J . Phys. Chem. 1989, 93, 3704-3710
3704
Properties and Structure of Microemulsions of the System Sodium Octyl Sulfate/n-Hexyl AlcohoVn-Decane/Brine Keizo Ogino, Masato Nakamae, and Masahiko Abe* Faculty of Science and Technology, Science University of Tokyo, 2641, Yamazaki, Noda, Chiba 278, Japan (Received: August 2, 1988)
The microemulsion phases formed in solutions of n-decane, sodium octyl sulfate ( S O S ) ,and n-hexyl alcohol ("A) with various brines were examined with dynamic light scattering (DLS), interfacial tension, electric conductivity, extractionspectrophotometry,Karl-Fisher moisture, and fluorescence-probe measurements. Plots of these experimental data with changing salinity show two discontinuous points at two characteristic salinities, where phase transitions take place (Winsor I Winsor 111 and Winsor 111 Winsor 11) in the multiphase microemulsion system. A salinity zone containing two different types of microemulsions exists in the vicinity of each of these two characteristic salinities. Our results imply the existence of three types of microemulsions that have different dispersion mediums: pure water, pure oil, and a water-oil mixed medium. The mixed medium may exist in a three-phase region for which the microemulsion is the middle phase. In addition, the phase transitions with changing salinity in the multiphase microemulsion system will take place continuously through two transitional stages within the two salinity zones.
-
-
I. Introduction Microemulsions are homogeneous mixtures of hydrocarbons and water with large amounts of surfactants.'V2 Frequently, an alcohol and/or other amphiphilic cosurfactant is necessary for microemulsion formation. Such emulsions, the droplet diameters of which are about 10-100 nm, are liquid-liquid dispersion systems3q4and are thermodynamically stable single phase^,^^^ differentiating them from macroemulsions that achieve equilibrium by separating into their original two mutually insoluble liquid phases. The surfactants create an extremely low interfacial tension between oil and ~ a t e r , ~ which , ~ , * promotes emulsification. It is now recognized that the application of microemulsion systems is of industrial and practical importance for enhanced oil recovery9J0and for preparation of ultramicroparticles." From the phase behavior of microemulsions, it is evident that a variety of phases can exist in equilibrium with one other. While in equilibrium, each phase will involve a different structure. The commonly observed Winsor-type ~ y s t e m s l ~ indicate -'~ that the microemulsions can exist in equilibrium with excess oil, excess water, or both. A Winsor I type system consists of a lower phase microemulsion and excess oil; a Winsor I1 type system consists of an upper phase microemulsion and excess water. The factors that affect the transition between different types of systerrls include the temperature, the salinity, the molecular structure of the surfactant and cosurfactant, the nature of the oil, and the oil to water ratio.I5 Under adequate conditions, the microemulsion system is miscible with both oil and brine. However, beyond its (1) Schulman, J. H.; Stoeckenius, W.; Prince, L. M. J. Phys. Chem. 1959, 63, 1677.
(2) Stoeckenius, W.; Schulman, J. H.: Prince, L. M. Kolloid-2. 1960, 169, 170. (3) Danielsson, 1.; Lindman, B. Colloids Surf. 1981, 3, 391. (4) Friberg, S.E. Colloids Surf. 1982, 4, 201. (5) Bellocq, A. M.; Bourbon, D.; Lemanceau, B.; Fourche, G. J. Colloid Interface Sei. 1982, 89, 427. (6) Hermansky, C.; Mackay, R. A. J. Colloid Interface Sei. 1980, 73, 324. (7) Kunieda, H.: Shinoda, K. Bull. Chem. SOC.Jpn. 1982, 55, 1777. (8) Pouchelon, A.; Chatenary, S . ; Meunier, J.; Langevin, D. J. Colloid Interface Sci. 1981, 82, 418. (9) A b , M.; Schechter, D.; Schechter, R. S.; Wade, W. H.; Weerasmriya, U.; Yiv, S. J. Colloid Interface Sci. 1986, I l l , 342. (10) Abe, M.; Schechter, R. S . ; Selliah, R. D.: Sheikh, B.: Wade, W. H. J. Dispersion Sei. Technol. 1987, 8, 157. (1 I ) Kandori, K.: Kon-no, K.; Kitahara, A. J. Colloid Interface Sci. 1987, 115, 579. (12) Winsor, P. A. Solvent Properties of Amphiphilic Compounds; Butterworth: London, 1954. (1 3) Abe, M.; Nakamae, M.; Ogino, K. Sekiyu Gakkaishi 1988.31.458. (14) Nakamae, M.; Abe, M.; Ogino, K. Sekiyu Gakkaishi 1988,171,466. ( 1 5 ) Shah, D. O., Ed.: Macro- and Microemulsions; ACS Symposium Series 272; American Chemical Society: Washington, DC, 1985.
0022-3654/89/2093-3704$01.50/0
solubilization limit, the microemulsion system partitions into three phases: namely, a surfactant-rich middle phase, a surfactant-poor brine phase, and an oil phase. This surfactant-rich middle phase (middle phase microemulsion16) is called Winsor I11 type microemulsion. The lower phase and the upper phase microemulsions will have structures where droplets (dispersed phase) disperse in a continuous phase: the former microemulsion is an oil-in-water (0/W) type, and the latter is a water-in-oil (W/O) type. But, the structure of the middle phase microemulsion is presently a matter of controversy. Olsson et aI.,l7 Chen et al.,l* Talmon et al.,I9 and Scriven20suggest various bicontinuous structures for the middle phase microemulsion. Clarifying the properties of the middle phase microemulsion is more difficult than for the other microemulsions. In this paper, we report the changes in the microemulsion microstructure and phase-transition mechanism due to changing salinity using dynamic light scattering (DLS), interfacial tension, electric conductivity, surfactant and moisture contents, and fluorescence-probe measurements. 11. Experimental Section I . Phase Behauior. Sodium octyl sulfate (SOS) was supplied from Nihon Surfactant Industries Co., Tokyo. It was recrystallized from n-propyl alcohol, extracted with diethyl ether in a Soxhlet extractor, and dried in a vacuum oven. The purity was ascertained and by surface tension measurements. n-Hexyl alcohol ("A) n-decane purchased from Tokyo Chemical Industry Co. and sodium chloride (NaCI) for brine from Wako Pure Chemical Industries were reagent grade and were used without further purifications. Pyrene as a fluorescent probe was reagent grade from Wako and was doubly recrystallized from ethyl alcohol. Doubly distilled water was used in all solutions. SOS (2.0 wt %), N H A (4.0 wt %), n-decane (47.0 wt %), and brine (47.0 wt 7%) having various salinities were placed in tubes and were sealed. The tubes were shaken and allowed to attain equilibrium over a period of 10 days at 35 f 0.1 O C . At this time, the microemulsion phase volumes were recorded. Reading of the tubes was repeated until no further changes in volume were observed. This was done to verify equilibrium conditions. Systems with maximum O/W and/or W / O macroemulsion stability were (16) Healy, R. N.; Reed, R. L.; Stenmark, D. G. SOC.Pet. Eng. J . 1976, 16, 147. (17) Olsson, U.; Shinoda, K.; Lindman, B. J. Phys. Chem. 1986, 90, 4083. (18) Chen, S. J.; Evans, D. F.; Ninham, B. W.; Mitchell, D. J.; Blum, F. D.; Pickup, S. J. Phys. Chem. 1986, 90, 842. (19) Talmon, Y.; Prager, S. J. Chem. Phys. 1978, 69, 2984. (20) Scriven, L. E. Micelliration, Solubilization, and Microemulsions; Plenum Press: New York, 1977, Vol. 2.
0 1989 American Chemical Society
Properties and Structure of Microemulsions identified as corresponding to the extremes of the three-phase region.21 The systems with minimum macroemulsion stability were also closely observed to locate the optimum system. 2. Dynamic Light Scattering (DLS). The dynamic light scattering (DLS) measurements were performed with a 4700-type submicron particle analyzer (Malvern Co.), with a multibit (8 X N ) Malvern correlator with delayed channels. The correlator is interfaced to a PC-AT IBM computer, allowing continuous control of the base line. The light source was an argon ion laser (Spectra Physics, Model 164), used at a wavelength of 488 nm and a power of 5 W or less. Samples were contained in the 8-mm-diameter high-precision Burchard cells, which were set in a temperature-controlled vessel at 35 OC. The viscosity value of water (0.72 cP) comes close to that of n-decane (0.80 cP) at this temperature. 3. Interfacial Tension. The interfacial tension measurements were performed with a spinning drop interfacial tensiometer (University of Texas, Model 300). The glass tube cell is filled with two phases: a microemulsion and an excess phase in the multiphase microemulsion system. The cell is rotated at a high speed, so that the lower density phase, which is usually an oily phase, becomes a cylindrical drop. As the cylinder becomes very long, we could obtain the interfacial tension by use of Vonnegut’s equationz2 y = ‘/4Apwzr2 (1) where y is the interfacial tension (mN m-I); Ap, the density difference between the two phases (g cm”); w, the angular velocity (rad s-l); and ro, the cylindrical radius (cm). The density of each phase in the multiphase microemulsion was measured with a digital density meter (Kyoto Electronics Co., Model DA-210) on the basis of the measurement of the characteristic frequency. 4 . Electric Conductivity. Electric conductivity was measured with a conductivity meter (TOA Electronics, Ltd., CM-40s) and an immersion cell (cell constant of 9.49 cm-I). 5 . Compositional Analysis. The anionic surfactant (SOS) contents in each phase of the multiphase microemulsion system were determined with an extraction-spectrophotometry method described previously.23 This method is based on the ion-pair formation of an anionic surfactant with a cationic dye (ethyl violet), its extraction in benzene, and the absorbance measurement of the ion pair in the benzene phase at the wavelength of the absorption maximum (610 nm). The moisture content in each microemulsion was measured with a Karl-Fischer moisture titrator (Kyoto, Model MKA-3p). 6. Fluorescent Intensity Ratio. As for the fluorescence-probe study, we prepared each multiphase microemulsion by use of a pyrene-solute n-decane (5.0 X lod mol dm-)) as described above. The fluorescence emission spectra of pyrene monomers in each type of microemulsion phase were measured with a fluorescence spectrophotometer (Shimadzu Co., Model RF-540; the excitation wavelength was 344.9 nm). Each spectrum has five predominant vibronic peaks, numbered 1-5 from the shorter to the longer wavelength. The fluorescent intensity ratio Zl/13, which is the ratio of the intensities of the first to the third peaks, can be calculated. The ratio [ ] / I 3is related to the polarity sensed by pyrene at its solubilization site in microemulsions.24~25 These measurements were all carried out at 35 f 0.1 OC.
111. Dynamic Light Scattering Theory Dynamic light scattering measurements are based on the time dependence of thermal fluctuations in fluid systems (random motion of colloidal particles). The dynamic study of the scattered light consists of measuring scattered electric field autocorrelation ~
~~~
~~~
(21) Graciaa, A.; Baraket, Y.; Schechter, R. S.; Wade, W. H.; Yiv, S . J . Colloid Interface Sci. 1982, 89, 217. (22) Vonnegut, B. Rev. Sci. Instrum. 1942, 13, 6. (23) Nakamae, M.; Ogino, K.; Abe, M . Colloid Polym. Sci. 1988, 266, 475. (24) Kalyanasundram, K.; Thomas, J. K. J . Am. Chem. SOC.1977, 99, 2039. ( 2 5 ) Lianos, P.; Lang, J.; Strazielle, C.; Zana, R. J . Phys. Chem. 1982, 86, 1019.
The Journal of Physical Chemistry, Vol. 93, No. 9, 1989 3105 functions.26 With use of the homodyne detection te~hnique,~’ the intensity autocorrelation function g(l)(T).isobtained. As for the dilute and monodisperse solutions of optically isotropic particles that are smaller than a wavelength of the light, or spherically symmetric, or both, the autocorrelation function is given by lg!I)(r)l = exp(-rs)
(2)
where
r
= Dq2
(3)
with D, the translational diffusion coefficient of the particles, r, the decay constant of the autocorrelation function, and q, the magnitude of the scattering vector. For polydisperse solutions, eq 2 must be generalized to a sum or distribution of exponentials
where
L=G(I’) d r = 1
(5)
with G(r), the distribution function of the decay rates. The average decay constant, can be written by
r,
r
=
Lmrc(r)d r
In the cumulative expansion for
(6)
Ig(l)(~)l by use of F
where
(9) Here, p2 and p3 are the second and the third moments, respectively, of the distribution function. The values of these moments are observed as deviations from the single exponential function. The hydrodynamic radius of the particle, R H ,is obtained from the Stokes-Einstein equation given by
RH = k T / 6 r g D
(10)
where g is the viscosity of the continuous phase in microemulsions; k , the Boltzmann constant; and T, the absolute temperature. In general, this technique can apply to infinite and dilute solutions where Rayleigh scattering occurs. Moreover, Ryan et aLZ8 have reported that the DLS technique is also applicable to high-concentration dispersed systems.
IV. Results Phase Behavior of the SOSINHAfn-DecanelBrine System. Figure 1 shows the changes of the volume fractions of the SOSINHAln-decanelbrine multiphase microemulsion with salinity. As can be seen in Figure l , Winsor type I, which is the two-phase region composed of a lower phase microemulsion and excess oil, appears at the salinity lower than 10.3%. Winsor type 11, which is the two-phase region made of an upper phase microemulsion and an excess water phase, forms at salinity values and higher than 15.7%. In the salinity region between 10.3% (SI) 15.7% (SI]),Winsor type 111, which is the three-phase region containing a middle phase microemulsion, is observed. At the two characteristic salinities (SIand SI]) where Winsor I Winsor I11 and Winsor 111 Winsor I1 phase transitions
-
-
(26) Berne, B. J.; Pecora, R. Dynamic Light Scattering Wiley-Interscience: New York, 1976. (27) Koppel, D. E. J . Chem. Phys. 1972, 57, 4814. (28) Ryan, V.; Hart, T. R.; Schiller, R. Biophys. J . 1980, 31, 313.
3106
The Journal of Physical Chemistry, Vol. 93, No. 9, 1989
Ogino et al.
1.o
Excess I oil
0.5 W
J
d
> 0
!:; I
n 8
9
10
11
12
13
15
14
17
16
le
........
.....
~~
Salinity
1 %
Figure 1. Volume fractions of each phase in SOS (2% by weight)/NHA (4%)/n-decane(47%)/brine (47%) multiphase microemulsion system as a function of salinity at 35 OC. ML represents the lower phase micro-
emulsion, MM the middle phase microemulsion, and M Uthe upper phase are the characteristic salinities where Winsor microemulsion. SIand SI, I Winsor 111 and Winsor 111 Winsor I1 phase transitions occur. 2, and ZIIare the salinity zones where two different types of microemulsions coexist: from SIto S,' and from SI,'to SII.Key: 0,microemulsion phase/excess oil phase interface; 0 , microemulsion phase/excess water phase interface.
-
2
0 0
-
take place, the phase volume of the middle phase microemulsion becomes a maximum. At the intermediate salinity between SI and Sll(optimum salinity for middle phase microemulsion formation), however, the phase volume becomes a minimum. One should note that two different types of microemulsions are in contact with each other in the vicinity of the characteristic salinities (SIand SII).A lower phase and a middle phase microemulsion (ML and MM) coexist at the salinity zone (Z,) from A middle phase and an upper phase 10.3% (SI)to 10.7% (SI'). microemulsion (MM and MU) coexist at the salinity zone (Zll) from 15.3% (Sll')to 15.7% (Sll). Diffusion Coefficient and Hydrodynamic Radius of Multiphase Microemulsion. Figure 2 depicts the reciprocal of the diffusion coefficient ( 0 - I ) as a function of the magnitude of the scattering vector ( q ) q = (47rn/X) sin (0/2)
(11)
where n is the refractive index of the solution; A, the wavelength of light; and 8, the scattering angle. The D-I of a spherical structure without interparticle interactions does not depend on q, while those of spherical structure with interparticle interactions As can be seen in and of nonspherical ones depend on Figure 2a, the scatterings from any type of multiphase microemulsions at which the salinity is a great distance from the two characteristic salinities (SIand SI,)show no dependence upon the scattering vector. In Figure 2b, however, as the characteristic salinities are approached, the D-' of Winsor I and Winsor I1 type systems depend on q. Even within the three-phase region, as the salinity moves away from the optimum salinity (13.0%) for middle phase microemulsion formation, a dependence of D-' upon q is found. This may be due to the increasing of interparticle interactions. We used 90' as the scattering angle in the following DLS measurements. Figure 3 exhibits the diffusion coefficients of each microemulsion vs salinity at 35 OC. There exists a distribution of diffusion coefficients among the values obtained from DLS measurements, so that we adopted the z-average values as representative values of the distribution. As can be seen in Figure 3, the D value initially decreases with increasing salinity until SI, ~
7
.
~
~
3
~
~
(29) Rushforth, D. S.;Sanchez-Rubio, M.; Santos-Vidals, L. M.; Wormuth, K. R.; Kaler, E. W.; Cuevas, R.; Puig, J. E. J . Phys. Chem. 1986, 90, 6668.
(30) Fijnaut, H. M . J . Chem. Phys. 1981, 7 4 , 6 8 5 7 .
Salinlty ( t )
2
1
3
1 103
4
H
Figure 2. Reciprocal of the diffusion coefficient of each microemulsion ( P )as function of the scattering vector ( 4 ) at 35 OC. Key: 0,ML;6 , MM;0,Mu. 0.4 !
I
Type
! Type I1
I11
Type
! I t
I
I
.
:
I
I
n
I. 8
i
I
9
10
S..!
SI, 11
'
.il
12
13
15
14
Salinity /
16
'7
.:
5,
Figure 3. Diffusion coefficients of each microemulsion as function of salinity at 35 OC. Key: 0, ML; 0 , MM;0 , Mu.
-
where the phase transition (type I type 111) occurs; beyond this point, the value increases and attains a maximum through the optimum salinity, decreases again and reaches a second minimum at SII,where the phase transition (type I11 -type 11) takes place, and increases linearly with increasing salinity. Figure 4 depicts the changes of the diffusion coefficient of each microemulsion with the salinity zones (Zl and ZII)where two different types of microemulsions coexist. In Z1(from 10.3% to 10.7%), the diffusion coefficients of both the lower phase and the middle phase microemulsions increase with an increase in salinity. On the other hand, the D values of both the middle phase and the upper phase microemulsions decrease with decreasing salinity in Zll (from 15.3% to 15.7%). Figure 5 exhibits the hydrodynamic radius (RH)of each microemulsion with changing salinity. Here, to get the hydrodynamic radii of middle phase microemulsions, the viscosity value of the continuous phase in the microemulsions is taken to be 0.76 cP. This value is an average of water and n-decane viscosities (0.72 and 0.80 cP, respectively). The apparent RH initially increases with increasing salinity until SI;beyond this point, the value decreases and attains a minimum through the optimum salinity for middle phase microemulsion formation, increases again and reaches a maximum at SII,and decreases with increasing salinity.
The Journal of Physical Chemistry, Vol. 93, No. 9, 1989 3707
Properties and Structure of Microemulsions 0.15
7
I
0.10 Ln N
E,
W
0 7
\
n
0.05
0 10
15
11
U
16
10
11
Salinity / %
15
16
Salinity / %
Figure 4. Changes of diffusion coefficient (D)with changing salinity within the salinity zones ZI and ZIIat 35 'C. Key: 0, ML; Q, MM; 0 , MU.
Figure 6. Changes of hydrodynamic radii with changing salinity within ZI and ZIIat 35 "C. Key: 0,M,; 0 , M,; 0 , MU. 1 .o
80 I
.....
-I-
Lo 0 Lo L
a 0
Y 3
0.5
Type
Type I
c
Type 11
111
3 Y 3
ci
h
L m
/SI,
,
,
,
,
j 5, 1 1
,
,
8 Salinity / %
Figure 5. Hydrodynamic radii of each microemulsion as function of salinity at 35 OC. Key: 0, ML; 0, M,; 0 , M u .
A similar tendency is recognized in results for sodium decyl sulfate and sodium dodecyl sulfate. The hydrodynamic radii of the middle phase microemulsion at the optimum salinity become longer with increasing alkyl carbon number of the surfactant. The hydrodynamic radii within ZI and ZIIare shown in Figure 6. In ZI, the radii of the lower phase and the middle phase microemulsions become shorter with an increase in salinity; in ZII, the RH of both the middle phase and the upper phase microemulsions increase with increasing salinity. Observation with the naked eye has detected that two different particle sizes of emulsions coexist in each region. Anionic Surfactant (SOS) Content in Each Phase of the Multiphase Microemulsion System. The partition ratios of surfactant (SOS) molecules vs salinity are shown in Figure 7. Surfactants in the multiphase microemulsion system are distributed to each separated phase; most of the SOS molecules exist in one phase (microemulsion), but very few are in the excess phases. Figure 8 depicts the partition ratios of SOS in each phase within ZI and ZII,where two different types of microemulsions are in contact with each other. As can be seen in Figure 8, SOS molecules begin to move from the lower phase to the middle phase in ZI and from the middle phase to the upper phase in ZI1with increasing salinity. SOS molecules within ZI (and/or ZII)are partitioned between the middle phase and the lower phase (and/or the upper phase), so that these two phases containing SOS become different types of microemulsions. Moisture Content in Multiphase Microemulsion. The moisture content in each microemulsion phase is shown in Figure 9. The contents decrease with an increase in salinity and indicate about
".d. .....
0 Lo-0-1 9
10
11
.,
12
13
14
15
16
17
15
Salinity / %
Figure 7. Partition ratios of SOS in each phase as a function of salinity at 35 'c. Key: A, ML; M,; 0 , Mu; A, excess water; 0, excess oil.
Salinity / %
Figure 8. Partition ratios of SOS in each phase within ZIand ZIIas a function of salinity a t 35 OC. Key: A, M,; W, M,; 0 , Mu; A, excess water; 0,excess oil.
50% at the optimum salinity, which is the intermediate salinity between SIand SI,. Figure 10 depicts the changes in the moisture contents of microemulsions with changing salinity in ZI and ZII. The moisture contents of the middle phase microemulsions, within both salinity zones, decrease with increasing salinity. On the other hand, those of the lower phase (in Z,) and the upper phase microemulsions
Ogino et al.
3708 The Journal of Physical Chemistry, Vol. 93, No. 9, 1989
U
8
9
10
11
:2
13
Salinity
14
15
16
?!
ld
/ %
Figure 9. Moisture content of each microemulsion as a function of salinity at 35 " C . Key: 0,ML;Q, MM; 0 , MU.
.
Salinity
/ %
Figure 11. Interfacial tensions between oil and water (0-w), excess oil
(.
and microemulsion (o-m), and excess water and microemulsion (w-m) as a function of salinity at 35 "C.
) .
4.2 3 3
>
10
11
Salinity
15
16
w
/ %
Figure 10. Moisture content of each microemulsion within ZI and ZI1 as a function of salinity at 35 "C. Key: 0,M,; 0, MM;0 , Mu.
(in ZII)decrease with an increase in salinity. Interfacial Tension in the Multiphase Microemulsion System. One of the most characteristic properties of the multiphase microemulsion systems is to show an ultralow interfacial tension between oil and water. Kunieda et al.' have obtained approximate values of the interfacial tensions between oil and water (yww)by use of the values between the microemulsion phase and the excess oil phase (-yo-,,,) [and/or between the microemulsion phase and the excess water phase (-yW-,,,)] in some multiphase microemulsion systems. In accordance with this approximation, we can regard ywwin two-phase regions as Y+,,,and/or y w m .Furthermore, yin a three-phase region can be also taken as ywm+ y w m . Figure 11 exhibits the interfacial tensions in the multiphase microemulsion system with changing salinity. -ywm decreases with an increase in salinity and reaches zero at the characteristic salinity, Sa; yw-,,, decreases with decreasing salinity and attains zero at SI. Consequently, yWw(dashed line in the figure) becomes a minimum (Y,,,,") at the optimum salinity for middle phase microemulsion formation. Electric Conductivity in Multiphase Microemulsion. The electric conductivities in each microemulsion, which will depend on its continuous phase, are shown in Figure 12. The conductivities in the lower phase microemulsion increase with increasing salinity, and those in the upper phase microemulsions are almost independent of salinity and are very low (below 650 &km-l). On the other hand, the electric conductivities in the middle phase microemulsions (in the three-phase region) decrease with an increase in salinity. Fluorescence Intensity Ratio of Pyrene in Multiphase Microemulsion. The fluorescence intensity ratios 11/13in each
Salinity
/ %
Figure 12. Electric conductivities of each microemulsion as a function of salinity at 35 O C . Key: 0,ML; Q, MM; 0 , Mu. 1.00
!
8
9
10
11
12
13
Sallnity
14
15
16
17
19
/ %
Figure 13. Fluorescence intensity ratios (Z1/13)of pyrene in each mi-
croemulsion as a function of salinity at 35 " C . Key: 0,ML; Q, MM; 0 , MU. microemulsion, which are obtained from the fluorescence emission spectra of pyrene monomers, are shown in Figure 13. The effective polarity in the pyrene environment increases as I , / I , increases. As can be seen in Figure 11, 11/13in two-phase regions (Winsor types I and 11) decreases with an increase in salinity; the
Properties and Structure of Microemulsions
The Journal of Physical Chemistry, Vol. 93, No. 9, 1989 3709
ratio indicates a minimum at the optimum salinity in the threephase region (Winsor type 111).
V. Discussion Microemulsion Structures in the Multiphase Microemulsion System. We consider the structure in the multiphase microemulsion based on a liquid-liquid dispersion system. Sanchez-Rubio et aL3’ have suggested that the structure of microemulsion in two-phase regions (Winsor types I and/or 11) would be an oil-in-water (O/W) and/or a water-in-oil (W/O) type. Many scientists”-20 have presented the idea that the structure of the middle phase microemulsion in a three-phase region is regular and have given models that are called “bicontinuous structures”. However, no concept has avoided controversy. As can be seen in Figures 3, 5 , 9 , 12, and 13, there exist drastic and Srr); changes in properties at the characteristic salinities (SI how these data change differs from one to the next depending upon each Winsor type. It can be postulated that microemulsions having different structures would exist in the multiphase microemulsion system. A large amount of water exists in the lower phase microemulsion, while the amount contained in the upper phase microemulsion is only very small (Figure 9). Pyrene molecules, used as a fluorescent probe, are usually insoluble in aqueous polar solvents. The polarity sensed by pyrene molecules will, therefore, be influenced by the moisture (polar solvent) content in the microemulsion. As can be seen in Figure 13, the fluorescence intensity ratio (11/13)of the pyrene dissolved in decane is large in the lower phase but small in the upper phase microemulsions. In addition, since the electric conductivities in the lower phase microemulsion increase with increasing salinity and since those in the upper phase one are very low and are almost independent of salinity (Figure 12), the continuous phase in the lower and/or upper phase microemulsion will be water (brine) and/or oil. This suggests that the structures of the lower and upper phase microemulsions will be O / W type (oil droplet dispersing in water) and W / O type (water droplet dispersing in oil). In the three-phase region, the moisture content decreases with increasing salinity and attains a 50% water content at the optimum salinity for the middle phase microemulsion formation. However, the fluorescence intensity ratio in it is not proportional to the amount of water and shows a minimum at the optimum salinity. This means that the middle phase microemulsion structure will be different from other microemulsions. Even within the threephase region, the moisture content and the fluorescence intensity ratio change with increasing salinity as mentioned above. With increasing salinity, the moisture content in the middle phase microemulsion decreases, but the 1,/13ratio in it is not proportional to the amount of water. As can be seen in Figure 12, furthermore, the electric conductivity of the microemulsion decreases with increasing salinity in the three-phase region. We suggest that the middle phase microemulsion structure does not always remain a constant; the middle phase microemulsion formed at the optimum salinity will have a more regular structure than that at lower and/or higher salinities within the three-phase region. Furthermore, as shown in Figure 3, we have been able to measure the diffusion coefficient of the middle phase microemulsion. Its particle size calculated from the values of the diffusion coefficient corresponds to the size due to apparent turbidity; the middle phase microemulsion in the three-phase region will have the character of an assembly of droplets, in analogy with the lower and/or the upper phase microemulsion. The changes in the interfacial tensions observed in the threephase region, -yo-,,, and -yw-,,, in Figure 10, would be mainly dependent on the interfacial tensions between the continuous phase of the middle phase microemulsion and the excess phases. This is because the surfactant contents in the microemulsion hardly change as can be seen in Figure 6 . The decrease in -y,, with (31) Sanchez-Rubio, M.; Santos-Vidals, L. M.; Rushforth, D. S.; Puig, J
E.J. Phys. Chem.1985, 89,411.
O’L,
“I
O/L
(
and
W/L
O p t i m u m salinity I Salinltv
Figure 14. Schematic illustration of models of microemulsion structures in three-phase region. Key: 0, water; V, oil; e-, surfactant (cosurfactant); L, mixed dispersion medium; L,, water-rich mixed dispersion medium; Lo, oil-rich mixed dispersion medium.
increasing salinity will be due to enhancement of cohesive forces between the middle phase microemulsion and the excess oil, owing to an increase in the oil fraction in the middle phase microemulsion with increasing salinity. On the other hand, the decrease in -yw-,,, with decreasing salinity will be caused by an increase in the water fraction in the middle phase microemulsion with decreasing salinity. In the salinity zones of ZI and ZIIwhere two microemulsion phases [a middle phase microemulsion and a lower (and/or an upper) phase microemulsion] can coexist, there will be positive interfacial tensions, though very low, between the two microemulsion phases. Thus, we consider that positive interfacial tensions will mean the existence of different continuous phases between the middle phase microemulsion and the lower (and/or the upper) phase microemulsion. Namely, the medium phase of the middle phase microemulsion will be neither water nor oil. The above suggestion is illustrated by the liquid-liquid dispersion model for these systems shown in Figure 14. The figure shows the middle phase microemulsion having oil and/or water droplets as a dispersed phase as well as the lower and/or the upper phase one. The dispersion medium of the middle phase microemulsion will be a mixed dispersion medium that is a mixture of oil, water, and surfactant (cosurfactant). This dispersion medium may be formed due to a partial fracture of oil/water interfaces in the middle phase microemulsion caused by stretching motions of the interfaces. Surfactant molecules in the middle phase microemulsion may enhance the affinity between oil and water and thus facilitate their mixing. Furthermore, the fractions of oil and/or water in the middle phase microemulsion would be factors that characterize its structure. As can be seen in Figure 9, the middle phase microemulsion in the vicinity of the optimum salinity contains nearly the same amounts of oil and of water, that is, the oil-water ratio in the mixed dispersion medium would also be unity. In this case, consequently, oil and/or water droplets would disperse in the mixed dispersion medium (L). On the other hand, when the salinity is lower than the optimum salinity, the middle phase microemulsion would have a water-rich mixed dispersion medium (Lw) and oil droplets as a dispersed phase. In contrast, at upper salinity, the middle phase microemulsion would have an oil-rich mixed dispersion medium (Lo) and water droplets. Effect of Interfacial Tension on Microemulsion Formation. Harkins et have reported that the sizes of droplets decrease with decreasing interfacial tension. Huh et state that the phase volumes of the microemulsion phase formed at the optimum (32) Harkins, W. D.; Brown, F. E. J. Am. Chem.SOC.1919, 41, 499. (33) Huh, C. J . Colloid Interface Sei. 1979, 71,408.
J. Phys. Chem. 1989, 93, 3710-3713
3710
salinity in the three-phase region increase with decreasing interfacial tension. As can be seen in Figures 1, 5, and 11, in the three-phase region, as ywwdecreases, the droplet size and the phase volume of the microemulsion decrease. However, the droplet size and the phase volume of the microemulsions in two-phase regions (in the vicinity of the three-phase region) increase in spite of the decreasing yww. A fundamental thermodynamic equation indicates that the interfacial tension (y) is a change in the free energy of the interface (AQ) with an increase in the unit interfacial area (.4,4):34 AGs/AA = y
(12)
As can be seen in this equation, the decreasing y results in in-
creasing interfacial area (AA). It can be postulated that the increasing interfacial area is caused by the increased number of microemulsion droplets or by the increased swelling of the microemulsion droplet due to increasing adsorption of decane, of course without any aggregation. In the three-phase region, the decrease of yew may result in the increase of the number of microemulsion droplets, while in these two-phase regions (in the vicinity of the three-phase region) it may bring about the swelling of microemulsion droplets. The effect of the interfacial tension on the microemulsion formation in the two-phase regions would be distinctly different from that in the three-phase region. Mechanism of Phase Transitions in Multiphase Microemulsion System. As mentioned previously, some experimental data show drastic changes at the characteristic salinities (SIand SI,);the structural transition of the multiphase microemulsion may occur (34) Adamson, A. W. Physical Chemistry of Surfaces; Wiley-Interscience: New York, 1982.
at these salinities. Furthermore, in Figures 4, 6, 8, and 10, the data within Z, and ZII, where two different types of microemulsions coexist, would be related to the mechanism of the phase transition in the multiphase microemulsion system, because the data in these zones show conspicuous changes. In ZI,growing of the middle-phase microemulsion and breaking of the lower phase microemulsion will take place. This is because within this zone, with increasing salinity, the surfactant (SOS) molecules move from the lower phase to the middle phase, as shown in Figure 8; meanwhile, the moisture content in the lower phase microemulsion increases, but that in the middle phase microemulsion decreases (Figure 10). The sizes of droplets in both the lower phase and the middle phase microemulsion decrease (Figure 6). On the other hand, the middle phase microemulsion will begin to break up and the upper phase microemulsion will begin to grow with an increase in salinity in ZII. Thus, Z, and ZIIwill be the transitional stages in the multiphase microemulsion system. The phase transition with increasing salinity in the multiphase microemulsion system will consequently take place through these transitional stages, because the dissociation of anionic surfactants may be changed continuously and the hydrophilic-lipophilic balance of the surfactant may also be changed continuously with increasing salinity.
Acknowledgment. This research was supported by Japan National Oil Corp. and Saneyoshi Scholarship Foundation. We thank Dr. S. Qutubuddin, Case Western Reserve University, for discussions concerning this paper. Registry No. NHA, 111-27-3; SOS, 142-31-4; NaC1, 7647-14-5; n-decane. 124-18-5.
High-pressure Studies of a Fluorescence Probe for the Critical Micelle Concentration in Sodium Dodecyl Sulfate Kimihiko Hara,* Hideo Suzuki, Department of Chemistry, Faculty of Science, Kyoto University, Sakyo- ku, Kyoto 606, Japan
and Noboru Takisawa Department of Chemistry, Faculty of Science and Engineering, Saga University, Saga 840, Japan (Received: August 5, 1988; In Final Form: October 11, 1988)
The utility of the fluorescence intensity ratio of vibronic bands of pyrene as a probe for determining the pressure effect on critical micelle concentration (cmc) and on the micropolarity of micelle interiors was investigated at high pressures up to 400 MPa for a typical anionic surfactant (sodium dodecyl sulfate). The effect of the probe on the transition from the singly dispersed state to the micellar state was also examined. The resulting cmc’s agree well with those from the conductivity method, exhibiting a maximum around 100 MPa. In the concentration region of so-called singly dispersed nonmicellar state, an anomalous decrease in the intensity ratio, observed with increasing pressure, may be explained by some pressure-induced aggregation.
( 1 ) Nakajima, A. Bull. Chem. Sac. Jpn. 1971, 44, 3272. (2) Birks, J. B. Photophysics of Aromatic Molecules; Wiley-Interscience:
expected to be quite useful for the study of nonionic micelle^.^ In this application, however, it is important to ensure the absence of any influence of the probe molecule on the micelle properties. This fluorescence probe method is also a valuable technique for studying the pressure effect on the microproperties of micelles. The intensity ratio of the first vibrational peak at 373 nm (Ii) to the third one at 384 nm (I,) in pyrene molecules is used as a parameter characterizing the micropolarity of the probe’s environment. In this paper the intensity ratio [ , / I 3 was investigated at various surfactant concentrations, and on this basis, the critical micelle
London, 1970. (3) Kalyanasundram, K.; Thomas, J. K. J . Am. Chem. Sac. 1977,99,2039. (4) Turro, N . J.; Kuo, P. L.;Somasundaran, P.; Wong, K. J. Phys. Chem. 1986, 90, 288.
(5) Ananthapadmanabhan, K. P.; Goddard, E. D.; Turro, N. J.: Kuo, P. L. Langmuir 1985, 1 , 352.
Introduction
The fluorescence intensity of the (0-0)vibronic band in pyrene monomer molecules is dependent on the polarity of the environment,’ and the behavior of the band is analogous to that of the “Ham” band observed originally in benzenes2 By making use of this fact, it has been found that it is possible to use pyrene as a probe molecule for detecting the polarity in micelle interiors, termed here “ m i ~ r o p o l a r i t y ” . ~Especially, ~~ this technique is
0022-3654/89/2093-37 10$01.50/0
0 1989 American Chemical Society
