Biodegradable naphthenic acid ionic liquids: Synthesis, characterization, and quantitative structure-biodegradation relationship

Article abstract of DOI:10.1002/chem.200800620

It has been confirmed that commonly used ionic liquids are not easily biodegradable. When ultimately disposed of or accidentally released, they would accumulate in the environment, which strongly restricts largescale industrial applications of ionic liquids. Herein, ten biodegradable ionic liquids were prepared by a single, one-pot neutralization of choline and surrogate naphthenic acids. The structures of these naphthenic acid ionic liquids (NAILs) were characterized and confirmed by ¹H and ¹³C NMR spectroscopy, IR spectroscopy, and elemental analysis, and their physical properties, such as densities, viscosities, conductivities, melting points (T_m), glass transition points (T_g), and the onset temperatures of decomposition (T_d), were determined. More importantly, studies showed that these NAILs would be rapidly and completely biodegraded in aquatic environments under aerobic conditions, which would make them attractive candidates to be utilized in industrial processes. To explore the underlying mechanism involved in the NAIL biodegradation reaction and seek prediction of their biodegradability under environmental conditions, four molecular descriptors were chosen: the logarithm of the n-octanol/ water partition coefficient (log P), van der Waals volume (K_vdW). energies of the highest occupied molecular orbital (E_HOMO), and energies of the lowest unoccupied molecular orbital (E_LUMO)-Through multiple linear regression, a general and qualified model including the biodegradation percentage for NAILs after the 28-day OECD 301D test (%B₂₈) and molecular descriptors was developed. Regression analysis showed that the model was statistically significant at the 99 % confidence interval, thus indicating that the %B₂₈ of NAILs could be explained well by the quantum chemical descriptor E_HOMO, which might give some important clues in the discovery of biodegradable ionic liquids of other kinds.

Full text of DOI:10.1002/chem.200800620

DOI: 10.1002/chem.200800620
Biodegradable Naphthenic Acid Ionic Liquids: Synthesis, Characterization,
and Quantitative Structure–Biodegradation Relationship
Yinghao Yu,[a, b] Xingmei Lu,[a] Qing Zhou,[a] Kun Dong,[a] Hongwei Yao,[a, b] and
Suojiang Zhang*[a]
Abstract: It has been confirmed that
commonly used ionic liquids are not
easily biodegradable. When ultimately
disposed of or accidentally released,
they would accumulate in the environ-
ment, which strongly restricts large-
scale industrial applications of ionic liq-
uids. Herein, ten biodegradable ionic
liquids were prepared by a single, one-
pot neutralization of choline and surro-
gate naphthenic acids. The structures
of these naphthenic acid ionic liquids
(NAILs) were characterized and con-
termined. More importantly, studies
showed that these NAILs would be
rapidly and completely biodegraded in
aquatic environments under aerobic
conditions, which would make them at-
tractive candidates to be utilized in in-
dustrial processes. To explore the un-
derlying mechanism involved in the
NAIL biodegradation reaction and
seek prediction of their biodegradabil-
ity under environmental conditions,
four molecular descriptors were
chosen: the logarithm of the n-octanol/
water partition coefficient (logP), van
der Waals volume (VvdW), energies of
the highest occupied molecular orbital
(EHOMO), and energies of the lowest un-
occupied molecular orbital (ELUMO).
Through multiple linear regression, a
general and qualified model including
the biodegradation percentage for
NAILs after the 28-day OECD 301D
test (%B28) and molecular descriptors
was developed. Regression analysis
showed that the model was statistically
significant at the 99% confidence inter-
val, thus indicating that the %B28 of
NAILs could be explained well by the
1
firmed by H and 13C NMR spectrosco-
quantum chemical descriptor EHOMO,
py, IR spectroscopy, and elemental
analysis, and their physical properties,
such as densities, viscosities, conductivi-
ties, melting points (Tm), glass transi-
tion points (Tg), and the onset temper-
atures of decomposition (Td), were de-
which might give some important clues
in the discovery of biodegradable ionic
liquids of other kinds.
Keywords: biodegradability · green
chemistry · ionic liquids · molecular
descriptors · reaction mechanisms
Introduction
and global climate change.[1] RTILs are showing more and
more promising perspectives in the diverse fields of synthe-
sis,[2] catalysis or biocatalysis,[3–7] materials science,[8] electro-
chemistry,[9] and separation technology[10–13] at the laboratory
level and even on the industrial scale.[14,15] With some
Due to their unique physicochemical characteristics, room-
temperature ionic liquids (RTILs) were designed as greener
solvents to replace conventional volatile solvents that were
believed to result in photochemical smog, ozone depletion,
ACHUTNGRENaNUG chieveACHTUNGTNERmNUGN ent of industrial processes and commercial applica-
tions, certain factors that used to be neglected in the whole
life cycle[16] of RTILs, such as ultimate disposal and environ-
mental risk assessment, have to be taken into account. One
of the most important factors in risk assessment is the bio-
degradability of RTILs, because biodegradation is a main
process for removal of chemicals from the environment.[17]
However, several studies[1,18,19] have shown that convention-
al or “first generation” RTILs are not biodegradable, which
means that when accidentally released, these ionic liquids
will stay intact and accumulate in the environment. This is
obviously against the tenth of the 12 Green Chemistry prin-
[a] Dr. Y. Yu, Dr. X. Lu, Dr. Q. Zhou, K. Dong, H. Yao,
Prof. Dr. S. Zhang
Key Laboratory of Green Process and Engineering
Institute of Process Engineering, Chinese Academy of Sciences
Beijing 100190 (PR China)
Fax : (+86)10-8262-7080
E-mail: sjzhang@home.ipe.ac.cn
[b] Dr. Y. Yu, H. Yao
Graduate University of Chinese Academy of Sciences
Beijing 100149 (PR China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200800620.
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FULL PAPER
ciples[20] and will strongly restrict the large-scale industrial
applications of RTILs.
Table 1. Structures of anions and the abbreviations used for the synthe-
sized NAILs.
Fortunately, according to several reviews,[16,21,22] we might
find a way to improve the biodegradability of RTILs
through rational molecular design,[23] structural adjustment,
and natural-sources screening. To produce biodegradable
ionic liquids, Gathergood et al.[24–27] attached an ether side
chain to the cationic core, Stolte et al.[28] introduced an octyl
group and its hydroxylated and carboxylated derivatives
into the side chain, and Harjani et al.[29] chose nicotinic acid,
an inexpensive natural product, as starting material, which
gives us a helpful hint to carry on our research.
Naphthenic acids, usually found in oil, were originally de-
fined as a complex mixture of monocarboxylic acids with
single or multiple rings, which comprise five or six carbon
atoms.[30–33] Previous studies[34–36] had shown that naphthenic
acids, especially low-molecular-weight surrogate naphthenic
acids, could be almost completely biodegraded to CO2 or
CH4 just by natural aging or by microbial activity in labora-
tory cultures.
Anions
Ionic liquids
Abbreviations
choline cyclopentane
carboxylate
[Ch]
[Ch]
ACHTUNGTRENNUNG[CPC]
choline cyclopentyl
acetate
ACHTUNGTRENNUNG[CPA]
choline cyclohexane
carboxylate
[Ch]
[Ch]
ACHTUNGTRENNUNG[CHC]
choline 3-cyclohexyl
propionate
ACHTUNGTRENNUNG[CHP]
choline benzoate
choline salicylate
[Ch][Be]
[Ch][Sa]
Herein, we chose ten surrogate naphthenic acids as anions
and biodegradable choline[37,38] as cation to prepare naph-
thenic acid ionic liquids (NAILs). The structures of the
anions and the abbreviations of the synthesized NAILs are
presented in Table 1. In view of the fact that halogen con-
taminations introduced by metathesis routes would dramati-
cally affect the physicochemical properties of the ionic liq-
uids, a neutralization method[39–41] was adopted to prepare
the desired ionic liquids from a single, one-pot reaction (see
Scheme 1). The biodegradability of the obtained NAILs was
studied by means of the closed-bottle test (OECD 301D).[42]
Furthermore, based on four molecular descriptors, a biodeg-
radation model for NAILs was established to identify the
rate-limiting step and explore the underlying mechanism in-
volved in the NAILs biodegradation reaction.
choline 2-naphthoxya-
cetate
[Ch]ACHTUNGTRENNUNG[NOA]
choline anthracene-9-
carboxylate
[Ch][AC]
choline deoxycholate
[Ch]ACHTUNGTRENNUNG[DOC]
Results and Discussion
Density, viscosity, and conductivity: The densities, viscosi-
ties, and conductivities of six NAILs that are liquid at room
temperature are presented in Table 2. Their densities range
from 1.03 to 1.18 gcmꢀ3 at 408C. It can be seen that: 1) 1[Ch]
[CPC] >1[Ch][CPA] and 1[Ch][CHC] >1[Ch][CHP]: this may be attributed
to the incorporation of longer alkyl chains into the rings,
which results in the increasing disruption of crystal effective
packing; 2) 1[Ch][Sa] >1[Ch][Be]: introduction of the OH group
into the ring can generate hydrogen bonds between the
anions, then increase the density of [Ch][Sa]; and 3) ionic
liquids with aromatic naphthenic acids as anions were much
denser than their aliphatic naphthenic acid counterparts,
due to their planar phenyl groups which resulted in stacking
effects and interactions between them.
choline lithocholate
[Ch][LC]
Scheme 1. Synthesis of NAILs.
The viscosity of an ionic liquid appears to be governed es-
sentially by van der Waals interactions and hydrogen
bonds.[43] Table 2 shows that increased van der Waals inter-
actions between the alkyl chains contribute to higher viscos-
ities of both [Ch]
ACHUTNGTRENNUG[CPA] and [Ch]AHCTUNGTREN[NUNG CHP] compared with
those of their homologues without alkyl chains. Similarly,
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S. Zhang et al.
Table 2. Some properties of six room-temperature NAILs.
NAILs are listed in Table 3,
and Figure 2 shows their differ-
ential scanning calorimetry
(DSC) traces. In most cases, the
glass transition temperatures
are in the region of ꢀ85 to
ꢀ508C for a wide range of
anions. The results indicate that
an increase of ion size and in-
corporation of an alkyl chain
[e]
ꢀ1[f]
Ionic liquids
1[a] [gcmꢀ3
]
1
[b] [gcmꢀ3
]
h[c] [mPas]
s[d] [Smꢀ1
]
Mw
Rꢀ
[ꢁ109 mꢀ1
]
[Ch]
[Ch]
[Ch]
[Ch]
[Ch][Be]
[Ch][Sa]
N
1.087904
1.068976
1.064432
1.046285
1.138713
1.174111
1.081890
1.062376
1.061194
1.038349
1.131199
1.168177
172.7103
249.7639
292.3849
676.9422
1096.7305
1192.3123
0.0958
0.0653
0.0497
0.0117
0.0139
0.0101
217
231
231
259
225
241
2.192
1.990
1.999
1.680
2.269
2.082
[a] Density at 258C. [b] Density at 408C. [c] Viscosity at 408C. [d] Conductivity at 408C. [e] Molar mass. [f] In-
verse of the anionic radius. In the case of choline it is a cation (R+ꢀ1): inverse of cholineꢂs radius; R+
=
ꢀ1
ꢀ1
1.955ꢁ109 mꢀ1. Rꢀ and R+ꢀ1 were calculated with the program Gaussian 03.
hydrogen bonding between the anions increases the viscosity
of [Ch][Sa] over [Ch][Be]. Generally, for the same cation,
the smaller the size of the anion, the lower the viscosity.
However, strong stacking effects and interactions between
aromatic rings dramatically increase the viscosities of
NAILs with an aromatic carboxylate as anion compared
with their aliphatic-ring counterparts, as is also the case for
the densities.
The conductivity is usually influenced by viscosity, density,
molecular weight, and ion size. Among these, viscosity plays
a dominating role; normally, the larger the viscosity, the
lower the conductivity. Besides the obvious influence of vis-
cosity, the effects of density and ion size must be stressed.
[Ch][Be], for instance, is more conducting than [Ch]ACTHUNTGRNEUNG[CHP],
Figure 1. Conductivities calculated from Equation (1) (scalcd) versus those
measured in the experiment (sexp).
in spite of its much higher viscosity. This might combine the
advantages of a larger density and a smaller anion size. Ad-
ditionally, the negative charge is delocalized more widely
over aromatic rings than aliphatic rings. The aromatic-ring
NAILs will therefore dissociate more easily than the corre-
sponding aliphatic-ring NAILs, which is another contributor
to higher conductivity.[44]
into the ring will result in a gradual increase of Tg. This con-
firms the increasing importance of van der Waals forces
over Coulomb attractions because the latter decrease with
increasing size of the ions.[51–53] It is expected at first that the
Tg of [Ch][Be] and [Ch][Sa] should increase dramatically,
presumably as a consequence of the phenyl ring component
in this anion structure. Surprisingly, the opposite is observed,
which could probably be attributed to larger electron deloc-
alization.[49]
Abbott et al.[45–47] used the Stokes–Einstein relation to
derive an expression for the conductivity of ionic liquids. In
this study, the conductivities s of NAILs are also calculated
by using Equation (1):
z2Fe 1
6ph Mw
ꢀ1
ꢀ1
ð1Þ
s ¼
ðRþ þ Rꢀ
Þ
where F is the Faraday constant, e is the electronic charge,
ꢀ1
and z is the charge on the ion; 1, h, Mw, R+ꢀ1, and Rꢀ are
all listed in Table 2. The relationship between calculated and
experimental conductivities is illustrated in Figure 1. An ex-
cellent correlation is observed, which means the conductivi-
ty of NAILs can be predicted with Equation (1) very well.
Melting and glass transition temperatures: Several stud-
ies[48–50] assumed that the introduction of carboxylic acid
groups to ionic liquids would readily produce strong hydro-
gen bonds, and thus result in high melting points (Tm) or
glass transition temperatures (Tg), so it was not appropriate
for carboxylic acid groups to be incorporated into the ions.
The fact is that all ten NAILs are obtained with the Tm or
Tg below 1008C, and six of them (shown in Table 2) are
liquid at room temperature. The Tm and Tg values of the
Figure 2. DSC curves for NAILs.
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Biodegradable Ionic Liquids
FULL PAPER
Table 3. Thermal properties of ten NAILs.
Ionic liquids
[Ch]
[CPC]
[Ch]
E
[Ch]
[CHC]
[Ch]
E
[Ch][Be]
[Ch][Sa]
[Ch]
[NOA]
[Ch][AC]
[Ch]
–
70.72
232.45
G
[Ch][LC]
Tg [8C]
Tm [8C]
Td [8C]
ꢀ84.48
–
178.76
ꢀ81.31
–
180.63
ꢀ67.67
11.84
177.64
ꢀ66.21
–
175.73
ꢀ63.26
–
203.15
ꢀ67.76
–
230.43
ꢀ53.07
–
210.20
–
–
86.22
201.35
88.45
218.50
Thermal stability: The onset points of decomposition (Td)
for the ten NAILs are given in Table 3, and Figure 3 shows
their thermogravimetric analysis traces. The four single-ali-
phatic-ring NAILs show only very minor weight loss until
above 1758C; from that point, a rapid and complete decom-
position is observed. As for the aromatic-ring NAILs, that
point is above 2008C, the highest being 2308C for [Ch][Sa].
biodegradation percentage after 28 days of incubation
(%B28) is higher than 60% is referred to as readily biode-
gradable; therefore, it should be assumed that such a chemi-
cal will be rapidly and completely biodegraded in aquatic
environments under aerobic conditions.[18,42] The biodegra-
dation data of all the test substances are illustrated in
Figure 4, which shows that almost all the NAILs pass the
closed-bottle test (%B28 >60%) during a 28-day incubation
The thermal decomposition traces of [Ch]ACTHNUTRGENU[GN DOC] and [Ch]
[LC] are a little different from those of the ionic liquids
mentioned above. These compounds decompose initially at
the first point (2328C for [Ch]ACTHUNTGRNEUNG[DOC] and 2188C for [Ch]
period, except for [Ch]ACTHNGUTERN[UNG NOA] and [Ch][AC]. Notably, as
the number of rings increases, biodegradability generally de-
creases; yet the two- and three-ring NAILs failed in the
closed-bottle test, but both of the four-ring NAILs passed.
Similar results[55] were obtained from naphthalene and phen-
anthrene under the MITI test, another aerobic biodegrada-
tion test method adopted by the Japanese Ministry of Inter-
national Trade and Industry. The negative results of [Ch]-
[LC]); after that, they have parallel humps in the decompo-
sition traces, perhaps indicating a similar decomposition
mechanism and products. Yoshizawa-Fujita et al.[54] found
that the thermal stability of an ionic liquid was primarily re-
lated to the nature of the anion, including the nucleophilici-
ty and Lewis basicity. They found that a stronger Lewis ba-
sicity of the anion tended to induce thermal decomposition
of RTILs at a lower temperature compared with typical
ionic liquids. In this study, the electron is delocalized more
widely over aromatic rings than aliphatic rings, so the ali-
phatic-ring anions have stronger Lewis basicity, which leads
to the lower onset temperatures of decomposition.
AHCTUNGTREG[NNNU NOA] and [Ch][AC] may be attributed to their toxicity to
microorganisms because only these two had lag phases (0–
14 days) in their incubation curves, although the test sub-
stance concentration is only about 2 mgLꢀ1. Another reason
might be the poor solubility of [Ch]ACTHNUTRGNEUNG[NOA] and [Ch][AC] in
the test medium, which results in a decrease of the actual
uptake of dissolved molecular oxygen.[19]
Under the same conditions, [BMIM]
[PF6] show a %B28 value lower than 25%. The %B28 of
[BMIM][HSO4] is even negative, which is probably due to
its strong inhibition of endogenous respiration of the micro-
organisms. The ultimate biodegradability of [BMIM][CHC],
[BMIM][Sa], and [BMIM][NOA] is much lower than that of
their [Ch] counterparts but significantly higher than that of
[BMIM][BF4], [BMIM][PF6], and [BMIM][HSO4]. To some
ACHTUNGTREN[NUNG BF4] and [BMIM]-
Ultimate biodegradability: The biodegradability of the ten
NAILs was studied by means of the closed-bottle test
(OECD 301D).[42] In this test, sodium dodecyl sulfate (SDS)
was used as reference compound. Other ionic liquids were
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
[BMIM]
[BMIM][PF6], [BMIM]
[Sa], and [BMIM][NOA]; some common volatile organic
A
(BMIM:
1-butyl-3-methylimidazolium),
ACHTUNGTRENNUNG
A
R
ACHTUNGTRENNUNG
U
A
N
ACHTUNGTRENNUNG
solvents (toluene, methylene chloride, acetonitrile, and etha-
nol) were tested for comparison. A test substance whose
extent, it might help us to conclude that the synthesized
ionic liquids would be biodegradable if the anions and cat-
ions are both easily biodegraded. Three out of four chosen
organic solvents pass the closed-bottle test. Except for etha-
nol, whose %B28 value reaches 95.1%, the biodegradability
of eight NAILs which pass the test is equivalent to or even
higher than that of the chosen common volatile solvents, so
the NAILs can be competent as greener solvents to replace
conventional volatile ones.
To identify the rate-limiting step and explore the underly-
ing mechanism involved in the NAIL biodegradation reac-
tion, four molecular descriptors were chosen and calculated;
the results, together with the %B28 values of the NAILs, are
listed in Table 4. Through stepwise regression analysis car-
ried out by SPSS software, the regression equation for %B28
can be expressed as Equation (2):
%B28 ¼ 119:294 þ 37:821EHOMO
ð2Þ
Figure 3. Thermogravimetric analysis traces for NAILs.
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S. Zhang et al.
Figure 4. Biodegradation of all test substances; the %B28 values are given on the corresponding columns.
conclude that the rate-limiting step of aerobic biodegrada-
tion of NAILs may be the attack on the ions by the electro-
philic oxygen of a monoxygenase or dioxygenase, because
EHOMO usually directs the nucleophilic reactivity of a com-
pound in the frontier molecular orbital theory. So the higher
the EHOMO, the easier it is for the compound to be attacked
by electrophilic oxygen, and the larger the %B value. Our
study and some others[56,57] on quantitative structure–biodeg-
radation relationships (QSBRs) confirmed that.
Table 4. %B28 values and some descriptors of ten NAILs.
Ionic liquids
%B28
ELUMO [eV]
EHOMO [eV]
logP
VvdW [ꢃ3]
[Ch]
[Ch]
[Ch]
[Ch]
[Ch][Be]
[Ch][Sa]
[Ch]
[Ch][AC]
[Ch][DOC]
[Ch][LC]
N
69.4
68.9
69.1
72.5
71.4
60.1
41.9
49.4
78.3
83.2
2.88
2.64
2.64
2.20
2.67
2.45
1.50
1.17
0.95
1.12
ꢀ1.20
ꢀ1.22
ꢀ1.22
ꢀ1.22
ꢀ1.61
ꢀ1.47
ꢀ1.90
ꢀ1.85
ꢀ1.12
ꢀ1.17
0.59
1.33
1.33
1.99
1.56
1.50
1.96
3.30
2.89
3.70
110.41
126.89
126.86
159.68
110.01
119.01
178.56
196.08
390.75
381.91
We can also explore the underlying mechanism and pre-
dict the most probable biodegradation procedure with the
frontier molecular orbital theory. Take [Ch]ACHTNUTRGNE[NUG CPA], for ex-
ample: complete dissociation is presumed due to its low
starting concentration (about 2 mgLꢀ1), so the interactions
between the cation and anion are not considered. According
to its structure, generally, the cation [Ch] does not have
other biodegradation pathways than the one illustrated in
Scheme 2. As for the anion [CPA], when attacked by elec-
trophilic oxygen there are six positions available. By analyz-
ing the HOMO surfaces (Figure 5, left), we find that only
three positions (1–3) are easily attacked, because these posi-
tions have higher EHOMO values. Combined with atomic
charge analysis (Figure 5, right), we know that the most
probable position to be attacked is position 3, because it has
the highest electron density. The most probable biodegrada-
n=10, R=0.875, S.E.=6.48, F=26.210>F0.01 (1, 8)=11.93,
and p=0.001, where n is the number of observations, R is
the correlation coefficient, S.E. is the standard error, F is
the Fisher criterion, and p is the significance level. Multiple
linear regression analysis shows that the model is statistical-
ly significant at the 99% confidence interval, thus indicating
that the %B28 of NAILs can be explained well by the quan-
tum chemical descriptor EHOMO. Under aerobic conditions,
the percentage of biodegradation is mainly governed by
three factors: hydrophobic, steric, and electronic parameters.
If a molecule or compound has proper hydrophobicity and
molecular volume, it will easily penetrate the membrane of
the cell, arrive at the active site of oxygenase, and then be
oxidized and degraded. In the model, EHOMO plays an impor-
tant role in the biodegradation reaction of NAILs. We can
tion pathway of [Ch]
ACHTUNGTRENN[UNG CPA] is illustrated in Scheme 2, which
needs to be proved further.
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Biodegradable Ionic Liquids
FULL PAPER
Scheme 2. Most probable biodegradation pathway of [Ch]ACTHUNTGRNEUNG[CPA].
tion period. That means the NAILs will be rapidly and com-
pletely biodegraded under environmental conditions. One
point we should keep in mind is that the closed-bottle test
method is extremely stringent (relatively low density of mi-
croorganism, relatively short incubation period, absence of
other carbon and nitrogen sources), so that biodegradation
values lower than 60% (larger than 40%) do not necessarily
mean the test compound will persist under environmental
conditions; it may be sufficiently removed by using an ap-
propriate treatment system.[58] Due to their good biodegrad-
ability, we anticipate that these new NAILs will be attractive
in large-scale industrial processes and commercial applica-
tions in the near future. By comparing the biodegradability
of NAILs with that of other ionic liquids and some common
organic solvents, we can conclude that biodegradable ionic
liquids can be designed as successful substitutes for volatile
solvents through natural-sources (biodegradable cations and
anions) screening.
Figure 5. HOMO surfaces (left) and atomic charges (right) of [Ch]ACTHNUTRGNEU[GN CPA].
Although the biodegradation reaction is complicated, in-
fluenced by many factors, and hard to interpret in a simple
equation, through descriptor selection, regression analysis,
and model development we can draw the conclusion that
EHOMO is a dominant parameter governing the biodegrada-
bility of NAILs, which gives us a clear direction to discover
biodegradable ionic liquids of other kinds.
Conclusion
Ten NAILs have been synthesized by neutralization of the
respective acid with choline hydroxide. The overall neutrali-
zation process is atom efficient and produces no waste.
Compared with metathesis routes, the neutralization method
provides a guideline for the synthesis of ionic liquids in a
very pure state without any halogen contamination. Further-
more, the simple preparation involved encourages commer-
cial applications of NAILs.
Experimental Section
Materials
Chemicals used in the synthesis of NAILs: Cyclopentane carboxylic acid,
cyclopentyl acetic acid, 3-cyclohexyl propionic acid, 2-naphthoxyacetic
acid, and anthracene-9-carboxylic acid were purchased from Lancaster.
Cyclohexane carboxylic acid, benzoic acid, salicylic acid, deoxycholic
acid, and choline hydroxide (45 wt.% aqueous solution) were purchased
from Acros Organics, and lithocholic acid was purchased from Fluka.
The densities, viscosities, conductivities, glass transition/
melting temperatures, and decomposition temperatures of
NAILs were measured. Studies indicate that these proper-
ties of NAILs are more influenced by van der Waals forces
and stacking effects of phenyl groups than Coulomb attrac-
tions and electron interactions. In addition, similar to other
ionic liquids, NAILs also have designable characteristics.
Their physicochemical properties can be adjusted by chang-
ing the following four factors: the alkyl chain connected to
the rings, the carbon number in the rings, the number of
rings, and whether aromatic or aliphatic rings. Therefore,
NAILs are more designable than common ionic liquids,
which can be adjusted just by the side chains connected to
the core.
Chemicals used for comparison in the ultimate biodegradability test:
SDS, toluene, methylene chloride, acetonitrile, and ethanol were all ana-
lytical grade and produced by the Sinopharm Chemical Reagent Beijing
Co., Ltd. [BMIM]
plied by Henan Lihua Pharmaceutical Co., Ltd. We synthesized [BMIM]-
[CHC], [BMIM][Sa], and [BMIM][NOA] and their preparation and char-
ACHTNUGTRNEU[GNN BF4], [BMIM]ACHTUNGTRENGUN[N PF6], and [BMIM]CAHTGUNTREN[NUGN HSO4] were sup-
A
ACHTUNGTRENNUNG
acterization can be found in the Supporting Information. All reagents
purchased were used as received without further purification.
General methods: Densities of NAILs were measured at 25 and 408C by
using a DMA 5000 density meter (Anton Parr) with a precision of
ꢁ10ꢀ6 gcmꢀ3. Viscosities were measured at 408C by using an automated
microviscometer (Anton Parr) with an uncertainty in experimental mea-
surement of ꢁ0.0001 mPas. Conductivities were measured with a DDS-
In the closed-bottle test, the biodegradation values of
most NAILs are higher than 60% during a 28-day incuba-
Chem. Eur. J. 2008, 14, 11174 – 11182
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
11179

S. Zhang et al.
307 conductometer (Shanghai Precision & Scientific Instruments). The
glass transition and melting temperatures were determined from DSC
thermograms during heating scans (heating rate of 108Cminꢀ1 from
ꢀ120 to +1008C) on a modulated DSC 2910 apparatus (TA Instru-
ments). Thermogravimetric analyses were conducted in an air atmos-
phere on a TGA 2050 thermogravimetric analyzer (TA Instruments) be-
892, 721, 668 cmꢀ1; elemental analysis calcd (%) for C12H25NO3·H2O: C
57.80, H 10.91, N 5.62; found: C 57.81, H 10.69, N 5.51.
[Ch]
1H; HOCH2CH2), 3.87 (m, 2H, J
2H, 3J
(H,H)=4.4 Hz; HOCH2CH2), 3.14 (s, 9H; N
3J(H,H)=15.6 Hz; OC(=O)CH2CC1CCCCC1), 1.66 (m, 5H, 3J
ACHTUNGTRENNUNG
[CHP]: 1H NMR (400 MHz, [D6]DMSO, 258C, TMS): d=7.70 (br,
ACHTUNGTRENNUNG
3
AHCTUNGTRENNUNG
A
ACHTUNGTRENNUNG
N
19.3 Hz; OC(=O)CCC1HCH2CCCC1H2), 1.33 (m, 2H, 3J
ACHTUNGTRENNUNG
tween 25 and 4008C (5708C for [Ch]ACTHNUTRGNEUGN[DOC] and [Ch][LC]) at a heating
rate of 58Cminꢀ1. The 1H NMR spectra were recorded on an ARX400
NMR spectrometer (Bruker) at 400 MHz with dimethyl sulfoxide
(DMSO) as solvent and TMS as internal standard. 13C NMR spectra
were recorded on the same instrument at 100 MHz. IR studies were con-
ducted with a Nicolet 380 FTIR spectrometer (Thermo Nicolet). Elemen-
tal analyses were conducted with a Vario EL elemental analyzer (Ele-
mentar).
22.3 Hz; OC(=O)CCH2C1CCCCC1), 1.14 (m, 4H, 3J
OC(=O)CCC1CCH2CCH2C1), 0.81 ppm (m, 2H, 3J
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
OC(=O)CCC1CCCH2CC1); 13C NMR (100 MHz, [D6]DMSO, 258C,
TMS): d=176.73 (OC(=O)CCC1CCCCC1), 67.39 (HOCH2CH2), 55.05
(HOCH2CH2), 53.28 (N
ACHTUGNERTN(NUNG CH3)3), 37.38 (OC(=O)CCC1CCCCC1), 36.13
(OC(=O)CCC1CCCCC1), 34.39 (OC(=O)CCC1CCCCC1), 33.05 (OC(=
O)
O)
ACHTUNGTRENNUNG
Synthesis: The obtained NAILs were prepared by neutralization of the
ACHTUNGTRENNUNG
respective surrogate naphthenic acids dissolved in ethanol with choline
(nsCH), 1558 (nasCOO), 1447 (dCH), 1394 (nsCOO), 1297, 1124 (nCN),
hydroxide. As an example, the synthesis procedure of [Ch]
A
1092, 1008, 957, 887, 668 cmꢀ1
;
elemental analysis calcd (%) for
follows. Cyclopentane carboxylic acid (17.12 g, about 0.15 mol) dissolved
in ethanol (50 mL) was loaded into a flask (250 mL) with a magnetic stir-
rer, and then, under vigorous stirring, an equimolar amount of choline
hydroxide aqueous solution was added dropwise to the flask in about
20 min. The reaction lasted for 2 h at 258C. The solvent was removed in
vacuo at or below 508C in a rotary evaporator. The resultant residue was
dried under vacuum over P2O5 for 48 h at 808C to afford a colorless
product.
C14H29NO3·H2O: C 60.61, H 11.26, N, 5.05; found: C 59.99, H 10.81, N
4.93.
[Ch][Be]: 1H NMR (400 MHz, D2O, 25 8C, TMS): d=7.75 (d, 2H, 3J-
ACHUTNGRENNUG CAHTUNGTRENNUNG
(H,H)=9.6 Hz; OC(=O)c1cHcccc1H), 7.38 (t, 1H, 3J
OC(=O)c1cccHcc1), 7.34 (t, 2H, 3J
c1ccHccHc1), 3.81 (m, 2H, 3J
3J
ACHTUNGTRENNUNG
N
ACHTUNGTRENNUNG
N
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
Characterization
140.76 (OC(=O)c1ccccc1), 129.05 (OC(=O)c1ccccc1), 128.66 (OC(=
O)c1ccccc1), 127.21 (OC(=O)c1ccccc1), 67.42 (HOCH2CH2), 55.16
[Ch]
1H; HOCH2CH2), 3.87 (m, 2H, J
2H, 3J
(H,H)=9.9 Hz; HOCH2CH2), 3.15 (s, 9H; N
3J(H,H)=23.8 Hz; OC(=O)C1HCCCC1), 1.63 (m, 4H, 3J
15.6 Hz; OC(=O)C1CH2CCC1H2), 1.52 (m, 2H, 3J
(H,H)=4.2 Hz; OC(=
(H,H)=11.5 Hz; OC(=
ACHTUNGTRENNUNG
[CPC]: 1H NMR (400 MHz, [D6]DMSO, 258C, TMS): d=7.75 (br,
3
(HOCH2CH2), 53.25 ppm (N
(nasCOO), 1558, 1486 (dCH), 1373 (nsCOO), 1136 (nCN), 1089, 1023,
957, 867, 829, 725, 672 cmꢀ1
elemental analysis calcd (%) for
ACHTUGNTERN(NUNG CH3)3); IR: n˜ =3203 (nOH), 1598
AHCTUNGTRENNUNG
T
ACHTUNGTRENNUNG
;
N
ACHTUNGTRENNUNG
C12H19NO3·H2O: C 59.24, H 8.74, N 5.76; found: C 59.46, H 8.55, N 5.78.
[Ch][Sa]: 1H NMR (400 MHz, D2O, 25 8C, TMS): d=7.69 (d, 1H, 3J-
AHCTUNGTRENNUNG
O)C1CCH2CC1), 1.40 ppm (m, 2H, 3J
ACHTUNGTRENNUNG
O)C1CCCH2C1); 13C NMR (100 MHz, [D6]DMSO, 258C, TMS): d=
179.31 (OC(=O)C1CCCC1), 67.40 (HOCH2CH2), 55.07 (HOCH2CH2),
(H,H)=9.6 Hz; OC(=O)c1cHcccc1O), 7.31 (t, 1H, 3J
ACHUTGTNRENNUG CAHTUNGTRENNUNG
OC(=O)c1ccccHc1O), 6.84 (m, 2H, 3J
c1ccHcHcc1O), 3.87 (m, 2H, 3J
2H, 3J
ACHTUNGTRENNUNG
53.19 (N
N
N
ACHTUNGTRENNUNG
25.71 ppm (OC(=O)C1CCCC1); IR: n˜ =3381 (nOH), 2956 (nCH), 2870,
1552 (nasCOO), 1479 (dCH), 1397 (nsCOO), 1325, 1138 (nCN), 1089,
N
NACHTUNGTRENNUNG
1007, 957, 867, 669 cmꢀ1
;
elemental analysis calcd (%) for
ACHTUNGTRENNUNG
C11H23NO3·2H2O: C 52.15, H 10.74, N 5.53; found: C 52.34, H 10.59, N
ACHTUNGTRENNUNG
5.49.
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
[Ch]
1H; HOCH2CH2), 3.86 (m, 2H, 3J
2H, 3J
(H,H)=5.0 Hz; HOCH2CH2), 3.15 (s, 9H; N
3J(H,H)=22.7 Hz; OC(=O)CC1HCCCC1), 1.87 (d, 2H, 3J
7.3 Hz; OC(=O)CH2C1CCCC1), 1.67 (m, 2H, 3J
(H,H)=7.2 Hz; OC
O) (H,H)=7.7 Hz; OC(=O)-
CC1CCCC1H2), 1.52 (m, 2H, 3J
CC1CH2CCC1), 1.45 (m, 2H, 3J
(H,H)=9.1 Hz; OC(=O)CC1CCCH2C1),
1.07 ppm (m, 2H, 3J CC1CCH2CC1); 13C NMR
(H,H)=13.8 Hz; OC(=O)
(100 MHz, [D6]DMSO, 258C, TMS): d=175.81 (OC(=O)CC1CCCC1),
67.52 (HOCH2CH2), 55.00 (HOCH2CH2), 53.21 (N(CH3)3), 45.55 (OC(=
O)CC1CCCC1), 37.80 (OC(=O)CC1CCCC1), 32.63 (OC(=O)-
CC1CCCC1), 24.84 ppm (OC(=O)CC1CCCC1); IR: n˜ =3419 (nOH),
[CPA]: 1H NMR (400 MHz, [D6]DMSO, 258C, TMS): d=7.87 (br,
(H,H)=8.9 Hz; HOCH2CH2), 3.46 (t,
(CH3)3), 2.12 (m, 1H,
(H,H)=
(=
AHCTUNGTRENNUNG
;
G
ACHTUNGTRENNUNG
C12H19NO4·0.85H2O: C 56.25, H 8.20, N 5.47; found: C 56.10, H 8.21, N
N
ACHTUNGTRENNUNG
5.51.
R
ACHTUNGTRENNUNG
E
G
[Ch]
ACHTUNGTRENNUNG
N
A
U
2H, 3J
ACHTUNGTRENNUNG
A
U
(H,H)=8.1 Hz; OC(=O)COc1ccHc2ccccc2c1), 7.42 (t, 1H, 3J
ACHUTGTNRENNUG ACHTUNGTRENNUNG
13.9 Hz; OC(=O)COc1ccc2cccHcc2c1), 7.31 (t, 1H, 3J
G
3
R
OC(=O)COc1ccc2ccHccc2c1), 7.13 (dd, 1H, J
G
E
COc1ccc2ccccc2c1H), 7.08 (d, 1H, 3J
ACHUTGTNRENNUG CAHTUNGTRENNUNG
E
A
ACHTUNGTRENNUNG
2952 (nCH), 2870, 1557 (nasCOO), 1487 (dCH), 1398 (nsCOO), 1319,
1134 (nCN), 1088, 1007, 956, 866, 667 cmꢀ1; elemental analysis calcd (%)
for C12H25NO3·2H2O: C 53.82, H 10.16, N 5.23; found: C 53.88, H 10.61,
N 5.19.
C
ACHTUNGTRENNUNG
A
NACHTUNGTRENNUNG
O)COc1ccc2ccccc2c1), 128.77 (OC(=O)COc1ccc2ccccc2c1), 128.10
(OC(=O)COc1ccc2ccccc2c1), 127.41 (OC(=O)COc1ccc2ccccc2c1), 126.45
(OC(=O)COc1ccc2ccccc2c1), 126.11 (OC(=O)COc1ccc2ccccc2c1), 123.01
(OC(=O)COc1ccc2ccccc2c1), 119.04 (OC(=O)COc1ccc2ccccc2c1), 106.63
(OC(=O)COc1ccc2ccccc2c1), 67.98 (OC(=O)COc1ccc2ccccc2c1), 67.16
[Ch]
1H; HOCH2CH2), 3.96 (m, 2H, J
2H, 3J
(H,H)=10.0 Hz; HOCH2CH2), 3.08 (s, 9H; N
1H, 3J(H,H)=11.2 Hz; OC(=O)C1HCCCCC1), 1.71 (m, 2H, 3J
10.4 Hz; OC(=O)C1CH2CCCC1), 1.62 (m, 2H, 3J
(H,H)=7.6 Hz; OC(=
O) (H,H)=12.0 Hz; OC(=O)-
C1CCCCC1H2), 1.21 ppm (m, 6H, 3J
C1CCH2CH2CH2C1); 13C NMR (100 MHz, [D6]DMSO, 258C, TMS): d=
178.86 (OC(=O)C1CCCCC1), 67.47 (HOCH2CH2), 55.03 (HOCH2CH2),
53.18 (N(CH3)3), 46.73 (OC(=O)C1CCCCC1), 30.55 (OC(=O)-
C1CCCCC1), 26.36 (OC(=O)C1CCCCC1), 26.13 ppm (OC(=O)-
C1CCCCC1); IR: n˜ =3419 (nOH), 3025, 2928 (nasCH), 2852 (nsCH), 1567
(nasCOO), 1539, 1404 (nsCOO), 1279, 1221, 1137 (nCN), 1087, 960, 930,
ACHTUNGTRENNUNG
[CHC]: 1H NMR (400 MHz, [D6]DMSO, 258C, TMS): d=7.90 (br,
3
AHCTUNGTRENNUNG
A
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
(HOCH2CH2), 55.02 (HOCH2CH2), 53.07 ppm (NACTHUNGTRNEUNG(CH3)3); IR: n˜ =3362
G
ACHTUNGTRENNUNG
(nOH), 1605 (nasCOO), 1471 (dCH), 1393 (nsCOO), 1260, 1216, 1179,
1121 (nCN), 1053, 953, 842, 752, 665 cmꢀ1; elemental analysis calcd (%)
for C17H23NO4·1.1H2O: C 62.73, H 7.75, N 4.31; found: C 2.89, H 8.00, N
4.33.
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
G
[Ch][AC]: 1H NMR (400 MHz, [D6]DMSO, 258C, TMS): d=8.29 (s, 1H;
OC(=O)c1c2ccccc2cHc3ccccc13), 8.18 (d, 2H, 3J
ACTHNUTRGNE(NUG H,H)=3.3 Hz; OC(=
11180
www.chemeurj.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 11174 – 11182

Biodegradable Ionic Liquids
FULL PAPER
O)
c1c2ccccHc2cc3cHcccc13), 7.43 (t, 2H, 3J
c1c2ccHcHcc2cc3ccccc13), 7.40 (t, 2H, 3J
c1c2ccccc2cc3ccHcHcc13), 6.86 (br, 1H; HOCH2CH2), 3.79 (m, 2H, 3J-
(H,H)=9.6 Hz; HOCH2CH2), 3.36 (t, 2H, 3J
(H,H)=9.9 Hz;
HOCH2CH2), 3.07 ppm (s, 9H;
(CH3)3); 13C NMR (100 MHz,
[D6]DMSO, 258C, TMS): d=171.10 (OC(=O)c1c2ccccc2cc3ccccc13),
134.58 (OC(=O)c1c2ccccc2cc3ccccc13), 131.20 (OC(=O)-
c1c2ccccc2cc3ccccc13), 127.96 (OC(=O)c1c2ccccc2cc3ccccc13), 126.76
c1c2ccccc2cc3ccccc13), 125.75 (OC(=O)c1c2ccccc2cc3ccccc13),
(OC(=O)c1c2ccccc2cc3ccccc13), 123.91 (OC(=O)-
N
E
150 rpm in an incubator. The depletion of dissolved molecular oxygen
was measured at the desired test intervals (0, 7, 14, 21, and 28 days) with
a JPSJ-605 dissolved oxygen analyzer (Shanghai Precision & Scientific
Instruments). The depletion data were corrected for uptake by the use of
a blank inoculum run in parallel and then expressed as a percentage of
the theoretical oxygen demand to give the percentage of biodegradation
(%B).
G
ACHTUNGTRENNUNG
E
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
N
NACHTUNGTRENNUNG
Calculation of descriptors—statistical analysis: The logarithm of the n-oc-
R
ACHTUNGTRENNUNG
tanol/water partition coefficient (logP) and van der Waals volume (VvdW
)
N
of the anions were calculated by using Molecular Modeling Pro Plus
(ChemSW, Inc.); energies of the highest occupied molecular orbital
ACHTUNGTRENNUNG
(EHOMO
) and energies of the lowest unoccupied molecular orbital
AHCTUNGTRENNUNG
(ELUMO) of the anions were all calculated with the program Gaussian 03
(Gaussian, Inc.). EHOMO and ELUMO were calculated at the B3LYP/6-311+
G* level so that the overlap of the orbitals was considered.
(nOH), 1588 (nasCOO), 1423 (dCH), 1387 (nsCOO), 1318, 1276, 1133
(nCN), 1087, 1011, 955, 887, 861, 800, 742, 656, 559 cmꢀ1; elemental anal-
ysis calcd (%) for C20H23NO3·1.3H2O: C 68.80, H 7.34, N 4.01; found: C
68.93, H 7.47, N 3.98.
All statistical analyses to develop the biodegradation model were carried
out by SPSS software (version 14.0, SPSS Inc.). Multiple linear regression
analysis with the stepwise method was used. The criterion for descriptors
to enter the model is the probability of F<=0.050; to remove, the proba-
bility of F>=0.100. Model quality was characterized by the number of
observations (n), the correlation coefficient (R), the standard error
(S.E.), the Fisher criterion (F), and the significance level (p).
[Ch]ACHTUNGTRENNUNG
[DOC]: 1H NMR (400 MHz, [D6]DMSO, 258C, TMS): d=7.30 (br,
1H; HOCH2CH2), 4.26 (br, 1H), 3.85 (m, 2H, 3J
G
3
HOCH2CH2), 3.79 (br, 1H), 3.56 (m, 1H, J
A
A
3J
9H; N
3.9 Hz), 1.69 (m, 2H, J
1.55 (m, 2H, 3J(H,H)=7.7 Hz), 1.47 (m, 2H, 3J
2H, 3J(H,H)=4.1 Hz), 1.36 (m, 3H, 3J
(H,H)=7.6 Hz), 1.19 (m, 2H, J
ACHTUNGTRENNUNG
A
R
3
3
ACHTUNGTRENNUNG(H,H)=8.2 Hz), 1.61 (m, 2H, JCAHTUNGTRENNNUG
N
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
3
E
E
3
9.5 Hz), 1.04 (m, 2H, J
0.88 (m, 4H, 3J
G
ACHTUNGTRENNUNG
Acknowledgements
U
ACHTUNGTRENNUNG
This work was supported financially by the National Science Fund for
Distinguished Young Scholars of China (grant no. 20625618), the Nation-
al Natural Science Foundation of China (grant nos. 20808083&20873152),
and the National 863 Program of China (grant no. 2006AA06Z376).
ACHTUNGTRENNUNG
G
R
G
ACHTUNGTRENNUNG
E
T
N
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
46.61, 46.01, 41.69, 36.32, 35.84, 35.72, 35.21, 33.87, 32.97, 30.24, 28.65,
27.05, 26.16, 23.63, 23.13, 17.32, 12.57 ppm; IR: n˜ =3239 (nOH), 3030,
2939, 2850, 1576 (nasCOO), 1452 (dCH), 1379 (nsCOO), 1291, 1215, 1093
(nCN), 1078, 1052, 1014, 958, 922, 866, 781, 714, 618, 607 cmꢀ1; elemental
analysis calcd (%) for C29H53NO5·1.0H2O: C 67.73, H 10.71, N 2.73;
found: C 67.68, H 10.65, N 2.71.
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18, 481–493.
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Angew. Chem. Int. Ed. 2000, 39, 3772–3789.
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155–169.
[Ch][LC]: 1H NMR (400 MHz, [D6]DMSO, 258C, TMS): d=7.50 (br,
3
1H; HOCH2CH2), 3.84 (m, 2H, J
N
3
3
1H, J
(m, 1H, J
(CH3)3), 1.93 (m, 1H, J
1.65 (m, 2H, 3J(H,H)=4.3 Hz), 1.61 (m, 2H, 3J
2H, 3J(H,H)=5.7 Hz), 1.35 (m, 4H, 3J(H,H)=14.1 Hz), 1.28 (m, 4H, 3J-
(H,H)=10.6 Hz), 1.24 (m, 4H, J
ACHTUNGTRENNUNG(H,H)=9.2 Hz), 3.41 (t, 2H, JACHTUNGTRENN(GUN H,H)=14.7 Hz; HOCH2CH2), 3.33
3
ACHTUNGTRENNUNG(H,H)=4.7 Hz), 3.28 (m, 1H, JCAHTUNGTRNE(NUGN H,H)=5.5 Hz), 3.11(s, 9H; N-
3
N
U
T
A
3
A
G
3
7.6 Hz), 1.15 (m, 3H, J
T
0.83 (d, 3H, 3J
ACHTUNGTRENNUNG
[D6]DMSO,
ACHTUNGTRENNUNG
A
70.03
(CC-
67.20
ACHTUNGTRENNUNG
(HOCH2CH2), 64.63, 64.19, 60.07, 56.27, 55.19 (HOCH2CH2), 53.29 (N-
(CH3)3), 42.35, 41.67, 36.38, 35.67, 35.53, 34.32, 30.45, 27.93, 27.01, 26.28,
ACHTUNGTRENNUNG
24.02, 23.38, 20.53, 18.60, 12.05 ppm; IR: n˜ =3280 (nOH), 2926, 2862,
1564 (nasCOO), 1467 (dCH), 1398 (nsCOO), 1341, 1256, 1242, 1167, 1088
(nCN), 1071, 1046, 1013, 964, 950, 869, 786, 723, 617 cmꢀ1; elemental
analysis calcd (%) for C29H53NO4·H2O: C 69.97, H 11.14, N 2.81; found:
C 71.20, H 10.87, N 2.80.
Ultimate biodegradability testing: The ultimate biodegradability of
NAILs was evaluated using the closed-bottle test (OECD 301D).[42] In
this test, solutions of test substances were added to an aerobic aqueous
medium inoculated with microorganisms derived from the second efflu-
ents of a local sewage treatment plant. The aerobic aqueous medium was
transferred into completely full, closed biochemical oxygen demand
(BOD) bottles. All transfer and filling operations were conducted by the
use of siphons to make the medium bubble-free. The closed BOD bottles
were kept in the dark and incubated at 258C at a rotation rate of
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Received: April 2, 2008
Revised: August 21, 2008
Published online: October 27, 2008
11182
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