Enthalpy of solution of hexaglyme in mixtures of water with N,N-dimethylformamide at 298.15 K

Article abstract of DOI:10.1016/j.molliq.2014.09.017

The enthalpies of solution of hexaglyme in water-N,N-dimethylforamide mixtures have been measured at 298.15 K. To analyze the process of hydrophobic hydration the cage model proposed by Mastroianni, Pical and Lindenbaum was used. The enthalpic effect of h

Full text of DOI:10.1016/j.molliq.2014.09.017

Journal of Molecular Liquids 199 (2014) 224–226
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Short Communication
Enthalpy of solution of hexaglyme in mixtures of water with
N,N-dimethylformamide at 298.15 K
a,
b
Małgorzata Jóźwiak ⁎, Andrzej Jóźwiak
a
University of Lodz, Faculty of Chemistry, Department of Physical Chemistry, Pomorska 165, 90-236 Lodz, Poland
University of Lodz, Faculty of Chemistry, Department of Organic Chemistry, Tamka 12, 91-403 Lodz, Poland
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 July 2014
Received in revised form 29 August 2014
Accepted 8 September 2014
Available online 11 September 2014
The enthalpies of solution of hexaglyme in water–N,N-dimethylforamide mixtures have been measured at
2
98.15 K. To analyze the process of hydrophobic hydration the cage model proposed by Mastroianni, Pical and
Lindenbaum was used. The enthalpic effect of hydrophobic hydration of hexaglyme in water has been calculated.
2014 Elsevier B.V. All rights reserved.
©
Keywords:
Hexaglyme
Water–N,N-dimethylforamide mixtures
Enthalpy of solution
Hydrophobic hydration
1
. Introduction
assumptions of Mastroianni et al.'s cage model of hydrophobic hydra-
tion [11,12] used in the presented paper. The DMF + W mixture can
be used to study the pure effect of hydrophobic hydration of compounds
showing hydrophobic properties [13].
Linear polyethers known as glymes with their general formula
3 2 2 n 3
CH O(CH CH O) CH constitute an interesting class of compounds.
They are used in many areas of science and industry, mainly as a solvent
but also as ligands in complexes [1–9].
2. Experimental section
Glymes have hydrophilic and hydrophobic groups in the molecules.
Therefore, in water or in mixed aqueous-organic solvents, they are
hydrophobically hydrated by water. The process of hydrophobic hydra-
tion plays a very important role in nature and has not yet been fully in-
vestigated. Therefore, the study of this process is of great importance.
The literature offers just few publications on the hydrophobic hydration
of glymes [9,10].
In our previous paper [10], the process of hydrophobic hydration of
monoglyme, diglyme, triglyme, tetraglyme and pentaglyme in the mix-
tures of water and N,N-dimethylformamide (DMF + W) at 298.15 K
was examined and the enthalpic effect of hydrophobic hydration of
these compounds was calculated using the cage model of hydrophobic
hydration [11,12]. It seemed appropriate to investigate the hexaglyme
2.1. Materials
2,5,8,11,14,17,20-Heptaoxahenicosane (hexaethylene glycol di-
methyl ether) was prepared from 2-methoxyethanol and tetraethylene
glycol ditosylate. 2-Methoxyethanol (5.33 g, 0.07 mol, Aldrich) was
added to a stirred suspension of sodium hydroxide (6.00 g, 0.15 mol)
in THF (300 ml) under argon atmosphere. After 1 h, tetraethylene glycol
ditosylate was added to the mixture (obtained as described previously
[14] 15.08 g, 0.03 mol) in THF (150 ml) dropwise over 1 h and stirring
was continued for 50 h at 66 °C. The precipitate was separated and
washed with THF (3 × 50 ml). The filtrate was concentrated under a re-
duced pressure to obtain crude hexaethylene glycol dimethyl ether that
was then purified by Kugelrohr distillation (oil bath 215–220 °C,
2.0 Torr; ref. [15], 145–149 °C 0.6 Torr; 5.22 g, yield 56%). 1H NMR
(Fig. 1), the next glyme of homologous series and compare the results
with those given in previous publications. In this paper we present the
solution enthalpy of hexaglyme in DMF + W at 298.15 K. As before,
for our studies we chose a mixture of DMF with water as a solvent.
DMF is a neutral solvent from the point of view of the hydrophobic
and hydrophilic properties; these properties almost undergo compen-
sation. Thus, this solvent in the mixture with water meets the
(CDCl
\OCH
trometer at 600 MHz and referenced to the residual CDCl
3
, 600 MHz): δ = 3.65–3.52 (24H, m, \OCH
2
CH
). H NMR spectra were recorded on a Bruker Advance III spec-
signal at
2
\), 3.36 (6H, s,
1
3
3
7.26 ppm. The purity of the compound is 0.99 mole fraction and was de-
termined with a TG DSC 111 (SETARAM) with indium as a standard.
N,N-dimethylformamide (Aldrich, anhydrous, 99.8%) was purified
and dried according to the procedures described in the literature [16,
17]. To prepare the aqueous solutions doubly distilled water was used.
⁎
Corresponding author.
E-mail address: mjozwiak@uni.lodz.pl (M. Jóźwiak).
http://dx.doi.org/10.1016/j.molliq.2014.09.017
167-7322/© 2014 Elsevier B.V. All rights reserved.
0

M. Jóźwiak, A. Jóźwiak / Journal of Molecular Liquids 199 (2014) 224–226
225
Fig. 1. The molecule of hexaglyme.
2
.2. Methods
The enthalpy of solution of hexaglyme in the DMF + W mixtures
was performed at (298.15 ± 0.01) K using an “isoperibol” type calorim-
eter as described in the literature. [18]. The calorimeter was verified on
the basis of the standard enthalpy of solution of urea and KCl (Calori-
metric standard US, NBS) in water at (298.15 ± 0.005) K [19,20] as
was described in our recent publication [21]. The concentration of
−
1
hexaglyme in the mixtures was from 0.00168 to 0.00416 mol kg
the mole per kilogram of solvent). Six to eight independent measure-
(
ments were performed for each investigation system. The uncertainties
in the measured enthalpies did not exceed ± 0.5% of the measured
value. No concentration dependence (outside the error limits) of the
measured enthalpies of solution was observed within the examined
range of hexaglyme content. For this reason, the standard solution en-
o
thalpy Δsol
H
was calculated as a mean value of the measured
enthalpies (Table 1).
3
. Results and discussion
The transfer enthalpies of hexaglyme from water to DMF + W mix-
tures are presented in Fig. 2 as a mole fraction of water (x ) in the
w
mixture. For comparison, Fig. 2 shows the transfer enthalpies of
monoglyme, diglyme, triglyme, tetraglyme and pentaglyme from
water to DMF + W mixtures. As seen in Fig. 2, the dissolution enthalpy
curves of glymes run according to a similar course. It is characterized by
Fig. 2. The transfer enthalpy of glymes: monoglyme, ■; diglyme, ●; triglyme, ▲;
tetraglyme, ▼; pentaglyme, ♦; and hexaglyme,◄ from water to DMF + W mixtures at
298.15 K as a function of the mole fraction of water (x ) at 298.15 K.
w
o
an almost linear course of the function (ΔsolH ) from the value in pure
organic solvent, and then, beyond the maximum, strong decrease of
o
Δ
solH values within the water-rich region. The drop in the exothermic
dissolution effect along with the increase of the water content in the
mixed solvent is connected with the hydrophobic hydration of glymes.
Fig. 2 analysis showed that for each mixed solvent composition
transfer enthalpy of glymes from water to the mixed solvent depends
linearly on the number of n—CH2CH2— groups (n—CH2CH2— is the number
The maximum at x ≈ 0.5 is connected with the structure of mixed sol-
w
vent DMF + W. In this region of mixture composition the interactions
between molecules being mixture components are the strongest. DMF
molecules react with water molecules through the formation of weak
hydrogen bonds, but the structures formed are not characterized in
detail. The wider discussion on this topic is presented in our previous
papers [22,23].
2 2
of \CH CH \ groups in the glyme molecules) with high regression co-
efficient. However, the analysis of coefficients of linear functions, as well
as the regression coefficient and the standard deviation does not give
interesting conclusions.
Using the standard molar enthalpies of solution of hexaglyme in
o
DMF + W, ΔsolH , and the cage model of hydrophobic hydration [11,
1
2], the enthalpic effect of hydrophobic hydration, Hb(W), of
hexaglyme was determined. As described in our earlier publications
24], using the diagrams of the standard dissolution enthalpies of
Table 1
[
Standard dissolution enthalpy of hexaglyme in DMF + W mixtures at
298.15 K.
hexaglyme, the values of dissolution enthalpy of the hypothetical sub-
o
−1
stances with no hydrophobic hydration were determined graphically,
x
w
ΔsolH /kJ mol
o
Δ
solH (W*) [25]. The enthalpic effect of hydrophobic hydration in
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
.00
.10
.20
.30
.40
.50
.60
.70
.80
.90
.92
.94
.96
.98
.00
−3.18 ± 0.04
−1.77 ± 0.06
−0.38 ± 0.08
1.00 ± 0.07
2.02 ± 0.05
2.46 ± 0.05
water, Hb(W) was calculated using Eq. (1).
À
ꢀÁ
o
o
HbðWÞ ¼ ΔsolH ðWÞ−ΔsolH
W
ð1Þ
0.98 ± 0.08
−5.10 ± 0.05
−17.00 ± 0.03
−34.82 ± 0.03
−39.25 ± 0.03
−43.39 ± 0.03
−48.27 ± 0.02
−54.05 ± 0.02
−60.52 ± 0.02
o
where ΔsolH (W) is the standard enthalpy of solution in water and
o
Δ
solH (W*) is the solution enthalpy of the solute in water when hydro-
phobic effect is absent.
The value of the enthalpic effect of hydrophobic hydration of
hexaglyme in water calculated in this way, Hb(W), is equal to
−
1
(−71.28 ± 0.06) kJ mol . Using analogous values for monoglyme,
diglyme, triglyme, tetraglyme and pentaglyme, the function Hb(W) =
f(n—CH CH —) is presented in Fig. 3. As shown in Fig. 3, the relationship
x
±
w
— the mole fraction of water in the mixtures.
is the standard deviation.
2
2

226
M. Jóźwiak, A. Jóźwiak / Journal of Molecular Liquids 199 (2014) 224–226
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diglyme, triglyme, tetraglyme, pentaglyme and hexaglyme are directly
proportional to the length of the chain in the glyme molecules.
The interactions between glyme molecules (monoglyme, diglyme,
triglyme, tetraglyme, pentaglyme and hexaglyme) and water molecules
as well as glyme molecules and DMF molecules are dependent on the
length of the chain in the glyme molecules.