Task-specific, biodegradable amino acid ionic liquid surfactants

Article abstract of DOI:10.1002/cssc.201100065

VersatAAIL Surfactants: Biodegradable, chiral amino acid ionic liquid surfactants (AAILSs) with a very high surface activity are synthesized and characterized. The AAILS can be applied as task-specific ionic liquids; two examples given are the mitigation of harmful algal blooms from sea water and the shape- and size-specific synthesis of nanomaterials. Copyright

Full text of DOI:10.1002/cssc.201100065

DOI: 10.1002/cssc.201100065
Task-Specific, Biodegradable Amino Acid Ionic Liquid Surfactants
Tushar J. Trivedi, K. Srinivasa Rao, Tejwant Singh, Subir Kumar Mandal, Narottom Sutradhar, Asit Baran Panda,
and Arvind Kumar*[a]
Besides several conventional uses, such as in cleaning prod-
ucts, food, beverages, dairy processing, water treatment,
healthcare, fuel and lubricant additives, and emulsifiers or sta-
bilizers in paints or cosmetics, there is an increasing interest in
the synthesis of new surfactants for high-end applications such
as gene transfection agents, the denaturation/encapsulation of
proteins or drugs, or templates for shape- or size-selective and
highly ordered nanomaterials.[1] Because of their modular
nature and unique physicochemical properties, ionic liquids
(ILs) have found widespread application and are used in many
areas of chemistry.[2] The inherent amphiphilic character of
some ILs has yielded surface-active properties that are different
(and better) than those of conventional surfactants, and thus
they have emerged as a superior class of surfactants.[3] Also,
conventional ionic surfactants suffer from phase separation,
owing to solubility limitations, and the fact that the creation of
hierarchical micellar systems is usually thermodynamically un-
favorable. This restricts their use in many applications, for ex-
ample, as templates for a desired nanomaterial architectures.[4]
In contrast, owing to their strong and directional polarizability
and excellent water solubility ILs have been shown to self-as-
semble into highly structured forms, useful for the preparation
of a variety of nanomaterials (e.g., metals, metal oxides, zeo-
lites).[5]
amino acids having a superior surface activity and solvent mis-
cibility are reported here for the first time. From the natural
amino acids, l-glycine, l-alanine, l-valine, l-glutamic acid, and
l-proline were chosen allow simple variations in the side chain
through branching, the addition of another ÀCOOH group, or
cyclization. Because the incorporation of an ester group into
an amino acid has been shown decrease the melting point
and significantly increase biodegradability,[9] esterification of
the amino acids was carried out by using either isopropyl or
isobutyl alcohol.
For esterification, thionyl chloride was slowly added to iso-
propyl or isobutyl alcohol at 08C. Amino acids were slowly
added to the reaction mixtures, which were then refluxed for
4 h. The reaction mixtures were concentrated in a rotary evap-
orator, and crude amino acid ester hydrochlorides were titurat-
ed with hexane at 08C. Pure crystals of amino acid ester hydro-
chlorides (AAECls) were obtained by recrystallization with
methanol/hexane. Equimolar amounts of the AAECls and SLS
were then dissolved in hot water. After the completion of reac-
tion, water was removed under vacuum and AAILSs were ex-
tracted by the addition of dichloromethane. All of the AAILSs
except the one obtained from glutamic butyl ester hydrochlo-
ride (white crystalline solid at room temperature) were clear
but slightly viscous liquids at room temperature. The reaction
sequence is shown in Scheme 1. Detailed preparation, wash-
ing, and drying procedures can be found in the Supporting In-
formation. The structures of the AAILSs were confirmed by
Although IL surfactants used so far are “green” in terms of
their negligible vapor pressure, they generally contain synthet-
ic quaternary nitrogen cations (such as alkylammonium, dialky-
limidazolium, or pyridinium) with halogen atoms as anions
(such as Cl or F). They can release HCl or HF by hydrolysis
under certain conditions, which may pose a hazard when they
are released into the environment through wastewater efflu-
ents. Therefore, toxicity and biodegradation are vital issues
when dealing with ILs.[6]
In this context, the green credentials of ILs have been tre-
mendously improved by the development of biobased ILs.[7]
Herein, we choose natural amino acids and sodium lauryl sul-
phate (SLS) as precursors for amino acid ionic liquid surfactant
(AAILS) architectures. Amino acid-based surfactants, featuring
amino acids modified with long aliphatic chains to generate
linear, dimeric, or glycerolipid-like structures, have been report-
ed extensively,[8] however, ionic liquid surfactants based on
Scheme 1. Synthesis of AAILSs.
[a] T. J. Trivedi, K. S. Rao, T. Singh, S. K. Mandal, N. Sutradhar, A. B. Panda,
Dr. A. Kumar
NMR (1H and 13C), elemental analysis (CHNS), and electrospray
ionization mass spectrometry (ESI-MS). Herein, we use general
symbols to represent the amino acids, and use the number of
carbon atoms to represent the alkyl groups of the ester (e.g.,
glycine propyl ester lauryl sulphate is denoted as GlyC3LS).
Physicochemical properties of the AAILSs are listed in
Table 1. Differential scanning calorimetry (DSC) analysis re-
Central Salt and Marine Chemicals Research Institute (CSIR)
G.B. Marg, Bhavnagar-364002, Gujarat (India)
Fax: (+91)278-2567562
E-mail: mailme_arvind@yahoo.com
arvind@csmcri.org
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201100065.
604
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ChemSusChem 2011, 4, 604 – 608

AAILSs
biodegradation
by
Table 1. Physicochemical properties of synthesized AAILSs.
marine bacteria (the status of
the marine bacteria in sterilized
and natural sea water, with or
without addition of AlaC3LS is
shown in Figure 3b. The surface
tension of all the AAILSs solu-
tions reached the value of natu-
ral sea water within 36 h, indicat-
ing rapid and complete biode-
gradability.
[b]
Sample
Tm
[8C]
Tg
[8C]
Td
[8C]
[a]3D0[a]
1[b]
k[b]
cac[b]
[mm]
gcac
[mNmÀ1
]
[gcmÀ3
]
[mScmÀ1
]
GlyC3LS
GlyC4LS
AlaC3LS
AlaC4LS
GluC3LS
GluC4LS
ValC3LS
ValC4LS
ProC3LS
ProC4LS
5
À18
À5
243
257
244
268
221
230
272
279
247
280
–
–
1.0503
1.0174
1.0469
1.0418
1.0503
1.0331
1.0261
1.0093
1.0468
1.0359
0.01015
2.415
0.08848
3.95
1.082
–
0.01308
1.814
0.059
1.023
0.63
0.43
0.53
0.49
0.67
–
0.68
0.28
0.48
0.42
26.0
27.0
25.7
28.5
30.6
–
27.5
30.6
27.8
30.5
35
À3
À12
30
56
26
À34
À33
À43
À19
À19
À27
À33
À46
+0.77
À1.03
+9.57
+8.83
+6.71
+5.86
À14.58
À18.60
26
À15
À22
Biosurfactants such as glycoli-
pids and lipoproteins, produced
from natural compounds by vari-
ous groups of microorganisms,
[a] Concentration: 2g per 100 mL MeOH. [b] At 258C.
vealed a melting temperature (Tm) <308C for all AAILSs except
GluC4LS (Tm =568C). Tm varies with the structure of amino
acids, being lowest for ProC3LS and highest for GluC4LS. The
glass transition temperature (Tg) also varied with the structure
of amino acids, and was normally <08C. All AAILSs were signif-
icantly thermally stable as can be seen from their decomposi-
tion temperature (Td) values of >2208C, obtained from ther-
mogravimetric analysis (TGA, Table S1). DSC and TGA profiles
of all AAILSs are provided in Figures S1 and S2. Since amino
acids and their ester hydrochloride salts have chiral centers
that are retained in the final products, we measured the specif-
ic rotations ([a]3D0) of the synthesized AAILSs (Table 1). These
were found to be only slightly less than those of the corre-
sponding amino acid precursors, implying their use as stereo-
selective catalytic media for organic reactions. Table 1 also
shows density and conductivity data. The density of the AAILSs
does not vary much with structural changes in amino acids
while there is a large variation in conductivity values. AAILSs
that have propyl group are slightly more dense and much less
conductive than their butyl analogues. A monotonic increase
of the conductivity was observed with the increase in tempera-
ture.
Figure 1. Temperature dependence of ionic conductivity in AAILSs: (&)
^
AlaC3LS, (*) GlyC3LS, (~) GluC3LS, (!) ProC3LS ( ) ValC3LS. Hollow symbols
correspond to butyl analogues of AAILSs. VFT data is plotted as solid lines.
Table 2. Comparison of cac’s of the synthesized AAILSs to amphiphilic
ionic liquids or conventional surfactants with similar alkyl chain lengths
(at 258C).
AAILSs
cac
Ionic liquid/conventional surfactant
cac
Ref.
[mm]
[mm]
AlaC3LS
GlyC3LS
0.63 1-dodecyl-3-methylimidazolium chloride 13.47 [13a]
0.43 1-dodecyl-3-methylimidazolium bromide 10
[13b]
[13b]
[13b]
[13b]
The conductivity data can be reasonably well fitted to the
Vogel–Fulcher–Tammann (VFT) equation.[10] The fitted curves
and experimental data are shown in Figure 1. The best-fit pa-
rameters for specific conductivity are given in Table S1. Consis-
tent with the literature,[11] the VFT temperatures are below the
Tg.
GluC3LS 0.53 1-dodecyl-3-methylpyridinium chloride
ProC3LS 0.49 1-dodecyl-3-methylpyridinium bromide
ValC3LS
AlaC4LS[a]
GlyC4LS
13
10
0.67 1-dodecyl-1-methylpiperidinium bromide 11
– 1-dodecyl-1-methylpyrrolidinium bromide 13.5 [13c]
0.68 sodium dodecyl sulfate 8.0 [13d]
GluC4LS 0.28 dodecyl trimethyl ammonium chloride
21.5 [13d]
ProC4LS 0.48
ValC4LS 0.42
–
–
–
–
The critical aggregation concentration (cac) and surface ten-
sion at cac (gcac) values of AAILSs are lower than analogous
conventional ionic surfactants by an order of magnitude.[12]
Table 2 compares the cac’s of the AAILSs to conventional sur-
factants or ionic liquids with the same alkyl chain length.[13]
Figure 2 shows the tensiometry profile of a representative
AAILS (ProC3LS). Transmission electron microscopy (TEM) and a
dynamic light scattering (DLS) plot (Figure 2, inset) revealed
stable spherical aggregates of ca. 4 nm in size.
[a] The cac of AlaC4LS could not be detected due to solubility limitations.
have been shown to be potential agents for managing harmful
algal blooms (HABs), which have severe adverse effects on fish-
eries, aquacultures, drinking water, tourism, and human
health.[14] However, these biosurfactants suffer from high pro-
duction and recovery costs, while mass production is also diffi-
cult. AAILSs, being highly biodegradable and surface active,
may mimic the effects of biosurfactants as anti-HAB agents
while retaining the advantage of ecological acceptability.
The biodegradability of the synthesized AAILSs was checked
by periodic surface tension measurements of solutions of
AAILSs in natural sea water (50 mgLÀ1). The increase in surface
tension with time (Figure 3a) gave a qualitative measure of
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Another potential application of AAILSs was explored: the
generation of shape- and size-specific nanostructured materials
of technological importance. CeO2, which is applied in
amongst others automotive three-way catalysis, solid oxide
fuel cells, metal-oxide semiconductor devices, and gas sensors
was chosen as test material. Imidazolium-based ionic liquids
with long alkyl chain lengths have been used as both template
and solvent for the synthesis of spherical aggregates of CeO2
nanocrystals or mesostructured crystalline CeO2.[15] However,
the use of AAILSs as structure-directing agents can be advanta-
geous because of not only their green nature, but also be-
cause they may allow distinct morphological control owing to
the structural and physical properties of amino acids, such as
their ability to coordinate to metal ions, the type and
length of the functional groups, or their influence on
intrinsic electric fields.[16] The synthesis of nearly mon-
odisperse CeO2 structures was carried out by using a
cerium ammonium carbonate solution precursor in
the presence of AlaC3LS under reflux or hydrothermal
conditions (see Supporting Information). Reflux con-
ditions produced monodisperse nanospheres, where-
as hydrothermal conditions generated monodisperse
cubes and star-shaped structures just by varying the
precursor concentration (see SEM images in Fig-
ure 5a–c).
Figure 2. Surface tension of aqueous solutions of a representative AAILS
(ProC3LS). The breaking point indicates the cac. Insets: TEM and DLS data, in-
dicating an aggregate size in the range of 4–5 nm.
All samples comprised nanocrystalites 5–14 nm in
size (Figure 5d and e). X-ray diffraction (XRD) of syn-
thesized samples calcined at 4008C revealed well-re-
Figure 3. (a) Time-dependent surface tension measurements of solutions of the AAILSs in
natural sea water (50 mgLÀ1). (b) Bacterial colony forming units in (1) sterilized sea water,
(2) natural sea water diluted by a factor of 100, (3) an AlaC3LS-sea water solution after
12 h, and (4) an AlaC3LS-sea water solution after 36 h. The volumes of inoculums was
100 mL.
solved diffraction peaks which can be indexed to the
(111), (200), (220), (311), (222), (400), and (331) planes
of a cubic fluorite structured CeO2 (JCPDS 34-0394)
(Figure 5 f). Other interesting and useful morpholo-
gies can be generated by varying parameters such as
temperature, reaction time, or type of AAILS.
Therefore, the application of representative AAILSs (AlaC3LS
and AlaC4LS) towards the mitigation of HABs comprising Am-
phidinium cartarae, a toxic phytoplankton (Dinoflagellate) was
studied. Treatments with 50 mm of AlaC3LS and AlaC4LS to in-
oculums of culture containing 1500Æ50 cells led to 100%
mortality in 24 and 72 h, respectively, indicating that AAILSs
with a propyl ester functionality are more effective
than butyl analogues (Figure 4a). Lower concentra-
tions, however, had little effect on cell viability in the
stipulated experimental time periods. The presence
and numbers of dead cells were determined by
Evan’s Blue staining. The cells were incubated for
10 min and then counted in an inverted microscope
in bright mode (Figure 4b shows the impact of AAILS
on the test organism; details concerning isolation of
the test organism and experimental methodology are
provided in the Supporting Information). Results
show that very low concentrations of AAILSs are suf-
ficient to provoke an algicidal effect while avoiding
detrimental environmental effects. The very high bio-
degradability of AAILSs ensures ensure their quick
In conclusion, we have synthesized novel highly biodegrad-
able amino acid-based ionic liquid surfactants with better sur-
face activity than conventional surfactants, and have demon-
strated their potential application in areas such as the mitiga-
tion of harmful algal blooms from sea water and the of shape-
and size-specific synthesis nanomaterials. Their high solubility
Figure 4. (a) Mortality (%) of Amphidinium cartarae as function of AAILS concentration
and time. (b) Impact of 50 mm AlaC4LS after 24 h (a dead cell is indicated by the arrow;
Evan’s Blue staining is indicative of dead cells).
disappearance from a marine environment after HAB
mitigation.
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Experimental Section
All reagents and solvents were analytical-grade materials pur-
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purification unless stated otherwise. NMR spectra were recorded in
[D6]DMSO on a Bruker 200 MHz instrument with tetramethyl silane
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Keywords: amino acids · ionic liquids · nanostructures ·
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Received: February 2, 2011
Published online on April 19, 2011
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