J. Phys. Chem. 1991, 95, 8861-8866
growth in the conductivity is observed to occur. This type of process has been previously suggested to explain the current doubling effect of a l ~ o h o l s . ~ ~The - ~ * injection of electrons from surface-adsorbed excited states of molecules on colloidal Ti02 has certainly been very well substantiated by the flash-photolysis TRMC studies of Kamat and Fessender~~~ and picosecond optical studies by Moser et a1.26
metry.I8 The subsequent relaxation of the conductivity back to the original, low level could be observed to take place on a time scale of many seconds on interrupting the radiation. A final effect worth noting can be seen for the last pulse of a 100-pulse, 5-Hz train for the isopropyl alcohol sample in Figure 2. A small growth in the conductivity signal is discernable within the first tens of nanoseconds after the pulse. This growth is absent in the case of dioxane and is not an artifact of the detection equipment. It has been suggested previously'*21 that alcohol radicals formed in the fluid phase can diffuse to the interface and transfer an electron to the particle. Specifically this has been suggested to occur for CH3C'(OH)CH3 radicals formed in an irradiated isopropyl alcohol sol of Ti02via CH3C'(OH)CH3 + TiOz
-
CH3C(0)CH3 + Ti02-
Conclusions Electrons initially formed within microcrystallites of Degussa P25 Ti02 have a mobility of 1 X lo4 m2/V s or larger. They undergo equilibrium localization with an intrinsic surface lattice state (possibly rutilelike) on a subnanosecond time scale. Further deep localization and subsequent recombination occur a t the surface in unadulterated samples. Recombination is retarded and extended to a time scale of tenths of seconds by a surface layer of isopropyl alcohol or paradioxane. As a result the surface state can be saturated and the bulk pumped with hundreds of mobile electrons by repetitively pulsing at a frequency on the order of 1 electron-hole pair per particle per second. Evidence is found for a small fraction of mobile electrons being produced by electron injection by free radical species formed in the isopropyl alcohol layer.
+ H+ (5)
If one makes the assumption that the viscosity of the liquid layer surrounding the particles is close to a normal liquid value of 1 cP, corresponding to molecular diffusion coefficients on the order of 1 X m2/s, then an average time for diffusion of radicals formed within the liquid layer can be derived by using as a first approximation the three-dimensional diffusion relationship r = R2/6D
8861
Acknowledgment. P.P. and N.S. are grateful to NATO for an exchange grant (No. 89746). Registry No. Ti02, 13463-67-7; isopropyl alcohol, 67-63-0.
(6)
For a liquid layer 8.5 nm thick, this yields a diffusion time on the order of 10-20 ns. This is exactly the time scale on which the
(22) Nosaka, Y.; Sasaki, H.; Norimatsu, K.; Miyama, H. Chem. Phys. Lett. 1984, 105, 456. (23) Morrison, S.R.; Freund, T. J . Chem. Phys. 1967, 47, 1543. (24) Miyake, M.; Yoneyama, H.; Tamura, H. Chem. Len. 1976, 635. (25) Fessenden, R. W.; Kamat, P. V. Chem. Phys. Leu. 1986, 123, 233. (26) Moser, J.; Graetzel, M.; Sharma, D. K.; Serpone, N . Helv. Chim. Acta 1985, 68, 1686.
(18) Nakabayashi, S.; Fujishima, A,; Honda, K. J. Am. Chem. Soc. 1985, 107, 250. (19) Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1982,86, 241. (20) Nishimoto, S.; Ohtani, B.;Kajiwara, H.; Kagiya, T. J . Chem. SOC. Faraday Trans. I 1985,81, 61. (21) Moser, J.; Gritzel, M. J . Am. Chem. SOC.1983, 105, 6547.
Effect of Salt on the Phase Behavior in a Water/Ionic Surfactant/Alcohol System Hironobu Kunieda* and Kazuyosbi Nakamura Department of Physical Chemistry, Division of Materials Science and Chemical Engineering, Yokohama National University, Tokiwadai 156, Hodogaya-ku, Yokohama 240, Japan (Received: April 3, 1991; I n Final Form: June 11, 1991)
There is a critical (plait) point, pld, in a relatively alcohol-rich region of a main miscibility gap above a critical end temperature in a water/R8S03Na/hexanoI system. A lamellar liquid crystal (LC) intrudes into the main miscibility gap and forms three-phase triangles below a certain temperature called the intrusion temperature (IT). The effect of added NaCl on the phase behavior in the ternary system was clarified above and below the IT. Upon addition of salt, the critical point, Pg, is continuously shifted from alcohol-rich to water-rich regions at 80 OC above the IT. On the other hand, the three-phase region containing the LC phase shrinks with increasing salinity at 50 "C below the IT and eventually terminated at a critical end point at which the critical point, P z , is originated. Thus, salt decreases the IT and the critical point, P!, is produced even at lower temperatures. Another critical point, PL,and a three-phase triangle consisting of excess water (W), surfactant (D'), and reversed micellar solution (Om) phases are formed in a water-rich region by adding salt to the water/R8S03Na/hexano1 system. The D' is an anomalous surfactant phase which corresponds to a so-called L3phase. This three-phase region remains even at higher salinities and the composition of the Om phase forming the W + D' + Om is shifted toward an water-rich region with increasing salinity.
Introduction It is known that middle-phase microemulsions (surfactant phases) coexist with excess water and oil phases in a brine/ionic surfactant/cosurfactant/oil system'V2 or in a water/nonionic surfactant/oil system.ss The microemulsions are very important for practical applications such as tertiary oil recovery." However, multicomponents are included in a microemulsion system and the *To whom correspondence should be addressed.
0022-3654/91/2095-8861$02SO/O
phase behavior is usually very complicated due to the formation of liquid crystals. Besides, even four coexisting phases are observed (1) Winsor, P. A., Solvent Properties of Amphiphilic Compounds; Butterworths: London, 1950; p 68. (2) Kunieda, H.; Shinoda, K. Yukagaku ( J . Jpn. Oil Chemists' Soc.) 1980,
29, 676. ( 3 ) Shincda, K.; Saito, H. J . Colloid Interface Sci. 1968, 26. 70. (4) Kunieda, H.; Friberg, S. E. Bull. Chem. SOC.Jpn. 1981, 54, 1010. ( 5 ) Kunieda, H.; Shincda, K. Bull. Chem. SOC.Jpn. 1982, 55, 1777.
0 1991 American Chemical Society
8862 The Journal of Physical Chemistry, Vol. 95, No. 22, 1991 in some systems7-" or tricritical phenomena are i ~ v o l v e d . ~ ~In -'~ order to understand the complex phase behavior systematically, it is very important to clarify the phase behavior in a system with fewer components. Although the phase behavior in a water/ionic surfactant/cosurfactant system had been extensively investigated at constant temperatuFe,16 it was not clear how the phase behavior is related to the formation of middle-phase microemulsions. Recently, the effect of temperature on the phase equilibria was clarified in the ternary system."-19 It was found that there are an azeotropic point of lamellar liquid crystal and a critical end point for coexisting isotropic phases in a water/sodium octanesulfonate/hexanol system.18J9 A lamellar liquid crystal intrudes into a main miscibility gap at a certain temperature called the intrusion temperature below which the phase behavior is complicated due to the formation of three-phase triangles including a lamellar liquid ~rysta1.I~ It is also known that an isolated isotropic phase (D'or L,) forms upon addition of an inorganic salt to a water/ionic surfactant/ cosurfactant system." In the case that a less hydrophilic ionic surfactant is used; the D' phase appears even without This anomalous phase also appears in water/nonionic surfactant/polar oil The D' phase is another type of microemulsion and forms four coexisting phases with a middle-phase microemulsion (surfactant phase, D).7-11It has not been known how the D' phase is created and terminated. Although the effect of added salt was also reported in a sodium dodecyl sulfate system, phase behavior was investigated only at temperatures close to the intrusion temperature and has not been clarified ~ystematically.'~ In this context, the phase behavior in a NaCl/water/ R8S03Na/hexanol system has been systematically investigated above and below the intrusion temperature of a lamellar liquid crystal. Experimental Section Materials. Extra-pure-grade sodium octanesulfonate (abbreviated as R8S03Na) and I-hexanol were obtained from Tokyo Kasei Kogyo Co. Sodium chloride was obtained from Junsei Chemical Co., and its purity was above 99%. All the chemicals were used without further purification. Procedures. Various amounts of salt, water, surfactant, and alcohol were sealed in ampules. The series of ampules kept in a thermostat were well shaken and left at constant temperature (f0.01"C)from several hours to 1 week depending on the stability of the emulsions. Liquid crystals were detected by polarizers and their types were determined by a polarized microscope.22 Results and Discussion Pbase Behavior in a Water/Ionic Surfachnt/Alcobol (Cosurfactant) System. Phase behavior in a water/ionic surfactant/ middle- or long-chain alcohol system had been extensively studied
Kunieda and Nakamura
I
.hexanol
\
111 ( W + L C * O m )
Figure 1. Schematic phase behavior in a water/R8S03Na/hexanoI system as a function of temperature at constant pressure. I, 11, and 111 are one-, two-, and three-phase regions, respectively. Wm and % mean an aqueous micellar solution phase and a nonaqueous reversed micellar solution phase. W is an excess water phase. D is a surfactant phase which is separated from the Om phase at point p'de. PM and Pm are azeotro ic point curve and azeotropic end point of lamellar liquid crystal (LC) Pc and pJe are critical point curve and critical end point between D and Om phases. Thick curves indicate the loci of respective phases forming a three-phase region. The line &-y' is a critical tie line. Typical three-phase tie triangfes are indicated by broken lines.
at constant temperature and pressure.16 Isotropic one-phase regions are expanded from both water- and alcohol-rich regions. A lamellar liquid crystalline phase exists in between. Thus, there is no critical point between isotropic phases in a main miscibility gap in many systems.I6 However, the phase behavior is largely influenced by a temperature ~ h a n g e . ' ~ ,The I ~ schematic phase behavior in the ternary water/R8S03Na/hexano1system is shown in Figure l.I9 The loci (thick curve) of respective phases forming three-phase triangles and the loci (dotted curve) of critical points and azeotropic points of a lamellar liquid crystal are shown in Figure 1. Liquid crystals except lamellar type and concentrated regions are omitted. Only one three-phase triangle (W LC + Om) exists in the dilute region below a critical end temperature (the bottom in Figure 1). This type of phase pattern is very common in many water/ionic surfactant/alcohol systems.I6 With the rise in temperature, the solubility of surfactant in the W phase of the W LC + Om region increases and it i s continuously changed to an aqueous micellar solution phase (Wm). An isotropic phase (Om) extended from an alcohol apex becomes wider with the rise in temperature. The Om phase splits into two isotropic phases (D and Om) at a critical endpoint, PzE, and another three- hase triangle, LC + D Om is produced on a critical tie line, P'cE+. D and O m phases are continuously connected in the single-phase region.I8 Since the critical point is located in a relatively a l a hol-rich region, tie lines are also oriented toward an alcohol apex. The two three-phase triangles containing the LC phase merge at an azeotropic end temperature (point PAZE)at which the Wm, D, and LC phases are oonnected. Above the P m ,the LC retreats to a concentrated region and the solubility curve surrounding the main miscibility gap is smoothly connected from water- to alcohol-rich regions (the top in Figure 1). The azeotropic point may
+
(6) Healy, R. N.; Reed, R. L. In Improved ( 3 1 Recowry by Surfoctmt and Polymer Flwding, Shah, D. O., Scbechter, R. S., Eds.;Academic h: New York, 1977, p 383. (7) Bennett, K. E.; Davis, H. T.; Scriven. L. E. f. Phys. Chem. 1982,86,
..
1017
_
