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NORMAN HACKERMAN AND E. L. COOK
Vol. 56
DUAL ADSORPTION OF POLAR ORGANIC COMPOUNDS ON STEEL' BY NORMAN HACKERMAN AND E. L. COOK^ Department of Chemistry, University of Texas, Austin, Texas Received June 7, 1861
A portion of the surface of steel powder is specific to the irreversible adsorption of alkyl carboxy acids, amines, alcohols and esters. Certain areas ap arently will irreversibly adsorb either acid or amine, whereas other areas are specific to the acid or to the amine. Alcohog and esters are not irreversibly ad'sorbed to the extent the acids are on fresh steel powder but appear to be irreversibly adsorbed on certain of the same sites which adsorb acids. Alcohols and esters are not irreversibly adsorbed by steel powder on which an acid has already been irreversibly adsorbed. The extent of total adsorption (reversible plus irreversible) of the second solute, for systems investigated in this study, is only slightly reduced by the pretreatment of the steel powder with another solute. The maximum amount of irreversible adsorption observed with the organic solutes is not sufficient to form a closed packed monolayer over the surface of the steel powder available for krypton adsorption.
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Previous studies on the adsorption of polar organic compounds from benzene solution on steel. powder showed that all of the adsorbed component could not be desorbed by repeated washings with benzene.* The amount of irreversible adsorption was shown to differ for the alkyl carboxy acids, amines, alcohols and certain esters, but was essentially constant within each homologous series. This behavior suggested that the irreversible adsorption process occurred on areas of the metal surface which were to some degree specific toward certain functional groups. The present investigation was undertaken to obtain further information on the degree of specificity of the surface of the steel powder toward the various functional groups.
Experimental The steel powder, polar organic compounds and benzene solvent used were taken from the same lots as in earlier work.3 The equipment and technique for measurin the amount of adsorption have aLso been described in tetail earlier. The procedure for this study was developed to permit a second adsorption process to be carried out on steel powder samples upon which a polar material had already been adsorbed. The procedure was as follows: Freshly washed and cleaned samples of steel (SAE 1020)powder were repeatedly contacted with benzene solution of a given polar organic compound until adsorption was complete as indicated by no further increase in weight of the steel powder samples after removal of excess solution from the powder and drying. After the adsorption had reached equilibrium, the steel owder samples were subjected to a desorption process wfich consisted of thoroughly washing the steel powder with benzene solvent until the dried samples showed no further weight loss. I t was observed that not all of the adsorbed component could be desorbed with repeated washings, the amount remaining irreversibly adsorbed being characteristic of the functional group of the polar organic compound and to a large extent independent of the molecular weight of the alkyl chain. Steel powder samples pretreated in this manner were then contacted with benzene solution containing polar compounds with a different functional group but having a hydrocarbon chain containing the same number of carbon atoms. The contacting was continued over a period of several days until no further adsorption was indicated by increases in weight of the dried steel powder samples. It was observed that the pretreated steel samples showed weight increases due to adsorption of solute from the second set of solutions. The steel powder samples were then subjected t o the desorption process and the washings were continued until no further weight loss was obtained.
Results I n Fig. 1 are shown results of the dual adsorption (1) Presented before the Division ofColloid Chemistry at the 115th National Meeting of the American Chemical Soaiety, which was held in Ssn Francisco, California, March, 1949. (2) Magnolia Petroleum Co., Dallas, Texas. (3) E. L. Cook and N. Hackerman, THISJOVRNAL,56, 649 (1951).
of stearic acid and octadecylamine. The stearic acid first adsorbed was only partially desorbed by the washing procedure as formerly de~cribed.~ Subsequent contacting of the pretreated powder with octadecylamine solutions gave a weight increase due to adsorbed octadecylamine. The weight increase of the dried powder overthe weight with the undesorbable stearic acid is shown as adsorbed octadecylamine. The amount of adsorbed amine is slightly less than the amount adsorbed on fresh steel powder which had not previously been contacted with stearic acid solutions. Desorption of amine by repeated washings until there was no further weight loss showed an incomplete desorption of the amine. Assuming no desorption or displacement of the firmly adsorbed stearic acid occurred during the subsequent contacts with amine solution and washings, the total amount of undesorbable material would account for the original undesorbable stearic acid and an additional amount of undesorbable octadecylamine being present on the surface at the same time. The amount of undesorbable amine present, however, was less than in the case where the amine had been the first component adsorbed. In Fig. 2 are shown results of adsorption of stearic acid on steel powder samples which had been pretreated with octadecylamine. The adsorption of stearic acid on the pretreated samples was slightly less than on fresh steel samples. Again assuming no desorption or displacement of the first adsorbed amine, the amount of undesorbable stearic acid was considerably less than that observed when fresh steel samples were used. A similar set of data were developed uslng lauric acid and dodecylamine as the adsorbed components. The results obtained with the C12-compoundscorresponded closely to the data obtained with the Cls-compounds. In Fig. 3 are shown the results of the adsorpti.on of octadecyl alcohol from benzene solution on steel powder samples which had been pretreated with stearic acid in the same manner as described above. The adsorption of octadecyl alcohol was slightly less than that observed when fresh steel samples were used. Upon being subjected to repeated washings with benzene, however, the weight losses observed indicated that all of the adsorbed alcohol was desorbed, if no desorption of the stearic acid occurred. Figure 4 shows the results obtained when stearic acid was adsorbed from benzene solution by steel powder which had been pretreated with octadecyl
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DUALADSORPTION OF POLAR ORGANIC COMPOUNDS ON STEEL
April, 1952 2.0
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525 I
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0
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ADSORPTION COSNCENTRAT~ON,MOLAR x 103.
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ADSORPT~ON&CENTRATION,
Fig. 1.-Dual adsorption of stearic acid and octadecylamine: total adsorption (-); irreversible adsorption (- -); 0,acid; , amine.
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L A R x 103.
Fig. 4.-Dual adsorption of octadecyl alcohol and stearic acid: total adsorption (-); irreversible adsorption (- - -); 0,acid; A, alcohol.
were extrapolated into the higher concentration range according to trends observed in systems with other solutes. A study of the dual solute system of stearic acid and methyl stearate ester (Figs. 5 and 6) indicated a behavior similar t o that observed with the stearic acid-octadecyl alcohol system.
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I 10
ADSORPTION5CONCENTRATION, MOLAR x IO3,
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Fig. 2.-Dual adsorption of octadecylamine and stearic irreversible adsorption acid: total adsorption (-); (- -); 0,acid; 0 ,amine.
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P
I I 5 in ADSORPTION -CONCENTRATION, ' M O L A R x 103.
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adsorption of stearic acid and methyl Fig. 5.-Dual stearate: total adsorption (-); irreversible adsorption (- -); 0,acid; V, ester.
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2.0
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OO
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ADSORPTION CONCENTRATION, MOLAR x 103.
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0 X
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Fig. ~3.-Dual adsorption of stearic acid and octadecyl irreversible adsorption alcohol: total adsorption (-); (- -); 0,acid; A, alcohol.
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alcohol. The adsorption of stearic acid was slightly less than that observed on fresh steel powder samples. Upon desorption by benzene washings, weight losses indicated that a portion of the stearic acid was not desorbed and remained on the powder along with the undesorbed alcohol remaining from the pretreatment. The total amount of undesorbed alcohol and acid was very nearly equivalent to the amount of stearic acid which could not be desorbed from fresh steel powder samples. Due to experimental mishap, results from the full range of solute concentrations covered by other expezments in this study were not obtained. The results
ADSORPTION CONCENTRATION, MOLAR x 103.
Fig. 6.-Dual a&orption of methyl stearate and stearic acid: total adsorption (-); irreversible adsorption (- -); 0,acid; V, ester.
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Discussion The results obtained with the acid-amine system (Figs. 1 and 2) suggest that the portion of the adsorbed components which could not be desorbed were, to some extent, situated on specific adsorption
526
TERRELL L. HILL
sites. It appears that certain areas adsorbed only acid or amine, whereas other areas could adsorb either acid or amine. The amount of the second component which was readily removable by solvent treatment was not altered greatly by the presence of the first adsorbed component. In either of the dual solute systems, acid-alcohol or acid-ester, if the acid were the first solute to be adsorbed on the fresh steel powder, no appreciable amount of the second solute, alcohol or ester, was irreversibly adsorbed. Since alcohols and esters have been previously shown to be capable of being irreversibly adsorbed on fresh steel powder, this is interpreted as an indicaiion that both alcohols and esters are adsorbed on some of the same adsorption sites a ~ the 3 acid. The acid, being the first adsorbed prevents any further irreversible adsorption of alcohol or ester. The assumption that displacement was not a major factor is supported by the fact that with the reverse system the total irreversibly held components is greater than for either one alone. The pretreatment of the powder with alcohol or ester, neither of which is irreversibly
Vol. 56
adsorbed on fresh steel powder to the extent observed with the acid, apparently does not interfere with further irreversible adsorption of the acid which is irreversibly adsorbed to a greater extent than alcohol or ester on fresh steel surfaces. The acid is adsorbed to the extent that the amount of the first irreversibly adsorbed component, either alcohol or ester, plus the then irreversibly adsorbed acid is approximately equivalent to the amount of acid which is irreversibly adsorbed on fresh steel powder. Based on the surface area of the steel powder which was determined to be 0.10 m.2/g. by low temperature adsorption of krypton, and assuming a cross sectional area of 21 for the adsorbed organic molecule, the maximum amount of irreversible adsorption observed in the dual systems did not exceed 70% of that required for a close packed monoIayer. Acknowledgment.-The authors wish to thank the Office of Naval Research for their financial support of this work, under contract N5 ori-136, T. 0. 11.
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STATISTICAL THERMODYNAMICS OF THE TRANSITION REGION BETWEEN TWO PHASES. I. THERMODYNAMICS AND QUASITHERMODYNAMICS'a2 BY TERRELL L. HILL Naval Medical Research Institute, Bethesda, Md. Received June 66, 1061
The thermodynamics and quasi-thermodynamics of plane and spherical surfaces are discuased, including an alternative formulation of Gibbs' surface thermodynamics and an improvement on the Tolman quasi-thermodynamic theory. The results of this paper will be applied later to approximate statistical mechanical calculations now in progress.
I. Introduction In these papers we shall investigate the statistical thermodynamics of interfacial regions using approximate methods which are more refined than those employed by Tolman.* Kirkwood and Buff4 have given a rigorous statistical discussion of the plane surface case, but their general equations are apparently too complicated for numerical applications. However, they have made calculations in the approximation that the gas and liquid densities are both assumed constant right up to a mathematical surface of density discontinuity. The present theory introduces an approximation in the statistical model at the outset but leads to equations which can be solved numerically and predicts a continuous transition in density in the interfacial region, as must be the case physically and as would be found from the general equations of Kirkwood and Buff if they could be applied. Tolman introduced the van der Waals equation as a (1) The opinions contained herein are those of the writer and do not necessarily reflect the views of t h e N a v y Department. (2) Presented a t a n American Chemical Society meeting, Boston, April, 1951. (3) R. C . Tolman, J. Chem. P h ~ s . 16, , 758 (1948); 17, 118, 333 (1949); see also F. 0.Xoenig, ibid., 18, 449 (1950). (4) J. G.Kirkwood and F. P. Buff,$bid., 17, 338 (1940).
statistical model, but in such a way that there is still some discontinuity in density at a mathematical surface. In earlier notes5we outlined the general approach, which is applicable to plane or curved interfaces with one or more components, spherical drops and bubbles, plane interfaces with one phase under an extra hydrostatic pressure, and physical adsorption. Numerical calculations on these topics are in progress, based on van der Waals and improved approximate theories of the liquid state. These results wiIl be published later.5a However, we may mention here that preliminary calculations, based on (a) a van der Waals models~sand (b) P hard sphere model6,' with a superimposed van der Waals type smoothed potential, give results in semiquantitative agreement with those of Tolman and Kirkwood and Buff for the magnitude of the surface tension and the location of the surface of tension (and hence for the superfieid density of matter at the surface of tension and for the dependence of (5) T. L. Rill. ibid., 19, 281 (1951); 19, 1203 (1951). (5a) The resulta for a plane surface with one Component have been publlhed: T. L. Hill, h i d . , 20, 141 (1952). (6) T. L. Hill, paper presented a t an American Chemical Society Meeting, Boston, Rlasa., April, 1951. (7) L. Tonks, P h y s . Rev., 60, 955 (1936).
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