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Isopiestic determination of the osmotic and activity coefficients of dilute aqueous solutions of the series MeEt3NI to HepEt3NI at 298.15 K
Fluid Phase Equilibria 249 (2006) 147–152
Isopiestic determination of the osmotic and activity coefficients of dilute aqueous solutions of the series MeEt3NI to HepEt3NI at 298.15 K Luis H. Blanco b,1 , Eliseo Amado a,∗ , Jos´e A. Avellaneda b,1 a
b
IBEAR, Universidad de Pamplona, Pamplona, Colombia LIB, Universidad Nacional de Colombia, Bogot´a, Colombia
Received 19 May 2006; received in revised form 20 September 2006; accepted 20 September 2006 Available online 26 September 2006
Abstract The osmotic coefficients of MeEt3 NI, Et4 NI, PrEt3 NI, BuEt3 NI, PenEt3 NI, HexEt3 NI and HepEt3 NI in dilute aqueous solutions were determined by the isopiestic method at 298.15 K. The osmotic coefficients of tetra-alkyl-ammonium iodides in aqueous solutions were analyzed by comparing them with calculated values from the Debye–H¨uckel limiting law. The osmotic coefficient data of MeEt3 NI, PrEt3 NI and BuEt3 NI show positive deviations from the Debye–H¨uckel limiting law. The osmotic coefficient data of HexEt3 NI, Et4 NI, PenEt3 NI and HepEt3 NI show negative deviations from the Debye–H¨uckel limiting law. The non-ideal behavior of these electrolyte systems was satisfactorily correlated by the Pitzer ion interaction model with two adjustable parameters β0 and β1 . © 2006 Published by Elsevier B.V. Keywords: Activity coefficient; Osmotic coefficient; Unsymmetrical quaternary ammonium iodides; Pitzer ion interaction model
1. Introduction The thermodynamic properties of tetra-alkyl-ammonium halides (TAAX, X = Cl, Br, I, F) and their hydrophobic interactions in aqueous solutions are related. Due to the fairly large solubility of the TAAX in water and the possibility of changing the length of the alkyl chains [1–3]. The isopiestic method was used currently as a useful experimental technique for the investigation of the effect of such electrolyte systems on the structure of water [4]. In earlier papers, the osmotic coefficients of aqueous solutions of the series Bu4 NBr, sec-Bu4 NBr, iso-Bu4 NBr, Bu2 Et2 NBr and Bu3 EtNBr at 298.15 K [5] and at 293.15 K [6] were analyzed by comparing them with the Debye–H¨uckel limiting law (DHLL). At both temperatures a positive deviation of the osmotic coefficients compared with DHLL was found. In the present work, results of the osmotic coefficients for aqueous solutions of MeEt3 NI, Et4 NI, PrEt3 NI, BuEt3 NI, PenEt3 NI, HexEt3 NI and HepEt3 NI at 298.15 K using the isopiestic method ∗
Corresponding author. Fax: +57 7 56805303. E-mail addresses: [email protected] (L.H. Blanco), [email protected] (E. Amado). 1 Tel.: +57 1 3150188. 0378-3812/$ – see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.fluid.2006.09.025
are reported and compared with DHLL. In addition the results are used to correlate the structure of the compounds, specially the chain length, with osmotic and activity coefficients. The Pitzer ion interaction model was used for modelling and correlating the non-ideal behavior of these electrolyte systems. 2. Experimental 2.1. Materials The TAAI salts were synthesized using a modification of the procedure recommended by Vogel [7]. Stoichiometric amounts of triethylamine and the selected alkyl iodide in methanol (Merck, zur analyze) solution were stirred in a reactor for 48 h. Usually a white sticky solid was obtained. The salts were crystallized from ethanol solution and dried under vacuum for 48 h. In all cases anion analysis was done by potentiometric titration with silver nitrate (Fischer Scientific Co.) and cation analysis was done by potentiometric titration with NaTPB [8]. Methyltryethylammonium iodide. Reagents: triethylamine (BDH Chemicals P.A.) and methyl iodide (Merck zur synthese). Melting temperature 583 K. Purity: cation analysis 99.3 ± 0.5 and anion analysis 99.5 ± 0.5. Propyltryethylammonium iodide. Reagents: triethylamine (BDH Chemicals P.A.)
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and propyl iodide (Merck zur synthese). Melting temperature 533 K. Purity: cation analysis 99.3 ± 0.5 and anion analysis 99.5 ± 0.5. Buthyltryethylammonium iodide. Reagents: triethylamine (BDH Chemicals P.A.) and buthyl iodide (J.T. Baker chemicals Co., R.A.). Melting temperature 482 K. Purity: cation analysis 99.8 ± 0.5 and anion analysis 99.9 ± 0.5. Penthyltryethylammonium iodide. Reagents: triethylamine (BDH Chemicals P.A.) and penthyl iodide (J.T. Baker chemicals Co., R.A.). Melting temperature 431 K. Purity: cation analysis 99.8 ± 0.5 and anion analysis 99.9 ± 0.5. Hexyltryethylammonium iodide. Reagents: triethylamine (BDH Chemicals P.A.) and hexyl iodide(J.T. Baker chemicals Co., R.A.). Melting temperature 397 K. Purity: cation analysis 99.8 ± 0.5 and anion analysis 99.9 ± 0.5. Hepthyltryethylammonium iodide. Reagents: triethylamine (BDH Chemicals P.A.) and hepthyl iodide (J.T. Baker chemicals Co., R.A.). Melting temperature 396 K. Purity: cation analysis 99.8 ± 0.5 and anion analysis 99.8 ± 0.5. The Et4 NI was an Eastman Kodak product.
3. Calculation procedure The osmotic coefficient was calculated form the isopiestic molalities (see Tables 1 and 2) by means of the following equation: φ=
m r φr m
(1)
The osmotic coefficient of NaCl was determined as a function of molality and temperature from the extended Bradley–Pitzer correlation [10]. The molality m is accurate to ±0.002 m. The Pitzer ion interaction model [11,12] has been successfully used for aqueous electrolyte solutions. It has the following form: 3/2 2ν+ ν− φ φ 2 2(ν+ ν− ) φ − 1 = |z+ z− |f + m B +m Cφ ν ν (2) where
2.2. Experimental procedure All solutions were prepared by weight using doubly distilled water at room temperature (293 ± 2) K. A Metler AT 261 balance was used to weigh the sample cups and solution samples. It has a precision of 1 × 10−5 g. Buoyancy corrections were applied. NaCl (analytical reagent grade) isopiestic reference standard stock solutions were prepared by weight. NaCl was oven-dried at 383 K. Molar mass of NaCl used for molality calculations was 58.443 g mol−1 . The isopiestic technique was previously described [9]. Air was removed from a modified glass vacuum desiccator until the pressure was around 4.5 kPa (see Fig. 1). The apparatus was held in a constant temperature bath (70 l) at (298.15 ± 0.1) K. The cups rested in depressions in a steel (AISI-316) block. The cups were weighed at intervals varying from 8 days to 2 weeks (depending on the time found necessary to reach equilibrium). Equilibrium was assumed to have been attained when the solutions in duplicate dishes arrived at the same molality (within 0.5%).
Aφ I 1/2 I + bI 1/2
(3)
Bφ = β0 + β1 exp(−αI 1/2 )
(4)
fφ = −
ln γ± = |z+ z− |f + m γ
2ν+ ν− ν
B +m γ
2
2ν+ ν− ν
Cγ (5)
β0 (kg/mol), β1 (kg/mol) and C (kg/mol)2 are Pitzer ioninteraction parameters. The constants α = 2 (kg/mol)1/2 and b = 1.2 (kg/mol)1/2 are parameters of the Pitzer ion interaction model. z+ and z− are positive and negative ionic charges. γ ± is the molality-scale mean ionic activity coefficient of the electrolyte. The Debye–H¨uckel constant for the osmotic coefficient, Aφ (kg/mol)1/2 , is defined as: Aφ =
1.4006 × 106 d 1/2 (εr T )3/2
(6)
Fig. 1. Diagram of the isopiestic apparatus. (a) Manometer; (b) air thermostat (90 cm × 50 cm × 90 cm); (c) water thermostat; (d) modified desiccator; (e) metal block; (f) metal cell; (g) metal cap; (h) resistance thermometer; (i) temperature control.
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Table 1 Activity and osmotic coefficients of aqueous solutions of MeEt3 NI to BuEt3 NI at 298.15 K NaCl
MeEt3 NI
Et4 NI
m (mol/kg)
m (mol/kg)
φ
γ±
m (mol/kg)
φ
γ±
m (mol/kg)
φ
γ±
m (mol/kg)
φ
γ±
0.0492 0.0591 0.0785 0.0922 0.1015 0.1521 0.1578 0.1646 0.1732 0.1881 0.2235 0.2810 0.3099 0.3527 0.3900 0.3940 0.4100 0.4375 0.4917 0.5617 0.5620 0.5690 0.6058 0.6500 0.6718 0.6871 0.7035 0.7359 0.7798 0.8069 0.8129 0.8208 0.8297 0.8323 0.8593
0.0478 0.0576 0.0766 0.0902 0.0995 0.1505 0.1558 0.1631 0.1721 0.1879 0.2254 0.2898 0.3240 0.3742 0.4177 0.4237 0.4447 0.4801 0.5528 0.6505 0.6561 0.6693 0.7286 0.8032 0.8482 0.8851 0.9205 0.9981 1.1005 1.2005 1.2198 1.2501 1.2901 1.3703 1.4703
0.7389 0.7142 0.6730 0.6485 0.6324 0.5635 0.5562 0.5482 0.5394 0.5233 0.4881 0.4397 0.4206 0.3911 0.3710 0.3683 0.3596 0.3468 0.3214 0.2944 0.2954 0.2915 0.2795 0.2652 0.2592 0.2549 0.2495 0.2410 0.2318 0.2255 0.2238 0.2226 0.2202 0.2191 0.5446
0.9777 0.9749 0.9713 0.9680 0.9643 0.9579 0.9563 0.9537 0.9525 0.9459 0.9398 0.9267 0.9180 0.9067 0.8800 0.8841 0.8735 0.8589 0.8252 0.7617 0.7511 0.7396 0.6908 0.6621 0.6309 0.5887 0.5869 0.5594 0.5459 0.5406 0.5394 0.5348 0.5333 0.5300 0.3438
0.0519 0.0633 0.0860 0.1019 0.1134 0.1748 0.1826 0.1916 0.2018 0.2218 0.2718 0.3586 0.3999 0.4738 0.5329 0.5415 0.5698 0.6153 0.7181 0.8512 0.8456 0.8671 0.9377 1.0322 1.0758 1.1085 1.1511 1.2235 1.3091 1.3732 1.3912 1.4035 1.4294 1.4419
0.8954 0.8781 0.8544 0.8443 0.8338 0.8060 0.8002 0.7950 0.7938 0.7836 0.7585 0.7217 0.7135 0.6852 0.6738 0.6698 0.6625 0.6548 0.6312 0.6091 0.6134 0.6058 0.5970 0.5826 0.5781 0.5741 0.5664 0.5580 0.5529 0.5465 0.5435 0.5442 0.5403 0.5373
0.7389 0.7142 0.6730 0.6485 0.6324 0.5635 0.5562 0.5482 0.5394 0.5233 0.4881 0.4397 0.4206 0.3911 0.3710 0.3683 0.3596 0.3468 0.3214 0.2944 0.2954 0.2915 0.2795 0.2652 0.2592 0.2549 0.2495 0.2410 0.2318 0.2255 0.2238 0.2226 0.2202 0.2191
0.0475 0.0570 0.0757 0.0889 0.0981 0.1471 0.1528 0.1597 0.1682 0.1838 0.2194 0.2793 0.3108 0.3581 0.4080 0.4103 0.4322 0.4691 0.5492 0.6807 0.6907 0.7103 0.8103 0.9083 0.9858 1.0811 1.1108 1.2205 1.3257 1.3881 1.4018 1.4280 1.4480 1.4619
0.9777 0.9749 0.9713 0.9680 0.9643 0.9579 0.9563 0.9537 0.9525 0.9459 0.9398 0.9267 0.9180 0.9067 0.8800 0.8841 0.8735 0.8589 0.8252 0.7617 0.7511 0.7396 0.6908 0.6621 0.6309 0.5887 0.5869 0.5594 0.5459 0.5406 0.5394 0.5348 0.5333 0.5300
0.8886 0.8846 0.8780 0.8736 0.8706 0.8532 0.8511 0.8483 0.8449 0.8385 0.8227 0.7937 0.7775 0.7526 0.7259 0.7247 0.7129 0.6932 0.6513 0.5870 0.5824 0.5735 0.5307 0.4931 0.4663 0.4370 0.4286 0.4005 0.3776 0.3658 0.3634 0.3589 0.3556 0.3534
0.0475 0.0571 0.0760 0.0898 0.0991 0.1499 0.1571 0.1638 0.1729 0.1890 0.2281 0.2930 0.3288 0.3800 0.4288 0.4350 0.4582 0.5067 0.5888 0.7326 0.7462 0.7600 0.8550 0.9926 1.0926 1.1323 1.1776 1.3052 1.5030 1.6450 1.6811 1.7381 1.7820 1.8108
0.9775 0.9735 0.9668 0.9582 0.9545 0.9400 0.9300 0.9298 0.9266 0.9199 0.9038 0.8831 0.8677 0.8544 0.8373 0.8339 0.8239 0.7952 0.7697 0.7077 0.6952 0.6912 0.6547 0.6059 0.5693 0.5620 0.5536 0.5231 0.4815 0.4562 0.4498 0.4394 0.4334 0.4279
0.8579 0.8494 0.8345 0.8245 0.8179 0.7829 0.7779 0.7733 0.7670 0.7558 0.7279 0.6804 0.6538 0.6156 0.5794 0.5749 0.5579 0.5228 0.4656 0.3738 0.3658 0.3577 0.3055 0.2399 0.1995 0.1850 0.1695 0.1315 0.0869 0.0636 0.0586 0.0514 0.0465 0.0434
PrEt3 NI
BuEt3 NI
d1 and D are the density and dielectric constant of the pure solvent, respectively. N0 , εr and k are Avogadro number, relative permittivity (static dielectric constant) of the solvent and the Boltzmann constant, respectively. The value of Aφ is 0.3915 (kg/mol)1/2 . I is ionic strength on a molality scale. 4. Results and discussion Fig. 2 shows that the osmotic coefficients for MeEt3 NI, PrEt3 NI, BuEt3 NI, BuEt3 NBr and Bu2 Et2 NBr are larger that calculated by DHLL. However, for m1/2 > 1, the osmotic coefficients of these TAAI seem to be lower than the DHLL results. This is especially remarkable for BuEt3 NI and PrEt3 NI. The values for the osmotic coefficients vary as follows: MeEt3 NI > PrEt3 NI > BuEt3 NI. In comparison with the osmotic coefficients of the TAAI, the osmotic coefficients of Bu2 Et2 NBr and BuEt3 NBr resemble the behavior of strong 1:1 electrolytes. Fig. 3 shows the osmotic coefficients for Et4 NI, PenEt3 NI, HexEt3 NI and HepEt3 NI salts. The occurrence of extensive ion–pair formation is suggested by the fact that
Fig. 2. Concentration dependence of the osmotic coefficients for aqueous solutions of TAAI salts at 298.15 K compared with DHLL. Osmotic coefficient of BuEt3 NBr and Bu2 Et2 NBr are from literature [5]. Experimental data: () MeEt3 NI; () PrEt3 NI; (♦) BuEt3 NI; (–) DHLL; () BuEt3 NBr; (*) Bu2 Et2 NBr.
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Table 2 Activity and osmotic coefficients of aqueous solutions of PenEt3 NI to HepEt3 NI at 298.15 K NaCl
PenEt3 NI
HexEt3 NI
m (mol/kg)
m (mol/kg)
φ
γ±
m (mol/kg)
φ
γ±
m (mol/kg)
φ
γ±
0.0531 0.0690 0.0775 0.0891 0.0933 0.1018 0.1250 0.1381 0.1389 0.1408 0.1879 0.2030 0.2157 0.2385 0.2697 0.2805 0.2920 0.2991 0.3001 0.3328 0.3678 0.3845 0.3957 0.4126 0.4573 0.4953 0.5295 0.5804 0.6015 0.6264 0.6875 0.7431 0.7659 0.7808 0.8105 0.8296
0.0539 0.0703 0.0798 0.0922 0.0976 0.1065 0.1320 0.1471 0.1482 0.1501 0.2076 0.2284 0.2454 0.2742 0.3148 0.3295 0.3456 0.3579 0.3614 0.4108 0.4718 0.4980 0.5140 0.5434 0.6238 0.6938 0.7814 0.9005 0.9578 1.0150 1.1988 1.3585 1.4650 1.5487 1.6875
0.9283 0.9203 0.9092 0.9030 0.8923 0.8910 0.8799 0.8712 0.8694 0.8697 0.8364 0.8208 0.8113 0.8020 0.7891 0.7841 0.7781 0.7696 0.7646 0.7457 0.7176 0.7107 0.7087 0.6991 0.6797 0.6580 0.6250 0.5952 0.5802 0.5705 0.5312 0.5076 0.4855 0.4684 0.4467
0.7805 0.7542 0.7407 0.7245 0.7178 0.7073 0.6797 0.6650 0.6639 0.6621 0.6135 0.5980 0.5860 0.5668 0.5420 0.5335 0.5246 0.5179 0.5161 0.4911 0.4633 0.4522 0.4456 0.4341 0.4051 0.3826 0.3574 0.3278 0.3152 0.3035 0.2713 0.2488 0.2361 0.2273 0.2147
0.0542 0.0712 0.0797 0.0926 0.0971 0.1070 0.1332 0.1502 0.1517 0.1530 0.2121 0.2320 0.2494 0.2836 0.3308 0.3473 0.3651 0.3804 0.3878 0.4518 0.5224 0.5588 0.5839 0.6237 0.7289 0.8228 0.9203 1.1014 1.1706 1.2748 1.5119 1.8404 1.9404 2.0403 2.2404 2.3440
0.9221 0.9085 0.9098 0.8987 0.8963 0.8864 0.8718 0.8528 0.8493 0.8537 0.8185 0.8080 0.7983 0.7755 0.7511 0.7439 0.7366 0.7240 0.7126 0.6781 0.6481 0.6334 0.6239 0.6091 0.5817 0.5549 0.5307 0.4866 0.4748 0.4543 0.4212 0.3747 0.3665 0.3556 0.3365 0.3294
0.7723 0.7434 0.7304 0.7123 0.7062 0.6936 0.6631 0.6452 0.6437 0.6424 0.5887 0.5729 0.5597 0.5355 0.5051 0.4952 0.4850 0.4764 0.4724 0.4399 0.4082 0.3933 0.3836 0.3690 0.3345 0.3081 0.2842 0.2478 0.2361 0.2204 0.1919 0.1648 0.1588 0.1536 0.1454 0.1423
0.0575 0.0770 0.0876 0.1022 0.1082 0.1205 0.1533 0.1727 0.1750 0.1805 0.2592 0.2917 0.3136 0.3583 0.4282 0.4477 0.4858 0.5029 0.5121 0.5894 0.6973 0.7870 0.8401 0.8900 1.1047 1.3415 1.5041 1.8032 1.9205 2.2050 2.5154 2.7900 2.9600 3.0000
0.8694 0.8400 0.8287 0.8146 0.8048 0.7873 0.7574 0.7421 0.7363 0.7234 0.6699 0.6425 0.6348 0.6137 0.5802 0.5770 0.5535 0.5476 0.5396 0.5198 0.4856 0.4497 0.4336 0.4268 0.3838 0.3403 0.3247 0.2972 0.2894 0.2626 0.2531 0.2471 0.2403 0.2418
0.6809 0.6340 0.6120 0.5845 0.5741 0.5540 0.5078 0.4845 0.4818 0.4756 0.4033 0.3798 0.3655 0.3394 0.3055 0.2972 0.2822 0.2760 0.2727 0.2482 0.2204 0.2015 0.1917 0.1833 0.1541 0.1309 0.1187 0.1013 0.0959 0.0851 0.0762 0.0702 0.0672 0.0666
the osmotic coefficients are lower than calculated from DHLL. The osmotic coefficient data varies as follows: Et4 NI > PenEt3 NI > HexEt3 NI > HepEt3 NI. The following relation can be deduced from Figs. 2 and 3 for solute–solvent interactions: MeEt3 NI > PrEt3 NI > BuEt3 NI > Et4 NI > PenEt3 NI > HexEt3 NI > HepEt3 NI Fig. 4 shows the activity coefficient data for MeEt3 NI, PrEt3 NI and BuEt3 NI. These graphs are lower than calculated
HepEt3 NI
by DHLL. The explanation may be that cations interact with water and disrupt its structure. In Fig. 5 the data from Et4 NI, PenEt3 NI, HexEt3 NI and HepEt3 NI form a group with negative deviations from DHLL. The negative deviations from the DHLL were confirmed for solutions of Et4 NI [13]. From our data it is not possible to confirm micelle formation in this range of concentration for these TAAI. Nevertheless, it is possible that the micelle formation may be present at higher concentrations. The micelle formation of TAAX salts in aqueous solution in relation with semiclathrates crystal structure has been used to explain the behavior of the TAA ions with long alkyl chains
Table 3 Values obtained for Pitzer ion-parameters for TAAI salts at 298.15 K Parameter
MeEt3 NI
Et4 NI
PrEt3 NI
BuEt3 NI
PenEt3 NI
HexEt3 NI
HepEt3 NI
β0 (kg/mol) β1 (kg/mol) σ(φ)a
−0.2688 0.9912 0.003
−0.1023 −0.9550 0.004
−0.2708 0.7935 0.007
−0.2474 0.4216 0.008
−0.1481 −0.8471 0.005
−0.1325 −1.2030 0.005
−0.1032 −1.9460 0.007
a
σ(φ) =
i
(φexp −φcal )2i n
(with n = number of experimental osmotic coefficient used data in the regression for each salt).
L.H. Blanco et al. / Fluid Phase Equilibria 249 (2006) 147–152
Fig. 3. Concentration dependence of the osmotic coefficients for aqueous solutions of TAAI salts at 298.15 K compared with DHLL. Osmotic coefficient of Et4 NBr is from literature [13]. Experimental data: () Et4 NI; () PenEt3 NI; (♦) HexEt3 NI; (+) HepEt3 NI; (–) DHLL; () Et4 NBr.
151
Fig. 6. Relation of β0 to β1 for TTAX salts (X: I and Br) at 298.15 K. Experimental data: (1) HexEt3 NI; (2) BuEt3 NI; (3) PrEt3 NI; (4) MeEt3 NI; (5) Et4 NBr, 1964; (6) Et4 NI, 1964; (7) BuEt3 NBr, 2005; (8) Bu2 Et2 NBr, 2005; (9) PenEt3 NI; (10) HepEt3 NI.
surrounding water molecules [14–18]. Also the hydration numbers and the formation of polyhydrates have been used as alternative arguments [19,20]. It is clear that the behavior of the osmotic and activity coefficient data of TAAX solutions have been proposed several times as a model to describe the changes of water structure [21–27]. Dimers and micelles were considered to be formed in Et4 NBr solutions above 4.0 m [28]. Table 3 shows the calculated values of β0 and β1 . The osmotic coefficient may be reproduced with an average error of 0.5% (standard deviation of σ(φ) = 0.004) for TAAI aqueous solutions in the range (0.04–2.33) mol/kg at 298.15 K. Fig. 6 shows the relationship between the two parameters β0 and β1 for the series MeEt3 NI to HepEt3 NI. The corresponding data for Et4 NI, Et4 NBr, Bu2 Et2 NBr and BuEt3 Br do not show any relationship.
Fig. 4. Logarithm of mean activity coefficient for TAAI salts vs. square root of molality at 298.15 K. Experimental data: () MeEt3 NI; () PrEt3 NI; () BuEt3 NI; (–) DHLL.
Fig. 5. Logarithm of mean activity coefficient for TAAI salts vs. square root of molality at 298.15 K. These TAAI present negative deviations compared with DHLL. Experimental data: () Et4 NI; (♦) PenEt3 NI; () HexEt3 NI; () HepEt3 NI; (–) DHLL.
5. Conclusions Experimental osmotic coefficient data were obtained for aqueous solutions of the series MeEt3 NI to HepEt3 NI by isopiestic method at 298.15 K. From the comparison of the data with the DHLL, the behavior of the osmotic and activity coefficients has been interpreted qualitatively in terms of solute–solvent interactions and ion–pair formation. From our data it was not possible to confirm micelle formation in this given range of concentration for these TAAI. The experimental osmotic coefficient data was satisfactorily correlated using the Pitzer ion interaction model and a relation between the two parameters β0 and β1 for the series MeEt3 NI to HepEt3 NI was found. Finally, it was observed that both the halide and concentration of the TAAX have an important effect on ion–pair formation, and the effect of the cation may be reversed at higher concentration. List of symbols a activity m molality (mol/kg) M molecular weight (g mol−1 ) T absolute temperature (K)
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Greek letters φ osmotic coefficient γ activity coefficient ν stoichiometric coefficient Subscripts o standard state r reference s solute Acknowledgements The authors are grateful to Dr. D. Archer, N.I.S.T., for the use of his computers programs. To Colciencias for its financial support, and to Universidad de Pamplona and Universidad Nacional de Colombia for support. References [1] [2] [3] [4]
B.M. Lowe, H.M. Rendall, Trans. Faraday Soc. (1971) 2318–2327. H.S. Frank, M.W. Evans, J. Chem. Phys. 13 (1945) 507–513. J. Turner, A.K. Soper, J. Phys. Chem. 101 (1994) 6116–6125. R.A. Horne, Water and Aqueous Solutions: Structure, Thermodynamics and Transport Processes, Wiley-Interscience, New York, 1972. [5] E. Amado, L.H. Blanco, Fluid Phase Equilib. 233 (2005) 230–233.
[6] E. Amado, L.H. Blanco, Fluid Phase Equilib. 243 (2006) 166–170. [7] A. Vogel, Vogel’s Textbook of Practical Organic Chemistry, John Wiley and Sons, New York, 1989. [8] J.G. Dick, Analytical Chemistry, International Student Edition, Tokyo, 1973. [9] K.S. Pitzer, Activity Coefficients in Electrolyte Solutions, CRC Press, Boca Raton, FL, 1991. [10] D.G. Archer, J. Phys. Chem. Ref. Data 28 (1999) 1–17. [11] K.S. Pitzer, J. Phys. Chem. 77 (1973) 268–277. [12] K.S. Pitzer, G.J. Mayorga, Phys. Chem. 77 (1973) 2300–2308. [13] S. Lindembaum, G.E. Boyd, J. Phys. Chem. 68 (1964) 911–917. [14] J.A. Burns, R.E. Verrall, Thermochim. Acta 9 (1974) 277–287. [15] Y. Nagano, M. Sakiyama, J. Phys. Chem. 92 (1988) 5823–5827. [16] T.G. Coker, J. Ambrose, G.J. Janz, J. Am. Chem. Soc. 92 (1970) 5293–5297. [17] D.L. Fowler, W.V. Loebenstein, D.B. Pall, C.A. Kraus, J. Am. Chem. Soc. 62 (1940) 1140–1142. [18] W. Wen, S. Saito, J. Phys. Chem. 68 (1964) 2639–2644. [19] H. Oyama, W. Shimada, T. Ebinuma, Y. Kamata, S. Takeya, T. Uchida, J. Nagao, H. Narita, Fluid Phase Equilib. 234 (2005) 131–135. [20] H. Nakayama, Bull. Chem. Soc. Jap. 54 (1981) 3717–3722. [21] J. Lipkowski, V.Yu. Komarov, T.V. Rodionova, A. Dyadinb, L.S. Aladkob, J. Supramolecular Chem. 2 (2002) 435–439. [22] R.M. Diamond, J. Phys. Chem. 64 (1963) 2513–2517. [23] W.Y. Wen, S. Saito, S.C.M. Lee, J. Phys. Chem. 70 (1966) 1244–1248. [24] A. Covington, D.J. Irish, Chem. Eng. Data 17 (1972) 175–176. [25] O. Bonner, J. Chem. Eng. Data 21 (1976) 498–499. [26] J. Barthel, R. Neueder, R.W. Kunz, Pure Appl. Chem. 65 (1993) 889–894. [27] D.M. Fox, L. Leifer, Fluid Phase Equilib. 213 (2003) 1–17. [28] H. Wirth, J. Phys. Chem. 71 (1967) 2922–2929.
Isopiestic determination of the osmotic and activity coefficients of dilute aqueous solutions of the series MeEt3NI to HepEt3NI at 298.15 K Luis H. Blanco b,1 , Eliseo Amado a,∗ , Jos´e A. Avellaneda b,1 a
b
IBEAR, Universidad de Pamplona, Pamplona, Colombia LIB, Universidad Nacional de Colombia, Bogot´a, Colombia
Received 19 May 2006; received in revised form 20 September 2006; accepted 20 September 2006 Available online 26 September 2006
Abstract The osmotic coefficients of MeEt3 NI, Et4 NI, PrEt3 NI, BuEt3 NI, PenEt3 NI, HexEt3 NI and HepEt3 NI in dilute aqueous solutions were determined by the isopiestic method at 298.15 K. The osmotic coefficients of tetra-alkyl-ammonium iodides in aqueous solutions were analyzed by comparing them with calculated values from the Debye–H¨uckel limiting law. The osmotic coefficient data of MeEt3 NI, PrEt3 NI and BuEt3 NI show positive deviations from the Debye–H¨uckel limiting law. The osmotic coefficient data of HexEt3 NI, Et4 NI, PenEt3 NI and HepEt3 NI show negative deviations from the Debye–H¨uckel limiting law. The non-ideal behavior of these electrolyte systems was satisfactorily correlated by the Pitzer ion interaction model with two adjustable parameters β0 and β1 . © 2006 Published by Elsevier B.V. Keywords: Activity coefficient; Osmotic coefficient; Unsymmetrical quaternary ammonium iodides; Pitzer ion interaction model
1. Introduction The thermodynamic properties of tetra-alkyl-ammonium halides (TAAX, X = Cl, Br, I, F) and their hydrophobic interactions in aqueous solutions are related. Due to the fairly large solubility of the TAAX in water and the possibility of changing the length of the alkyl chains [1–3]. The isopiestic method was used currently as a useful experimental technique for the investigation of the effect of such electrolyte systems on the structure of water [4]. In earlier papers, the osmotic coefficients of aqueous solutions of the series Bu4 NBr, sec-Bu4 NBr, iso-Bu4 NBr, Bu2 Et2 NBr and Bu3 EtNBr at 298.15 K [5] and at 293.15 K [6] were analyzed by comparing them with the Debye–H¨uckel limiting law (DHLL). At both temperatures a positive deviation of the osmotic coefficients compared with DHLL was found. In the present work, results of the osmotic coefficients for aqueous solutions of MeEt3 NI, Et4 NI, PrEt3 NI, BuEt3 NI, PenEt3 NI, HexEt3 NI and HepEt3 NI at 298.15 K using the isopiestic method ∗
Corresponding author. Fax: +57 7 56805303. E-mail addresses: [email protected] (L.H. Blanco), [email protected] (E. Amado). 1 Tel.: +57 1 3150188. 0378-3812/$ – see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.fluid.2006.09.025
are reported and compared with DHLL. In addition the results are used to correlate the structure of the compounds, specially the chain length, with osmotic and activity coefficients. The Pitzer ion interaction model was used for modelling and correlating the non-ideal behavior of these electrolyte systems. 2. Experimental 2.1. Materials The TAAI salts were synthesized using a modification of the procedure recommended by Vogel [7]. Stoichiometric amounts of triethylamine and the selected alkyl iodide in methanol (Merck, zur analyze) solution were stirred in a reactor for 48 h. Usually a white sticky solid was obtained. The salts were crystallized from ethanol solution and dried under vacuum for 48 h. In all cases anion analysis was done by potentiometric titration with silver nitrate (Fischer Scientific Co.) and cation analysis was done by potentiometric titration with NaTPB [8]. Methyltryethylammonium iodide. Reagents: triethylamine (BDH Chemicals P.A.) and methyl iodide (Merck zur synthese). Melting temperature 583 K. Purity: cation analysis 99.3 ± 0.5 and anion analysis 99.5 ± 0.5. Propyltryethylammonium iodide. Reagents: triethylamine (BDH Chemicals P.A.)
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and propyl iodide (Merck zur synthese). Melting temperature 533 K. Purity: cation analysis 99.3 ± 0.5 and anion analysis 99.5 ± 0.5. Buthyltryethylammonium iodide. Reagents: triethylamine (BDH Chemicals P.A.) and buthyl iodide (J.T. Baker chemicals Co., R.A.). Melting temperature 482 K. Purity: cation analysis 99.8 ± 0.5 and anion analysis 99.9 ± 0.5. Penthyltryethylammonium iodide. Reagents: triethylamine (BDH Chemicals P.A.) and penthyl iodide (J.T. Baker chemicals Co., R.A.). Melting temperature 431 K. Purity: cation analysis 99.8 ± 0.5 and anion analysis 99.9 ± 0.5. Hexyltryethylammonium iodide. Reagents: triethylamine (BDH Chemicals P.A.) and hexyl iodide(J.T. Baker chemicals Co., R.A.). Melting temperature 397 K. Purity: cation analysis 99.8 ± 0.5 and anion analysis 99.9 ± 0.5. Hepthyltryethylammonium iodide. Reagents: triethylamine (BDH Chemicals P.A.) and hepthyl iodide (J.T. Baker chemicals Co., R.A.). Melting temperature 396 K. Purity: cation analysis 99.8 ± 0.5 and anion analysis 99.8 ± 0.5. The Et4 NI was an Eastman Kodak product.
3. Calculation procedure The osmotic coefficient was calculated form the isopiestic molalities (see Tables 1 and 2) by means of the following equation: φ=
m r φr m
(1)
The osmotic coefficient of NaCl was determined as a function of molality and temperature from the extended Bradley–Pitzer correlation [10]. The molality m is accurate to ±0.002 m. The Pitzer ion interaction model [11,12] has been successfully used for aqueous electrolyte solutions. It has the following form: 3/2 2ν+ ν− φ φ 2 2(ν+ ν− ) φ − 1 = |z+ z− |f + m B +m Cφ ν ν (2) where
2.2. Experimental procedure All solutions were prepared by weight using doubly distilled water at room temperature (293 ± 2) K. A Metler AT 261 balance was used to weigh the sample cups and solution samples. It has a precision of 1 × 10−5 g. Buoyancy corrections were applied. NaCl (analytical reagent grade) isopiestic reference standard stock solutions were prepared by weight. NaCl was oven-dried at 383 K. Molar mass of NaCl used for molality calculations was 58.443 g mol−1 . The isopiestic technique was previously described [9]. Air was removed from a modified glass vacuum desiccator until the pressure was around 4.5 kPa (see Fig. 1). The apparatus was held in a constant temperature bath (70 l) at (298.15 ± 0.1) K. The cups rested in depressions in a steel (AISI-316) block. The cups were weighed at intervals varying from 8 days to 2 weeks (depending on the time found necessary to reach equilibrium). Equilibrium was assumed to have been attained when the solutions in duplicate dishes arrived at the same molality (within 0.5%).
Aφ I 1/2 I + bI 1/2
(3)
Bφ = β0 + β1 exp(−αI 1/2 )
(4)
fφ = −
ln γ± = |z+ z− |f + m γ
2ν+ ν− ν
B +m γ
2
2ν+ ν− ν
Cγ (5)
β0 (kg/mol), β1 (kg/mol) and C (kg/mol)2 are Pitzer ioninteraction parameters. The constants α = 2 (kg/mol)1/2 and b = 1.2 (kg/mol)1/2 are parameters of the Pitzer ion interaction model. z+ and z− are positive and negative ionic charges. γ ± is the molality-scale mean ionic activity coefficient of the electrolyte. The Debye–H¨uckel constant for the osmotic coefficient, Aφ (kg/mol)1/2 , is defined as: Aφ =
1.4006 × 106 d 1/2 (εr T )3/2
(6)
Fig. 1. Diagram of the isopiestic apparatus. (a) Manometer; (b) air thermostat (90 cm × 50 cm × 90 cm); (c) water thermostat; (d) modified desiccator; (e) metal block; (f) metal cell; (g) metal cap; (h) resistance thermometer; (i) temperature control.
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Table 1 Activity and osmotic coefficients of aqueous solutions of MeEt3 NI to BuEt3 NI at 298.15 K NaCl
MeEt3 NI
Et4 NI
m (mol/kg)
m (mol/kg)
φ
γ±
m (mol/kg)
φ
γ±
m (mol/kg)
φ
γ±
m (mol/kg)
φ
γ±
0.0492 0.0591 0.0785 0.0922 0.1015 0.1521 0.1578 0.1646 0.1732 0.1881 0.2235 0.2810 0.3099 0.3527 0.3900 0.3940 0.4100 0.4375 0.4917 0.5617 0.5620 0.5690 0.6058 0.6500 0.6718 0.6871 0.7035 0.7359 0.7798 0.8069 0.8129 0.8208 0.8297 0.8323 0.8593
0.0478 0.0576 0.0766 0.0902 0.0995 0.1505 0.1558 0.1631 0.1721 0.1879 0.2254 0.2898 0.3240 0.3742 0.4177 0.4237 0.4447 0.4801 0.5528 0.6505 0.6561 0.6693 0.7286 0.8032 0.8482 0.8851 0.9205 0.9981 1.1005 1.2005 1.2198 1.2501 1.2901 1.3703 1.4703
0.7389 0.7142 0.6730 0.6485 0.6324 0.5635 0.5562 0.5482 0.5394 0.5233 0.4881 0.4397 0.4206 0.3911 0.3710 0.3683 0.3596 0.3468 0.3214 0.2944 0.2954 0.2915 0.2795 0.2652 0.2592 0.2549 0.2495 0.2410 0.2318 0.2255 0.2238 0.2226 0.2202 0.2191 0.5446
0.9777 0.9749 0.9713 0.9680 0.9643 0.9579 0.9563 0.9537 0.9525 0.9459 0.9398 0.9267 0.9180 0.9067 0.8800 0.8841 0.8735 0.8589 0.8252 0.7617 0.7511 0.7396 0.6908 0.6621 0.6309 0.5887 0.5869 0.5594 0.5459 0.5406 0.5394 0.5348 0.5333 0.5300 0.3438
0.0519 0.0633 0.0860 0.1019 0.1134 0.1748 0.1826 0.1916 0.2018 0.2218 0.2718 0.3586 0.3999 0.4738 0.5329 0.5415 0.5698 0.6153 0.7181 0.8512 0.8456 0.8671 0.9377 1.0322 1.0758 1.1085 1.1511 1.2235 1.3091 1.3732 1.3912 1.4035 1.4294 1.4419
0.8954 0.8781 0.8544 0.8443 0.8338 0.8060 0.8002 0.7950 0.7938 0.7836 0.7585 0.7217 0.7135 0.6852 0.6738 0.6698 0.6625 0.6548 0.6312 0.6091 0.6134 0.6058 0.5970 0.5826 0.5781 0.5741 0.5664 0.5580 0.5529 0.5465 0.5435 0.5442 0.5403 0.5373
0.7389 0.7142 0.6730 0.6485 0.6324 0.5635 0.5562 0.5482 0.5394 0.5233 0.4881 0.4397 0.4206 0.3911 0.3710 0.3683 0.3596 0.3468 0.3214 0.2944 0.2954 0.2915 0.2795 0.2652 0.2592 0.2549 0.2495 0.2410 0.2318 0.2255 0.2238 0.2226 0.2202 0.2191
0.0475 0.0570 0.0757 0.0889 0.0981 0.1471 0.1528 0.1597 0.1682 0.1838 0.2194 0.2793 0.3108 0.3581 0.4080 0.4103 0.4322 0.4691 0.5492 0.6807 0.6907 0.7103 0.8103 0.9083 0.9858 1.0811 1.1108 1.2205 1.3257 1.3881 1.4018 1.4280 1.4480 1.4619
0.9777 0.9749 0.9713 0.9680 0.9643 0.9579 0.9563 0.9537 0.9525 0.9459 0.9398 0.9267 0.9180 0.9067 0.8800 0.8841 0.8735 0.8589 0.8252 0.7617 0.7511 0.7396 0.6908 0.6621 0.6309 0.5887 0.5869 0.5594 0.5459 0.5406 0.5394 0.5348 0.5333 0.5300
0.8886 0.8846 0.8780 0.8736 0.8706 0.8532 0.8511 0.8483 0.8449 0.8385 0.8227 0.7937 0.7775 0.7526 0.7259 0.7247 0.7129 0.6932 0.6513 0.5870 0.5824 0.5735 0.5307 0.4931 0.4663 0.4370 0.4286 0.4005 0.3776 0.3658 0.3634 0.3589 0.3556 0.3534
0.0475 0.0571 0.0760 0.0898 0.0991 0.1499 0.1571 0.1638 0.1729 0.1890 0.2281 0.2930 0.3288 0.3800 0.4288 0.4350 0.4582 0.5067 0.5888 0.7326 0.7462 0.7600 0.8550 0.9926 1.0926 1.1323 1.1776 1.3052 1.5030 1.6450 1.6811 1.7381 1.7820 1.8108
0.9775 0.9735 0.9668 0.9582 0.9545 0.9400 0.9300 0.9298 0.9266 0.9199 0.9038 0.8831 0.8677 0.8544 0.8373 0.8339 0.8239 0.7952 0.7697 0.7077 0.6952 0.6912 0.6547 0.6059 0.5693 0.5620 0.5536 0.5231 0.4815 0.4562 0.4498 0.4394 0.4334 0.4279
0.8579 0.8494 0.8345 0.8245 0.8179 0.7829 0.7779 0.7733 0.7670 0.7558 0.7279 0.6804 0.6538 0.6156 0.5794 0.5749 0.5579 0.5228 0.4656 0.3738 0.3658 0.3577 0.3055 0.2399 0.1995 0.1850 0.1695 0.1315 0.0869 0.0636 0.0586 0.0514 0.0465 0.0434
PrEt3 NI
BuEt3 NI
d1 and D are the density and dielectric constant of the pure solvent, respectively. N0 , εr and k are Avogadro number, relative permittivity (static dielectric constant) of the solvent and the Boltzmann constant, respectively. The value of Aφ is 0.3915 (kg/mol)1/2 . I is ionic strength on a molality scale. 4. Results and discussion Fig. 2 shows that the osmotic coefficients for MeEt3 NI, PrEt3 NI, BuEt3 NI, BuEt3 NBr and Bu2 Et2 NBr are larger that calculated by DHLL. However, for m1/2 > 1, the osmotic coefficients of these TAAI seem to be lower than the DHLL results. This is especially remarkable for BuEt3 NI and PrEt3 NI. The values for the osmotic coefficients vary as follows: MeEt3 NI > PrEt3 NI > BuEt3 NI. In comparison with the osmotic coefficients of the TAAI, the osmotic coefficients of Bu2 Et2 NBr and BuEt3 NBr resemble the behavior of strong 1:1 electrolytes. Fig. 3 shows the osmotic coefficients for Et4 NI, PenEt3 NI, HexEt3 NI and HepEt3 NI salts. The occurrence of extensive ion–pair formation is suggested by the fact that
Fig. 2. Concentration dependence of the osmotic coefficients for aqueous solutions of TAAI salts at 298.15 K compared with DHLL. Osmotic coefficient of BuEt3 NBr and Bu2 Et2 NBr are from literature [5]. Experimental data: () MeEt3 NI; () PrEt3 NI; (♦) BuEt3 NI; (–) DHLL; () BuEt3 NBr; (*) Bu2 Et2 NBr.
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Table 2 Activity and osmotic coefficients of aqueous solutions of PenEt3 NI to HepEt3 NI at 298.15 K NaCl
PenEt3 NI
HexEt3 NI
m (mol/kg)
m (mol/kg)
φ
γ±
m (mol/kg)
φ
γ±
m (mol/kg)
φ
γ±
0.0531 0.0690 0.0775 0.0891 0.0933 0.1018 0.1250 0.1381 0.1389 0.1408 0.1879 0.2030 0.2157 0.2385 0.2697 0.2805 0.2920 0.2991 0.3001 0.3328 0.3678 0.3845 0.3957 0.4126 0.4573 0.4953 0.5295 0.5804 0.6015 0.6264 0.6875 0.7431 0.7659 0.7808 0.8105 0.8296
0.0539 0.0703 0.0798 0.0922 0.0976 0.1065 0.1320 0.1471 0.1482 0.1501 0.2076 0.2284 0.2454 0.2742 0.3148 0.3295 0.3456 0.3579 0.3614 0.4108 0.4718 0.4980 0.5140 0.5434 0.6238 0.6938 0.7814 0.9005 0.9578 1.0150 1.1988 1.3585 1.4650 1.5487 1.6875
0.9283 0.9203 0.9092 0.9030 0.8923 0.8910 0.8799 0.8712 0.8694 0.8697 0.8364 0.8208 0.8113 0.8020 0.7891 0.7841 0.7781 0.7696 0.7646 0.7457 0.7176 0.7107 0.7087 0.6991 0.6797 0.6580 0.6250 0.5952 0.5802 0.5705 0.5312 0.5076 0.4855 0.4684 0.4467
0.7805 0.7542 0.7407 0.7245 0.7178 0.7073 0.6797 0.6650 0.6639 0.6621 0.6135 0.5980 0.5860 0.5668 0.5420 0.5335 0.5246 0.5179 0.5161 0.4911 0.4633 0.4522 0.4456 0.4341 0.4051 0.3826 0.3574 0.3278 0.3152 0.3035 0.2713 0.2488 0.2361 0.2273 0.2147
0.0542 0.0712 0.0797 0.0926 0.0971 0.1070 0.1332 0.1502 0.1517 0.1530 0.2121 0.2320 0.2494 0.2836 0.3308 0.3473 0.3651 0.3804 0.3878 0.4518 0.5224 0.5588 0.5839 0.6237 0.7289 0.8228 0.9203 1.1014 1.1706 1.2748 1.5119 1.8404 1.9404 2.0403 2.2404 2.3440
0.9221 0.9085 0.9098 0.8987 0.8963 0.8864 0.8718 0.8528 0.8493 0.8537 0.8185 0.8080 0.7983 0.7755 0.7511 0.7439 0.7366 0.7240 0.7126 0.6781 0.6481 0.6334 0.6239 0.6091 0.5817 0.5549 0.5307 0.4866 0.4748 0.4543 0.4212 0.3747 0.3665 0.3556 0.3365 0.3294
0.7723 0.7434 0.7304 0.7123 0.7062 0.6936 0.6631 0.6452 0.6437 0.6424 0.5887 0.5729 0.5597 0.5355 0.5051 0.4952 0.4850 0.4764 0.4724 0.4399 0.4082 0.3933 0.3836 0.3690 0.3345 0.3081 0.2842 0.2478 0.2361 0.2204 0.1919 0.1648 0.1588 0.1536 0.1454 0.1423
0.0575 0.0770 0.0876 0.1022 0.1082 0.1205 0.1533 0.1727 0.1750 0.1805 0.2592 0.2917 0.3136 0.3583 0.4282 0.4477 0.4858 0.5029 0.5121 0.5894 0.6973 0.7870 0.8401 0.8900 1.1047 1.3415 1.5041 1.8032 1.9205 2.2050 2.5154 2.7900 2.9600 3.0000
0.8694 0.8400 0.8287 0.8146 0.8048 0.7873 0.7574 0.7421 0.7363 0.7234 0.6699 0.6425 0.6348 0.6137 0.5802 0.5770 0.5535 0.5476 0.5396 0.5198 0.4856 0.4497 0.4336 0.4268 0.3838 0.3403 0.3247 0.2972 0.2894 0.2626 0.2531 0.2471 0.2403 0.2418
0.6809 0.6340 0.6120 0.5845 0.5741 0.5540 0.5078 0.4845 0.4818 0.4756 0.4033 0.3798 0.3655 0.3394 0.3055 0.2972 0.2822 0.2760 0.2727 0.2482 0.2204 0.2015 0.1917 0.1833 0.1541 0.1309 0.1187 0.1013 0.0959 0.0851 0.0762 0.0702 0.0672 0.0666
the osmotic coefficients are lower than calculated from DHLL. The osmotic coefficient data varies as follows: Et4 NI > PenEt3 NI > HexEt3 NI > HepEt3 NI. The following relation can be deduced from Figs. 2 and 3 for solute–solvent interactions: MeEt3 NI > PrEt3 NI > BuEt3 NI > Et4 NI > PenEt3 NI > HexEt3 NI > HepEt3 NI Fig. 4 shows the activity coefficient data for MeEt3 NI, PrEt3 NI and BuEt3 NI. These graphs are lower than calculated
HepEt3 NI
by DHLL. The explanation may be that cations interact with water and disrupt its structure. In Fig. 5 the data from Et4 NI, PenEt3 NI, HexEt3 NI and HepEt3 NI form a group with negative deviations from DHLL. The negative deviations from the DHLL were confirmed for solutions of Et4 NI [13]. From our data it is not possible to confirm micelle formation in this range of concentration for these TAAI. Nevertheless, it is possible that the micelle formation may be present at higher concentrations. The micelle formation of TAAX salts in aqueous solution in relation with semiclathrates crystal structure has been used to explain the behavior of the TAA ions with long alkyl chains
Table 3 Values obtained for Pitzer ion-parameters for TAAI salts at 298.15 K Parameter
MeEt3 NI
Et4 NI
PrEt3 NI
BuEt3 NI
PenEt3 NI
HexEt3 NI
HepEt3 NI
β0 (kg/mol) β1 (kg/mol) σ(φ)a
−0.2688 0.9912 0.003
−0.1023 −0.9550 0.004
−0.2708 0.7935 0.007
−0.2474 0.4216 0.008
−0.1481 −0.8471 0.005
−0.1325 −1.2030 0.005
−0.1032 −1.9460 0.007
a
σ(φ) =
i
(φexp −φcal )2i n
(with n = number of experimental osmotic coefficient used data in the regression for each salt).
L.H. Blanco et al. / Fluid Phase Equilibria 249 (2006) 147–152
Fig. 3. Concentration dependence of the osmotic coefficients for aqueous solutions of TAAI salts at 298.15 K compared with DHLL. Osmotic coefficient of Et4 NBr is from literature [13]. Experimental data: () Et4 NI; () PenEt3 NI; (♦) HexEt3 NI; (+) HepEt3 NI; (–) DHLL; () Et4 NBr.
151
Fig. 6. Relation of β0 to β1 for TTAX salts (X: I and Br) at 298.15 K. Experimental data: (1) HexEt3 NI; (2) BuEt3 NI; (3) PrEt3 NI; (4) MeEt3 NI; (5) Et4 NBr, 1964; (6) Et4 NI, 1964; (7) BuEt3 NBr, 2005; (8) Bu2 Et2 NBr, 2005; (9) PenEt3 NI; (10) HepEt3 NI.
surrounding water molecules [14–18]. Also the hydration numbers and the formation of polyhydrates have been used as alternative arguments [19,20]. It is clear that the behavior of the osmotic and activity coefficient data of TAAX solutions have been proposed several times as a model to describe the changes of water structure [21–27]. Dimers and micelles were considered to be formed in Et4 NBr solutions above 4.0 m [28]. Table 3 shows the calculated values of β0 and β1 . The osmotic coefficient may be reproduced with an average error of 0.5% (standard deviation of σ(φ) = 0.004) for TAAI aqueous solutions in the range (0.04–2.33) mol/kg at 298.15 K. Fig. 6 shows the relationship between the two parameters β0 and β1 for the series MeEt3 NI to HepEt3 NI. The corresponding data for Et4 NI, Et4 NBr, Bu2 Et2 NBr and BuEt3 Br do not show any relationship.
Fig. 4. Logarithm of mean activity coefficient for TAAI salts vs. square root of molality at 298.15 K. Experimental data: () MeEt3 NI; () PrEt3 NI; () BuEt3 NI; (–) DHLL.
Fig. 5. Logarithm of mean activity coefficient for TAAI salts vs. square root of molality at 298.15 K. These TAAI present negative deviations compared with DHLL. Experimental data: () Et4 NI; (♦) PenEt3 NI; () HexEt3 NI; () HepEt3 NI; (–) DHLL.
5. Conclusions Experimental osmotic coefficient data were obtained for aqueous solutions of the series MeEt3 NI to HepEt3 NI by isopiestic method at 298.15 K. From the comparison of the data with the DHLL, the behavior of the osmotic and activity coefficients has been interpreted qualitatively in terms of solute–solvent interactions and ion–pair formation. From our data it was not possible to confirm micelle formation in this given range of concentration for these TAAI. The experimental osmotic coefficient data was satisfactorily correlated using the Pitzer ion interaction model and a relation between the two parameters β0 and β1 for the series MeEt3 NI to HepEt3 NI was found. Finally, it was observed that both the halide and concentration of the TAAX have an important effect on ion–pair formation, and the effect of the cation may be reversed at higher concentration. List of symbols a activity m molality (mol/kg) M molecular weight (g mol−1 ) T absolute temperature (K)
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Greek letters φ osmotic coefficient γ activity coefficient ν stoichiometric coefficient Subscripts o standard state r reference s solute Acknowledgements The authors are grateful to Dr. D. Archer, N.I.S.T., for the use of his computers programs. To Colciencias for its financial support, and to Universidad de Pamplona and Universidad Nacional de Colombia for support. References [1] [2] [3] [4]
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