Micropolarity and Hydrogen-Bond Donor Ability of Environmentally Friendly Anionic Reverse Micelles Explored by UV/Vis Absorption of a Molecular Probe and FTIR Spectroscopy

Article abstract of DOI:10.1002/cphc.201701264

In the present work we show how two biocompatible solvents, methyl laurate (ML) and isopropyl myristate (IPM), can be used as a less toxic alternative to replace the nonpolar component in a sodium 1,4-bis-2-ethylhexylsulfosuccinate (AOT) reverse micelles (RMs) formulation. In this sense, the micropolarity and the hydrogen-bond ability of the interface were monitored through the use of the solvatochromism of a molecular probe (1-methyl-8-oxyquinolinium betaine, QB) and Fourier transform infrared spectroscopy (FTIR). Our results demonstrate that the micropolarity sensed by QB in ML RMs is lower than in IPM RMs. Additionally, the water molecules form stronger H-bond interactions with the polar head of AOT in ML than in IPM. By FTIR was revealed that more water molecules interact with the interface in ML/AOT RMs. On the other hand, for AOT RMs generated in IPM, the weaker water–surfactant interaction allows the water molecules to establish hydrogen bonds with each other trending to bulk water more easily than in ML RMs, a consequence of the dissimilar penetration of nonpolar solvents into the interfacial region. The penetration process is strongly controlled by the polarity and viscosity of the external solvents. All of these results allow us to characterize these biocompatible systems, providing information about interfacial properties and how they can be altered by changing the external solvent. The ability of the nontoxic solvent to penetrate or not into the AOT interface produces a new interface with attractive properties.

Full text of DOI:10.1002/cphc.201701264

DOI: 10.1002/cphc.201701264
Articles
Micropolarity and Hydrogen-Bond Donor Ability of
Environmentally Friendly Anionic Reverse Micelles
Explored by UV/Vis Absorption of a Molecular Probe and
FTIR Spectroscopy
[
a]
Valeria R. Girardi, Juana J. Silber, Ruben Darꢀo Falcone,* and N. Mariano Correa
In the present work we show how two biocompatible solvents,
methyl laurate (ML) and isopropyl myristate (IPM), can be used
as a less toxic alternative to replace the nonpolar component
in a sodium 1,4-bis-2-ethylhexylsulfosuccinate (AOT) reverse
micelles (RMs) formulation. In this sense, the micropolarity and
the hydrogen-bond ability of the interface were monitored
through the use of the solvatochromism of a molecular probe
interface in ML/AOT RMs. On the other hand, for AOT RMs gen-
erated in IPM, the weaker water–surfactant interaction allows
the water molecules to establish hydrogen bonds with each
other trending to bulk water more easily than in ML RMs, a
consequence of the dissimilar penetration of nonpolar solvents
into the interfacial region. The penetration process is strongly
controlled by the polarity and viscosity of the external sol-
vents. All of these results allow us to characterize these bio-
compatible systems, providing information about interfacial
properties and how they can be altered by changing the exter-
nal solvent. The ability of the nontoxic solvent to penetrate or
not into the AOT interface produces a new interface with at-
tractive properties.
(1-methyl-8-oxyquinolinium betaine, QB) and Fourier transform
infrared spectroscopy (FTIR). Our results demonstrate that the
micropolarity sensed by QB in ML RMs is lower than in IPM
RMs. Additionally, the water molecules form stronger H-bond
interactions with the polar head of AOT in ML than in IPM. By
FTIR was revealed that more water molecules interact with the
1
. Introduction
Reverse micelles (RMs) are a class of self-assemblies system
generally described as nanometer sized water droplets dis-
persed in a nonpolar solvent with the aid of a surfactant mon-
[
1,2]
olayer.
Polar and nonpolar substances can be solubilized by
RMs and this ability confer to them several applications in
[
2]
fields such as chemical and enzymatic reactions, nanomateri-
[
3–7]
[8]
als synthesis,
in drug delivery systems, among others. Dif-
ferent kinds of surfactants (anionic, cationic and nonionic)
have been used to formulate RMs in nonpolar solvents. Partic-
ularly, the surfactant most employed is the anionic sodium 1,4-
[
2,3,9–14]
bis-2-ethylhexylsulfosuccinate (AOT, Scheme 1).
This sur-
factant has the ability to forms spherical RMs in aromatic and
aliphatic nonpolar solvents without the addition of a cosurfac-
tant and, variable amount of water (quantified as W =[H O]/
0
2
[Surfactant]) can be solubilized depending on the chemical
nature of the external solvent and temperature, among other
[
9]
factors. Even that RMs have been explored and characterized
since several decades ago, actually the main critical problem is
still the potential toxicity of their components; particularly the
[a] Dr. V. R. Girardi, Prof. J. J. Silber, Dr. R. D. Falcone, Dr. N. M. Correa
Departamento de Quꢀmica
Universidad Nacional de Rꢀo Cuarto
Agencia Postal # 3, C.P. X5804BYA Rꢀo Cuarto (Argentina)
E-mail: rfalcone@exa.unrc.edu.ar
Scheme 1. Molecular structures of AOT, IPM, ML and QB.
Supporting Information and the ORCID identification number(s) for the
author(s) of this article can be found under:
https://doi.org/10.1002/cphc.201701264.
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external solvent which is the major component in the solution.
This difficulty reduces their potential application in industries
such as food, cosmetic, and pharmaceutical. In these sense,
many efforts have been made to formulate non-toxic RMs cre-
ated in different oils. In particular, RMs prepared in biocompat-
ible solvents such as isopropyl myristate (IPM, Scheme 1), ethyl
myristate, ethyl palmitate and ethyl oleate appears as promis-
98% purity), were stored over molecular sieves before use. Ul-
trapure water was obtained from Labonco equipment model
90901-01.
1
-methyl-8-oxyquinolinium betaine (QB) was synthesized ac-
[25]
cording to the procedure reported by Ueda and Schelly.
[
10,15–18]
ing replacement to traditional nonpolar solvents.
These
long chain fatty acid esters are environmentally friendly with
Methods
[
15,19–21]
low toxicity and highly biodegradable.
One of the main
The stock solutions of AOT in IPM and ML were prepared by
mass and volumetric dilution. Aliquots of these stock solutions
were used to make individual RMs solutions with different
goals is to produce non-toxic RMs that preserves the unique
properties of the very-well known AOT RMs in traditional sol-
vents.
[
18]
amount of water, defined as W =[water]/[AOT]. The incorpora-
Previous studies performed in our group have shown a
peculiar and unexpected behavior of the AOT RMs formed in
methyl laurate (ML, Scheme 1), in comparison with the RMs
formulated in IPM despite the similar chemical structure of
these external solvents. Thus, it was observed that the anionic
surfactant is completely soluble in both solvents in absence of
0
tion of water into each micellar solutions were performed
using calibrated microsyringes. To obtain optically clear solu-
tions they were shaken in a sonicating bath.
À2
For the absorption experiments, a 1ꢂ10 m solution of QB in
methanol (Sintorgan HPLC quality) was prepared. The appro-
water (W =0). The maximum amount of water dissolved (de-
0
priate amount of this solution to obtain a concentration of 3ꢂ
max
fined as W0 ) for both RMs were different being, the ML/AOT
À4
10
m for QB in the AOT RMs was transferred into a volumetric
RMs able to dissolve approximately the double amount of
water than IPM/AOT. From the dynamic light scattering (DLS)
experiments, was observed an increase in the diameter of the
droplets (dapp) when the W0 increases in IPM and ML RMs,
proving that the RMs are formed, hence the water molecules
are effectively entrapped by the AOT layer. However, the sizes
flask, and the methanol was evaporated by bubbling dry N2;
then, the surfactant RMs stock solution was added to the resi-
due and agitated in a sonicating bath until the RMs was opti-
cally clear. For the experiments varying the AOT concentration
at W constant (0 and 10), a stock solution of surfactant 0.2m
0
was used. Thus, to the cell baring 2 mL of QB of the same con-
centration in the nonpolar solvent, was added the appropriate
amount of surfactant and molecular probe stock solution to
obtain a given concentration of surfactant in the micellar
media. Therefore, the absorbance values of the molecular
probe were not affected by dilution. For the experiments vary-
ing the water content, an AOT RMs solution prepared at
were dissimilar between both RMs. For instance, at W =10 the
0
dapp value for IPM/AOT RMs was 3.1 nm but 7 nm in the case
of ML/AOT RMs. The aggregation numbers (Nagg) of both RMs
were also very different. A Nagg of 49 was obtained for IPM/
AOT/water RMs at W =15, being around 182 for ML/AOT/
0
water. Interestingly, we observed similarities between these
biocompatible solvents and those traditionally used to create
[
AOT]=0.1m was employed.
max
RMs such as n-heptane and benzene. In this sense, the W0
For FTIR experiments, monodeuterated water (HOD) as polar
phase was used, which was prepared by stirring a solution of
values observed in ML/AOT and n-heptane/AOT were quite
similar and, for IPM/AOT and benzene/AOT only slightly differ-
ent but both are able to dissolve less water than ML/AOT or n-
1
0% D O (>99% purity from Sigma) in H O at room tempera-
2
2
[26]
[
4,22]
ture for 1 h in order to allow the exchange of H.
heptane/AOT RMs.
Additionally, the similar Nagg values ob-
[
23]
tained for IPM/AOT/water and benzene/AOT/water, around
[
24]
3
1
0, and for ML/AOT/water and n-heptane/AOT/water around
80 reinforces the idea that IPM and ML have analogous be-
General
havior in AOT RMs that benzene and n-heptane, respectively.
In order to explore more in detail on the unique microenvir-
onment created in these RMs, we investigate the water entrap-
ped inside IPM/AOT and ML/AOT RMs by using two ap-
proaches: i) absorption spectroscopy using 1-methyl-8-oxyqui-
nolinium betaine (QB, Scheme 1) as molecular probe; and ii)
Fourier transform infrared spectroscopy (FTIR).
UV/Vis absorption spectra were recorded using a spectropho-
tometer Shimadzu 2401 with a thermostated sample holder.
The path length used in absorption experiments was 1 cm. All
experimental points were measured three times with different
prepared samples.
FTIR spectra were recorded with a Nicolet IMPACT 400 spec-
trometer. The FTIR measurements for the RMs samples were
taken in Irtran-2 cell of 0.5 mm path length from Wilmad Glass
Experimental
(Buena, NJ). FTIR spectra were obtained by co-adding 200
À1
spectra at a resolution of 0.5 cm . The n spectral band of
OD
Materials
HOD was superimposed on a finite background. It was as-
sumed that this background could be approximated with the
Sodium 1,4-bis (2-ethylhexyl) sulfosuccinate (AOT) from Sigma
[26]
(
>99% purity), was dried under vacuum prior use. Isopropyl
spectrum of 100% H O in the n spectral region. Therefore
2 OD
myristate (IPM) and methyl laureate (ML) both from Sigma (>
the reference sample, at each W values, was a surfactant solu-
0
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tion containing exactly the same W but adjusted with pure
0
H O.
2
All the experiments were carried out at 25Æ0.58C.
2
. Results and Discussion
2.1. QB in Nontoxic Lipophilic Solvents/AOT Reverse Mi-
celles
We have chosen QB as molecular probe because its solvato-
chromic behavior is highly sensitive to the microenviron-
[
27–29]
ment
and, it was previously used to obtain valuable infor-
[
14,27,28,30]
mation about aqueous and nonaqueous AOT RMs.
QB
has an UV-visible absorption spectrum with two electronic ab-
sorption bands: an absorption band in the visible region, B1,
that is primarily sensitive to polarity, and another band peaking
at shorter wavelength in the UV, B , which reflects the hydro-
2
[
27,28]
gen bond donor capability of the solvent.
The absorbance
of the B band is highly sensitive to the molecule’s environ-
2
ment hence, the absorbance ratio of B to B bands (Abs B /
2
1
2
Abs B ), in combination with the absorption bands shifts, pro-
1
vides an effective method to determine the properties of the
[
14,27,28,30]
microenvironment surrounding the probe.
In the present contribution, the QB behavior in non-toxic lip-
ophilic solvents/AOT RMs was explored, performing all the ex-
periments at different surfactant concentration; first in absence
of water addition (W =0) and later in presence of the entrap-
0
ped water (W =10). Finally, the variation of the water content
0
when the RMs is formed ([AOT]=0.1m) was investigated.
Figure 2. Variation of the B
1
2
band maximum (A) and the B band maximum
(
[
B) of QB in IPM/AOT RMs as a function of AOT concentration at W
QB]=3ꢂ10 M.
0
=0.
2.1.1. QB in IPM/AOT RMs
À4
Figure 1 shows the QB absorption spectra in IPM/AOT RMs
À4
À3
varying [AOT] at [QB]=3ꢂ10 m and W =0. As QB is soluble
>10 m the absorption values remain practically constant.
This behavior may denote that the QB microenvironment is
not changing after the RMs formation. As the location of the
probe is crucial to infer RMs properties and, as QB is soluble in
the non-polar solvent, when the micelle is formed (concentra-
tions above the CMC) QB can undergo a partition process be-
tween the two different pseudophases: the RMs interface and
the non-polar organic solvent, changing the surfactant concen-
0
in pure IPM, the surfactant concentration was varied from 0
(pure non-polar solvent) to 0.2 M. In Figure 2 A and B are plot-
ted the absorption maximum of B and B bands when the sur-
1
2
factant concentration is increased, respectively.
As evident in Figures 2A,B, there are hypsochromic shifts of
B and B bands when the [AOT] increases. However, at [AOT]
1
2
[28]
tration. Figure 1 shows that there is no isosbestic point for
QB spectra in IPM/AOT RMs at W =0 and, Figures 2A,B clearly
0
À3
.
show the lack of spectral changes at [AOT] >10 M Both
facts suggest that, even in absence of water, QB is anchored to
the interface and the partition process between the two pseu-
dophases is negligible. Additionally, the CMC value (4.2ꢂ
À3
10
m) for the IPM/AOT RMs can be obtained from the inflec-
tion point of data plotted in Figure S1 in the Supporting Infor-
mation. Similar plots were obtained in all the systems explored
(
results not shown).
The variation of the QB absorption spectra as a function of
AOT] in presence of water added in the IPM/AOT RMs is
[
potted in Figure S2. Figures S3A–B summarize the lmax B1 and
lmax B2 values for QB in IPM/AOT/water RMs, respectively, vary-
ing [AOT] at W =10. For comparison, the l
B1 values vary-
max
Figure 1. QB absorption spectra in IPM/AOT RMs as a function of AOT con-
0
À4
.
centration at W
0
=0. [QB]=3ꢀ10
M
ing the surfactant concentration at W =0 and 10 are present-
0
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both biocompatible systems help to organize the RMs at lower
surfactant concentration (low CMC) than at W =0.
0
The results observed in Figure 3B indicate an increment on
the micropolarity sensed by QB in ML/AOT RMs when the sur-
factant concentration increases. Moreover, when W increases
0
[27]
QB is sensing the presence of water at the interface.
In order to make comparison between both biocompatible
systems and conventional AOT RMs and, to understand the in-
fluence of the external solvent on the properties of the RMs
studied we have decided to compare the results at [AOT]=
0.1m, which is above the value of the CMC. In Table 1 are gath-
Table 1. Maxima absorption wavelength of B
systems. [QB]=3ꢂ10 M; [AOT]=0.1 M.
1
values of QB in different
À4
l
max 1
B [nm]
System
W
0
=0
0
W =10
IPM/AOT
ML/AOT
neat IPM
neat ML
neat water
490
496
552
573
441
465
468
[
a]
[
a] Data obtained from Refs. [27,28].
ered the values of lmax B1 values for QB in IPM/AOT/water and
ML/AOT/water RMs at [AOT]=0.1m at W =0 and 10. The
0
spectroscopic values for QB in neat IPM, ML and water are also
shown in Table 1.
Analyzing the QB lmax B1 values data gathered in Table 1 it
can be seen that the lmax B1 value is smaller for IPM/AOT than
for ML/AOT at W =0. This suggests that the micropolarity of
0
Figure 3. Variation of the B
1
band maximum of QB in IPM/AOT/water (A) and
=0 and 10.
the interface is higher in the former system. When water is
added to both RMs, the micropolarity sensed by QB in ML RMs
is still lower than in IPM RMs.
ML/AOT/water (B) RMs as a function of AOT concentration at W
0
À4
.
[QB]=3ꢂ10
M
In comparison with traditional non-polar external solvents,
QB detects differences between both biocompatible solvents
ed in Figure 3A. As it can be observed, the presence of water
entrapped in the RMs modified the spectral behavior of QB.
and n-hexane or benzene. For example, at W =0 the l
B1
max
0
Even that the tendencies are similar to the system at W =0,
appears at 506 nm and 510 nm in n-hexane/AOT and benzene/
0
[28]
that is, shifting to lower wavelengths when the surfactant con-
AOT RMs, respectively.
Table 1 shows a lmax B1 value of
centration increases, at W =10 the micropolarity sensed by
490 nm and 496 nm for AOT RMs formed in IPM and ML, re-
spectively. Even that the micropolarity in benzene/AOT RMs is
lower than in n-hexane/AOT RMs (same tendency that IPM and
ML RMs), both traditional solvents produce different interfacial
microenvironment that the biocompatible solvents.
0
QB is larger (red points in Figure 3A) that the media in ab-
sence of water (black points in Figure 3A). This difference de-
notes that the molecular probe senses the water at the inter-
[
27]
face.
In order to provide more information about the influence of
the external solvent on the water entrapped in both RMs, we
varied the water content at fixed [AOT]. Figures S8 and S9,
2.1.2. QB in ML/AOT RMs
Similar experiments were performed for QB in ML/AOT RMs.
Thus, in Figures S4–S7 are plotted the spectra of QB, the B1
and B maximum values as a function of [AOT] at W =0 and
show the QB absorption spectra with varying W in IPM/AOT/
0
water and ML/AOT/water RMs, respectively. The spectral
changes of QB show that both bands undergo hypsochromic
shifts when the amount of water increases. On the other hand,
it is very interesting to analyze the AbsB /AbsB ratio values as
2
0
10. Figure 3B summarizes the lmax B1 values varying the surfac-
tant concentration at W =0 and 10. Additionally, in Table S1
0
2
1
are shown the CMC values obtained for both systems. It is im-
portant to note that all CMC values are comparable to those
reported for AOT RMs formulated in solvents such as benzene
a function of W since this is only sensitive to the H-bond abili-
0
[14,27,28,30]
ty of the environment.
Figure 4 summarizes the AbsB2/
AbsB ratio values for QB in IPM/AOT/water and ML/AOT/water
1
[
14,28]
or n-heptane.
Moreover, the presence of water (W =10) in
RMs.
0
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Figure 5. FTIR spectra in the region of O-D stretching frequency of HOD in
IPM/AOT/water RMs upon increasing the W values. The IPM bands have
0
been subtracted. [AOT]=0.1 M.
Figure 4. Variation of the absorbance ratio between B
2 1
and B bands of QB
in IPM/AOT/water and ML/AOT/water RMs as a function of W
0
.
À4
.
[
QB]=3ꢂ10 M [AOT]=0.1 M.
Figure 4 shows two different profiles when W increase in
0
both RMs. The AbsB /AbsB values are practically constant in
2
1
the IPM RMs but there is a significant diminishing in the values
with the increasing of the water content in the ML RMs. The
data for IPM RMs suggests a very poor interaction by H-bond
interaction between QB and water molecules at the interface.
On the other hand, the results observed in ML system are con-
sistent with an important H-bond donor ability of the region
around the molecular probe. At the W0 maximum value
reached, QB detects an AbsB /AbsB value equal to 2.4 in ML/
2
1
AOT/water, while for IPM/AOT/water RMs, the ratio value is
around 4. Particularly, this last value is quite far from that
[
28]
found in pure water (AbsB /AbsB =1.1). Apparently, these
2
1
results denote that water forms stronger H-bond interaction
with the polar head of AOT in ML than in IPM. Consequently,
the structure of the interfacial water in IPM/AOT RMs should
be different from those found in the ML/AOT RMs.
0
Figure 6. Shift of O-D stretching frequency for HOD upon increasing the W :
IPM/AOT/HOD (&) and ML/AOT/HOD (*). The nOD value for pure HOD (–) is
included for comparison. [AOT]=0.1 M.
tained in ML/AOT/water RMs. Figure 6 plots the nOD values as a
function of the water content for both RMs.
2
.2. Nontoxic Lipophilic Solvents/AOT/Water Reverse Mi-
As can be seen from Figures 5 and S10, for HOD entrapped
celles Explored by FTIR
in IPM/AOT and ML/AOT RMs at all the W values investigated
0
To gain more insights about the biocompatible RMs and the
water-surfactant interaction we investigated the systems using
the noninvasive technique FTIR spectroscopy, which allows us
a symmetric band was observed and attributed to only one
kind of water present in the RMs. Additionally, from Figure 6
two interesting facts can be observed: i) all the O-D stretching
[31,32]
to infer, for example, interactions existing at the interface.
frequency values in both RMs are larger than the correspond-
[
26,31,32]
À1 [34]
As it was performed in previous works
we use monodeu-
ing to neat HDO (2519 cm ) specially at low W and, ii) the
0
terated water as polar phase in our RMs. Thus, we follow the
nO-D values at the same water content are different between
both RMs.
water O-D stretching mode (n ) in the AOT RMs. It must be
OD
pointed out that the main stretching bands corresponding to
The nO-D values larger than neat water observed when this
polar solvent is entrapped in both non-toxic RMs, suggest that
water molecules are interacting with the surfactant at the in-
À [26,32,33]
AOT (C=O, symmetrical and asymmetrical SO )
cannot
3
be monitored in the RMs formed in these systems since they
appear in the same region of the lipophilic ester non-polar sol-
vents.
[31,32]
terface breaking its H-bond network.
This is probably due
À
to the interaction mainly with the polar heads group (SO ) of
3
[31,32]
Figure 5 shows the FTIR spectra in the region corresponding
AOT.
to the O-D stretching band (n ) of HOD in IPM/AOT/water
Figure 6 also shows that although in both RMs the nO-D
OD
system at different W values. In Figure S10 (supporting infor-
values shift to lower frequencies as W increases, the values
0
0
mation section) is shown the corresponding FTIR study ob-
obtained are different from each other. For example, at W =2
0
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in IPM RMs the frequency of the O-D band appears at
voked to explain why the penetration of IPM into the interface
is larger than ML making the interface of IPM/AOT RMs more
polar but less H-bonding that ML/AOT RMs.
À1
2
565 cm , while in RMs formed in ML the n
value is
O-D
À1
2
585 cm . The same tendency is observed at W =10; being
0
À1
À1
the band at 2536 cm in IPM and at 2548 cm in ML RMs.
These results suggest a difference in the magnitude of the
water-anionic surfactant interaction at the interface. In compar-
ison with traditional non-polar solvents, the O-D stretching
mode of the water entrapped also show similarities with the
biocompatible solvents explored here. For example, in ben-
Thus, upon confinement the water entrapped in IPM RMs
seems to be more associated with other water molecules than
with the surfactant polar head group. This result is completely
different to those found for ML RMs where the confinement
makes the interfacial water less self-associated than the bulk
À
one because of the strong water—SO3 interaction. In
À1
zene/AOT RMs the frequency of the O-D band is 2563 cm at
Scheme 2 is represented how the penetration external solvent
affect the water structure at the interface in both RMs.
[
32]
W =2. On the other hand, at the same water content but in
0
À1 [26]
isooctane/AOT RMs the O-D band appears at 2570 cm .
Analyzing the data showed in Figure 6, a stronger water-sur-
factant interaction is present in ML RMs while for IPM RMs this
interaction seems to be weaker. This would lead to more water
molecules interacting with the interface in ML/AOT RMs with
its H-bond network more disrupted. On the other hand, for
AOT RMs generated in IPM a weaker water-surfactant interac-
tion allows the water molecules to establish H-bond to each
other trending to bulk water more easily than in ML RMs.
These evidences can be produced by the dissimilar penetration
3
. Conclusions
The present work demonstrates how two biocompatible sol-
vents can be used to replace non-polar solvent in the AOT
RMs formation. Particularly, the polarity and viscosity of the ex-
ternal solvents impact strongly in the interfacial properties of
these RMs. All these results allow us to characterize these bio-
compatible systems, providing information about interfacial
properties such as micropolarity and H-bond donor ability and
how they can be altered changing the external solvent. Thus,
the ability of the non-toxic solvent to penetrate or not into the
AOT interface produces a new interface with attractive proper-
ties. In this sense, actually we are performing experiments
about enzymatic catalysis and nanoparticles synthesis inside
IPM and ML AOT RMs. Finally, we demonstrate that the non-
toxic AOT RMs systems have similar properties than the tradi-
tional AOT RMs made in hydrocarbons. That is, the water is ef-
fectively entrapped in the polar interior and, they have differ-
ent and unique properties in comparison with bulk water.
[
18]
of the non-polar solvents into the interfacial region. Two fac-
tors can be invoked to explain this phenomenon: the viscosity
and polarity of the external solvents.
When more viscous is the solvent, greater is its ability to
penetrate to the interface in RMs. For example, common non-
polar solvents to create RMs such as benzene and n-heptane
[
18]
show this tendency. Benzene is more viscous than n-heptane
[
4]
and penetrates more to the interface in AOT. In this sense,
[
18]
the larger viscosity of IPM in comparison with ML allow to
penetrate more toward the AOT RMs interface.
As it can be seen from the QB study, IPM is more polar (lmax
B1 =552 nm) than ML (lmax B1 =573 nm) and consequently can
penetrate deeper the polar interface than ML. Furthermore, Acknowledgements
the IPM penetration into the interfacial zone affect also the H-
bond interactions of the region as is reflected by the high
value of the AbsB /AbsB ratio obtained (Figure 4). Thus, the
Financial support from the Consejo Nacional de Investigaciones
Cientꢀficas y Tꢁcnicas (PIP CONICET 112–201101-00204, PIP CONI-
CET 112–2015–0100283), Universidad Nacional de Rꢀo Cuarto
2
1
viscosities and polarities of these non-toxic solvents can be in-
Scheme 2. Schematic representation of the penetration of the IPM (left) and ML (right) in the AOT RMs interfaces.
&
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Articles
(
PPI-UNRC 2016–2018), Agencia Nacional de Promociꢂn Cientꢀfica
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2
012, 13, 115–123.
y Tꢁcnica (PICT 2012-0232, PICT-2012-0526 and PICT 2015-0585),
and Ministerio de Ciencia y Tecnologꢀa, gobierno de la provincia
de Cꢂrdoba (PID 2013) is gratefully acknowledged. J.J.S., N.M.C.
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Revised manuscript received: December 27, 2017
Version of record online: && &&, 0000
2011, 56, 3698–3702.
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&
These are not the final page numbers! ÞÞ

ARTICLES
V. R. Girardi, J. J. Silber, R. D. Falcone,*
N. M. Correa
The biocompatible solvents methyl
laurate (ML) and isopropyl myristate
(
IPM) were used as a less toxic alterna-
&& – &&
tive to replace the nonpolar component
in a sodium 1,4-bis-2-ethylhexylsulfosuc-
cinate (AOT) reverse micelles (RMs) for-
mulation. The micropolarity and the hy-
drogen-bond ability of the interface
were monitored using the solvato-
chromism of a molecular probe (1-
methyl-8-oxyquinolinium betaine) and
by Fourier transform infrared spectros-
copy.
Micropolarity and Hydrogen-Bond
Donor Ability of Environmentally
Friendly Anionic Reverse Micelles
Explored by UV/Vis Absorption of a
Molecular Probe and FTIR
Spectroscopy
&
ChemPhysChem 2018, 19, 1 – 8
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8
ꢁ 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!