Ionic Liquids Derived from Proline: Application as Surfactants

Article abstract of DOI:10.1002/cphc.201800735

Ionic liquids derived from prolinium esters, previously described as fully green and stable, were found to decompose in the presence of water by ester hydrolysis. To avoid this problem, a new family of these biodegradable salts incorporating an alcohol instead of the ester group is proposed. From this family, two novel ionic liquids that incorporate the prolinolium cation [HOPro] and the [DS] or [DBS] anion were selected (DS=dodecylsulfate; DBS=dodecylbenzenesulfonate). Both salts are liquid at room temperature, a property not usually found in ionic surfactants, and are also chemically and thermally stable. Moreover, they are more effective in reducing the surface tension of water than the corresponding traditional surfactants in the form of sodium salts, being useful for applications related to their aggregation capacity. They were tested for surfactant enhanced oil recovery and an optimal formulation for reservoirs at high salinity and temperature, able to produce ultra-low interfacial tension, was found with [HOPro][DBS].

Full text of DOI:10.1002/cphc.201800735

Accepted Article
Title: Ionic Liquids derived from proline: application as surfactants
Authors: Verónica Fernández-Stefanuto, Raquel Corchero, Iria
Rodríguez-Escontrela, Ana Soto, and Emilia Tojo
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: ChemPhysChem 10.1002/cphc.201800735
Link to VoR: http://dx.doi.org/10.1002/cphc.201800735
A Journal of
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10.1002/cphc.201800735
ChemPhysChem
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Ionic Liquids derived from proline: application as surfactants
Verónica Fernández-Stefanuto[a], Raquel Corchero[b], Iria Rodríguez-Escontrela[b], Ana Soto[b], and
Emilia Tojo*[a]
Abstract: Ionic liquids derived from prolinium esters, previously
described as fully green and stable, were found to decompose in the
presence of water by ester hydrolysis. To avoid this problem, a new
family of these biodegradable salts incorporating an alcohol instead
of the ester group is proposed. From this family, two novel ionic liquids
that incorporate prolinolium cation, [HOPro], and [DS] or [DBS] anion
were selected. Both salts are liquid at room temperature, a property
not usually found in ionic surfactants, as well as chemically and
thermally stable. Moreover, they are more effective in reducing the
surface tension of water than the corresponding traditional surfactants
in the form of sodium salts, being useful for applications related to
their aggregation capacity. They were tested for surfactant enhanced
oil recovery and an optimal formulation for reservoirs at high salinity
and temperature, able to produce ultra-low interfacial tension, was
found with [HOPro][DBS].
SAILs show a wide number of applications that derive from
the two fundamental properties of surface active agents in
aqueous solution: adsorption at the surface or interface, and
aggregation. Conventional ionic surfactants, being salts with
melting points below 373.15 K, have only rather recently been
seen as ILs. Such is the case of dodecyl-dimethylbenzyl-
ammonium chloride, which is widely used as a bactericide. In fact,
even solvents used for decades in the large-scale production are
only recently considered as ILs able to produce
a safe
environment.[9] The attention paid in recent decades to ILs is
leading to the synthesis of new surfactants able to be used in
applications where the choice of conventional surfactants is
limited. Collins et al.[10] were pioneering on presenting synthetic
methods and applications of SAILs. Depending on the
hydrophilic-lipophilic balance index, they cited some applications
that include their use for the preparation of foams for mobility
control, the dispersion of oil spills, as emulsifiers or wetting agents,
as hydrophobic solvents, as lubricants or heat transfer agents in
drilling fluids, as inhibitors of gas hydrates or corrosion in oil
production, as recovery-enhancing additives in oil recovery,
hydraulic fluids, milling or cutting fluids, phase transfer agents or
drug delivery agents, and as chemical reaction media (e.g. for
micellar catalysis). Besides the general advantages of ILs, the
delocalized charges associated to the special head-groups of
these compounds can also deliver special interfacial properties to
the systems in which they are included.[11]
Introduction
Decades ago, Ionic Liquids (ILs) started to attract a lot of attention
due to their “green” properties. However, the scientific community
immediately began to notice that this “green” character was solely
due to their practically negligible vapor pressure, associated with
an absence of air contamination. ILs are frequently rather toxic
and often present limited biodegradability.[1] Instead, the main
advantage of ILs is that they can be functionalized in order to get
a specific physical, chemical or biological property or even for a
specific application.[2,3] Thus, cations, anions, the length and
shape of alkyl side-chains, and functional groups can be selected
in order to obtain, for instance, a green (in the widest sense of the
word) IL or a Surface Active Ionic Liquid (SAIL).
Amino acid ILs have been highlighted as ‘‘fully green’’
alternative solvents to traditional ILs having synthetic chemical
components.[4] Their use has been proposed as solvents or
catalysts,[4] for CO2 capture,[5] and for the separation of the main
compounds of lignocellulosic biomass.[6,7] Moreover, as they have
the general advantages of ILs but also possess better
biocompatibility for enzyme molecules, their application in
enzyme immobilization, enzymatic biocatalysis, and enzyme
biosensor are very promising.[8]
Trivedi et al.[12] chose natural amino acids and sodium lauryl
sulfate as precursors for amino acid ILs surfactant architectures.
These authors synthesized biobased SAILs with a superior
surface activity and solvent miscibility for the first time. To achieve
that aim, the esterification of the amino acids was carried out to
decrease the melting point and to increase the biodegradability of
the generated salts.
Perhaps one of the most promising applications of SAILs is
their usage to overcome the current limitations of surfactant
flooding Enhanced Oil Recovery (EOR).[13,14] Obtaining a bio-
based SAIL to design an optimal formulation, stable in aqueous
salt solutions and able to obtain ultra-low interfacial tensions,
could mean a definitive advance in EOR with surfactants.
Therefore, in this work, the limitations of the SAILs proposed by
Trivedi et al.[12] were studied and novel SAILs able to overcome
these limitations were synthesized for the first time. The most
stable SAILs were selected, characterized, and their capacity to
be used in EOR was analyzed.
[a]
[b]
V. Fernández Stefanuto, Prof. E. Tojo
Department of Organic Chemistry
Universidade de Vigo
Marcosende, As Lagoas, 36210 Vigo (Spain)
*E-mail: etojo@uvigo.es
R. Corchero, Dr. I. Rodríguez-Escontrela, Prof. A. Soto
Department of Chemical Engineering
Universidade de Santiago de Compostela
15782 Santiago de Compostela (Spain)
Results and Discussion
Synthesis
Supporting information for this article is given via a link at the end of
the document.
Taking into account the promising properties previously shown for
ILs derived from the iso-propyl and iso-butyl esters of (L)-
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10.1002/cphc.201800735
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prolinium (1) and dodecylsulfate anion,[12] the starting point of this
work was the synthesis of similar structures introducing different
alkyl chains in the ester group (Scheme 1). The alkyl chains
selected were n-butyl to give the IL [nC4Pro][DS] (6), iso-butyl to
give [iC4Pro][DS] (7), 2-butyl-octyl to give [C4C8Pro][DS] (8) and
2-decyl-tetradecyl to give [C10C14][DS] (9). Their synthesis was
carried out in two steps. The first one involved the esterification of
L-proline (1) by treatment with a suitable alcohol in the presence
of thionyl chloride (SOCl2) to obtain the corresponding esterified
prolinium chlorides 2-5 with high yields. A later methatesis
reaction with sodium dodecylsulfate gave the desired ILs 6-9. To
our knowledge, all these salts except those derived from [iC4Pro]
cation are described in this work for the first time. The structures
of all synthesized ILs were confirmed by 1H and 13C NMR
spectroscopy (see supporting information) as well as low and/or
high MS spectrometry. The ILs with
a short alkyl chain
[nC4Pro][DS] (6) and [iC4Pro][DS] (7), were found to be water
soluble, while those with long alkyl chains, [C4C8Pro][DS] (8) and
[C10C14Pro][DS] (9), formed two phases when water was added.
O
H2N+
O
n
Cl-
-butanol
nC4
2
Pro]Cl
( )
[
O
iso-butanol
+
H2N
O
Cl-
O
O
R1
H2N+
O
iC
HN
OH
3
Pro]Cl
( )
[
4
SOCl2
Na[C12SO4
]
R2
-
C12SO4
O
1
L-proline
( )
2-butyl-
1-octanol
+
H2N
O
5
2
nC
6
Cl-
[
4Pro][DS]
( )
=
=
H
R1
H5
R
C2
,
2
iC
7
[
4Pro][DS]
( )
4
C8
[C4 Pro]Cl
( )
=
=
R1
R
2
CH3
CH3
,
8
( )
4C
[C 8Pro][DS]
=
=
R1
H13
R
H9
C4
C6
,
2
O
9
( )
10C
2-decyl-1-
tetradecanol
[C
14Pro][DS]
+
=
=
R1
H25
R
H21
C12
C10
,
2
H2N
O
11
8
Cl-
5
Pro]Cl
( )
C14
[C10
Scheme 1. General synthetic procedure applied to obtain the esterified prolinium dodecylsulfates
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Although a previous report about [iC4Pro][DS] (7) showed a
high thermal stability (onset temperature 553.15 K),[12] what is
promising from the point of view of the thermal stability of this
SAIL, in this work some decomposition was observed after
several weeks when it was kept at room temperature in the
presence of water. To go into more detail about the decomposition
process, a water solution of the IL was heated for some days at
hydroxide, dodecylsulfonic acid and isobutyl alcohol (Scheme 2).
Liquid-liquid extraction with CH2Cl2 allowed the separation of
dodecylsulfonic acid (organic phase) from the prolinium hydroxide
(aqueous phase), which was confirmed by the 1H, 13C NMR and
MS data of the isolated compounds (see Supporting Information).
A similar problem of degradation was previously reported.[15] As it
is shown in Table 1, when the IL is heated at 313.15 K for 4 days
or at 373.15 K for 1 day, the decomposition is only partial; but the
decomposition is total after keeping the temperature at 373.15 K
for 4 days.
1
313.15 and at 373.15 K. The analysis by H NMR spectroscopy
showed that when [iC4Pro][DS] (7) is heated in water, the ester
group undergoes hydrolysis to give the corresponding prolinium
O
O
H2N+
H2N+
+
H
2O
HO
O
OH
HO-
+
H
SO4
-
Tª
C12SO4
iC
[
7
Pro][DS]
( )
4
Scheme 2. Decomposition of [iC4Pro][DS] (7) by hydrolysis.
Knowing the high stability of ILs derived from
dodecylbenzenesulfonate anion [DBS],[16] the IL [iC4Pro][DBS] (10)
was synthesized by treatment of the corresponding chloride
[iC4Pro]Cl (3) with sodium dodecyl benzenesulfonate (Scheme 3).
Its structure, no previously reported in literature, was confirmed by
1H and 13C NMR spectroscopy as well as HRMS spectrometry (see
Supporting Information). This IL was found to be also soluble in
water. However, the analysis of its stability when heating in water
showed that although it is more stable than the [DS] analogue (7),
there is a partial decomposition after heating at 373.15 K for 5 days
(Table 1).
In order to avoid the ester hydrolysis, the synthesis of ILs
derived from (L)-prolinol (11) was then approached (Scheme 4).
Starting from (L)-proline (1), it was first reduced by treatment with
LiAlH4 to afford (L)-prolinol (11), that after protonation with an
aqueous solution of HCl 2M gave (S)-(+)-2-(hydroxymethyl)
pirrolidinium chloride [HOPro]Cl (12).
Table 1. Decomposition of [iC4Pro][DS] (7) and [iC4Pro][DBS] (10) when
heating in water.
IL
Temperature (K)
313.15
Days
Decomposition
No
[iC4Pro][DS] (7)
[iC4Pro][DS] (7)
[iC4Pro][DS] (7)
[iC4Pro][DS] (7)
[iC4Pro][DBS] (10)
[HOPro][DS] (13)
[HOPro][DS] (13)
[HOPro][DBS] (14)
1
4
1
4
5
4
4
5
313.15
Partial
Partial
Complete
Partial
No
373.15
373.15
373.15
313.15
373.15
Partial
No
373.15
O
O
-
SO3
+
H2N+
H2N
O
O
Na[DBS]
Cl-
11
iC
iC
10
Pro][DBS]
(
3
( )
[
)
4
[
Pro][Cl]
4
Scheme 3. Synthesis of [iC4Pro][DBS] (10) from the corresponding chloride.
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The anion exchange of [HOPro]Cl (12) by reaction with the
suitable sodium salt (sodium dodecylsulfate Na[DS] or sodium
dodecyl benzenesulfonate Na[DBS]) allowed obtaining of (S)-(+)-
prolinolium dodecylsulfate [HOPro][DS] (13) and (S)-(+)-2-
prolinolium dodecylbenzene sulfonate [HOPro][DBS] (14). Their
resolution MS spectrometry. As expected, no hydrolysis of
[HOPro] cation was observed in any of the two ILs, even after
heating. Only a partial degradation of [DS] anion in [HOPro][DS]
(13) was observed after heating for 4 days at 373.17 K (Table 1),
due to the equilibrium that is produced between this anion and the
corresponding dodecylsulfonic acid in aqueous solutions (see
Supporting information).
structures, no previously described, were confirmed by 1H and 13
C
NMR spectroscopy (see Supporting Information) as well as high
+NH2
-
SO4
10
OH
+
NH2
Cl-
OH
O
Na[DS]
NH
NH
HCl
LiAlH4
[HOPro][DS] (13)
Na[DBS]
-
OH
OH
(L)-prolinol (11)
SO3
+NH2
[HOPro][Cl] (12)
(L)-proline (1)
11
OH
[HOPro][DBS] (14)
Scheme 4. Synthetic procedure applied to prepare ILs derived from (L)-prolinol (11).
Thermal properties
Due to their stability in aqueous solutions, [HOPro][DS] (13) and
[HOPro][DBS] (14) were the most interesting alternatives for
applications where water is involved. DSC analyses (Figure 1)
were carried out to identify the phase transitions for both ILs. The
heating/cooling cycles ranged between 190 and 370 K, at rates of
5 K·min−1. In the case of [HOPro][DS] (13), one endothermic peak
was observed in the heating ramp at 305 K. The onset
temperature of the peak, 295 K, was considered as the melting
temperature of the IL. For the SAIL with the [DBS] anion no peaks
were observed. However, there is a step in the heat, which is
indicative of a glass transition temperature that was determined
as the inflexion point of the step in the DSC curve. A temperature
of 201 K was obtained.
Decomposition temperatures were determined by TGA. In the
weight vs. temperature plots, a double-step decomposition was
observed, with a 5% onset decomposition temperature (onset
temperature for a 5% weight loss of the original mass) of 454 K
for the [HOPro][DS] (13) and 564 K for [HOPro][DBS] (14) (Figure
2). In the case of [iC4Pro][DS] (7) a value of about 478 K was
found.[12]
190
220
250
280
310
340
370
190
220
250
280
310
340
370
T (K)
T (K)
Figure 1. DSC thermograms of [HOPro][DS] (13) (left) and [HOPro][DBS] (14) (right).
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10.1002/cphc.201800735
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100
100
80
60
40
20
0
5%
5%
80
60
40
20
0
T
d, 5% onset= 564K
T
d, 5% onset= 454 K
300
400
500
600
700
800
300
400
500
600
700
800
T (K)
T (K)
Figure 2. TGA thermograms of [HOPro][DS] (13) (left) and [HOPro][DBS] (14) (right). The TGA thermogram also shows the calculation of the 5% onset
decomposition temperature: the crossing of the tangent at the point of 5% weight loss with the stable weight of initial sample before the starting of decomposition.
Aggregation behavior in aqueous solution
Surface tension was used to check the surface active character
of the studied ILs and to determine the critical micelle
concentration (cmc) in aqueous solution at 298.15 K. Figure 3
shows the change in surface tension (γ) as a function of the
logarithm of the concentration in water of [HOPro][DS] (13) left)
and [HOPro][DBS] (14) (right). Usually, surface tension
decreases until a constant value of this property is achieved and
the breakpoint is the cmc. However, as it can be seen in Figure 3
(left), for [HOPro][DS] (13) a minimum in the surface tension was
found before stabilisation of the property. This behavior is
common with [DS] surfactants and it is widely accepted that the
minimum is caused by the presence of the highly surface-active
lauryl alcohol, due to the hydrolysis of the [DS] anion in aqueous
solution.[17] For this SAIL, and as is usual, the cmc is defined as
the concentration where the surface tension goes up and reaches
the equilibrium value at increasing surfactant concentration.
Values of cmc and the corresponding surface tension are given in
Table 2 for both SAILS and their parent compounds (common
surfactants with sodium cations). The cmc obtained for the SAILs
is lower than for their sodium analogues: sodium dodecylsulfate
(Na[DS]) and sodium dodecylbenzenesulfonate (Na[DBS]).
Comparing cmc of [HOPro][DS] (13) and [iC4Pro][DS] (7), the
latter has a clearly lower value (0.48mM[18]). As Rao et al.[18]
indicated, more hydrophobic counterions normally lead to the
reduction in cmc. In this case [HOPro] cation is more hydrophilic
due to the presence of the group OH.
70
60
65
55
45
35
25
C (mmol/kg)
50
40
30
0,01
0,1
1
10
0,01
0,1
1
10
C (mmol/kg)
C (mmol/kg)
Figure 3. Surface tension as function of [HOPro][DS] (13) (squares, left) and [HOPro][DBS] (14) (circles, right) concentration in aqueous solution at 298.15 K.
The effectiveness of a surfactant to lower the surface tension
of water can be evaluated by means of two parameters: pC20 (C20
being the concentration required to reduce the surface tension of
pure water by 20 mN·m-1) and the surface pressure at cmc (Πcmc),
pC20= -log C20 (1)
the difference between surface tensions of pure water and the
surfactant solution at the cmc:
Πcmc= γwater-γcmc (2)
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Table 2. Parameters derived from surface tension data for [HOPro][DS] (13) and [HOPro][DBS] (14) SAILs and parent compounds at 298.15 K and atmospheric
pressure.
SAIL
cmc (mM)
2.74
γ
cmc (mN/m)
pC20
3.59
2.51
4.26
2.6
Πcmc (mN/m)
42.70
37.1
Γm (μmol/m2)
1.72
asm (Å2)
96.4
56
[HOPro][DS] (13)
Na[DS][19]
29.30
34.9
28.75
38
8.2
3.16
[HOPro][DBS] (14)
Na[DBS][20]
1.25
43.25
34
0.29
570.14
74
2.90
2.3
Results shown in Table 2 indicate that at the same concentration
of surfactant, the SAILs produce a higher reduction of the surface
tension than the corresponding parent compounds. Moreover,
comparing the values for both SAILs, [HOPro][DBS] (14) is more
efficient than the [DS] analogue.
The surface excess concentration of a surfactant (Γ1) and the
area per molecule ( ꢀ1ꢁ ) at the water–air interface can be
determined on the basis of the adsorption isotherm using the
Gibbs equation:[19]
C1 is the surfactant concentration (mol/kg), γ the surface tension
(mN/m), N is the Avogadro constant. The derivative in equation
(3) is obtained by fitting surface tension data up to the cmc to a
second order polynomial. Table 2 presents values of the excess
of surfactant concentration at the water–air interface (Γm) and the
area per molecule at the interface (ꢀꢉꢁ ), at surface saturation, for
[HOPro][DS] (13), [HOPro][DBS] (14) and the corresponding
parent compounds. From this Table, it can be seen that Γm is
lower and ꢀꢉꢁ higher than values for the parent compounds. It
means that the effectiveness of SAILs adsorption at water–air
interface is lower than traditional surfactants.
Aqueous solutions of [HOPro][DS] (13) and [HOPro][DBS]
(14) were prepared with a concentration 20 times higher than the
cmc and were analyzed at room temperature by means of
Dynamic Light Scattering. Results are shown in Figure 4 showing
a hydrodynamic radius of aggregates of 1-3 nm.
3
10
∂γ
Γ1 − 4.606·ꢂ·ꢃ ∂ ꢄꢅꢆꢇ
�
�
(3)
(4)
1
ꢃ
26
10
ꢀ1ꢁ =
ꢈ Γ
1
Figure 4. Size distribution of aggregates in aqueous solutions of [HOPro][DS] (13) (left) and [HOPro][DBS] (14) (right) by DLS and TEM.
The problem of the minimum found using surface tension to
determine the cmc of the SAIL [HOPro][DS] (13), can be solved
using another physical property affected by the formation of
micelles. For this reason, electrical conductivity measurements of
aqueous solutions of prolinolium-based SAILs were also carried
out to determine cmc values, expanding the study to analyze the
influence of temperature. Figure 5 shows the conductivity
aqueous solutions of the amino-acid SAILs at different
concentrations and temperatures ranging from 288.15K and
318.15K. The conventional method, using the intersection of the
two straight lines (obtained by linear regression of the
experimental data at low and high concentrations) was applied to
determine cmc values.
As shown in Figure 6, the cmc initially decreases and then
increases with temperature, with the minimum in the 290-300 K
range for [DS] anion and 305-315K for [DBS] anion. A similar
behavior has been found with conventional surfactants and also
with other proline-based SAILs.[18,19]
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1000
800
600
400
200
200
160
120
80
40
0
0
0
2
4
6
8
10 12 14 16
0.0
0.5
1.0
1.5
2.0
2.5
C (mmol/kg)
C (mmol/kg)
Figure 5. Conductivity as function of [HOPro][DS] (13) (left) or [HOPro][DBS] (14) (right) concentration in aqueous solution at different temperatures and atmospheric
pressure. Closed circles: 288.15 K, open circles: 298.15 K, triangles down: 308.15 K, triangles up: 318.15 K.
5.2
5.0
4.8
4.6
4.4
4.2
4.0
3.8
1.315
1.310
1.305
1.300
1.295
285
295
305
315
325
285
295
305
315
325
T (K)
T (K)
Figure 6. Plot of cmc as function of temperature for [HOPro][DS] (13) (left) [HOPro][DBS] (14) (right). Line drawn to guide the eye.
The tendency of surfactants to form micelles can be established
on the basis of standard free energy of micellisation (∆ꢊꢉ0 ꢋꢌ):
Calculated values of ꢍ, ∆ꢊꢉ0 ꢋꢌ,∆ꢓꢉ0 ꢋꢌ and ∆ꢔꢉ0 ꢋꢌ are given in Table
3 and Table 4 for [HOPro][DS] (13) and [HOPro][DBS] (14),
respectively. The Gibbs energy of micellisation decreases with
temperature. Therefore, the micellisation process becomes more
spontaneous. The Gibbs energy of micellisation for SAILs is less
negative (less spontaneous micellisation) than for parent
compounds Na[DS] and Na[DBS].[19] Enthalpy of micellisation
(∆ꢓꢉ0 ꢋꢌ) was positive at lower temperatures and negative at higher
ones. The same behavior was found for conventional
surfactants.[19,20] Values of ꢏ∆ꢔꢉ0 ꢋꢌ indicate that the aggregation
process is mainly entropy-driven at lower temperatures and
mainly enthalpy driven at higher temperatures (see Tables 3 and
4).
0
(
)
(
)
∆ꢊꢉꢋꢌ = 1 + ꢍ ꢎꢏꢐꢑ ꢒꢌꢉꢌ (5)
being ꢒꢌꢉꢌ the mole fraction of SAIL at the cmc and β the degree
of counterion binding. Basic equations can be applied to calculate
standard enthalpy (∆ꢓꢉ0 ꢋꢌ) and entropy (∆ꢔꢉ0 ꢋꢌ) of micellisation:
0
G
ꢖꢗꢘ
ꢕ�
�
ꢙ
∆ꢓꢉ0 ꢋꢌ
=
=
(6)
(7)
1
ꢕ� �
ꢙ
0
0
mic
∆H
−∆G
∆Sm0 ic
mic
T
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Table 3. Parameters derived from conductivity data for [HOPro][DS] (13) aqueous solution at different temperatures and atmospheric pressure.
T (K)
cmc (mmol/kg)
4.20
β
ΔG°mic (kJ/mol)
-34.62
∆H°mic KJ/mol)
10.59
TΔS°mic (kJ/mol)
45.21
288.15
298.15
308.15
318.15
0.53
0.54
0.53
0.54
4.01
-36.27
-0.13
36.14
4.38
-36.87
-10.16
26.71
5.02
-37.91
-19.56
18.35
Table 4. Parameters derived from conductivity data for [HOPro][DBS] (14) aqueous solution at different temperatures and atmospheric pressure.
T (K)
cmc (mmol/kg)
1.309
β
ΔG°mic (kJ/mol)
-31.46
∆H°mic KJ/mol)
30.70
TΔS°mic (kJ/mol)
62.16
288.15
298.15
308.15
318.15
0.23
0.23
0.24
0.17
1.303
-32.52
-2.36
30.16
1.299
-33.87
-33.27
0.59
1.310
-32.80
-62.24
-29.44
Phase behavior in aqueous/oil mixtures
Focusing on EOR, salinity scans with both SAILs ([HOPro][DS]
(13) and [HOPro][DBS] (14) were carried out. The first one, the
SAIL with [DS] anion, formed in the presence of n-octane, with a
WOR~1 and 2 %wt surfactant, microemulsions type Winsor I up
to 15 %wt NaCl at 298.15 K. When samples were tested at 323.15
K the same behavior was observed. So the SAIL seems to be too
hydrophilic to be used alone for EOR applications.
where C=0.3, ꢡꢅ =Vo/Vs. According to this correlation, this
microemulsion should produce ultralow IFT of ~3·10-3 mN/m. So,
the amino acid-based SAIL [HOPro][DBS] (14) is promising for
EOR applications at high temperatures and salinity with probably
low enough interfacial tensions to displace
proportion of oil.
a substantial
However, with the [DBS] anion, the IL generated a transition
from Winsor type I to Winsor type II microemulsions at the tested
temperatures. Figure 7 shows photographs of salinity scans at
298.15 (top) and 348.15 K (bottom) for aqueous solutions of
[HOPro][DBS] (14) equilibrated with equal volumes of n-octane,
with an overall surfactant concentration of 2%. At 298.15 K, the
separation of the phases in the Winsor type III region takes a long
time. After one month, at 5% wt NaCl, a middle phase
microemulsion with really low solubilisation parameter was found
(~2). The behavior was similar at 323.15 K. However, at 348.15
K, the SAIL generated classical microemulsion phase behavior,
Winsor I-III-II transition. Figure 8 shows solubilisation parameters
for this temperature (Vo/Vs and Vw/Vs) as a function of salinity. An
optimal salinity of ~8.5 %wt NaCl was found with a solubilisation
parameter of ~10 for the middle phase microemulsion. Chung-
Huh correlation (equation 8) was used to calculate IFT from the
solubilisation parameters.[21]
Figure 7. Salinity scans at 298.15 and 348.15 K for 2 %wt [HOPro][DBS] (14),
WOR~1, n-octane, NaCl brine.
ꢚꢛ = ꢜ/(ꢝꢞ)ꢟ
(ꢠ)
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10.1002/cphc.201800735
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synthetic reactions was dried in an oven at 333.15 K during 24 h before its
use. The evolution of the reactions was monitored by thin layer
chromatography (t.l.c.) employing silica-gel sheets (Merck, TLC Silica gel
60 F254) and MeOH:CH2Cl2 5 % as eluent; t.l.c. plates were visualised by
treatment with a solution of nynhidrine as revealing agent. 1H and 13C NMR
spectra were recorded on a BRUKER ARX 4CO spectrometer at 400.1621
(1H) and 100.6314 (13C) MHz, respectively. CDCl3 (ACROS Organics,
99.6+ atom % D) and D2O (ACROS Organics, 99.8+ atom % D) were
employed as deuterated solvents as received from the supplier. Chemical
shifts are quoted in parts per million (ppm) relative to the signals
corresponding to the residual non-deuterated solvents (CDCl3: δH =7.26
ppm, δC = 77.16 ppm, D2O: δH=4.79 ppm). Mass spectra were recorded
on a BRUKER FTMS APEXIII mass spectrometer. A purity greater than 99
wt% is estimated for all the ILs synthesized in this study.
30
25
20
15
10
5
0
0
2
4
6
8
10
12
14
16
% NaCl
Figure 8. Solubilisation parameters (
Vw/Vs,
Vo/Vs) from salinity
scans at 348.15 K for 2 %wt [HOPro][DBS] (14), WOR~1, n-octane, NaCl brine.
Differential scanning calorimetry (DSC) experiments were run in a TA
Instruments Q2000 differential scanning calorimeter, with heating and
cooling rates of 5 K·min−1. N2 (Praxair, 99.999 %) was used as sample
purge gas with 50 mL/min flow. Three cycles were carried out, being
second and third cycle coincident. Thermal stability of the samples was
measured in a TA Instruments Q500 thermogravimetric analysis (TGA)
Conclusions
apparatus. Samples were initially heated (5 K·min−1
) from room
Amino acid ILs share the promising advantages of ILs, and
being derived from a biomolecular structure, they could be
considered true green solvents. Some ILs obtained from amino
acids by carboxylic acid esterification were presented to the
scientific community as stable and highly biodegradable, and
some applications were also suggested, most of them involving
an aqueous environment. However, the studies carried out in this
work on the stability of similar structures derived from proline
esters, have shown that the ester group incorporated into the
cation undergoes hydrolysis in the presence of water.
A new family of biodegradable amino acid derived ILs
avoiding carboxylic acid esterification by its reduction to an
alcohol is proposed in this work. Resulting ILs are not only
thermally but also hydrolytically stable. Prolinolium cation [HOPro]
was selected as cation and combined with anions able to confer
a surface active character to the ILs: dodecylsulfate and
dodecylbenzenesulfonate. The first SAIL, [HOPro][DS], showed
the same problems as the traditional surfactant Na[DS] regarding
the hydrolysis of the anion. However, [HOPro][DBS] shown to be
fully stable. Both SAILs form micelles in aqueous solutions and
are more effective to reduce the surface tension of water than the
traditional corresponding parents (Na[DS], Na[DBS]). However,
the effectiveness of SAILs adsorption at the water-oil interface is
lower. These Room Temperature Ionic Liquids are recommended
for applications taking advantage of its capacity of aggregation.
Both SAILs were tested for EOR applications. An optimal
formulation, useful for reservoirs at high salinities and
temperatures, was found with [HOPro][DBS]. An ultra-low
interfacial water-oil of ~3·10-3 mN/m is expected. This low value
is able to reduce the capillary forces that trap the oil in the pores
of the rock pushing it out of the reservoir.
temperature to 348 K, then being held isothermally at this temperature for
30 min to help removing water and other volatile compounds that might be
present. After that isothermal step, the samples were further heated, at a
rate of 5 K·min-1, up to 773 K, under a constant nitrogen gas flow of 60
mL/min. DSC and TGA thermograms were evaluated using the Universal
Analysis 2000 software (Version 4.5.0.5A) by TA Instruments.
Surface tension measurements were carried out using a Krüss K11
tensiometer based in the Wilhelmy plate method. All measurements were
carried out at 298.15 K using a Julabo F12 cryogenic thermostat for
temperature control. Electric conductivity was measured using a Basis 30
Crison electric conductivimeter connected to a thermostatic bath (Julabo
F12–EH). Measurements were carried out at four different temperatures
ranging from 288.15 to 318.15 K at 10 K interval. To check micelles size,
some samples (surfactant solutions with concentration ca. 20 times above
the cmc) were analyzed at room temperature via a Malvern Zetasizer Nano
ZS apparatus to perform Dynamic Light Scattering measurements. The
estimated uncertainty in aggregates diameter is 0.2 nm. In the case of
Transmission Electron Microscopy tests, samples were prepared by
putting a drop of surfactant solution (with concentration ca. 20 times above
the cmc) on the carbon-coated Nickel grid (400 mesh). Samples were
imaged under a JEOL JEM1010 electron microscope at a working voltage
of 100 kV.
Salinity scans were carried out in encased glass pipettes adding 1 cm3 of
aqueous surfactant solution (4%wt SAIL) and 1 cm3 of n-octane as model
oil. Tests at room temperature were carried out mixing the samples
approximately 24 h using a rotary mixer and letting them to equilibrate until
the phases volume remained constant. The same procedure was used
with tests at higher temperature but a Julabo F12-EH was used for
temperature control. Solubilization parameters and optimal salinity were
calculated from the equilibrated phase volumes. Further details of the
experimental set-up can be found in [14].
Acknowledgements
Experimental Section
A. Soto acknowledges the Ministry of Economy and
Competitiveness (Spain) for financial support throughout project
CTQ2015-68496-P (including European Regional Development
The reagents and solvents were purchased from commercial suppliers and
employed without further purification. The glass material employed in the
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Ionic Liquids derived from proline,
application as surfactants: unlike
ionic liquids derived from prolinium
esters, prolinolium dodecylsulfate and
dodecylbenzenesulfonate have shown
to be chemically and thermally stable,
and have a high aggregation capacity.
An optimal formulation for EOR was
designed.
Verónica Fernández-Stefanuto, Raquel
Corchero, Iria Rodriguez-Escontrela,
Ana Soto, Emilia Tojo*
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Title
Liquids derived from proline: application
as surfactants
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