either that both hydrophilic and hydrophobic hydration effects have not any predominating influence on the water structure in solutions compared or that, in the cases considered, a sign at the parameter (∂E∞ p,ϕ,2/∂T)p is not a sufficient condition to “identify” the overall solvent structure enhancement or destruction. In this respect it is interesting to look at the recently published data26−28 on the solution (hydration) heat capacity, Δsolc∞ p,2, for the solutes in question. In the first place, it is worth −1 −1 26 noting that Δsolc∞ p,2 is positive for TU (ca. 19 J·mol ·K ) and −1 −1 27 negative for U (ca. −19 J·mol ·K ), whereas Δsolc∞ p,2 of both 1,3-DMTU and 1,3-DMU are positive and significantly higher, the former displaying a decisively higher value (∼151 J·mol−1·K−1 as against ∼136 J·mol−1·K−1).26,28 In view of the structure-making and structure-breaking model,28,60 one can say that hydrophobic hydration is the prevailing effect in both 1,3-DMTU and 1,3-DMU. In turn, proceeding from the Δsolc∞ p,2 values, TU and U act oppositely toward water structure. That is, unlike the hydrophilic-hydrated U, the hydrophobic interactions are predominant in TU hydration due to the “intrinsic lower capability of thiourea’s nitrogen region to form hydrogen bonds with water”.28 However we consider that these structure-distinguishable effects should be additionally tested. Thus, a variety of thermodynamic properties for solutes under comparison must be considered simultaneously, as we made an attempt to do above.

Such a behavior of the discussed thermodynamic characteristics appears in agreement with the fact that TU and 1,3DMTU have slightly higher dipole moments μ than U and 1,3DMU (by ca. 1.2·10−30 C·m and ca. 0.4·10−30 C·m, respectively, in the gaseous state) as reported by Kaatze et al.36 as well as Kumler and Fohlen.34 (Herewith the μ values for unsubstituted molecules were found to be lower compared to those for Nmethylated molecules.36) According to calculations,35 these differences become somewhat higher at infinite dilution in water. Authors56,57 concluded that, in contrast to U, the thiourea’s C−N bond is enhanced, while the polarity of C−S bond increases. It was also shown that the total interaction energies are likely the same in the region of carbonyl and thionyl groups.58 This feature is in agreement with an enhanced bonding ability (primarily, by H-bonding) of the sulfur atom brought about by the weakening of the C−S bond mentioned above. Concerning 1,3-DMTU and 1,3-DMU molecules, the hydrogen-bonding ability of the >N−H groups is sterichindered and the thionyl−water or carbonyl−water interactions can be considered as the dominating contribution in the hydrophilic hydration. If one takes into account that the methylation is directly related not only to the intrinsic volume increment but also to the hydrophobic hydration (being located near the apolar groups where the more structured cluster-like solvent entities are to be formed, especially at low temperatures),14,22,26−28,47 it makes sense that the latter effect is influenced by the ability of >CS and >CO groups to form the hydrogen bonds in different ways. This inference is supported by the fact that, unlike N-methylureas, the hydration enthalpies of [NH−CS−NH] and [NH−CS−NH2] fragments are practically identical.28 Again, by contrast with 1,3-DMTU, 1,3-DMU displays a significant and modulated by methyl groups topography contribution of nitrogen atom to the hydrophilic hydration, resulting in less exothermic effect of the solute hydration, as a whole.26,28 The above speculations could explain in many respects the revealed differences in volume properties of aqueous ureas and thioureas in question. One cannot exclude also that the aqueous structure seems to be better suited for incorporating both TU and 1,3-DMTU molecules, a factor favoring the decrease in ∞ Vw,2/V∞ ϕ,2 (Figure 2) and increase in |ΔhydrH2 | (see above) compared to U and 1,3-DMU molecules, respectively. For this reason, the interactions in the last two infinite-diluted aqueous solutions should take place against the background of the more pronounced configurational rearrangements resulting in a denser molecular packing of the hydrogen-bonded hydration complexes formed. Limiting Apparent (Partial) Molar Expansibility. In addition, the temperature dependence of the limiting apparent ∞ ∞ (partial) molar expansibility, E∞ p,ϕ,2(≡E̅ p,2) = (∂Vϕ,2/∂T)p, for each of solutes considered can be used to extract useful information on the capability of the solute to promote or destroy the water structure, based on the thermodynamic relation proposed primordially by Hepler59 (∂ c ̅p∞,2/∂p)T = −T (∂ 2V2̅ ∞/∂T 2)p = −T (∂E ̅p∞,2 /∂T )p

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. Tel.: +7 4932 351859. Fax: +7 4932 336246. Notes

The authors declare no competing financial interest.

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where c∞ p̅ ,2 is the limiting (at infinite dilution) partial molar isobaric heat capacity of the dissolved component. On this basis (being mediately related to the pressure-dependent heat capacity of water)59 structure-breaking and structure-making

ACKNOWLEDGMENTS

Authors are grateful to Dr. E. Yu. Lebedeva for help in densimetric measurements. G



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Apparent Molar Volumes and Expansibilities of Thiourea, 1,3

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