Effects of hydration on the thermodynamic properties of aqueous ethylene glycol ether solutions

Article abstract of DOI:10.1016/j.jct.2004.10.006

The densities and isobaric specific heat capacities of binary mixtures of water with various open-chain and cyclic ethylene glycol ethers have been measured at 298.15 K using vibrating tube densimetry, and flow or differential scanning calorimetry, respectively. Excess molar volumes were derived over the whole composition range. Molar isobaric heat capacities and the relative apparent thermodynamic quantities were determined in the water-rich region. The data reflect the changes in the structure and hydrogen-bond dynamics of water caused by these non-ionic solutes. The observed effects are discussed in terms of the influence of hydrophobic hydration on the thermodynamic properties of aqueous solutions. Correlations are given that enable the prediction of the thermodynamic properties of open-chain and cyclic oligo(ethylene oxide) ethers in their pure liquid state and at infinite dilution in water.

Full text of DOI:10.1016/j.jct.2004.10.006

J. Chem. Thermodynamics 37 (2005) 513–522
www.elsevier.com/locate/jct
Effects of hydration on the thermodynamic properties of
aqueous ethylene glycol ether solutions
a
Simon Schr o¨ dle , Glenn Hefter , Richard Buchner
b
a,
*
a
Institut f u¨ r Physikalische und Theoretische Chemie, Universit a¨ t Regensburg, D-93040 Regensburg, Germany
b
Chemistry Department, DSE Murdoch University, Murdoch, WA 6150, Australia
Received 23 August 2004; accepted 8 October 2004
Available online 13 December 2004
Abstract
The densities and isobaric specific heat capacities of binary mixtures of water with various open-chain and cyclic ethylene glycol
ethers have been measured at 298.15 K using vibrating tube densimetry, and flow or differential scanning calorimetry, respectively.
Excess molar volumes were derived over the whole composition range. Molar isobaric heat capacities and the relative apparent ther-
modynamic quantities were determined in the water-rich region. The data reflect the changes in the structure and hydrogen-bond
dynamics of water caused by these non-ionic solutes. The observed effects are discussed in terms of the influence of hydrophobic
hydration on the thermodynamic properties of aqueous solutions. Correlations are given that enable the prediction of the thermo-
dynamic properties of open-chain and cyclic oligo(ethylene oxide) ethers in their pure liquid state and at infinite dilution in water.
ꢀ
2004 Elsevier Ltd. All rights reserved.
Keywords: Hydrophobic hydration; Glycol ethers; Cyclic ethers; Apparent heat capacity; Excess volume
1
. Introduction
Straight chain oligo(ethylene glycol)s and their
mono- and dimethyl ether analogues are miscible with
water over the whole composition range, apparently
without forming higher aggregates like micelles. As
such, they provide a series of solutes with systematically
varying characteristics, which can be used to probe the
influences of hydrophobic hydration and the H-bond
donor–acceptor ratio on the dynamics and structure of
the H-bond network. Comparisons of the thermody-
namic properties of aqueous solutions of open-chain
oligo(ethylene glycol) ethers with those of related, but
more rigid, cyclic ethers such as 1,4-dioxane and the
crown ethers can in addition provide insights into the
structural effects of organic solutes on water properties.
This investigation compares the molar excess vol-
Water shows distinctive properties relative to other
solvents due to its strong intermolecular forces, which
produce an extensive three-dimensional network of
hydrogen (H) bonds. H-bonding in water is a highly
cooperative phenomenon, mainly due to the small size
of the water molecule and its considerable charge sepa-
ration (high dipole moment) and its 1:1 ratio of donor to
acceptor sites. The presence of non-ionic compounds in
aqueous solutions typically results in significant changes
to the local and long-range water structure. Particularly
large thermodynamic effects occur in the presence of
hydrophobic solutes. Such effects are of outstanding
importance in understanding many biological processes
and in polymer, surface and colloid science.
E
umes, V , and the relative apparent heat capacities,
DCp,U,2, of mixtures of water with oligo(ethylene glycol)
monomethyl ethers, C E OH, and dimethyl ethers,
*
1 n
Corresponding author. Tel.: +49 9419434031.
E-mail address: richard.buchner@chemie.uni-regensburg.de
R. Buchner).
C E C , of oligomerisation degree 1 6 n 6 5. For some
1 n 1
(
of the compounds, the volume properties have been
0
021-9614/$ - see front matter ꢀ 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jct.2004.10.006

514
S. Schr o¨ dle et al. / J. Chem. Thermodynamics 37 (2005) 513–522
2
D
5
reported for the aqueous solutions by other investiga-
tors. However, because of serious inconsistencies among
the reported values, it was considered appropriate to
measure them again systematically with samples of
well-established purity.
mole fraction (H O 6 500 ppm, n ¼ 1:4481) was
2
collected.
Pentaethylene glycol dimethyl ether (C E C ) was
prepared in a similar way from ethylene glycol monom-
ethyl ether and 1,2-bis-(2-chloro-ethoxy)-ethane: 69 g
sodium was cut into small pieces and added under stir-
ing to 500 g ethylene glycol monomethyl ether
1
5 1
(C E OH; destilled from CaH ). This mixture was
1 1 2
2
. Experimental
heated to 100 ꢁC until the sodium dissolved. Anhydrous
NaI (1 g, finely powdered) and 281 g 1,2-bis-(2-chloro-
ethoxy)-ethane (dropwise) were added, and heating
2
.1. Materials
(
100 ꢁC) continued for 3 days. After evaporation of ex-
1,4-Dioxane (Merck, pro analysi) was purified by
refluxing over solid calcium hydride (CaH ) for several
hours [1] and subsequently distilled over a 40 cm col-
umn at atmospheric pressure, resulting in a final purity
cess C E OH the remainder was dissolved in water and
1
1
extracted with dichloromethane (4 Æ 100 mL). The or-
ganic layers were combined, dried over anhydrous so-
dium sulfate and evaporated. The residue was then
distilled at 0.04 mbar. The fraction boiling between (95
and 119) ꢁC was heated with 100 mL of ꢀ5% hydrochlo-
ric acid for a few hours to decompose byproducts (ace-
tals), washed with n-pentane (3.50 mL) and the water
evaporated at atmospheric pressure. 300 mL dichloro-
methane was added to the residue. After stirring over
anhydrous potassium carbonate for several hours the
dichloromethane was removed under vacuum. The raw
product was fractionated over a 40 cm column at 0.03
mbar and the center fraction (b.p. ꢀ105 ꢁC) was further
distilled at ꢀ0.008 mbar using a Fischer Spaltrohr appa-
ratus (60 theoretical plates at 1013 mbar). The pentaeth-
ylene glycol dimethyl ether (73 g) so obtained had a
2
of 0.9997 mole fraction (g.c.); H O 6 30 ppm (Karl
2
Fischer coulometric titration). Ethylene glycol monom-
ethyl ether (C E OH; Fluka; 0.999 mole fraction;
1
1
H O 6 300 ppm), ethylene glycol dimethyl ether
2
(
and triethylene glycol monomethyl ether (C E OH;
C E C ; Sigma; 0.9994 mole fraction; H O 6 80 ppm)
1
1
1
2
1
3
Fluka; 0.997 mole fraction; H O 6 300 ppm) were used
2
as received. Diethylene glycol dimethyl ether (Merck,
for synthesis) was heated with solid CaH , then frac-
2
tionally distilled at atmospheric pressure using a 25
cm column, giving a product with a purity of 0.999 mole
fraction; H O 6 60 ppm. Triethylene glycol dimethyl
2
ether (C E C ; Merck, for synthesis) was distilled at
ꢀ
1
3 1
1 mbar using a 65 cm column. The fraction used for
measurements had a purity of 0.998 mole fraction;
purity of 0.997 mole fraction (H O 6 500 ppm;
2
2
D
5
n
¼ 1:4376).
H O 6 500 ppm.
The structures of the pentaethylene glycol ethers were
13
2
1
confirmed by n.m.r. spectroscopy ( H, C and
13
Pentaethylene glycol monomethyl ether (C E OH)
1
C
5
was prepared from diethylene glycol and 1-(2-chloro-
ethoxy)-2-(2-methoxy-ethoxy)-ethane as follows. Using
stirring throughout, a stochiometric amount of sodium
metal (12.7 g) and 0.1 g of powdered, anhydrous sodium
iodide (NaI) were added to 233.5 g diethylene glycol
DEPT 135ꢁ).
Note that all reactions were carried out with anhy-
drous reagents under a dry nitrogen atmosphere. All dis-
tillation employed vacuum-jacketed Vigreux colums.
Aqueous mixtures were prepared gravimetrically on
an analytical balance without buoyancy corrections.
Concentrations are thus accurate to four significant fig-
ures. Water, deionized and purified with a Millipore
Milli-Q system (with a final pass through a 0.22 lm fil-
ter) was used throughout. All pure liquids were carefully
degassed at low pressure prior to mixture preparation.
(
ꢁ
1
dried by distillation). The mixture was heated to 120
C until all the sodium dissolved. Then 100.5 g of
-(2-chloro-ethoxy)-2-(2-methoxy-ethoxy)-ethane (pre-
pared from triethylene glycol monomethyl ether and
thionyl chloride [2], fractionated at ꢀ0.1 mbar) was
added dropwise and the resulting mixture heated (120
ꢁ
C) for several hours. After standing overnight, sodium
chloride was removed by filtration and washed with
methanol. The filtrate was evaporated and the excess
diethylene glycol removed by distillation at ꢀ0.03 mbar
using a 25 cm column. Phosphoric acid (ꢀ20% w/w, 25
mL) was added to the residue and the aqueous phase ex-
tracted with dichloromethane (4 Æ 100 mL). The com-
bined extracts were evaporated and the residue was
fractionated at 0.02 mbar. The middle fraction was fur-
ther purified using a short-path molecular distillation
apparatus at ꢀ3 Æ 10ꢁ mbar. Pentaethylene glycol
monomethyl ether (58 g) with a final purity of 0.994
2.2. Apparatus and procedure
Solution densities, q, were determined by vibrating
tube densimetry (Anton Paar DMA602/mPDS4000 sys-
tem) and are accurate to ꢀ5 Æ 10 kg Æ m . The temper-
ature was kept constant to ±4 mK with a circulating
thermostat (Julabo F33 SD). The densimeter was cali-
brated using water and air as references. The density
of water at 298.15 K was taken as 997.047 kg Æ m con-
sistent with the IAPWS-IF97 value [3], and that of air
was calculated from generally accepted equations [4].
ꢁ
3
ꢁ3
ꢁ
3
5

S. Schr o¨ dle et al. / J. Chem. Thermodynamics 37 (2005) 513–522
515
At least two independent density measurements were
performed with each solution.
needed to heat or cool the reference and sample cells
between (297.65 and 298.65) K (1 K step) were deter-
mined by integration of the heat flow signals. An inter-
Isobaric volumetric heat capacities were measured
relative to water using a Picker flow calorimeter (Sodev,
Sherbrooke, Canada), consisting of a specific heat unit,
model CP-Cpr, a thermal detector, model DT-C, and a
temperature control/program unit, model CT-L. A peri-
staltic pump (Gilson Minipulse 3) was used to control
the flow rate (ꢀ0.5 mL Æ minꢁ ). The output voltage
from the detector was digitized by a voltmeter (Agilent
ꢁ
1
ꢁ1
nal reference (n-heptane; c = 2244.4 J Æ K Æ kg [10])
p
was used. Heat capacities were calculated from two
independent experiments, each consisting of several
heating and cooling cycles. The reproducibility of the
cp values was usually better than 0.1%, the overall
uncertainty of the measurement estimated from calibra-
tion and test experiments is assumed to be ꢀ0.3%. No
allowances were made for sample vaporization and gas
phase corrections as the vapor volume was kept below
10% of the total cell volume and all substances have
quite low vapor pressures at the measurement temper-
ature [11].
1
3
ment, two independent values were determined for the
4401A) connected to a computer. From each measure-
ꢂ
ꢂ
ꢂ
relative volumetric heat capacity Dr/r = (r ꢁ r )/r ,
ꢂ
where r and r are the heat capacities per unit volume
of the sample and reference liquid, respectively [5]. The
values of c , the isobaric specific heat capacity, were cal-
p
culated from these values using the following expression
[
6]:
3. Data treatment
ꢂ
ꢂ
ꢂ
c
p
¼ c ꢃ ð1 þ Dr=r Þ ꢃ q =q:
ð1Þ
p
Experimental results for the densities, q, and specific
The calorimeter was calibrated electrically at the end of
each series of measurements. Water was used as a refer-
heat capacities, c , of the pure compounds are given in
p
table 1; those of the mixtures of the various ethers with
water are summarized in tables 2 to 4. From these data,
apparent thermodynamic quantities relative to the pure
liquid state of the solute,. DYU,2, were obtained by the
following equation:
ꢂ
ꢁ1
ꢁ1
ence liquid, assuming c ¼ 4181:3 J ꢃ K ꢃ kg accord-
p
ing to the IAPWS-95 formulation [7]. No correction
was made for heat losses [5].
A Tian-Calvet type heat-conduction microcalorime-
ter (MicroDSC II, Setaram, France) was utilized to
measure the heat capacities of the pure organic materi-
als with an accuracy sufficient for the determination of
thermodynamic excess quantities. Specially designed
batch cells (sample volume ꢀ0.8 mL) made from stain-
less steel with a perfluoroelastomer O-ring seal (Kalrez
ꢂ
Y ꢁ x ꢃ Y
1
1
ꢂ
DY U;2
¼
ꢁ Y :
ð2Þ
2
x
2
Here, Y denotes the molar quantities of the mixtures,
ꢂ
and Y the corresponding properties of the pure sub-
stances. For the systems studied, the relative apparent
quantities are more helpful than the absolute quanti-
ties for comparison of compounds of different chain
length.
4
079) were used. A discontinuous step method was em-
ployed with heating/cooling rates of 0.1 K/min and an
isothermal delay of 3600 s [8,9]. The power difference
TABLE 1
p
Densities, q, and isobaric specific heat capacities, c , of the pure liquid ethers, obtained in this work along with the literature values, all at 298.15 K
ꢁ
3
)
ꢁ1
ꢁ1
)
Compound
M
q/(kg Æ m
c
p
/(J Æ K Æ kg
ꢁ
3
ꢁ1
1
0
kg Æ mol
This work
1028.043
Literature
This work
1719.9
Literature
1,4-Dioxane
88.11
76.10
90.12
1027.92 [12]
1713.2 [10]
1696.7 [14]
1
027.98 [13]
028.07 [15]
960.34 [16]
1
C
C
C
1
E
1
E
1
E
1
1
2
OH
960.164
861.199
938.529
2295.3
2124.8
2304.9 [17]
2289.8 [19]
960.16 [18]
960.101 [19]
C
C
1
861.506 [20]
2144.4 [17]
2121.0 [22]
2121.8 [24]
2083.8 [25]
2079.8[27]
8
61.32 [21]
61.48 [23]
8
1
134.17
938.73 [25]
38.4 [26]
9
C
C
1
E
E
3
OH
164.20
178.23
1042.965
981.686
1043.0 [28]
980.82 [29]
2174.5
2074.6
1
3
C
1
2069.2 [17]
2073.3 [32]
2071.4 [29]
981.17 [30]
80.01 [33]
9
C
C
1
E
E
5
OH
252.30
266.33
1069.437
1024.793
2128.8
2055.4
1
5
C
1

516
S. Schr o¨ dle et al. / J. Chem. Thermodynamics 37 (2005) 513–522
TABLE 2
Densities, q, and isobaric specific heat capacities, c
p
, of {water (1) + 1,4-dioxane (2)} mixtures at 298.15 K
x
2
x
O,2
q
c
p
x
2
x
O,2
q
c
p
ꢁ
3
ꢁ1
ꢁ1
ꢁ3
ꢁ1
ꢁ1
kg Æ m
J Æ K Æ kg
kg Æ m
J Æ K Æ kg
{
Water (1) + 1,4-dioxane(2)}
0
0
997.05
1000.99
1003.44
1009.01
1017.66
1023.79
1027.12
1030.99
4181.3
4102.4
4050.7
3925.2
3702.3
3507.9
3388.0
3222.6
0.2000
0.2857
0.4000
0.5000
0.6667
0.8000
0.8889
1
0.3333
0.4444
0.5714
0.6667
0.8000
0.8889
0.9412
1
1034.98
1036.85
1036.18
1034.66
1031.91
1029.87
1028.76
1028.04
2979.3
2709.2
0
0
0
0
0
0
0
.00990
.01639
.03226
.06250
.09091
.1111
0.01961
0.03226
0.06250
0.1177
0.1667
0.2000
0.2500
.1429
1719.9
TABLE 3
Densities, q, and isobaric specific heat capacities, c
p
, of {water (1) + oligoethylene glycol monomethyl ethers (2)} mixtures at 298.15 K
x
2
x
O,2
q
c
p
x
2
x
O,2
q
c
p
ꢁ
3
ꢁ1
ꢁ1
ꢁ3
ꢁ1
ꢁ1
kg Æ m
J Æ K Æ kg
kg Æ m
J Æ K Æ kg
{
Water (1) + ethylene glycol monomethyl ether (2)}
0
0
997.05
998.08
998.70
4181.3
4157.0
4142.6
4090.9
4061.0
4008.0
3952.0
3894.8
0.1111
0.1429
0.2000
0.3333
0.5000
0.6667
0.8000
1
0.2000
0.2500
0.3333
0.5000
0.6667
0.8000
0.8889
1
1006.32
1006.98
1005.88
997.80
985.96
975.40
968.51
960.17
3811.2
3680.8
3456.0
3073.8
0
0
0
0
0
0
0
.01316
.01961
.03846
.04762
.06250
.07692
.09091
0.02597
0.03846
0.07407
0.09091
0.1176
1000.68
1001.65
1003.11
1004.34
1005.32
0.1429
0.1667
2295.3
{Water (1) + triethylene glycol monomethyl ether (2)}
0
0
0
0
0
0
0
0
0
997.05
1007.68
1012.43
1024.71
1029.57
1036.38
1041.50
1045.55
4181.3
4082.0
4031.6
3883.0
3812.9
3704.7
3607.5
3519.1
0.1111
0.1429
0.2000
0.3333
0.5000
0.6667
0.8000
1
0.3333
0.4000
0.5000
0.6667
0.8000
0.8889
0.9412
1
1049.55
1053.42
1055.74
1053.92
1050.27
1047.07
1045.27
1042.97
3413.8
3265.7
.01316
.01961
.03846
.04762
.06250
.07692
.09091
0.05063
0.07407
0.1379
0.1667
0.2105
0.2500
0.2857
2174.5
{Water (1) + pentaethylene glycol monomethyl ether (2)}
0
0
0
0
0
0
0
997.05
1007.31
1012.04
1024.63
1043.39
1058.63
4181.3
4097.2
4056.2
3937.7
3731.4
3517.5
0.09091
0.1667
0.3333
0.5000
0.6667
1
0.3750
0.5455
0.7500
0.8571
0.9231
1
1068.43
1076.08
1075.06
1072.73
1071.22
1069.44
3324.1
2976.3
.00662
.00990
.01961
.03846
.06250
0.03846
0.05660
0.10714
0.1936
0.2857
2128.8
As mole fraction is not fully satisfactory as a measure
of composition for oligomeric systems, the mole fraction
of oxygen atoms from the non-ionic solute
4. Results
4.1. Volumes
n
O;2 ꢃ x
2
n
O;2 ꢃ x
2
xO;2 ¼ n
¼
ð3Þ
The densities for the pure compounds are mostly in
good agreement (better than 50 ppm, table 1) with the
literature data. The only exceptions are C E C for
O;2 ꢃ x
was used, where (1) denotes water and (2) the ether com-
2
þ x
1
1 þ x ꢃ ðnO;2 ꢁ 1Þ
2
1
1 1
pound containing nO,2 oxygen atoms.
Excess molar volumes (figures 1 to 3) were derived
from the density values by the equation
which the present result is 240 ppm lower and C E C
3 1
1
where it is 1000 ppm higher than the average of the re-
ported values, although both values are still within 3r of
the literature means. The source of these differences is
unknown but purity is probably the most critical factor.
All the glycol ethers are sensitive to oxygen and readily
absorb significant amounts of water from air. Special
X
where M is the molar mass of the pure liquid component i.
x M þ x M
E
id
1
1
2
2
ꢂ
V
¼ V ꢁ V
¼
ꢁ
x
i
V ;
ð4Þ
i
q
i
i

S. Schr o¨ dle et al. / J. Chem. Thermodynamics 37 (2005) 513–522
517
TABLE 4
Densities, q, and isobaric specific heat capacities, c
p
, of {water (1) + oligoethylene glycol dimethyl ethers (2)} mixtures at 298.15 K
x
2
x
O,2
q
c
p
x
2
x
O,2
q
c
p
ꢁ3
ꢁ1
ꢁ1
ꢁ3
ꢁ1
ꢁ1
kg Æ m
J Æ K Æ kg
kg Æ m
J Æ K Æ kg
{
Water (1) + ethylene glycol dimethyl ether (2)}
0
0
997.05
993.84
992.67
989.98
988.87
987.00
985.05
982.88
4181.3
4177.4
4171.4
4137.6
4113.0
4064.6
4010.6
3953.1
0.1111
0.1429
0.2000
0.3333
0.5000
0.6667
0.8000
1
0.2000
0.2500
0.3333
0.5000
0.6667
0.8000
0.8889
1
979.24
972.59
958.93
929.51
903.35
885.19
873.97
861.20
3864.4
3726.8
3474.8
3042.5
0
0
0
0
0
0
0
.01316
.01961
.03846
.04762
.06250
.07692
.09091
0.02597
0.03846
0.07407
0.09091
0.1177
0.1429
0.1667
2124.8
{Water (1) + triethylene glycol dimethyl ether (2)}
0
0
0
0
0
0
0
0
0
997.05
1003.92
1007.09
1015.26
1018.21
1021.75
1023.84
1024.79
4181.3
4102.0
4060.4
3928.5
3862.1
3757.5
3661.3
3575.8
0.1111
0.1429
0.2000
0.3333
0.5000
0.6667
0.7936
1
0.3333
0.4000
0.5000
0.6667
0.8000
0.8889
0.9389
1
1024.83
1022.94
1017.20
1005.21
995.86
989.62
986.18
981.69
3462.3
3292.3
3054.9
.01316
.01961
.03846
.04762
.06250
.07692
.09091
0.05063
0.07407
0.1379
0.1667
0.2105
0.2500
0.2857
2074.6
3324.2
{
Water (1) + pentaethylene glycol dimethyl ether (2)}
0
0
0
0
0
0
0
997.05
1005.40
1009.29
1019.64
1034.39
1044.63
4181.3
4107.3
4071.1
3963.5
3770.1
3546.0
0.09091
0.1667
0.3333
0.5000
0.6667
1
0.3750
0.5455
0.7500
0.8571
0.9231
1
1048.94
1046.87
1037.64
1031.91
1028.40
1024.79
.00662
.00990
.01961
.03846
.06250
0.03846
0.05660
0.1071
0.1936
0.2857
2055.4
0.0
0.0
-0.2
-0.4
-0.6
-0.8
-1.0
-1.2
-1.4
-1.6
-1.8
-
-
-
-
0.5
1.0
1.5
2.0
(
a)
(b)
-2.5
0.0
0.2
0.4
0.6
0.8
1.0
0.0
0.2
0.4
0.6
0.8
1.0
x2
x2
E
FIGURE 1. Molar excess volumes, V , of mixtures of water (1) + C
1 2 1 1 1 1
E C (2) (a) and C E C (2) (b). The markers represent the experimental values:
(
d) this work, (m) reference [26], (.) reference [30], (·) reference [36], (n) reference [37] and (,) reference [38].
care therefore needs to be taken with regard to purifica-
tion and storage (see section 2). No densities appear to
have been reported previously for C E OH and C E C .
Given the generally good agreement among the liter-
ature (and present) densities of the pure compounds
(table 1), it is surprising to find that major discrepan-
cies exist in the densities (and related excess molar vol-
umes) of their mixtures with water. This is illustrated
in figure 1 for mono- and diethylene glycol dimethyl
ether, C E C and C E C . The present data clearly
1
5
1 5 1
1
1
1
1 2 1

518
S. Schr o¨ dle et al. / J. Chem. Thermodynamics 37 (2005) 513–522
0.0
TABLE 5
Densities, q, of {water (1) + diethylene glycol dimethyl ether (2)}
mixtures at 298.15 K
-
-
-
-
-
-
-
-
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
x
2
x
O,2
q
ꢁ3
kg Æ m
{Water (1) + diethylene glycol dimethyl ether (2)}
0
0
997.05
999.33
1002.88
1006.59
1003.91
998.18
993.70
986.48
981.26
976.53
971.16
965.91
956.27
947.49
942.93
938.53
0
0
.01495
.03497
0.04355
0.09806
0.2069
0.3002
0.3800
0.4287
0.5002
0.5502
0.5956
0.6476
0.7000
0.7984
0.8935
0.9453
1
0.08002
0
0
0
0
0
.1251
.1696
.2001
.2501
.2897
0
.0
0.2
0.4
0.6
0.8
1.0
xO, 2
0.3293
.3798
0
E
FIGURE 2. Molar excess volumes, V , of mixtures of water
0.4375
0.5689
0.7366
(
1) + C OH (j),
OH (m) and C
1
E
1
C
1
E
2
OH [43] (dotted line),
(n).
C
1
E
3
OH (d),
C
1
E
5
1
E
5
C
1
0
1
.8522
0.0
-
-
-
-
-
-
-
-
-
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
turbing that none of the more recent papers cite the
earlier work.
Figures 2 and 3 show the excess molar volumes de-
rived from the present density data for mixtures of
water with the compounds of interest, along with se-
E
lected literature values for related compounds. All V
values are negative across the entire composition range.
E
Their minima, V , are shifted to higher xO,2 values
with increasing degree of polymerisation, n, figure 4.
min
E
Simultaneously, V , decreases from (ꢁ1.0 to ꢁ1.4)
min
3
ꢁ1
cm Æ mol for the monomethyl ethers, whereas only
moderate changes were observed for the dimethyl
ethers. For the cyclic ethers, where only limited data
are available, there is some evidence that they behave
0.0
0.2
0.4
0.6
0.8
1.0
xO, 2
E
FIGURE 3. Molar excess volumes, V , of mixtures water (1) + 1,4-
dioxane (r), 12-crown-4 [36] (dashed line), C C (h), C
1
1
E
1
1 2 1
E C (,),
C
1
E
3
C
1
(s), C [30] (dotted line) and C E C (n).
1 5 1
1
E
4
C
1
2
4
0
.64
.60
support the older determinations by Wallace and
Mathews [26,37] rather than the more recent vibrating
tube study by Dethlefsen and Hvidt [30] and Marchetti
et al. [38], even though the former was obtained by the
less accurate [31] pycnometric method. The source of
this discrepancy is unknown but it is noted that Wal-
lace and Mathews distilled their ethers from sodium
immediately prior to use whereas Dethlefsen and Hvidt
used an untreated commercial sample with a stated
purity of only 0.98 mole fraction. The lithium alumin-
ium hydride method employed by Marchetti et al. [38]
for purification of ethers is known to be ineffective [1].
It is of concern that the less accurate data for C E C
-
1.0
1.1
-
0
-1.2
1.3
-1.4
-
0.56
0
.52
.48
-1.5
-1.6
-1.7
0
1
2
3
n
4
5
1
2
1
[
[
30] have been utilized in several other investigations
36,39]. Similar considerations apply to the mixtures
of water with C E C . In each case the present results
FIGURE 4. Shift of the minimum value of the molar excess volumes,
E
min
V
(h), and the minimum position, xO,2,min (s) of mixtures of water
1
1
1
(1) + homologous series of oligo(ethylene glycol) monomethyl ethers
(full symbols) and dimethyl ethers (open symbols) at 298.15 K.
(
tables 4 and 5) confirm the older values and it is dis-

S. Schr o¨ dle et al. / J. Chem. Thermodynamics 37 (2005) 513–522
519
more like the monomethyl ethers [36]. Apparently, the
4.2. Heat capacities
E
min
V
of C E OH and C E C as a function of n are
1 n 1 n 1
approximately parallel for n P 3. The comparison of
The isobaric specific heat capacities of the pure com-
pounds are given in table 1. Where comparison is possi-
ble, the present data are in good agreement (better than
5 J Æ K Æ kg ) with the means of the literature values,
with the partial exception of 1,4-dioxane which differs
slightly (<2%). Interestingly, the available literature heat
capacities have been classified as having an accuracy of
ethylene glycol monomethyl ether, C E OH, with its
dimethyl analogue, C E C , shows, that the terminal
1
1
1
n 1
ꢁ1
ꢁ1
–
ume contribution.
CH –OMe groups give rise to a strong negative vol-
2
The relative apparent molar volumes, DVU,2, show
distinct minima at xO,2 ꢄ 0.12 to 0.15, figure 5. It is
striking that at higher dilution the volume effects of
the C E C compounds almost coincide with those of
only 2% [10,17].
The molar excess heat capacities, C , calculated from
the present data are positive over the composition range
E
p
1
n
1
their C En + 2OH homologues, especially for the higher
1
members. From the extrapolation of the DVU,2, values
studied, so are the relative apparent quantities, DCp,U,2,
figure 6. As for the volumes (figure 5), aqueous solutions
ꢂ
of the molar volume at infinite dilution, DV , can be
U;2
obtained (table 6). Close agreement of our data with
the available literature values was found. The present re-
sults appear to be the first estimates for the pentaethyl-
ene glycol derivatives. Their estimated accuracy is
of the C E C compounds have relative apparent heat
1
n 1
capacities that are very close to their C En + 2OH homo-
1
logues at xO,2 ꢄ 0.15.
ꢂ
Heat capacities at infinite dilution in water, DC
were determined by extrapolation. The values so
,
p;U;2
ꢁ
6
3
ꢁ1
ꢀ
0.15 Æ 10 m Æ mol
.
0
3
50
00
-
-
-
-
2
4
6
8
3
250
2
1
1
00
50
00
-
-
-
-
-
-
10
12
14
16
18
20
5
0
0
0.0
0.0
0.2
0.4
0.6
0.8
1.0
0.1
0.2
0.3
0.4
0.5
xO, 2
xO,2
FIGURE 5. Relative apparent molar volumes, DVU,2, of mixtures of
water (1) + 1,4-dioxane (r), C OH (j), C (h), C OH (d),
(s), C OH (m) and C (n) at 298.15 K.
FIGURE 6. Relative apparent molar heat capacities, DCp,U,2, of
mixtures of water (1) + 1,4-dioxane (r), C OH (j), C (h),
OH (d), C (s), C OH (m) and C
1
E
1
1
E
1
C
1
1
E
3
1
E
1
1 1 1
E C
C
1
E
3
C
1
1
E
5
1
E
5
C
1
C
1
E
3
1
E
3
C
1
1
E
5
1
E
5
C
1
(n) at 298.15 K.
TABLE 6
Relative apparent molar volumes and heat capacities of various ethers at infinite dilution in water at 298.15 K
ꢁ
6
3
ꢁ1
p,U,2ꢂ
ꢁ1
ꢁ1
Compound
n
O,2
DVU,2ꢂ/(10 m Æ mol
)
DC
/(J Æ K Æ mol
)
This work
Literature
This work
71.9
Literature
70.9 [40]
1
,4-Dioxane
2
2
ꢁ4.73
ꢁ4.77 [40]
ꢁ
4.75 [41]
ꢁ4.14 [18]
4.08 [16]
C
1
E
1
OH
ꢁ4.04
115.9
116.3 [16]
112.0 [18]
ꢁ
115.3 [45]
C
C
C
1
1
1
E
E
E
1
3
3
C
1
2
4
4
ꢁ8.87
ꢁ8.17
ꢁ8.75 [34]
ꢁ8.02 [43]
ꢁ11.85 [35]
188.4
196.4
276.5
188.5 [45]
OH
C
1
ꢁ11.97
ꢁ
11.65 [44]
11.72 [42]
ꢁ
C
C
1
E
E
5
OH
6
6
ꢁ12.13
ꢁ16.33
285.4
365.3
1
5
C
1

520
S. Schr o¨ dle et al. / J. Chem. Thermodynamics 37 (2005) 513–522
obtained are in close agreement with the literature esti-
mates where these are available (table 6).
Similar linear correlations were found between chain
length and the relative apparent quantities of oligo(eth-
ylene glycol) monomethyl and dimethyl ethers at infinite
4
.3. Ethylene oxide group increments
dilution in water, figure 7(c) and (d). The average incre-
ꢂ
ments of DV
per E group found for the C E OH and
1 n
U;2
ꢂ
ꢁ6
The molar volumes, V , and isobaric molar heat
C E C compounds were (ꢁ2.04 and ꢁ2.07 Æ 10
)
i
1 n 1
3
ꢂ
ꢁ1
.
capacities, C , of the pure compounds increase virtually
linearly within the various homologous series, figure 7(a)
and (b). Thus, the increments V i in 10 m Æ mol
upon addition of one E (–CH CH O–) group are 39.2,
m Æ mol
For the corresponding DC
p;i
ꢂ
values, increments of
(42.4 and 44.2) J Æ mol Æ K were obtained. The value
of DC for C E C , which deviates from the linear
1 1 1
trend because of its two –OMe groups, was not used
p;U;2
ꢂ
ꢁ6
3
ꢁ1
ꢁ1
ꢁ1
ꢂ
2
2
p;U;2
ꢂ
3
8.9 and 37.5, whereas for C the corresponding values
p;i
ꢁ
1
ꢁ1
are 90.9, 89.2 and 87.3 J Æ K Æ mol (omitting 1,4-
dioxane) to, respectively, the mono-, di- (present work)
and cyclic-ethers [46].
for the fit.
With the increments given above it should be possible
to predict volumetric properties and heat capacities of
6
5
5
4
4
3
3
2
2
00
50
00
50
00
50
00
50
00
2
2
2
2
1
1
1
1
75
50
25
00
75
50
25
00
7
5
4
6
8
1
4
50
00
(
a)
(b)
-
-
-
3
3
2
2
1
1
5
50
00
50
00
50
00
0
-
-
-
-
10
12
14
16
(
c)
(d)
2
3
4
5
6
2
3
4
5
6
nO
nO
ꢂ
ꢂ
p;2
FIGURE 7. (a) Molar volumes, V , and (b) molar heat capacities, C , for various homologous series of pure liquid cyclic ethers (d), oligo(ethylene
2
ꢂ
glycol) monomethyl ethers (j) and dimethyl ethers (m). (c) Relative apparent molar volumes, DV , and (d) relative apparent heat capacities,
U;2
ꢂ
DC
values at 298.15 K.
O
, at infinite dilution in water, for the same ethers. n denotes the number of oxygen atoms per molecule of the organic compound [43,44,46]; all
p;U;2

S. Schr o¨ dle et al. / J. Chem. Thermodynamics 37 (2005) 513–522
521
other ethylene glycol ethers, including those of polydis-
perse industrial mixtures.
infinite dilution in water. For the pure compounds, the
molar volume and heat capacity increment is almost
independent of the molecular structure. The trend of
the thermodynamic properties of the fully hydrated
monomeric molecules is strongly dependent on their
molecular shape and flexibility.
5
. Discussion
It is always problematic to draw structural conclu-
The effects observed can be well explained by the
assumption of a strengthening of the water structure in
the vicinity of non-ionic solutes. This is clearly revealed
by the measured heat capacity data. Volume effects are
less easy to interpret, but show some distinct features
that support the general picture.
sions from thermodynamic data. However, in this case
the volume defects observed are consistent with a con-
traction of the water structure near the hydrophobic sur-
E
min
faces of the –OMe or –O–CH CH –O– groups. As V
is
2
2
located close to the 1:1 ratio of water molecules to
H-bond acceptor sites on the solutes, the formation of
a packed collection of water with oligo(ethylene glycol)
dimethyl ether molecules can be proposed. Because of
the highly flexible nature of the open-chain ethers, these
groupings are probably not strongly associated. Dielec-
tric relaxation studies indicate that, although the dynam-
ics of these systems slow down significantly, they are still
governed by rapidly fluctuating H-bond networks [47].
At high dilution in water, the effects of non-ionic
compounds on the thermodynamic properties of the
mixtures cannot be easily extracted from molar excess
quantities. In this region it is more helpful to discuss
the relative apparent molar values. Such quantities can
be interpreted as the molar volumes of the fully hy-
drated, non-associated solute.
Acknowledgements
The authors thank Dr. J u¨ rgen Seidel and Prof. Gerd
Wolf (Institut f u¨ r Physikalische Chemie, TU Bergakade-
mie Freiberg, Freiberg, Germany) for providing access
to their MicroDSC II calorimeter. S.S. appreciates the
support of the Verband der Chemischen Industrie e.V.
(VCI). We gratefully acknowledge experimental assis-
tance from Dr. Josef Duschl (Regensburg), Thomas
Rowland (Murdoch) and Ethan See (Murdoch).
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