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Apparent molal volumes of HMT and TATD in aqueous solutions around the temperature of maximum density of water
J. Chem. Thermodynamics 45 (2012) 28–34
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Apparent molal volumes of HMT and TATD in aqueous solutions around the temperature of maximum density of water J.A. Clavijo Penagos ⇑, L.H. Blanco Laboratorio de Investigaciones Básicas, Departamento de Química, Facultad de Ciencias, Universidad Nacional de Colombia Sede Bogotá, Colombia
a r t i c l e
i n f o
Article history: Received 18 July 2011 Received in revised form 23 August 2011 Accepted 25 August 2011 Available online 3 September 2011 Keywords: Apparent molal volumes Partial molar volumes at infinite dilution HMT TATD Aqueous solutions Water structure
a b s t r a c t Apparent molal volumes V / have been determined from density measurements for several aqueous solutions of 1,3,5,7-tetraazatricyclo[3.3.1.1(3,7)]decane (HMT) and 1,3,6,8-tetraazatricyclo[4.4.1.1(3,8)]dodecane (TATD) at T = (275.15, 275.65, 276.15, 276.65, 277.15, 277.65 and 278.15) K as function of composition. The infinite dilution partial molar volumes of solutes in aqueous solution are evaluated through extrapolation. Interactions of the solutes with water are discussed in terms of the effect of the temperature on the volumetric properties and the structure of the solutes. The results are interpreted in terms of water structure-breaking or structure forming character of the solutes. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Great interest has been given to the study of thermodynamic properties of dilute aqueous solutions of small model compounds as they provide useful information about the understanding of the nature of the interactions between polar and non-polar groups and water and contribute to know the behaviour of more complex systems in aqueous solutions, like biological systems. In particular, knowledge of the volumetric properties of aqueous solutions of non-electrolyte solutions gives important information on solvent–solute and solute–solute interactions. For this purpose density measurements of high accuracy have to be performed [1]. In our laboratory lately we have been using macrocyclic aminals as solutes. This is so because the two more common HMT and TATD are non-electrolytes that have high symmetry, are non-polar and are quite soluble in water. All these characteristics made them very interesting solutes when one wants to study their effect on water structure. Several physical chemistry studies suggest that HMT is a structure making agent [2–6] though one especially shows structure breaking behaviour of HMT similar to any alcohols, diols and sucrose [7]. Temperature influence on the behaviour of volumetric properties of aqueous solutions has often been used to obtain information about solute structural effects on water structure.
⇑ Corresponding author. E-mail address: [email protected] (J.A. Clavijo Penagos). 0021-9614/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jct.2011.08.024
The temperature dependence of the partial molar volume at infinite dilution is discussed in terms of solute hydration and the balance between hydrophobic and hydrophilic interactions between solute and water [8–16]. Since the 1970s, it has been proposed to use the temperature dependence of the variation of the partial molar volume at infinite dilution as a classification of the solutes as liquid water structure formers (SM) or structure breakers (SB) [17] and since then, many criteria have been proposed for classification based on diverse properties without general agreement to date on the topic. However, volumetric studies of this type of solutions at low concentration, of less than 0.05 mol kg1, are very scarce, and around the temperature of maximum density of water are almost non-existent in the available literature; data for macrocyclic aminal HMT in aqueous solutions are found for partial molar volumes and for partial molar volume at infinite dilution at T = (283.15, 288.15, 298.15, 308.15, and 318.15) K using extrapolation of apparent volume data at molar concentrations above 0.05 mol dm3 [2], and for apparent volumes at concentrations above 0.24 mol kg1 [18] and above 0.008 mol kg1 at T = (278.15, 288.15, and 298.15) K [7]. The most recent volumetric data for aqueous solutions of HMT is found for density and apparent molar and partial molar volume at temperatures above T = 288.15 K [30]. Volumetric data for TATD in aqueous solution are inexistent to the best of our knowledge. This article deals with the volumetric study of HMT and TATD in aqueous solutions at molal concentrations between 0.001 and 0.2 mol kg1. The densities of solutions were determined using
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J.A. Clavijo Penagos, L.H. Blanco / J. Chem. Thermodynamics 45 (2012) 28–34
magnetic float densimetry and the dependence with temperature allowed the calculation of the volumetric properties reported in this paper. This work was undertaken because the properties of this type of solutions have little mention in the published literature; in the case of the volumetric properties of aqueous solutions HMT and TATD around the temperature of maximum density of water there are no contributions in the current available literature. 2. Experimental The 1,3,5,7-tetraazatricyclo[3.3.1.1(3,7)]decane (HMT) was obtained from J.T. Baker, Baker analysed ACS reagent (purity > 0.99 mass fraction – see table 1). 1,3,6,8-Tetraazatricyclo[4.4.1.1(3,8)]dodecane (TATD) was synthesized following the instructions available in the literature [23]. The TATD preparation was characterised giving good agreement with literature: 1H NMR d 3.98 (s, 8H), 3.26 (s, 8H); 13C NMR d 73.74, 58.57 [23] and 1H NMR d 3.89 (s, 8H), 5 3.17 (s, 8H); 13C NMR d 73.54, 58.58 (this work, purity > 0.9 mass fraction – see table 1). The solids were stored over CaCl2 for at least 24 h at room temperature. The water used was treated by means of a Barnstead Easy-Pure RoDI 3321 water purifier. Then it was degassed by ultrasound treatment before use. The solutions were prepared by weight by means of a Mettler AT261 balance dual range with sensibility of 105 g in the range of (0 to 60) g, and from (60 to 100) g, 104 g. The solutes concentrations were (0.001, 0.005, 0.010, 0.025, 0.050, 0.100, 0.150 and 0.200) mol kg1. As noted above, to obtain the apparent molal volumes for HMT and TATD, the density q of the solutions must be determined with high accuracy, by which the magnetic float technique was used. The full description of this technique is easily found in literature [19–22]. The main theoretical framework of the magnetic float densimetry states that if an object placed within a pure liquid solvent or a solution can be vertically risen or sunken by means of a magnetic force, the density of the liquid is proportional to the
current needed to rise or to sink the object, provided that the magnetic characteristics of the magnetic source and the volume of the object are known (calibration of the densimeter) and the reproducibility of the vertical balance of forces is assured and very good temperature control is used at all desired temperatures. Our densimeter was built and calibrated following well-known literature procedures and models, using water as reference liquid [1,19–22]. The densities of water for densimeter calibration and operation were taken from the literature [24]. A constant temperature bath built for the purpose was used. For all the calibrations and measurements, the temperature of the measurement cell was controlled to better than ±0.03 K. All the experimental work was realised at atmospheric pressure of 75.06 kPa but the measurement cell always was tightly closed to the atmosphere, to avoid undesired temperature gradients, changes of concentration or pollution of the samples. The magnetic float densimetry requires calibration of the equipment at every temperature before measuring the densities of the solutions to assure that the values obtained for pure water reproduce the value taken as the reference, which guarantees that the experimental values of density of the solutions are adapted to calculate apparent volume [19–22]. Densities q for pure water obtained in this work are presented in table 2 together with values from the literature [24]. They agree satisfactorily with published values. Density values for the aqueous solutions of both macrocyclic aminals are the average of at least three independent measurements and they have a standard uncertainty of 3.2 106 g cm3, applying the law of propagation of uncertainties to our experimental density data [25].
3. Results and discussion Experimental data for densities of the binary aqueous mixtures of macrocyclic aminals as a function of molality at the selected
TABLE 1 Chemical samples used in this study. Chemical name
Source
Initial mole fraction purity
Purification method
1,3,5,7-Tetraazatricyclo [3.3.1.13,7]decane (HMT) 1,3,6,8-Tetraazatricyclo [4.4.1.1(3,8)]dodecane (TATD)
J.T. Baker, solid Synthesis, solid
0.990
Distillation Acetonea
Final mole fraction purity
Analysis method
0.99
NMRb
All the work was done under atmospheric pressure of 75.06 kPa. a TATD cannot be recrystallized from hot liquid acetone because it decomposes very easily in hot acetone; the solid was synthesized according to literature procedures, then it was separated by decantation and finally it was washed with cold acetone until the NMR spectra was consistent (Synthesis literature source and NMR data reported in the text). b 1 H and 13C NMR.
TABLE 2 Comparison of measured densities with literature values for pure water around the temperature of maximum density of water. T/K
Reference [4]
Experimental1
275.15 275.65 276.15 276.65 277.15 277.65 278.15
0.9999429 0.9999571 0.9999672 0.9999731 0.9999750 0.9999728 0.9999668
Experimental2
q/g cm
10 dq/g cm
q/g cm3
106 dq/g cm3
0.9999437 0.9999604 0.9999690 0.9999719 0.9999738 0.9999749 0.9999660
0.81 3.4 1.7 1.2 1.2 2.1 0.92
0.9999418 0.9999590 0.9999685 0.9999726 0.9999725 0.9999740 0.9999657
1.1 1.9 1.3 0.50 2.5 1.2 1.2
3
6
3
dq indicate the uncertainties in water density, obtained as the absolute difference between our experimental value and the value taken from reference [4]. All the work was done under atmospheric pressure of 75.06 kPa. 1 Water density values obtained in this work on having calibrated and having had measured the densities of the HMT aqueous solutions. 2 Water density values obtained in this work on having calibrated and having had measured the densities of the TATD aqueous solutions.
30
TABLE 3 Density q and apparent molal volume V / of HMT in aqueous solution around the temperature of maximum density of water. T/K 275.15 ± 0.02
275.62 ± 0.01
276.14 ± 0.01
276.67 ± 0.01
q ± 106 dq/(g cm3)
V / ± dV / / (cm3 mol1)
m/(mol kg1)
q ± 106 dq/(g cm3)
V / ± dV / / (cm3 mol1)
m/(mol kg1)
q ± 106 dq/(g cm3)
V / ± dV / / (cm3 mol1)
m/(mol kg1)
q ± 106 dq(g cm3)
V / ± dV / / (cm3 mol1)
0.00121 0.00500 0.00994 0.02457 0.05001 0.10004 0.15093 0.19869
0.999954 ± 1.9 1.000095 ± 2.0 1.000238 ± 2.1 1.000702 ± 2.1 1.001508 ± 1.8 1.003075 ± 2.1 1.004487 ± 2.0 1.006157 ± 2.0
131.30 ± 1.60 109.78 ± 0.39 110.51 ± 0.21 109.21 ± 0.08 108.73 ± 0.04 108.54 ± 0.02 109.59 ± 0.02 108.25 ± 0.01
0.00121 0.00531 0.00994 0.02491 0.05070 0.10004 0.15093 0.19869
0.999987 ± 2.0 1.000120 ± 2.0 1.000270 ± 1.8 1.000746 ± 2.0 1.001562 ± 1.7 1.003112 ± 1.7 1.004560 ± 1.8 1.006178 ± 2.2
115.36 ± 1.73 109.57 ± 0.37 108.72 ± 0.18 108.44 ± 0.09 108.37 ± 0.04 108.32 ± 0.02 109.20 ± 0.02 108.21 ± 0.01
0.00131 0.00531 0.00994 0.02491 0.05070 0.10004 0.14965 0.20015
0.999996 ± 2.0 1.000121 ± 2.0 1.000283 ± 1.8 1.000749 ± 2.0 1.001557 ± 1.7 1.003112 ± 1.7 1.004626 ± 1.8 1.006192 ± 2.2
118.21 ± 1.49 111.21 ± 0.37 108.37 ± 0.18 108.71 ± 0.08 108.66 ± 0.03 108.42 ± 0.02 108.56 ± 0.01 108.42 ± 0.01
0.00134 0.00622 0.00981 0.02343 0.05176 0.10004 0.14965 0.20015
1.000009 ± 3.10 1.000184 ± 2.20 1.000263 ± 2.50 1.000696 ± 2.00 1.001608 ± 3.10 1.003125 ± 2.30 1.004643 ± 2.30 1.006252 ± 2.30
113.19 ± 2.28 106.30 ± 0.36 110.57 ± 0.26 109.27 ± 0.09 108.43 ± 0.06 108.34 ± 0.02 108.48 ± 0.02 108.14 ± 0.01
T/K 277.14 ± 0.01
277.65 ± 0.01
278.15 ± 0.02
m/(mol kg1)
q ± 106 dq/(g cm3)
V / ± dV / /(cm3 mol1)
m/(mol kg1)
q ± 106 dq/(g cm3)
V / ± dV / /(cm3 mol1)
m/(mol kg1)
q ± 106 dq/(g cm3)
V / ± V / /(cm3 mol1)
0.00134 0.00622 0.00981 0.02343 0.05176 0.10004 0.14965 0.20015
1.000012 ± 2.1 1.000166 ± 2.5 1.000269 ± 2.6 1.000683 ± 2.3 1.001632 ± 3.2 1.003181 ± 3.2 1.004721 ± 3.2 1.006247 ± 2.4
112.32 ± 1.55 109.46 ± 0.39 110.23 ± 0.27 109.91 ± 0.10 108.00 ± 0.06 107.80 ± 0.03 107.97 ± 0.02 108.18 ± 0.01
0.00099 0.00506 0.00966 0.02556 0.05176 0.10004 0.14965 0.20015
0.999993 ± 1.90 1.000116 ± 2.00 1.000232 ± 2.10 1.000727 ± 2.40 1.001621 ± 2.00 1.003112 ± 2.00 1.004577 ± 1.90 1.006038 ± 2.10
119.85 ± 1.93 111.86 ± 0.39 113.30 ± 0.21 110.60 ± 0.09 108.18 ± 0.04 108.47 ± 0.02 108.92 ± 0.01 109.23 ± 0.01
0.00100 0.00500 0.00993 0.02556 0.05176 0.10004 0.14965 0.20042
0.999990 ± 2.0 1.000120 ± 1.9 1.000299 ± 3.0 1.000723 ± 3.0 1.001556 ± 2.0 1.003076 ± 2.2 1.004570 ± 2.4 1.006139 ± 2.9
117.34 ± 1.95 109.49 ± 0.39 106.71 ± 0.31 110.52 ± 0.12 109.31 ± 0.04 108.77 ± 0.02 108.93 ± 0.02 108.72 ± 0.01
Values within parenthesis indicate the uncertainties in T. m, molality of HMT. q, density of solution, g cm3. dq indicate the uncertainties in density, 106 g cm3. V / , apparent molal volume, 106 m3 mol1. dV / indicate the uncertainties in V / . All the work was done under atmospheric pressure of 75.06 kPa.
J.A. Clavijo Penagos, L.H. Blanco / J. Chem. Thermodynamics 45 (2012) 28–34
m/(mol kg1)
TABLE 4 Density q and apparent molal volume V / of TATD in aqueous solution around the temperature of maximum density of water. T/K 275.65 ± 0.03
276.15 ± 0.01
276.65 ± 0.01
m/(mol kg1)
q ± 106 dq/(g cm3)
V / ± dV / / (cm3 mol1)
m/(mol kg1)
q ± 106 dq/(g cm3)
V / ± dV / / (cm3 mol1)
m/(mol kg1)
q ± 106 dq/(g cm3)
V / ± dV / / (cm3 mol1)
m/(mol kg1)
q ± 106 dq/(g cm3)
V / ± dV / / (cm3 mol1)
0.00100 0.00513 0.01021 0.02492 0.05060 0.09984 0.14961 0.19952
0.999973 ± 2.0 1.000103 ± 1.9 1.000262 ± 1.8 1.000723 ± 2.0 1.001529 ± 2.2 1.003073 ± 2.0 1.004634 ± 2.1 1.006199 ± 2.0
137.77 ± 2.01 137.02 ± 0.37 136.91 ± 0.17 136.80 ± 0.08 136.67 ± 0.04 136.45 ± 0.02 136.24 ± 0.01 136.02 ± 0.01
0.00100 0.00513 0.01021 0.02492 0.05060 0.09984 0.14961 0.19952
0.999990 ± 1.8 1.000119 ± 1.8 1.000277 ± 1.7 1.000735 ± 1.8 1.001534 ± 1.6 1.003068 ± 1.6 1.004617 ± 1.6 1.006172 ± 1.8
135.18 ± 1.84 136.70 ± 0.35 136.86 ± 0.16 136.91 ± 0.07 136.84 ± 0.03 136.65 ± 0.02 136.44 ± 0.01 136.24 ± 0.01
0.00100 0.00513 0.01021 0.02492 0.05060 0.09984 0.14961 0.19952
0.999999 ± 1.7 1.000126 ± 1.7 1.000283 ± 1.7 1.000735 ± 1.7 1.001525 ± 1.8 1.003040 ± 1.8 1.004571 ± 1.7 1.006106 ± 1.8
136.16 ± 1.74 137.19 ± 0.34 137.29 ± 0.16 137.31 ± 0.07 137.23 ± 0.04 137.03 ± 0.02 136.83 ± 0.01 136.62 ± 0.01
0.00102 0.00520 0.00985 0.02534 0.05060 0.09984 0.14961 0.19952
0.999993 ± 2.2 1.000125 ± 2.5 1.000209 ± 2.7 1.000737 ± 2.1 1.001620 ± 2.5 1.003042 ± 2.6 1.004486 ± 2.3 1.006042 ± 2.6
148.93 ± 2.20 138.90 ± 0.47 144.29 ± 0.27 137.99 ± 0.08 135.47 ± 0.05 137.07 ± 0.03 137.44 ± 0.02 136.98 ± 0.01
T/K 277.14 ± 0.01 1
m/(mol kg 0.00102 0.00520 0.00985 0.02534 0.05060 0.09984 0.14565 0.19637
277.64 ± 0.02 )
6
3
3
q ± 10 dq/(g cm )
V / ± dV / /(cm
0.999999 ± 2.2 1.000130 ± 2.3 1.000283 ± 2.1 1.000802 ± 3.2 1.001681 ± 3.2 1.003171 ± 2.2 1.004720 ± 2.2 1.006251 ± 3.2
144.21 ± 2.14 138.34 ± 0.44 136.95 ± 0.22 135.47 ± 0.13 134.29 ± 0.06 135.78 ± 0.02 135.00 ± 0.02 135.42 ± 0.02
mol
Values within parenthesis indicate the uncertainties in T. m, molality of TATD. q, density of solution, g cm3. dq indicate the uncertainties in density, 106 g cm3. V/, apparent molal volume, 106 m3 mol1. dV / indicate the uncertainties in V / . All the work was done under atmospheric pressure of 75.06 kPa.
1
)
1
m/(mol kg 0.00093 0.00488 0.01002 0.02508 0.04864 0.09984 0.14565 0.19637
278.16 ± 0.02 )
6
3
3
q ± 10 dq/(g cm )
V / ± dV / /(cm
0.999987 ± 2.1 1.000117 ± 2.2 1.000259 ± 2.5 1.000756 ± 2.0 1.001466 ± 2.4 1.003080 ± 2.1 1.004423 ± 2.0 1.005952 ± 2.7
153.34 ± 2.26 138.74 ± 0.45 139.59 ± 0.25 136.89 ± 0.08 137.32 ± 0.05 136.68 ± 0.02 137.06 ± 0.01 136.96 ± 0.01
mol
1
)
m/(mol kg1)
q ± 106 dq/(g cm3)
V / ± dV / /(cm3 mol1)
0.00101 0.00488 0.00986 0.02508 0.04864 0.09984 0.14565 0.19637
0.999983 ± 2.4 1.000096 ± 2.0 1.000246 ± 2.0 1.000741 ± 2.3 1.001465 ± 2.0 1.003052 ± 2.0 1.004347 ± 2.0 1.005913±1.9
152.75 ± 2.35 141.80 ± 0.40 139.90 ± 0.20 137.27 ± 0.09 137.23±0.04 136.91 ± 0.02 137.55 ± 0.01 137.13±0.01
J.A. Clavijo Penagos, L.H. Blanco / J. Chem. Thermodynamics 45 (2012) 28–34
275.16 ± 0.02
31
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J.A. Clavijo Penagos, L.H. Blanco / J. Chem. Thermodynamics 45 (2012) 28–34
temperatures and the results for apparent volumes are presented in tables 3 and 4. Apparent molal volumes V / were calculated from density measurements at each temperature according to the following equation:
FIGURE 1. Plot of apparent molal volume (V / ) against molality (m) of HMT (T = 275.16 K) and TATD (T = 275.15 K).
FIGURE 2. Plot of apparent molal volume (V / ) against molality (m) of HMT (T = 275.65 K) and TATD (T = 275.62 K).
FIGURE 3. Plot of apparent molal volume (V / ) against molality (m) of HMT (T = 276.15 K) and TATD (T = 276.14 K).
V/ ¼
M2
q
1000ðq q0 Þ ; mqq0
ð1Þ
where q is the density of the solution experimentally determined, q0 is the density of pure water (taken as reference for calibration),
FIGURE 4. Plot of apparent molal volume (V / ) against molality (m) of HMT (T = 276.65 K) and TATD (T = 276.67 K).
FIGURE 5. Plot of apparent molal volume (V / ) against molality (m) of HMT and TATD (T = 277.14 K).
FIGURE 6. Plot of apparent molal volume (V / ) against molality (m) of HMT (T = 277.64 K) and TATD (T = 277.65 K).
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J.A. Clavijo Penagos, L.H. Blanco / J. Chem. Thermodynamics 45 (2012) 28–34
m is the molality of the solution, M2 is the molar mass of macrocyclic aminal and V / is the apparent molal volume of the solute. Figures 1 to 7 show the dependence of apparent molal volume of the binary aqueous mixtures of HMT and TATD as function of molality at the selected temperatures. In order to evaluate the standard uncertainty in determinations of the apparent molal volume values, by combining the uncertainties in m and q, according to equation (1), the law of propagation of uncertainties was used [25]. These results also are shown in tables 2 and 3. As expected, the uncertainty in apparent molal volume is greater at low concentrations and almost constant, for both aqueous systems, at concentration above 0.025 mol kg1. From figures 1 to 7, it is clear that the composition dependence of V / is linear above 0.025 mol kg1 for all aqueous systems. Also it is clear that the behaviour of the property at very diluted concentration (0 mol kg1 < m < 0.025 mol kg1) is very complex to be easily interpreted, for which we do not contribute any simple explanation to this subject; we want only to remark that the apparent increasing of V / with increasing dilution is a trend already shown for aqueous solutions of HMT [2] and for other non-electrolytic solutes in aqueous solution [26], in contrast to the behaviour found for electrolytic ones, which usually show that V / decreases with increasing dilution [27]. Examples illustrate the complexity of the behaviour of V / in the most diluted region of many aqueous solutions. Partial molar volumes of macrocyclic aminals in water at infinite dilution at the different temperatures were obtained by extrapolation of the adjusted data of apparent molal volumes by least squares. The results are presented in table 5 and figure 8. It is necessary to highlight that the intervals of concentration used for the extrapolations reported in this work begin at
m = 0.025 mol kg1, which is a lower value than the usually taken ones in the literature for HMT’s watery solutions [2,5,7,18]. This gives major reliability to the values of obtained by extrapolation of the linear region of the dependence of V / with m. Because no results have been found in the literature for volumetric properties of aqueous solutions of HMT and TATD around the temperature of maximum density of water, there is no possibility of comparison of the results we are reporting in this paper. However, we think our results are in good agreement with data of several researchers, as seen in table 6. The small differences can be attributed to the differences in the concentration range used in extrapolation and the resulting differences in least squares fitting of experimental data. The temperature dependence of partial molar volumes of macrocyclic aminals in water at infinite dilution within the temperature range studied is presented in figure 8. The standard uncertainty in this values amounts to 0.39 cm3 mol1. Partial molar volumes at infinite dilution of HMT and TATD show a slight increase with temperature, and the effect of temperature on the partial molar volumes at infinite dilution of the solutes in aqueous solution is well described by a polynomial second order equation. According to the Hepler criteria for the second derivative of the partial molar volumes at infinite dilution with temperature [17], we obtain negative values for HMT and TATD and this suggests they have a breaking effect on water structure. The calculated values are not reported because they are smaller than the estimated uncertainty for infinite dilution partial molar volumes.
FIGURE 8. Plot of partial molal volume at infinite dilution against temperature for aqueous solutions of HMT and TATD around the temperature of maximum density of water.
o FIGURE 7. Plot of apparent molal volume (V / ) against molality (m) of HMT (T = 278.15 K) and TATD (T = 278.16 K).
TABLE 5 Infinite dilution partial molar volumes for HMT and TATD around the temperature of maximum density of water. HMT
TATD
T/K
3 V1 2 =cm mol
275.15 275.62 276.14 276.67 277.14 277.65 278.15
109.50 108.63 108.86 108.98 108.74 109.36 109.47
1
T/K
3 V1 2 =cm mol
275.16 275.65 276.15 276.65 277.14 277.64 278.16
136.96 136.89 137.33 137.83 138.31 137.52 138.03
1
TABLE 6 Comparison between reported values of infinite dilution partial molar volumes for HMT. T/K
3 V1 2 =cm mol
275.62 276.14 276.67 276.81 277.14 277.65 278.15 278.15 283.15 288.15 288.15 289.22 298.15 298.15
108.63 108.86 108.98 108.86 [18] 108.74 109.36 108.87 [7] 109.47 109.9 [2] 109.76 [7] 110.5 [2] 109.80 [18] 110.58 [7] 110.66 [5]
1
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J.A. Clavijo Penagos, L.H. Blanco / J. Chem. Thermodynamics 45 (2012) 28–34
Finally, from the apparent volume and partial molar volume at infinite dilution data for HMT and TATD we are reporting (tables 3 to 5), and although the following proposition is in this case a qualitative one, it appears plausible to assume the existence of strong solute-solvent interactions when HMT and TATD are dissolved in water around the temperature of maximum density of water, breaking the water structure when happens the dissolution process and forming a more ordered structures around them, as proposed before [2]. Undoubtedly, it is necessary to gather values of several thermodynamic and physicochemical properties of the aqueous solutions around the temperature of maximum density of water in order to define a more valid classification of the solutes as liquid water structure breakers or makers [28,29].
4. Conclusions In the present work, the apparent molal volume of macrocyclic aminals HMT and TATD in aqueous solution are reported at T = (275.15, 275.65, 276.15, 276.65, 277.15, 277.65, and 278.15) K as function of composition together with the partial molar volumes at infinite dilution as function of temperature. The reported values of apparent molal volumes and partial molar volumes at infinite dilution indicate the presence of strong solute-solvent interactions in the solutions, as expected by the presence of four nitrogen atoms in the HMT and TATD molecules which can establish hydrogen bonds with water. The application of Hepler’s criterion to the values of obtained in this work allows the suggestion that the solutes seem to behave as liquid water structure breakers, which agrees with previous reports for HMT [7]. Finally, the information that we are reporting in this paper is an absolutely new contribution to the knowledge.
The work described here was carried out by J.A. Clavijo Penagos under thesis direction of Ph.D. Luis H. Blanco at Laboratorio de Investigaciones Básicas of Universidad Nacional de Colombia, in partial fulfilment of his Ph.D. in Science – Chemistry program. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]
Acknowledgements [26]
The authors express their sincere gratitude to the reviewer and the editor for making critical suggestions which helped us to improve the quality of the manuscript. J.A. Clavijo thanks to the Lord Jesus Christ for all the help, material and spiritual, to reach the final of his Ph.D. studies, and dedicates to Him this work.
[27] [28] [29] [30]
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JCT 11-305
Contents lists available at SciVerse ScienceDirect
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Apparent molal volumes of HMT and TATD in aqueous solutions around the temperature of maximum density of water J.A. Clavijo Penagos ⇑, L.H. Blanco Laboratorio de Investigaciones Básicas, Departamento de Química, Facultad de Ciencias, Universidad Nacional de Colombia Sede Bogotá, Colombia
a r t i c l e
i n f o
Article history: Received 18 July 2011 Received in revised form 23 August 2011 Accepted 25 August 2011 Available online 3 September 2011 Keywords: Apparent molal volumes Partial molar volumes at infinite dilution HMT TATD Aqueous solutions Water structure
a b s t r a c t Apparent molal volumes V / have been determined from density measurements for several aqueous solutions of 1,3,5,7-tetraazatricyclo[3.3.1.1(3,7)]decane (HMT) and 1,3,6,8-tetraazatricyclo[4.4.1.1(3,8)]dodecane (TATD) at T = (275.15, 275.65, 276.15, 276.65, 277.15, 277.65 and 278.15) K as function of composition. The infinite dilution partial molar volumes of solutes in aqueous solution are evaluated through extrapolation. Interactions of the solutes with water are discussed in terms of the effect of the temperature on the volumetric properties and the structure of the solutes. The results are interpreted in terms of water structure-breaking or structure forming character of the solutes. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Great interest has been given to the study of thermodynamic properties of dilute aqueous solutions of small model compounds as they provide useful information about the understanding of the nature of the interactions between polar and non-polar groups and water and contribute to know the behaviour of more complex systems in aqueous solutions, like biological systems. In particular, knowledge of the volumetric properties of aqueous solutions of non-electrolyte solutions gives important information on solvent–solute and solute–solute interactions. For this purpose density measurements of high accuracy have to be performed [1]. In our laboratory lately we have been using macrocyclic aminals as solutes. This is so because the two more common HMT and TATD are non-electrolytes that have high symmetry, are non-polar and are quite soluble in water. All these characteristics made them very interesting solutes when one wants to study their effect on water structure. Several physical chemistry studies suggest that HMT is a structure making agent [2–6] though one especially shows structure breaking behaviour of HMT similar to any alcohols, diols and sucrose [7]. Temperature influence on the behaviour of volumetric properties of aqueous solutions has often been used to obtain information about solute structural effects on water structure.
⇑ Corresponding author. E-mail address: [email protected] (J.A. Clavijo Penagos). 0021-9614/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jct.2011.08.024
The temperature dependence of the partial molar volume at infinite dilution is discussed in terms of solute hydration and the balance between hydrophobic and hydrophilic interactions between solute and water [8–16]. Since the 1970s, it has been proposed to use the temperature dependence of the variation of the partial molar volume at infinite dilution as a classification of the solutes as liquid water structure formers (SM) or structure breakers (SB) [17] and since then, many criteria have been proposed for classification based on diverse properties without general agreement to date on the topic. However, volumetric studies of this type of solutions at low concentration, of less than 0.05 mol kg1, are very scarce, and around the temperature of maximum density of water are almost non-existent in the available literature; data for macrocyclic aminal HMT in aqueous solutions are found for partial molar volumes and for partial molar volume at infinite dilution at T = (283.15, 288.15, 298.15, 308.15, and 318.15) K using extrapolation of apparent volume data at molar concentrations above 0.05 mol dm3 [2], and for apparent volumes at concentrations above 0.24 mol kg1 [18] and above 0.008 mol kg1 at T = (278.15, 288.15, and 298.15) K [7]. The most recent volumetric data for aqueous solutions of HMT is found for density and apparent molar and partial molar volume at temperatures above T = 288.15 K [30]. Volumetric data for TATD in aqueous solution are inexistent to the best of our knowledge. This article deals with the volumetric study of HMT and TATD in aqueous solutions at molal concentrations between 0.001 and 0.2 mol kg1. The densities of solutions were determined using
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J.A. Clavijo Penagos, L.H. Blanco / J. Chem. Thermodynamics 45 (2012) 28–34
magnetic float densimetry and the dependence with temperature allowed the calculation of the volumetric properties reported in this paper. This work was undertaken because the properties of this type of solutions have little mention in the published literature; in the case of the volumetric properties of aqueous solutions HMT and TATD around the temperature of maximum density of water there are no contributions in the current available literature. 2. Experimental The 1,3,5,7-tetraazatricyclo[3.3.1.1(3,7)]decane (HMT) was obtained from J.T. Baker, Baker analysed ACS reagent (purity > 0.99 mass fraction – see table 1). 1,3,6,8-Tetraazatricyclo[4.4.1.1(3,8)]dodecane (TATD) was synthesized following the instructions available in the literature [23]. The TATD preparation was characterised giving good agreement with literature: 1H NMR d 3.98 (s, 8H), 3.26 (s, 8H); 13C NMR d 73.74, 58.57 [23] and 1H NMR d 3.89 (s, 8H), 5 3.17 (s, 8H); 13C NMR d 73.54, 58.58 (this work, purity > 0.9 mass fraction – see table 1). The solids were stored over CaCl2 for at least 24 h at room temperature. The water used was treated by means of a Barnstead Easy-Pure RoDI 3321 water purifier. Then it was degassed by ultrasound treatment before use. The solutions were prepared by weight by means of a Mettler AT261 balance dual range with sensibility of 105 g in the range of (0 to 60) g, and from (60 to 100) g, 104 g. The solutes concentrations were (0.001, 0.005, 0.010, 0.025, 0.050, 0.100, 0.150 and 0.200) mol kg1. As noted above, to obtain the apparent molal volumes for HMT and TATD, the density q of the solutions must be determined with high accuracy, by which the magnetic float technique was used. The full description of this technique is easily found in literature [19–22]. The main theoretical framework of the magnetic float densimetry states that if an object placed within a pure liquid solvent or a solution can be vertically risen or sunken by means of a magnetic force, the density of the liquid is proportional to the
current needed to rise or to sink the object, provided that the magnetic characteristics of the magnetic source and the volume of the object are known (calibration of the densimeter) and the reproducibility of the vertical balance of forces is assured and very good temperature control is used at all desired temperatures. Our densimeter was built and calibrated following well-known literature procedures and models, using water as reference liquid [1,19–22]. The densities of water for densimeter calibration and operation were taken from the literature [24]. A constant temperature bath built for the purpose was used. For all the calibrations and measurements, the temperature of the measurement cell was controlled to better than ±0.03 K. All the experimental work was realised at atmospheric pressure of 75.06 kPa but the measurement cell always was tightly closed to the atmosphere, to avoid undesired temperature gradients, changes of concentration or pollution of the samples. The magnetic float densimetry requires calibration of the equipment at every temperature before measuring the densities of the solutions to assure that the values obtained for pure water reproduce the value taken as the reference, which guarantees that the experimental values of density of the solutions are adapted to calculate apparent volume [19–22]. Densities q for pure water obtained in this work are presented in table 2 together with values from the literature [24]. They agree satisfactorily with published values. Density values for the aqueous solutions of both macrocyclic aminals are the average of at least three independent measurements and they have a standard uncertainty of 3.2 106 g cm3, applying the law of propagation of uncertainties to our experimental density data [25].
3. Results and discussion Experimental data for densities of the binary aqueous mixtures of macrocyclic aminals as a function of molality at the selected
TABLE 1 Chemical samples used in this study. Chemical name
Source
Initial mole fraction purity
Purification method
1,3,5,7-Tetraazatricyclo [3.3.1.13,7]decane (HMT) 1,3,6,8-Tetraazatricyclo [4.4.1.1(3,8)]dodecane (TATD)
J.T. Baker, solid Synthesis, solid
0.990
Distillation Acetonea
Final mole fraction purity
Analysis method
0.99
NMRb
All the work was done under atmospheric pressure of 75.06 kPa. a TATD cannot be recrystallized from hot liquid acetone because it decomposes very easily in hot acetone; the solid was synthesized according to literature procedures, then it was separated by decantation and finally it was washed with cold acetone until the NMR spectra was consistent (Synthesis literature source and NMR data reported in the text). b 1 H and 13C NMR.
TABLE 2 Comparison of measured densities with literature values for pure water around the temperature of maximum density of water. T/K
Reference [4]
Experimental1
275.15 275.65 276.15 276.65 277.15 277.65 278.15
0.9999429 0.9999571 0.9999672 0.9999731 0.9999750 0.9999728 0.9999668
Experimental2
q/g cm
10 dq/g cm
q/g cm3
106 dq/g cm3
0.9999437 0.9999604 0.9999690 0.9999719 0.9999738 0.9999749 0.9999660
0.81 3.4 1.7 1.2 1.2 2.1 0.92
0.9999418 0.9999590 0.9999685 0.9999726 0.9999725 0.9999740 0.9999657
1.1 1.9 1.3 0.50 2.5 1.2 1.2
3
6
3
dq indicate the uncertainties in water density, obtained as the absolute difference between our experimental value and the value taken from reference [4]. All the work was done under atmospheric pressure of 75.06 kPa. 1 Water density values obtained in this work on having calibrated and having had measured the densities of the HMT aqueous solutions. 2 Water density values obtained in this work on having calibrated and having had measured the densities of the TATD aqueous solutions.
30
TABLE 3 Density q and apparent molal volume V / of HMT in aqueous solution around the temperature of maximum density of water. T/K 275.15 ± 0.02
275.62 ± 0.01
276.14 ± 0.01
276.67 ± 0.01
q ± 106 dq/(g cm3)
V / ± dV / / (cm3 mol1)
m/(mol kg1)
q ± 106 dq/(g cm3)
V / ± dV / / (cm3 mol1)
m/(mol kg1)
q ± 106 dq/(g cm3)
V / ± dV / / (cm3 mol1)
m/(mol kg1)
q ± 106 dq(g cm3)
V / ± dV / / (cm3 mol1)
0.00121 0.00500 0.00994 0.02457 0.05001 0.10004 0.15093 0.19869
0.999954 ± 1.9 1.000095 ± 2.0 1.000238 ± 2.1 1.000702 ± 2.1 1.001508 ± 1.8 1.003075 ± 2.1 1.004487 ± 2.0 1.006157 ± 2.0
131.30 ± 1.60 109.78 ± 0.39 110.51 ± 0.21 109.21 ± 0.08 108.73 ± 0.04 108.54 ± 0.02 109.59 ± 0.02 108.25 ± 0.01
0.00121 0.00531 0.00994 0.02491 0.05070 0.10004 0.15093 0.19869
0.999987 ± 2.0 1.000120 ± 2.0 1.000270 ± 1.8 1.000746 ± 2.0 1.001562 ± 1.7 1.003112 ± 1.7 1.004560 ± 1.8 1.006178 ± 2.2
115.36 ± 1.73 109.57 ± 0.37 108.72 ± 0.18 108.44 ± 0.09 108.37 ± 0.04 108.32 ± 0.02 109.20 ± 0.02 108.21 ± 0.01
0.00131 0.00531 0.00994 0.02491 0.05070 0.10004 0.14965 0.20015
0.999996 ± 2.0 1.000121 ± 2.0 1.000283 ± 1.8 1.000749 ± 2.0 1.001557 ± 1.7 1.003112 ± 1.7 1.004626 ± 1.8 1.006192 ± 2.2
118.21 ± 1.49 111.21 ± 0.37 108.37 ± 0.18 108.71 ± 0.08 108.66 ± 0.03 108.42 ± 0.02 108.56 ± 0.01 108.42 ± 0.01
0.00134 0.00622 0.00981 0.02343 0.05176 0.10004 0.14965 0.20015
1.000009 ± 3.10 1.000184 ± 2.20 1.000263 ± 2.50 1.000696 ± 2.00 1.001608 ± 3.10 1.003125 ± 2.30 1.004643 ± 2.30 1.006252 ± 2.30
113.19 ± 2.28 106.30 ± 0.36 110.57 ± 0.26 109.27 ± 0.09 108.43 ± 0.06 108.34 ± 0.02 108.48 ± 0.02 108.14 ± 0.01
T/K 277.14 ± 0.01
277.65 ± 0.01
278.15 ± 0.02
m/(mol kg1)
q ± 106 dq/(g cm3)
V / ± dV / /(cm3 mol1)
m/(mol kg1)
q ± 106 dq/(g cm3)
V / ± dV / /(cm3 mol1)
m/(mol kg1)
q ± 106 dq/(g cm3)
V / ± V / /(cm3 mol1)
0.00134 0.00622 0.00981 0.02343 0.05176 0.10004 0.14965 0.20015
1.000012 ± 2.1 1.000166 ± 2.5 1.000269 ± 2.6 1.000683 ± 2.3 1.001632 ± 3.2 1.003181 ± 3.2 1.004721 ± 3.2 1.006247 ± 2.4
112.32 ± 1.55 109.46 ± 0.39 110.23 ± 0.27 109.91 ± 0.10 108.00 ± 0.06 107.80 ± 0.03 107.97 ± 0.02 108.18 ± 0.01
0.00099 0.00506 0.00966 0.02556 0.05176 0.10004 0.14965 0.20015
0.999993 ± 1.90 1.000116 ± 2.00 1.000232 ± 2.10 1.000727 ± 2.40 1.001621 ± 2.00 1.003112 ± 2.00 1.004577 ± 1.90 1.006038 ± 2.10
119.85 ± 1.93 111.86 ± 0.39 113.30 ± 0.21 110.60 ± 0.09 108.18 ± 0.04 108.47 ± 0.02 108.92 ± 0.01 109.23 ± 0.01
0.00100 0.00500 0.00993 0.02556 0.05176 0.10004 0.14965 0.20042
0.999990 ± 2.0 1.000120 ± 1.9 1.000299 ± 3.0 1.000723 ± 3.0 1.001556 ± 2.0 1.003076 ± 2.2 1.004570 ± 2.4 1.006139 ± 2.9
117.34 ± 1.95 109.49 ± 0.39 106.71 ± 0.31 110.52 ± 0.12 109.31 ± 0.04 108.77 ± 0.02 108.93 ± 0.02 108.72 ± 0.01
Values within parenthesis indicate the uncertainties in T. m, molality of HMT. q, density of solution, g cm3. dq indicate the uncertainties in density, 106 g cm3. V / , apparent molal volume, 106 m3 mol1. dV / indicate the uncertainties in V / . All the work was done under atmospheric pressure of 75.06 kPa.
J.A. Clavijo Penagos, L.H. Blanco / J. Chem. Thermodynamics 45 (2012) 28–34
m/(mol kg1)
TABLE 4 Density q and apparent molal volume V / of TATD in aqueous solution around the temperature of maximum density of water. T/K 275.65 ± 0.03
276.15 ± 0.01
276.65 ± 0.01
m/(mol kg1)
q ± 106 dq/(g cm3)
V / ± dV / / (cm3 mol1)
m/(mol kg1)
q ± 106 dq/(g cm3)
V / ± dV / / (cm3 mol1)
m/(mol kg1)
q ± 106 dq/(g cm3)
V / ± dV / / (cm3 mol1)
m/(mol kg1)
q ± 106 dq/(g cm3)
V / ± dV / / (cm3 mol1)
0.00100 0.00513 0.01021 0.02492 0.05060 0.09984 0.14961 0.19952
0.999973 ± 2.0 1.000103 ± 1.9 1.000262 ± 1.8 1.000723 ± 2.0 1.001529 ± 2.2 1.003073 ± 2.0 1.004634 ± 2.1 1.006199 ± 2.0
137.77 ± 2.01 137.02 ± 0.37 136.91 ± 0.17 136.80 ± 0.08 136.67 ± 0.04 136.45 ± 0.02 136.24 ± 0.01 136.02 ± 0.01
0.00100 0.00513 0.01021 0.02492 0.05060 0.09984 0.14961 0.19952
0.999990 ± 1.8 1.000119 ± 1.8 1.000277 ± 1.7 1.000735 ± 1.8 1.001534 ± 1.6 1.003068 ± 1.6 1.004617 ± 1.6 1.006172 ± 1.8
135.18 ± 1.84 136.70 ± 0.35 136.86 ± 0.16 136.91 ± 0.07 136.84 ± 0.03 136.65 ± 0.02 136.44 ± 0.01 136.24 ± 0.01
0.00100 0.00513 0.01021 0.02492 0.05060 0.09984 0.14961 0.19952
0.999999 ± 1.7 1.000126 ± 1.7 1.000283 ± 1.7 1.000735 ± 1.7 1.001525 ± 1.8 1.003040 ± 1.8 1.004571 ± 1.7 1.006106 ± 1.8
136.16 ± 1.74 137.19 ± 0.34 137.29 ± 0.16 137.31 ± 0.07 137.23 ± 0.04 137.03 ± 0.02 136.83 ± 0.01 136.62 ± 0.01
0.00102 0.00520 0.00985 0.02534 0.05060 0.09984 0.14961 0.19952
0.999993 ± 2.2 1.000125 ± 2.5 1.000209 ± 2.7 1.000737 ± 2.1 1.001620 ± 2.5 1.003042 ± 2.6 1.004486 ± 2.3 1.006042 ± 2.6
148.93 ± 2.20 138.90 ± 0.47 144.29 ± 0.27 137.99 ± 0.08 135.47 ± 0.05 137.07 ± 0.03 137.44 ± 0.02 136.98 ± 0.01
T/K 277.14 ± 0.01 1
m/(mol kg 0.00102 0.00520 0.00985 0.02534 0.05060 0.09984 0.14565 0.19637
277.64 ± 0.02 )
6
3
3
q ± 10 dq/(g cm )
V / ± dV / /(cm
0.999999 ± 2.2 1.000130 ± 2.3 1.000283 ± 2.1 1.000802 ± 3.2 1.001681 ± 3.2 1.003171 ± 2.2 1.004720 ± 2.2 1.006251 ± 3.2
144.21 ± 2.14 138.34 ± 0.44 136.95 ± 0.22 135.47 ± 0.13 134.29 ± 0.06 135.78 ± 0.02 135.00 ± 0.02 135.42 ± 0.02
mol
Values within parenthesis indicate the uncertainties in T. m, molality of TATD. q, density of solution, g cm3. dq indicate the uncertainties in density, 106 g cm3. V/, apparent molal volume, 106 m3 mol1. dV / indicate the uncertainties in V / . All the work was done under atmospheric pressure of 75.06 kPa.
1
)
1
m/(mol kg 0.00093 0.00488 0.01002 0.02508 0.04864 0.09984 0.14565 0.19637
278.16 ± 0.02 )
6
3
3
q ± 10 dq/(g cm )
V / ± dV / /(cm
0.999987 ± 2.1 1.000117 ± 2.2 1.000259 ± 2.5 1.000756 ± 2.0 1.001466 ± 2.4 1.003080 ± 2.1 1.004423 ± 2.0 1.005952 ± 2.7
153.34 ± 2.26 138.74 ± 0.45 139.59 ± 0.25 136.89 ± 0.08 137.32 ± 0.05 136.68 ± 0.02 137.06 ± 0.01 136.96 ± 0.01
mol
1
)
m/(mol kg1)
q ± 106 dq/(g cm3)
V / ± dV / /(cm3 mol1)
0.00101 0.00488 0.00986 0.02508 0.04864 0.09984 0.14565 0.19637
0.999983 ± 2.4 1.000096 ± 2.0 1.000246 ± 2.0 1.000741 ± 2.3 1.001465 ± 2.0 1.003052 ± 2.0 1.004347 ± 2.0 1.005913±1.9
152.75 ± 2.35 141.80 ± 0.40 139.90 ± 0.20 137.27 ± 0.09 137.23±0.04 136.91 ± 0.02 137.55 ± 0.01 137.13±0.01
J.A. Clavijo Penagos, L.H. Blanco / J. Chem. Thermodynamics 45 (2012) 28–34
275.16 ± 0.02
31
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J.A. Clavijo Penagos, L.H. Blanco / J. Chem. Thermodynamics 45 (2012) 28–34
temperatures and the results for apparent volumes are presented in tables 3 and 4. Apparent molal volumes V / were calculated from density measurements at each temperature according to the following equation:
FIGURE 1. Plot of apparent molal volume (V / ) against molality (m) of HMT (T = 275.16 K) and TATD (T = 275.15 K).
FIGURE 2. Plot of apparent molal volume (V / ) against molality (m) of HMT (T = 275.65 K) and TATD (T = 275.62 K).
FIGURE 3. Plot of apparent molal volume (V / ) against molality (m) of HMT (T = 276.15 K) and TATD (T = 276.14 K).
V/ ¼
M2
q
1000ðq q0 Þ ; mqq0
ð1Þ
where q is the density of the solution experimentally determined, q0 is the density of pure water (taken as reference for calibration),
FIGURE 4. Plot of apparent molal volume (V / ) against molality (m) of HMT (T = 276.65 K) and TATD (T = 276.67 K).
FIGURE 5. Plot of apparent molal volume (V / ) against molality (m) of HMT and TATD (T = 277.14 K).
FIGURE 6. Plot of apparent molal volume (V / ) against molality (m) of HMT (T = 277.64 K) and TATD (T = 277.65 K).
33
J.A. Clavijo Penagos, L.H. Blanco / J. Chem. Thermodynamics 45 (2012) 28–34
m is the molality of the solution, M2 is the molar mass of macrocyclic aminal and V / is the apparent molal volume of the solute. Figures 1 to 7 show the dependence of apparent molal volume of the binary aqueous mixtures of HMT and TATD as function of molality at the selected temperatures. In order to evaluate the standard uncertainty in determinations of the apparent molal volume values, by combining the uncertainties in m and q, according to equation (1), the law of propagation of uncertainties was used [25]. These results also are shown in tables 2 and 3. As expected, the uncertainty in apparent molal volume is greater at low concentrations and almost constant, for both aqueous systems, at concentration above 0.025 mol kg1. From figures 1 to 7, it is clear that the composition dependence of V / is linear above 0.025 mol kg1 for all aqueous systems. Also it is clear that the behaviour of the property at very diluted concentration (0 mol kg1 < m < 0.025 mol kg1) is very complex to be easily interpreted, for which we do not contribute any simple explanation to this subject; we want only to remark that the apparent increasing of V / with increasing dilution is a trend already shown for aqueous solutions of HMT [2] and for other non-electrolytic solutes in aqueous solution [26], in contrast to the behaviour found for electrolytic ones, which usually show that V / decreases with increasing dilution [27]. Examples illustrate the complexity of the behaviour of V / in the most diluted region of many aqueous solutions. Partial molar volumes of macrocyclic aminals in water at infinite dilution at the different temperatures were obtained by extrapolation of the adjusted data of apparent molal volumes by least squares. The results are presented in table 5 and figure 8. It is necessary to highlight that the intervals of concentration used for the extrapolations reported in this work begin at
m = 0.025 mol kg1, which is a lower value than the usually taken ones in the literature for HMT’s watery solutions [2,5,7,18]. This gives major reliability to the values of obtained by extrapolation of the linear region of the dependence of V / with m. Because no results have been found in the literature for volumetric properties of aqueous solutions of HMT and TATD around the temperature of maximum density of water, there is no possibility of comparison of the results we are reporting in this paper. However, we think our results are in good agreement with data of several researchers, as seen in table 6. The small differences can be attributed to the differences in the concentration range used in extrapolation and the resulting differences in least squares fitting of experimental data. The temperature dependence of partial molar volumes of macrocyclic aminals in water at infinite dilution within the temperature range studied is presented in figure 8. The standard uncertainty in this values amounts to 0.39 cm3 mol1. Partial molar volumes at infinite dilution of HMT and TATD show a slight increase with temperature, and the effect of temperature on the partial molar volumes at infinite dilution of the solutes in aqueous solution is well described by a polynomial second order equation. According to the Hepler criteria for the second derivative of the partial molar volumes at infinite dilution with temperature [17], we obtain negative values for HMT and TATD and this suggests they have a breaking effect on water structure. The calculated values are not reported because they are smaller than the estimated uncertainty for infinite dilution partial molar volumes.
FIGURE 8. Plot of partial molal volume at infinite dilution against temperature for aqueous solutions of HMT and TATD around the temperature of maximum density of water.
o FIGURE 7. Plot of apparent molal volume (V / ) against molality (m) of HMT (T = 278.15 K) and TATD (T = 278.16 K).
TABLE 5 Infinite dilution partial molar volumes for HMT and TATD around the temperature of maximum density of water. HMT
TATD
T/K
3 V1 2 =cm mol
275.15 275.62 276.14 276.67 277.14 277.65 278.15
109.50 108.63 108.86 108.98 108.74 109.36 109.47
1
T/K
3 V1 2 =cm mol
275.16 275.65 276.15 276.65 277.14 277.64 278.16
136.96 136.89 137.33 137.83 138.31 137.52 138.03
1
TABLE 6 Comparison between reported values of infinite dilution partial molar volumes for HMT. T/K
3 V1 2 =cm mol
275.62 276.14 276.67 276.81 277.14 277.65 278.15 278.15 283.15 288.15 288.15 289.22 298.15 298.15
108.63 108.86 108.98 108.86 [18] 108.74 109.36 108.87 [7] 109.47 109.9 [2] 109.76 [7] 110.5 [2] 109.80 [18] 110.58 [7] 110.66 [5]
1
34
J.A. Clavijo Penagos, L.H. Blanco / J. Chem. Thermodynamics 45 (2012) 28–34
Finally, from the apparent volume and partial molar volume at infinite dilution data for HMT and TATD we are reporting (tables 3 to 5), and although the following proposition is in this case a qualitative one, it appears plausible to assume the existence of strong solute-solvent interactions when HMT and TATD are dissolved in water around the temperature of maximum density of water, breaking the water structure when happens the dissolution process and forming a more ordered structures around them, as proposed before [2]. Undoubtedly, it is necessary to gather values of several thermodynamic and physicochemical properties of the aqueous solutions around the temperature of maximum density of water in order to define a more valid classification of the solutes as liquid water structure breakers or makers [28,29].
4. Conclusions In the present work, the apparent molal volume of macrocyclic aminals HMT and TATD in aqueous solution are reported at T = (275.15, 275.65, 276.15, 276.65, 277.15, 277.65, and 278.15) K as function of composition together with the partial molar volumes at infinite dilution as function of temperature. The reported values of apparent molal volumes and partial molar volumes at infinite dilution indicate the presence of strong solute-solvent interactions in the solutions, as expected by the presence of four nitrogen atoms in the HMT and TATD molecules which can establish hydrogen bonds with water. The application of Hepler’s criterion to the values of obtained in this work allows the suggestion that the solutes seem to behave as liquid water structure breakers, which agrees with previous reports for HMT [7]. Finally, the information that we are reporting in this paper is an absolutely new contribution to the knowledge.
The work described here was carried out by J.A. Clavijo Penagos under thesis direction of Ph.D. Luis H. Blanco at Laboratorio de Investigaciones Básicas of Universidad Nacional de Colombia, in partial fulfilment of his Ph.D. in Science – Chemistry program. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]
Acknowledgements [26]
The authors express their sincere gratitude to the reviewer and the editor for making critical suggestions which helped us to improve the quality of the manuscript. J.A. Clavijo thanks to the Lord Jesus Christ for all the help, material and spiritual, to reach the final of his Ph.D. studies, and dedicates to Him this work.
[27] [28] [29] [30]
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JCT 11-305