ADSORPTION OF ORQANIC COMPOUNDS ON STEEL
549
(17) JOLIBOIS,P . : Compt. rend. 199, 53 (1934). (18) JOST, W.: Diffusion und chemisehe Reaktion in festen Slogen, p. 16. Photo-Lithoprint, Edwards Brothers, Ann Arbor, Michigan (1943). (19) LAVIN,G . I . , A N D STEWART, T. B.: Proc. x a t l . Acad. sci. U. s. 16, 839 (1929). (20) LISDER,E. G . : Phys. Rev. 98, 679 (1931). A. E . , A N D LAVROV, F. A , : 2.Physik 69, 690 (1930). (21) MALINOVSKI, (22) MOTT,N. F.: Trans. Faraday SOC. 86, 472 (1940). R . E.: J. Inst. Metals 29, 529 (1923). (23) PILLIKG,N. B., A N D BEDWORTH, (24) RHISES, F. K.,JOHNSON, W. A , , A N D ASDERSON,W. A , : Trans. .4m. Inst. Mining hlet. Engrs. 147, 205 (1942). (25) RODEBUSH, W.H., A N D WAHL,ILI. H . : J. Chem. Phys. 1, 696 (1933). (26) STVECKLEN, H.: Handbuch der Physik, Vol. XIV, p . 108.J. Springer, Berlin (1927). (27) THOMPSON, M.DE K A Y :The Total and Free Energies of Formation of the Ozides o j Thirty-Two Metals. The Electrochemical Society, Inc., New York (1942). (28) UREY,H.C., A N D LAWN,G . I.: J. Am. Chem. SOC.61, 3286,3290 (1929). (29) VOLUER,M.: K i n e t i k der Phasenbildung. Photo-Lithoprint, Edwards Brothers, A n n Arbor, Michigan (1943). (30) WAQNER,C., A N D GRUENEW~ALD, K.: z. physik. Chem. BM, 455 (1938). (31) WILKINS,F. J . , ASD RIDEAL,E . K.: Proc. Roy. SOC.(London) 128. 394 (1930).
ADSORPTION OF POLAR ORGANIC COMPOUNDS ON STEEL' E. L. COOK2
AND
X O R M S S HACKERM.4S
Department of Chemistry, University o j Texas, Auslin, Teras Received April 87, 1950
Adsorption from solution of polar organic compounds on metal surfaces has been studied by numerous investigators because of its importance in corrosion prevention and in lubrication. Rhodes and Kuhn (7) studied the corrosioninhibiting character of acridine and some of its derivatives and found that the higher-molecular-weight compounds of the same homologous series were generally better inhibitors against acid attack of metal, presumably because of greater extent of adsorption. Mann, Lauer, and Hultin ( 5 ) demonstrated the inhibition of acid attack on steel with mono-, di-, and tri-alkylamines containing from one to five carbon atoms in the alkyl group. They found that both increasing the number of substituent radicals and increasing the chain length of the radical improved the effectiveness of the inhibitor. Zisman (11) showed that fatty acids, the more basic amines, and any other polar molecules capable of ionizing at an oil-water interface were many times more adsorbable than were alcohols, esters, ketones, or other molecules not capable of ionization at such an interface. Later work by Bigelow, Glass, and Zisman (1) showed these same large differences in adsorptivity at oil-metal interfaces. Presented before the Division of Colloid Chemistry a t the 115th National Meeting of the American Chemical Society, which was held in San Francisco, California, March, 1919. ' Present address : Magnolia Petroleum Company, Dallas, Texas.
550
E. L. COOK AND NORMAN HACKERMAN
Bowden, Gregory, and Tabor (2) showed that transition from smooth to stick-slip action of a sliding block occurred at or near the melting point of a fatty acid lubricant when used on an unreactive surface such aa platinum or silver. Use of a more reactive surface such as iron permitted the temperature to be increased to approximately 3OOC. above the melting point of the acid used before the stick-slip action occurred. The higher temperature was about the same as the melting point of the respective metal soap of the fatty acid. Further work by Tingle (10) with the same type of apparatus revealed that even reactive surfaces such as cadmium, copper, and magnesium were not lubricated above the melting point of the fatty acid if the metal surface had been freshly cut in an oxygen-free and moisture-free system. Thus, physical adsorption or chemical adsorption or reaction waa obtained, depending on the condition of the surface. In all of the studies mentioned above, the extent of adsorption at the metalsolution interface was indicated indirectly by some property of the system, i.e., extent of corrosion or lubrication characteristics. Very few direct determinations of the adsorption of polar organic compounds on metal surfaces have been reported. This is largely because of the low ratio of surface area to weight inherent in such nonporous materials. The adsorption of several high-molecularweight fatty acids on platinum and nickel has been measured by titration of the equilibrium solution (8). A recent paper (3) describes similar work for other metals and sorbates wherein the change in solution concentration waa determined with a surface balance. The present study was undertaken to develop a method whereby the extent of adsorption from solution could be determined directly from the change in weight of the adsorbent and to obtain further information on the nature of adsorption. EXPERIMENTAL
Materials A 150- to 200-mesh particle-size fraction of SAE 1020 steel powder’ was used aa the adsorbent throughout the study. The metal from which the powder was prepared by atomization had the following composition: 0.20 per cent carbon; 99.1 per cent iron; the remainder manganese with traces of phosphorus and sulfur. The specific surface area, obtained by the B.E.T. method of gas adsorption using krypton at - 195.8OC., was found to be 0.10 m.$/g. Baker’s C.P. thiophenefree benzene was used aa solvent in all of the experiments. The polar organic compounds studied were limited to n-alkyl carboxylic acids, amines, and alcohols, and esters of the long-chain acids. The acids, alcohols, and esters were of C.P. grade, obtained from leading supply houses. The amines were of the “distilled” grade produced by h o u r and Company and were of 90 per cent purity or better, the impurities being amines containing two CH1 groups more or leas than the subject compound. Method The type of apparatus used is shown in figure 1. Approximately 25 g. of steel powder was placed in the adsorption tube and 200 ml. of benzene poured over
* Obtained from Exomet
Incorporated, Connesut, Ohio.
ADSORPTION OF ORGANIC COMPOUNDS ON STEEL
55 1
the powder and through the tube. A fritted-glass plug (Corning Pyrex, E porosity) at the bottom of the tube permitted free flow of solution, but retained the metal powder quantitatively. The tube with steel sample was placed over a vacuum cup, and air, dried with Drierite, was pulled through the powder until all the benzene had been evaporated. The tube and contents were accurately weighed to within 0.1 mg. The difference in weight of the sample tube with and without the steel was taken as the weight of steel powder. Washing of the powder with benzene waa repeated and the sample reweighed until constant weight was !+XK AN0 ROO SUPPORT FOR RASING AND LOWER ING THE NBE
SOLUTKW L E V U (WITH POWDER TUBE LOWERED) PWDER TUBE STEEL PWC€R FRITTED GLASS
PLUG
FIG.1. Adsorption apparatus obtained, the final weight being considered aa that of the clean powder prior to any adsorption. The adsorption process was carried out by immersing the sample tube containing the steel powder in a benzene solution of the nonvolatile compound to be adsorbed. The solution level was such that it permitted the solution to pass through the fritted-glass plug and completely surround the steel powder. The tube was suspended at the desired level by an iron wire which passed through a piece of capillary tubing in an enclosing cork stopper to an external support. -4 glass tube inserted through the cork stopper extended to the bottom of the
552
E. L. COOK AND NORhtAK HACKERMAN
solution tube and permitted removal and refilling with solution in the tube without having to remove the powder sample tube. The whole 200 ml. of solution was removed and replaced with fresh solution if any significant change in concentration occurred. At intervals, the powder tube was raised by means of the wire to a level which allowed the solution to drain from the powder, and then lowered to its original position. The whole adsorption tube assembly was immersed in a thermostat held a t 30°C. The powder sample tube was removed from the solution tube and placed on P, vacuum cup, where the solution mechanically held up in the steel powder was pulled through by the vacuum, the air passing through the powder first being dried over Drierite. The removal of the entrained liquid took only 30-60 sec. and air was pulled through the powder for another 5-10 min. to dry the powder completely. The sample tube and contents were weighed and the increase in weight over the original weight of the tube and powder taken as the weight of nonvolatile organic compound left on the powder. To determine whether the adsorption process was at equilibrium, the sample tube and powder were again suspended in the solution tube and the immersion procedure repeated. The whole process was repeated until a constant weight increase over the original weight was obtained. Blank runs on the empty sample tube showed adsorption on the glass to be negligible. The amount of solute left deposited (not adsorbed) on the powder due to evaporation of the benzene during the drying process was determined by weighing the tube and powder before and after the final evaporation. The weight of solvent evaporated multiplied by the concentration of the solution gave the amount of solute deposited in this manner. This correction was never greater than about 5 per cent of the total amount of adsorption observed and was neglected, since the overall accuracy of the method was of the same order. During the adsorption studies, it Ivas found that a portion of the adsorbed phase was not easily desorbed. Indeed, for practical purposes, a portion of the adsorption proved to be irreversible. A desorption procedure developed for this study was as follows: 100 ml. of benzene was poured through the sample tube containing the steel powder, the powder being in contact with the benzene,for a t least 45 min. The powder was then dried again by the above given procedure and the sample reweighed. The loss in weight indicated the amount desorbed, the weight in excess of the initial weight of the powder being due to the irreversibly adsorbed component. A repetition of the desorption process consistently showed no further desorption. The adsorption-desorption cycle could be repeated any number of times. Fresh powder showed no increase in weight after repeated and sustained contacts with the benzene, thereby showing that there was no measurable effect because of the solvent alone. RESULTS
Adsorption of acids Stearic, palmitic, lauric, capric, and benzoic acids were studied. Figure 2 shows the adsorption isotherms for stearic and capric acids. Total adsorption
553
ADSORPTION OF ORGANIC COMPOUNDS ON STEEL
increased slightly with molecular weight but the amount of irreversible adsorption was the Same for each acid. Isotherms of lauric and palmitic acids displayed similar characteristics. The adsorption between the desorption and total adsorption levels mas reversible; however, successive desorption treatments failed to remove any more of the firmly adsorbed material. Caproic and benzoic acids (isotherms not shown) gave results similar to each other but markedly different from that of the other acids studied. A continued decrease in weight of the metal powder was observed, along with the development of a blue-green coloration of the benzene solution. Apparently the benzoic and caproic acids reacted with the powder and formed the respective ferrous salts. The attack was more severe at higher acid concentrations. No coloration
(j 1.5-
a W
0 IB
0
1.0-
X
P
v)
w
0
OO
I
ADSORPTION
I
I
I
5
IO
'5
CONCENTRATION, MOLAR x lo3
FIG.2. Adsorption of acids: tot31 adsorption (--);
irreversible adsorption (- - -);
0 , CIS; 0 , ClO.
of the benzene adsorption solutions of capric, lauric, palmitic, or stearic acid was observed.
Adsorption of amines The adsorption isotherms and values obtained after desorption for octadecyl-, decyl-, and octyl-amines are shown in figure 3. As might be anticipated, total adsorption decreased with decreasing molecular weight. A gradual transition from the type of isotherm obtained with octadecylamine to that observed for decylamine was obtained with hexsdecyl-, tetradecyl-, and dodecyl-amines. These data are not shown, as no further unusual behavior occurred. Desorption of the amines gave rise to the same irreversible adsorption behavior observed with the acids, except that the maximum amount remaining undesorbed was about 0.28 X mole/g. as compared to 0.38 X molelg.
554
E. L. COOK AND NORMAN HACKERMAN
of undesorbed acid in those casea where no reaction was indicated by dissolution of the metal. No evidence of metal dissolution as a result of reaction with amine
was observed. Adsorption of ahhole Figure 4 shows similar data for octadecyl, tetradecyl, and decyl alcohols. The total adsorption was comparable to that for the corresponding amine whereas the amount remaining undesorbed was slightly less. Hexadecyl and dodecyl alcohols behaved similarly to those shown.
0: W
n
>
1.0-
x
n
W
I5
10
f 0
ADSORPTION
CONCENTRATION, MOLAR x lo3
FIQ.3. Adsorption of amines: total adsorption (-); 0 , CIS;0 ,CIO;A , Ca.
irreversible adsorption (-
- -);
Adsorption of esters Figure 5 shows the adsorption-desorption data obtained with methyl, ethyl, and butyl stearata. The effect of increasing the siee of the alcohol group on total absorption is apparent. An undesorbable portion was again observed with the esters, the amount of methyl and ethyl stearate being comparable to the alcohols. Butyl stearate showed considerably less of the undesorbable type of attachment. DISCUSBION
The amount of total adsorption was shown to increase slightly with increase in molecular weight within a given homologous series, whereas the amount of irreversible adsorption was independent of molecular weight and characteristic of the functional group. Although differing only to a small extent, the total adsorption for the series studied was in the decreasing order acids, amines, alcohols, and esters. The amount of irreversible adsorption was found to follow the same order, but more markedly.
555
hDSOFZTION OF ORQANIC COMPOUNDB ON
2 .o -I
u W
I-
cn (j 1.5
a W
a
-
'OO 1.0 X
n w m go.5
cn
n 4
m Y o oo I
5
FIG.4. Adsorption of alcohols: total adsorption (-); 0 , Cia; 0 , C l r ; A , Cio.
5
ADSORPTION
15
IO
CONCENTRATION, MOLAR x lo3
ADSORPTION
irreversible adsorption (- --);
10
I5
CONCENTRATION, MOLAR x lo3
FIQ.5 . Adsorption of esters: total adsorption (-); irreversible adsorption (0, butyl stearate; 0 , ethyl stearate; A , methyl stearate.
- -);
The surface area of the powder (0.10 m.*/g.) would require 0.79 X 10-Bmole/g. of adsorbate to form a complete monolayer, assuming 21.0 A? the crosssectional area per adsorbed molecule. The data show that the amount of the irreversibly adsorbed material was not sufficient to make up a close-packed monolayer of any one component. Further, the amount of irreversible adsorp-
556
EL L. COOK
AND NORMAN H h c g E W
tion was constant throughout a given homologous series and the adsorption was specific for a given functional group. This indicates that the irreversibly adsorbed material is oriented with the functional group adjacent to the metal surface. Total adsorption a t higher concentrations in this and subsequent studies' showed amounts adsorbed in excess of a monolayer by 20-70 per cent in most cases. The principal exception was with octylamine. I t is known that benzene solutions of the type of compounds studied herein are made up of dimer particles as well as of the single molecules (6, 9). The dimers are believed to be of linear form and held through the functional group. The possibility exists for adsorption of such dimers either on top of an adsorbed monolayer region, by condensation, or onto the metal surface directly. The former case appears more likely, since adsorption of dimer particles tail to tail with the molecules already present would produce an adsorbed phase simulating the crystal structure of the compound and enhance the van der Waals forces between neighboring hydrocarbon chains. Electron diffraction studies have shown that stearic acid adsorbed on steel from hexane solution exhibits such a surface structure (4). This does not eliminate the possibility of an incomplete monolayer of irreversibly adsorbed molecules existing along with an incomplete monolayer of reversibly adsorbed molecules and polylayers built up on either irreversibly or reversibly adsorbed molecules, or any combination thereof. SUMMARY
The adsorption from solution of higher-molecular-&.eight aliphatic acids, amines, alcohols, and certain esters on SAE 1020 steel powder with a specific surface area of 0.10 m?/g. has been studied. Two types of adsorption, irreversible and reversible, were observed for these systems. Total adsorption and irreversible adsorption were in the following decreasing order: acids, amines, alcohols, esters. The extent of total adsorption was a function of molecular weight and the polar group. I t was found to exceed the calculated amount necessary for a complete close-packed monolayer by 20-70 per cent, depending on the compound. The extent of irreversible adsorption was independent of molecular weight, and characteristic of the functions1 group of the series. Calculated surface coverage in these cases was between 10 and 50 per cent. The authors wish to thank the Office of Naval Research for their financial support of this work. They are pleased to express their gratitude to Dr. A. L. McClellan of the University of California, Berkeley, for making the measurements of surface area. REFERENCES (1) BIGELOW, W. C . , G ~ s s s E , . , ~ N ZISMAN, D W. A , : J. Colloid Sci. 1, 513 (1916). (2) BOWDEN, F.P., GREGORY, J. N.,A N D TABOR, D . : S a t u r e 156, 97 (1945).
' Manuscript
in prepitration for This Journal.
PARTICLE AGGLOMERATION I N TUNGSTEN POWDER
557
(3) GREENHILL, E. B . : Trans. Faraday SOC. 46, 625 (1949). NORMAN, A N D SCHMIDT, H . R . : J . Phys. & Colloid Chem. 65, 629 (1949). (4) HACKERMAN, (5) MANX,C. A., LAWER, B. P . , AND HULTIN,C. T . : Ind. Eng. Chem. 28, 159 (1936). (6) RALSTON, A. w., AND HOERB,C. w.:J. Org. Chem. 10, 170 (1945). F. H . , A N D KUHN,W. E.: Ind. Eng. Chem. 21,1066 (1929). (7) RHODES, H.A , , A N D FWZEK, J. F . : J . Am. Chem. SOC.88, 229 (1946). (8) SMITH, (9) STEPANEKO, S . , AGRANAT, v., ASD Nov1Kov.4, T.:Acta Physicochim. U.R.S.S. 20, 923 (1943);Chem. Abstracts 41, 7178d. (10) TINGLE, E.D . : Nature 160, 710 (1947). (11) ZISMAN,W.A . : J . Chem. Phys. B, 534,729, 789 (1941).
PARTICLE AGGLOMERATIOS IK TUKGSTEK METAL POWDER BERSARD KOPELXIAS
~ N D C.
C. GREGG
Sblvania Elecirzc Products, Inc., Metallurgical Laboralorzes, Bayside, New York Received April 87, 1950 INTRODUCTION
Of the several methods available for particle-size distribution or analysis, such as sedimentation, turbidimetry, gas or liquid elutriation, none will yield an analysis in agreement with the microscopic examination of those metal powders which are prepared by the reduction of their oxides at elevated temperatures. Microscopic examination of such powders, which have been carefully dispersed in a liquid by spatulation of the slurry on a slide, invariably reveals agglomerates as well as single particles. The agglomerates are much more numerous in the fines, say, those particles having diameters of 1-3 microns. These agglomerates may be due to two factors: ( I ) sintering of the fine particles at the elevated temperatures of reduction, resulting in an agglomerate whose intraparticle strength approaches that of a grain boundary in a metal; ( 2 ) flocculation due to electrical forces, the degree of flocculation increasing with a decrease in particle size. In many applications of such powders it is desirable to know whether theEe agglomerates are mere flocculates or clusters of strongly sintered particles, since the latter would behave as single particles while the former would not. It is the primary purpose of this paper to show that aggregates seen in the microscope, after the tungsten powder has been carefully dispersed with a spatula, are clusters of sintered particles. These clusters may be assumed equivalent to single particles of the same diameter. THEORY
I t has been known for a long time that hydrophilic solids will be deflocculated in liquids of high dielectric constant (2) and in liquids whose interfacial tension against water is small (1). Flocculation will then occur in liquids of low dielectric constant and high interfacial tension. To date, all these observations have
