J . Phys. Chem. 1987, 91, 5937-5940 more sensitive to the existence of oxygen lattice vacancies and more stable to heating.& The weak peak at 1628 cm-’ observed when COz was adsorbed onto the preoxidized surface may be attributed to model A, which might be formed by the reaction of CO, with residual surface OH groups. The other bands assigned to model A would be hidden among those assigned to models B and C. N o evidence for the carbonate ion, model D, was found. It is interesting that the IR absorption in the carbonate region was stronger for pure CeOz (Figures 6 and 7) than for platinized ceria (Figure 5 ) . This is consistent with the COz TPD observation that more COz desorbed from pure CeO, than from platinized ceria at high temperature, 700 K. Although the pure CeOZhas a slightly larger surface area than the platinized ceria samples, the effect is too great for this explanation. One explanation, that the decomposition of COz,at the interface on the platinized samples fills the lattice vacancies where C02might have remained as carbonate had the Pt not been there, is consistent with the observations.
5937
5. Conclusions 1. C O is adsorbed on Pt supported by CeO, to form at least three linear species interpreted as on terraces, steps, and kinks. 2. C O on metallic Pt is easier to remove (oxidize) than C O on oxidized Pt surfaces. 3. C02decomposes at the interface between Pt and the support, CeO,, to produce CO adsorbed on Pt and to fill a surface oxygen vacancy on the CeO,. The amount of decomposition depends upon the oxidation state of the local CeOZ interface. 4. COz is adsorbed on CeO, to form both carbonate and bidentate carbonate. The CO, adsorption is enhanced by prereduction, suppressed by preoxidation, and not affected by preadsorption of water.
Acknowledgment. This research was supported in part by the Office of Naval Research and by the Robert A. Welch Foundation. Registry No. Pt, 7440-06-4; CeOz, 1306-38-3; CO, 630-08-0; C02, 124- 3 8-9.
An Investigation of the Micropolarity of Several Aqueous Nematic Lyotropic Liquid Crystals V. Ramesh, Hai-Shan Chien, and M. M. Labes* Department of Chemistry, Temple University, Philadelphia, Pennsylvania 191 22 (Received: December 29, 1986)
The shift in the absorption maximum of methyl orange with solvent polarity is used as a probe to characterize the micropolarity of several aqueous nematic lyotropic liquid crystals: sodium decyl sulfate (SDecS), potassium laurate (KL), myristyltrimethylammonium bromide (MTAB), and disodium cromoglycate (DSCG). Lyophase aggregates of SDecS, KL, and MTAB appear to be less polar than the corresponding dilute micellar dilute solution aggregates. The DSCG lyophase is found to be a very high polarity microenvironment.
phenolbetaine ET(30),10 merocyanine” and coumarin dyes,l2 2-(4-hydroxyphenylazo)benzoic acid,I3 and methyl orange14have been employed as polarity ,probes for micelles, inverted micelles, microemulsions, phospholipid bilayers, and polysoaps. In the present study, the sensitivity of the absorption maximum (kmax) of methyl orange (MO) to changes in solvent polarity has been used as a probe for the micropolarity of lyomesophase aggregates formed by SDecS, KL, MTAB, and DSCG in water. The structures of the surfactants, methyl orange, and reference compounds are given in Figure 1.
Introduction Sodium decyl sulfate (SDecS), potassium laurate (KL), and myristyltrimethylammonium bromide (MTAB) are among a group of surfactants that form nematic lyotropic liquid crystalline phases in specific concentration-temperature These lyomesophases are concentrated surfactant solutions composed of anisotropic micellar aggregates which are disklike (NL) or rodlike (Nc) in shape.4 In contrast to dilute micellar solutions in which a relatively porous cluster of surfactants are in equilibrium with dispersed monomers, the lyomesophase aggregates are both denser and larger in size.5 Recently, we have demonstrated that lyotropic liquid crystalline phases of SDecS, KL, and MTAB used as solvents for chemical reactions hold potential for reactivity control? Since lyomesophase aggregates are “extended micelles”, an understanding of their microenvironment with regard to both their micropolarity and microviscosity is of importance. The use of indicator dyes to probe the micropolarity of amphiphile aggregates is conventional in surfactant chemistry. Dodecylpyridinium iodide,’ bromophenol blue: chlorophenol red:
Methods and Materials M O (Aldrich) and DSCG (Fisons Inc.) were used as received. SDecS (Eastman Kodak) and MTAB (Aldrich) were recrystallized twice from 95% ethanollwater and dried under vacuum. KL was synthesized and purified by literature methods.” To ensure a constant pK, for MO, triply distilled water buffered to pH 7 (phosphate buffer) was used in the preparation of micellar and liquid crystalline samples.
(1) Yu, L. J.; Saupe, A. J. Am. Chem. SOC.1980,102,4879. (2) Yu, L. J.; Saupe, A. Phys. Rev. Lett. 1980,45, 1000. (3) Boden, N.; Radley, K.;Holmes, M. C. Mol. Phys. 1981, 42, 493. (4) Forrest, B. J.; Reeves, L. W. Chem. Rev. 1981,81,1, and references therein. (5) (a) Fendler, F. M. Acc. Chem. Res. 1979,12,111. (b) Fendler, F. M. Acc. Chem. Res. 1980, 13,7. ( 6 ) (a) Ramesh, V.; Labes, M. M. J . Am. Chem. SOC.1986, 108,4643. (b) Ramesh, V.; Labes, M. M. Mol. Cryst. Liq. Cryst. 1987,144, 257. (c) Ramesh, V.; Labes, M. M. J. Am. Chem. SOC.1987, 109,3228. (7) (a) Mukerjee, P.; Ray, A. J . Phys. Chem. 1966,70, 2144. (b) Mukerjee, P.; Cardinal, J. R.; Desai, N. R. In Micellization, Solubilization and Microemulsions, Vol. 1, Mittal, K.L., Ed.; Plenum: New York, 1977; p 241.
(8) (a) Funasaki, N. J . Colloid Interface Sci. 1977,60, 54. (b) Funasaki, N. J. Phys. Chem. 1979,83,1998. (9) Mackay, R. A.; Jacobson, K.; Tourian, J. J . Colloid Interface Sci. 1980,76,515: (10) Zachariasse, K. A.; Phuc, N. V.; Kozankiewicz, B. J . Phys. Chem. 1981,85, 2676. (11) deMayo, P.; Amiri, A. S.; Wong, S.K. Can. J . Chem. 1984.62,1001. (12) Fernandez, M. S.;Fromherz, P. J. Phys. Chem. 1977, 81, 1755. (13) Baxter, J. H. Arch. Biochem. Biophys. 1964,108,375. (14) (a) Kunitake, T.; Shinkai, S.;Hirotsu, S. J . Org. Chem. 1977.42,306. (b) Takagishi, T.; Nakata, Y.; Kuroki,N. J . Polym. Sci., Polym. Chem. Ed. 1974,12,807. (15) Saupe, A,; Boonbrahm, P.; Yu, L. J. J . Chim. Phys. 1983, 80, 7.
0022-3654/87/2091-5937$01SO/O 0 1987 American Chemical Society
5938
Ramesh et al.
The Journal of Physical Chemistry, Vol. 91, No. 23, 1987
TABLE I: Absorption Maximum
(L,J and Transition Energy (E,) Values for Methyl Orange in SDecS,KL, MTAB, and DSCG medium
surfactant SDecS
phase NC NL
_. 425 425 425 453
- . ._ 67.7
26.5 26.5 24.0 11.0
67.2 67.2 69.8 89.0
6.3 6.3 6.3
24 15 28 24
423 423 423 460
67.6 67.6 67.6 62.2
micellar
39.6 30.0 35.0 1.7
60.4 60.0 60.0 98.3
24 24 24 24
423 423 423 432
67.6 67.6 67.6 66.2
NC
15.0
85.0
24 24
476 463
60.2 61.9
micellar NC NL I
3.8 1.9
6.2 3.1
HZO CH,- (CH,),,-CH~-
CH,~CH,),- CH,OSO;N;
I~(cH,),B~
MTAB
SOecS
P-CH-cHz\
a T!
CH,-(CH,),-CH~-COOK+
NaOOC'
0
0
&)'COONa
OSCG
Methyl
E,, kcal mol-'
24 19 24 24
I
KL
nm
A,
7.0 7.0 7.0
NC NL
DSCG
temp, " C
56.0 56.0 60.0 98.0
micellar
MTAB
NH4Br
37.0 37.0 33.0 2.0
I
KL
5% composition HzO I-decanol
surfactant
Orange
0
4-oxo-4H ~-benroPyran2-carboxylic a c i d
0
4-Chromanone
67.3 67.3 63.2
TABLE 11: Observed Wavelength of Maximum Absorption of Methyl Orange in Various Reference Solvents at 25 O C and Their Corresponding ET Values
acetone I-butanol 1-propanol acetonitrile ethanol methanol ethanol/water (80/20) ethylene glycol ethanol/water (50/50) methanol/water (50/50) methanol/water (40/60) methanol/water (30/70) water " E Tvalues obtained from ref 16 and
0
413 412 415 416 418 420 428 436 447 453 459 46 1 463
42.2 50.2 50.7 46.0 51.9 55.5 53.6 56.3 54.6 57.6 58.9 58.4 63.1
11,
T
82
Chromone
Figure 1. Structures of the surfactants, methyl orange, and reference compounds.
The exact composition of the components, viz. surfactant, 1decanol, water, and in some case ammonium broinide, used in the preparation of the lyomesophases is indicated in Table I. A lo4 M solution of M O in the lyomesophases was made by stirring together for 12 h weighed amounts of the components and MO. After centrifugation, 250 pL of the liquid crystalline solvent was introduced into a 1-mm path length UV cuvette and examined under crossed polarizers with a Nikon microscope to confirm the nematic (NL and N,) nature of the samples. Samples were thermostated at the given temperature (Table I) for 10 min and absorption spectra recorded on a Perkin-Elmer 330 spectrophotometer interfaced with a Model 3600 data station.
Results and Discussion The absorption maximum of MO is sensitive to changes in solvent polarity. With increasing polarity of solvent, the intense, well-resolved long wavelength absorption band of M O moves to lower energy (Table 11). The sensitivity of & of M O to changes in solvent polarity can be a polarity probe for microenvironments provided the transition energy E p (calculated from) ,A correlates with the Dimroth-Reichardt solvent polarity parameter ET.I6 (16) (a) Reichardt, Ch. Angew. Chem., Inr. Ed. Engl. 1965, 4, 29. (b) Dimroth, K.;Reichardt, Ch.; Siepmann, T.;Bohlmann, F. Justus Liebig's Ann. Chem. 1963, 661, 1.
EP( kcal/moI) Figure 2. Plot of ET vs E , for methyl orange absorption in alcohol and alcohol/water mixtures: (1) 1-butanol,(2) I-propanol, (3) ethanol, (4) methanol, (5) ethanol/water: 80/20, (6) ethylene glycol, (7) ethanol/
water:
50/50, (8)
formamide, (9) methanol/water: 50/50, (10) meth-
anol/water: 40/60, (1 1) methanol/water: 30/70.
Since alcohols and alcohol/water mixtures are the best model solvents for surfactant systems,'O a correlation between E , and ET for alcohol and alcohol/water mixtures was attempted (Figure 2). A satisfactory linear correlation is obtained, with the exception of the data on methanol, which substantiates the validity of MO
Micropolarity of Lyotropic Liquid Crystals as a polarity probe. In fact, shifts in the absorption maximum of M O have been used previously as a polarity probe for the hydrophobic microenvironment of polymer micelles or polys~aps.'~ The absorption maximum (A,,,) and transition energy (E,) values for the nematic lyomesophases of SDecS, KL, MTAB, and DSCG are presented in Table I. The corresponding values for dilute micellar solutions are also indicated for comparison. The exact composition of the components used in the preparation of each of the lyomesophases is also indicated. N L represents the nematic lyomesophase composed of disklike micellar aggregates (these exhibit a bilayer morphology); N c represents the nematic phase comprising of cylindrical aggregates; and I represents the isotropic phase consisting of spherical micellar aggregates. SDecS, KL, and MTAB. Four significant features are evident from the results presented in Table I. 1. Since & for M O in micellar aggregates formed from dilute surfactant solutions is higher than that in lyomesophase aggregates, M O encounters a more polar environment in the former than in the latter. Hence, micelles formed from dilute aqueous solutions of SDecS, KL, and MTAB appear to be more polar than the corresponding lyomesophase aggregates. 2. In case of SDecS and KL, A, of M O shifts by about 27 nm from dilute micellar to lyomesophase aggregates, whereas in MTAB the shift is only 9 nm. In going from dilute micellar to lyomesophase aggregates in anionic surfactants (SDecS and KL), there appears to be a very significant change in polarity encountered by MO; whereas, in the cationic surfactant, MTAB, the polarity change is of a lower magnitude. 3. Since ,A, of M O in the bilayer (NL), cylindrical (Nc), and spherical (I) aggregates is identical, there is no apparent difference in micropolarity between NL, Nc, and I phases. 4. Dilute micellar solutions of SDecS and KL are more polar than MTAB, but lyomesophase aggregates of SDecS, KL, and MTAB reveal no micropolarity difference. In all three surfactant lyomesophases, M O encounters a polarity equivalent to that of ethanol. The fact that M O experiences a less polar environment in the lyomesophase aggregates of SDecS, KL, and MTAB is an indication that M O is located in the hydrophobic interior of the aggregate. DSCG. DSCG does not possess the amphiphile structure of conventional lyotropic mesogens, with hydrophilic head groups and hydrophobic alkyl chain tails, but is a disodium salt of an aromatic system which possesses hydrophilic substituents. Solutions of DSCG in water form a lyotropic nematic liquid crystalline phase at room temperature of low viscosity which can be easily aligned in a magnetic field." Hartshorne and Woodard,Is who first observed the lyophase, suggested a structure based on X-ray diffraction studies in which the nematic phase consisted of a translationally disordered cylinders of approximately 20 8, diameter and 200 8, length. As to the precise configuration of the dianionic molecules forming this lyophase, Hui and LabesI9 concluded, based on infrared studies, that the dianions form a conventional micellar aggregate with the polar heads of the V-shaped molecule pointing outward to the water continuum. In the center of this cylindrical extended micelle, one would expect to find "internal" water, which is hydrogenbonded to the secondary hydroxyl group of the glyceryl bridge between the chromone moieties. LydonZ0and Attwood and LydonZ1have suggested two other models for the DSCG aggregate, the first of which they have recently rejected as being inappropriate. The rejected model is that of a hollow square cylinder in which the sodium ions form salt bridges within the walls of the cylinder between dianions. The newer model consists of simple stacks of extended molecules in (17) (a) Lee, H.; Labes, M. M. Mol. Crysf.Liq. Crysf.1982,84, 137. (b) Lee, H.; Labes, M. M. Mol. Crysf.Liq. Crysf. 1983, 91, 53. (18) Hartshorne, N. H.; Woodard, G. D. Mol. Cryst. Liq.Cryst. 1973,23, 343. (19) Hui, Y . W.; Labes, M. M. J . Phys. Chem. 1986, 90, 4064. (20) Lydon, J. E. Mol. Cryst. Liq. Cryst. 1980, 64, 19. (21) Attwood, T. K.; Lydon, J. E. Mol. Cryst. Liq. Cryst. 1986, 4 , 9.
The Journal of Physical Chemistry, Vol. 91, No. 23, 1987 5939 TABLE 111 A,x, of Methyl Orange ( lo4 M) as a Function of DSCG Concentration in H20at pH 7 and 25 OC
DSCG concn, wt % 0.0 (H20)
molarity. M
A,.
nm
463
3.9 x 10-3 7.8 x 10-3
0.2 0.4 1.o
2.0 x 10-2
2.0
4.0 X
4.0
8.1 X
465 465 470 472 474
1.7 X lo-' 2.2 x 10-1 3.4 x 10-1
476 476
8.0 10.0 15.0
476
TABLE I V A- of Methyl Orange (lo4 M) in Aqueous Solutions (pH 7, 25 "C) of Various Substrates substrate concn. M A,. nm
..
chromone 4-chromanone 4-oxo-4H-1-benzopyran-2-carboxylic acid
benzoic acid 18-crown-6 H20 DSCG
1
x 10-3
1 x 10-3 1X 1 x 10-3 2.5 X lo4
463 463 463 463 463 463
476
3.4 X lo-'
columns which are tilted in a herringbone array with respect to one another. Although the earlier Lydon model would allow for internal water, the new model features water existing only between stacks. To assist in distinguishing among these models, information was sought regarding the microenvironment by studying guest dye molecule solubilization and spectroscopy. Relative to water, a bathochromic shift of 13 nm in the A,, of M O is observed in the nematic liquid crystalline phase of DSCG. Such a bathochromic shift has been observed previously only in a few polysoapsL4and is therefore an unusual situation. It implies that M O encounters a highly polar microenvironment in these lyomesophase aggregates in contrast to that in SDecS, KL, and MTAB. To ascertain whether effects other than polarity are involved in causing the observed bathochromic shift, a number of possibilities were examined. A,, of M O as a function of DSCGZ2concentration is shown in Table 111. At about the reported cmc of DSCG ( 1% by wt), a shift of 5 nm in the A, of M O is observed. Thereafter, A, increases steadily and plateaus at -8% by wt, the composition where the N c is formed. These observations indicate that DSCG of MO. aggregation is responsible for the observed shift in ,A, Spitzer and WolfP3 studied the interaction of DSCG with cationic dyes, e.g. methylene blue, and observed a bathochromic shift in the ,A, when solubilized in 8 mM DSCG solutions. The observed shifts were rationalized as due to the formation of ion-pair complexes between the cationic dye and the cromoglycate anion. In the case of methyl orange such an ion-pair complex is impossible, since the anionic dye would be repelled by the anionic surface of DSCG cylindrical aggregates. However, there exists the possibility that 7-complexation between M O and the aromatic moiety of DSCG could occur leading to a bathochromic shift in the electronic absorption maximum. In order to examine this of M O was determined in 4-chromanone, chropossibility, ,A, mone, 4-oxo-4H- 1-benzopyran-2-carboxylicacid, and benzoic acid (Table IV). A,, of M O remains the same as that observed in water, indicative that 7-complexation is either not occurring or, if it does, is not responsible for the observed bathochromic shift N
in A,,.
If DSCG lyophases contain internal water, one intriguing possibility is that the sodium cation would be included in the interstices of the canal, thereby being separated from the anion which would be located in the water continuum. Such an effect is rather similar to crown ether complexes. An experiment was performed in which M O was examined in an 18-crown-6 solution. (22) Goldfarb, D.; Labes, M. M.; Luz, Z.; Poupko, R. Mol. Cryst. Liq. Cryst. 1982, 87, 259. (23) Spitzer, J. C.; Wolff, D.A. Heterocycl. Chem. 1979, 16, 845.
J. Phys. Chem. 1987, 91, 5940-5943
5940
However, no bathochromic shift in the A,, of MO (Table IV) was observed. It therefore appears most likely that DSCG lyophases show an unusually high micropolarity as a consequence of internal water, and that MO is solubilized in that internal water to some extent. As has been pointed out in our earlier infrared study19 of DSCG lyophases, the secondary hydroxyl group of the chromone moiety
is probably involved in hydrogen bonding with this internal water, making it a rather special environment.
Acknowledgment. This work was supported by the US.Army Research Office under Contract No. DAAG29-84-K-0036. Registry No. SDecS, 142-87-0; KL, 10124-65-9; MTAB, 1119-97-7; DSCG, 15826-37-6; methyl orange, 547-58-0.
Surface-Enhanced Raman Spectra of Phthalarlne. Anlon-Induced Reorientation on a Silver Electrode Machiko Takahashi,* Hirotaka Furukawa, Masato Fujita, and Masatoki Ito Department of Chemistry, Faculty of Science and Technology, Keio University, Hiyoshi 3-14- 1 , Kohoku-ku, Yokohama 223, Japan (Received: January 5, 1987)
A remarkable potential-dependent SERS of phthalazine adsorbed on a silver electrode has been observed from a series of aqueous solutions containing C1-, I-, Br-, F,SCN-, S042-, or C104- as electrolyte anion. The Raman intensity of certain b2 modes was greatly enhanced accompanied by anion desorption during the cathodic sweep. However, the b2 species were absent from the SER spectra when the surfaces were fully covered with anions. The specific adsorption of anions influences the orientations of the adsorbed phthalazine. The importance of the charge-transfer effect on SERS is also suggested from the potential and excitation-wavelength-dependent spectral changes.
1. Introduction
Since the initial discovery of SERS from pyridine adsorbed on a silver electrode, a number of investigations for pyridinesilver systems have been reported and piles of information about the mechanism of SERS have been obtained. In order to elucidate the mechanism of SERS in more detail, it is quite important to examine the SERS from a series of molecules, because each SER spectrum reflects the characteristic adsorption properties of the molecule. We have recently investigated the SERS from several kinds of heterocyclic compounds. The S E R spectra from quinoxaline,l phthalazine,2 and phenazine3 adsorbed on silver surfaces exhibited the large intensity enhancement of certain nontotally symmetric vibrations relative to totally symmetric ones in contrast with the results from the pyridine-silver system. We also suggested that orientations of adsorbed quinoxaline and phthalazine can be controlled by changing the solution concentration or the applied potential. Recently, this has also been confirmed from S H G experiment of silver-phthalazine system by Voss et aL4 Here we report the SERS from phthalazine adsorbed on a silver electrode surfacc in various kinds of electrolyte anion solutions. The solvent and excitation wavelength dependencies of S E W were also observed. The important roles of the specific electrolyte anion adsorption are pointed out and the molecular orientations of the adsorbates are discussed. Finally, the importance of the charge-transfer effect is emphasized. 2. Experimental Section
Potential dependency and anion-induced spectral change of SERS were examined with an Ar ion laser (514.5 nm) excitation. The excitation wavelength dependency was obtained by an Ar ion laser (457.9, 514.5 nm) and a dye laser (R6G; 600.2 nm). Detection systems were the same as described in previous papers.'S2 A Ag working electrode was polished with alumina and rinsed with distilled water. When an organic solvent was used, a Ag (1) Takahashi, M.; Ito, M. Chem. Phys. Lett. 1984,103, 512. Takahashi, M.; Sakai, Y . ;Fujita, M.; Ito, M. Surf. Sci. 1986, 176, 351. (2) Takahashi, M.; Fujita, M.; Ito, M. Chem. Phys. Lett. 1984, 109, 122. (3) Takahashi, M.; Goto, M.; Ito, M. Chem. Phys. Lett. 1985, 121, 458. (4) Voss, D. F.; Nagumo, M.; Goldberg, L. S.; Bunding, K. A. J . Phys. Chem 1986, 90, 1834
0022-3654/87/2091-5940$01.50/0
electrode was dried by use of N 2 gas. The reference electrode was SCE. Phthalazine concentration was fixed to mol/L. The supporting electrolytes were KI mol/L), KBr ( 5 X and mol/L), KCI (10-I mol/L), K F (10-1 mol/L), KSCN (5 X mol/L), Na2S04 (10-1 mol/L), and LiC104 (10-I mol/L). The roughening process consisted of one oxidation-reduction cycle (ORC) a t 50 mV/s between respective potentials and 0.25 V in KCI, KBr, KF, and KSCN solutions, and 0.55 V in KI, Na2S04, and LiC104 solutions. The pH of the solution was adjusted by injecting the necessary amounts of HCl and NaOH solutions. Organic solvents, MeOH and acetonitrile, were dehydrated by using zeolite. The capacitance was measured by an AC impedance method.s A frequency response analyser ( N F S-5720) and a digital potentiostat (TOHOGIKEN PS-2000) were used.
3. Results and Discussion 1 . Effect of Anion Adsorption. The relative intensity varies remarkably with the electrode potentials in the S E R spectrum of adsorbed phthalazine in a lo-' M KCI aqueous solution as shown in Figure 1. At 0 V, all of the prominent lines are assignable to totally symmetric species and the overall spectral feature has a striking resemblance to the normal Raman spectrum of phthalazine in an aqueous solution. Raman signals at around 502 and 1257 cm-' exhibited a large apparent intensity enhancement with the cathodic potential sweep. These bands are assigned to b2 species. The a l bands near 940, 1150, 1300, and 1490 cm-', and the b, band at 755 cm-I, also increased in intensity. The maximum intensity of these bands was attained at about -0.7 V. The SERS intensity gradually decreased at more negative potentials. The electrolytic reduction of phthalazine began near -1.0 V4 and, at potentials more negative than -1.0 V, several unassignable bands appeared in the SER spectrum with appreciable intensity. In the present paper, our attention is focused on the adsorption of phthalazine and, therefore, the potential regions were confined to potentials more positive than -1 .O V. The Raman signal near 200 cm-I appeared with large intensity and broad band shape at 0 V. The band reduced in its line width with (5) Shimura, K.; Nishihara, H.; Aramaki, K. Boshoku Gijutsu 1986, 35, 289.
0 1987 American Chemical Society
