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Thermochemical behaviour of crown ethers in the mixtures of water with organic solvents. Part II. Enthalpy of solution of benzo-15-crown-5 ether in the mixtures of water and DMSO, DMF, DMA and HMPA at 298.15 K
I?YE%&JLAR LIQUIDS
ELSEVIER
Journal of Molecular Liquids 81 (1999) 261-268
Thermochemical behaviour of crown ethers in the mixtures of water with organic solvents. Part II. Enthalpy of solution of benzo-1%crown-5 ether in the mixtures of water and DMSO, DMF, DMA and HMPA at 298.15 K* Maigorzata Joiwiak Department of Physical Chemistry, University of Eodi, Pomorska 165, 90-236 Lodi, Poland Received 4 March 1999; accepted 31 May 1999 Abstract Enthalpies of solution of benzo-IS-crown-5 ether (Bl X5) in the mixtures of water with dimethylsulphoxide (DMSO), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA) and hexamethylphosphortriamide (HMPA) have been measured at 398.15 K. The enthalpies of solution of B 15C5 in these mixtures exhibit a maximum at xw = 0.5 for DMSO, DMF, DMA and xw z 0.8 for HMPA. The results were discussed on the basis of the ,,cage model” of hydrophobic hydration.
0 1999 Elsevier Science B.V. All rights reserved.
Introduction The present paper is a continuation of our earlier studies on crown ethers in water-organic solvent mixtures [ 1, 31. Crown ethers are known to complex cations in a selective way [3, 41. The formation of crown ether complexes depends, among others, on the properties of solvent in which the complex formation process takes place, thus on the solvation process [2]. These studies were intended to examine the effect of the properties of organic and aqueous-organic solvents on the solvation of crown ethers. Due to the fact that crown ethers exhibit hydrophobic properties [ 11, it seemed particularly interesting to examine the process of hydrophobic hydration, The previous paper related to the enthalpy of solution of 15-crown-5 ether (15C5) in the mixtures of water with DMSO, DMF, DMA and HMPA. The results obtained were analysed by means of the ,,cage model” of hydrophobic hydration [5], as proposed by Mastroianni, Pikal and Lindenbaum, and the enthalpic effect of 15C5 hydrophobic hydration in water, Hb(W), was calculated. Choosing benzo-15-crown-5 ether (BlX5) to continue these studies, it was expected that the obtained results would allow to evaluate the effect of the benzene ring on the interactions between crown ether and the mixed solvent components. The present paper contains the results of calorimetric investigations of B15C5 solution enthalpy in the mixtures of water with aprotic solvents such as DMSO, DMF’, DMA and HMPA.
* Part I ref. [ 11. 0167-7322/99/$ see front matter 0 1999 Elsevier Science B.V. All rights reserved. PI1 SO167-7322(99) 00097-5
262
Experimental Benzo-l5-crown-5 ether was synthesised Chemistry of the University of Lodi (m.p. Dimethylsulphoxide (Fluka >99%) was dried, [7]. under reduced pressure CaHz
and purified at the Department of Organic 351-353 K; literature data: 352-352.5 K). using molecular sieves 5A, and distilled over NJ-dimethylformamide (Aldrich 99%)
N,N-dimethylacetamide (Aldrich 99%) and ,,purum” hexamethylphosphortriamide (Fluka) were purified and dried according to the procedures described in the literature [S-IO], Calorimetric measurements were carried out at (298.15 + 0.01) K, using an jsoperibol” type calorimeter, as described in the literature [ll]. The uncertainties in the measured enthalpies did not exceed +0.5% of the measured value. The enthalpies of solution of Bl5C5 were measured within the range 5 - 100 mol% of organic co-solvent. Six to eight independent measurements were performed in each investigated mixture, and the final concentration of the B15C5 solutions was below 0.009 mol kg-l. The solubility of B 15C5 in pure water and in the mixtures with very high water contents above 95 mol% was so low that precise calorimetric measurements were impossible. Results and discussion In the mixtures of water with DMSO, DMF, DMA crown ether concentration range, no dependence of the Bl5C5 concentration was observed (outside error limits), dissolution enthalpy in the mixtures of water and the same
and HMPA within the investigated B15C5 dissolution enthalpy on the similarly as in the case of the 15C5 solvents [ 11. Therefore, the standard
enthalpy of solution of B15C5, AsolHo, was calculated as an average value of the measured enthalpies of solution of B15C5 in a mixed solvent with the given composition. The results of the standard enthalpy of solution versus the mixed solvent composition are given in Table 1 and shown in Fig. 1. As can be seen, the relationships AsolHo(B15C5) = f(x,)
show similar
courses in all the investigated mixed solvents. Within the range with a low water content, i.e. 0 5 xw < 0.4 for DMSO, DMF and DMA and 0 I xw < 0.7 for HMPA, the enthalpy of solution versus the mixed solvent composition is characterised almost by a linear course. Beyond a soft maximum at x, = 0.5 for DMSO, DMF, DMA and xw = 0.8 for I-IMPA, up to pure water, a sharp decrease of the enthalpy of solution of B15C5 in all the systems under investigation is observed. This characteristic course of curves within the water rich range seems to be caused by the hydrophobic hydration of crown ether [12]. Within the organic solvent rich range, the endothermic effect of B 15C5 dissolution in the systems under investigation decreases in the following sequence: DMSO-water z DMA-water > DMF-water > HMPA-water. The same shape of the dissolution enthalpy curves and the same sequence of the AsolHo effects was observed in the case of 15C5 dissolved in the same mixed solvents [ 11. It is worth noticing that within the organic solvent rich range, i.e. 0 5 xw I 0.55, the curves AsolHo= f(xw) for B15C5 and 15C5 in the given mixed solvent are almost parallel, which
263
means that the difference AsoJHo(B15C5) - AsolHo(l 5C5) depends only to a slight extent on the mixed solvent composition. Table 1 Standard enthalpy of solution of B15C5 (kJ mol-l) DMF, DMA and HMPA at 298.15 K
in the mixtures of water with DMSO,
AsolHO XW
DMF
DMSO
1.00
-2.oa
0.95 0.90 0.80 0.70 0.60 0.50 0.40 0.30 0.20 0.10 0.00
6.79 12.92 21.65 27.15 30.63 31.55 3 1.05 30.43 29.81 29.13 28.64
DMA
-2.oa + 0.06 If:0.04 + 0.09 + 0.07 li: 0.10 +0.07 + 0.09 * 0.05 + 0.05 k 0.09 + 0.06
6.36 10.89 19.48 25.73 29.85 30.93 30.65 29.63 28.68 27.82 27.08
f 0.03 & 0.04 f 0.07 + 0.05 + 0.10 f 0.10 I!Z0.10 i 0.03 + 0.10 k 0.10 f 0.08
-2.oa 7.75 I!I0.07 13.33 + 0.10 22.78 f 0.10 29.07 + 0.06 31.72 + 0.09 31.77 f 0.05 31.67 ?I 0.05 30.79 + 0.06 30.06 k 0.10 28.73 f 0.08 27.71 f 0.09
HMPA -2.oa 19.10 25.37 27.47 27.33 26.91 26.53 26.11 25.61 25.08 24.76 24.52
* 0.07 + 0.05 If:0.10 f 0.09 + 0.07 f 0.10 + 0.08 C 0.05 f 0.10 + 0.09 * 0.07
a Value estimated in this work (see the text) As was mentioned above, the curves of the B15C5 dissolution enthalpies have a shape which is characteristic of dissolution of hydrophobic substances or substances having hydrophobic fragments in their molecules [12]. For this reason it was reasonable to use the ,,cage model” of hydrophobic hydration proposed by Mastroianni, Pikal and Lindenbaum [ 1, 51 to analyse the obtained results. The ,,cage model” describes the hydrophobic hydration as the formation of a clathrate-like cage of water molecules around the solute. Based on the assumptions of the ,,cage model” [ 1, 51, the enthalpy of solution of the hydrophobic substance in the mixed water-organic solvent (W + Y) may be described by the equation (1) [ 12-151 containing only two adjustable parameters: Hb(W) - the enthalpic effect of hydrophobic hydration of a non-polar substance in water and ,,n” associated with the number of water molecules participating in the hydrophobic cage formation [5].
+(l-xw) AsolH”V> +(xt: - xw> H’JW) As&%’+V = xw AsolH”(W where: AsolHO(W + Y) - the standard enthalpy of solution in the mixed solvent, Aso$IO(W) - the standard enthalpy of solution in water,
(1)
264
AsolHO(Y) - the standard enthalpy of solution in a pure organic solvent, Xw
molar fraction of water in the mixed solvent,
-
xk, - the probability that ,,n” water molecules will be at the same time around the hydrophobic site in the molecule of the hydrophobic substance, J%(W) - the enthalpic effect of hydrophobic hydration in pure water.
0
-10 L 0.0
a
I 0.2
I 0.4
I 0.6
I 0.8
-1.0
Fig 1. Standard enthalpies of solution of B15C5 in the mixtures of water with DMSO (V), DMF (m)_ DMA (0) and HMPA (A) at 298.15 K. Schematic representation of the method of linear extrapolation to obtain Hb(W) in the mixtures DMA-water (see the text)
265
1. Application qf the ,, cage model” in the Jystems: BISCS-water-DMSO, DMF and BISCS-water-DMA
B15C.Swater-
The assumptions of the ,,cage model” are fulfilled in the mixtures of water and DMSO and DMF [ 12, 151 and to a lesser extent in the water-DMA mixture due to a weak hydrophobic hydration of the organic component of the latter mixture [ 15, 161. Thus, the use of this model to describe the experimental data obtained in this study seems to be justified in relation to the above mentioned mixtures. In order to calculate the l%(W) and ,,n” parameters in equation (I), it is obvious to know the enthalpy of solution of B15C5 in pure water, AsolHo(W). As was already mentioned, B 1SC5 is hardly soluble in water and it was impossible to determine experimentally this value. Therefore, it was necessary to change the form of equation (1). According to Fig. 1, the enthalpic effect of the hydrophobic hydration, I%(W), is equal to the difference: Hb(W) = AsolHo(W) - AsolHo(W*)
(2)
where: AsolHO(W*) is the enthalpy of solution of the solute in water when hydrophobic effects is absent. Due to the linear course of the enthalpy of solution of B 15C5 within the range of low water content, the value AsolHo(W*) can be found by extrapolation (the linear regression) as shown in Fig. 1. Substituting equation (2) in equation (I), we obtain expression (3): hoPoW
+
Y) = xw AsolH”OV +U-x,)
+( xt:- xw)(AsolH”V’l
A,,lH”W+ - As,lH”V*N
(3)
This expression contains two unknown values: AsolHo(W) and ,,n” which were calculated by the method of non-linear regression as adjustable parameters. After applying this procedure to the systems: B 1SCS-water-DMSO, B 1SCS-water-DMF and B 1SCS-water-DMA the values of AsolHo(W) equal to (-1.89 f 0.45) kJ mol-I,
(-1.89 + 0.65) kJ mol-1 and (-1.47 f 0.46)
kJ mol-1 respectively were obtained. Then, substituting the average value of A,,lHO(W) = -1.75 + 0.22 kJ mol-1 and the value of I%(W) calculated from equation (2) into equation (l), the parameter ,,n” was recalculated for each of the examined systems. The obtained data are given in Table 2 together with analogous values for 15C5 in the same mixed solvents [ 11. The values of ,,n” for B15C5 in the mixtures DMSO-water, DMF-water and DMA-water are similar to each other, their average value amounts to 5.01 + 0.41, The average value of ,,n” for 15C5 in the same mixtures is equal to 4.33 + 0.31 [l]. Thus, these value are almost the same within the error limits. So, it can be assumed that the presence of a benzene ring has no effect on the value of ,,n” in the systems under investigation. It is worth noticing that in the systems BlSCS-water-DMSO, B 1SCS-water-DMF and B 1SCS-water-DMA, the number ,,n” corresponds to the number of -CH2CH2- groups in the crown ether ring.
266 The values of the enthalpic effect of hydrophobic hydration of B 1X5, Hb(W), are negative and similar to each other in the mixtures of water and DMSO, DMF and DMA. The comparison of the Hb(W) values for Bl5C5 in the examined mixtures with those for 15C5 in the same mixtures indicates a decrease in the exothermic effect of the enthalpy of hydrophobic hydration of BlSCS in relation to 15C5 (Table 2). This seems to be caused by the presence of the benzene ring in the B15C5 molecule. The calculated differences in the enthalpic effect of hydrophobic hydration in water for B 15C5 and 15C5: GHb(W) = Hb(W)(B 15C5) - Hb(W)( 15C5)
(4)
in the mixtures of water and DMSO, DMF and DMA are almost the same within the error limits and their average value amounts to (12.31 i 0.52) kJ mol-1. Thus, there is no influence of the mentioned organic solvents on the value of this effect. 2. Analysis of the properties of the system: B IX%water-HA&PA HMPA is a strongly hydrophobic solvent [ 16, 171 and therefore the mixture HME’A does not satisfy the ,,cage model” assumption that the number of water a mixed solvent which are capable of hydrating the molecules of a dissolved proportional to xw. This is confirmed by the non-linear course, (versus
of water and molecules in substance is the mixture
composition), of the enthalpies of solution of NaI, NaCl, KI and KC1 in the HMPA-water mixtures [ 18, 193. However, due to the fact that the curve of the enthalpy of solution of B 15C5 in HMPA-water shows a similar shape to those obtained in the remaining systems discussed earlier, it was intended to describe the curve AsolHo(B 15C5) = f(xw) using the same equation, A similar analysis was performed previously in the HMPA-water-15C5 system [ 11. and The parameters ,,n” and Hb(W) obtained for the system HMFA-water-B15C5 calculated in the same way as for the mixtures of water and DMSO, DMF and DMA containing B15C5 are given in Table 2. As can be seen, the value of ,,n” for the system HMPA-water-B15C5 is much higher than ,,n” obtained for the remaining systems. It is worth noticing that the HMPA molecule itself is strongly hydrophobic and in this case the ,,n” value can be connected not only with the crown ether molecule but also with HMPA one. However, it can be said that ,,n” characterises, in some way, the number of water molecules participating in the hydrophobic hydration in the system. The value of Hb(W) concerning the system BlSCS-HMPA-water amounts to -30.34 kJ mol-l. As can be noticed (Table 2) this value is less negative than that calculated from the data related to the enthalpy of solution of B15C5 in the mixtures of water with DMSO, DMF and DMA. The change in this value seems to be associated with the competitive interaction of HMPA and B 1SC5 molecules with water. The molecules of B 15C5 interact first of all with those water molecules which are not directly associated with the HMFA molecules. Therefore, the crown ether molecules, being surrounded by a lower number of water molecules, are not hydrated to such an extent as in the mixed solvent containing DMSO, DMF, DMA and water. This results in a decreased effect of the hydrophobic hydration of crown ether. The effect of benzene ring on the value of Hb(W) was calculated from the data of the enthalpy of solution of B 1X5 in the HMPA-water mixture as in the other mixed solvents,
267 Table 2 The coefficient ,,n” and the enthalpic contribution due to the hydrophobic hydration of B 15C5 and 15C5 in water l%(W) (kJ mol-*) at 298.15 K B15C5
15c5
Solvent
n
AsolH’(W*)
I-NW)
na
DMSO
5.27 + 0.10
34.59 +_0.10
-36.34
4.39 + 0.25
48.82
DMF
4.53 & 0.12
35.93 * 0.26
-37.68
4.00 * 0.13
49.41
DMA
5.22 + 0.10
37.78 + 0.46
-39.53
4.61 +0.17
-52.25
22.93 + 0.21
28.59 + 0.09
-30.34
12.01 f 0.19
-42.95
HMPA
Hb(W)a
aRef. [l] The difference GHb(W) (see equation 4) due to the presence of benzene ring amounts to 12.61 kJ mol-l and is similar to that obtained in the remaining mixtures examined in this study. This allows one to draw a conclusion that the properties of organic solvent do not affect to a decisive extent the energetic contribution of the benzene ring in the hydrophobic hydration of crown ether in water. Acknowledgement The author would like to thank Professor H. Piekarski for his helpful suggestions and discussions, This study was partially supported by Lodi University grant no. 505/491 and 5051641. References 1. M. Jbiwiak, H. Piekarski, J. Mol. Liqiud., in press. 2. H. Piekarski, M. Joiwiak, J. Therm. Anal. 48 (1997) 1283-1291. 3. Y. Takeda, K. Katsuta, Y. Inoue and T. Hakushi, Bull. Chem. Sot. Jpn., 61 (1988) 627 4. Y. Takeda and T. Kumazawa, Bull. Chem. Sot. Jpn., 61 (1988) 655. 5. M.J. Mastroianni, M.J. Pikal and S. Lindenbaum, J. Phys. Chem., 76 (1972) 3050. 6. C. J. Pedersen, J. Am. Chem. Sot., 89 (1967) 7017. 7. R.J. Ouellette, Can. J. Chem., 43 (1965) 707. 8. SC. Chan and J.P. Valleau, Can. J. Chem., 46 (1968) 853. 9. CD. Schmulbach and R.S. Drago, J. Amer. Chem. Sot., 82 (1960) 4484. 10. S. Taniewska-Osidska and M. Joiwiak, J. Chem. Thermodynamics, 18 (1986) 339. 11, H. Piekarski and D. Waliszewski, J. Thermal. Anal., 47 (1996) 1639. 12. C. de Visser and G. Somsen, J. Phys. Chem., 78 (1974) 1719. 13. W.J.M. Heuvesland and G. Somsen, J. Chem. Thermodyn., 8 (1976) 873. 14. C. de Visser, W.J.M. Heuvesland and G. Somsen, J. Solution Chem., 4 (1975) 3 11. 15. W.J.M. Heuvesland, C. de Visser and G. Somsen, J. Phys. Chem., 82 (1978) 29. 16. A.M. Zaichikov and Yu.G. Bushuev, Zh. Fiz. Khim., 69 (1995) 1942.
268 17. A.L. Zaicev, E.A. Nogovicyn, A.M. Zaichikov, N.I. Zheleznyak and G.A. Krestov, Zh. Fiz. Khim., 65 (1991) 906. 18. S. Taniewska-Osinska and M. Jbiwiak, J. Chem. Thermodynamics, 18 (1986) 339. 19. S. Taniewska-Osinska and M. Jhiwiak, J. Chem. Sot., Faraday Trans. 1, 84 (1988) 1-077.
ELSEVIER
Journal of Molecular Liquids 81 (1999) 261-268
Thermochemical behaviour of crown ethers in the mixtures of water with organic solvents. Part II. Enthalpy of solution of benzo-1%crown-5 ether in the mixtures of water and DMSO, DMF, DMA and HMPA at 298.15 K* Maigorzata Joiwiak Department of Physical Chemistry, University of Eodi, Pomorska 165, 90-236 Lodi, Poland Received 4 March 1999; accepted 31 May 1999 Abstract Enthalpies of solution of benzo-IS-crown-5 ether (Bl X5) in the mixtures of water with dimethylsulphoxide (DMSO), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA) and hexamethylphosphortriamide (HMPA) have been measured at 398.15 K. The enthalpies of solution of B 15C5 in these mixtures exhibit a maximum at xw = 0.5 for DMSO, DMF, DMA and xw z 0.8 for HMPA. The results were discussed on the basis of the ,,cage model” of hydrophobic hydration.
0 1999 Elsevier Science B.V. All rights reserved.
Introduction The present paper is a continuation of our earlier studies on crown ethers in water-organic solvent mixtures [ 1, 31. Crown ethers are known to complex cations in a selective way [3, 41. The formation of crown ether complexes depends, among others, on the properties of solvent in which the complex formation process takes place, thus on the solvation process [2]. These studies were intended to examine the effect of the properties of organic and aqueous-organic solvents on the solvation of crown ethers. Due to the fact that crown ethers exhibit hydrophobic properties [ 11, it seemed particularly interesting to examine the process of hydrophobic hydration, The previous paper related to the enthalpy of solution of 15-crown-5 ether (15C5) in the mixtures of water with DMSO, DMF, DMA and HMPA. The results obtained were analysed by means of the ,,cage model” of hydrophobic hydration [5], as proposed by Mastroianni, Pikal and Lindenbaum, and the enthalpic effect of 15C5 hydrophobic hydration in water, Hb(W), was calculated. Choosing benzo-15-crown-5 ether (BlX5) to continue these studies, it was expected that the obtained results would allow to evaluate the effect of the benzene ring on the interactions between crown ether and the mixed solvent components. The present paper contains the results of calorimetric investigations of B15C5 solution enthalpy in the mixtures of water with aprotic solvents such as DMSO, DMF’, DMA and HMPA.
* Part I ref. [ 11. 0167-7322/99/$ see front matter 0 1999 Elsevier Science B.V. All rights reserved. PI1 SO167-7322(99) 00097-5
262
Experimental Benzo-l5-crown-5 ether was synthesised Chemistry of the University of Lodi (m.p. Dimethylsulphoxide (Fluka >99%) was dried, [7]. under reduced pressure CaHz
and purified at the Department of Organic 351-353 K; literature data: 352-352.5 K). using molecular sieves 5A, and distilled over NJ-dimethylformamide (Aldrich 99%)
N,N-dimethylacetamide (Aldrich 99%) and ,,purum” hexamethylphosphortriamide (Fluka) were purified and dried according to the procedures described in the literature [S-IO], Calorimetric measurements were carried out at (298.15 + 0.01) K, using an jsoperibol” type calorimeter, as described in the literature [ll]. The uncertainties in the measured enthalpies did not exceed +0.5% of the measured value. The enthalpies of solution of Bl5C5 were measured within the range 5 - 100 mol% of organic co-solvent. Six to eight independent measurements were performed in each investigated mixture, and the final concentration of the B15C5 solutions was below 0.009 mol kg-l. The solubility of B 15C5 in pure water and in the mixtures with very high water contents above 95 mol% was so low that precise calorimetric measurements were impossible. Results and discussion In the mixtures of water with DMSO, DMF, DMA crown ether concentration range, no dependence of the Bl5C5 concentration was observed (outside error limits), dissolution enthalpy in the mixtures of water and the same
and HMPA within the investigated B15C5 dissolution enthalpy on the similarly as in the case of the 15C5 solvents [ 11. Therefore, the standard
enthalpy of solution of B15C5, AsolHo, was calculated as an average value of the measured enthalpies of solution of B15C5 in a mixed solvent with the given composition. The results of the standard enthalpy of solution versus the mixed solvent composition are given in Table 1 and shown in Fig. 1. As can be seen, the relationships AsolHo(B15C5) = f(x,)
show similar
courses in all the investigated mixed solvents. Within the range with a low water content, i.e. 0 5 xw < 0.4 for DMSO, DMF and DMA and 0 I xw < 0.7 for HMPA, the enthalpy of solution versus the mixed solvent composition is characterised almost by a linear course. Beyond a soft maximum at x, = 0.5 for DMSO, DMF, DMA and xw = 0.8 for I-IMPA, up to pure water, a sharp decrease of the enthalpy of solution of B15C5 in all the systems under investigation is observed. This characteristic course of curves within the water rich range seems to be caused by the hydrophobic hydration of crown ether [12]. Within the organic solvent rich range, the endothermic effect of B 15C5 dissolution in the systems under investigation decreases in the following sequence: DMSO-water z DMA-water > DMF-water > HMPA-water. The same shape of the dissolution enthalpy curves and the same sequence of the AsolHo effects was observed in the case of 15C5 dissolved in the same mixed solvents [ 11. It is worth noticing that within the organic solvent rich range, i.e. 0 5 xw I 0.55, the curves AsolHo= f(xw) for B15C5 and 15C5 in the given mixed solvent are almost parallel, which
263
means that the difference AsoJHo(B15C5) - AsolHo(l 5C5) depends only to a slight extent on the mixed solvent composition. Table 1 Standard enthalpy of solution of B15C5 (kJ mol-l) DMF, DMA and HMPA at 298.15 K
in the mixtures of water with DMSO,
AsolHO XW
DMF
DMSO
1.00
-2.oa
0.95 0.90 0.80 0.70 0.60 0.50 0.40 0.30 0.20 0.10 0.00
6.79 12.92 21.65 27.15 30.63 31.55 3 1.05 30.43 29.81 29.13 28.64
DMA
-2.oa + 0.06 If:0.04 + 0.09 + 0.07 li: 0.10 +0.07 + 0.09 * 0.05 + 0.05 k 0.09 + 0.06
6.36 10.89 19.48 25.73 29.85 30.93 30.65 29.63 28.68 27.82 27.08
f 0.03 & 0.04 f 0.07 + 0.05 + 0.10 f 0.10 I!Z0.10 i 0.03 + 0.10 k 0.10 f 0.08
-2.oa 7.75 I!I0.07 13.33 + 0.10 22.78 f 0.10 29.07 + 0.06 31.72 + 0.09 31.77 f 0.05 31.67 ?I 0.05 30.79 + 0.06 30.06 k 0.10 28.73 f 0.08 27.71 f 0.09
HMPA -2.oa 19.10 25.37 27.47 27.33 26.91 26.53 26.11 25.61 25.08 24.76 24.52
* 0.07 + 0.05 If:0.10 f 0.09 + 0.07 f 0.10 + 0.08 C 0.05 f 0.10 + 0.09 * 0.07
a Value estimated in this work (see the text) As was mentioned above, the curves of the B15C5 dissolution enthalpies have a shape which is characteristic of dissolution of hydrophobic substances or substances having hydrophobic fragments in their molecules [12]. For this reason it was reasonable to use the ,,cage model” of hydrophobic hydration proposed by Mastroianni, Pikal and Lindenbaum [ 1, 51 to analyse the obtained results. The ,,cage model” describes the hydrophobic hydration as the formation of a clathrate-like cage of water molecules around the solute. Based on the assumptions of the ,,cage model” [ 1, 51, the enthalpy of solution of the hydrophobic substance in the mixed water-organic solvent (W + Y) may be described by the equation (1) [ 12-151 containing only two adjustable parameters: Hb(W) - the enthalpic effect of hydrophobic hydration of a non-polar substance in water and ,,n” associated with the number of water molecules participating in the hydrophobic cage formation [5].
+(l-xw) AsolH”V> +(xt: - xw> H’JW) As&%’+V = xw AsolH”(W where: AsolHO(W + Y) - the standard enthalpy of solution in the mixed solvent, Aso$IO(W) - the standard enthalpy of solution in water,
(1)
264
AsolHO(Y) - the standard enthalpy of solution in a pure organic solvent, Xw
molar fraction of water in the mixed solvent,
-
xk, - the probability that ,,n” water molecules will be at the same time around the hydrophobic site in the molecule of the hydrophobic substance, J%(W) - the enthalpic effect of hydrophobic hydration in pure water.
0
-10 L 0.0
a
I 0.2
I 0.4
I 0.6
I 0.8
-1.0
Fig 1. Standard enthalpies of solution of B15C5 in the mixtures of water with DMSO (V), DMF (m)_ DMA (0) and HMPA (A) at 298.15 K. Schematic representation of the method of linear extrapolation to obtain Hb(W) in the mixtures DMA-water (see the text)
265
1. Application qf the ,, cage model” in the Jystems: BISCS-water-DMSO, DMF and BISCS-water-DMA
B15C.Swater-
The assumptions of the ,,cage model” are fulfilled in the mixtures of water and DMSO and DMF [ 12, 151 and to a lesser extent in the water-DMA mixture due to a weak hydrophobic hydration of the organic component of the latter mixture [ 15, 161. Thus, the use of this model to describe the experimental data obtained in this study seems to be justified in relation to the above mentioned mixtures. In order to calculate the l%(W) and ,,n” parameters in equation (I), it is obvious to know the enthalpy of solution of B15C5 in pure water, AsolHo(W). As was already mentioned, B 1SC5 is hardly soluble in water and it was impossible to determine experimentally this value. Therefore, it was necessary to change the form of equation (1). According to Fig. 1, the enthalpic effect of the hydrophobic hydration, I%(W), is equal to the difference: Hb(W) = AsolHo(W) - AsolHo(W*)
(2)
where: AsolHO(W*) is the enthalpy of solution of the solute in water when hydrophobic effects is absent. Due to the linear course of the enthalpy of solution of B 15C5 within the range of low water content, the value AsolHo(W*) can be found by extrapolation (the linear regression) as shown in Fig. 1. Substituting equation (2) in equation (I), we obtain expression (3): hoPoW
+
Y) = xw AsolH”OV +U-x,)
+( xt:- xw)(AsolH”V’l
A,,lH”W+ - As,lH”V*N
(3)
This expression contains two unknown values: AsolHo(W) and ,,n” which were calculated by the method of non-linear regression as adjustable parameters. After applying this procedure to the systems: B 1SCS-water-DMSO, B 1SCS-water-DMF and B 1SCS-water-DMA the values of AsolHo(W) equal to (-1.89 f 0.45) kJ mol-I,
(-1.89 + 0.65) kJ mol-1 and (-1.47 f 0.46)
kJ mol-1 respectively were obtained. Then, substituting the average value of A,,lHO(W) = -1.75 + 0.22 kJ mol-1 and the value of I%(W) calculated from equation (2) into equation (l), the parameter ,,n” was recalculated for each of the examined systems. The obtained data are given in Table 2 together with analogous values for 15C5 in the same mixed solvents [ 11. The values of ,,n” for B15C5 in the mixtures DMSO-water, DMF-water and DMA-water are similar to each other, their average value amounts to 5.01 + 0.41, The average value of ,,n” for 15C5 in the same mixtures is equal to 4.33 + 0.31 [l]. Thus, these value are almost the same within the error limits. So, it can be assumed that the presence of a benzene ring has no effect on the value of ,,n” in the systems under investigation. It is worth noticing that in the systems BlSCS-water-DMSO, B 1SCS-water-DMF and B 1SCS-water-DMA, the number ,,n” corresponds to the number of -CH2CH2- groups in the crown ether ring.
266 The values of the enthalpic effect of hydrophobic hydration of B 1X5, Hb(W), are negative and similar to each other in the mixtures of water and DMSO, DMF and DMA. The comparison of the Hb(W) values for Bl5C5 in the examined mixtures with those for 15C5 in the same mixtures indicates a decrease in the exothermic effect of the enthalpy of hydrophobic hydration of BlSCS in relation to 15C5 (Table 2). This seems to be caused by the presence of the benzene ring in the B15C5 molecule. The calculated differences in the enthalpic effect of hydrophobic hydration in water for B 15C5 and 15C5: GHb(W) = Hb(W)(B 15C5) - Hb(W)( 15C5)
(4)
in the mixtures of water and DMSO, DMF and DMA are almost the same within the error limits and their average value amounts to (12.31 i 0.52) kJ mol-1. Thus, there is no influence of the mentioned organic solvents on the value of this effect. 2. Analysis of the properties of the system: B IX%water-HA&PA HMPA is a strongly hydrophobic solvent [ 16, 171 and therefore the mixture HME’A does not satisfy the ,,cage model” assumption that the number of water a mixed solvent which are capable of hydrating the molecules of a dissolved proportional to xw. This is confirmed by the non-linear course, (versus
of water and molecules in substance is the mixture
composition), of the enthalpies of solution of NaI, NaCl, KI and KC1 in the HMPA-water mixtures [ 18, 193. However, due to the fact that the curve of the enthalpy of solution of B 15C5 in HMPA-water shows a similar shape to those obtained in the remaining systems discussed earlier, it was intended to describe the curve AsolHo(B 15C5) = f(xw) using the same equation, A similar analysis was performed previously in the HMPA-water-15C5 system [ 11. and The parameters ,,n” and Hb(W) obtained for the system HMFA-water-B15C5 calculated in the same way as for the mixtures of water and DMSO, DMF and DMA containing B15C5 are given in Table 2. As can be seen, the value of ,,n” for the system HMPA-water-B15C5 is much higher than ,,n” obtained for the remaining systems. It is worth noticing that the HMPA molecule itself is strongly hydrophobic and in this case the ,,n” value can be connected not only with the crown ether molecule but also with HMPA one. However, it can be said that ,,n” characterises, in some way, the number of water molecules participating in the hydrophobic hydration in the system. The value of Hb(W) concerning the system BlSCS-HMPA-water amounts to -30.34 kJ mol-l. As can be noticed (Table 2) this value is less negative than that calculated from the data related to the enthalpy of solution of B15C5 in the mixtures of water with DMSO, DMF and DMA. The change in this value seems to be associated with the competitive interaction of HMPA and B 1SC5 molecules with water. The molecules of B 15C5 interact first of all with those water molecules which are not directly associated with the HMFA molecules. Therefore, the crown ether molecules, being surrounded by a lower number of water molecules, are not hydrated to such an extent as in the mixed solvent containing DMSO, DMF, DMA and water. This results in a decreased effect of the hydrophobic hydration of crown ether. The effect of benzene ring on the value of Hb(W) was calculated from the data of the enthalpy of solution of B 1X5 in the HMPA-water mixture as in the other mixed solvents,
267 Table 2 The coefficient ,,n” and the enthalpic contribution due to the hydrophobic hydration of B 15C5 and 15C5 in water l%(W) (kJ mol-*) at 298.15 K B15C5
15c5
Solvent
n
AsolH’(W*)
I-NW)
na
DMSO
5.27 + 0.10
34.59 +_0.10
-36.34
4.39 + 0.25
48.82
DMF
4.53 & 0.12
35.93 * 0.26
-37.68
4.00 * 0.13
49.41
DMA
5.22 + 0.10
37.78 + 0.46
-39.53
4.61 +0.17
-52.25
22.93 + 0.21
28.59 + 0.09
-30.34
12.01 f 0.19
-42.95
HMPA
Hb(W)a
aRef. [l] The difference GHb(W) (see equation 4) due to the presence of benzene ring amounts to 12.61 kJ mol-l and is similar to that obtained in the remaining mixtures examined in this study. This allows one to draw a conclusion that the properties of organic solvent do not affect to a decisive extent the energetic contribution of the benzene ring in the hydrophobic hydration of crown ether in water. Acknowledgement The author would like to thank Professor H. Piekarski for his helpful suggestions and discussions, This study was partially supported by Lodi University grant no. 505/491 and 5051641. References 1. M. Jbiwiak, H. Piekarski, J. Mol. Liqiud., in press. 2. H. Piekarski, M. Joiwiak, J. Therm. Anal. 48 (1997) 1283-1291. 3. Y. Takeda, K. Katsuta, Y. Inoue and T. Hakushi, Bull. Chem. Sot. Jpn., 61 (1988) 627 4. Y. Takeda and T. Kumazawa, Bull. Chem. Sot. Jpn., 61 (1988) 655. 5. M.J. Mastroianni, M.J. Pikal and S. Lindenbaum, J. Phys. Chem., 76 (1972) 3050. 6. C. J. Pedersen, J. Am. Chem. Sot., 89 (1967) 7017. 7. R.J. Ouellette, Can. J. Chem., 43 (1965) 707. 8. SC. Chan and J.P. Valleau, Can. J. Chem., 46 (1968) 853. 9. CD. Schmulbach and R.S. Drago, J. Amer. Chem. Sot., 82 (1960) 4484. 10. S. Taniewska-Osidska and M. Joiwiak, J. Chem. Thermodynamics, 18 (1986) 339. 11, H. Piekarski and D. Waliszewski, J. Thermal. Anal., 47 (1996) 1639. 12. C. de Visser and G. Somsen, J. Phys. Chem., 78 (1974) 1719. 13. W.J.M. Heuvesland and G. Somsen, J. Chem. Thermodyn., 8 (1976) 873. 14. C. de Visser, W.J.M. Heuvesland and G. Somsen, J. Solution Chem., 4 (1975) 3 11. 15. W.J.M. Heuvesland, C. de Visser and G. Somsen, J. Phys. Chem., 82 (1978) 29. 16. A.M. Zaichikov and Yu.G. Bushuev, Zh. Fiz. Khim., 69 (1995) 1942.
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