J. Phys. Chem. 1980, 84, 1987-1991
1987
Photosyntlhetic Production of H2 and H202on Semiconducting Oxide Grains in Aqueous Solutlons M. V. Rao, K. Rajeshwar,* V. R. Pal Verneker,+ and J. DuBow Department of Electrical Engineering, Colorado State University, Fort Collins, Colorado 80523 (Received December 18, 1979) Publication costs assisted try Colorado State University
Photosynthetic production of H2 and H202 is demonstrated on UV-irradiated ZnO/water and Ti02/water suspensions. The rates of H2 and H202production were monitored by a combined analytical-gas chromatographic procedure as a function of irradiation time and solid content. In the case of ZnO, evidence is presented which indicates that photocorrosion of ZnO proceeds simultaneously with photosynthesis of H2 and HzOD The present data are rationalized with the aid of energy band diagrams showing the relative positions of the semiconductor band edges and the solution redox levels.
Introductioin There has lbeen much recent interest in photocatalytic and photosynthetic processes at the a,emiconductor/liquid interface-l Interest in photosynthetic processes stems primarily from the possibility of utillizing these reactions to convert sunlight to storable forms of energy. Two reactions which have received particular attention in this regard are
Both of these reactions are thermodynamically “uphill” and involve respectively free-energy changes of 2.43 and 1.26 eV per molecule. Photoelectrochemical (PEC) methods h a w offered a ready means of overcoming the thermodynamic barriers and have einabled production of fuels by reactions 1 and 2. Several rleviews of this field of research have appeared in the recent literaturea2An attractive alternative to the PEC approach involves the socalled photoriynthetic m0de.l’ This approach utilizes semiconducting grains which are suspended in the appropriate reaction media and thereby affords a more practical means of carrying out the energy conversion process. Each illuminated grain may be regarded as a short-circuited PEC cell, and the reactions at the interface of each grain and the liquid medium constitute the “local cell”ld,fJprocesses. Photosynthetic production of Hz02according to reaction 2 has been investigated on various semiconducting materials including ZnO?s5 CdS,4 HgS: ZnS,4 CdTe,6 CdSe,4 and Ga2S3.4ZnO, in particular, has been the focus of considerable a t t e n t i ~ n . ~Notwithstanding ,~ the controversies that remain on the mechanism of photosynthesis of Hz02at the ZnO/liquid i ~ i t e r f a e eit , ~was ~ particularly of interest to note that previous authors had reported no traces of hydrogen in the reaction products.3c In view of the importance of reaction 1 to energy conversion and storage and the preponderance of this reaction at oxide semiconductor/electrolyte interfaces in general (vide infra), we decided to reexamine the products of UV-generated processes at the powdered ZnO/liquid interface. A second aspect of relevance to this paper concerns reaction 2. Pirevious authors had observed H202produc‘Currently with Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore, India. 0022-3654/80/2084-1987$01 .OO/O
tion at the Ti02/electrolyte interface subjected to UV and visible irradiation.6 These experiments were, however, conducted in the photoelectrochemical mode. With powdered Ti02 grains suspended in aqueous solution, no evidence for reaction 2 was obtained in previous studies.3a This finding was surprising particularly in view of the good correlation that was observed between photoelectrochemical cell reactions of Ti02and the corresponding processes on powdered TiO,.lh-j~m~o We were intrigued, therefore, by the possibility of photosynthesizing Hz and H20zat the granular Ti02/liquid interface. Experimental SBction Photochemical measurements were performed at room temperature in a specially designed reaction vessel made of either quartz or Pyrex glass (Ace Glass Co., NJ). The light source was a Hanovia high-pressure Hg lamp and was housed in a double-walled quartz or Pyrex glass envelope as schematically shown in Figure 1. Prepurified Nz was used as the transport and gas chromatograph carrier gas for the experiments (cf. Figure 1). A constant N2flow was also employed in the reaction chamber. This served to keep the semiconductor grains in suspension and also maximized their exposure to the UV radiation. A gas chromatograph with automatic sampling accessory (Model 111, Carle Instruments Inc., Fullerton, CA) was used to monitor the photoproduction of Ha. The column consisted of a 6-ft. stainless steel tube filled with molecular sieve 5A 45/60. The output of the gas chromatograph was recorded on a strip-chart recorder (Omnigraphic 3000 recorder). The peak areas were calibrated by using standard mixtures of H2 and N2 (Scientific Gas Products, NJ). Hydrogen peroxide was analyzed by an iodimetric procedure described e1sewhere.I Reduction of peroxide by iodide ions was carried out on 20-mL aliquots of the reaction mixture in 1 N H,S04 medium. A few drops of ammonium molybdate served to catalyze the reduction reaction. The iodine liberated was detected by addition of starch and the solution analyzed spectrophotometrically. The energy output of the light source was measured by an actinometric procedure. The decomposition of monochloroacetic acid in UV light was utilized for this purpose. The C1 ions liberated in the reaction were estimated by the Volhard methods8 A value of quantum efficiency ($) equal to 0.62 f 0.04 was taken from previous work on the UV photolysis of monochloroacetic acidag In all of the experiments reported here, the reaction mixtures were irradiated with the full intensity of the lamp (11.04 X 0 1980 Amerlcan Chemlcal Society
1988
The Journal of Physical Chemistry, Vol. 84, No. 15, 1980
Rao et al.
CARRIER GAS GC CARRIER GAS
-
CHROMATOGRAPH CONTROLLER TRANSPORT GAS - ' - I S OU i F E A
,
TRANSPORT GAS FLOWMETER
,
AMPLIFIER
AUTO-SAMPLING ACCESSORY
INTEGRATOR
STRIP-CHART RECORDER
Figure 1. Schematic of the setup employed for photosynthetic measurements.
einstein/min) as filtered by the Pyrex glass envelope (in some cases) and water in the cooling jacket (cf. Figure 1). In some experiments, a quartz jacket was substituted for the Pyrex envelope; no difference could be observed in the nature of the reaction products. The light source employed in some early experiments was subject to a slow diminishing of intensity due to gradual aging of the mercury arc lamp. For this reason, although the results given in any one series of measurements are consistent within themselves, the data in one set of experiments are not necessarily comparable with those in another. Titanium dioxide (99.998%) was obtained in powder form from Materials Research Corporation, Orangeburg, NY. Reduction of the starting material was carried out in a conventional furnace by heating for -6 h at 700-800 "C in a flowing atmosphere of Hz. The reduced material was light blue in color. Zinc oxide was obtained in granular form from commercial sources (Materials Research Corporation, Orangeburg, NY) and was used as received. Some samples of ZnO from Pfaltz and Bauer Inc., Stamford, were also investigated. The reaction medium in all experiments was deionized water (resistivity: -18 MQ). The initial pH of the reaction mixtures was usually in the range 6.4-6.7 as measured on a Model 701A Orion Research pH meter. The use of buffered solutions to control pH was avoided to preclude effects arising from the UV photolysis of buffer reagents. The results described below also pertain to the case where no organic additives were intentionally added to the reaction mixture to enhance the photosynthesis of H20p3 0 2 gas was, however, passed through the reaction mixture in a few experiments of ZnOz which were designed to duplicate measurements previously reported.
Results hydrogen Contrary to the results of a previous production on UV irradiation of ZnO/water mixtures seemed to be quite facile. In the absence of ZnO, however, no traces of Hzand HzOzcould be found. Figure 2a illustrates typical results which show that a limiting rate of H2 evolution is attained after approximately 1 h of irradiation.1° This limiting rate is proportional to the mass of ZnO as shown in Figure 2b, although at high levels of solid content in the reaction mixture ( > l o g in 700 mL of water), this dependence falls off, and the rate becomes
20
15
ct
(a)
XIO-7
:5t ,,/ E
,'
-
=II 5
20
0;
io
60
Eo
IO0
I20
I40
I60
I60
IRRADIATION TIME, min
0
>
20
(b)
XIO-7
2 1
:
'0
5
10
15
20
25
30
35
AMOUNT OF ZnO, gm
Figure 2. (a) Rate of H2 evolution as a function of Irradiation tlme for ZnOlwater suspensions; grams of ZnO in 700 mL of water. (b) Limiting rate of H2 evolution as a function of the amount of ZnO. The solution volume in all cases was 700 mL.
insensitive to a further increase in ZnO concentration. A gradual decay in the rate of Hz evolution was also noted on prolonged irradiation (Figure 2a). Analysis of the reaction mixtures at various stages of the illumination revealed the presence of H2O2in all cases. Passage of 02 through the reaction mixture also brought about a substantial increase in the concentration of Hz02. Addition of -5 pol of H20zto the reaction mixture followed by 1 h of illumination brought about a twofold decrease in the concentration of H202. These results are in essential agreement with those of previous worker^.^ The apparent quantum yield for production of Hz on ZnO was calculated for the conditions employed in the present to be 1.13 X set of experiments. Figure 3 illustrates the rate of H2 evolution of Ti02 for three levels of solid content in the reaction mixture. A limiting rate of Hz production is attained after -2 h of
The Journal of Physical Chemistry, Vo/. 84, No. 15, 1980 9989
Photosynthetic Production of H, and H202
r
5 x10-0
’
/
(a)
7, 20
40
60
80
100
120
140
160
180
200
(b)
4TION TIME, rnin
Figure 3. Rate of evolution of H, in Ti02/water suspensions for three levels of solid content: (a) 10 g, (b) 20 g, and (c) 30 g. Solution volume in all cases was -700 mL. .-
E VI
-e
E 3-’
IRRADIATION TIME, rnin
z
;*-
.
z
2
Figure 5. (a) Amount of H202formed in TiO,/water suspensions as a function of irradiation time. (b) Rate of H202formation in TiO,/water suspensions as a function of irradiation time. Data are shown for three levels of solid content in 700 mL of water: (0) 10 g, (A) 20 g, and (0) 30 9.
o H,O,
H,
I-
LL
0
Y d LL 00
I
I
I
IO 20 AMOUNT OF TiOp, grn
I
I
30
Figure 4. Dependence (of limiting rates of H2 and H202formation on the amount of TiO, in 700 mL of solution.
illumination. The behavior is similar to the case of ZnO discussed above. This limiting rate is directly proportional to the concentration of TiOz as shown in Figure 4. Figure 5a illustrates the concentration of HzOzin the illuminated TiOz/water suspensions as a function of time. A limiting concentration of HzOzis attained after -3 h of illumination, and concomitantly the rate drops gradually to zero as shown in Figure Sb. Control experiments carried out in the absence of TiQz revealed no traces of either Hz gas or HZOz. The behavior seen in the case of TiOz in the present study is similar to that reported by previous authors for Zn0,3a3dph The apparent quantum yields for production of Hz and HzOzon TiOz were calculated to be 3.08 X lo4 and 3.17 X respectively.
Discussion To our knowledge, the results reported in the present study represent the first instance where Hz evolution in the case of ZnlO and Hz and HzOzproduction in the case of TiOz have been observed in the photosynthetic mode. HzOzformation was noted, however, by previous authors on TiOz electrodes in PEC cells.6 Phlotoproduction of Hz on TiOz electrodes in PEC cells is also a well-documented result.’ The present results, however, seem to be contradictory to those obtained by previous investigators, Markham and Laidler%report that “under the conditions described in this paper, no peroxide can be found in irradiated suspensions of titanim dioxide either in presence or absence of phenolic compounds.” No mention is made in this paper on the nature of the TiO, samples that were employed, i.e., whether they were reduced or not before irradiation. Our experience with TiOz which was not
subjected to prior reduction indicates that the negative results in the above-mentionedstudy could be rationalized on the basis that the samples employed by Markham and Laidler were not sufficiently conductive. No HzOzor Hz, for that matter, could be detected in the present study with unreduced TiOz. Calvert et al.3calso report that Hz could not be detected in irradiated 02-freeZnO/water suspensions. The reason for this observation is not presently clear but may well be linked to the closed nature of their reaction system and the rather stringent requirements imposed therein on the sensitivity of the Hz detection system. (These authors employed a mass-spectrometric procedure for their analysis. We believe that our gas-chromatographic system is more sensitive to Hz at the concentration levels that are commonly encountered in these studies.) The results obtained in the present study are compelling proof for the existence of a direct correlation between photoelectrochemical processes on semiconducting electrodes and the corresponding photosynthetic reactions on the same materials in granular form. Such a correlation, however, hinges to a large extent on the relative positions of the semiconductor band edges and the energy levels corresponding to the individual local cell (redox) reactions. For the water-splitting reaction (reaction l), the semiconductor condition and valence-band edges (EcBand Em, respectively) should ideally straddle the levels corresponding to the half-reactions” 2H+(solv) + 2e OH-(solv) + ht
-
-
Hz(g)
‘/zHzO(l)
+
(3)
(4) For reaction 2, the interfacial energetics will have to be such that the conduction and valence band edges straddle respectively the level corresponding to reaction 3, i.e., Hf/Hz redox energy, and the Oz/HzOzredox level. It is pertinent to note that the above criterion will hold independent of the pH of the reaction medium since the band edges and the redox levels move in a manner relative to each other on the pH scale. The apparently facile pro‘/402(g)
1990
The Journal of Physical Chemistry, Vol. 84, No. 15, 1980 ENHE
t
- I
TiOp (RUTILE)
TiO, (ANATASE)
rca rcB H,O/H,
-
Figure 8. Semiconductor band energies and solution redox levels for reduction of Ht ions, oxidation of OH- ions, and reduction of 0,. All energies are referred to the electrochemical energy scale (normal hydrogen electrode). Decomposition Fermi energies for ZnO and T i 4 are also shown, and the data are those calculated by Gerischer (ref 12a,b). The band positions and redox levels are adjusted for pH 7.
duction of Hz that was observed on TiO, grains in the present study contradicts the situation observed for rutile electrodes in PEC cells where a small bias is required to sustain H2 evolution.2 This bias is necessitated by the fact that the H+/H2equilibrium potential lies slightly negative of the flat-band potential, V , (V, N EcB/eofor heavily doped material), for rutile. The anatase form of TiOz, however, has a slightly larger energy band ap (E,) than rutile ( E = 3.23 vs. 3.02 eV, respectively).' This would presumahy shift the flat-band potential in a more negative direction with the result that ECB now becomes more negative than the H+/H2level. An X-ray diffractogram of the T i 0 2 samples that were used for the experiments revealed the material to be a mixture of both rutile and anatase forms. The interfacial energetics are illustrated in Figure 6 for both TiO, (anatase and rutile) and ZnO. The energetics are shown for the case where no band bending exists at the grain/liquid interface. Data for construction of the band edges for Ti02and ZnO and the various energy levels were obtained from ref 11 and 12. Several features of the energy diagram are worthy of note: (i) Oxidation of water can proceed on Ti02 since the O,/H,O level (cf. reaction 4 is negative with respect to Em. (ii) The 02/H202level is also negative with respect to Em for Ti02;photosynthesis of HzOzis therefore thermodynamically feasible. (iii) Both the Oz/H20 and the Oz/HzOz levels lie negative of the decomposition Fermi energy, ,,ED, for Ti02. Reactions 2 and 4 therefore will occur more readily than corrosion of TiOz for kinetic reasons which have been discussed by Gerischer.12" (iv) Although the 02/HzOz and 0 2 / H z 0levels are negative with respect to EVBfor ZnO, the decomposition Fermi energy also lies more negative than the corresponding level for Ti02 with the result that corrosion reactions become more important for this material. The above predictions are consistent with the findings of the present study. Although corrosion of ZnO cannot be unequivocally proved by the present results, two findings are relevant in this regard: (i) A gradual decay in the rate of Hzevolution was observed for ZnO but not for TiOz (compare Figures 2a and 3), and (ii) an infrared spectrum of the material after the irradiation experiments and subsequent drying at -110 "C showed intense bands
8
Rao et ai.
corresponding to OH- groups. These bands were virtually absent in the spectra for the starting material and for material kept in contact with water for approximately the same period as the irradiation experiments and subsequently dried. These results are consistent with a corrosion reaction of the type: 2Zn0
+ 4H20 + 4h+
-
2Zn(OH),
+ O2 + 4H+
(5)
The Fermi energy level corresponding to this reaction has been calculated by GerischerlZband is shown in Figure 6. Photocorrosion reactions of the type represented by eq 5 would account for the complex trends in the dependence of Hz evolution rate on the mass of ZnO (cf. Figure 2b). We postulate the following sequence of processes to explain photosynthetic production of H2 and H202on TiO, and ZnO. Many features in the proposed mechanism are common to the scheme previously described by Harbour and Hair for Zn0.3h Light, of energy greater than the band gap of the oxide creates electron-hole pairs. For n-type semiconductors,the interfacial energetics are such that the bands are bent upward; holes therefore move to the grain surface where they are available for oxidation reactions, and electrons move to the bulk of the grain. Therefore, the various steps in the sequence will consist of
+ + + -
semiconductor
hu
e-
+ h+
(6)
followed by reaction 4 and
Ods) + e-
(OZ-),
H+(sdv) + (OH,.),
(HO,.), (HOZ-),
(7)
(02-)s
e-
H+(solv)
(HO,-),
(H,O,),
(8) (9)
HzO2
(10)
The subscript "s" above denotes a surface absorbed state. Evidence for formation of 0; and HO; radicals is found in a recent study by Harbour and Hair.3h The reduction reactions represented by eq 7 and 9 could be mediated through surface states in the band gap of the material. In fact, recent work on the chemisorption of O2 on Ti02 reports evidence for the transfer of electrons from the conduction band of Ti02 to adsorbed 0, to create a chemisorbed 02- state.13 The formation of such a state could introduce a level within the band gap of the host material. An additional source of surface states would be defects which are preponderant in polycrystalline materials such as the samples employed in the present study. The oxygen required for the formation of H,02 by reaction 10 is provided either by reaction 4 or externally, e.g., by bubbling O2 through the reaction mixture. The reduction process represented in eq 3 also effectively competes with reactions 7 and 9 for the photogenerated electrons. Photocorrosion reactions (e.g., reaction 5), particularly in the case of ZnO, compete with reaction 4 for the photogenerated holes in the semiconductor. The data shown in Figure 5b on the gradual decay in the rate of H20z production on Ti02 are similar to the results obtained by previous workers on Zn0.3a7d This falloff in rate merely reflects the increased contribution from back-reactions of the type: (HzO2),
-
HZO(1) + '/202(g)
(11)
The saturation observed in the rate of HzOzand Hz production as a function of Ti02content (Figure 4) is probably related to factors originating from surface coverage limitations at high levels of solid content. More work is needed to unravel the origin of this effect.
J. Phys. Chem. 1980, 84, 1991-1995
Back-reactions of the type represented by eq 11 and radical reactilons such as H+(solv) OH-(solv)
+ e-
+ h+
-
+
13.
(12)
OH-
(13) (14)
He + OH. H20(1) +
become especially important in the photosynthetic mode. A clear advantage of PEC cells in this regard lies in an efficient meains of separating the reaction products; Le., oxidation and reduction reactions ace carried out at different sites (at the photoanode and counterelectrode, respectively, for the case of n-type semiconducting electrodes) rather than on the same grain as in the case of the photosynthetic mode. Other loss mechanisms such as recombination of photogenerated carriers in the semiconductor bulk also become more predominant in the photosynthetic mode particularly because a field-assisted means of separating the carriers (e.g., by means of an external bias) does not readily exist unlike in the case of PEC cells. Partial metallization of the semiconducting grains would,, however, serve to reduce recombination losses in the bulk. ‘This approach has worked well with other photocatalytic and photosynthetic reactions on Ti02 and SrTi0314 Work is continuing in this laboratory on the effect of metallization of H202 and H2 yields utilizing various semiconducting materials. Acknowledgment. This research program was funded by the U.S.Department of Energy under Grant G-77-C02-4258. References and Notes (1) (a) S. R. Morrlson and T. Freund, J. Chern. Phys., 47, 1543 (1967); (b) W. P. Oomes, T. Freund, and S. R. Morrison, Surf. Sci., 13, 201 (1968); (c) (3.Gerischer and K. Cammann, Ber. Bunsenges. Phys. Chem., 7 6 , 385 (1972); (d) H. Yoneynama, Y. Toyoguchi, and H. Tamura, J. Phys. Chem., 76, 3460 (19’72); (e) M. Miyake, H. Yoneyama, and H. Tamura, Electrochim. Acta, 21, 1065 (1976); (f) M. Miyake, H. Yoneyama, and H. Tamura, ibid., 22, 319 (1977); (9)
I991
S. N. Frank and A. J. Bard, J . Am. Chem. Soc., 99, 303 (1977); (h) M. Mjake, H. Yoneyama, and H. Tamura, Bull. Chem. SOC.Jpn., 50, 1492 (1977); (i) 8. Kraeutler and A. J. Bard, J. Am. Chem. Soc., 99, 7729 (1977); (i)ibM., 100,2239 (1978); (k) ibM., 100, 4317 (1978); (I) ibid., 100, 5985 (1978); (m) M. Miyake, H. Yoneyama, and H. Tamura, J. Catal., 58, 22 (1979); (n) H. Reiche, W. W. Dunn, and A. J. Bard, J. Phys. Chem., 63, 2248 (1979); (0)F. F. Fan and A. J. Bard, J . Am. Chem. Soc., 101, 6139 (1979). (a) A. J. Nozik, Annu. Rev. Phys. Chem., 29, 89 (1978); (b) W.A. Gerrard and L. M. Rouse, J . Vac. Sci. Techno/., 15, 1155 (1978); (c) L. A. Harris and R. H. Wilson, Annu. Rev. Mater. Sci., 8, 99 (1978); (d) K. Rajeshwar, P. Singh, and J. DuBow, Electrochim. Acta, 23, 1117 (1978); (e) H. P. Maruska and A. K. Ghosh, Sol. Energy, 20, 443 (1978); (f) M. Tomkiewicz and H. Fay, Appl. Phys., 18, 1 (1978). (a) M. C. Markam and K. J. Laldler, J. Phys. Chem., 57, 363 (1953) (seealso references cited in this article); (b) T. R. Rubin, J. G. Cakert, G. T. Rankin, and W. M. MacNevin, J. Am. Chem. SOC.,75, 2850 (1953); (c) J. 0. Cakert,K. Theurer, G. T. Rankin, and W. M. MacNevin, ibid., 76, 1575 (1954); (d) 0. A. Korsumovskii and Yu. S. Lebedev, Russ. J. Phys. Chem. (Engl. Trans/.),35, 528 (1961); (e) T. Freund and W. P. Gomes, Cafal. Rev., 3, 1 (1969); (f) D. R. Dixon and T. W. Healv. Aust. J. Chem.. 24. 1193 (1971): (a) M. D. Archer, J . Appl. Eiectrochem., 5, 17 (1975); (h) J. R. Ha6our and M. L. Hair, J . Phys. Chem., 83, 652 (1979). R. E. Stephens, B. Ke, and D. Trklch, J. Phys. Chem., 59,966 (1953). J. J. Rowlette, Sol. Energy, 7, 8 (1963); (a) M. S. Wrlghton, D.S. Ginley, A. B. Ellis, P. T. Wolcranskl, D. L. Morse, and A. Linz, Proc. Natl. Acad. Sci. U.S.A., 72, 1518 (1975); (b) P. Clechet, C. Martelet, J. R. Martin, and R. Olier, €lectroch/m. Acta, 24, 457 (1979). D. J. Savage, Analyst (London), 78, 224 (1951). 1. M. Koltholi and P. J. Eking, Eds., “Treatise on Analytical Chemistry", Vol. 7, Part 11, Intersclence, New York, p 7. L. Kuechler and H. Pick, Z . Phys. Chem. Abt. B, 45, 116 (1939). The equilibration time for the gas chromatograph was found to be -20 min from calibration experiments. Thls Induction period is shown as dotted lines in Figures 2a and 3. H, yields quoted in this paper are also uncorrected for the gas dissolved in the solution. The initial saturation of the evolved gas in the solution phase would account for part of the induction period. “Semiconductor Liquid-JunctionSolar Cells”, The Electrochemical Society, Princeton, NJ, 1977, Chapter VII, p 272. (a) H. Gerlscher in “Solar Power and Fuels”, J. R. Bolton, Ed., Academic Press, New York, 1977, Chapter 4; (b) H. Gerischer, J . Vac. Sci. Techno/., 15, 1422 (1978); (c) W. M. Latimer, “Oxidation Potentials”, Prentice-Hall, Englewood Cliffs, NJ, 1952. H. W. Gundlach and K. E. Heisler, Z . Phys. Chem. (Wiesbaden), 112, 101 (1978). M. S. Wrighton, P. T. Wolczanskl, and A. B. Ellis, J. SolM State Chem., 22, 17 (1977).
Kinetics OF Phase Separatlon in Binary Liquid Mixtures J. Wenzel, U. Limbach,t G. Bresonik, and G. M. Schnelder” University of Bochum, Department of Chemistiy, Bochum, West Germany (Received November 13, 1979) Publicatlon costs assisted by the University of Bochum
The kinetics of liquid-liquid phase separation was studied in some binary liquid mixtures (2-butoxyethanol-H20; triethylamine-H20; nitrobenzene-2-methylbutane) by observing light scattering due to the formation of droplets of the new phase. Demixing was initiated by relaxation techniques (pressure-or temperature-jump);here pressure or temperature was changed within less than 0.1 ms to get very rapidly from the homogeneous into the heterogeneous region. After such a jump the intensity of the scattered light ran through a maximum value as a function of time, and the time t, between passing the coexistence curve and the occurrence of the intensity maximum varied between and lo1 s depending on how far the jump entered the miscibility gap. The dependence of t, on jurnp width was very strong for low supersaturations but became much smaller with increasing quench depth. The transition between these two types of behavior was found to occur at a definite superheating temperature T,which is assumed to be related to the so-called spinodal curve. These findings are supported by experiments with a1 different method using light absorption by dyes.
I. Introduction In the thermodynamic theory of liquid-liquid &mixing1 the limit between one-phase and two-phase regions in the +Deceased.
isobaric T-x diagram is called the coexistence or connodal curve where the chemical potentials of each component in the two coexisting phases are equal (see Figure 1); in the following the temperatures on this curve will be denoted by TE. The mutual solubility of liquids is also
0022-3654/80/2084-1991$01.00/00 1980 American Chemical Society
