Izabela Karpiuk 0 1 2
Hanna Wilczura-Wachnik 0 1 2
Adam Mys´lin´ ski 0 1 2
0 National Medicines Institute , Chelmska 30/34, 00-725 Warsaw , Poland
1 Faculty of Chemistry, University of Warsaw , Pasteura 1, 02-093 Warsaw , Poland
2 & Hanna Wilczura-Wachnik
In this paper the calorimetric studies on a-tocopherol/AOT/alkane/water systems are presented. Structural representations of surfactant AOT (a) and a-tocopherol (b) are shown below.
SO3– Na+ H3C
a-Tocopherol (antioxidant) is a form of vitamin E that is
preferentially absorbed and accumulated in humans. AOT
is an anionic type of surfactant. Its molecule contains two
non-polar hydrocarbon tails connected to the polar head.
The paper contains new experimental data of heat of
mixing (Q) for heterogeneous system
a-tocopherol/AOT/nalkane with and without water measured at 309.2 K. The
heat of mixing for AOT (sodium bis (2-ethylhexyl)
sulfosuccinate) and a-tocopherol solutions in n-alkane (heptane,
decane, hexadecane) as the function of surfactant
concentration and parameter R given as the water to surfactant
molar ratio R = [H2O]/[AOT] was measured. It can be
seen that the Q values grow significantly with temperature
increase which indicates on entropic origin of this process.
The significant influence of surfactant concentration on
Q values was noticed. Unexpectedly, the water presence in
the reverse micelles did not have any noticeable influence
on the heat of mixing values measured at 309.2 K. There
was not noticed the significant hydrocarbon chain length
influence on the thermal effect of mixing a-tocopherol and
AOT solutions. The thermodynamic parameters: the
ing constant (K) and the molar enthalpy of transition (DHtr)
of a-tocopherol between solvent and AOT reverse
micelles, were calculated from the heat of mixing data as a
function of surfactant concentration using the nonlinear
regression method. The new calorimetric data measured at
309.2 K were compared to results obtained earlier at
298.2 K, as well as have been discussed in the context of
the hydrocarbon chain length and water presence influence
on a-tocopherol solubility in AOT reverse micelles.
Generally in all investigated systems have been not found a
significant influence of water concentration in AOT reverse
micelles on a-tocopherol molecules distribution between
organic phase and micellar phases. The exothermic values
of a-tocopherol molecules transition from organic to
micellar phase indicate on spontaneous motion of
a-tocopherol towards AOT aggregates. For the systems with
n-heptane and n-decane as a solvent, the (DHt0r) values are
between -51 and -52 kJ mol-1 whereas for n-hexadecane
around -43 kJ mol-1. Such tendency can be explained
through hydrocarbon chains orientation effect existing in
hexadecane which probably is responsible for a
a-tocopherol molecules freedom motion limitation. Moreover, the
solubility of the a-tocopherol in AOT reverse micelles
measured with calorimetric technique has been compared
to the literature data obtained, respectively, with UV
spectrophotometer for reverse micelles, and for
phospholipids bilayer by other techniques. Finally, transferring of
a-tocopherol from solvent to AOT reverse micelles,
intermolecular interactions between a-tocopherol and AOT
micelles were discussed, and a privileged place of
atocopherol in the palisade layer of AOT reverse micelle has
It is known that a water addition to a solution of surfactant
AOT (sodium bis (2-ethylhexyl) sulfosuccinate) in alkanes
and next shaking during a few minutes induces a
spontaneous microemulsion formation [
]. Among many, S.P.
Moulik research group belongs to the pioneer workers in
the field of this type of microemulsions, but not only. They
published many essential papers regarding formation,
structure, phase behaviour, stability and thermodynamics
of water/surfactant/oil systems [
] also including some
aspects of modelling biological microemulsions .
Such a three-component system (AOT/alkane/water) is
thermodynamically stable although some evidence has
recently been obtained suggesting that AOT water-in-oil
microemulsions formed in decane can separate into two
phases on standing without stirring for a long time (even a
few months) [
]. Besides, microemulsions as separate
classes of systems containing surfactants aggregates also
normal and reverse micelle solutions are selected.
Generally it is accepted that the surfactant concentration is the
criterion of such selection. The micelles are distinguishable
in a narrow surfactant concentration range (usually above
critical micellization concentration, CMC). The
phenomenon of normal and reverse micelle formation is due to
the amphiphilic nature of surfactant molecules, which have
separate lyophilic and lyophobic groups. Having both types
of such groups makes a surfactant molecule both
amphiphilic and amphipathic. A result of the micellization
process is the two-phase system formation consisting of the
continuous solvent phase and pseudophase containing
micellar structures. Such pseudophase formation is a kind
of approximation because of dynamic equilibrium between
single-phase isotropic of surfactant solution and
thermodynamically inverted surfactant aggregates which are in
dynamic equilibrium with the surfactant single molecules
(surfactant monomers) existing in the solution. Generally,
when the solvent appears chemical affinity to surfactant
headgroups (which usually are polar) normal micelles are
formed whereas in non-polar solvents the reverse micelles
formation occurs [
Sodium bis (2-ethylhexyl) sulfosuccinate (AOT) is a
very popular anionic surfactant whose molecule consists of
a polar head-group and two non-polar tails (built with
hydrocarbon chains) [
]. The structural formula as well
as the size and the shape of AOT reverse micelles formed
in aliphatic hydrocarbons or other non-polar solvents have
a good representation in the literature [
]. Until now
reverse micelles made from water and AOT are commonly
studied experimentally as models of aqueous
microenvironments. They are small enough for individual reverse
micelles to also be studied by molecular dynamics
simulation, which yields detailed insight into their structure,
size and properties .
One of the most important among many
physicochemical properties is a solvation of a wide range of hydrophilic
compounds in the core of reverse AOT micelles. Because
of this property, reverse micelles are of interest in
], catalysis and in a variety of fields of
study in biology and chemistry also at the molecular level
] recently. In addition, reverse micelles have opened
up the possibility of developing new biotechniques to
extract proteins from a liquid medium , for example.
Recently it has been shown that natural membranes except
lipid bilayers have some domain areas of reverse micelle
]. It has been recognized that the
AOT/nalkane/water system mimics in a simple way the domain
structural peculiarities that have been found in natural
membranes as for example micro-surroundings of such
biomolecules as enzymatic proteins [
]. According to
recent research, the natural cell membranes have a domain
structure and except for such domain patterns as lipid rafts,
caveoles are present and also some areas having structure
of reverse micelles (hexagonal phase second, HII) [
This finding opened a new possibility for very simple
lifemimicking systems. Although processes in natural cells are
very complicated and in their exploration many parameters
need to be taken into account, such a simple structural
model as reverse micelles is useful at the stage of deducing
the fundamental rules occurring as close as possible to the
natural conditions. As was mentioned earlier [
] in our
investigations, the AOT reverse micelles are discussed as
an interface defined by the hydrophobic tailgroups
separating the hydrophilic headgroups and water pools (in
water-containing reverse micelles) from the organic
nonpolar solvent phase. The AOT reverse micelles can host a
different kind of hydrophilic molecules inside the micelle
core and hydrophobic in the palisade layer formed with
non-polar tailgroups [
]. This peculiarity of AOT reverse
micelles led us to explore the natural antioxidant favourite
places in such a model structure of domains of natural
membranes as close as possible to the natural structural
The presented work is an extension of our investigation
on natural antioxidant solubility in AOT reverse micelle
solutions in alkanes. Previously calorimetric data for
] and a-tocopherol [
] at 298.2 K were
In this paper are reported the new calorimetric results on
a-tocopherol solubility in AOT/n-heptane/water,
AOT/ndecane/water and AOT/n-hexadecane/water systems
measured at 309.2 K. Obtained data are compared to results at
298.2 K for AOT/n-heptane/water reported previously [
and then discussed in the context of the temperature, and
alkane chains length influence on a-tocopherol solubility in
AOT reverse micelles, as well as the water contents
influence on its localization in AOT reverse micelles. The
presented data indicate that: (1) the molar enthalpy of
transfer of a-tocopherol from the solvent to AOT reverse
micelles decreases in the order: n-heptane [
n-decane n-hexadecane; (2) the significant influence of
temperature on the molar enthalpy of transition (DHtr) of
a-tocopherol between solvent and AOT reverse micelles
has been noticed; (3) it has been confirmed that the palisade
layer is the privileged place of a-tocopherol localization in
domain structure mimicking with AOT reverse micelles.
The paper also contains a comparison of calorimetric data
] to the literature data obtained with UV
] as well as for natural cell membrane structures
like the phospholipid bilayer obtained by other techniques
Materials and methods
99% aerosol OT was purchased from Sigma-Aldrich Co.,
USA, and vacuum-dried at 25 C for 24 h, and then the
AOT container was kept over molecular sieves.
a-Tocopherol was obtained from SIGMA as 95%, and
continuously kept at temperature between 2 and 8 C as well as
protected from air and light. For calorimetric
measurements, solvents and water were prepared as follows:
n-heptane, n-decane and n-hexadecane (Merck) were dried
over molecular sieves and then purified by fractional
distillation (distillation in n-hexadecane case was done under
vacuum). The final purity checked by GLC was 99.97% for
n-heptane and n-decane, and 99.95% for n-hexadecane.
Water was deionized and bidistilled from a laboratory
All investigated solutions were prepared gravimetrically
using a balance with ± 0.00005 g accuracy.
The flow microcalorimeter UNIPAN 600 [
] was used for
the heat of mixing measurements in all investigated
systems. Using the diathermic (stable state) method, thermal
effects occurring during mixing two solutions (suitable
antioxidant and surfactant in n-alkane) were detected at
309.2 K. Two peristaltic pumps were used to drive both
investigated solutions into the calorimetric chamber.
During all experiments (including calibration), the flow rates of
a-tocopherol/n-alkane and AOT/n-alkane solutions were
constant and equal 0.2 mL min-1. The numeric value of
the flow rate was chosen in order to assure the occurrence
of the totality of the thermal effect inside the calorimeter
chamber (vessel). Before each heat of mixing measurement
(Q) as a standard procedure, the base line signal (U0) was
detected. During base line detection, one pump drove
solvent (n-alkane) and the second AOT/n-alkane solution at a
given suitable molality. In this way, the base line detection
caused the main thermal effect correction with heat of
dilution of the AOT/n-alkane solution. The calibration
procedure using electric method was described elsewhere
in details [
The thermal effects occurring in the calorimetric vessel
during base line detection (U0), as well as during mixing
the two solutions (U) were measured in terms of voltage,
and recorded. Afterwards, collected data were processed to
calculate the heat of mixing effect using the prescribed way
]. In all measurements, accuracy of the thermal effects
determination is estimated to be within ± 2 J mol-1.
The amount of water dissolved in AOT solutions is
given with R parameter defined as a ratio of water to AOT
molality (R = [H2O]/[AOT]). The heat of mixing (Q) was
measured as the surfactant concentration function for
different R. In all investigated systems, a-tocopherol solution
concentration was invariable and equal to 0.02 mol kg-1.
The molalities of AOT solutions were above CMC
characteristic for a given solvent and below 0.4 mol kg-1.
They were above the suitable CMC value to be sure that
surfactant molecules are aggregated in reverse micelles
structure. The numerical values of CMC for AOT reverse
micelle formation in n-alkane used as solvents are given in
Table 1. To avoid eventual phase separation, the AOT
solutions with R higher than 10 were continuously mixed
with a magnetic stirrer until the mixing chamber dosage
was reached. Shimadzu pumps LC-10AD (the flow rate
accuracy ± 2 lL min-1) were used for driving both
solutions into the calorimetric vessel.
Results and discussion
Our investigation was focused on the system: a-tocopherol/
n-alkane/AOT/water. Normal alkanes: n-heptane, n-decane
and n-hexadecane were used as solvents. The experimental
heat of mixing data (Q) for a-tocopherol/n-alkane solution
with AOT/n-alkane/water system measured
calorimetrically at 309.2 K as a function of AOT molality and R
parameter are present in Fig. 1. Q values recorded in all
systems were strongly exothermic and decreasing with
AOT concentration. A particularly rapid decrease is
observed in the range of AOT concentrations below
0.05 mol kg-1. In all studied systems, we did not notice
any influence of water dissolved in AOT reverse micelles
on Q values.
The results obtained at 309.2 K for n-heptane were
compared to data obtained previously at 298.2 K [
(Fig. 2). It can be seen that the Q values grow significantly
with temperature increase which indicates on entropic
origin of this process. The significant influence of
surfactant concentration on Q values was noticed. Unexpectedly
the water presence in the reverse micelles did not have any
noticeable influence on the heat of mixing values measured
at 309.2 K (see Fig. 2a). In contrary, the surfactant
concentration increase from 0.02 to 0.11 mol kg-1 caused a
significant decrease in heat of mixing values. Similar
regularity has been observed in the systems containing
Method of measurement UV–vis – –
n-decane and n-hexadecane as solvents. There was not
noticed the significant hydrocarbon chain length influence
on the thermal effect of mixing a-tocopherol and AOT
solutions (Fig. 1).
The total thermal effect concomitant with two solutions
mixing as a function of surfactant concentration can be
discussed in the context of a-tocopherol molecule
distribution between AOT reverse micelles and bulk solvent.
Magid et al. [
] proposed a thermodynamic model
describing phenol solvation in AOT reverse micelles
measured with the UV method. According to it, Yekta et al.
] and Kwan et al. [
] made the assumption that the
mean number of molecules binding to a single micelle can
be described by the Poisson distribution function and they
T = 298.2 K
[AOT] = 0.02 mol kg–1
[AOT] = 0.1 mol kg–1
[AOT] = 0.3 mol kg–1
0 10 20 30 40 50 60
R = [H2O]/[AOT]
T = 309.2 K
0,00 0,05 0,10 0,15 0,20 0,25 0,30 0,35
Fig. 2 Heat of mixing of 0.02 mol kg-1 a-tocopherol/n-heptane with
AOT/n-heptane/water solution as a function of AOT/n-heptane
concentration: o—data at 298.2 K [
]; filled points mark data at
309.2 K (where: blue dots, red triangles, green triangles and pink
diamonds mark data for R = 0; 1.46; 5 and 50). a Heat of mixing of
0.02 mol kg-1 a-tocopherol/n-heptane with AOT/n-heptane/water
solution as a function of R at 309.2 K for chosen AOT/n-heptane
defined the ratio of the concentration of molecules bonded
with micelles to molecules unbound as directly
proportional to the total surfactant concentration. The
proportionality coefficient was the association constant with the
same value for each association step. D’Aprano et al. [
successfully used this model for describing thermal effects
connected with n-pentanol transition from heptane to AOT/
In the present paper, the total value of heat of mixing of
two solutions: a-tocopherol/n-heptane with AOT/n-heptane
has been considered as the sum of a two contributions: heat
of dilution of a-tocopherol and the heat of a-tocopherol
and AOT intermolecular interaction. It needs to be noticed
that the heat of dilution of AOT solution was covered in the
base line. When AOT concentration tends to infinite
dilution, the a-tocopherol/n-heptane solution is mixed with
almost pure solvent. Thus, at AOT infinite dilution we can
discuss the calorimetrically measured heat of mixing as a
sum of the contributions of a-tocopherol solution dilution
and the intermolecular interactions between a-tocopherol
and AOT molecules. Following Magid et al. [
assumed that the mean number of a-tocopherol molecules
connected to one AOT reverse micelle is described in the
terms of the Poisson distribution function. The final
relation used in the numerical analysis with nonlinear
regression of heat of mixing experimental data was the following:
Qtot ¼ Qf þ ðQb
Qf ÞKCAOTð1 þ KCAOTÞ 1
where Qtot is the total heat of mixing value (experimental
data), Qf is a heat of dilution of a-tocopherol solution, Qb is
the heat of a-tocopherol and AOT intermolecular
interaction, K is the binding constant of a-tocopherol molecule
solubilized in the AOT reverse micelles and CAOT is the
AOT molality. The experimental data (Qtot) are correlated
to Eq. (1) with three fitted parameters: Qb, Qf and K
suitable. In the nonlinear regression procedure, values of
binding constant (K) and the heats contributed to Qtot (Qf,
Qb) were obtained. The molar enthalpy of transition (DHtr)
of a-tocopherol from solvent to AOT reverse micelles was
calculated as a difference between Qb and Qf. The
numerical values of all fitted parameters and DHt0r are given
in Table 2.
Reported in previous paper [
], experimental data
registered at 298.2 K were analysed with the procedure
described above and compared to binding constant and
molar enthalpy of transition values founding in NLREG
procedure employed to Nernstian law formalism [
]. As it
can be seen in Table 2 obtained in both procedures,
parameters K and (DHt0r) for a-tocopherol/n-heptane/AOT
system are comparable and we conclude that the
procedures are compatible.
Next the a-tocopherol distribution constants (K0) were
calculated using the relation:
ðK0Þ ¼ MAOT ð2Þ
where K is a binding constant, MAOT is a surfactant
The obtained values are given in Table 2. For systems
with n-heptane as solvent, the difference between
distribution constant values obtained at 309.2 and 298.2 K is
not significant. For systems with R = 0, the distribution
constant value (K0) changes in order heptane \
decane hexadecane. Additionally the lack of any
dependence between the distribution constant and the
R parameter can be noticed in the systems with n-heptane
as solvent. For n-decane K0 value doesn’t change with R in
the range R \ 075 [ and then decreases for R = 30. For
n-hexadecane systems K0 are stable for R \ 071.46 [.
Generally in all investigated systems have been not found a
significant influence of water concentration in AOT reverse
micelles on a-tocopherol molecules distribution between
organic phase and micellar phases (Fig. 3 and Table 2).
This finding indicates that the privileged place of
a-tocopherol molecules in AOT reverse micelles is the micellar
palisade layer between the alkyl chains of surfactant.
Moreover, the small values of Qb indicate that
intermolecular interactions between a-tocopherol and AOT are
rather weak. As a consequence, a-tocopherol molecules
move towards the external part of the palisade layer and
they do not compete with water molecules (in the systems
with R [ 0) for the binding sites at the water surfactant
interface. The exothermic values of a-tocopherol
molecules transition from organic to micellar phase indicate on
spontaneous motion of a-tocopherol towards AOT
aggregates. For the systems with n-heptane and n-decane as a
solvent, the (DHt0r) values are between -51 and
-52 kJ mol-1 whereas for n-hexadecane around
-43 kJ mol-1. Such tendency can be explained through
hydrocarbon chains orientation effect existing in
] which probably is responsibly for a
a-tocopherol molecules freedom motion limitation.
On the basis of the presented data, we can say that the
thermodynamic parameters of a-tocopherol solution in
AOT reverse micelles obtained from calorimetric data led
us to deduce that a-tocopherol is preferentially solubilized
in the palisade layer, not inside AOT reverse micelles.
Such hypothesis was postulated in our previous paper [
(K0 values are independent of the R parameter for a
different AOT molality). Our findings also confirmed results
obtained by Avellone et al. [
] who investigated the
binding of a-tocopherol to water-containing reverse
micelles using UV–vis spectrophotometry, as well as by
Fukuzawa et al. [
], Urano et al. [
] and Wassall et al.
] who found that vitamin E molecules are located in the
hydrophobic part of the phospholipid bilayer of cell
membranes. Recently Gunaseelan et al. [
results referring to a-tocopherol solubility in
water/surfactant/octane system. Although they used a different kind
of surfactants (forming regular micelles), their results are
consistent with our data presented in this paper.
In this paper, the new calorimetric studies on a-tocopherol/
n-alkane/AOT system at 309.2 K are presented. Using the
flow calorimetric method, the a-tocopherol solubility in
AOT reverse micelles, both with and without water, was
investigated. Using the experimental heat of mixing data,
the binding constant and the heat of dilution of
a-tocopherol as well as the heat of a-tocopherol and AOT
intermolecular interaction at R = 0, 1.46, 5.0, 30 and 50
with nonlinear regression were calculated. The obtained
results were compared to data at 298.2 K.
The present paper is a continuation of our investigations on
a-tocopherol solubility in the AOT/n-heptane/water system.
Previously it has been postulated that a-tocopherol molecules
are located preferentially in the palisade layer of AOT reverse
micelles. Present data are in a good agreement with the
literature UV absorption spectroscopy data, as well as with results
published for phospholipids bilayer structure.
As it has been emphasized previously, the knowledge on
a-tocopherol localization in such a simple model of
interface as AOT reverse micelles may be useful in mimicking
the domain structural peculiarities in the presence of
The presented data indicate that a-tocopherol molecules
are located between palisade layers of AOT reverse
micelles towards its external part. In addition, it has been
postulated not competition in the interaction with the polar
heads of the AOT between molecules of water that were
located in the interior of micelles, and a-tocopherol in the
palisade layer. Data presented in this paper indicate also
that AOT reverse micelles mimic some aspects of the
domain structure of biomembranes successfully.
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