J . Phys. Chem. 1988, 92, 4699-4702
4699
Study of Thermally Induced Vanadate Dispersion D. Honicke* and J. Xut Engler- Bunte- Institut der Universitat Karlsruhe (TH), Kaiserstrasse 12, 7500 Karlsruhe, West Germany (Received: September 22, 1987; In Final Form: February 24, 1988)
The possibility for the dispersion of V,05 on different oxidic supports by heating physical mixtures of them to 773 K in dry air has been studied by means of X-ray diffraction and laser Raman spectroscopy. It is shown that monolayer dispersion occurs on TiOz (anatase) as a support. This dispersion is considered to occur by a surface transport process. By contrast, no evidence for V205dispersion was obtained with y-Al,03, SiO,, SiO2-AI2O3,SnO,, ZrO,, and Nb2O5 as supports, whereas a V20S/Mg0mixture led to the formation of the “three-dimensional” binary oxide Mg,V,O,.
Introduction
TABLE I: Survey of Prepared Samples
Vanadium oxides have been used extensively as catalysts in gas-phase oxidations of hydrocarbons,’-8 sulfur dioxide, and carbon monoxide as well as in the reduction of nitric oxide with ammonia.”’ These catalysts can be doped with promoters and/or supported by carriers in order to enhance their activities and lifetimes. Furthermore, changes in catalytic properties of vanadia catalysts can be caused by interactions of vanadia with the support. For example, V205supported by the anatase modification of TiO, is superior to unsupported V20s in the selective oxidation of certain h y d r o c a r b o n ~ . ~ ~Recent ~ ~ ~ J studies ~ J ~ have revealed that anatase modifies the properties of vanadium oxide by forming a monolayer of surface vanadia species (“supported V,Os”) and/or V205as crystallites (“bulk V205”).1520 Moreover, it has been shown that supported V20s in V,O5/TiO2 catalysts was the active site in the partial oxidation of hydrocarbon^'^-'^ and gave a higher selectivity and activity than bulk V205in many hydrocarbon oxidation reactions. Several methods for the preparation of vanadia monolayer catalysts were tested, in which liquid or gaseous vanadium compounds have been used. A relatively new approach is the solid/solid reaction between the active component and the support by thermal treatment (calcination) of physical mixtures of them. Thus, the formation of supported Mo0321-2Sand supported V20$6-29 by thermal treatment have been reported. In the case of supported vanadia, one could obtain a thin layer of vanadia, probably a monomolecular layer, which covered the TiO, (anatase) support. The present study has focused on the interactions between V,05 and a variety of metal oxides as supports by thermal treatments of physical mixtures of them. The results of such treatments were examined by X-ray diffractions (XRD), laser Raman spectroscopy (LRS), and BET measurements.
Experimental Section Materials. The following supports were used: y-A1203(99%), S i 0 2 (99.8%), Si02-A1203 (ratio 12/88, 99%), MgO (99.5%), ZrO, (99+%), Nb2O5 (99+%), and SnO, (99.9%) obtained from Ventron as well as TiO, (anatase) (99.9+%) obtained from Aldrich. The support materials were fine powders with the exception of pelletized y-A1203 and Si02-A1203. As the active compound, finely powdered vanadium pentoxide (99.995%) from Ventron was used. Preparation Methods. V20s and the respective support were admixed in the proportions listed in Table I and subsequently powdered in an agate mortar. The amount of V,Os was in each case calculated to be less than that required for monolayer coverage. The physical mixture obtained in the mortar was placed into a quartz tube (Figure 1 ) connected to flowing dry air (60 cm3 min-I, molecular sieve 3 A) and as a rule kept constant at 773 K in a liquid tin bath for 48 h. In some cases lower temperatures and shorter residence times have been used, too, as ‘Permanent address: Department of Chemical Engineering, Tianjin University, PR China.
0022-3654/88/2092-4699$01.50/0
surf. area v2°5
sample V2°5/y-A1203
V205/Si02 V2O5/SiO2-AI2O3 V2°5/Sn02 V2°5/Zr02
V205/Nb205
V205/MgO V205/Ti02
(BET), m2 g-’
av surf. coverageb as a fract v
content,’ unof V - O atoms wt 5% treated treated monolayer nm-2 4.8 16.4 13.6 0.67 0.4 0.15 3.1 0.78
“Mass of support: 1-3 g.
96 465 355 6.4 3.8 1.8 34 10
94 326 287
0.35 0.35 0.35
3.4 3.3 3.1
6.3 3.7 1.7
0.73 0.74 0.6
7.0 7.2 5.8
36 8.8
0.59 0.6
5.7 5.9
b@vo2,s= 10.3 X
IO4 pm2.
described in the results. The samples were covered with quartz wool. (1) Hucknall, D. J. Selective Oxidation of Hydrocarbons; Academic Press: London, 1974. (2) Higgins, R.; Hayden, P. Catalysis 1977, 1 , 168. (3) Bielanski, A.; Haber, J. Catal. Rev. 1979, 1 , 19. (4) Cole, D. J.; Cullis, C. F.; Hucknall, D. J. J . Chem. SOC.,Faraday Trans. 1 1976, 72, 2185. (5) Vanhove, D.; Blanchard, M. Bull. SOC.Chim. Fr. 1971, 9, 3291. (6) Vejux, A,; Cortine, P. J . Solid State Chem. 1978, 23, 93. (7) Bond, G. C.; Sarkany, J.; Parfitt, G. D. J . Catal. 1979, 57, 476. (8) Murakami, Y.; Inomata, M.; Miyamoto, A.; Mori, K. In Proceedings of the 7th International Congress on Catalysis, Tokyo; Elsevier: Amsterdam, 1981; p 1344. (9) Inomata, M.; Miyamato, A.; Murakami, Y. J. Catal. 1980, 62, 140. (10) Shikada, T.; Fujimoto, K.; Kunugi, T.; Tominaga, H.Ind. Eng. Chem. Product Res. Dev. 1981, 20, 91. (1 1) Bauerle, G. L.; Wu, S. C.; Nobe, K. Ind. Eng. Chem. Product Res. Dev. 1975. 14. 268. (12) Grabdwski, R.; Grzybowska, B.; Haber, J.; Sloczynski, J. React. Kinet. Catal. Lett. 1975, 2, 8 1. (13) Bond, G. C.; Konig, P. J. Catal. 1982, 77, 309. (14) Bond, G. C.; Bruckman, K. Faraday Discuss. Chem. SOC.1981, 72, 235. (15) Van Hengstum, A. J.; Van Ommen, J. G.; Bosch, H.; Gellings, P. J. Appl. Catal. 1983, 8, 369. (16) Gasior, M.; Gasior, J.; Grzybowska, B. Appl. Catal. 1984, 10, 87. (17) Wachs, I. E.; Saleh, R. Y.; Chan, S. S.; Chersich, C. C. Appl. Catal. 1985, 15, 339. (18) Saleh, R. Y.; Wachs, I. E.; Shirley S. Chan; Chersich, C. C. J. Catal. 1986, 98, 102. (19) Roozeboom, F.; Mittelmeijer-Hazeleger, M. C.; Moulijn, J. A,; Medema, J.; de Beer, V. H. J.; Gellings, P. J. J . Phys. Chem. 1980, 84, 2783. (20) Kozlowski, R.; Pettifer, R. F.; Thomas, J. M. J. Phys. Chem. 1983, 87, 5176. (21) Xie, Youchang; Gui, Linlin; Liu, Yingjun; Zhao, Biying; Yang, Naifang; Zhang, Yufen; Guo, Quinlin; Duan, Lianyun; Huang, Huizhong; Cai, Xiaohai; Tang, Youchi. Proc. Inr. Congr. Catal., 8th, 1984 1984, 5, 147. (22) Xie, Youchang; Gui Linlin; Liu Yingjun; Zhang, Yufen; Zha, Biying; Yang, Naifang; Guo, Quinlin; Duan, Lianyun; Huang, Huizhong; Cai, Xiaohai; Tang, Youchi. In Adsorption and Catalysis on Oxide Surfaces; Che, M., Bond, G. C., Eds.; Elsevier: Amsterdam, 1985; p 139. (23) Margraf, R.; Leyrer, J.; Knozinger, H.; Taglauer, E. Surf. Sci. 1987, 1891190, 842.
0 1988 American Chemical Society
Honicke and Xu
4700 The Journal of Physical Chemistry, Vol. 92, No. 16, 1988 dry a l r
e f f l u e n t gas
1 r quartz woo1 llquld tln
bath
saw 1e
Figure 1. Scheme of the experimental setup.
BET and Spectroscopic Measurements. BET measurements were performed with a Strohlein Areameter using nitrogen. A series of X-ray diffractions and laser Raman spectra were obtained on each binary oxide system before and after the thermal treatment was carried out. All spectra were recorded in air at ambient conditions. X-ray diffraction patterns were obtained with a Philips diffractometer using Cu K a radiation. The Raman spectra were excited by using approximately 50 mW of laser radiation at the sample location from the 514.5-nm line of a Spectra Physics argon ion laser (Type 171-07). The spectra were dispersed and recorded by using a Jobin-Yvon double grating monochromator with a cooled photodetector and an adapted microscope for Raman microanalyses. The spectrometer slits were set at 1000 pm. The spectra were recorded at scanning speeds of 0.5 and 0.2 cm-I s-l with integration times of 0.2 and 0.5 s, respectively.
Results and Discussion In order to detect changes in surface areas caused by the thermal treatment, the BET surface areas of the treated and untreated samples were measured (Table I). Markable decreases of BET surface areas of about 35% occurred by thermal treatment of the samples having high surface areas, viz., V205/Si02and VZO5/SiO2-Al2O3. By contrast, thermal treatment of Si02 alone led only to a slightly lower surface area and thermal treatment of SiO2-AlZO3 did not lead to any decrease of the surface area. Therefore, the considerable decreases in surface areas in the former cases are probably caused by sintering of the supports. This is catalyzed by VzO5 and/or by dispersion of vanadia which leads to plugging of micropores of these supports. V205/y-A1203and other samples having low and intermediate initial surface areas did not show any significant changes in BET areas except for V205/Ti02. The latter exhibited a loss in surface area of about 20% by thermal treatment. This is in accord with a previous observation'* in which a 7% V205/Ti02(anatase) catalyst upon heat treatment suffered 20% loss in surface area, probably caused by V205-catalyzed sintering of Ti02. The expected average surface coverage of the supports with vanadia after dispersion was calculated on the basis of the surface areas obtained after the thermal treatment (Table I). The values resulted in 0.35 as a fraction of V-0 monolayer in the samples having y-A1203,S i 0 2 or SiO2-Al2O3 as supports, while those in the other samples amounted to 0.59-0.74. The corresponding calculated average numbers of V atoms per unit of surface area (24) Leyrer, J.; Zaki, J.; Knozinger, H. J . Phys. Chem. 1986, 90, 4775. (25) Stampfl, S. R.; Chen, Y . ;Dumesic, J. A.; Niu, C.; Hill, C. G. J . Catal. 1987, 105, 145. (26) Haber, J.; Machej, T.; Czeppe, T. Surf.Sci. 1985, 151, 301. (27) Haber, J. In Surface Properties and Catalysis by Non-Metals; Bonnelle, J . P., Delmon, B., Derouane, E., Eds.; Reidel: Dordrecht, 1983; p 1 . (28) Haber, J. Proc. Int. Congr. Catal. 8th, 1984 1984, I , 85. (29) Haber, J. Pure Appl. Chem. 1984, 56, 1663.
amounted to 3.1-3.4 nm-2 for the high surface area samples and to 5.7-7.2 nm-2 for the samples having low and intermediate surface areas. These values are lower than the possible number of V atoms per unit of surface area, which has been previously calculated to be 9.7 nm-2 on the basis of the units in pure crystallized V2OS. Therefore, the numerical prerequisite for the achievement of a monolayer coverage is fulfilled. The results of the XRD and LRS examinations will be described and discussed for the individual samples in the following. V205/y-A1203. The relatively high Vz05 content (Table I) of the physical mixture of V205/y-A1203led to an XRD pattern and a laser Raman spectrum in which all diffraction peaks of V205 are present. The corresponding pattern and spectrum of treated V205/yA1203are unchanged and thus provided no evidence for dispersion of V2OS on y-A120,.30 V 2 0 5 / S i 0 2 V2O5/SiO2-AI2O3. , The XRD patterns of the physical mixtures consisting of V205/Si02and V205/Si02-A1203 showed all diffraction peaks of V205. The diffraction patterns of the thermally treated samples showed the same signals albeit with slightly lower intensities. The laser Raman (LR) spectra of untreated and treated V205/Si02 exhibited all the bands of V205. However, in the spectrum of the treated sample the bands in the range of 300-700 cm-' appeared weaker and broadened. The LR spectrum of untreated V2O5/SiO2-AI2O3exhibited only three weak bands due to strong fluorescence, whereas that of the treated sample showed all V2O5 bands due to reduced fluorescence. The above-mentioned changes in the spectra may be indicative of a partial spreading of V2OS on S i 0 2 and Si02-A1203, respectively. The fact that such a spreading did not show up in the LR spectra of the treated samples may be due to the low extent of spreading and additionally to the strong fluorescence in the case of V2O5/SiO2-AI2O3. V 2 0 5 / S n 0 2V, 2 0 5 / Z r 0 2V, 2 0 5 / N b 2 0 5The . XRD patterns of V 2 0 5 / Z r 0 2 and V 2 0 5 / N b 2 0 5 showed none, and those of V205/Sn02showed only one of the expected signals of V205,both in the untreated and in the treated form. This is probably due to the low V2O5 content (Table I). By contrast, the laser Raman spectra of untreated and treated V205/Sn02exhibited all bands of crystalline V2O5 due to the excellent sensitivity of the metaloxygen vibrations of small V205 crystallites as well as noncrystalline vanadia phases.I7-l9 The laser Raman spectrum of untreated V205/Zr02showed only part of the expected V2OS bands due to overlap of bands with those of crystalline Z r 0 2 below 650 cm-I. The spectrum of the thermally treated V205/Zr02exhibited the same V2O5 bands, however, two of them with decreased intensity. These results give no indication for a dispersion of vanadia.)O In the LR spectrum of V205/Nb205no bands of V2O5 could be definitely assigned due to overlap with the multitude of bands of crystalline niobia. Hence, no information concerning possible interactions between V205and N b 2 0 5 could be obtained. V 2 0 5 / M g 0 . The XRD patterns of V205/Mg0, untreated (Figure 2a) and treated at 773 K (Figure 2e), showed the intense signal at 243 pm (20 = 36.9') of MgO, corresponding to diffraction by (1 11) planes. The pattern of untreated V 2 0 5 / M g 0 showed additionally the diffraction peaks of crystalline V205, viz., an intense peak at 438 pm (20 = 20.26') corresponding to the (001) planes and less intense peaks at 576, 409, 340, 288, and 276 pm. These latter five V2O5 peaks had completely disappeared and the former had nearly disappeared in the XRD pattern of the samples treated at 773 K. However, in the range between 250 and 340 pm (28 = 36-26') the diffraction pattern exhibited new peaks of low intensities, which are due to a-Mg2V207as the major and @-Mg2V207 as the minor ~ o m p o n e n t . ~Additional ~ signals of low intensity could not be assigned. Samples of V,O,/MgO, treated at the intermediate temperatures of 623, 673, and 723 K, gave the XRD patterns b, c, and ~
~~~~~~~~~
(30) Nevertheless, partial dispersion cannot be excluded. For, it was reported recently that ion scattering spectroscopy revealed partial dispersion of MOO, on A1203although XRD and LR spectra gave no indication for such spreadingz3 (31) Clark, G. M.; Morley, R. J . Solid State Chem. 1916, 16, 429.
The Journal of Physical Chemistry, Vol. 92, No. 16, 1988 4701
Thermally Induced Vanadate Dispersion
J 1 d
a
b
C
C
I
I
30
25
20
15
10
2e/srad Figure 4. X-ray diffraction patterns of T i 0 2 (a) and physical mixtures of 0.78% V 2 0 5 / T i 0 2before (b) and after thermal treatment (c).
d
e 1
l
40
20
30
10
2Wgrad Figure 2. X-ray diffraction patterns of physical mixture of 3.1% V205/Mg0 before (a) and after thermal treatment for 48 h at 623 K (b), 673 K (c), 723 K (d), and 773 K (e).
a I
d
I
1200
'
"
"
1000
"
"
800
'
600
400
200
'
arR/cm-l
Figure 3. Raman spectra of physical mixture of 3.1% V20S/Mg0 before (a) and after thermal treatment at 723 K for 24 h (b) and 773 K for 48 h (c).
d, respectively, of Figure 2. It is evident from these that with increasing temperature the peaks of V2Os decreased and those of Mg2V207increased in intensity. The laser Raman spectrum of untreated V 2 0 S / M g 0 (Figure 3a) exhibited the bands of crystalline VZOS at 997, 702, 528,408, 285, and 145 cm-I. Of these, the sharp band at 997 cm-I is associated with the symmetric stretching vibration of terminal
V=O in crystalline V2Os. Thermal treatment of V205/Mg0 at 723 K led to a drastic change in the L R spectrum (Figure 3b) even after short reaction times of 24 h. The bands of V2O5 decreased in intensity and a series of new and intense bands appeared in the range of 800-960 cm-I. Thermal treatment of V205/Mg0 at 773 K for 48 h led to the disappearance of all V2Os bands (Figure 3c), and the spectrum contained only two groups of bands appearing at 800-1000 and 250-400 cm-', both of which are due to a- and P-Mg2V207.32 The calculated z / a value, Le., the ratio of the cationic charge of MgO to the sum of ionic radii of oxygen anion and support ~ a t i o n ,is~in~ line , ~ ~with the observed reaction between V2Os and MgO. The z / a value of MgO is nearly one, which is the lowest value of all the supports used in this study. The difference in z / a values of V2O5 and MgO amounted to 1.61 corresponding to a high affinity of the support oxide for the solid/solid reaction. Therefore, the reaction can occur at relatively low temperatures which was indeed observed. The spectroscopic results obtained in the foregoing provided no evidence for a distribution of V205 on Mg0.30 V 2 0 s / T i 0 2 .XRD patterns of TiO2 (Figure 4a) and of the untreated (Figure 4b) and treated (Figure 4c) V205/Ti02showed an intensive peak at 352 pm (28 = 25.28') corresponding to diffraction by planes (101) of T i 0 2 (anatase) and a weak peak at 325 pm (28 = 27.4') corresponding to diffraction by planes (1 10) of T i 0 2 (rutile). The intensity ratio of these two peaks was the same in all diffraction patterns. This indicates that the thermal treatment did not change the crystallographic phase of TiO,. The XRD pattern of the untreated sample (Figure 4b) exhibited additionally a weak peak at 438 pm (28 = 20.26') corresponding to V2O5. This peak had disappeared in the diffraction pattern of the treated sample (Figure 4c). Signals which could be indicative of other oxides of vanadium or of ternary TixV,,O, compounds were not observed. The laser Raman spectrum of Ti02 exhibited all bands for Ti02 anatase35 at 144, 199, 315, 395, 520, 640, 695, and 794 cm-'. Figure 5a shows only the range of 700-1200 cm-' of this spectrum which contains the weak anatase band at 794 cm-'. The spectrum of untreated V,OS/TiO2 (Figure 5b) showed the sharp V2Os band at 997 cm-I in addition to the anatase band at 794 cm-'. The spectrum of treated V2OS/TiO2(Figure 5c) showed only a very weak broad band between 950 and 1015 cm-', whereas (32) Kristallov, L. V.; Fotiev, A. A.; Tsvetkova, M. P. Russ. J. Inorg. Chem. 1982, 27, 1714.
(33) Roozeboom, F.;Fransen, T.; Mars, P.; Gellings, P. J. Z . Anorg. Allg. Chem. 1979, 449, 25.
(34) Fransen, T.; Van Berge, P. C.; Mars, P. In Preparation of Catalysts; Delmon, B., Jacobs, P. A., Poncelet, G., Eds.; Elsevier: Amsterdam, 1976; p 405. (35) Beattie, I. R.;Gilson, T. R. Proc. R.SOC.A 1968, 307, 407.
J . Phys. Chem. 1988, 92, 4702-4706
4702
a
I
I
1200
1000
800
A YR/Cm-l
Figure 5. Raman spectra of Ti02 (a) and physical mixtures of 0.78%
V205/Ti02before (b) and after thermal treatment (c) as well as of 1.4% V205/Ti02(d) after thermal treatment (signal averaging by addition of three scans). the band of crystalline V2OS at 997 cm-' had completely disappeared. The weak broad band is assigned to a two-dimensional surface polyvanadate based on the arguments given in ref 33 and 36. Since it is known that such bands are up to one order of
magnitude weaker in intensity than those of crystalline VZOS,the observed low intensity was not surprising. In order to ascertain the assignment of this band to surface vanadia, the original V205content of 0.78 wt % was increased to about 1.4 wt %. The sample of this physical mixture in which the V,Oscontent now corresponded to an approximately complete monolayer coverage was thermally treated under the same conditions. The LR spectrum of such a treated sample (Figure 5d) showed the signal between 930 and 1040 cm-' at enhanced intensity. It can be concluded from these results that V205has undergone complete dispersion of VZOS on TiOz (anatase) with the concurrent formation of a V20smonolayer. Thermally induced V2Os dispersion on Ti02 (anatase) has been observed previously but only at higher temperatures in the range of 823-923 K.26 For the dispersion of Moo3 on A1203,gas-phase transport has been ruled out on the basis of the low vapor pressure of MOO^.,^ Since the vapor pressure of V2OS is even lower at 773 K, it is assumed that the spreading of V2Os on TiO, does not occur by gas phase but rather by surface transport. The mechanism of this transport could not be revealed on the basis of the experimental data available from this study. Acknowledgment. We thank Dr. E. Gantner (Institut fur Radiochemie, Kernforschungszentrum Karlsruhe) for preparing computer programs and Mr. D. Steinert for recording the laser Raman spectra, J.X. thanks the International Seminar for a fellowship. Registry No. Vz05,1314-62-1;Ti02, 13463-67-7; Mg2VZO7, 1356863-3; MgO, 1309-48-4. (36) Roozeboom, F.; Medema, J.; Gellings, P. J. Z . Phys. Chem. (Wiesbaden) 1978, 111, 2 15.
Change of Organized Solution (Microemulsion) Structure with Small Change in Surfactant Composition As Revealed by NMR Self-Diffusion Studies Bjorn Lindman,**t Kozo Shinoda,* Mikael Jonstromer,t and Akira Shinoharat Physical Chemistry 1 , Chemical Center, Lund University, P.0.B 124, S-221 00 Lund, Sweden, and Department of Applied Chemistry, Faculty of Engineering, Yokohama National University, Tokiwadai, Hodogaya, Yokohama 240, Japan (Received: October 15, 1987; In Final Form: February 24, 1988)
A surfactant system was designed which has the capability of mixing large amounts of hydrocarbon and aqueous electrolyte with solution into homogeneous solution. In the system Cl2H2~OCH2CH2S0~Cal,2/I'-C~H1,0CH2CH(OH)CHzOH/water 8 wt % CaC12/decane, 3 wt 7%of total surfactant can mix hydrocarbon and aqueous solution over the entire range of solvent composition. Microemulsion microstructure investigations by self-diffusion give evidence for dramatic structural changes as surfactant composition is varied; changing the molar fraction of nonionic surfactant from 0.35 to 0.45 produces a change in the ratio of water-to-oil self-diffusion by more than a factor of lo4. The microstructure is of the oil droplets in water type at lower amounts of nonionic surfactant and of the water droplets in oil type at higher amounts. At intermediate surfactant ratios the surfactant mixture is balanced and bicontinuous microemulsion with low mean curvature surfactant layers is formed. The structural changes are discussed inter alia in conjunction with earlier studies of single nonionic surfactant systems where temperature determines the balancing of the surfactant.
Introduction Microstructure of microemulsions is a field of much current research activity.'-' Particularly intriguing are those surfactant systems which may mix equal amounts of water and oil into homogeneous solution and which may thus be denoted balanced.s-10 Of special interest are clearly such surfactants which *To whom correspondence should be addressed. 'Chemical Center, Lund. *Yokohama National University.
0022-3654/88/2092-4702$01.50/0
can achieve this mixing at low amounts of surfactant. By suitable choice of surfactant and of conditions (temperature, salinity, (1) Langevin, D.; Meunier, J., Eds. Physics of Amphiphilic Layers; Springer: Heidelberg, 1987; Springer Proc. P ~ Y s .Val. , 21. ( 2 ) Safran, S . A,; Clark, N. A., Eds. Physics of Complex and Supermolecular Fluids; Wiley-Interscience; New York, 1987. (3) Mittal, K. L.; Lindman, B., Eds. Surfactants in Solution; Plenum: New York, 1984; Vol. 3, pp 1501-1924. (4) Mittal, K. L.; Bothorel, P., Eds. Surfactants in Solution; Plenum: New York, 1987; Vol. 6, pp 1137-1484.
0 1988 American Chemical Society
