First Room Temperature Chiral Anionic Liquid Forming Micelles and Reverse Micelles

Article abstract of DOI:10.1021/acs.jpcb.6b11964

We report the first chiral surface active anionic liquid, T12M, derived from biodegradable tartaric acid, and its unusual properties. T12M features unprecedented combination of characteristics not found in other ionic liquids (ILs): (a) T12M is the first surface active ionic liquid that is fully chiral, by virtue of the presence of chirality in both anionic headgroup and the counterion; (b) T12M remains as room temperature IL for 3 days and then transforms to a semisolid with melting point at ～55 °C. The d-spacings in solid T12M, and T12M lyophilized from its aqueous solution, are 13.89 and 14.54 ?, respectively. (c) Tartaric acid is unconventional and unprecedented starting material for the synthesis of ILs. (d) T12M dissolves in both hydrogen bonding (water) and non-hydrogen bonding (chloroform) solvents and forms anionic chiral micellar aggregates (CMAs) and reverse-CMAs, at very low concentrations 0.32 mM and ～10 mM, respectively. (e) CMAs of T12M adopt structures ranging from spherical to lamellar in shape in water in the 10-200 mM range; however, the zeta potential remained constant at ～ -13 mV. The alkyl chains, are interdigited in the CMAs of T12M in water to form lamellar structures and are extended outward to form reverse micelles in CHCl₃.

Full text of DOI:10.1021/acs.jpcb.6b11964

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Article
The First Room Temperature Chiral Anionic
Liquid Forming Micelles and Reverse Micelles
Vijay Raghavan, and Prasad Leela Polavarapu
J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.6b11964 • Publication Date (Web): 25 Jan 2017
Downloaded from http://pubs.acs.org on January 27, 2017
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The First Room Temperature Chiral Anionic Liquid
Forming Micelles and Reverse Micelles
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Vijay Raghavan and Prasad L Polavarapu*
Department of Chemistry, Vanderbilt University, Nashville, Tennessee 37235
ABSTRACT: We report the first chiral surface active anionic liquid, T12M, derived from
biodegradable tartaric acid, and its unusual properties. T12M features unprecedented
combination of characteristics not found in other ionic liquids (ILs): (a) T12M is the first surface
active ionic liquid (SAIL) that is fully chiral, by virtue of the presence of chirality in both anionic
head group and the counter-ion; (b) T12M remains as room temperature IL for 3 days and then
o
transforms to a semi-solid with melting point at ~55 C. The d-spacings in solid and lyophilized
aqueous T12M, are 13.89 Å and 14.54 Å respectively; (c) Tartaric acid is unconventional and
unprecedented starting material for the synthesis of ILs. (d) T12M dissolves in both hydrogen
bonding (water) and non-hydrogen bonding (chloroform) solvents and forms anionic chiral
micellar aggregates (CMAs) and reverse-CMAs, at very low concentrations 0.32 mM and ~10
mM, respectively. (e) CMAs of T12M adopt structures ranging from spherical to lamellar in
shape in water in the 10-200 mM range; however, the zeta potential remained constant at ~ -
13mV. The alkyl chains, are interdigited in the CMAs of T12M in water to form lamellar
structures and are extended outwards to form reverse micelles in CHCl3.
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INTRODUCTION
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All most all ionic liquids (ILs) reported so far contain achiral imidazolium, pyrrolidinium or
ammonium cation group.1-9 Surface active ionic liquids (SAILs) are a class of ionic liquids that,
as the name implies, are also surface active10-15. Many researchers have also reported surface
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¾
activity of achiral ILs containing short alkyl chains , but their critical micelle concentrations
(CMCs) and pC20 are inferior by several orders in comparison to those for conventional
surfactants.16, 17 Such short chain ILs cannot serve as standalone surfactants and the formation of
microemulsions was not even possible18-20. Both the ILs and SAILs derived from imidazolium
or related groups were popular for their —green chemistry“, but have recently come under
criticism as 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 effluents.21-24 Also the
syntheses of ILs/SAILs derived from imidazolium or related groups involve multistep processes
involving corrosive reactants and potentially harmful chemicals and are therefore not truly green
solvents.
However, SAILs cannot be ignored in view of their extraordinary properties both as solvents
and superior surfactants and their uses in various applications. SAILs (or ILs), have negligible
vapor pressure, low melting point, large electrochemical window, and thermal stability.25, 26 In
spite of several advantages, the broader uses of ILs/SAILs as solvents or surface active solvents
are limited because of their limited ability to, solvate compounds or dissolve in solvents of
varying polarity, such as water and chloroform. Some ILs such as 1-methyl-3-octylimidazolium
chloride show limited solubility even in water (< 5mM)16. From empirical observations27, ILs are
known to be immiscible in nonpolar solvents such as diethyl ether and hexane.
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The research on chiral ionic liquids (CILs) is in its infancy28-31 and have applications in
asymmetric synthesis32, highly selective transport membranes33, 34, stationary phases for gas
chromatography35, 36 and in the design of chiral nano materials using chiral micellar aggregates
(CMAs) as templates. Most of the CILs known to date are derived from chiral organic cation
and achiral inorganic counter-ion with limited scope for structural modifications. CILs from
achiral imidazolium, pyridinium, tetrabutylammonium and pyrrolidinium cations and chiral
anions (such as amino acids, lactic acid and tartaric acid) have been reported.37-40 The CILs with
alkyl chain varying between 0-8 carbons atoms37-40 conceivably would have no surface activity
or may have inferior surface activity equivalent to that of a co-surfactant. A few CILS were also
derived from surface active achiral lauryl sulfate anion and chiral amino acid cation.21, 41 Such
systems may not be classified as surface active CILs, as the influence of the chirality of counter
ion may only be limited to the Stern layer. Furthermore, just as for ILs/SAILs, the limited
solubility of CILs in solvents of varying polarity limits the broader use of CILs in various
application including asymmetric synthesis.19, 38, 42
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As SAILs are generally high molecular weight compounds they are not expected to remain as
liquids at room temperature43. Some high molecular weight room temperature SAILs are known
to exist but are certainly rare. 31
To the best of our knowledge, CILs with chirality endowed to both anion and cation, and are
surface active, have not been reported. In these contexts, we report the discovery and unusual
properties of the first chiral surface active anionic liquid (T12M) derived from biodegradable
tartaric acid. Its preparation is facile and —green“ and can be designed to completely eliminate the
bi-products. Most importantly, unlike other SAILs/ILs, T12M can dissolve in both hydrogen
bonding (water) and non-hydrogen bonding (chloroform) solvents at very low concentrations,
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and can form CMAs. This is also the first report demonstrating that biodegradable tartaric acid
moiety can be used to synthesize surface active CILs, opening a pathway for a novel family of
surface active CILs. It is to be noted that chiral tartaric acid has been used before to derive
surfactants,44-47 to provide counter ion for IL forming achiral cations37-39 and to study the
entrapment of its esters in achiral reverse micelles,48 but not to derive surface active CILs. Due to
the ubiquitous use of tartaric acid in enantioselective reactions49, 50, tartaric acid based surface
active CILs can offer widespread applications.
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EXPERIMENTAL
L-tartaric acid (99%) was obtained from Eastman Organic Chemicals. D-tartaric acid (99%),
dodecanol (98%), concentrated sulfuric acid (95-98%), pyrene (99%), 3,4-
dimethylbenzophenone (99%), and benzene (>99%) were obtained from Sigma-Aldrich. Diethyl
ether (anhydrous, 99.9%) was obtained from Fisher Scientific and Cetylpyridinium chloride
(>98.5%) from TCI America.
Synthesis of Diacetyl-L-tartaric anhydride (L-DTA), Lauryl monoester of diacetyl-L-
tartaric acid (T12OH) and chiral anionic liquid (T12M): L-DTA was first synthesized to
prepare lauryl monoester of diacetyl-L-tartaric acid. The synthesis of L-DTA was accomplished
by reaction between 10 g (0.028 mole) of L-tartaric acid and 15.8 mL of acetic anhydride (0.167
mole) containing 0.15 mL of concentrated sulfuric acid under reflux conditions for 15 minutes.
The crude product was cooled in an ice bath and was collected on a Büchner funnel. The product
was washed with benzene twice and then with diethylether. The product was dried under
vacuum.
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T12OH was synthesized (see Scheme 1) by reacting 1.6 mL of dodecanol (6.9 mmole) and 3 g
of L-DTA (13.9 mmole) under inert atmosphere in chloroform under reflux conditions. L-DTA
was added in small increments of 0.5g. The reaction was allowed to stir overnight. The crude
product was collected and was washed with 0.1 M hydrochloric acid several times. The washing
was performed by using vortex shaker and centrifuge. Finally, chloroform was added and the
organic layer was extracted. The CHCl3 was evaporated under vacuum. The diethyl ether was
added to dissolve the product and dried over MgSO4. The product was obtained by evaporating
diethyl ether under vacuum.
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T12OH was mixed with 1 equivalence of (S)-(-)-.-methylbenzylamine (MBA), to obtain the
chiral ionic liquid, T12O-MBA+, which is designated as T12M.
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1H, C NMR and 2D ROESY experiments: H, C NMR and 2D-ROSEY experiments
were performed using a Bruker 400 MHz spectrometer equipped with a 9.4 Tesla Oxford magnet
which is controlled by a Bruker AV-400 console.
Differential Scanning Calorimetry: Differential scanning calorimeter (DSC) experiments
were run at 10oC/min under nitrogen on a TA Instruments Q1000.
Tensiometry: Surface tension measurements were performed on a Fisher Scientific
tensiometer (Model 21), employing a platinum du Nuoy ring as the probe. The surface tension
values reported here are the averages of at least three measurements and represent the
equilibrium values.
Vibrational and Electronic circular dichroism: Vibrational circular dichroism (VCD) and
associated vibrational absorption (VA) measurements were made using ChiralIR spectrometer. A
50 ꢀm fixed path length cell, with BaF2 windows was used. Electronic circular dichroism (ECD)
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and associated electronic absorption (EA) measurements were made using a JASCO 720
spectrometer using a 0.01 -1 cm quartz cells.
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Steady-State Fluorescence quenching (SSFQ) experiments: SSFQ experiments were
performed to determine the aggregation number of T12M in water and to estimate the CMC of
T12M in CHCl3.
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Estimation of aggregation numbers: Pyrene was used as the fluorescence probe.
Cetylpyridinium chloride (CPC) quencher was used for T12M concentrations at 10 and 50 mM.
3,4-dimethylbenzophenone (DMBP) was used as quencher for T12M concentrations at 100 and
200mM. In view of low solubility of pyrene in water (1 þ 10œ7 M), a required volume of stock
solution of pyrene in methanol (1 þ 10œ4 M) was pipetted into a standard flask, and a thin film of
pyrene was deposited on the side of the flask through evaporation of solvent and purging with N2
gas. The effective concentration of pyrene was maintained at 1 þ 10œ6 M in all of the solutions.
The concentration of the quencher was varied from 9x10-5 M to 12x10-4. The fluorescence
spectra of pyrene in T12M solutions and in the presence of quencher at different concentrations
were recorded with a JASCO spectrofluorometer FP-8300. The slit widths of excitation and
emission were maintained at 5 nm. The excitation wavelength (ꢁexc) was set at 337 nm and that
of emission was set at 374 nm. The aggregation numbers, N, of T12M were determined using the
Equations 1 and 251.
[q]
I
m
o q L ATL m
Iq
q……………………(1)
[micelle]
[c - CMC]
[micelle]
0 L ATL m
q ……………………..(2)
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where I0 and Iq are the intensities of fluorescence emission at 374 nm of pyrene in the absence
and presence of the quencher of concentration [q], respectively. c is the total surfactant
concentration. All measurements were performed at 24 ± 0.1 °C.
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Estimation of CMC of T12M in CHCl3: T12M solutions in CHCl3 in the 1x10-3 - 200 mM
range were added to flasks containing pyrene. The effective concentration of pyrene was
maintained at 1x10-6 M. The pyrene was deposited in the same manner as it was deposited to
estimate aggregation number. The ratio of the intensities of the third I3 (384 nm) to first I1 (373
nm) vibronic peak of the fluorescence spectrum of the pyrene probe was used as an estimate of
the micropolarity of the pyrene microenvironment.
Dynamic Light Scattering Measurements (DLS): DLS measurements were carried out to get
an estimate of the size of aggregated structures and to estimate the CMC for T12M solutions.
The measurements were performed using a DLS Malvern Nano ZS. The samples were filtered
using 0.45 ꢀm membrane filter.
X-ray diffraction measurements: The X-ray diffraction patterns of T12M were measured by
Scintag X-1 powder X-ray diffractometer using a zero background sample holder and a
monochromatic Cu Ka (45 kv, 40.8 mA) radiation (1.54 A°) in 2ꢂ ranges of 1 to 25°.
Computational Methods: The conformation of T12M was generated, utilizing the
conformational search with CONFLEX program as follows. Since CONFLEX program does not
handle the ionic species properly,52 a molecular model of diacetyl-L-tartaric acid was input into
the CONFLEX program and conformational search was undertaken using MMFF94S molecular
mechanics force field. The 115 conformers obtained within 7 kcal/mol are further optimized at
B3LYP/aug-cc-pVDZ/PCM level. The minimum energy conformer determined in this manner
was used to fix the structure of the head group and to build the structure of T12M by replacing H
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However, the use of reactants at 1:1 mole ratio and without solvents yielded the product with
purity > 97% and without any bi-products. The purity of T12OH was reproducible only when
freshly synthesized L-DTA was used, instead of the commercially available L-DTA. The
commercially available L-DTA degraded with time on storing and led to issues with
reproducibility. The purity of T12OH could not be increased any further by simple washings. It
is worth mentioning that T12OH foams in water, so sparingly soluble T12OH and its highly
soluble sodium salt, T12O-Na+, can be used as chiral surfactants. Therefore, our attempts to
remove diacetyl-L-tartaric acid from T12OH using water did not work due to the formation of
emulsion. The emulsion could not be broken with addition of NaCl solution.
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Figure 1. (A) 1H and (B) 13C NMR of diacetyl-L-tartaric anhydride
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B
Figure 2. (A) 1H and (B) 13C NMR of lauryl monoester of diacetyl-L-tartaric acid (T12OH)
However, the use of 0.1 M HCl solution for washing selectively decreased the solubility of
T12OH in water, which facilitated the removal of diacetyl-L-tartaric acid impurities. It may be
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noted that the emulsion formed by T12OH in 0.1 M HCl could be easily broken by
centrifugation.
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It is worth mentioning that T12OH is a liquid, even though diacetyl-L-tartaric acid and
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dodecanol exist as a solid and paste respectively, at room temperature (~24+1 C). This
observation indicates that diacetyl tartaric acid when attached to long chain alcohol of suitable
length can destabilize the solid-phase crystals, similar to the properties of imidazolium ions1.
T12M, obtained by mixing T12OH with liquid (S)-MBA, also exits as a liquid for 3 days. The
use of any non-toxic and biodegradable liquid counter ion, in lieu of MBA used here for proof-
of-principle, is expected to make the resulting SAIL completely non-toxic and biodegradable.
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0
-1
-2
-3
-4
-100
-50
0
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100
Temperature o
C
Figure 3. DSC trace of T12M on day 5.
T12M exits as liquid at room temperature for 3 days, becomes pasty on fourth day and finally
appears as wet solid on 5th day. The differential scanning calorimetry (DSC) trace of T12M after
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equilibration for 5 days (Figure 3) shows only one endotherm with onset temperature at 51 C,
which has also been verified with the melting point of solid T12M. Absence of other endotherms
indicates that all of T12M exists in one phase on day 5. The existence of T12M as room
temperature IL for 3 days is therefore uncommon. Nevertheless, T12M can offer both the
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benefits of room temperature and high temperature IL. High Temperature SAILS can be hugely
beneficial for emulsion polymerization techniques. The emulsion polymerization is usually
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performed at 60-70 C. At this temperature T12M can act both as solvent or surfactant. After
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polymerization, excess T12M can be recovered by simply storing the emulsion at room
temperature as described elsewhere 53 for conventional surfactants.
The structure of T12M solid, after 5 days of equilibration, was investigated using low angle
powder X-ray diffraction data, as governed by Bragg‘s law (Equation 3):54
nl=2dhkl sinqhkl------------(3)
where n is an integer; ꢁ is the wavelength of probe radiation; d is the spacing between diffraction
planes defined by Miller indices h, k, and l; and ꢂ is the diffraction angle. The d-spacings of
successive peaks of lamellar and hexagonal structures appear in the ratio (1: 0.5: 0.33) and (1:
0.71: 0.58) respectively. 54, 55
The powder X-ray diffraction data of T12M solid, along with that of T12M lyophilized from
200mM aqueous solution, are shown in Figure 4. The data of T12M lyophilized from 200mM
aqueous solution show peak positions in the ratio 1: 0.5: 0.33 which indicates the presence of
lamellar structure. The corresponding ratios for T12M solid, 1: 0.52: 0.34, indicate slightly
distorted lamellar structures. The d-spacings for T12M lyophilized from water (200 mM) and for
solid, respectively, are at 14.54 Å and 13.89 Å, and are less than the length of hydrophobic
chain, 16.68 Å, derived from Tanford equation (Equation 4)56:
Lmax=1.5+1.265 n …….
(4)
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where Lmax is maximum stretching length of alkyl chain and n is number of carbons in the alkyl
chain. This observation indicates that alkyl chains are interdigited in both (i.e. T12M after 5 days
of equilibration and T12M lyophilised from 200 mM aqeous solution). In other words, T12M
liquid upon dissolution in water at 200 mM, and lyophilization, acquires well organized lamellar
structure which is evident from the X-ray difraction data of the lyophilized sample. On the
otherhand, the slow trnsformation of T12M liquid, upon equilibration, to solid involves the
transformation of disorganised liquid structure to partially organized lamellar structure. A single
chain surfactant, with a relatively short chain, forming lamellar structure is rare.57 Also, as
mentioned earlier, high molecular weight room temperature SAILs are rare.31, 43 It is unclear if
the high molecular weight room temperature SAILs31 reported in literature remain as liquid for
prolonged periods or transform to pasty substances with time as T12M did, because SAILS have
propensity to aggregate.
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14.54 Å
(a)
(b)
4.85 Å
7.27 Å
13.89 Å
14.32 Å
4.78 Å
7.16 Å
5
10
15
20
25
2q (degrees)
Figure 4. Powder X-ray diffraction data of T12M (a) lyophilized from 200 mM aqueous solution and (b) solid.
The chiroptical properties of T12M were characterized using VCD/VA and ECD/EA spectra
shown in Figure 5. In the vibrational spectra of aqueous T12M solution at 200mM, the band at
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1743 cm-1 is associated with the ester carbonyl group; the band at 1623 cm-1 is associated with
COO- group; the bands at 1562 cm-1, 1500 and 1458 cm-1 are associated with C=C stretching
vibrations of the MBA; the band at 1384 cm-1 is associated with CH bending vibrations. The
VCD spectrum shows a bisignate couplet corresponding to the carbonyl group stretching
vibrations at 1743 cm-1 and a negative VCD associated with stretching vibration of COO- group.
The electronic spectra of aqueous T12M solution at 10 and 200mM spectrum shows absorbance
peak, associated with negative ECD, at ~212 cm-1. Further discussion on the abovementioned
CD bands and extensive investigations on chiroptical properties of the enantiomers and
diastereomers of T12M will be reported in a future article.
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A
B
2
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6
0
10 mM
200 mM
-6
-2
1900 1800 1700 1600 1500 1400
-12
200
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20000
10000
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0.5
0.0
1900 1800 1700 1600 1500 1400
Wave numbers (cm-1)
200
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275
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Wavelength[nm]
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Figure 5. Circular dichroism spectra of T12M. (A) VCD (top) and vibrational absorption (bottom) spectra at 200 mM in
D2O; (B) ECD (top) and (B) electronic absorption (bottom) spectra at 10 and 200 mM concentrations in H2O.
5
6
Surface Properties, Micellization Parameters and Aggregation Behavior: The plot of
surface tension (ꢀ) with molar concentration of T12M on the log scale (log C) is shown in Figure
6. It is observed that ꢀ undergoes very steep decrease from about 72 mN/m to about 36 mN/m
over the narrow concentration range of 1 þ 10Þ5 to 3.2 þ 10Þ4 M (Figure 6). Above the
concentration of 3.2 þ 10Þ4 M, ꢀ remains almost constant at 38 mN/m and this is indicative of
coverage of the air/solution interface with the monolayer of T12M. The concentration
corresponding to the onset of ꢀc is taken as the critical micelle concentration (CMC), which is
3.2 þ 10Þ4 M; it is the concentration at which the slope of the decrease in surface tension meets
equilibrium surface tension. The value of pC20, which measures the efficiency of adsorption of
the surfactant at the air-water interface51 was high at 4.2. The low CMC of T12M in comparison
to those for conventional surfactants is due to mutifunctionality in the head group. Several SAILs
and cationic/anionic surfactants with multifunctionality in the head group were endowed with
low CMC and better interfacial characteristics 4, 58-63. It was reported that COOH-functionalized
imidazolium-based SAILs possess lower CMC and superior surface activity compared to
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homologous
non-functionalized
SAILs4.
Long-chain
ꢃ-hydroxy-ꢄ-alkyloxy-N-
methylimidazolium ILs58 have superior surface activity compared with the simple alkyl-
substituted derivatives. Several cationic/anionic surfactants have been synthesized before with
mutifunctionlaity in the head group which conferred low CMC and better interfacial
characteristics to the surfactant59-62. In fact a cationic surfactant from tyrosine, with
mutifunctionality in the head group, had physiochemical properties similar to nonionic
surfactants and with low CMC at ~10-5 M.63, 64
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Applying Gibbs adsorption isotherm equation (Equation 5)51, the surface coverage (+) and
minimum surface area (a) of packing/molecule have been estimated (Table 1).
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ꢄ = ÞnRT×+×ln×C = Þ2.303nRT×+×log×C, a=1023/N+ …………..(5)
where ꢀ refers to surface tension in mN/m, R refers to gas constant = 8.31 J molÞ1 KÞ1, T refers to
temperature = 298.15 K, + refers to surface excess concentration, C refers to concentration in
mol/L, a refers to area per molecule at the interface in Å2 and n=(1+.), where . is the fractional
dissociation. For 1:1 ionic surfactants, . =1 and n=2. In the presence of swamping of electrolyte
containing common non surfactant ion51 n=1. As . for T12M is not assessed, the packing
characteristics of T12M are summarized in Table 1 for both cases of n=1 and 2.
Table 1: Adsorption characteristics of T12M
n
1
2
+ (µmol/m2)
a (Å2/ molecule)
3.1
1.6
53
106
75
60
45
30
-6
-5
-4
-3
-2
-1
0
log [T12M] in water (M)
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Figure 6. Surface tension profile of T12M.
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The CMC derived from tensiometry was corroborated with correlation coefficient vs time at
different concentrations, and photon counts vs concentrations, using DLS (Figure 7). The plot of
correlation coefficient vs. time is most important but often ignored data that can be obtained
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1.0
A
0.1 mM
0.5mM
1mM
0.8
0.6
0.4
0.2
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10mM
50mM
100mM
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10-1
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109
Time (ms)
B
100
10
10-4
10-3
10-2
10-1
100
101
102
103
[T12M] in Water (mM)
Figure 7. Plot of the correlogram (A) and photon counts (B) as a function of concentrations of T12M in water.
from DLS. The correlation coefficient vs time is extremely sensitive and can be used to identify
the presence of contaminants and in the estimation of size of the particles.65-67 The plot of
correlation coefficient vs. time for T12M at 0.1 mM œ 200 mM is shown in Figure 7A. The
photon counts from back scattered light by T12M solution remains constant (see Figure 7B) at
~11.6 kc/s at concentrations 0.001- 0.1 mM. At concentrations >0.1 mM, the intensity of the
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scattered light increased due to the presence of well-defined particles in the T12M solution,
resulting in increased correlation coefficient with increase in concentration. The correlogram for
T12M at 0.1 mM is not well defined indicating the absence of aggregates at 0.1mM. A well-
defined correlogram for T12M could be seen at 0.5 mM indicating the formation of micelles at
this concentration. The correlograms also show the presence of a slow mode, as revealed by a
small hump, at ~102 µs which could be due to unavoidable contaminants such dust particles,
micro foam bubbles, viruses or even bacteria.65-67 The data from photon counts of laser light
scattered by T12M solution in Figure 7B suggest that micelles begin to form at concentration >
0.1mM and the size of micelles increases with the surfactant concentration. The CMC deduced
from the correlograms and photon counts in Figure 7 is in close agreement with that from
tensiometry.
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DLS data suggest that the mean micelle size of T12M, increased from 1.4-3.1 nm in the
concentration range of 10-200 mM (Figure 8). The aggregation number (N) for T12M in the 10-
200 mM range, determined from total surface area of the micelle (estimated from the size of
micelles from DLS at respective concentrations) and the surface area per molecule estimated
from tensiometry, would be in the range of 4-6. Since N cannot be so small, it is inferred that the
size of the micelles is underestimated by DLS. The underestimated size of micelles is due the
interference caused by the scattering of light by the large unavoidable particulate contaminants,
the presence of which is confirmed by the presence of a small hump in the correlogram presented
in Figure 7A. The contaminants can interfere with the scattering of light by micelles because (a)
micelles do not have high refractive index and (b) they are very small in comparison to
contaminants. As per the Rayleigh approximation the intensity of the scattered light is
proportional to 6th power of its diameter66. As a result, the light from the larger contaminants can
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overwhelm the light scattered by the smaller ones making it difficult to currently estimate the
size of the smaller particles with sufficient accuracy. 66
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Although the size of the aggregates arrived from DLS appears to be under-estimated, the trend
in the growth of aggregates is real, as the increase in size of aggregates coincides with the
aggregation numbers determined using SSFQ technique (Figure 8). The aggregation numbers of
T12M are 57, 84, 110 and 218 for respective concentrations at 10, 50, 100 and 200 mM. These
aggregation numbers can be useful in accessing the shape of the aggregates. For instance, the
aggregation number ~ 90 is attributed to rod like micelle and < 90 is attributed to spherical
micelles68. The lamellar structures are known to have large aggregation number ~20057.
Therefore, the aggregation numbers of T12M in water determined from SSFQ demonstrate that
CMAs of T12M exist as spherical micelles at 10 mM and as rod like micelles at 50 mM and in
lamellar structures at 200mM. The existence of T12M in lamellar structures at 200 mM
corroborates with X-ray diffraction data of the lyophilized T12M solution at 200 mM. The zeta
potential of T12M at 10-200 mM is almost constant at -13.8+ 0.8 mV (Figure 8). Importantly,
zeta potential data demonstrate that aggregates are anionic in nature. In other words, all the
MBAs do not form intimate ion pair with T12OH. Also, a constant zeta potential with increase in
the size of micelles indicates that the ratio of T12OH:MBA is maintained constant with increase
in the size of micelles.
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240 3.5
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80 1.5
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40 1.0
0
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100
150
200
[T12M] in water (mM)
Figure 8. Micelle size (black, squares) and zeta potential (blue, circles) from DLS, aggregation number (pink,
triangles) from SSFQ, for T12M at various concentrations in water.
7.390
m-ArH
7.370
7.350
1.270
CH
2
1.248
1.225
0.900
CH
3
0.865
0.830
1
10
100
[T12M] in CHCL (mM)
3
Figure 9. Chemical shift values (ppm) of various groups of T12M as a function of concentration in CDCl3.
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1.14
1.12
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1.06
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10-4
10-3
10-2
10-1
100
101
102
103
[T12M] in CHCl (mM)
3
Figure 10. I1/I3 ratio of pyrene fluorescence quenching for T12M in CHCl3.
In non-aqueous solvent, chloroform, T12M formed reverse micelles. The CMC of T12M in
1
chloroform, determined from H NMR (Figure 9), was ~37 mM; that from fluorescence
quenching experiments with pyrene (Figure 10) was ~10 mM; the former value is probably an
overestimate because of dominant changes in ppm values of protons due to ring current effect of
the aromatic groups of MBA explained vide infra. The scattering intensity of reverse micelles of
T12M in chloroform was too low to get meaningful results from DLS.
Structural Analysis from NMR and computational studies: 2D-ROESY data and the ring
current effect associated with aromatic group has been used to arrive at the architecture of
micelles and biomembranes69-71. The ring current associated with phenyl ring of MBA can shield
or deshield neighboring protons through space which would cause13 respective up-field or down-
field chemical shift values (/), revealing the conformation of the surfactants in micelles72. The
ring current effect is governed by Equation 6.
AåÖ L ©":s F u ?KO6 à;ꢀN7) ………..(6)
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where —i“ is a ring-current factor, B is the constant of proportionality, ꢂ is the angle (between
aromatic electron cloud and proton), and r is the distance (from aromatic ring to proton) The
chemical shifts for methine (CH) and acetyl (OAc) protons of tartaric acid and for .-CH2 and ꢃ-
CH2 protons of alkyl chain (Figures 11 and SI.1.) of T12M progressively shift up-field with
increase in concentration from 5-200 mM indicating that these protons are in close proximity to
MBA and present in the shielded region. Only the methyl group protons of the alkyl chain
showed downfield / (Figure 11) indicating its presence in the de-shielded region of MBA. It is
conceivable that the methylene protons adjacent to methyl protons of the alkyl chain would be
present in the deshielded region of MBA; the methylene protons adjacent to the .-CH2 protons of
the alkyl chain are expected be present the shielded region of MBA. The presence of some CH2
protons of the alkyl chain in the shielded and some in the deshielded regions of MBA renderers
overall line broadening (see SI, Figure S1E) and random changes in their chemical shift
values(/) with concentration (Figure 11g).
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5.410
5.390
5.370
5.66
5.64
5.62
(a1)
(a2)
0
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200
200
200
0
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200
4.148
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2.145
2.115
2.085
(b)
(c)
0
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1.630
1.585
1.563
1.540
(d)
(e)
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0
(g)
1.258
1.256
1.255
0.867
0.863
0.860
(f)
0
0
[T12M] in D O (mM)
2
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Figure 11. (A) Chemical shift (w) values of T12M at 200mM in D2O from H NMR spectra showing proton peaks for:
(a1, a2) CH of tartaric acid; (b) .-CH2 of alkyl chain, (c) OCH3 of tartaric acid, (d) CH3 of MBA, (e) ꢀ-CH2 of alkyl
chain, (f) other CH2 of alkyl chain and (g) CH3 of alkyl chain.
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1
The 2D H-1H ROESY NMR (Figure 12A) of 200mM T12M solution show cross peaks
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corresponding to MBA and all protons of T12M indicating that all protons of MBA and T12M in
the aggregate at 200mM are within the distance of 5-6 ). In other words, the proximity of
aromatic protons of MBA and alkyl chain protons of T12M, including the methyl protons,
determined by ring current effects is supported by 2D ROESY experiments (Figure 12A). The
A
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Figure 12. 1H-1H 2D ROESY spectra of T12M at 200 mM in (A) D2O and (B) CDCl3. Inset shows the expanded view
of cross correlation peaks from 7.2-7.45 ppm.
protons of methyl group of the alkyl chain of T12M can come in proximity of the MBA only in 2
ways: (a) when the alkyl chain of T12M loops around; or (b) when the aggregates are
interdigited. The large up-field shift of methyl groups in the alkyl chain of T12M and the results
from 2D experiments can be only supported by option (b) as it would be impossible for the most
of alkyl chain of T12M to loop around. In fact, option (b) corroborates the X-ray data.
1
2D H-1H ROESY NMR experiments of T12M in CDCl3 (Figure 12B) show no cross
peaks corresponding to MBA and the methyl protons in alkyl chain of T12M which indicates
that the protons of methyl group and aromatic protons of MBA are far away from each other.
The methyl protons in the alkyl chain of T12M can be far away from MBA only when they (a)
don‘t form micelles or (b) when they form reverse micelles. Data from change in chemical shift
values (ppm) of various groups of T12M as a function of concentration in CDCl3 (Figure 9) and
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pyrene fluorescence quenching for T12M in CHCl3 (Figure 10), indicate the formation of
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1
reverse micelles in chloroform. Thus, it could be inferred that 2D H-1H ROESY NMR
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experiments of T12M in CDCl3 support the presence of reverse micelles.
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Figure 13. Rendering of micellar arrangement for T12M at 200mM in water.
One of the most important results from the NMR experiments is the progressive change in / of
protons with increase in concentration of T12M (Figure 11) which is indicative of increase in
the packing density of micelles. The extent of change in / is highest between 5 and 10 mM and
relatively less between 100 and 200 mM. This observation indicates that spherical/rod like
micelles at ~10-50 mM have more compact arrangement than lamellar structures at 200mM. The
arrangement for CMAs of T12M at 200 mM that is consistent with all of the experimental data is
depicted in Figure 13. This rendering enlightens as to why almost all protons of T12M are in the
shielded or de-shielded region.
CONCLUSIONS
We have discovered and characterized the first chiral SAIL from tartaric acid, T12M, that (a).
dissolves and forms aggregates in both hydrogen bonding (water) and non-hydrogen bonding
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(chloroform) solvents; (b). serves as both room temperature and high temperature IL; (c). is truly
green and is also biodegradable. This discovery paves the way for novel applications and for the
syntheses of additional new ILs and SAILs from tartaric acid.
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ASSOCIATED CONTENT
Supporting Information.
The following files are available free of charge.
SI 1 (A) 1H NMR spectra of T12M at 200mM in D2O showing proton peaks of (A) ArH of
MBA, (B) CH of tartaric acid, (C) .-CH2 of alkyl chain, (D) OCH3 of tartaric acid, (E) CH3 of
MBA, (F) ꢃ-CH2 of alkyl chain, (G) CH2 of alkyl chain and (H) CH3 of alkyl chain. (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: Prasad.L.Polavarapu@Vanderbilt.edu
Funding Sources
The authors thank National Science Foundation for financial support (CHE 1464874).
Notes
¾
Many of the conventional anionic SAILS can be merely considered as co-surfactants and not as
conventional surfactants given the fact that their CMCs are over 100 mM. In this work, the use
of —SAIL“ is meant to refer to ILs that are really surface active and can behave as standalone
surfactants.
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ACKNOWLEDGMENT
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We thank Dr. Donald Stec for help with ROESY experiments and Professor Anil Kumar for the
use of tensiometer.
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ABBREVIATIONS
IL ionic liquids; T12M dodecyl ester of diacetyltartaric acid; SAIL surface active ionic liquid;
CMA chiral micellar aggregates; CMC critical micelle concentration; CIL chiral ionic liquids;
T12OH diacetyl-L-tartaric acid; MBA (S)-(-)-.-methylbenzylamine; NMR Nuclear magnetic
resonance spectroscopy; 2D-ROSEY Two-dimensional nuclear magnetic resonance
spectroscopy, VCD Vibrational circular dichroism; VA vibrational absorption; ECD Electronic
circular dichroism; EA electronic absorption; SSFQ Steady-State Fluorescence quenching; CPC
Cetylpyridinium chloride; DMBP 3,4-dimethylbenzophenone; DLS Dynamic Light Scattering
Measurements; DSC Differential scanning calorimetry; L-DTA diacetyl-L-tartaric anhydride.
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