J. Phys. Chem. 1991, 95, 451-457 always form a dimer in its pure liquid as clarified by the IR spectrum measurements: The acid dimer should have a larger value of at least twice'J that obtained for the long spacing. The incomprehensive value for the long spacing can be explained, as indicated in Figure 1 1 , by a unique arrangement proposed for the dimerized molecules in the clusters. The dimerized acid molecules in the clusters arrange longitudinally and alternately: The dimerized carboxyl groups in one dimer and the methyl groups at the terminal of the neighboring dimers arc aligned alternately in a same lateral plane. Such alignment in the longitudinal direction for the acid molecules is the same as that for dodecanoic acid molecules in A-form crysta1.30*3' The structure somewhat resembles the smectic liquid crystal. Thus we call it a quasi-smectic liquid crystal structure. If dimerized acid molecules take this structure, segments near the end of the acid molecule, i.e., methyl and methylene groups near the 18th position, can move significantly, because the movement of the carboxy groups in the neighbor dimer is restricted as shown in the upper illustration in Figure 8. The lateral distance among neighboring dimers in this structure would increase with an increase in temperature. Consequently, it is reasonable for the hydrodynamic radius for the acid molecule (30) Lomer, T. R. Acta Cryslallogr. 1963, 16, 984. (31) Goto, M.; Ashida, E. Bull. Chem. Soc. Jpn. 1978, 51, 70.
45 1
to increase with increasing temperature (see Figure 3). In addition, when the molecules are cooled, they are liable to assemble into a crystal, because their arrangement does not so apart from the crystal structure. Therefore, the relatively large local viscosity and the high crystallization temperature of cis-9-octadecenoic acid in the temperature range between the melting point and 30 OC are well explained by the presence of the quasi-smectic liquid crystal structure. A consecutive rise in temperature would lead the aligned acid molecules to a more disordered structure and finally to an isotropic one. Acknowledgment. We express our deepest thanks to Professor Hiroshi Kobayashi for his helpful discussion and encouragement throughout the present experiment, Dr. Midori Goto, National Chemical Laboratory for Industry, for her helpful discussion about X-ray diffraction results, Dr. Yasuaki Kawamura, the Institute of Physical and Chemical, for his valuable discussion about liquid crystals, Dr. Michihiro Yamaguchi, the Research Center of Shiseido Cosmetic Co., for his help in the 'HNMR measurement, and Dr. Hideyo Matsuzawa, Mr. Yoshio Ogura, Mr. Toshiyuki Kato, Miss Mugiko Iwasaki, Mr. Kazuo Saito, and Mr. Yoshinori Hayashi for their help in the DSC, density, viscosity, and polarization measurements. This work was supported in part by a Grant-in-Aid for Scientific Research (No. 02640356) from the Japanese Ministry of Education.
Chemical Reactions in Microemuisions: Probing the Local Dielectric Number of the Dispersed Water Reinhard Schomacker Max- Planck-Institut fur biophysikalische Chemie, Postfach 2841, 0-3400 Gottingen, FRG (Received: March 6, 1990; In Final Form: June 25, 1990)
The experiments described in this paper were performed for answering the question how to distinguish between a microemulsion and a weakly structured solution. The local dielectric number of water dispersed in ternary mixtures of water, oil, and amphiphile is probed by studying two chemical reactions sensitive for this physical property. The kinetics of the decomposition of murexide and the pK, value of Thymol blue are used as chemical probes. Two hydrophilic probes are chosen to ensure that the probe is located in the aqueous domains and not at the interface or in the nonpolar domains. If the probe studied in the ternary mixture experiences an environment similar to bulk water, the mixture is classified as a microemulsion. If the dielectric number of the environment of the probe is much lower than that of water, the ternary mixture is classified as a weakly structured solution. A borderline can be drawn at that concentration of water needed to hydrate the head groups of the amphiphile molecules. The presence of microstructure in the weakly structured solution can also be shown by studying a probe reaction. The complexation reaction of NiZ+with the hydrophobic ligand pyridin-2-azodimethylaniline(PADA) is a sensitive probe for the presence of hydrophobic aggregates in a solution.
Introduction
In a recently published review article, Kahlweit and co-workers' suggested microemulsions to be defined as stable colloidal dispersions of either water or oil droplets sufficiently large for the dispersed solute to exhibit the properties of a bulk phase. The size of the droplets appears to be inversely proportional to the square root of the interfacial tension between the water-rich and the oil-rich phase in the presence of a saturated monolayer of the amphiphile. Because the interfacial tension decreases with increasing efficiency of the detergents, and depends, furthermore, sensitively on temperature and the chemical nature of the oil as well as on the brine concentration, the transition from weakly structure homogeneous solutions to microemulsions in the above narrower sense is gradual. This raises the question how to determine the borderline experimentally. In this paper we suggest ( I ) Kahlweit, M.; Strey, R.; Busse, G. J . Phys. Chem. 1990, 94, 3881.
0022-3654/91/2095-045 1$02.50/0
studying chemical reactions in the dispersed phase for probing its properties. For water in oil dispersions (w/o) this can be done by measuring the kinetics of the decomposition of murexide in the presence of an acid, which permits determination of the effective relative dielectric permittivity (dielectric number) in the water domains as it depends on temperature, the efficiency of the amphiphile, and the carbon number of the surrounding oil phase. Experiments show that the phase behavior of ternary mixtures of water oil and nonionic nonionic amphiphiles depends sensitively on the nature of the oil and the amphiphile and on the temperaturee2 In Figure 1 (top) a schematic phase prism summarizes the general patterns of the phase behavior of such mixtures. This phase prism is discussed on the basis of two vertical sections through the prism. Figure 1 (middle) shows a section at a constant (2) (a) Kahlweit, M.; Strey, R.; Firman, P.; Haase, D.; Jen, J.; Schomacker, R. Langmuir 1988, 4, 499. (b) Kahlweit, M.;Strey, R.; Schomacker, R.; Haase, D. Langmuir 1989, 5, 305.
0 1991 American Chemical Society
Schomacker
452 The Journal of Physical Chemistry, Vol. 95, No. 1, 1991
T
t H20
phiphile
oi I 0
-a[wt%l
a = const.
/ I
2
T
Tu
t
100
r;
-
In y
Figure 2. Top: Mean domain size [ at constant y vs cy. Bottom: Mean domain size [ at constant cy versus y (schematic). (Reprinted with permission from ref 9. Copyright 1989 Springer-Verlag.)
solution consists of a stable dispersion of water droplets in oil (w/o microemulsion). For the mean domain size 4 in a random twophase medium, one evaluates'
T
t
0
-a
100
Figure 1. Schematic phase prism (top) for mixtures of water, oil, and nonionic amphiphiles, with vertical sections at constant cy (middle) and constant y (bottom). The section at constant y shows the isotropic channel from (Y = 0 to cy = I 0 0 wt %. The microstructure in the isotropic channel is visualized. (Reprinted with permission from ref 9. Copyright I989 Springer-Verlag.)
ratio of oil to water, cy, and varying amphiphile concentration y. At low temperatures two phases are observed with the amphiphile mainly dissolved in the water-rich phase. At high temperatures the amphiphile is dissolved mainly in the upper oil-rich phase. Within a well-defined intermediate temperature interval T, to Tu the mixtures separate into three liquid phases. An amphiphile-rich middle phase is in equilibrium with a water-rich lower phase and an oil-rich upper phase. If at the mean temperture T the amphiphile concentration y is increased further than y, a single-phase solution is obtained. This solution shows the highest mutual solubility between water and Figure 1 (bottom) shows a vertical section through the prism at a constant amphiphile concentration higher than 7. A channel of homogeneous isotropic solutions is obtained extending from the water-rich side to the oil-rich sidc of the prism. These macroscopically homogeneous solutions are microscopically heterogeneous. On the water-rich side they consist of a stable dispersion of oil droplets in water (o/w microemulsions). At a 4 0 wt %, the solutions become spongelike, with water and oil domains of about equal size, as described in refs 4-6. As one increases cy further, that is, proceeds to the oil-rich side, the (3) Kahlweit, M.; Strey, R. Angew. Chem., Int. Ed. Engl. 1985, 24,654. (4) Lichterfeld, F.; Schmeling, T.; Strey, R. J. Phys. Chem. 1986,90,5762. ( 5 ) Jahn, W.; Strey, R. J . Phys. Chem. 1988, 92, 2294. (6) Olsson, U.; Shinoda, H.; Lindman, B. J . Phys. Chem. 1986,90,4083.
-
$(I - - $ ) / ( W V where 6 is the volume fraction of one of the phases, in our case that of the oil in the mixture of water and oil, and (A/V) is the internal interface, that is, the area of the water-oil interface per unit volume. Assuming that in the three-phase temperature interval all the amphiphile molecules are located in the internal interface, one has A / V = N,a,
4
where N , is the number density of amphiphile molecules (being proportional to y) and a, is the area occupied by each molecule. Accordingly one has
4
-
441 - $I/?
At constant amphiphile concentration y, as, e.g., in the isotropic channel, the dependence of the mean domain size of the microemulsion on cy (=$) can thus be represented as a parabola as shown schematically in Figure 2 (top). At constant cy, the mean domain size decreases as y-' (Figure 2,,bottom). Consequently, the higher the efficiency of the amphiphile, that is, the lower 7, the larger is the mean domain size. This description of the microstructure is supported by electron micrographs, scattering experiments, conductivity and NMR measurements? The change of the structure from the water-rich to the oil-rich side of the isotropic channel can thus be visualized as shown in Figure 1 (bottom). On the water-rich side one finds at low temperatures a dispersion of oil droplets that appear to coagulate with increasing temperature. With increasing oil concentration a,water and oil form a bicontinuous spongelike structure. On the oil-rich side one finds at elevated temperatures a dispersion of water droplets that appear to coagulate as one decreases temperature. Figures 1 and 2 are taken from ref 9 and discussed there in more detail. (7) Andelman, D.; Cates, M. E.; ROUX,D.; Safran, S. A. J . Chem. Ph$. 1987,87, 7229. (8) Kahlweit, M.; Strey, R.; Haase, D.; Kunieda, H.; Schmeling, T.; Faulhaber, B.; Borkovec, M.; Eicke, H. F.; Busse, G.; Eggers, F.; Funck, Th.;
Richmann, H.; Magid, L.; Siiderman, 0.;Stilbs, P.; Winkler, J.; Dittrich, A.; Jahn, W. J. Colloid Interface Sei. 1987, 118, 436. (9) Kahlweit, M.; Strey, R.; Schomacker, R. In Reactions in Compurtmentalized Liquids; Knoche, W., Schomacker, R., Eds.; Springer-Verlag: Berlin, 1989.
Chemical Reactions in Microemulsions
In the experiments, the size of the aqueous domains was varied either by varying the oil to water ratio a at a constant, y, or by varying the amphiphile concentration y at constant a. The variation of the domain size as a function of the parameter a was studied in the system H20-octane-C12E5. From SANS experiments and electron microscopy, a domain size of 80 nm was determined for a sample of equal volumes of water and octane (a = 40 wt %) and an amphiphile concentration of y = 7 wt %.5,8 Decreasing one of the volume fractions yields decreasing domain sizes as described by the parabola in Figure 2 (top). The variation of the domain size as a function of the parameter y was studied for different systems. In the first experiment the concentration of C,,E5 was increased in the system H20-octane-C12E5 at constant a = 40 wt %. For the second experiment, a series of amphiphiles of decreasing chain length was chosen. Each amphiphile of this series shows with water and decane a three-phase body at a temperature around 50 “C. The minimum concentration .5. for obtaining homogeneous solutions varies from 12 wt % for CI2E6to 60 wt % for C4EI. Increasing the chain len th of the alkanes with a given amphiphile makes both 4 and increase. Systems of water, C8E3, and different alkanes were chosen for this experiment, adjusting the temperatures and the amphiphile concentrations as required by the corresponding alkanes. The phase diagrams of the systems studied are described in detail in ref 8 (H20-n-octane-C12E5), ref 10 (H20-n-alkanes-C8Ej), and ref 11 (H20-n-decane-CiEj).
%
Experimental Section ( a ) Materials and Methods. The nonionic amphiphiles CiEj were supplied by Bachem AG (Bubendorf, Switzerland) and Nikkol (Japan). All amphiphiles appeared to contain small amounts of acidic impurities, because decomposition of murexide was observed already before the addition of acid. Therefore, the amphiphiles were carefully purified to remove these impurities. The procedure is described in ref 12. In solutions prepared from purified amphiphiles, no decomposition of murexide is observed without addition of acid. The alkanes and indicators (murexide, Thymol blue, pyridin-2-azodimethylaniline)were supplied by Merck (Darmstadt). The composition of the ternary mixtures of water (A), oil (B), and amphiphile (C) are given in weight percent. The concentration of amphiphile is y = 1 0 0 C / ( A + E + C‘) The ratio of oil to water is expressed as a = 100A/(A + B ) The photometric measurements were performed on a Kontron Uvikon 860. The kinetics of the decomposition of murexide was followed at X = 520 nm. The extinction coefficient of murexide in aqueous solutions a t this wavelength is 1.38 X lo4 mol-’ dm3 A single-exponential decay of the absorbance was observed over a period of about 3-4 7 . The samples were prepared in a Beckman cuvette and thermostated at the temperature of the experiment. When the samples had become homogeneous, a small volume of an aqueous IW3 M stock solution of murexide was added and the solution mixed by shaking. The equilibrium constants of the complexation reaction of Nia+ with pyridin-2-azodimethylaniline(PADA) were determined from photometric titrations. The absorbance at X = 550 nm of the complex formed was determined as a function of the concentration of Ni(N03)2. The concentration of PADA was 2 X mol dm-3 in all samples. Acidic impurities have to be avoided, because the pK, of PADA is 4.2and the protonation of PADA competes with the complexation reaction. The extinction coefficients at X = 550 nm of the species involved are 2 X lo3 mol-’ dm3 cm-I (PADA), 4.04 X lo4 mol-’ dm3 cm-I (NiPADAZ+),and 4.34X IO4 mol-’ (IO) Kahlweit, M.; Strey, R.; Haase, D.; Firman, P. Langmuir 1988, 4, 785. ( I I ) Kahlweit, M.; Strey, R.; Firman, P. J . Phys. Chem. 1986, 90, 671. (12) Schubert, K . V.; Strey, R.; Kahlweit, M. J . Colloid Interface Sci., in press. (13) Knoche, W.; Rees, N. H. J . Chem. Educ. 1984,6l, 724.
The Journal of Physical Chemistry, Vol. 95, No. I, 1991 453
Murexide
Uramil Alloxan Figure 3. Molecular structure of murexide, uramil, and alloxan. dm3 cm-’ (PADAH+). The equilibrium constant was independently determined from the kinetics of the reaction, employing a stopped-flow experiment. ( b ) The Probe Reactions. The decomposition reaction of murexide (Mu-) follows the equation
k
M ~ H u +A (1) The kinetics and mechanism of this reaction have been studied in detail in ref 13. There, it was found that, under pseudofirst-order conditions, Le., [H’] >> [Mu-], the rate of the reaction is given by MU-
+ H+&
1 / = ~ kqH+lf12; k’= Kk
(2) where K is the dissociation constant of purpuric acid (MuH) formed as an intermediate and k is the rate constant of the decomposition of purpuric acid to uramil (U) and alloxan (A). fl is the activity coefficient of the H+ and Mu- ions, estimated by using the Davies f0rmu1a.I~ The ionic strength of the aqueous fractions was used for these estimations offi. In aqueous solutions one finds at 25 OC; k’= 1.1 dm3 mol-’ The activation energy of k’ was determined to be 54 kJ/mol.15 The kinetics of the reaction in water-dioxane mixtures was studied by Stickdorn and Knoche.” They found a linear dependence of In k’ on the reciprocal dielectric number of the mixtures, described by In k’ = 95/tr - 1.0 (3) This was evaluated according to Born’s equation for the dependence of dissociation constants on the dielectric number of the reaction mediumI6
(4) With the assumptions 6 In k’/6(l/er) = 6 In K/6(l/tr) and 6 In k/6( l/tr) = 0, an ion pair radius of a = 1 nm is derived. From this value it is concluded that the assumptions given above satisfactorly described the dependence of k’ on tr. Application of eq 3 thus permits determining c, in the environment of the probe. Because murexide is strongly hydrophilic it is expected to be located only inside the aqueous domains of mixtures of water, oil and nonionic amphiphiles; for the molecular structure, see Figure 3. If tr is found to be the dielectric constant of bulk water, the mixture is called a microemulsion. If t is much lower, the system is called a weakly structured mixture. A typical experimental result is shown in Figure 4. The rates 1 / were ~ determined as function of the proton concentration in the aqueous fraction. The slope gives a rate constant of 2.15 f 0.1 dm3 mol-’ s-’. For each composition of the ternary mixture a different temperature had to be chosen in order to find a single-phase solution. For comparing the results, the corresponding rate constants a t 25 O C were calculated assuming the activation en(14) Davies, C. W. Ion Associution; Butterworths: London, 1962. (15) Stickdorn, K. Dissertation, Bielefeld, 1990. (16) Robinson, R. A.; Stokes, R. H. Electrolyte Solutions, 2nd ed.; Butterworths; London, 1970.
454
The Journal of Physical Chemistry, Vol. 95, No. 1, 1991 1.51
,
I
Schomacker
I
I
60
A
50 -
A
LO -
, -
A
30 -
0
A
20 -
2
0
6
L
A
10 -
$1 Figure 4. Plot of reciprocal relaxation times vs proton concentration in a ternary mixture of water, octane. and CI2E5(a = 60 %, y = 7 wt %; [H'] [10'3mol
A A
' A
:i
T = 34.0 "C). L5
1
1
,
I
I
,
I
,
Figure 6. The local dielectric number of water in the system water%tane-C,,E, as function of a (vertical bars with filled circles) and the dielectric number determined macroscopically (A). The dashed line gives the dielectric number of bulk water at the temperatures of the experi-
I
1s
Lo
ments.
0
20
60
LO _c
80
100
OiWl%1
0
10
20
-v
0
20
LO
60
80
30
LO
[wt%l
100
alwt%l
Figure 5. Top: The isotropic channel of the system water-~ctane-C,~E, at constant y = 7 wt %. Crosses mark the compositions of the samples
probed for the local dielectric number of the aqueous domains. Bottom: Rate constants of the decomposition of murexide as function of a,calculated for 25 OC. ergies to be independent of the composition of the solution. The presence of structure in these mixtures was probed by the reversible complexation of Ni2+ with PADA (pyridin-2-azodimethylaniline). PADA is an uncharged oil-soluble molecule. The equilibrium constant of the reaction is independent of the dielectric number of the medium. If the reaction medium contains unpolar domains the PADA is dissolved therein and the equilibrium constant is found to decrease strongly. This was shown for micellar solutions and microemulsions in a number of ~tudies.l'-~~ The reaction is described by PADAw
+ Ni2'
kl
k -,
NiPADA',
(5)
PAD& (17) Carbone, A. I.; Cavasino, F. P.; Di Dio, E.; Sbriziolo, C. Inr. J. Chem. Kine!. 1986, 18, 609. (18) Fletcher, P. D. 1.: Robinson, B. H. J . Chem. Soc., Furuduy Truns. I 1984, 80, 2417.
(19) Holzwarth, J.; Knoche, W.; Robinson, B. H. Ber. Bumen-Ges. Phys. Chem. 1978, 82, 1001.
1
Chemical Reactions in Microemulsions H2O - 01,
The Journal of Physical Chemistry, Vol. 95, No. 1 , 1991 455
- C,E,
H2O
- B, - C8E,
80 r 060 Er
1
-10 -
E
LO
C6E3
P
-20 -
CLEl 20
-30
0 0 10 20 30 LO 50 60 70
-
f
6'
I 0 10 20 30 LO 50 60 70
y[wt%l
Figure 8. The local dielectric number of the water in mixtures of water,
decane, and nonionic amphiphiles of different chain lengths.
discrepancy between both values is found. The reason is that macroscopic e , contain contributions of both water and oil, with the microstructure determining that component giving the major contribution. At a constant ratio a = 40 wt %, the local t, is measured as function of the concentration of CI2ES.Figure 7 (top) shows the phase diagram of the system with crosses marking the composition and the temperature of the samples probed for the local dielectric number of the aqueous domains. In Figure 7 (bottom) the difference of the local t r to e, of bulk water is plotted vs the concentration of C12ESin the sample. For amphiphile concentrations higher than 20 wt %, the local e, is decreased in comparison to bulk water. ( b ) The Local Dielectric Number of Systems Containing Different Amphiphiles CiEj. The dielectric numbers of the aqueous domains of mixtures stabilized by different amphiphiles of the series C,Ej were studied to find the transition from weakly structured solutions to microemulsions. The compositions of the mixtures were equal amounts of oil and water (a = 50 %) and an amphiphile concentration 2 wt % higher than the minimum concentration (4)to stabilize a single-phase mixture. For every mixture decane was the oil component, and the temperature was about 50 OC. For the mixtures stabilized by C12E6,CloEs,and C8E4,the dielectric number of the aqueous domains is the same as the dielectric number of water; within experimental error. For the mixtures stabilized by C6E3and C4El the dielectric number of the aqueous domains is significantly decreased in comparison to bulk water (Figure 8). In these systems the local water structure is insufficiently developed to yield a local dielectric number close to that of bulk water. An experiment presented below, section e, shows still the presence of a microstructure in these systems, separating polar and nonpolar domains. The abscissa in Figure 8 is scaled according to the concentrations of each amphiphile in the mixtures. The transition from microemulsions to weakly structured mixtures can be seen between CsE4 and CsE3. The transition is gradual, so that no sharp boundary can be drawn. For the system H20-decane-CsE4, the amphiphile concentration was increased up to 60 wt %. For y > 40 wt %, k'is observed to increase and t, to decrease correspondingly. ( c ) The Local Dielectric Number of Systems Containing Different Alkanes. The chain length of the alkane was varied in systems of water-alkane-C8E3. For n-hexane a concentration of 16 wt % C8E3is needed to stabilize a solution containing equal amounts of water and hexane. If octadecane is chosen as oil component, the minimum concentration of CBE3is increased to 46 wt %. In the first case the mean temperature of the three-phase interval is IO OC, in the latter 45 O C . The results in Figure 9 are given as the difference of the dielectric number of the aqueous domains to the dielectric number of bulk water at the temperature of the experiment. For alkanes longer than tetradecane, am-
Iwt%l
-y
Figure 9. The local dielectric number of the water in mixtures of water, CsE3, and alkanes of different chain lengths. HZO n 2 , ,
- 610 - C,E, ,
,
IT=5O0CJ
,
I
- 0.L -0.6
of the protonated Thymol blue (IH) is determined spectrophotometrically at X = 550 nm. The extinction coefficient is 3.3 X lo4 mol-' dm3 cm-'. The pH was calculated from the proton concentration in the aqueous fractions of the ternary mixtures. Figure 10 shows the shift of the pKa value of Thymol blue in the aqueous fraction of the ternary mixtures against the pK, value in aqueous solution. The abscissa is scaled according to the concentrations of the amphiphiles in the mixtures. For the mixtures stabilized by CI2E6,CloE5,and CsE4, the pKa is similar to the pK, in bulk water, while for mixtures stabilized by C6E3 and C4E, a significant deviation is observed. In the same figure the A(1og k? for the decomposition kinetics of murexide are plotted. The expression A(log k? should be equal to the ApK, for purpuric acid. Both experiments show a good agreement.
456
The Journal of Physical Chemistry, Vol. 95, No. 1, 1991 0.8 I
Schomacker
I
60
t -
I
OT
IO-^
-
10-~
30 -
IO-,
10''
.
Er
[Ni"] [mol I"]
mol PADA with Ni2+in aqueous Figure 11. Titration curve of 2 X solution (full symbol) and in a tenary mixture (open symbol) of water, octane and C4EI(a = 50 wt %, y = 60 wt %).
Y
I
-
20
0
LO
60
80
J 100
C,E, [ v o l % l
Figure 13. Dielectric number of mixtures of water and C4EIdetermined
macroscopically (full symbols) and by the probe reaction (open symbols).
"
0
-
1
2
3
L
t
5
[ N I ~ ' ]I10-3m~lI'l
Figure 12. Reciprocal relaxation time of the complexation of PADA with Ni2+ in aqueous solution (squares) and in a ternary mixture (circles) of water, octane, and C4EI (a = 50 %, y = 60 wt %).
( e ) Probing a Weakly Structured Mixture of Water, Octane, and C4El. For mixtures of water, octane, and C4E, no microstructure can be detected with scattering experiments. Studying the complexation reaction of Ni2+ with PADA in this mixture shows a strongly decreased stability constant of the NiPADAZ+ complex in comparison to an aqueous solution. Figure 11 shows titration curves of PADA with Ni2+ in an aqueous solution and in the ternary mixture of water, octane, and C4El. The stability constant calculated for the aqueous solution is 2 X lo4 mol-, dm3, while that for the ternary mixture is 700 mol-' dm3. This shift of the stability constant can be explained by dividing the volume of the ternary mixture into two subvolumes and the PADA partitioning into the nonaqueous subvolume. An exact evaluation of the partition coefficient is not possible, because the fractions of the subvolumes can only be estimated. A study of the kinetics of the complexation yields the rate constants for formation and dissociation of the complex, so that the stability constant can be calculated from this experiment (Figure 12). For experiments performed with [Ni2+]>> [PADA], the observed reciprocal relaxation time I / T is 1 / T = k-,
+ k , [Ni2+]C#Jw-'(C#Jw + PC#JO)-l
(7)
with dwand do are the volume fractions of the aqueous and nonaqueous domains, respectively. The intercept is k-, = 0.5 s-l, and the slope k,4w-'(4w PC#Jo)-l= 350 s-I mol-' dm3, yielding a stability constant of 700 f 100 mol-, dm3. The agreement of the stability constants obtained from the kinetic and from the titration experiment confirms the mechanism proposed in eq 5 , with the reaction occurring in the aqueous domains and the PADA partitioning between the aqueous and nonaqueous domains. This experiment only demonstrates the presence of domains of different polarity, and no information can be obtained about the size. U, Probing Mixtures of Water and C4El. In aqueous solutions amphiphiles like C12E6or C8E3 form micelles. The critical micelle concentration of nonionic amphiphiles can be determined by various methods, Le., surface tension, incorporation of dyes, or light scattering. Air-solution surface tensions of water-C,E,
+
0.1
1
0 1 0
0 0
'
'
10 20
' ' ' ' ' 30 LO 50 60 70
-
10 20 30 LO
50 60 70
C,E, [vol %I
Figure 14. Top: Absorbance of the NiPADA2+complex at X = 550 nm mol and [Ni2+],= 2.5 X IO4 mol of solutions of [PADA], = 2 X in mixtures of water and C4E, of different volume fractions. Bottom:
Stability constant of the NiPADA2+complex in mixtures of water and C4E,.
mixtures plotted as u vs log [C4El] yield a monotonically decreasing u with increasing concentration of C4E1, until a concentration of 11 wt % is reached. Beyond this concentration u is substantially constant.20 A study by Kilpatrick et al. of the I3C N M R chemical shift of the 4-CH2 carbon resonance in water-C4El mixtures indicates an aggregation of C4E, at conccntrations higher than 1 1 wt %.20 Here the macroscopic and the local dielectric number of these mixtures are studied. In Figure 13 both values are plotted as function of the volume fraction of C4EI. The dielectric number determined macroscopically shows a linear dependence on the volume fraction of C4E,. The molecular probe monitors higher dielectric constants than determined macroscopially. This may be interpreted as an aggregation of the C4EI lcaving the water as almost bulk water. Experiments (20) Kilpatrick, P. K.; Davis, H. T.; Scriven, L. E.; Miller, W.G. J . Colloid
Interface Sci.1987, 118,270.
The Journal of Physical Chemistry, Vol. 95, No. I, 1991 451
Chemical Reactions in Microemulsions
TABLE I: The Commition Limit of the Microemulsion Reeion limiting composition system
a
H20-BE-C lzES (y = 7 wt lo; (Y = var) H20-Bg-Cl2ES ( a = 40 wt %; y = var) H2O-B ,&E4 (a = 50 wt 5%; y = var) H 20-Bk-CB E, (a = 50 wt %; y = var)
85
7
9
40
23
6
50
30
5
50
35
4.5
Y
H,O/EO
studying the complexation of Ni2+ with PADA in mixtures of watcr and C,E, give strongly decreasing stability constants at C4E1 conccntrations higher than IO wt % (Figure 14). The interprctation of this effect is the same as for mixtures of water, C4E,, and octanc. This aggregation effect was also found in mixtures of watcr with C4E2, terr-butyl alcohol, and propanol, respectively.
Discussion Three chemical reactions were studied for probing the physical propertiesof the water domains in ternary mixtures of water, oil, and amphiphiles. The decomposition kinetics of murexide and the pK, of Thymol blue are probes for the transition from microemulsions to weakly structured solutions. The presence of a weak microstructure dividing the solutions into hydrophilic and hydrophobic domains was probed by the complexation reaction of Ni2+ with the hydrophobic ligand PADA. The kinetics of the decomposition of murexide is sensitive to the local dielectric number of the environment of the probe. Since murexide is a very hydrophilic molecule, it is located inside the aqueous domains of the ternary systems and so it probes the local dielectric number of the dispersed water. Over a wide range of compositions of homogeneous ternary mixtures the probe measures the dielectric number of bulk water. If the size of the aqueous domains is decreased below a certain limit, a decreasing dielectric number is observed. Table 1 summarizes these composition limits
for every series of experiments. For this composition, the number of water molecules per ethylene oxide unit is calculated, yielding 5-9 H 2 0 molecules per EO group. This figure is in good agreement with the number of water molecules hydrating the amphiphile head group, determined by N M R self-diffusion experiments.21 Thus, if a system contains less than these five water molecules per EO group there is no water available for forming an aqueous core, exhibiting the properties of bulk water. A similar result was obtained by Eicke studying microemulsions stabilized by the ionic amphiphile AOT.22 The vapor pressure of water was measured over water-in-oil microemulsions as function of the water concentration. If the ratio R = [H20]/[AOT] was increased to R > 12, the vapor pressure of bulk water was found. Since the microstructure is expected to be symmetric with respect to I$ = 50 vol %, a similar result is expected when the physical properties of the oil domains in o/w dispersions are probed. This result may be taken as an experimental support for the suggestion of defining microemulsions.’ The results of this study are also of importance for applying microemulsions as solvents for chemical reactions. Microemulsions may be applied as solvents for reactions between water-soluble and oil-soluble reactants.23 The water-soluble reactants are in general salts, the solubility of which decreases strongly with decreasing dielectric number of the solvent. Microemulsions are also applied as solvent for incubating enzymes with water-insoluble s ~ b s t r a t e s . If ~ ~the dielectric number in the water domains is much lower than in bulk water, enzymes may be denatured and thus lose their activity.25 Acknowledgment. This work was carried out in the laboratory of Prof. M. Kahlweit, to whom I am indebted for his support and helpful discussions. I am further indebted to D. Luckmann for drawing the figures. (21) Nilsson, P. G.; Lindman, B. J . Phys. Chem. 1983, 87, 4756. (22) Kubik, P.; Eicke, H. F.; JBnson, B. Helu. Chim. Acta 1982,65, 170. (23) Schomacker, R. Prog. Colloid Polym. Sci. 1990, 81, 131. (24) Luisi, P. L.; Giomini, M.; Pileni, M.; Robinson, B. H. Biochim. Biophys. Acta 1988, 947, 246. (25) Schomacker, R.; Robinson, B. H.; Fletcher, P. D. 1. J . Chem. SOC., Faraday Trans. I 1988, 84, 4212.
