Heat of solution of 15-crown-5 ether in the mixtures of water with DMSO, DMF, DMA and HMPA at 298.15K

joumal of MOLECULAR

LIQUIDS ELSEVIER

Journal of MolecularLiquids 81 (1999) 63-70

Heat o f solution o f 15-crown-5 ether in the mixtures o f water with D M S O , DMF, D M A and H M P A at 298.15K Malgorzata J6~eiak and Henryk Piekarski Department of Physical Chemistry, University of L6d2, Pomorska 165, 90-236 L6d~, Poland

Enthalpies of solution of 15-crown-5 ether in dimethylsulfoxide-water, N,Ndimethylformamide-water, N,N-dimethylaeetamide-water, and hexamethylphosphoramidewater systems have been measured at 298.15K. The plot of enthalpy of solution of 15C5 in these mixtures exhibits maximum at xw ~ 0.5 for DMSO and DMA, xw e 0.4 for DMF, xw 0.7 for HMPA, respectively. The results were discussed on the basis of the ,,cage model" of hydrophobic hydration. © 1999 ElsevierScience B.V.All rights reserved. 1. INTRODUCTION Crown ethers constitute a very interesting class of compounds due to their properties, especially their capability to complex cations m a selective way [1, 2]. Nevertheless, very few papers have been dedicated to studies on the enthalpy of solution of crown ethers both in pure organic solvents (e.g., [3-7]) and in organic solvent-water mixtures [8-10]. Such studies are of great importance in explaining the interactions between crown ether and solvent which significantly affect the energeties and stability of the resultant complexes with cations [10]. Due to their molecular structure, crown ethers show some hydrophobic properties. As is known, hydrophobic subs~nees or those containing large apolar groups in their molecules exhibit characteristic changes of the thermodynamic functions when they are dissolved in water as well as in some organic solvent-water s3~tems [11]. In our studies on the effect of mixed solvent properties on the formation of macroeyclic ether-cation complexes it seemed advisable to examine thermochemieally solutions of crown ethers themselves in selected organic solventwater mixtures. The present paper contains the results of measurements of the dissolution enthalpies of 15crown-5 ether (15C5) in mixtures of water with aprotic solvents such as: dimethylmifoxide (DMSO), N,N-dimethylformamide (DMF), N,N-dimethylaeetamide (DMA) and hexarnethylphosphoramide (HMP,at). 2. EXPERIMENTAL

15-crown-5 (Aldrich) 98% was used as received. Dimethylmlfoxide (Fluka > 99°/,) was dried using the molecular sieves 5A and distilled over CaI-I2under reduced pressure [12]. N,Ndimethylformamide (Aldrich 99%), N,N-dimethylacetamide (Aldrich 99°/,) and ,,purum" hexamethylphosphoramide (Fluka) were purified and dried by the methods described m the literature [13 - 15]. 0167-7322/99/$ - see front matter © 1999 ElsevierScience B.V. All rights reserved. S0167-7322(99) 00032-X

PH

64 The calorimetric measurements were carried out using an ,,isoperibol" type calorimeter as described in our earlier paper [16] at 298.15 ± 0.005 K. The uncertainties in the measured enthalpies did not exceed :t: 0.5% o f the measured value. The dissolution enthalpies o f 15C5 were measured within the whole range o f the mixed solvent composition. The concentration range o f the crown ether was 0.002 + 0.009 tool kg -~.

3. RESULTS AND DISCUSSION No concentration dependence (outside the error limits) of the measured enthalpies of solution was otr~rved within the examined range o f the crown ether content. For this reason, the standard dissolution enthalpies AsoiHO o f 15C5 m the mixed solvent were calculated as mean values of the measured enthalpies. The results obtained are given in Table 1 and shown in Fig. 1 versus the mixed solvent composition. The determined values of the dissolution enthalpies o f 15C5 in water, AsolHO = - 40.64 ± 0.07 ld moi -1, and in DMA AsolHO = 0.65 ± 0.03 ld mo1-1 are in good agreement with the literature values, -39.71 ± 0.21 ld tool-~ [18] and 0.75 ± 0.4 kJ tool-~ [17], respectively. Table 1 Standard enthalpies of solution o f 15C5 (ld mo1-1) in water-organic solvent mixtures at 298.15 K

t~oli-io xw 0

DMSO 0.72-+0.01

DMF -0.12±0.03

O. 10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 0.92 0.94 0.95 0.96 0.97 0.98 0.985

1.54~0.1M 0.82±0.02 2.24:L-0.02 1.65±0.02 2.91±0.05 2.66±0.03 3.75i-0.09 3.3 9±0.03 4.07±0.10 3.28±0.01 3.1(K-0.04 1.03±0.01 -1.31±0.10 -4.70±0.02 -11.71±0.03 -14.06±O.03 -25.74±0.03 -26.06±0.05 -28.99AO.06 -28.49±0.04 -32.36±0.06 -31.48±0.04 . . . -35.26~q).05 -34.66±0.04 ---38.04±0,05 -37.54±0.03 ---

1.000

-40.64±0.07

a From fit. [17]

DMA

-40.64±0.07

0.65±0.03 0.75±0.4 a 1.68±0.01 2.75±0.03 3.87±0.04 5.06±0.03 5.66L-0.02 4.83±0.03 0.43±0.02 -9.06±0.01 -23.13±0.02 -26.97±0,06 -30.29-~,03

HMPA -0.08~0.01

-32.90~,04 --37.31+0,04 --

0.26~0.01 0.47±0.02 0.67±0.03 0.86±0.02 1.16~0.01 1.55±0.02 1.67~q).02 -0.28-~.01 -9.49a:0.03 --20.90±0.02 -24,60-~.03 -28.14±0.03 --34.44=L-0.03

--40.64±0.07

-40.64=L-0.07

.

65

10

ol

-10 Hb(W)

1" -20

-30

-40

0.0

0.2

0.4

0.6

0.8

1.0

x. Figure 1. Enthaipies of solution of 15C5 in the mixtures of water with DMSO (V), DMF (i), DMA (O) 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).

66 The relatively high value of the exothermic dissolution effect of 15C5 in water seems to indicate a strong hydration of this compound. On the other hand, the considerably less exothermic effect of 15C5 dissolution in the examined organic solvents, despite their being less structured than water, indicates a relatively weak solvation of the crown ether in these solvents. As is seen in Fig. 1, the dissolution enthalpy curves of 15C5 in all examined mixtures run according to a similar course. It is characterised by a strong increase of the AsolH° 15C5 values within the water-rich region, and then, beyond the maximum, almost linear course of the function to the value in pure organic solvents. The drop in the exothermic dissolution effect along with the mcrease of the water content in the mixed solvent is connected with the hydrophobic hydration of crown ether [11 ]. This drop is the strongest in mixtures containing HMPA due to the competitive action of the strongly hydrophobic organic cosolvent. In the same, HMPA-water mixture the most exothermic dissolution effect of 15C5 is also observed within the organic solvent-rich region. Moreover, the enthalpy of solution of 15C5 changes only ms/enificantly from that in pure HMPA to the maximum. Probably, the strong interaction between water molecules and HMPA causes 15C5 to be solvated mainly by the organic cosoivent within the range of high HMPA content, up to xw = 0.7. In the other investigated systems, the organic cosolvent molecules in the crown ether solvation shell are probably gradually replaced by water molecules when water is added to the organic solvent. As a result the enthalpy of solution of 15C5 clearly increases from the value in pure solvent up to the maximum beyond which a main contribution to the observed dissolution effect is given by the above mentioned hydrophobic hydration. The course of the dissolution enthalpy of 15C5 in the presented water-organic solvent mixtures is typical for compounds hydrophobically hydrated in water and water-rich mixtures [ 11]. Thus, one may assume that the results of our investigations can be analysed by means of the ,,cage model" of hydrophobic hydration as proposed by Mastroianni, Pikal and Lindenbaum [ 19]. The essential assumption of this model are as follows: 1. In a mixed aqueous-organic solvent, the probability that molecules of one of the solvents will occupy solvation positions around the solute is proportional to the molar fraction of this solvent in the mixture. 2. The effect of hydrophobic hydration takes place only when ,,n" water molecules (required to form the hydration cage around the solute non-polar portion) occupy ,,n" solvation positions at the same time. 3. The enthalpic effect of the hydration cage formation around the solute non-polar portion is equal to Hb(W) both in water and the mixed solvent. Based on the above assumptions, the enthalpy of solution of the hydrophobic substance in the mixed solvent (W + Y) may be given by the following equation [11, 20-22]: AsolH°(W + Y) = (1-Xw) AsolH°(Y) + xw AsolH°(W) + ( x n - Xw) Fro(W) where: AsolH°(W + Y) - the standard enthalpy of solution in the mixed solvent, AsolI-I°(Y)- the standard enthalpy of solution in a pure organic solvent, AsolHO(W) - the standard enthalpy of solution in water, xw - molar fraction of water in the mixed solvent,

(1)

67 the probability that ,,n" water molecules will be at the same time around the hydrophobic site in the molecule of the hydrophobic substance, Hb(W) - the enthalpic effect of hydrophobic hydration in pure water. As follows from the above mentioned assumptions, the cage modd may be used to analyse data in those mixed solvents where the organic cosolvent shows no specific reaction with water and rio distinct hydrophobic properties. Of the examined mixed solvents, DMSO-water and DMF-water mixtures satisfy the model requirements [11, 22], while DMA-water do it to a lesser extent mixture [22, 23]. HMPA shows clear hydrophobic properties [23, 24] and its mixtures with water do not satisfy the first assumption of Mastroianni, Pikal and Lindenbaum's model. Due to the fact that the curves AsolHO(15C5) = f(Xw) have similar shapes in all the examined mixed solvents, we have decided to use the cage model also to HMPA-water mixtures to be able to observe the effect of the organic solvent hydrophobicity on the determined values of Hb(W) and ,oa". In order to solve equation (1) in relation to ICe(W) and ,,n", the method of non-linear regression and the method of linear extrapolation were used. In the latter method, extrapolating linearly the relationship AsolH°(15C5) = f(l - Xw) from the region with a low water content in the mixture (0 < xw < 0.4) to x w = 1, we determined .Hb(W) (Fig. 1). Then, substitutmg the obtained value to equation (1), we selected parameter ,,n" so that this equation described the experiraental data as well as possible within the whole range of mixture composition. The values of Hb(W) and ,,n" found by both mentioned methods are given in Table 2. Hb(W) and ,,n" values determined by the method of non-linear regression include higher errors than the data obtained by the second method. Therefore, the second series of dats will be used m further discussion. n Xw

_

Table 2 The coefficient ,0a" and the enthalpic contribution due to the hydrophobic hydration of 15C5 in water Hb(W) 0d tool-1) at 298.15 K Solvent

na

DMSO

3.66_+0.31

-56.30+3.48

4.39+0.25

-48.82+0.14

DMF

3.47:t:0.16

-55~85 + 1 . 9 2

4.00±0.13

-49.41 +0.18

DMA

4.07±0.22

-57.66 + 2.10

4.61 -1-0.17

-52.25+0.15

Hb(W)a

HMPA 11.65+0.21 -43.77+0.32 a calculated by the method of non-linear regression b calculated by the method of linear extrapolation

rlb

12.01+0.19

Hb(W) b

-42.95+0.09

As follows from Table 2, the values of ,,n" for 15C5 in DMSO-water, DMF-water and DMA-water mixtures are very much the same ranging from 4.0 to 4.6 (+ 0.2). Similarly, the values of Hb(W) found in the same systems are close to each other, especially for the systems containing DMSO and DMF. This would suggest that the cage model may be used to analyse hydrophobic properties of the crown ether. As is known, various meamngs are ascribed to the parameter ,,n" (its physical sense is not clear enough). There is an opinion that ,,n" is the number of water molecules forming

68 a hydrophobic cage around the whole hydrophobic molecule [19] or average number of water molecules forming single hydrophobic cage around each apolar group in the molecule [19, 22]. It may be also assumed that ,n" is equal to the number of active hydrophobic sites in a hydrophobic molecule. The molecule of 15C5 contains 5 -C2H4- groups. Thus, the value of ,,n" found by us, i.e. ,n" e 5, would confirm the latter opinion. The values of ,n" and Hb(W) found for the system: 15C5 - water - HMPA are different from those mentioned previously. This is due to the hydrophobic properties of the organic cosolvent - HMPA. In the aqueous solution containing 15C5 and HMPA, the number of hydrophobic sites is considerably higher. If we assume that each -CH3 group in the HMPA molecule constitutes such a site, their total number should be 5 + 6 = 11. The value found by us: ,,n" = 12 is surprisingly well consistent with that number. The values of the enthalpy of hydrophobic hydration of Hb(W) 15C5 in all the examined systems are negative as expected. Taking into account the fact that a single molecule of 15C5 contains ten -CH2- groups, the enthalpic effect of the hydrophobic hydration per one -CH2group is -5 kJ mo1-1. This value is similar to those found by Henvesland and Somsen per one C atom in Et4NBr, n-Pr4NBr , n-Bu4NBr and n-Pen4NBr, ranging from -4.8 to -3.9 kJ mo1-1 [22], as well as to those found by Rouw and Somsen per one methyl group of N-methylsubstituted ureas, ranging from -7.1 to --6.2 kJ mo1-1 [25]. The less exothermic effect of the hydrophobic hydration of the crown ether in the presence of HMPA may be accounted for by the competitive action of the amide molecule. The formation of a hydrophobic sheath around the 15C5 molecule within the water-rich region requires the hydrophobic sheath of non-polar groups in the molecule of HMPA to be destroyed at least partly. This would provide the endothermic contribution to the observed effect ofbydration. Using the measured of the enthalpies of solution of 15C5 in the examined mixtures we have calculated the enthalpic pair interaction coefficients for crown ether-organic cosolvent pairs in water, hxy, by the method described previously [26]. The standard enthalpy of solution of 15C5 in water-organic mixtures, AsolHO(15C5 in W + Y) within the range of low organic cosoivent content was fitted to the equation: ~o,HO0 5C5 m w + Y) = ~o,HO0 5C5 in W ) + b % +COy2

(2)

where: AsolHO(15C5 in W) denotes the standard enthalpy of solution of 15C5 in pure water, COy is the mass fraction of cosolvent Y, b and c are coefficients that can be determined by the leastsquare method. Parameter b that represents the limiting slope of function AsolHO(15C5 in W + Y) is connected with McMillan-Mayer's interaction coefficient hxy:

b = 2 hxy(6my/&o y)% -.,o

(3)

Denoting the molar mass of the cosolvent by My, we have for dilute solutions

(amy/~,)~:,0 = lm,.y Hence

(4)

69 hxy= b My/2

(5)

As is known, the solute molecules interact in solution with the participation of solvent molecules. The interaction is connected with the mutual approach of the interacting pair molecules and partial overlapping of their solvation sheaths [27]. As a result of this process, a number of solvent molecules is ,,squeezed out" from the hydration zones of interacting molecules. Thus, one may assume that the enthalpic pair interaction coefficients of solute X and organic cosolvent Y constitute a measure of the thermal effect associated with three basic processes [26, 28, 29]: 1) partial dehydration of solute X - endothermic process, 2) partial dehydration of organic cosolvent Y - endothermic process, 3) interaction between solute and organic cusolvent - exothermic process. The calculated pair interaction coefficients for the 15C5-organic cosolvent molecule Y in water, hxy, are given in Table 3. These coefficients are positive in all the examined systems. This would mean that in an aqueous solution, the energetic effects of dehydration prevail over the exothermic effect of direct interaction between the crown ether and the organic cosolvent molecule. The values of hxyconcerning the interactions of 15C5 with DMF, DMSO and DMA are similar. Since the dipole moments of the above mentioned cosolvents are close to each other, one may assume that the effect of direct interaction of 15C5-Y is similar for these pairs. In the fight of the presented model one can suppose that the hydration effects ofDMSO, DMF and DMA are also similar, though DMA seems to be hydrated to the greatest extent in this group,

Table 3 Enthalpic pair interaction coefficients, hxy (kJ kg tool-2) between the 15C5 (X) and the cosolvent Y in water-organic solvent mixtures at 298.15 K Solvent

hxy

DMSO

1.131 :t: 0.009

DMF

1.394 + 0.023

DMA

1.491 + 0.014

HMPA

3.970 + 0.072

The most disadvantageous interaction in aqueous solutions is observed in the case of 15C5HMPA, although the high dipole moment of HMPA (l~ = 5.5) [30] suggtms that a strong interaction effect of 15C5-HMPA dipole could be expected. This may be due to the strong hydrophobic hydration of HMPA molecules resulting in strong endothermic contribution to the enthalpic coefficient for this pair. Thus, the above conclusions resulting from the analysis of the enthalpic pair interaction are consistent with the observed enthalpies of solution of 15C5 in the system under investigation.

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