Anal. Chem. 1981, 53, 1889-1894 (1 1) Moore, W. S. Estuarine, Coastal Shelf Sci., 1981, 12, 713-723. &net. Sd. Lett. 1979, (12) Reid, D. F.; Key. R. M.; SChink, D. R. 43, 223-226. (13) Broecker, W. 8. “Sympcusium Diffusion in Oceans and Fresh Waters”; Ichlye, T., Ed.; Lamont Geol. Obs.: Palisaides, NY, 1975; PP 116-145. (14) Mlchel, J.; Moore, W. S. Health Phys. 1980, 38, 663-671.
RECEIVED for review March 16, 1981. Accepted jUly 7, 1981, This work was supporteld in part by funds provided by the
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Office of Water Research and Technology, Project No. OWRT-B-127-SC, u.S. Department of the Interior, Washington, DC, as authorized by the Water Research and Development Act of 1978. Additional support was provided by the Water Resources Research Institute, Clemson Univeristy, and by the College of Science and Mathematics, University Of South
Cooperative and Competitive Adsorption of Propane and Butane on Graphitized Carbon Black Jon F. Parcher” and Ping J. Lln Chemistry Department, The UniversiV of Mississippi, Unlver!sity, Mlsslsslppi 38677
Mass spectrometrlc tracer pulse chromatography is used to determlne the gas-solid equllibrlum Isotherms of propane on two graphitized carbon blacks, Carbopack B and C, over (9 temperature range of 0-100 ‘C at pressures of up to 2000 torr. The specific retentlom volumes of lnflnitely dilute samples of n-butane were measured with propane as a component of the carrier gas to determine the effect of adsorbed propane on the sampling capaclty and chromatographic properties of the adsorbents. Adsorbed propane has a tremendous effect upon the retentlon volume of butane, especlally at low temperatures. The maxlmum retentlon volumes were observed for both adsorbents wlth no propane In the carrler and the retention volumes were dlmlnlshed signlflcantly by small amounts of propane. However, another maxlmum occurred at less than monolayer coverage wlth propane at the lower temperatures. Thls cooperative adsorptlon effect is caused by lateral Interactions hietween the butane and propane preadsorbed on the surfaces.
Solid adsorben ts are used extensively in the field of analytical chemistry for both sampling and chromatographic separation schemes. Tenax-GC, XAD resins, and charcoal are the most commonly used adsorbents for collection of air and water samples, and graphitized carbon blacks (GCB) are probably the most commonly used adsorbents for gas chromatographic separation of these samples. Although these materials are very useful adsorbents, the adsorption mechanisms of the individual components of a complex mixture on these adsorbents are complex and not well understood. Adsorbate interactions may occur on the surface of the adsorbent; Le., one component may influence the adsorption of another component. Very polar components or a predominant component in a mixture may have a tremendous effect on the capacity of the adsorbent €or minor or trace components. For example, several authors (1-4) have shown that a very small amount of a chromatographicliquid phase coated on GCB can have a profound influence on the adsorption characteristics of the solid. These “liquid-modified”adsorbents are also very popular chromatographic stationary phases because the adsorbed liquids decrease the retention time and peak asymmetry of the volatile solutes. In a multicomponent sample, the different solutes must compete for the available adsorption sites on the adsorbent. This effect is not significant at low concentrations; however,
if some components atre present at high concentrations, such as water or carbon dioxide in an air sample, this competitive adsorption may diminish the sampling capacity and retention times for many comlponents in the mixture in a generally unpredictable manner. The extent of the change in capacity or retention varies with the chemical characteristics of the different components. In general, the presence of a preadsorbed component on the surface of an adsorbent will diminish the capacity of the adsorbent for other 13olutes(5). However, there are some unique systems in which adsorbate interactions can lead to enhanced adsorption. This phenomenon has been observeid for graphitized carbon blacks ( 1 4 ) as an increase in both the retention volume and the isosteric heat of adsorption wit:h surface coverage at less than monolayer coverages. The generally accepted explanation for this “cooperative” type olf adsorption is that on a homogeneous surface lateral interactions between adsorbate molecules can cause enhanced adsorption of some adsorbates. Di Corcia and Liberti (6) h a w reviewed the complex area of gas-liquid-solid Chromatography and have suggested that cooperative adsorption can be experimentally observed only under certain conditions. Thew conditions are that the surface must be homogeneous, there must be sufficiently strong lateral interactions, and the affinit?, of the adsorbate for the residual surface homogeneities must not be too great. Cooperative adsorption effects have been observed for nonvolatile modifiers, such as squalane, dihexyloctadecane (41, and PEG-1500 (3) and also for volatile modifiers, such as benzene (3, cyclohexane (8,9), and n-hexane (7,9). Brunei. et al. (2) have shown that preadsorbed hexane and squalano both have the same enhancing effect on the heat of adsorption of hexane. Some workers have used volatile modifiers to eliminate solid support adsorption or to change the polarity of the liquid phase in gas-liquid chromatography. Nonaka (10) has re.. viewed the applications of steam as a carrier gas, and Tsuda and Ishii (11-23) have used hexane, butyl chloride, ethyl1 acetate, ethanol, benzene, and carbon tetrachloride as carrier gases. The vapors have profound effects on the retention of‘ some solutes and very little effect on others. The use of‘ modifiers is advantageous because the chromatographic results1 are usually decreased rietention times and increased efficiency (symmetrical elution peaks) due to the elimination of retention by the solid support. Other applications have involved the use of a condensable component in the carrier gas to produce binary liquid phases for gas-liquid chromatography (14,15).
0003-2700/81/0353-1889$01.25/00 1981 American Chemlcal Society
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ANALYTICAL CHEMISTRY, VOL.
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The major limitations of these techniques are the need for a detection system which will not be swamped by the modifier and the uncertainty of the effect of the modifier on the retention volumes of the different components of a complex mixture. In the present investigation, the technique of mass spectrometric tracer pulse (MSTP) chromatography (16) was used to study cooperative and competitive adsorption of two simple hydrocarbons on different graphitized carbon blacks. Propane was used as the volatile modifier and the effect of this modifier on the retention of very small samples of butane was determined over a range of temperatures and pressures. The main objectives of the investigation were to evaluate the experimental technique for multicomponent systems, to investigate cooperative and competitive adsorption effects, and to compare the effect of volatile and nonvolatile modifiers on the adsorptive and chromatographic properties of different graphitized carbon blacks.
Table I. Specific Retention Volumes (mL/ma) of Propane and Butane on Graphitized Carbon Blacks
Carbopack B temp, "C propane butane 0
Carbopack C propane butane
11.4
2.16
0.14
3.08 0.83
0.54
0.061
0.28
10
0.36
40 70
90
0.052
100 110 130 150
0.29 0.20
0.033 0.022
0.15 0.088 0.053
01015
I
-1
d
EXPERIMENTAL SECTION Apparatus. The GC/MS system used for this investigation
was a Hewlett-Packard Model 5985A, and the experimental procedure of MSTP chromatography was the same as that described previously (16).The instrumentation was improved by the addition of a gas sampling valve for introduction of the inert gases and isotopic solutes and two pressure transducers with digital readout (Setra Systems, Inc., type 304)to monitor the inlet and outlet pressures continuously. Reagents. The graphitized carbon blacks used in this work were Carbopack B and Carbopack C (Supelco,Inc.) with surface areas of 100 m2/g and 12 m2/g, respectively. The columns were l/g in. copper columns 1-3 m long with packing weights of 0.8-4 g. The shorter columns were used for the low-temperature experiments in order to achieve reasonable retention times. The experimental data were determined to be independent of the column length and the weight of packing for each column. The stable isotope of propane was propane-2,2-d2 (Merck & Co., Teterboro, NJ). The inert gases were all Linde Research grade. Procedure. Some of the isotherms in this investigation were measured at subambient temperatures attained by the use of a liquid carbon dioxide cryogenic unit on the gas chromatograph in the GC/MS system. Subatmcsphericpressures of propane were obtained by two different techniques. In the first case, the chromatographic column was operated at low pressures of pure propane by venting the column outlet to vacuum. In the other case, the column was operated at high pressures with a carrier gas which was a mixture of propane and helium. No measurable difference was observed in the experimental data for these different techniques. The retention time t , of a hypothetical unretained solute was a critical factor in the accurate determination of isotherm data in this study. In this investigation, tm was determined from the retention times of four inert gases (Ne, Ar, Kr, and Xe) fit to a nonlinear least-squares regression of the following equation:
where tRi and Mi are the retention time and molecular weight of the inert gas i, a and b are constants, and +pi is a potential energy parameter. The modified Buckingham potential parameter (17) gave the best fit to eq 1 and was used in this and previous investigations (16, 18). The elution peaks were symmetrical for both propane and butane on Carbopack C at each temperature. However, the elution peaks were not symmetrical for butane on Carbopack B at the lower temperatures. Two techniques were used to produce meaningful retention data for butane, i.e., data independent of sample size, at these low temperatures. At higher temperatures, the elution peaks were symmetrical, the retention times were independent of sample size, and the heat of adsorption was constant so that a plot of In Vgovs. 1/T was linear. This linear relation could be used to extrapolate Vgodata to lower temperatures. Another approach to the same problem was to extrapolate retention times at several sample sizes to get a hypothetical value
m Propano Prersure (Torr 1
Flgure 1. Adsorption isotherms of propane on Carbopack B: 0, 0 'C; 0 ,40 OC; A, 70 OC; 0 , 100 OC.
for zero sample size. In this study, the two extrapolation techniques yielded approximately the same retention data.
RESULTS AND DISCUSSION The specific retention volumes (expressed as mL/m2) of infinite dilution samples of propane and butane were measured on Carbopack B and C over a range of temperatures, and these data are presented in Table I. These experiments were carried out by normal elution techniques with nitrogen as the carrier gas and with a flame ionization detector. On the GC/MS system, another series of experiments was carried out with propane or a mixture of propane and helium as the carrier gas. The injected sample was a gaseous mixture of the four inert gases, n-butane, and propane-d2. All of these solutes could be detected, in the presence of propane, by the mass spectrometer operated in the selected ion mode. The retention times of the inert gases were used to determine the retention time of a hypothetical unretained solute. The retention time of the propane isotope was used to measure the amount of propane adsorbed on the stationary phase (IS), and the common specific retention volume of butane was determined from the retention time of that solute. These measurements were carried out over a range of temperatures and propane pressures, and the data are presented in Table I1 and Figures 1 and 2. The limiting (P 0) heats of adsorption of both propane and butane were determined from the temperature dependence of the specific retention volumes. Plots of In Vgovs.
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 12, OCTOBER 1981
Proi,me ~ r e r s u n(Torr)
Figure 2. Adsorption isotherms of propane on Carbopack C: 0, 10 O C ; 0 40 O C ; A, 70 O C ; 0 , 100 O C .
o L 1
1
1
2 5 4 Propiin. Adwrbad
1
5 (ymol/m')
,
I
0
7
1691
8
Figure 4. Specific reterition volume of n-butane as a function of the amount of propane adsorbed on Carbopack 8. Legend is the sanie as Figure 1.
+
0
A Pn-ne
.L
1
1 4 Adwrbed ( p m o l / d )
5
Figure 5. ( I
3 4 R o p a n , ~Adsorbed (ymel/m')
Figure 3. Isosterlc heats of adsorption of propane on graphitized carbon blacks: 0, Carbopack B; 0,Carbopack C.
1/T were all linear with correlation coefficients of 0.99 or better, These limiting heats are given in Table 111,along with some previously determined literature values. These are in good agreement with the values obtained in this study. The isosteric heats of adsorption of propane at fiied surface coverages can be determined from the adsorption isotherms as the slopes of plots of lnl P(fixed surface coverage) vs. 1/T. These plots were linear over the-range of temperatures and pressures studied, except for the data for Carbopack C at the lowest temperature. The cause of this discrepancy is not known; however, the data for the other three temperatures were linear and the calculated isosteric heats of adsorption are given in Table IV and Figure 3, along with the limiting data obtained from the specific retention volumes of propane. Figure 3 is a plot of the isosteric heat vs. the amount of propane adsorbed on the surface, and the maxima at about 1pmol/m2 is indicative of cooperative adsorption effects. This cooperative adsorption is evident on both of the adsorbents, even though the Carbopacks do not have the homogeneous surface normally required for cooperative adsorption. Bruner et al. (1) have shown that Carbopack C, in particular, will not support cooperative adsorption of pentane by squalane unless the surface is reduced with hydrogen a t high temperatures to remove oxides from the adsorbent surface. Other GCBs, such as Sterling F'F G would support cooperative adsorption with or without the hydrogen treatment. The maxima in the heat of adsorption at submonolayer coverages has been observed for a variety of modifiers; how-
Specific retention volume of n-butane as a function of the amount of propane adsorbed on Carbopack C. Legend is the same as Figure 2. ever, propane is the lowest molecular weight modifier studied to date. The magnitude of the effect of cooperative adsorption on the heat of adsorption is proportional to the molecular weight of the modifier. In the study of Bruner et al. (2), the increase in the heat of adsorption of hexane was 4 kcal/mol with squalane as the modifier and 2 kcal/mol with hexane as the modifier. In the present study, the enhancement in the heat of adsorption for propane was only about 0.5 kcal/mall on both adsorbents. Although the magnitude of the effect is lower for propane, the surface homogeneity requirements are less and the effect is easier to observe on adsorbents which would not support coolperative adsorption with high molecular weight, nonvolatile modifiers. Cooperative adsorption is maximal at less than half of the monolayer coverage for propane and there it3 no sharp decrease a t the point of monolayer coverage a8 observed for other, higher molecular weight modifiers (1-4). In general, volatile modifiers apparently can bring about the same type of cooperative adsorption as nonvolatile modifiers, although the magnitude of the enhancement is less and the effect can be observed on lesi3 homogeneous adsorbents. The adsorption isotherms were measured in the range of temperatures normally encountered in sampling and chromatographic procedures, rather than the higher pressures and temperatures of engineering interest. The isosteric heats ojf adsorption are easy to determine experimentally, but these are not the only physical measures of the sampling capacity or chromatographic retention properties of the adsorbente. The retention volume of a solute is also a good measure of these properties and includes both enthalpy and entropy
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 12, OCTOBER 1981
Table 11. Adsorption and Retention Data for Propane and Butane Carbopack B propane pressure, torr
propane adsorptn, Mmol/m2
Carbopack C spec retention vol of butane, mL/mZ
propane pressure, torr
100 "C 0.0
79.0 121 275 278 421 433 834 839 881 1010 1040 1050 1060 1120 1270 1540 1740 0.0
63.0 93.0 104 191 359 491 751 1010 1100 1460 1810
0.0
0.16 0.27 0.41 0.52 0.74 0.79 1.5 1.4
32.0 46.0 66.0 109 150 445 458 789 974 1090 1210 1270 1330 1490 1620 1730
0.20 0.16
0.13
0.0
105 596 1010 1230 1580
0.0
m1/m2
0.0
0.15 0.62 1.3 1.4 2.1
0.11 0.09 0.08 0.08 0.07 0.07
0.11
1.7 1.7 1.5 1.6 1.6 1.9 2.2 2.4 3.0 70 "C 0.0
0.28 0.41 0.47 0.87 1.3 1.8 2.7 3.0 3.6 4.3 4.7
0.10
0.11 0.09 70°C
0.54 0.32 0.37 0.35
0.0
71.8 122 545 1080 1470
0.0
0.33 0.49 0.69 1.2 1.2 2.8 3.0 3.8 4.2 4.4 4.9 4.9 5.4 5.4 5.5 5.6 0.0
1.1 1.6 2.2 2.9 3.1 3.6 3.9 4.2 5.5 5.6 6.5 8.1 8.3
0.0
0.21 0.36 1.2 2.2 2.5
0.28 0.21 0.23 0.17 0.15 0.12
0.28 0.24 0.17 40 "C 2.16 1.08 1.13 1.12 1.26
0.0
91.0 134 524 954 1070 1700
0.0
0.61 0.96 2.1 2.9 3.0 3.8
0.83 0.70 0.72 0.39 0.24 0.21 0.14
0.58 0.33
0.35
0 "C
35.0 40.0 56.0 75.0 100 212 325 403 758 866 1150 1830 1830
spec retention vol of butane,
100 "C
40 "C 0.0
propane adsorptn, pmol/m2
10 "C
11.4 3.79 6.56 6.51 4.83 2.54 1.61 0.63 0.38
0.0
11.1 13.7 34.3 43.6 61.9 67.2 107 262 608 1030 1630
0.0
0.21 0.25 0.57 0.81 1.4 1.4 1.9 2.8 3.4 3.7 4.5
3.08 2.31 2.43 2.18 2.55 2.89 2.88 2.24 1.23 0.52 0.27 0.18
ANALYTICAL CHEMISTRY, VOL. 53, NO. 12, OCTOBER 1981
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-____
Table 111. Limiting Heats of Adsorption of Propane and Butane on Graphitized Carbon Black
limiting heats of
solute
adsorbents
-DroDane butane
adsorption, lit. values, kcal/mol kcal/mol
Carbopack B
Carbopack C Clarbopack B
-6.35 -5.64 -8.35
Clarbopack C
-7.77
-6.5 (19)
-8.0 (19), -7.9 ( 2 0 ) a -7.0 ( 4 ) , -8.0 ( 2 )
a
Spheron 6 ( $ 9 mz/g).
* Stering FT ( 1 5 mz/g).
Table IV. Isosteric Heats of Adsorption of Propane on Graphitized Carbon Black heat of adsorption, surface kcal/mol coverage., pmol/m2 Carbopack B Carbopack C
-
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5
-6.35 -6.75 -6.98 -6.75 -6.64 -6.48 -6.09 -4.61 -3.50 -3.05 ,-2.58 -2.57
-5.64 -6.05 -6.11 -5.32 -4.56 -3.92
contributions. The specific retention volumes of infinitely dilute samples of butane were determined u8 a function of the amount of propane adsorbed, and the data are presented in Table I1 and Figures 4 and 5. Propane has a drastic effect on the retention volume of butane, so the plots are given as In Vgovs. the amount of plropane adsorbed on the surface of the GCBs. The mechanism of the retention or adsorption of butane is very complex and is dependent upon the temperature, pressure, and surface properties of the adsorbent. The maximum retention volume is observed for the unmodified (no propane) adsorbent at each temperature. However, at lower temperatures, another maximum is observed at 0.5-2 pmol/m2. The magnitude of the maximum is greater at low temperatures and it occurs at a higher surface coverage. This type of adsorption is probably a combination of competitive and cooperative adsorption. Competitive adsorption is responsible for the dramatic decrease in the retention volume (shown as the dotted line) with the amount of propane preadsorbed at very high and very low surface coverage, and cooperative adsorption effects cause the enhanced retention a t finite surface coverages of less than a monolayer. In this case, it is not possible to determine the isosteric heat of adsorption of butane chromatographically from the temperature dependence of Vgobecause the heat of adsorption is not constant at a fixed aiurface coverage. The heat of adsorption is determined by ithe adsorption mechanism at that coverage and the mechanirim will vary with the temperature in these systems. It is also impossible to describe this type of retention behavior with the simple equation often proposed for use with gassolid or gas-liquidsolid chromatography(21). (1) V , == KiV1 + Ki,A1, + K,A,, Where K,, Kb, and K, represent partition and adsorption constants for the liquid, gas-liquid interface, and solid support, and V and A are the volume of bulk liquid and the surface
ICnpmna P n s s r r * (TW)
Flgure 6. Specific retentlon volume of n-butane as a function of the partial pressure of propane In the carrier gas: 0, Carbopack B at ID O C ; a, Carbopack C at 10 O C .
areas of the liquid and solid, respectively. In these system8, not enough propane is adsorbed to form a liquid layer, so the first two terms of the equation are negligible. The exact interactions between the adsorbate and the surface or other adsorbates are uncertain; however, it is obvious that the solid surface adsorption coefficient is dramatically influenced by the amount of propane adsorbed on the surface. The very high specific retention volumes of butane on th8 unmodified GCBs are indicative of the presence of specific active sites on the surface which are covered, or blocked, by a small amount of a moderator. One of the main difference13 between nonvolatile and volatile modifiers is that the latteir can cause cooperative stdsorption on this type of heterogeneous adsorbent, whereas the former cannot. Thus, it is possible that the necessary condition for cooperative adsorption is not surface homogeneity but rather uniform distribution of the modifier on the adsorbent surface. This condition will be easiest to fulfill with volatile, nonpolar modifiers which are in dynamic equilibrium with the surface. The ideal adsorbent for air and water sampling would be an adsorbent which could be “switched” on and off for collection and desorption of the sample. The systems studied1 in this investigation hiave that capability to a very limited. extent. The effect of propane on the retention volume of‘ butane is very large at low pressures. This is illustrated in Figure 6 which is a plot of Vgobutane as a function of the! partial pressure of propane in the carrier gas, rather than the amount of propane adsorbed. The major excursions of Vgo occur at pressures of less than 100 torr. Small changes in the pressure of propane can induce very large changes in the sampling capacity of the adsorbent for n-butane. The phenomenon could also be used to advantage by pressure or flow programming of the carrier gas to increase the amount of propane adsorbed and diminish the retention of other solutes in a technique similar to common temperature programming. This is possible with volatile modifiers but not with the higher molecular weight, nonvolatile modifiers. The major limitation is the obvious need for a detection system which is not responsive to the particular volatile modifier. The experimental technique of mass spectrometric tracer pulse chromatography can be used to investigate these types of multicomponent systems which are not amenable to other
Anal. Chem. WS1, 53, 1894-1899
1894
experimental procedures. In this study only two simple hydrocarbons were investigated; however, the results were far from simple. The complex interactions of solutes on the surface of adsorbents are not well understood, in part because of a paucity of experimental data, and further investigations are under way to provide additional information, especially for systems involving polar solutes and modifiers.
(10) Nonaka, Akira In “Advances in Chromatography”; Giddlngs, J. C., et al., Marcel Dekker: New York, 1975; Vol. 12, pp 223-260. (11) Tsuda, Takao; Tokoro, Nobuo; Ishil, Daido J . Chromatogr. 1970, 46, 241-246. (12) Tsuda, Takao; Ishii, Daido J. Chromatogr. 1973, 87,554-558. (13) Tsuda, Takao; Yanaglhara, Hideo; Ishil, DaMo J. Chromatogr. 1974, 707. 95-102. (14) Siu, K. W. Michael; Aue, Walter A. J . Chromatogr. 1960, 789, 255-258. (15) Parcher, Jon F.; Westlake, Theodore N. J. Phys. Chem. 1977, 87, 307-313. (16) Parcher, Jon F.; Selim, Mustafa I. Anal. Chem. 1979, 51, 2154-2156. (17) Nakahara. Tomoko; Chappeiear, Patsy S.;Kobayashi, Rikl Ind. Eng. Chem. Fundam. 1977, 16, 220-228. (18) Parcher, Jon F.; Johnson, David M. J. Chromatogr. Scl. W80, 78, 267-272. (19) Ross, Sydney; Olivler, James P. “On Physical Adsorption”; Interscience: New York, 1964; p 245. (20) Chknside, G. C.; Pope, C. G. J. Phys. Chem. 1964, 68, 2377-2379. (21) Conder, John R.;Locke, Davld C.; Purneil, J. Howard J. Phys. Chem. 1969, 73,700-708.
LITERATURE CITED (1) Bruner, Frabrizio; Bertoni, Giullano; Clccloli, Paolo J . Chromatogr. 1976, 120, 307-319. (2) Bruner, Frabrizio; Ciccioli, Paolo; Crescentlni, Giancarlo; Pistoiesl, Maria Teresa Anal. Chem. 1973, 45, 1851-1859. (3) Di Corcia, Antonio; Liberti, Arnaldo; Samperi, Roberio Anal. Chem. 1973, 45, 1228-1235. (4) Bruner, Fabrizio; Bertoni, Giullano; Montaii, R.; Severini, C. Ann. Chim. (Rome) 1978, 68, 565-573. (5) Bertoni, Giuliano; Bruner, Fabrizio; Llbertl, Arnaldo; Perrlno, C. J. Chromatogr. 1981, 203,263-270. (6) Di Corcia, Antonio; Llbertl, Arnaldo In “Advances in Chromatography”; Giddings, J. C., et ai., Eds.; Marcel Dekker: New York, 1978; Vol. 14, pp 305-366. (7) Kiseiev, Andrei V.; Yashin, Yakov I . “Gas-Adsorption Chromatography”; Plenum Press: New York, 1969; Chapter IV. (8) Dondi, Francesco; Gonnord, Marie-France; Gulochon, Oeorges J. Cd/OMInterface Sci. 1977, 62,316-328. (9) Von Rybinski, W.; Findenegg, 0. H. Ber. Bunsenges. Phys. Chem. 1979, 83,1127-1130.
RECEIVED for review April 13,1981. Accepted July 16,1981. Acknowledgment is made to The National Science Foundation (Grant No. CHE-7809918) and to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research.
Extraction of Alkali Metal Cations from Aqueous Solutions by a Crown Ether Carboxylic Acid Jerzy Strzelbickl’ and Richard A. Bartsch” Department of Chemlstty, Texas Tech University, Lubbock, Texas 79409
Solvent extraction of alkali metal cations from aqueous soiutions by sym-dibenzo-16-crown-5-oxyacetlc acid in chioroform Is reported. Influence of pH, metal ion identity and concentration in the aqueous phase, and anion identlty in the aqueous phase upon the concentrations of metal and compiexing agent in the organic phase are assessed for single ion extractions. Extraction efficiency is insensitive to changing the aqueous phase anion from chloride to sulfate which demonstrates that the anion is not transferred to the organic phase during extraction. Extraction efficiency is very sensitive to the pH of the aqueous phase. For competltive ion extractions, the selectivity is K > Na > Rb 2 Cs > Li for pH 6-7 and Na > K > Rb = Cs > Li for pH 8-12. The crown ether carboxylic acid exhibits extraction efficiencies and selectivities which surpass those of symdibenzo-16-crown-5 methyl ether or phenoxyacetic acid.
The influence of the anion upon the ability of a macrocyclic compound to extract metal ions from aqueous media into organic solvents has important bearing upon the potential practical applicationsof crown ethers for the selective removal of specific metal ions from natural and industrial water sources. For process solvent extraction, the anions normally encountered are chloride, nitrate, and sulfate. Distribution coefficients for metal chlorides, nitrates, and sulfates between Present address: Institute of Inorganic Chemistry and Metallurgy of Rare Elements, Technical University of Wroclaw, 60-370 Wroclaw, Poland.
an aqueous phase and a hydrocarbon or chlorocarbon phase which contains crown ethers are too low to be useful (1-4)Recent efforts to control these unfavorable distribution coefficients involve the addition of compounds to the organic phase which can hydrogen bond with the anion and make it more lipophilic (1-3). These additives include tributyl phosphate, bis(2-ethylhexy1)phosphoric acid, didodecylnaphthylenesulfonic acid, nonylphenol, and 2-ethylhexanol. Substantial synergistic effects are noted for extraction of cesium and strontium nitrates into organic media using both crown ethers and anion lipophilizing agents (3). In another study, protic organic phase solvents, such as rn-cresol, which can themselves solvate the anions are utilized (5). An alternative solution to this problem is the design of new macrocyclic complexing agents for which metal ion extraction does not involve transfer of the aqueous phase anion into the organic phase. Such compounds would resemble the acyclic carboxylic ionophores monensin and nigericin which, when ionized, effectively transport mono- and divalent metal cations across membranes (6). Although several crown ether cmpounds which bear ionizable groups have been synthesized (7-9),no systematic study of metal ion extraction from aqueous media into organic solvents using such complexing agents has appeared. We now report the results of an investigation in which the crown ether carboxylic acid 1 is utilized to extract alkali metal cations from aqueous solutions of the metal chlorides or sulfates into chloroform. This investigation of alkali metal cation extraction from an aqueous phase into chloroform utilizing 1 includes: (a) determination of single cation extractabilities for Li, Na, K,
0003-2700/81/0353-1894$01.25/00 I981 American Chemical Society
