Thermodynamic Study of the Solvation States of Acid and Base in a Protic Ionic Liquid, Ethylammonium Nitrate, and Its Aqueous Mixtures

Article abstract of DOI:10.1246/cl.2010.578

Ethylammonium nitrate (EAN) is a typical protic ionic liquid (PIL) known for a long time. In order to investigate acidbase reaction mechanisms in PIL, thermodynamic quantities of a reaction, which corresponds to autoprotolysis in amphoteric solvents, has been determined in neat EAN. Unlike H₃O ⁺ and OH^- in water, proton donor and acceptor species in EAN are both neutral; this makes acid-base reaction mechanisms in EAN distinct from that in water. EAN-water mixtures have also been studied.

Full text of DOI:10.1246/cl.2010.578

doi:10.1246/cl.2010.578
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Published on the web May 8, 2010
Thermodynamic Study of the Solvation States of Acid and Base in a Protic Ionic Liquid,
Ethylammonium Nitrate, and Its Aqueous Mixtures
1
2
2
2
Ryo Kanzaki,* Xuedan Song, Yasuhiro Umebayashi, and Shin-ichi Ishiguro
Graduate School of Science and Engineering, Kagoshima University, Korimoto, Kagoshima 890-0065
Department of Chemistry, Faculty of Sciences, Kyushu University, Hakozaki, Higashi-ku, Fukuoka 812-8581
1
2
(
Received January 26, 2010; CL-100077; E-mail: kanzaki@sci.kagoshima-u.ac.jp)
Ethylammonium nitrate (EAN) is a typical protic ionic
xEAN = 1.0
liquid (PIL) known for a long time. In order to investigate acid­
base reaction mechanisms in PIL, thermodynamic quantities of a
reaction, which corresponds to autoprotolysis in amphoteric
80
60
40
0
.9
∆
H°
0
.5
+
0.25
solvents, has been determined in neat EAN. Unlike H3O and
∆G°
¹
OH in water, proton donor and acceptor species in EAN are
both neutral; this makes acid­base reaction mechanisms in EAN
distinct from that in water. EAN­water mixtures have also been
studied.
T∆S°
2
0
0
Recently, protic ionic liquids (PILs) have received increas-
-
20
(a)
ing attention as a new protic solvent.1,2 As the composing ions of
(b)
1.0
PILs have both a proton accepting and a donating ability PILs
have characteristics of an amphoteric solvent. Not only as novel
acid­base reaction media, PILs are expected to be candidates for
fuel cell electrolytes due to excess electrochemically available
0
.0 0.5 1.0
0.0
0.5
EAN
n /n
x
tit ini
Figure 1. Calorimetric titration curves of neutralization in EAN
and EAN­water mixture (a), and obtained ¦G°, ¦H°, and T¦S° of
autoprotolysis plotted against xEAN (b).
2
protons. In amphoteric solvents, autoprotolysis is a peculiar
reaction, through which a proton is transferred between two
solvent species to generate a conjugate acid and base of the
¹
+
solvent. If we consider that A and HB , that are products of a
proton transfer from Brønsted acid (HA) to base (B), are the
solvent species in PILs, the following equilibrium is equivalent
to autoprotolysis.
are completely ionized in EAN to generate equimolar HNO3
and C2H5NH2, respectively. Calorimetric neutralization mea-
surements were carried out at 298 K by means of an online-
controlled titration and data acquisition system developed
3
in our laboratory. A basic solution (30 cm ) containing ca.
ꢀ
þ
A þ HB ꢀ HA þ B
ð1Þ
¹
3
1
0 mmol dm
C H NH was titrated with aliquots of ca.
3 7 2
¹
3
As HA and B practically act as the proton donor and acceptor for
acid­base reaction in PILs, the acidity of HA and the basicity of
B are the relevant properties of PIL as an acid­base reaction
100 mmol dm acidic HTfO solution. The heats of reaction at
each titration point were determined with errors less than 0.02 J.
Calorimetric titration curves in neat EAN and EAN­water
mixtures of xEAN (the molar fraction of EAN) = 0.25, 0.5, and
0.9 are shown in Figure 1a. In this figure, Q/nini are plotted
against ntit/nini where Q is the total heat evolution to each
titration point, and nini and ntit are the moles of C H NH in the
initial solution and of the total HTfO added, respectively. As
shown, Q increases linearly with increasing ntit and then
becomes constant at the equivalent point (n /n = 1). If eq 1
media. From this point of view, ¦pK is a plausible indicator to
a
specify PILs defined as a gap of pKas, or of proton-free energy
+
3,4
level, between HA and HB in an aqueous solution. However,
acid­base reaction mechanisms in neat PILs cannot be simply
extrapolated from those in an aqueous phase due to absolutely
different solvent environment. In the present study, we have
investigated the solvation state of HA and B by means of direct
thermodynamic measurements in neat PIL, ethylammonium
3
7
2
tit ini
is taken into account, the following reaction corresponds to
autoprotolysis in EAN:
5
,6
nitrate (EAN), which is a typical PIL known for a long time,
and EAN­aqueous mixtures. Although an anhydrous condition
is preferable for electrochemical applications, EAN­water
mixture is also useful as a promising reaction medium for
C2H5NH3 þ NO3ꢀ ! C2H5NH2 þ HNO3
þ
ð2Þ
The reaction enthalpy of eq 2 is given by ¦H2° = ∂(Q/nini)/
∂(ntit/nini) because reaction (2) is the backward reaction of a
neutralization in EAN. The obtained ¦H ° is 82.7 kJ mol
(standard deviation is 0.6 kJ mol ), a little larger than that
estimated in terms of van’t Hoff plot. We have already
demonstrated that the product of the concentrations,
[HNO3][C2H5NH2], gives a constant KS, similarly to the
autoprotolysis constant of amphoteric solvents. Thus the
reaction free energy and entropy of eq 2 are obtained by
¦G2° = ¹RT ln KS and ¦S2° = (¦H2° ¹ ¦G2°)/T, respectively.
7
biomolecules and for understanding PILs as an extensively
concentrated electrolyte solution.8
¹1
2
¹
1
EAN was prepared from aqueous solutions of ethylamine
1
0
and nitric acid. Details about the preparation procedure have
9
been mentioned elsewhere. The concentration of excess HNO
3
in the final product was separately determined by potentiometric
titration. Doubly distilled water was used to prepare EAN­water
mixture. Trifluoromethanesulfonic acid (HTfO) and propyl-
amine (C3H7NH2) were used after distillation. These compounds
9
Chem. Lett. 2010, 39, 578­579
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

5
79
Table 1. Reaction thermodynamic quantities at 298.15 Ka
decrease of T¦S° than ¦H° with decreasing xEAN from 1.0 to
0.9 consequently results in an apparently larger ¦G° or pKS in
xEAN = 0.9 mixture than that in neat EAN.
¹1
¦H°/kJ mol¹1
T¦S°/kJ mol¹1
¦
G°/kJ mol
C2H5NH3+ + NO3 ¼ C2H5NH2 + HNO3 in EAN
¹
Both ¦H ° and ¦S ° decrease almost linearly with
3
3
54.9(3)
83(2)
28(2)
decreasing xEAN in parallel among the midterm EAN content.
As a result, pKS is kept almost constant through a wide xEAN
range, unlike the EAN-rich region. In addition, reaction (3)
continuously stands corresponding to autoprotolysis. These facts
make EAN­water mixture a good solvent for acid­base reactions
with stable acid­base properties in a wide EAN­water compo-
sition. Considering that EAN is known as a hydrogen-bonding
solvent showing water-like properties and is miscible with water
+
¹
2
H2O ¼ H3O + OH in water11
79.9
55.9
­24.1
C2H5NH3+ + NO3 ¼ C2H5NH2 + HNO3 in water11
¹
64
67 3
aThe values in parentheses refer to three standard deviations.
12
in a regular fashion, EAN and water are possibly merged by
mutually exchanging hydrogen-bonding pairs without drastic
change in the solvent structure. The linear changes of ¦H3° and
¦S3° imply the successive transition of the fraction of the
Here, it should be pointed out that KS generally denotes
autoprotolysis constant in an amphoteric solvent, although
reaction (2) does not exactly belong to protolysis.
+
+
solvating species on C H NH , H O, and H O depending on
2
5
3
2
3
Reaction thermodynamic quantities of several related
reactions are summarized in Table 1. The large and positive
xEAN. However, we have no direct evidence. The solvent­solvent
interaction and the solvation manner in EAN­water mixture
should be clarified by further investigations.
¦
G ° and ¦H ° indicate the similarity of reaction (2) to
2 2
autoprotolysis in water. That is, C2H5NH2 and HNO3 are minor
+
species in EAN under neutral conditions, as well as H3O and
This work has been financially supported by Grant-in-Aids
for Scientific Research Nos. 20350037, 20031024, and
21118516 from the Ministry of Education, Culture, Sports,
Science and Technology of Japan.
¹
OH in pH 7 water, and this is mainly contributed by a large and
positive reaction enthalpy. In contrast, ¦S2° is large and positive
while that of autoprotolysis in water is large and negative.
+
¹
Through reaction (2) in EAN, C2H5NH3 and NO3 are
converted to C H NH and HNO . In other words, ions are
2
5
2
3
References and Notes
uncharged in an ionic atmosphere. As a result, the solvation is
weakened and positive ¦S2° is given. On the other hand, ions are
generated by autoprotolysis in water. Ions are tightly hydrated in
aqueous phase not only by the neighboring water molecules but
also those in the further hydration shell. Consequently, the
reaction entropy of autoprotolysis has an opposite sign between
in EAN and in water. The effect of entropic term (T¦S°) on ¦G°
1
a) C. A. Angell, N. Byrne, J.-P. Belieres, Acc. Chem. Res. 2007, 40, 1228.
b) D. R. MacFarlane, J. M. Pringle, K. M. Johansson, S. A. Forsyth, M.
Forsyth, Chem. Commun. 2006, 1905. c) B. Nuthakki, T. L. Greaves, I.
Krodkiewska, A. Weerawardena, M. I. Burgar, R. J. Mulder, C. J.
Drummond, Aust. J. Chem. 2007, 60, 21. d) B. Nuthakki, T. L. Greaves,
I. Krodkiewska, A. Weerawardena, M. I. Burgar, R. J. Mulder, C. J.
Drummond, Aust. J. Chem. 2007, 60, 21.
a) M. A. B. H. Susan, A. Noda, S. Mitsushima, M. Watanabe, Chem.
Commun. 2003, 938. b) A. Noda, M. A. B. H. Susan, K. Kudo, S.
Mitsushima, K. Hayamizu, M. Watanabe, J. Phys. Chem. B 2003, 107,
2
¹
1
is raised to larger than 50 kJ mol at 298 K. Note that both ¦H2°
4
4
024. c) J.-P. Belieres, D. Gervasio, C. A. Angell, Chem. Commun. 2006,
799. d) H. Nakamoto, A. Noda, K. Hayamizu, S. Hayashi, H.
and T¦S ° are significantly larger than those for the equivalent
2
reaction in aqueous phase. Hence, it is emphasized that the acid­
base properties of PILs cannot be predicted by extrapolating
those for the constituent ions in aqueous phase without taking
the solvation structure into account.
Hamaguchi, M. Watanabe, J. Phys. Chem. C 2007, 111, 1541.
M. Yoshizawa, W. Xu, C. A. Angell, J. Am. Chem. Soc. 2003, 125, 15411.
J.-P. Belieres, C. A. Angell, J. Phys. Chem. B 2007, 111, 4926.
a) S. Sugden, H. Wilkins, J. Chem. Soc. 1929, 1291. b) P. Walden, Bull.
Acad. Imp. Sci. St.-Petersbourg 1914, 8, 405.
a) R. G. Horn, D. F. Evans, B. W. Ninham, J. Phys. Chem. 1988, 92, 3531.
b) L. Crowhurst, P. R. Mawdsley, J. M. Perez-Arlandis, P. A. Salter, T.
Welton, Phys. Chem. Chem. Phys. 2003, 5, 2790. c) S. Bouguerra, I. B.
Malham, P. Letellier, A. Mayaffre, M. Turmine, J. Chem. Thermodyn.
3
4
5
As mentioned in the previous paper,9 H3O acts practically
as a proton donor in an EAN­water mixture. Thus, the following
reaction corresponds to autoprotolysis instead of eq 2 in EAN.
a
+
6
C2H5NH3 þ H2O ! C2H5NH2 þ H3Oþ
þ
ð3Þ
2008, 40, 146. d) Y. Umebayashi, W.-L. Chung, T. Mitsugi, S. Fukuda, M.
Takeuchi, K. Fujii, T. Takamuku, R. Kanzaki, S. Ishiguro, J. Comput.
Chem. Jpn. 2008, 7, 125.
D. K. Magnuson, J. W. Bodley, D. F. Evans, J. Solution Chem. 1984, 13,
Figure 1b shows ¦G°, ¦H°, and T¦S° of autoprotolysis of
xEAN. Note that the actual formula ascribing autoprotolysis
changes depending on xEAN. Interestingly, pKS for eq 3 (or
7
8
583.
a) D. F. Evans, S.-H. Chen, G. W. Schriver, E. M. Arnett, J. Am. Chem.
Soc. 1981, 103, 481. b) D. F. Evans, A. Yamauchi, R. Roman, E. Z.
Casassa, J. Colloid Interface Sci. 1982, 88, 89. c) D. Mirejovsky, E. M.
Arnett, J. Am. Chem. Soc. 1983, 105, 1112.
¦
G °) in the x
= 0.9 mixture is larger than that for eq 2 in
EAN
3
+
neat EAN; this means H3O has an apparently stronger acidity
than HNO3. As HNO3 is a stronger acid than H3O in an
+
9
a) R. Kanzaki, K. Uchida, X. Song, Y. Umebayashi, S. Ishiguro, Anal. Sci.
aqueous phase, the acidity order is reversed. This can be
explained by a solvation effect on the proton donor species,
2008, 24, 1347. b) R. Kanzaki, K. Uchida, S. Hara, Y. Umebayashi, S.
Ishiguro, S. Nomura, Chem. Lett. 2007, 36, 684.
+
10 J. A. Bautista-Martinez, L. Tang, J.-P. Belieres, R. Zeller, C. A. Angell, C.
Friesen, J. Phys. Chem. C 2009, 113, 12586.
11
H3O and HNO3. That is, positive ¦H3° and ¦S3° at xEAN = 0.9
indicate the solvation structure is broken through reaction (3).
However, smaller ¦H ° and ¦S ° than ¦H ° and ¦S ° indicate
J. J. Christensen, L. D. Hansen, R. M. Izatt, Handbook of Proton
Ionization Heats and Related Thermodynamic Quantities, Wiley-Inter-
science, New York, 1976, p. 79 and p. 126.
3
3
2
2
that the solvation of H2O is less extensively weakened when
+
¹
12 a) M. Allen, D. F. Evans, R. Lumry, J. Solution Chem. 1985, 14, 549. b)
M. Biquard, P. Letellier, M. Fromon, Can. J. Chem. 1985, 63, 3587. c) G.
Perron, A. Hardy, J.-C. Justice, J. E. Desnoyers, J. Solution Chem. 1993,
22, 1159.
converted to H3O than NO3 converted to HNO3 because
+
¹
H O is still solvated by negatively charged solvents, NO3
.
3
¦H° and T¦S° are almost canceled out, but a slightly larger
Chem. Lett. 2010, 39, 578­579
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/