ROBERT A. ERB
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The Wettability of Gold’ by Robert A. Erb Chemistrv Department, The Franklin Institute Research Laboratories, Philadelphia, Pennsylvania 19108 (Received November 18,1967)
Experiments were run to determine the‘inherent wettability of gold by water. Measurements in pure steam at 101” gave a contact angle on gold of about 65” after 23,648 hr of continuous condensation. Radiotracer studies with oleic acid added to the refluxing water showed 0.209 monolayer on filmwise copper and 0.0130.015 monolayer on dropwise gold, indicating that the nonwetting behavior of gold is not caused by organic contamination. The average contact angle on freshly electropolished gold surfaces was found to be 62.6 f 3.4”. Heating experiments in quartz apparatus showed three sources of erroneously low contact angles that could readily be present in this type of experiment: (1) inorganic contamination of the surface; (2) surface roughness; and (3) equilibrium not established with water vapor. Cumulative evidence indicates that the equilibrium contact angle of water on a clean, smooth gold surface is between 60 and 65”.
Introduction I n 1962 we undertook a program to develop permanent systems for producing dropwise condensation of steam on metal condenser surfaces.2 This program has been aimed toward the goal of increasing the heattransfer efficiency of distillation processes for desalination. Early in the study we were surprised to find that silver produced dropwise condensation of steam despite extensive measures taken toward maintaining the purity of the refluxing water and the cleanliness of the apparatus. This experience was contrary to the statementa that all pure liquids would spread spontaneously on high-energy surfaces, with the exception of certain organic liquids which were “autophobic” or hydrolyzed on contact with the solid surface. Nevertheless, because of the persistence of the results with silver, we predicted that gold also (from Cu-Ag-Au in the periodic table) would exhibit some hydrophobic behavior. Earlier experimental papers had reported both high (Plaksin and B ~ S S O ~ 61-62”> O V , ~ and low (Bartell and Smith,6 7”) contact angles of water on gold. White6 then produced new experimental evidence for a high contact angle of water on an oxide-free surface of gold and for a sharp contrast between results on gold and results on base metals. New concepts by Fowkes’ have provided a theoretical basis for understanding the nonwetting behavior on gold by consideration of the dispersion-force nature of the interaction between water and the oxide-free metal surface. Our continuous-condensation experiments in pure steam confirmed that gold was nonwettable under these conditions.8 I n addition, other noble metals, such as palladium and rhodium, were also seen to produce dropwise condensation. This is in contrast to some 13 different base metals tested, all of which produced filmwise condensation. Table I lists the average advancing contact angles9 measured on the same 99.9% gold sample after 3650 hr and after 23,648 hr under continuous-conThe Journal of Physical Chemistry
densing conditions. The average angle varied less than 1” in more than 2 years. In 1965, Bewig and Zisman’” published experimental results in which there were consistent 0” contact angles of water on gold and platinum after heating treatments in high-purity atmospheres. They concluded that if a water drop does not spontaneously wet a “clean” metal surface, it is probably an indication of measurable contamination by a hydrophobic organic impurity The following three experimental sections describe some of our programs designed to help clarify the situation by giving additional information toward the following questions: (1) is the dropwise condensation on gold caused by preferential attachment of organic contamination (as compared with filmwise base metals) ; (2) what factors cause lowering of the contact angle on gold, particularly in the heating-type experiments; (3) what is the best value for the equilibrium contact angle of water on a clean, smooth gold surface? (1) Paper presented a t the Symposium on Wettability of Metal Surfaces, Division of Colloid and Surface Chemistry, 153rd National Meeting of the American Chemical Society, Miami Beach, Fla., ,April 1967. XI 12) R. A. Erb and E. Thelen, Ind. Eng. Chem., 57, No.10,49(1965). (3) H . W. Fox, E. F. Hare, and W. F. Zisman, J . Phys. Chem., 59, 1097 (1955). (4) I. N. Plaksin and 8 . V. Bessonov, Dokl. Akad. Nauk SSSR, 61, 865 (1948). (5) F. E. Bartell and J. T. Smith, J . Phys. Chem., 57, 165 (1953). (6)M. L.White, ibid., 68, 3083 (1964). (7) F. M. Fowkes, ibid., 67, 2583 (1963); Ind. Eng. Chem., 56, No. 12,40 (1964). (8) R.A. Erb, J . Phys. Cham., 69, 1306 (1965) (9) The “advancing” and “receding” angles on the vertical surface in ref 8 are both advancing in the sense that the drops are aIways growing; they actually are “upper” and “lower” angles which are smaller and greater, respectively, than the equilibrium contact angle on a horizontal surface because of the gravitational distortion. An average of the “upper” and “lower” angles is used in Table I as the reported contact angle. (10) K. W. Bewig and W. A. Zisman, J . Phys. Chem., 69, 4238 (1965).
THEWETTABILITY OF GOLD
2413
Table I: Contact Angle of Water on 99.9% Gold under Continuous-Condensing Conditions (101')
Lower angle, deg Upper angle, deg Av advancing angle, deg
Time, 3650 hr (0,42year)
Time, 23,648 hr ( 2 . 7 year)
85 f 3 46 zk 2 65.5
77.6 f 1 . 0 51.8 zk 1 . 9 64.7
CU-NI , , ON
Experimental Program. I. Radiotracer Studies under Continuous Condensing Conditions An experimental procedure was devised to resolve the question as to whether gold as a condensing surface in steam picks up and retains organic hydrophobic materials more readily than base metals (a possible explanation for its dropwise behavior). Oleic acid, a classical promoter of dropwise condensation, was chosen to be used in a radiotracer study. Oleic acid-l-l*C with a specific activity of 9.0 mCi/mmol (Tracerlab Inc.) was used in this study. A solventless 0.0331-mg (0,001 mCi) sample was introduced into 6 1. of pure water (redistilled from alkaline permanganate) refluxing in a closed system at 101", giving a concentration of 0.005 mg/l. Previously, 16 flats, 2.5 X 7.5 cm, had been attached to a cooled core. At a period of 30 min after the introduction of the tagged oleic acid, small samples of steam and water were taken from the apparatus and were found to be radioactive. After 10 days the radioactivity in the water was greatly diminished, indicating that adsorption of the oleic acid had taken place on walls and condensing surfaces. Measurements of the surface concentration of oleic acid were made on the samples in a cell with fixed geometry by means of a Geiger counter with the substrate types previously calibrated for backscatter. Fractional monolayer coverages were calculated by using a value of 40 A2/molecule for a close-packed monolayer, which corresponds to 2.5 X 1014molecules/cm2. Figure 1 shows some of the experimental results. The surface concentration was 0.209 monolayer on a copper condensing surface (which showed the usual filmwise condensation) and only 0.015 and 0.013 monolayer on two gold surfaces (with the usual dropwise condensation). This is a strong indication that the nonwetting behavior of gold is not caused by organic contamination. The highest surface coverage was noted on the sample which is a mixture of poly(tetrafluoroethy1ene) (TFE) and aluminum oxide (Tufram, General Magnaplate). However, the oleic acid is mostly removed from the surface of the TFE polymer after one change of water. The chromium is an example of a wettable system which shows low adsorption of the oleic acid. Titanium,
Au
t ~
AWa
.
ON
CU-NI,
Au
t
A~OI
Figure 1. Radiotracer study of the pickup of oleic acid contamination by sufaces of gold and other materia!s under continuous condensing conditions: (left-hand side) surface coverage 10 days after adding 0.0311 mg of oleic acid to 6 1. of refluxing water, 101'; (right-hand side) surface coverage after two sLibsequent changes of water.
molybdenum, and type 316 stainless steel samples also showed wettable behavior and low adsorption. Palladium and rhodium, on the other hand, showed dropwise behavior and low adsorption, tending to confirm the conclusion that the nonwettability of noble metals under continuous-condensation conditions is not due to organic contamination.
Experimental Program I1 : Wettability of Electropolished Gold Surfaces The basic problem in the measurement of the contact angle of water on gold is in the preparation of a smooth surface which is free from both hydrophobic organic contamination and from hydrophilic inorganic contamination. One way to accomplish this is by deep electropolishing, in which 50 p or more of the surface is removed after the last mechanical polishing step. This technique was used by Trevoy and Johnson,ll who obtained angles of 0-11" for water on aluminum, copper, nickel, zinc, and other metals. We experimented with several types of electropolishing baths for gold and found that the most consistent results were obtained with the Meakin12 solution, which is made of glacial acetic acid, hydrochloric acid, and water. This produces smooth, specularly bright surfaces, which are free of possible residues from phosphoric acid or potassium ferrocyanide such as are found in cyanide baths. We operated the electropolishing at room temperature a t applied potentials of from about 20 to 23 V and at currents of 450-800 mA for the small cylindrical samples which are about 0.6-0.7 cm in diameter and 0.4-0.7 cm in height. After electropolishing, the sample is rinsed in water, with the final rinse in water which has been redistilled by alkaline permanganate. The water is shaken off the (11) D. J. Trevoy and H. Johnson, J . Phya. Chem., 62, 833 (1968). (12) J. D. Meakin, Rev. Sci. Instrum., 35,763 (1964).
Volume 78, Number 7 July 1968
ROBERTA. ERB
2414 sample and the contact angle of an applied sessile drop is measured rapidly, with a telemicroscope with a protractor eyepiece. The following gold materials were studied: (1) 99.99% pure monocrystalline gold (Semi-Elements Inc.), (2) 99.999% pure monocrystalline gold (Materials Research Corp.); and (3) 99.9999% pure polycrystalline gold (Semi-Elements Inc.) . The measured contact angle of water on gold (average for six samples, 21 drops) on surfaces freshly electropolished with removal of 50-p surface layer, was 62.6 f 3.4". There did not appear to be any significant wettability differences between mono- and polycrystalline surfaces or among the three levels of purity. The only great deviations (lowered angles) from the 60-65" region were observed when the sample surface became discolored, hazy, or roughened from improper current density or from improper agitation during the electropolishing process.
Experimental Program I11 : Wettabiiity Studies on Gold Heated in Pure Atmospheres As noted above, our experiments under continuouscondensing conditions and with freshly electroplated surfaces support a contact angle of water on gold of about 60-65". However, the experimental results of Bewig and Zisman,'O with zero contact angles on gold samples heated to near the melting point in pure atmospheres, have raised many basic questions, not the least of which is, "Is the inherent equilibrium contact angle zero or isn't it?" White and Drobek13 have provided some insight into the divergence of results with their recent paper on the effect of residual abrasives on the wettability of polished gold surfaces. Residual aluminum oxide abrasives, shown to be present by electron diffraction, were seen to cause lowered contact angles. They measured angles of 62" on clean, vacuum-deposited gold with and without heating to 1000" in oxygen. (Even with residual oxides present, the reported values were not less than 34" .) Two new apparatus were built for our study. Figure 2 shows a quartz apparatus for the induction heating of the gold sample in pure atmospheres and for the measuring of the contact angle without opening the system to the laboratory air. The second quartz apparatus, shown in Figure 3, uses a vertical tube furnace for heating the sample, which is then elevated to a cooler zone for measurement of contact angles. One function of the second apparatus was to determine whether any special surface phenomena were caused by induction heating per se. No differences were found between the two methods. As steps to avoid organic contamination: (1) greasefree fittings were used throughout; (2) all inner surfaces of the apparatus were cleaned with sodium dichromate-sulfuric acid; and (3) all helium and hydrogen The Journal of Physical Chemistry
SWAGELOK TEE FITTING QUARTZ VESSEL QUARTZ DELIVERY TUBE FOR WATER WATER COOLED COILS
GOLD SAMPLE
THERMOCOUPLE WIRE GROUND JOINT
i[b
GAS INLET (FROM LIQUID NITROGEN TRAP 1
HOLES THROUGH INNER TUBE
\
SWAGELOK TEE FITTINO
5CM
'
-[-THERMOCOUPLE
LEAD,
Figure 2. Quartz apparatus for the induction heating of gold specimens in pure atmospheres and for the measuring of wettability without subsequent openings. 0
GAS TIGHT SYRINGE
GOLD SAMPLE THERMOCOUPLE
I
GAS OUTLET TO CHECK VALVE AND FLOWYETER
11 1
1-QUARTZ
TUBE QUARTZ INNER TUBE SUPPORT FOR SAMPLE (VERTICAL MOVEMENT)
Figure 3. Quartz apparatus for heating gold sample in the tube furnace in a pure atmosphere, with subsequent movement of the sample to a cool zone for wettability measurement.
gas flows were passed through a double liquid nitrogen trap filled with glass beads. (Dry Ice-acetone was used with oxygen.) The contact angles were measured at room temperature in the gas stream flowing a t about 100 cm3/min. Table I1 gives some results on gold samples which had been prepared with a final step of deep electropolishing (greater than 50-p removal) to avoid the (13) M. L. White and J . Drobek, J . Phys. Chem., 7 0 , 3432 (1966).
2415
THEWETTABILITY OF GOLD presence of abrasive residue on the surface. The samples were held a t the indicated temperature for 3 min. A number of things can be noted in the table. First, no zero angles were obtained on any of these gold samples. However, the angles observed, mostly in the range from 6 to 21", were consistently lower than these obtained in our other experiments, or those of White and Drobek.'3 Other things of note are: (1) there was a very irregular trend toward lower angles with high
Table 11: Contact Angles after Heat Treatment"
Sample
1. 6-9's polyxn gold
(sequence without repolishing)
2. 6-9's polyxn gold 3. 5-9's monoxn gold (sequence) 4. 99.45% nickel, final diamond polish (sequence)
Atrno-
Temp,
sphere
"C
Range angles, deg
Helium Helium Helium Helium Helium Helium Oxygen Oxygen with water in the base of the cell Helium Helium
500 600 700 800 900 1000 1000 500
14,7-19.3 18.3 7.1-16.3 6.0-14.3 15,1-30.5 5.0-19.7 12.5 30.6-37.7
1000 400 700 950 1000 1000 1000
15.O-20.8 25.0 9.9-18.0 13.6 0 0 9.5-21 . 0
Oxygen Helium Hydrogen
polishing. Another, very serious, contamination possibility comes from concentration of oxide-forming bulk impurities a t the surface when the sample is heated to near its melting point. Plumb and Thakkar14 indicate that even at room temperature a gold sample containing only 0.01% copper will rapidly become coated with a layer of copper oxide. The diffusion coefficients increase greatly with increasing temperature, and it might be reasonable to expect that bulk impurities, even as low as 1 ppm, could lead to serious surface contamination of a thick gold sample when heated to just below the melting point (1063"). I n the work of Bewig and Zisman,'O where zero angles were obtained, the solid-gold samples were heated "white hot," which was described as a condition just a t the edge of melting. I n their work, finite contact angles were observed when the samples were heated to "dull red heat" (which is roughly from 605 to 850"). This difference might be explained in large part as due to surface contamination by bulk impurities.
Table 111: Sources of Reduced Contact Angle Experimental problem
a Measured a t room temperature in the dry gases given in the table; the gold was mechanically polished, followed by Meakin electropolish.
temperatures; (2) there were no great differences between monocrystalline and polycrystalline gold; (3) when a second drop was placed over a first drop, the contact angle of the second drop was always higher than that of the first drop; (4)the highest angle was for the one experiment in which water was kept in the base of the induction-heating cell during heating and angle measurement (these results in (3) and (4) pointed to the important effect of water vapor, which is discussed later) ; and (5) the nickel samples heated in oxygen and helium produced a zero contact angle, indicating freedom from organic contamination after heating and reduction; in hydrogen, nonzero angles were obtained, which might be attributed to the nonwettability of an oxide-free metal surface, as seen with gold. Table I11 lists eight possible causes of reduced contact angle on gold surfaces. The first five are related to inorganic (hydrophilic) contamination of the surface. The effect of residues of abrasives has been covered we11 by White and Drobek.13 This can be readily avoided by their methods or by deep electro-
Solution
I. Inorganic Contamination of Surface 1. Residues of abrasives Deep electropolishing ( S O P)
2. Impurities from bulk moving to surface under elevated temperatures
Use high-purity gold (6-9's); avoid too high temperatures 3. Deposition of material from wall of Use quartz rather vessel under high temperatures than borosilicate glass 4. Deposition of material from gas Avoid presence of stream (fibrous glass or trap packfinely divided main@;) terial in gas stream 5 . Nonvolatile residues from electroUse volatile electropolishing baths (such as those polishing reagent containing phosphoric acid, (e.y., Meakin soluferrocyanide) tion)
11. Surface Roughness 1. Surface not smooth as prepared Smoothing by polishing or otherwise 2. Surface roughening by thermal Check surface by elecetching tron microscopy; avoid conditions of temperature and atmosphere which produce thermal etching 111. Equilibrium Not Established with Water Vapor 1. Gold sample heated and then cooled Introduce saturating and contact angle measured in vessel in gas stream gas stream free from water vapor past the cold trap (passed through liquid nitrogen trap); contact angle observed is not the equilibrium contact angle ~~
(14) R. C. Plumb and N. Thakkar, J.Phys. Chem., 69, 439 (1966). Volume 7.9, Number 7 July 1968
2416
ROBERTA. ERB
-_.
I n our various experiments, the contact angle upon addition of water vapor has increased by 2040" over the angle measured upon cooling in the dry gas. Table IV: EKect of Addition of Water Vapor to the Atmasphere for the Measurement of Contact Angle Sample: Monocrystalline gold (69's), eleetropolished; heated in helium at 960'; cooled under dry helium Contact angles under dry helium: 11.9-13.8° Contact angle 15 min after water vapor added to helium stream: 53.2'
Figure 4. Electron rnirrogmplr i d mrinre oi previously electropolished !M.!)!I!l!)~~&I specimen niter heating for 3 min in oxygen at 1000'.
Contamination of the sample surface by deposition from borosilicate walls, by fibrous glass particles, and by electropolishing residues are also lisbed. We have observed particles of fibrous glass on one sample, with resulting lowered contact angles, and have replaced this packing with glass beads in the saturating vessel in Figure 3. We have also noticed adverse effects from nonvolatile residues from electropolishing in cyanide baths. The second section in Table 111 lists sources of reduced contact angle from surface roughness. The roughness may be considered to be related to contact angle by Wenzel's law," which states that the roughe,. . We have observed a ness r = cos B.,.,.t/cos number of instances in which a detectable roughness was introduced in electropolished gold surfaces by heating in pure atmospheres (oxygen and helium). Figure 4 is an electron micrograph of the surface of one 99.9999% gold sample after heating to 1OOO" in oxygen for 3 min. The faceted surface structure indicates that thermal etching has taken place. The third section in Table I11 lists one of the most serious sources of error leading to the reporting of abnormally low contact angles in the heating type of experiment. For an equilibrium contact angle to be measured, the measuring cell should be essentially saturated with water vapor. The heating experiments (ours included) in which liquid nitrogen traps are used have no water in the flowing dtmosphere, and after the heating has driven all adsorbed water from the gold surface, there is no opportunity for the adsorbed water to be restored. When a drop is placed on the gold substrate in the flowing stream of dry helium, water only then becomes present in the system, although at a very low partial pressure in the flowing gas atmosphere; this presence of water is associated with a higher contact angle when a second drop is placed on the surface, as noted earlier. Table IV shows an example of what happens when water vapor is introduced directly into the gas stream after heating and cooling is complete. The Journal OJ P h u d Chnniatru
The basis for the apparently substantial effect of water vapor on the contact angle of water on a previously heated surface of gold is not clearly understood. However, it would be reasonable to expect a higher contact angle if the solid-to-gas interfacial tension of gold is lowered, as it would be by adsorption of water in the presence of a high partial pressure of water vapor. It is to be noted that a duplex layer of water on the gold is not present (which would lead to liquid water spreading over bulk water, with a resultant 0' contact angle). These sources of lowered contact angle are cumulative and can indeed cause erroneous zero angles. To illustrate, we heated one 99.9999'% gold sample, previously electropolished, just up to the melting point in helium such that the edges were seen to deform slightly before the temperature was reduced. Surface roughening could be seen under an optical microscope, and we would also suspect migration of base metals from the bulk to the surface. When the sample was cooled in flowing dry helium, a zero contact angle was observed. However, one of the contributing factors to the low angle was removed by adding water to the Table V: Summary of Experimental Vslues Supporting the 6045' Contact Angle on Clean, Smooth Gold Angle.
Investi(lbt0ra
Plakain and Bessenov (1948)
Erb (1965) White and Drobek (1966)
Experimental mnditiom
dcg
61 61 62 65.5 62
In oxygen In nitrogen In CO, Condensation, 3650 hr (101") Vacuum deposited, with and without heating in Oz at
61
Diamond polished and heated in O2at 1MH)O Condensation, 23,648 hr (101') Electropalished (Meakin solution)
1 m o
Erh (1967)
64.7 62.6
(16)
R. N. Wenrel. I d . Ew.Chm.. 28,QgS (1936).
RADIATION (LASER)-~NDUCED DEGRADATLON OF AROMATIC COMPOUNDS saturating vessel. The contact angle was measured after 30 min and was found to be no longer zero but 31.7'.
Conclusion The experimental evidence discussed here helps to provide an explanation for low contact angles of water on gold which have previously been reported in the literature and supports a conclusion that water has a high equilibrium contact angle on a smooth, uncontaminated surface of gold. Table V is a compilation of results from several different investigators and experimental procedures. The best value for the equilibrium
2417
contact angle of water on gold a t room temperature appears to be about 62". A value of this magnitude is also in line with theoretical considerations of Fowkes7 and Thelen16on the interfacial relationships with oxidefree metal surfaces.
Acknowledgment. This work was supported by the Office of Saline Water, U. S. Department of the Interior, under Contract No. 14-01-0001-744. Special acknowledgment is due to Harold L. Heller for major contributions to the wettability studies and to Hagop E(.Kevorkian for the radiotracer studies. (16) E. Thelen, J . Phys. Chem., 71, 1946 (1967).
Focused, Coherent Radiation (Laser)-Induced Degradation of Aromatic Compounds by Richard H. Wiley and P. Veeravagu Department of Chemistru, Hunter College of the City University of New Y o T ~New , York, N e w York i002$ (Received November 10,1067)
The principal gaseous decomposition products formed from aromatic compounds in a focused laser beam experiment are methane and acetylene. Minor amounts of ethane, ethylene, propane, propene, allene, and a four-carbon acetylenic hydrocarbon are also formed. The ratio of methane to acetylene is very low (4:85) for polycyclic types (naphthalene, anthracene, phenanthrene, and biphenyl) and higher (40 : 10) for monocyclic (tolulene, xylene, and mesitylene) types. This difference suggests a reactant-dependent decomposition mechanism. The possibility of a nonsource-dependent, energetic carbon plasma-type reaction is suggested by the formation of high yields of acetylene from dissimilar substances and the absence of marked differences for decompositions run in the presence of nitrogen, oxygen, or hydrogen.
The physical conditions existing at the focal point of a focused, coherent radiation (laser) beam have been the subject of some spe~ulation.'-~ I n theory the beam can be focused to a lo-" cm2 area to provide, from normal (millisecond) burst, power densities of l O I 5 W/cm2 and electromagnetic field (optical wavelengths) of 10" V/cm. The temperature rise associated with this phenomenon is difficult to estimate,l12 and "temperature" assignments of l O 9 O E ( are probably not ~~ the meaningful. Electrical breakdown O C C U ~ S " and phenomenon presumably involves plasma formation. a Some experiments designed to evaluate the nature and magnitude of these effects have been reported. From a chemical point of view, the exposure of a material to such an intense power concentration should (and in fact does) result in extensive degradation.8 It is of some interest to determine, if possible, whether such focused-laser degradations show product patterns
which correlate with extrapolated thermodynamic data or with data on reactions of fragments and atoms (1) A. L. Shawlow, Science, 149, 13 (1965). (2) C. H. Townes, Bwphys. J . , 2 , 325 (1962). (3) J. Berkowitz and W. A. Chupka, J . Chem. Phys., 40, 2735 (1964) ; see also ref 4. (4) M. W. Dowley, K. B. Eisenthal, and W. L. Peticolas, Phys. Rev. Lett., 18, 531 (1967). These authors describe conditions involving laser-produced dielectric breakdown in liquids. These data indicate, and the authors are indebted to a reviewer for pointing this out, degradation in solids via internal scattering and absorption of energy on the interfaces. It is difficult to see, however, how crystal structures can survive under these conditions so as to participate in controlling energy-transfer schemes. (5) R. H. Wiley, Ann. N . Y . Acad. Sci., 122, 685 (1965). (6) R. C. Rosen, M. K. Healy, and W. F. McNary, Jr., Science, 142, 236 (1963). (7) A. G. Meyerand, Jr., and A. F. Haught, Phys. Rev. Lett., 11, 401 (1963). (8) R . H. Wiley, N. Dunski, and T . K. Venkatachalam, J . HeterocycEic Chem., 3, 117 (1966).
Volume 79, Number 7 July 1068
