Structure and properties of water-oil microemulsions

Article abstract of DOI:10.1023/A:1020721909581

The parameters determining the structure and properties (surface potential and size of microdrops) of oil-water microemulsions containing various surfactants, and also the distribution of components between the dispersed phase and dispersing medium were d

Full text of DOI:10.1023/A:1020721909581

Russian Journal of General Chemistry, Vol. 72, No. 7, 2002, pp. 1007 1011. Translated from Zhurnal Obshchei Khimii, Vol. 72, No. 7, 2002,
pp. 1077 1081.
Original Russian Text Copyright
2002 by Mirgorodskaya, Kudryavtseva, Zuev, Idiyatullin, Fedotov.
Structure and Properties of Water Oil Microemulsions
A. B. Mirgorodskaya, L. A. Kudryavtseva, Yu. F. Zuev,
B. Z. Idiyatullin, and V. D. Fedotov
Arbuzov Institute of Organic and Physical Chemistry, Kazan Scientific Center, Russian Academy
of Sciences, Kazan, Tatarstan, Russia
Institute of Biochemistry and Biophysics, Kazan Scientific Center, Russian Academy of Sciences,
Kazan, Tatarstan, Russia
Received July 23, 2001
Abstract The parameters determining the structure and properties (surface potential and size of microdrops)
of oil water microemulsions containing various surfactants, and also the distribution of components between
the dispersed phase and dispersing medium were determined by the kinetic method and by high-resolution
1H NMR spectroscopy with a pulse magnetic field gradient.
Microemulsions, i.e., isotropic optically homoge-
neous dispersions of oil in water or water in oil (by oil
is meant a hydrocarbon) are formed spontaneously
when a hydrocarbon, water, a surfactant, and a co-
reaction. Among the diversity of structural methods,
our attention was attracted by Fourier-transform H
NMR spectroscopy with pulse magnetic field gradient
[9], allowing measurement of the diffusion coeffi-
1
surfactant (usually a lower alcohol) are mixed in def- cients of microemulsion components.
inite proportions. Thanks to the high solubilizing
As reaction media for base hydrolysis of esters
power of microemulsions and extremely developed in-
terface ensuring an efficient contact between reactants
with different solubility in aqueous and organic me-
dia, these systems are actively used as media for
chemical reactions [1 3]. It is very important to reveal
what characteristics of microemulsions determine the
rates and direction of chemical reactions in them. This
can be done by various physical and physicochemical
methods [4, 5].
[p-nitrophenyl acetate (I) and bis-p-nitrophenyl meth-
ylphosphonate (II)], we chose microemulsions with
equal volumes of disperse phases but different charges
of the microdrop surface. As surfactants we used cat-
ionic cetyltrimethylammonium and cetylpyridinium
bromides, anionic sodium dodecyl sulfate, and non-
ionic decaoxyethylated oleyl alcohol (Brij-97). The
content of a surfactant in all microemulsions was
5.05 wt %. Apart from a surfactant, the microdrops
consist of n-hexane (1.97 wt %), which forms a non-
polar core, and n-butanol (5.05 wt %). The volume
fraction of the disperse phase was 0.13.
This work continues and develops our studies on
the use of oil water microemulsions in nucleophilic
substitutions [6, 7]. Our goal was to study the prop-
erties of direct microemulsions based on various sur-
factants and to reveal factors affecting chemical pro-
cesses in these systems.
The general scheme of base hydrolysis of the esters
can be presented as follows.
The features of microemulsions as media for chem-
ical reactions can be evaluated directly by studying
the kinetics of chemical reactions, i.e., by measuring
the rate constants of processes in these media. This
method can also be used for determining physico-
chemical parameters of microemulsion media, e.g.,
the surface potential of microdrops [8]. The data on
structural features of microemulsions, in particular, on
the size of microdrops and phase distribution of sys-
tem components, are extremely useful for choosing
the best microemulsion for performing a particular
CH3C(O)OC6H4NO2 + 2OH = CH3C(O)O + OC6H4NO2
I
CH3P(O)(OC6H4NO2)2 + 2OH
II
= CH3P(O)(OC6H4NO2)O
+
OC6H4NO2
Kinetic data describing base hydrolysis of esters I
and II in microemulsions are given in the table
and in Fig. 1. It should be taken into account that the
esters are localized mainly in oil drops {e.g., the dis-
1070-3632/02/7207-1007$27.00 2002 MAIK Nauka/Interperiodica

1008
MIRGORODSKAYA et al.
Second-order rate constants (k2, ) of base hydrolysis of esters I and II and surface potential of microdrops ( )
(COH 0.001 0.01 M, 25 C)
Ester I
Ester II
Surfactant
k2, , l mol 1 s
, mV
k2, , l mol 1 s
, mV
1
1
Cetyltrimethylammonium bromide
Cetylpyridinium bromide
Sodium dodecyl sulfate
Brij-97
8.8
9.6
1.1
3.4
24.4
26.6
29.9
0
29.4
30.0
2.7
30.4
29.2
30.9
0
9.0
tribution factor of ester I between hexane and water is
Two components of the effect of microemulsions
10.8 [5]}, whereas highly hydrophilic hydroxide ions on the reaction rate can be distinguished: the electro-
are concentrated in the dispersion medium. This sug-
gests that the chemical reaction takes place on the in-
static component associated with interaction of hy-
droxide ions with the charged surface of microdrops
terface. Irrespective of the nature of a surfactant which and the nonelectrostatic component describing the ef-
forms a microemulsion, the observed hydrolysis rate
constants (kobs) linearly depend on the alkali concentra-
tion in the dispersion medium within the entire con-
centration range studied (Fig. 1), which allows estima-
tion of the second-order rate constants (k2, ) of this
process (see table). With cationic surfactants, the hy-
drolysis of esters I and II is accelerated, as compared
to aqueous solutions with the same alkali concentra-
tion, which is apparently due to the accumulation of
negatively charged hydroxide ions near the microdrop
surface. The negative charge of the surface in the case
of anionic surfactants decelerates this reaction; the
microemulsions with nonionic surfactants have no ap-
preciable effect on the hydrolysis rate (Fig. 1).
fect of the medium.
ln k2, (ISA) = e /kB T + ln k2, (NSA).
(1)
Here e is the electron charge; , effective surface
potential; kB, Boltzmann constant; T, absolute tem-
perature; k2, (ISA) and k2, (NSA), second-order rate
constants determined in microemulsions with ionic
and nonionic surfactants, taking into account the phase
volume. As nonionic surfactant, we used Brij-97.
From the kinetic data on base hydrolysis of I, using
Eq. (1), we determined the effective surface poten-
tials in microemulsions containing ionic surfactants
(see table). The potentials in these systems are less
by 50 100 mV (in absolute values) than in micellar
solutions [10 12]. This is due to the presence of neu-
tral butanol molecules in the surface layer of micro-
drops. The coincidence of values obtained in the ex-
periments with esters I and II suggests that the loca-
tion of these substrates in microdrops is similar.
The surface potential is an important parameter
describing microemulsions as reaction media, because
it determines the role of Coulombic interactions in
solubilization of reactants. Specifically the surface
potential exerts a decisive effect on the shift of acid
base equilibria, as it is related to the solubilizing
selectivity with respect to neutral and protonated
species. We have clearly demonstrated it by the ex-
ample of cetylamine: the pH of its half-neutralization
point is 8.5, 8.7, and 10.9 in microemulsions based
on cetylpyridinium bromide, Brij-97, and sodium
dodecyl sulfate, respectively. Hence, the higher the
microdrop surface potential, the greater the content
of the neutral amine species in the system, which, in
turn, affects the behavior of amines in chemical pro-
cesses, in particular, their activity in cleavage of ester
bonds [6, 7].
Fig. 1. Observed rate constant (25 C) of hydrolysis of
ester I (1 3) in microemulsions containing (1) cetylpyr-
idinium bromide, (2) Brij-97, and (3) sodium dodecyl sul-
fate and (4) in aqueous solution, as a function of the alkali
concentration.
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STRUCTURE AND PROPERTIES OF WATER OIL MICROEMULSIONS
1009
1
Fig. 2. Diffusion falloffs of signals in the H NMR spectra of microemulsions based on cetylpyridinium bromide: (1) hexane,
(2) butanol, and (3) water. (G) Magnetic field gradient.
It should be noted that, although the surface po-
havior of microdrops based on sodium dodecyl sulfate
tential is a characteristic parameter of microemulsions, and cetylpyridinium bromide differ essentially. Never-
it can be affected by solubilized agents. This may be
due to a change in binding of head groups and coun-
terions of a surfactant in a microdrop, and also to re-
distribution of microemulsion components between
the dispersed phase and the dispersion medium under
the action of added components.
theless, without analyzing this difference in detail,
we can state that these dependences reflect diffusion
of hard spherical particles [13, 14], i.e., indeed,
DSA Ddrop. The concentration dependences of DSA
also allow us to determine by extrapolation the micro-
drop diffusion coefficient at infinite dilution and to
Information on the distribution of microemulsion
components can be obtained by NMR spectroscopy.
The combination of high-resolution NMR with the
technique of pulse magnetic field gradient makes it
possible to determine diffusion falloffs of individual
components of microemulsions (Fig. 2) and to com-
pute their diffusion coefficients (D).
The procedure of studying distribution of micro-
emulsion components between various phases requires
the diffusion coefficients of microdrops to be deter-
mined first. As a rule, the diffusion coefficients of
a surfactant (DSA) and a drop (Ddrop) in microemul-
sions are approximately equal, in contrast to micellar
solutions. It is clearly seen from the dependences of
DSA on the dispersed phase fraction for two micro-
emulsions under study (Fig. 3). As seen from Fig. 3,
the concentration dependences of the diffusion be-
Fig. 3. Diffusion coefficient of a surfactant as a function of
the dispersed phase fraction in microemulsions based on
(1) cetylpyridinium bromide and (2) sodium dodecyl sul-
fate.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 72 No. 7 2002

1010
MIRGORODSKAYA et al.
calculate the size of microdrops using the Stokes Ein-
stein equation. We have determined the radii of micro-
drops based on cetylpyridinium bromide (3.7 nm) and
sodium dodecyl sulfate (2.7 nm).
structure and properties of oil water microemulsions
used as media for hydrolytic cleavage of ester bonds.
EXPERIMENTAL
The fact that butanol, which is added to a micro-
emulsion to stabilize the surface layer, is not com-
pletely concentrated in microdrops is seen even from
its experimental diffusion coefficients. For a micro-
emulsion with cetylpyridinium bromide, Dbut is
The solvents and substrates I and II were purified
by common procedures. Surfactant samples were
twice reprecipitated with diethyl ether from ethanol
solution.
10
1
5.9 10
sodium dodecyl sulfate, Dbut is 4.7 10
These values are greater than the corresponding
m2 s , and for a microemulsion with
The reaction kinetics were studied spectrophoto-
metrically with a Specord UV-Vis instrument in tem-
perature-controlled cells. The reaction course was
monitored by variation of the optical density of solu-
tions at 400 nm (formation of p-nitrophenolate anion).
10
1
m2 s .
11
diffusion coefficients of microdrops (4.8 10
and
4.9 10 m2 s ) by almost an order of magnitude.
Thus, motion of butanol both in microdrops and in
the aqueous dispersion medium contributes to its
measured diffusion coefficient. Under the conditions
of fast (in the NMR time scale) exchange of butanol
between these two states, the observed diffusion co-
efficient is determined by Eq. (2).
11
1
5
The initial substrate concentration was 5 10 M,
and the conversion exceeded 90%.
We determined the observed pseudo-first-order rate
constants (kobs) from the relationship log(D
D ) =
0.434kobs + const, where D and D are the optical
densities of solutions at instant and after the reaction
completion. The kobs values were calculated using
the least-squares technique. The second-order rate
constants (k2, ) were calculated for a linear portion
of the dependence of kobs on the alkali concentration
in the dispersion medium (COH ) by the equation
free
Dbut = pbond Ddrop + (1
pbond)D
.
(2)
but
Here pbond is the butanol fraction participat-
free
ing in formation of microdrops, and D
10
= 7.2
but
m2 s is the butanol diffusion coefficient in
11
1
k2, = kobs /COH
.
an aqueous medium, as determined in independent
measurements. We found that, in the case of micro-
emulsions with cetylpyridinium bromide, only 20%
of butanol participates in the dispersed phase forma-
tion, and in the case of microemulsions with sodium
dodecyl sulfate this fraction is 38%. Compounds I
and II and alkali in concentrations used in the ki-
netic experiments do not affect the diffusion charac-
teristics of the system. However, with a microemul-
sion based on sodium dodecyl sulfate as example, we
found that its components are considerably redistrib-
uted at a high alkali content. At COH 0.2 M, the frac-
tion of bound butanol is lowered (29%), and the ra-
dius of microdrops decreases (2.2 nm). This should
cause an increase in the absolute value of the sur-
face potential and, as a consequence, a decrease in
the second-order rate constants of base hydrolysis.
Indeed, k2, for the hydrolysis of I in 0.2 N alkali
We measured the diffusion coefficients of micro-
emulsion components with a modified Tesla BS-587A
high-resolution NMR spectrometer at the proton res-
onance frequency of 80 MHz. The spectrometer was
equipped with a unit of pulse magnetic field gradient,
which allows creation of a field gradient of up to
1
50 G cm . The device and experimental procedures
used for studying microemulsions are described else-
1
where [15, 16]. We recorded the H NMR spectra and
measured the diffusion coefficients at 30 C. In micro-
emulsions studied in this work by NMR, we used
a dispersion phase consisting of 98 vol % D2O and
2 vol % double-distilled H2O.
ACKNOWLEDGMENTS
This work was financially supported by the Rus-
sian Foundation for Basic Research (project nos. 99-
03-32037a and 01-03-33224).
1
solution appeared to be 0.86 l mol 1 s (cf. data in
table).
Thus, we have demonstrated the possibilities of
using the kinetic method and high-resolution 1H NMR
spectroscopy with pulse magnetic field gradient to
determine the characteristics (surface potential and
size of microdrops) of microemulsions and the dis-
tribution of components between the dispersed phase
and dispersion medium. These data determine the
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