June, 1963
SURFACE AREASBY ADSORPTION OF QUATERSARY AMMONIUM HALIDE
higher plasticizer concentrations the present modification ceases to be effective, leading to a very peculiar result. Acknowledgments.-Thanks are due to Dr. K. Ninomiya, Japan Synthetic Rubber Co., Dr. A. Kishimoto,
1235
Department of Fisheries, Kyoto University, and Dr. M. Kurata, Institute for Chemical Research, Kyoto University, for their interest in this study. Support in part came from the Ministry of Education, to which grateful acknowledgment is made.
SURFACE AREAS BY ADSORPTION OF A QUATERNARY AMMOSIUM HALIDE FROM AQUEOUS SOLUTION BY J. KIVEL,F. C. ALBERS,D. A, OLSEN,AND R. E. JOHNSON Minneapolis-Honeywell Research Center, Hopkins, Minnesota Received November 19, 1961
A technique has been developed for determining the surface area of powders and flat surfaces by their adsorption of a carbon-14 labeled quaternary ammonium halide. Hexadecyl-l-C~4-trirrrlethylammoniumbromide has been adsorbed from an aqueous solution onto flat plates and crushed samples of lead glass and onto vacuum deposited iron-nickel thin films. The rate of adsorption and the adsorption isotherms have been determined for iM and the critical micelle conthese systems. A “monolayer” plateau was found to occur between 3 X centration in all systems studied. The effect of the tetrasodium salt of ethylenedirtrninetetraacetic acid (EDTA) on the effective surface area of lead glass has also been determined. EDTA treatment has been shown to result in a significant roughening of the glass. washed once with distilled water, twice with analytical reagent Introduction grade methanol, and stored in a desiccator. One approach to obtaining information on surface 2. Flat Glass.-Coirning Code No. 2915 microscope cover properties has been to adsorb a monomolecular layer glasses and 0120 lead glass. 3. Iron-Nickel Thin Films.-Iron-nickel thin films were of a long-chain polar organic compound onto the solid mm. The alloy was prepared in a bell jar evacuated to surface in the manner of Langmuir and B1odgett.l heated in an alumina crucible (morganite) by resistance heating These polar organic compounds, in particular, stearic and evaporated onto Corning Code Xo. 2915 microscope cover acid, have been used to determine the specific surface glasses. The experimental procedure is reported elsewhere7 area of powders of such materials as metals2 and clays.3 in greater detail. B. Surfactant.-The hexadecyl-l-Ci4-trimethylammonium The availability of radioisotopes of many elements bromide was obtained from the Nuclear-Chicago Corp. Analyses with specific activities high enough to yield statistically by radiochromatography revealed no other radioactive species. reliable results from a square centimeter of surface has -4“stock” solution wa8 prepared by diluting the labeled HMAB made it possible to study surface reactions and deterwith inactive HMAB (Eastman Organic Chemical Co.) t o a specific activity of 0.50 pcurie/mniole. mine surface areas that were previously difficult or imC. EDTA.-The tetrasodium salt of ethylenediaminetetrapossible to analyeea4 Techniques utilizing carbon-14 acetic acid was obtained from the Dow Chemical Company or tritium labeled polar organic compounds, such as and used without further purification. stearic have made possible determinations of Procedure.-The steps in the adsorption of HMAB on a specific surface area of very small powder samples, nonsurface and the subsequent measurement were as follows: 1. The initial activity in solution was determined. Aqueous porous materials, and flat surfaces. solutions containing 25 ml. of varying concentrations of HMAB I n this study, the properties of he~adecyl-l-C~~-triwere prepared in glass tubes that had been pretreated with an methylammonium bromide (HMAB), an aqueous identical HMAB solution. Small aliquots (usually 10 p l ~ ) soluble ionic surfactant, have been utilized to determine were removed from the solution, placed onto a circular area of 0.25 cm.%in the center of a 1.25 inch diameter aluminum disk, surface areas of various materials, to determine the and evaporated to dryness. The samples were then counted effect of particle size on roughness, and to determine the using an Atomic Accessories, Inc., Model FC-72A windowless effect of the tetrasodium salt of ethylenediaminetetragas-flow counter to determine the initial activity per microliter acetic acid (EDTA) on lead glass. of solution. 2. The time necessary for adsorption equilibrium to be Experimental Materials. A. Surfaces.-The surfaces used in the study were the following. 1. Crushed Glass.-The techniques used to prepare the glass samples were those recommended by the American Ceramic Society.6 Samples of Corning 0010 lead glass were placed in a flat bottomed steel mortar and crushed by dropping a weight from a constant height onto the samples. The glass was then sieved, and the desired mesh size was transferred t o a piece of paper: Foreign particles were removed by magnetic and manual techniques. The sample was then transferred back to the sieve, (1) K. B. Blodgett, J . A m . Chem. Soc., 57, 1007 (1935); I . Langmuir and V. J. Schaefer, ibid., 58, 284 (1936). (2) E. B. Greenhill, Trans. Faraday Soc., 45, 625 (1949). (3) C. Orr, Jr., and P. T. Bankston, 3 . Am. Ceram. Soc., 56, 58 (1952). (4) J. E. Willard, J . Phys. Chem., 67, 129 (1953). ( 5 ) M. C. Kordecki and M. B. Gandy, Intern. J . A p p l . Radiation Isotopes, 12, 27 (1962). (6) D. E. Sharp. Bull. Am. Geram. S o c . , 14, 181 (1935).
reached was determined, and the procedure follows for crushed glass. Approximately two grams of crushed glass was added t o the 25 ml. of HMAB Rolution a t 25 f 0.1’. The mixture was stirred manually. Small aliquots of solution were withdrawn a t definite time intervals, evaporated onto aluminum disks, and counted. 3. Isotherms for adsorption of HMAB were determined. The crushed glass samples were treated with aqueous solutions of to 2.3 X M HMAB a t 25” for an adsorption 1.0 X time of 30 minutes. Samples of the initial and final solutions were withdrawn, and the adsorption isotherm was determined from the change in activity in solution. The flat glass plates and the iron-nickel deposits were dipped into solutions of similar concentration until equilibrium was reached and then vertically withdrawn a t a rate of 2 mm./minute by means of a small motor. At this rate of withdrawal, the samples emerged dry; water did (7) R. J. Prosen, J. 0. Wolmen, B. E. Gran, and T. 6 . Cebulla, J . A p p l . Phya., 83, 1150 (1962).
J. KIVEL,F. C. ALBERS,D. A. OLSEN, AND R. E. JOHNSON
1236
e
4.06 I O - ~ M 2.64~ IC4# 1.24I ~
0
I
0
Fig. 1.-Rate
15
-0.84
x
I
~
164M I
30 45 60 ADSORPTION T I M E f M I N U T E S ) .
75
of adsorption of HMAB on 25-40 mesh 0010 lead glass a t 25'.
2000
1600
z
; 1200 + 3 0
800
400
0 0
4
8 12 16 20 24 CONCENTRATION H M A B (MOLAR x I O 4 ) ,
28
Fig. 2.-Adsorption isotherms of HMAB on (A) crushed 0010 lead glass, (B) iron-nickel thin films, and (C) 2915 microscope cover glasses. not wet the surface. I n the few cases where droplets formed, they were removed by touching a corner of tissue paper to the drops. These adsorption isotherms for the glass and metal plates were determined from the corrected counts/cm.2 on the plates. 4. The surface area of each sample was determined. Samples were treated with a 5.72 X l o p 4A[ H M S B solution in the manner described above. Flat samples were withdrawn vertically from solution and counted, whereas the adsorption by crushed glass samples was determined by comparison of the activity of the initial and final solutions. 5 . EDTA Treatment.-Squares of 0120 lead glass and samples of 7-8 and 50-60 mesh 0010 lead glass were treated with a 0.131 M EDTA aqueous solution of p H 12.3 a t 96'. Samples were removed from the EDTA solution at time intervals from 0 to 15 hours. The experimental procedure for the EDTA treatment is reported in greater detail elsewhere.* 6 . Radioactivity Measurement.-All samples were compared t o the average standards obtained by pipetting several 10-pl. samples of each initial HMAB solution onto aluminum disks, drying them, counting them, and determining the average activity. These standards were calibrated against a 0.19 f 0.019-@curiecarbon-14 source, assuming a 50% geometry factor, and a backscattering factor of 23%. The backscattering factor of 23y0 has been determined by othersgto be the same for aluminum and glass.
Experimental Results A. Rate of Adsorption.-The rate of adsorptioii was studied as a function of HMAB concentration and (8) D. A. Olsen, R. E. Johnson, J. Kivel, and E". C. Albers, to be published. (9) J. W. Shepard and J. P. Ryan, J . P ~ w Chem., . 6S, 1729 (1959).
~
Vol. 67
nature of adsorbent. For crushed glass samples a plateau was reached within 30 minutes for all but the most concentrated solutions of HMAB. Maximum adsorption was obtained for the flat glass and the ironnickel thin films within ten minutes of immersion time. Figure 1 shows the rate of adsorption of HMAB on 25-40 mesh 0010 lead glass as a function of HMAB concentration. Other samples of crushed glass yielded similar curves. B. Adsorption Isotherms.-Adsorption isotherms were obtained for all materials studied in this investigation. Isotherms obtained for 25-40 mesh 0010 lead glass, 2915 cover glasses, and iron-nickel thin films are Mshown in Fig. 2. The activity noted for the crushed glass in Fig. 2 is a relative value. The other activities are actual values obtained per sample of cover glass and iron-nickel deposit. C. Area Determinations.-The surface areas of six different particle sizes of 0010 lead glass have been determined and are listed in Table I. The areas were calculated by determining the fraction of activity in the solution lost due to the adsorption of HMAB onto the glass and multiplying this quantity by the total number of molecules originally present in the solution. The effective area of the molecule has been taken as 20.5 AO2. Since this value was not redetermined in the present investigation, the quantities reported here should be taken as relative rather than as absolute values. Roughness factors have been determined for the crushed glass and are also included in Table I. The roughness factor in this investigation is defined as the ratio of surface area determined by using HMAB to that determined by using geometric calculations. Geometric areas were obtained for the crushed glass samples by assuming spherical particles. TABLE I AREAASD ROUGHKESS FACTORS OF CRUSHED LEADGLASS Mesh range
7-8 20-25 40-45 50-60 80-100 140-170
HMAB surface area (om.%.)
Geometrical surface area (cm.Vg.)
Roughness factor
121 f 11 209 f 29 255 f 28 296 f 11 725 f 40 1078 f 105
8.3 33.4 56.8 82.4 133 224
14.5 6.3 4.5 3.6 5.4 4.8
Surface areas and roughness factors were also determined for the iron-nickel thin films, the cover glasses, and the 0120 lead glass squares. Roughness factors for these materials were found to be 2.8 for iron-nickel thin films, 2.5 for 2915 cover glags, and 4.4 for 0120 lead glass. An effective area of 20.5 A.2 was again attributed to each molecule adsorbed. D. EDTA Treatment.-The surface area of lead glass was found to increase upon treatment with EDTA. The area of flat 0120 lead glass plates treated with EDTA was found to increase almost linearly with length of treatment for the first seven hours from a roughness factor of 4.4 for the untreated glass to 86 after seven hours treatment. Beyond 7 hr. the increase is more gradual. After 15 hr., the roughness factor of the EDTA treated glass was determined to be 115. Similar results were obtained for crushed lead glass. Table I1 shows the effect of a 10-hr. EDTA treatment on
SURFACE AREASBY ADSORPTION O F QUATERNARY AIMMONIUM HALIDE
June, 1963
7-8 and 50-60 mesh 0010 lead glass and on 0120 lead glass plates. TABLEI1 EFFECT OF 10-HR.EDTA TREATMENT O N LEADGLASS
Glass
7-8 mesh
Untreated surface area
Geometrical area
cm.2 8.3g.
50-60 mesh
82.4 -
0120 plate
1.0 cm.2
g.
cm.2 121 f 11 -g.
cm
296 f 11 -.L
g.
4.4
=!z
0 . 2 om.*
EDTA treated surface area
cm.2 811 3: 93g.
cm.2
1541 A: 29 g. 91 i:6 cm.2
Discussion A. The Adsorption Isotherm.--Most adsorption isotherms have a plateau or an inflection. The plateau or the beginning of the linear portion above the inflection is believed to represent “first degree satura . tion”1° of the surface and is the condition in which all possible sites in the original surface have been filled. Further adsorption can take place only on new surfaces. This degree of coverage is often called a complete “monolayer,” although generally the layer may contain solvent as well as solute molecules, consist only of isolated clusters adsorbed on the most active sites, or consist of ionic micelles.11 Therefore, it is difficult to define the term “monolayer.” Willard4 has arbitrarily defined a monolayer as “the number of ions required tjo cover the macro surface area of the sample if each ion covers an area equal to the square of its ionic diameters.” The iirst degree saturation by HMAB has been shown12l 3 to be due to adsorption of long chain cations. Saraga12 has presented a mechanism of attachment of HMAB onto glass in which the glass is hydrolyzed to yield ESiOH groups. These groups are, in turn, to some extent dissociated to =SiO- and H+. The glass is attacked by the quaternary ammonium cation to produce, according to Saraga, a “surface salt,’’ =&ON( C H ~ ) ~ C I S HSexsmith ~~. and White13 interpret the initial adsorption of HMAB as a cationic exchange. Beyond the concentration range of first degree saturation, which is shown in Fig. 2 to occur approximately in the region 3 to 8 x 10-4 M , an increase in the amount of adsorption occurs. This has been attributed to an adsorption of ion pairs.12l3 I n the monolayer concentration range the glass is neutral; there is no charge on the surface. Beyond this range, however, the presence of the HMAB cations on the glass gives rise to a positive charge. Therefore, when the glass is removed from the solution it should carry negative ions (Br- and OH-) along with it. Saraga12 in a study of quaternary ammonium bromides, using radiobromine-82, noted that the attachment of bromine onto the glass surface began a t concentrations just below the critical micelle concentration. The c.m.c. for HMAB is 1 X M,12which is about that of the concentration a t the end of the monolayer plateau. Comparison of the adsorption isotherms for iron(10) S. Brunauer, “The Adsorption of Gases and Vapors,” Oxford Univ. Press, London, 1944, p. 287. (11) C. H Giles, T. H. MacEwan, S N. Nakhwa, and D. Smith, J . Chem. Soc., 3973 (1960). (12) L. Ter Minassian-Saraga, J . c h s n . p h y s . , 57, 10 (1960) (13) F. H. Sexsmith and H. E. Whlte, Jr., J . Collozd Scz., 14, 598 (1959).
1237
nickel thin films and the various types of glass indicates that a similar mechanism is responsible for adsorption in these systems. The data for the adsorption of HMAB obtained by Sexsmith and White13 show that cotton, cordura, and viscose filaments begin to retain bromide and yield a corresponding increase in cation adsorption a t approximately 4 to 6 X M. This concentration range is not very different from that observed in this investigation for the end of the monolayer plateau, again implying a similar type of adsorption mechanism. Preliminary studies in this Laboratory on other substrates also indicate similar adsorption isotherms. B. Surface Area Measurements.-In order to determine a value for the surface area, the area occupied or “blocked” by each molecule must be known. The value of 2O.5Aa2has been chosen since it has been notedll that the irregular shaped projection of long chain hydrocarbons usually results in the effective covering of an area of 20.5 even though the cross-sectional area of the -CH2groups on the hexadecyl chain and the area of the trimethylammonium group are calculable to smaller values. In addition, the value obtained for surface area is influenced by the size of the molecule. A large molecule may not be able to enter pores or fissures in the surface that, are accessible to smaller molecules and, therefore, yield a smaller area for the surface. Finally, the effect of the solvent on the substrate must be considered. The similarity between the adsorption isotherms of HMAB on iron-nickel thin films deposited on 2915 cover glasses and on the glass has been noted above. Figure 2 shows that the amounts of HMAB adsorbed on the two materials do not differ greatly. This result is expected if the deposited metal follows the contours of the glass. C. EDTA Treatment.-Prolonged treatment of lead glass with EDTA appears to increase appreciably the surface that can adsorb HMAB. Roughness factors of over 100 were noted after 15 hr. treatment. The kinetics of EDTA. removal of lead, potassium, and silicon from lead glass to produce the increased surface are reported elsewhere.* Interferometric studies of lead glass a t this Laboratory have also substantiated these data. The interference patterns were obtained by placing a lead-glass plate on an optical flat, illuminating it with a helium light, and photographing the resultant fringes. After the EDTA treatment, the fringe pattern, which had been sharp and distinguishable, became considerably less distinct and showed jagged variations in intensity along the fringe boundary. These jagged and even discontinuous characteristics of the fringes indicate a microroughness of the surface, in this case a result of the chemical etching of the EDTA salt. The adsorption of HMAB froin aqueous solution has been utilized to determine surface areas of several types of glass and iron-nickel thin films. Similar adsorption isotherms were obtained for these dissimilar systems, indicating that this surface area determination technique may be applicable to other systems. Adsorption isotherms obtained previously for other substrate^^^^^^ tend to confirm this view. Additional experimental studies with other types of surfaces using HMAB and related compounds are planned. Acknowledgments.-The authors wish to acknowl-
1238
S. LINDENBAUM AND G. E. BOYD
edge the assistance of Mr. L. L. Egan and Mr. B. E. Gran, and, in particular, the suggestions of Professor
Vol. 67
R. S. Haiisen of Iowa State University that initiated this investigation.
SPECTROPHOTOAIETRIC INVESTIGATION OF THE EXTRACTION OF TRANSITION METAL HALO-COMPLEX IONS BY AMINE EXTRACTANTS BYS. LINDENRAUM AND G. E. BOYD Chemistry Divasion, Oak Ridge National Laboratory, Oak Ridge, Tennessee Received November 19, 1969 The spectra of the extracted species in the distribution of the halo-complex ions of Fe(III), Co(II), Cu(II), Mn( II), and Ni(I1) between organic amine solutions and aqueous chloride and bromide solutions were studied. In each instance the only species observed in the organic phase waR the four-coordinated complex ion. In several cases (Fe(III), Co(II), Cu(II), and Ni(I1)) it was shown that even when the extraction was from aqueous solutions whose spectra indicated the complete absence of the tetrahedrally coordinated species, the spectra observed in the organic phase were in excellent agreement with spectra shown t o be representative of the MCla-2 and MClr- ions. It was not possible t o prepare an aqueous solution of Ni(I1) which showed any indication of the spectrum characteristic of the NiClr-2 ion. The spectrum of Xi(I1) extracted by a toluene solution of a tertiary amine hydrochloride from 13 M LiCl was, however, identical t o spectra of compounds containing NiCl,-2.
Introduction The generally large affinity of quaternary ammonium anion-exchange resins for the transition metal chlorocomplex ions, and the dependence of this specificity on the ligand concentration in aqueous solution, has been the basis for many anion-exchange column separations. It has been shown by Smith and Page2 and others3-6 that a large selectivity also exists for “liquid ion exchange” systems prepared by dissolving tertiary amine hydrochlorides in water-immiscible solvents. Further, a remarkable similarity in the relative orders of selectivity between liquid and resinous anion exchangers may be demonstrated by comparing published data for a liquid amine extraction system6 with that for an anion-exchange resin’ in a plot of the distribution coefficients us. the aqueous hydrochloric acid concentration. It was the objective of this study to determine the nature of the species involved in the ion-exchange extraction of the halo-complex ions of some of the 3d transition metals and to obtain an understanding of the origins of the large selectivities frequently observed. A liquid amine system was chosen for study because it lends itself more readily to spectrophotometric investigation. Spectra of ions absorbed onto ionexchange resins have been measured recently.8-12 In general, it has been found that the species on the anion-exchange resin was the same as that in the liquid amine extractant. In one case, however, Ryan1’ observed that for the anion-exchange resin absorption of hexavalent uranium from nitrate solution, both tri(1) K. A. Kraus and F. Neleon, Proc. Intern. Conf. Peaceful Use8 of Atomic Energy, Geneua, 7 , 113 (1955). (2) E . L. Smith and J. E. Page, J. Soc. C h e m Ind., 67, 48 (1948). (3) G. W. Leddicotte and F. L. Moore, J. Am. Chem. Soc., 7 4 , 1618 (1952). (4) F. L. Moore, “Liquid-Liquid Extraction with High Molecular Weight Aminea.” National Acad. of Sei. Series on Nucl. Sci., Deo. 15, 1960. (5) K. B. Brown, C. F. Coleman, D. J. Crouse, C. A. Blake, and A. D. Ryon, Proc. Intern. Conf. Peaceful Uses of Atomic Energy, I n d , Geneva, 8, 472 (1958). (6) H. A. Mahlman, G. W. Leddiootte, and F . L. Moore, Anal. Chem., 26, 1939 (1954). (7) K. A. Kraus and G. E. Moore, J. Am. Chem. Soc., 75, 1460 (1953). (8) D. K. Atwood and T. DeVries, ibid., 83, 1509 (1961). (9) E. Rutner, J . Phys. Chem., 65, 1027 (1961). (10) J. L. Ryan, ibid., 64, 1375 (1960). (11) J. L. Ryan, ibid., 66, I099 (1961). (12) J. L. Ryan, ibid., 65, 1856 (1961).
nitrato and tetranitrato complex ions occurred in the resin phase. The liquid amine system, however, extracts only the trinitrato complex ion UOr(N03)3-.1a Spectral measurements on ion-exchange resins, however, are beset by difficulties involving scattering of light, obtaining a reproducible blank, and adjusting the concentration of the extracted species. Furthermore, for the extraction of ions from concentrated aqueous solution not all of the ionic species taken up by the resin are necessarily associated with the exchanger sites; some tire present in the electrolyte solution imbibed by the resin phase as the result of Donnan “invasion.” Measurements have shownl4 that the solubility of electrolytes in solutions of tri-n-octyl amine salts in toluene is quite small. Experimental Materials.-Triisooctylamine (TIOA) as obtained from Union Carbide Chemicals Company was pale yellow in color, and its neutral equivalent was 375.5 (theor. 357.7). Solutions of TIOB in toluene were shaken with excess aqueous HCI or HBr to form the hydrochloride or hydrobromide. Tri-n-octylamine hydrochloride, TOA.HC1, was obtained as a white crystalline solid from Enstman Kodak Co., Rochester, Kew York. TOA. HBr was prepared from redistilled tri-n-octylamine (Eastman) by equilibrating an ether solution of the amine with aqueous HBr. The ethereal solution was dried and the ether evaporated to obtain the pure solid TOA.HBr. Cupric bromide and ferric bromide were prepared by the repeated addition of HBr to the nitrate salt and evaporating t o dryness. All other chemicals used were reagent grade. Spectrophotometric Measurements.-Spectral measurements were performed with a Cary Model 14 recording spectrophotometer. Equal volumes of aqueous HCl or HBr solutions containing known amounts of transition metal chloride or bromide were equilibrated with amine-toluene solutions. Absorption spectra of the organic phase and the original aqueous phase were determined, using for the reference cell a solution identical with that in the sample cell except for the presence of the transition metal compound. All measurements were made with 1-cm. path length cells.
Results Extraction of Fe(II1) from HCl and HBr Solutions.Trivalent iron was extracted from 12 M HC1 and 1 M HC1 by 0.225 M TIOA.HC1 in toluene. The spectra taken on the aqueous and organic phases are shown in (13) W. F. Keder, J. L. Ryan, and A. S. Wilson, J. Inorg. Nucl. Chem., 20, 131 (1961). (14) S. Lindenbaum and G. E. Boyd, J. Phys. Chem., 66, 1383 (1962).
