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Anal. Chem. 1091, 63,1018-1023
Solution Studies of 0-Cyclodextrin-Pyrene Complexes under Reversed-Phase Liquid Chromatographic Conditions: Effect of Alcohols as Mobile-Phase Comodifiers Arsenio Muiioz de la Peiia,' Thilivhali T. Ndou, Vincent C. Anigbogu? and Isiah M. Warner* Department of Chemistry, Emory University, Atlanta, Georgia 30322
Studies of pyrene complexes wHh @cyclodextrln, using reversed-phase (C,J lipukl chromatography require a relatively more nonpolar moMle phase than water (mixtures of methanol-water >55 % methanol) In order to achkve a reasonable retenth t h e . AlUlough methanol has a very low association constant wlth @-cydodextrln,It becomes strongly competttlve at hlgh concentratlons, resuttlng In very weak Interaction between pyrene and cyclodextrln. The presence of teri4)utyl alcohol or cyclopentanol In the m e d h Increases the strength of the B-cyclodextrln-pyrene complex by various orders of magnttude due to the formatlon of a ternary complex. I n the presence of these alcohols as moble-phase comodlflers, the Interaction between 8-cyclodextrin and pyrene becomes evldent at methanol concentrations in the range of practlcal use for HPLC.
INTRODUCTION Cyclodextrins (CDs) have previously been employed as mobile-phase modifiers in liquid chromatographic separations (1-14). The potential of CD-bonded phases in high-performance liquid chromatography (HPLC) has been demonstrated in the separation of optical, geometrical, and structural isomers (15-1 7). The advantages of CDs over micelles are that they do not foam when purging solutions with nitrogen. Furthermore, the fixed cavity sizes of the cyclodextrins govern the mechanism of solubilization/retention of the analyte by forming species of defined stoichiometry. Chromatographic separations, using CDs as mobile-phase modifiers, are largely the resuit of selectivity in the formation of inclusion complexes. The elution time of a given solute is a function of the strength of these complexes. Several intermolecular interactions are responsible for the complex formation of cyclodextrin inclusion complexes. These driving forces act synergistically and are related to the properties of the guest. Since the formation constant is dependent on many factors, a high degree of selectivity has been demonstrated when using CDs as mobilephase modifiers. The high stereoselectivity afforded by cyclodextrin complexation has been widely used for isomeric separation by HPLC (3-14). It has also been suggested that marked sensitivity improvements in fluorescence chromatographic detection may result from using cyclodextrin aqueous mobilephase modifiers, which allow moderate inclusion of the analytes (18). The growing interest in the use of cyclodextrins in chromatographic methodologies lead to studies of the behavior of several compounds as models for insight into the complex chemical interactions that occur on the chromato*Author for corres ondence. Present address: hepartment of Analytical Chemistry, Universit of Extremadura, Badajoz, Spain. :Present address: Department of Chemistry, Austin Peay State University, Clarksville, T N 37044. 0003-2700/91/0363-1018$02.50/0
graphic column when using cyclodextrins as mobile-phase modifiers. Recently, the retention behavior of a group of polycyclic aromatic hydrocarbons (PAHs) was investigated by using reversed-phase HPLC (19), in methanol-water mixtures and in the presence of 0-CD as a mobile-phase modifier. The formation constants of the inclusion complexes of several compounds with 0-CD have been determined by using reversed-phaseHPLC in methanol-water mixtures (20). Using dynamic coupled liquid chromatography, we determined the formation constants of cyclodextrins with several PAHs (21).
It should be noted that pyrene is often used as a probe of the polarity of a microenvironment (22-25). The vibrational fine structure of the pyrene monomer fluorescence spectrum undergoes significant changes in different media, especially with regard to relative band intensities. It has been shown that the ratio of the intensity of the third or fifth vibronic band with the first band (I/III or I/V) yields a sensitive measure of polarity. The intensity ratio parameter of pyrene cited above has been used in chromatographic studies to assess the microenvironment of adsorbed molecules of pyrene on polypropylene (261,polymeric C2 and C18stationary phases (27), and polymeric and monomeric CI8reversed phases, over a wide range of mobile-phase conditions (28). Street and Acree (29) have investigated the pyrene solvent scale for binary solvent mixtures used in reversed-phase liquid chromatography. Also, the micropolarity scale of pyrene using a-,p-, and -&De as mobile-phase modifiers has been investigated (18). In our laboratory, the I/III ratio has also been used to study the influence of mobile-phase alcohol modifiers in HPLC separations of PAHs using bonded-phase cyclodextrin columns. Dramatic changes in the chromatography were reported in the presence of the alcohol (30). It has been previously suggested that the effect of alcohols on the complexation may prove to be useful in CD chromatography, where alcohols could be used to selectively enhance the retention of some compounds. This process could be effectively used to manipulate retention of specific compounds (31). In this laboratory, we have previously shown the effect of solvent polarity on the I/III ratio of pyrene using 5% methanol and 100% water (30). The difference between the I/III ratios for 5% methanol and 100% water was minimal, suggesting that 5 % methanol varies only slightly in solvent polarity and average dipole moment from 100% water. As a result of this initial study, we further examined the changes on the I/III ratio of pyrene under more practical HPLC chromatographic conditions using p-CD in the mobile phase. The competitive equilibria between pyrene and methanol for the cyclodextrin is also discussed. In our previous study, we also examined the effect of tert-butyl alcohol (t-BuOH) as a mobile-phase comodifier using cyclodextrin-bonded phase. In the work reported here, the presence of tert-butyl alcohol and cyclopentanol, as mobile-phase comodifiers of the system, is investigated. Apparent formation constants for the complexes formed at different methanol concentrations and in the 0 1991 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 63, NO. 10, MAY 15, 1991
presence of tert-butyl alcohol or cyclopentanol are reported, and some implications on the chromatographic behavior of these systems are discussed. EXPERIMENTAL SECTION Apparatus. Steady-state fluorescence measurements were performed on a Perkin-Elmer LS-5 fluorescence spectrophotometer, controlled by an IBM AT microcomputer. Solutions were excited at 335 nm, with excitation and emission bandwidths of 5 and 3 nm, respectively. Since the slit width affects the calculated I/III ratio values (32), our values are -15% lower than those reported by Dong and Winnik (33) using a 1-nm emission slit width for the pyrene polarity scale. The original spectra were not smoothed, and the I/III ratio values are an average of three scans. All values had a standard deviation of f0.03 or better. Materials. Pyrene (99+%) was obtained from Aldrich and used as received. The 8-cyclodextrin was obtained from American Maize Products (Hammond, IN) and was recrystallized twice from water before use. Methanol (Burdick & Jackson, High Purity solvent), tert-butyl alcohol, and cyclopentanol (Aldrich, High purity grade) were used as received. HPLC grade water was purchased from Fisher and used in all experiments. Methods. A 5.0 X 10-4M stock solution of pyrene was prepared in cyclohexane. Aqueous or methanolic pyrene solutions were prepared by pipeting an aliquot of the stock solution into a 1WmL flask. The cyclohexane was then evaporated using dry nitrogen and the flask diluted with water or methanol to give the desired pyrene concentration. A. Methanol-Water Mixtures. Influence of Methanol Concentration. An aliquot of the aqueous pyrene solution was transferred into a 10-mL flask to give a 1.0 X lo-' M pyrene concentration. A weighed quantity of P-CD was added to give or 2.5 X M 8-CD Concentration. The necessary a 1.5 X amount of methanol was added to give the desired alcohol concentration. Methanol concentrations were varied between 0 and 70% by volume. Influence of @-CDConcentration. An aliquot of the methanolic pyrene solution was transferred into a 10-mL flask to give a 1.0 X lo-? M pyrene solution after dilution to volume. A weighed quantity of /3-CD was added to give the desired 8-CD concenand 6.0 X M. The necessary trations between 1.0 X amounts of methanol were added to give the desired alcohol concentrations. Methanol concentrations were varied between 15 and 65% by volume. B. Methanol-t -BuOH a n d Methanol-Cyclopentanol Mixtures. Influence of p-CD Concentration. An aliquot of the appropriate pyrene solution in methanol was transferred into a 10-mL flask to give a 1.0 X lo-? M pyrene solution after dilution to volume. A weighed quantity of 8-CD was added to give the desired 0-CD concentrations ranging from 5.0 X lo4 to 6.0 X M. The necessary amount of methanol was added to give the desired alcohol concentrations. Methanol concentrations were varied between 54 and 63% by volume. The necessary amount of tert-butyl alcohol or cyclopentanol was added to give the desired alcohol concentrations of 1% or 2% by volume. Influence of tert-Butyl Alcohol or Cyclopentanol Concentrations. An aliquot of the appropriate pyrene solution in methanol was transferred into a 10-mL flask to give a 1.0 x lo-? M pyrene solution after dilution to volume. A weighed quantity of @-CD was added to give a 3.0 X low3M concentration. The necessary amount of methanol was added to give a 55% or 60% solution by volume. The tert-butyl alcohol and cyclopentanol were each varied from 0 to 10% by volume. RESULTS A N D DISCUSSION Influence of Methanol on the B-CD-Pyrene Complex. It has been previously shown that pyrene predominantly forms a 2:1 (P-CD:pyrene) complex with 0-CD in pure water (34)and that, in the presence of alcohols (1%v/v), ternary complexes of stoichiometry 2:1:2 @-CD:pyrene:alcohol are formed (35). I t is suggested that the alcohols are positioned at the open ends of the cyclodextrin cavities in these complexes. It was concluded from this work that the size and molecular volume of the alcohol plays an important role in the formation of the ternary complex. The overall formation constant of the 2:l
1010
1.40 1-24 -
-:
1.08 1
0.92-
0.76 0.60
A
.
A
.
1
A
A I,
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ANALYTICAL CHEMISTRY, VOL. 63,NO. 10, MAY 15, 1991 1.60 1
I
I1*40* nn4
0.80
0.60
y ' 0
10
I
30
20
40
50
60
70
% METHANOL (v/v)
Flgure 2. Changes in the V I 1 1 ratio of pyrene with varying methanol concentratlon. (0)I n the absence of P-CD and (A)In the presence of 2.5 x 10-3 M p-CD. 1.60
I
1
Flgwe 4. Typical three-dimensional plot of the micropolarity changes when increasing both P C D and methanol concentrations.
Table I. Apparent Formation Constants for j3-CD-Pyrene in the Presence of Varying Methanol Concentrations
1.42 1.24 1.06
methanol, % v/v
log K2*
0 15 30
5.93 5.56 5.14
40
4.44
0.88
0.70
'
0
I
1
2
3
4
6
6
[8-Cytlodextrlnl (mM)
Flgure 3. Influence of varying S-CD concentration on the I/III ratio of pyrene using different methanol-water mixtures. (0) 65% v/v, (0) 60% vlv, (V)55% V/V,(0)50% v/V, (A)40% vlv, (0)30% vlv, and (+) 15% v/v.
lution of 2.5 X M 8-CD, suggesting that pyrene is included inside the 0-CD cavity (34). Upon addition of methanol, the I/III ratio increases until it essentially reaches the I/III ratio characteristic of pyrene in the methanol-water mixture. Therefore, it can be inferred that the addition of large quantities of methanol breaks up the complex and displaces pyrene from the 0-CD cavity. At around 6 0 4 5 % v/v methanol, in the presence of /3-CD, the I/III ratio is only slightly lower than that of pyrene in pure 60-65% v/v methanol solutions. Effect of @-CDConcentration, Maintaining a fixed methanol concentration a t 65% v/v, the influence of 6-CD concentration on the I/III ratio of pyrene was investigated. The increase of the 0-CD concentration does not have any significant effect on the I/III ratio of pyrene. These data strongly support the results shown in Figure 2. At 65% v/v methanol, the I/III ratio in the presence of 0-CD is only slightly lower than in the absence of P-CD. This suggests that, in 65% v/v methanol, practically all of the pyrene prefers the solvent as opposed to the cyclodextrin cavity. The opposite is true in pure water where pyrene is largely included in the nonpolar CD cavity. Formation Constants of 8-CD-Pyrene Complexes in Varying Water-Methanol Mixtures. It is well established that changes in the I/III ratio of pyrene with increasing CD concentration are correlated with the degree of complexation (34-37). Figure 3 shows the influence of varying P-CD concentration on the I/III ratio of pyrene using methanol-water mixtures ranging from 15 to 65% v/v. The decrease in the I/III ratio becomes less pronounced with increasing methanol concentration, thereby suggesting a weaker complex. Figure 4 represents a typical three-dimensional plot of the micro-
polarity changes when increasing both 0-CD and methanol concentrations. The decrease in the I/III ratio with increasing /3-CD concentration suggests that the microenvironment around pyrene is altered, giving rise to the formation of a binary complex. At high methanol concentrations (250% v/v), methanol becomes strongly competitive for complexation with P-CD, decreasing the degree of complexation between pyrene and p-CD. These data suggest a gradual decrease in the degree of complexation in going from 15 to 40% v/v methanol. For higher concentrations of methanol, the competitive effect of the solvent becomes predominant. This in turn results in a dramatic decrease of the complexation process. It could be inferred that, for 50% v/v methanol, the amount of pyrene complex formed at the highest CD concentration is very small and, for higher methanol concentrations, there is practically no complexation between pyrene and p-CD. The stoichiometry of the binary 0-CD-pyrene complex in pure water is 2:l (34). By using similar expressions as in ref 34, the stoichiometry and apparent formation constants of the complex in 15%, 30%,and 40% v/v methanol concentrations were calculated. The 2 1 stoichiometry was confirmed in these methanol-water mixtures by typical double-reciprocal plots. The observed I/III ratio (R) is related to the P-CD concentration by the following equation:
R=
+ R2K2*[CD]oz 1 + Kz*[CD]02
Ro
where the parameters Ro and R2 denote the ratio for pyrene in the solvent and in the 2 1 complex, respectively, K2* is the apparent overall formation constant, and [CD], is the initial P-CD concentration. By using this equation, the experimental data were directly fitted and the apparent formation constants were calculated by using nonlinear regression analysis (SAS/STAT, Release 6.03, SAS Institute Inc., Cary, NC, 1988). The statistical analysis was performed by using Marquadt's iterative method. Due to relatively small changes in the I/III ratio at methanol concentrations greater than 40% v/v, we could not estimate the apparent formation constants for these methanol concentrations. The calculated apparent formation constants are summarized in Table I.
ANALYTICAL CHEMISTRY, VOL.
In Table I, log Kz* decreases with increasing methanol concentration. It should be noted that at high methanol concentration (60 and 65% v/v) the change in the I/III ratio is not pronounced. The error associated with these measurements does not have any significant effect on the trends in the measured K values. The decrease in the apparent formation constant of the P-CD-pyrene complex with increasing methanol concentration is rationalized in terms of the competitive equilibria between methanol and pyrene for the cyclodextrin cavity. In the presence of another potential guest (e.g., methanol), only a fraction of the cyclodextrin cavities can interact with pyrene, resulting in a decrease in the amount of complexed pyrene. The formation constant of the 1:l P-CD-methanol complex was estimated independently by Matsui and Mochida (38)and Buvari et al. (39),using the spectrophotometric examination of the inhibitory effect of methanol on the association of 0-CD with sodium 4-[ (4-hydroxy-l-naphthyl)azo]-l-naphthalenesulfonate and phenolphthalein, respectively. Values of 0.32 and 0.40 M-' were reported, respectively. Buvari et al. (39) correlated the apparent stability constant of the 0-CDphenolphthalein complex with alcohol concentration and with the true formation constant in pure water, working at alcohol concentrations small enough to avoid any solvent effect. Assuming that the same considerations apply to our system (to a first approximation),the following simple binary complex pattern is used to analyze the data:
P
+ 2CD Me
P(CD),,
+ CD + CDMe,
[CDMe]
KMecD =
[Me1[CDI
(3)
where CD, P, and Me denote P-CD, pyrene, and methanol, respectively, P(CD12and MeCD are the pyrene and methanol complexes formed, respectively, and K2and KMeCD are the corresponding stability constants. The connection between the apparent stability constant K2*and the true formation constant can be expressed as (4)
If we take into consideration the possible formation of higher stoichiometries between @-CDand methanol, we can derive the following equation: K2* =
K2
1 + K M ~ C D [ M+~KMecD[MeI2 ] + ...
(5)
The existence of complexes of stoichiometries 1:2,1:3, and even 1:4 (CDalcohol) has been deduced for small alcohols such as methanol, ethanol, n-propyl alcohol, and n-butyl alcohol. The formation constants for those complexes have been reported for ethanol, n-propyl alcohol, and n-butyl alcohol (37). For 1-pentanol and other alcohols with longer carbon chains, high stoichiometries (e.g., 1:2 complexes) are less favorable. In that manuscript, the authors assumed a similar situation with methanol. However, the low stabilities of complexes of higher orders did not allow for further calculation of formation constants over K M e p In the present work, use of eq 4 gives a calculated formation constant for p-CD-methanol of 0.35 M-' at 15% v/v methanol-water mixtures. This is in agreement with the literature values (38,391. However, the calculated formation constants for 1:l P-CD-methanol complexes a t 30 and 40% v/v methanol are higher, suggesting that other processes must be taken into consideration for this high methanol concentrations. If
63,NO. 10, MAY 15, 1991
1021
higher orders of 0-CD-methanol complexes are formed, methanol may further displace pyrene from the cavity. The formation of such complexes will more likely take place at higher methanol concentrations. In addition, at high methanol concentrations, the properties of the bulk solvent begin to change substantially. Consequently, the presence of the alcohol will likely make the solvent more favorable to pyrene than a simple aqueous system. Furthermore, the difference in hydrophobicity of the solvent and CD cavity will become smaller, making pyrene complexation with CD less favorable. Either or both of these effects could account for the dramatic decrease in complex formation when the methanol content in the mixture is further increased. The formation constant of the 1:l pyrene-alcohol complexes was estimated by Limos and Georghiou (40)and Nakajima (41). The estimated weak formation constants were rationalized in terms of hydrogen bonding between pyrene and the alcohols. This is in agreement with the observation of Opallo et d. (42),who reported the complexation of 2,3-anthracenedicarboxylate(ADC) with @- and 7-CD in water and in water-organic solvent mixtures, using induced circular dichroism. They showed that an increase in the organic solvent ratio causes the degradation of the (3-CD-ADC complex. They also reported that, the higher the hydrophobicity of the cosolvent, the weaker the complexation of ADC by @-CD. This is also consistent with Lamparczyk et al. (20),who reported only small changes in the retention behavior of several PAHs in methanol-water mixtures (50-70% v/v methanol in water), in the presence and absence of @-CDin the mobile phase. The effect is rationalized in terms of inductive and dispersive interaction between the mobile phases and the solutes. This in turn suggests that the formation of inclusion complexes with P-CD has a relatively slight effect on the retention behavior. In view of our findings, the interpretation presented above is more applicable when using relatively high methanol-water mixtures (>55%). Again, this suggests that, under real HPLC conditions, at methanol concentrations greater than 55%, the presence of P-CD alone in the mobile phase should not have any effect upon the retention time of pyrene. Influence of t-BuOH and Cyclopentanol as MobilePhase Comodifiers. The data presented above demonstrate that under normal conditions used for HPLC experiments pyrene does not interact with P-CD, or the interaction is very weak. It has been previously demonstrated that the presence of small amounts of alcohols increases the stability of the binary @-CD-pyrene complex in pure water, by various orders of magnitude, due to ternary association (34). The strongest complexes are formed in the presence of large and bulky alcohols, e.g., the apparent formation constants obtained for t-BuOH and cyclopentanol are log K' = 8.45 and 9.07 M-?, respectively. The effect of adding small amounts of alcohols to P-CDpyrene solutions at methanol-water mixtures of practical use in HPLC was investigated. Again, the complexation process was monitored by following the changes in the I/III ratio of pyrene as the p-CD concentration is increased. Equations similar to those in ref 35 were used for the calculation of the apparent formation constants. These data follow a 2:l PCD-pyrene model. Figure 5 shows the plots of l / ( R o - R ) versus 1/[@-CDInfor n = 1and n = 2, respectively. Clearly, the observed linear relationship obtained demonstrates that the stoichiometric ratio between pyrene and 8-CD does not change when changing the solvent from pure water to the methanol-water-t-BuOH mixture. Similar results were found for all the mixtures investigated. These data were analyzed by using nonlinear regression to calculate the apparent formation constant via eq 1. In this case, the parameters Ro and R, denote the ratio for pyrene in the methanol-water mixture
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ANALYTICAL CHEMISTRY, VOL. 63,NO. 10, MAY 15, 1991
,
10.0 0
l/IP-CDl,2 10
20
x
m
W2 30
I\
lm50
40
1.35
/I \
i
1
a I
6.0
0.90 2-o 0.0
p"0
14
56
42
l l [ ~ - C D ] , x 10-1
70
54 54 59 59 63 63
t-BuOH, % v/v
cyclopentanol, % v/v
log K2*
1
5.18 5.61
1
1
4.80 5.24 4.78
2
5.05
1
2
4
6
8
10
Figure 6. Influence of alcohol concentration on the M I 1 ratio of pyrene in 3.0 X M 0-CD. (0)Cyclopentanol and (A)t-BuOH.
M-1
Table 11. Apparent Formation Constants of B-CD-Pyrene Complexes in Different Methanol-Water Mixtures Containing either t-BuOH or Cyclopentanol 90 v/v
I 2
% ALCOHOL v l v
I 28
Flgure 5. Double-reciprocal plots between pyrene and P C D in methanol-water-t-BuOH mixtures. (A)An upward concave curvature when the data are plotted assuming a 1:l B-CD-pyrene stoichlometry and (0)a linear relationship when the data are plotted assuming a 2: 1 P-CD-pyrene stoichiometry.
methanol,
' 0
and in the 2:l complex, respectively. The parameter K2* is the apparent overall formation constant in the presence of mobile-phase comodifiers, and [CD], is the initial 0-CD concentration. The results obtained are summarized in Table 11. From Table 11, it can be inferred that the interaction between pyrene and P-CD under HPLC conditions becomes evident in the presence of t-BuOH or cyclopentanol as mobile-phase comodifiers. The strong ternary complexes formed in the presence of these alcohols counteract the diminishing effect of high methanol concentrations, allowing for the inclusion of pyrene inside the cyclodextrin cavities at P-CD concentrations of practical use in HPLC. The apparent formation constants decrease with increasing methanol concentration in the medium. The interaction between pyrene and P-CD is stronger in the presence of cyclopentanol than in the presence of t-BuOH. The formation constants in the presence of 1% v/v t-BuOH or cyclopentanol are 5.18 and 5.61 M-*, respectively. These constants reflect the higher stability observed for the ternary complexes in water (35). An increase in the t-BuOH or cyclopentanol concentration from 1% to 2% v/v allows the use of still higher methanol concentrations in the medium. The high P-CD-pyrene formation constant obtained by using 63% v/v methanol-water mixture reflects the weakened interaction between pyrene and the stationary phase, after addition of 2% v/v t-BuOH or cyclopentanol as mobile-phase comodifier. This observation could be useful under HPLC conditions for decreasing the retention time of pyrene and thus the analysis time. Influence of t-BuOH and Cyclopentanol Concentration. The use of t-BuOH and cyclopentanol as mobile-phase comodifiers has allowed for the determination of strong PCD-pyrene ternary complexes at high methanol concentrations (>55% v/v). The optimum amount of alcohol as mo-
bile-phase comodifiers is investigated, using the variation in the I/III ratio of pyrene. The I/III ratio is monitored for a fixed P-CD (3.0 X M) concentration using 55% v/v methanol and for varying amounts of t-BuOH or cyclopentanol in the medium. It should be noted that, in the absence of P-CD, the alcohols, at the concentrations used here, do not have any significant influence on the I/III ratio. This in turn suggests that there is little or no association between pyrene and the alcohol. The results obtained are summarized in Figure 6. The I/III ratio of pyrene first decreases when the amount of t-BuOH or cyclopentanol in the medium is increased and then increases for higher t-BuOH or cyclopentanol concentrations. The decrease of the I/III ratio suggests the formation of ternary complexes between P-CD-pyrene and the alcohol. At higher alcohol concentrations, the competition of t-BuOH or cyclopentanol becomes evident. This competition is likely due to a change in the hydrophobicity of the medium making the association between pyrene and 0-CD less favorable. The optimum concentration of alcohol is around 3% v/v for tBuOH and around 2% v/v for cyclopentanol. Similar results were obtained for 60% v/v water-methanol mixtures.
CONCLUSIONS Steady-state fluorescence measurements give detailed information about the stability of the P-CD-pyrene inclusion complex of pyrene under chromatographic conditions. The formation constant can still be obtained by using relatively high methanol concentrations (>55% v/v), despite the competition between pyrene and methanol for the CD cavity. The fact that the interaction is only evident in the presence of comodifiers opens the interesting possibility of quantitatively distinguishing solutes under HPLC conditions, by varying their retention behavior. The increased formation constant of the inclusion complexes of PAHs will result in shorter retention times as well as improved selectivity in separations. This is the result of an increased affinity of the solute for the cyclodextrin cavity caused by the interaction of alcohol modifiers. Hence, this will likely increase the amount of time that the solute will spend in the mobile phase. ACKNOWLEDGMENT We are grateful to G. A. Reed of American Maize Products for providing the CDs used in this work. Registry No. t-BuOH, 75-65-0; methanol, 67-56-1; cyclopentanol, 96-41-3. LITERATURE CITED (1) Hinze, W.
L.; Armstrong, D. W. Anal. Lett. 1980, 73, 1093-1104.
(2) Hinze, W. L. S e p . Purlf. Mehods 1981. 70, 159-237. (3) Debowski, J.; Sybilska, D.; Jurczak, J. Chromatographia 1982, 16,
198-200. (4) Debowski, J.; Sybilska, D.; Jurczak, J. J . Chromatogr. 1982, 237, 303-306.
Anal. Chem. 1991, 63, 1023-1027 (5) Nobuhara, Y.; Hirano, S.; Nakanishi. Y. J . Chrmfogr. 1983. 258, 276-279. (6) Sybilska, D.; Llpkowski, J.; Wojclkowski, J. J . Chromatog. 1082, 253, 95-100. (7) Debowski, J.; Sybiiska, D.; Jurczak, J. J . Chmmafogr. 1983. 282, 83-88. (8) Sybiiska, D.; Debowski, J.; Jurczak, J.; Zukowski, J. J . Chromafogr. 1984, 286, 163-170. (9) Zukowski. J.; Sybilska, D.; Jurczak, J. Anal. Chem. 1085, 5 7 , 2215-2219. (IO) Debowski. J.; Jurczak, J.; Sybilska, D.; Zukowski, J. J . Chromatogr. 1985, 322, 206-210. (1 1) Debowski, J.; Grassinl-Strazza, G.; Sybllska, D. J . Chromatogr. 1985, 349, 131-136. (12) Sybiiska, D.; Zukowski, J.; Bojarski, J. J . Lip. Chromatogr. 1986, 9 , 591-606. (13) Oazdag, M.; Szepesi, G.; Huszar, L. J . Chromafcgr. 1986, 357, 128-135. (14) Debowski, J.; Sybiiska. D. J . Chromafogr. 1086, 353, 409-416. (15) Armstrong, D. W.; Demond, W. J . Chromatogr. Sci. 1084. 2 2 , 441-41 5. (16) Armstrong, D. W.; Aiak, A,; Bui, K.; Demond, W.; Ward, T.; Riehl, T. E.; Hinze. W. L. J . Inclusion Phenom. 1984. 3. 533-545. (17) Armstrong, D. W. Anal. Chem. 1987, 5 9 , 84A-91A. (18) Street, Jr., K. W. J . Liq. Chfomatogr. 1087, 70, 655-662. (19) Mohseni, R. M.; Hurtublse, R. J. J . C h f m f o g r . 1990. 499, 395-410. (20) Lamparczyk, H.; Zarzycki, P.; Ochocka, R. J.; Sybilska, D. Chromafographia 1990, 30, 91-94. (21) Blyshak, L. A.; Dodson. K. Y.; Patonay, G.; Warner, I . M.; May, W. E. Anal. Chem. 1980, 67, 955-960. (22) Nakajlma, A. Specfrochlm. Acta 1974, 30A, 860-862. (23) Kalyanasundaram, K.; Thomas, J. K. J . Am. Chem. SOC. 1977, 9 9 , 2039-2044. (24) Dong, D. C.; Winnik, M. W. Phofochem. Photobbl. 1982, 35, 17-21. (25) Waris, R.; Acree, W. E., Jr.; Street, K. W., Jr. Analyst 1988, 713, 1465- 1467.
(26) (27) (28) (29) (30)
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Rice, M. R.; Gold, H. S. Anal. Chem. 1084, 56, 1438-1440. Stahlberg, J.; Almqen, M. Anal. Chem. 1985, 57, 817-821. Carr, J. W.; Harris, J. M. Anal. Chem. 1986, 58, 626-631. Street, K. W., Jr.; Acree. W. E., Jr., J . LiquM Chromafogr. 1088, 9 , 2799-2808. Tarr, M. A.; Nelson, G.; Patonay, 0.; Warner, I . M. Anal. Left. 1088, 2 1 843-846. Nelson, G.; Patonay. G.; Warner, I . M. Anal. Chem. 1088, 60, 274-279. Tucker, S. A.; Acree, W. E.. Jr.; Street, K. W.. Jr.; Fetzer. J. C. Appl. Specfrosc. 1989, 43, 162-164. Dofig, D. C.; Wlnnlk. M. A. Can. J . Chem. 1084, 62, 2560-2561. Munoz de la Peza, A.; Ndou, T. T.; Zung, J. B.; Warner, I. M. J . Phys. Chem., in press. Muzoz de la Peiia, A.; Ndou, T. T.; Zung, J. B.; Greene. K. L.; Live, D. H.; Warner, I . M. J . Am. Chem. Soc. 1991, 773, 1572-1577. Aimgren, M.; &lesser. F.; Thomas, J. K. J . Am. Chem. Soc. 1979. 70 7 , 279-29 1, Kusumoto, Y. Chem. Phys. Left. 1087, 736, 535-538. MatSul, Y.; Mochlde, K. &I. Chem. Soc.Jpn. 1970, 52, 2808-2814. Buvarl, A.; Szejtli. J.; Barcza, 1. J . Inclusbn phenom. 1983, 1 , 15 1-1 57. Lianos, P.; Oeorghiou, S. Photo&". photobbl. 1970, 29, 843-846. Nakajima, A. Bull. Chem. Soc.Jpn. 1083, 5 6 , 929-930. Opailo, M.; Kobayashi, N.; Osa, T. J . Incl. phenom. 1989, 6 , 413-422. ~
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RECEIVED for review December 6,1990. Accepted February 21,1991. This work was supported by the National Science Foundation (CHE-9001412). A.M.P. acknowledges support from the D.G.I.C.Y.T. of the Ministry of Education and Science of Spain for the grant that made possible his research in Professor Warner's laboratory.
Selective Concentration of Lead(I I) Chloride Complex with Liquid Anion-Exchange Membranes Takashi Hayashha* and Richard A. Bartsch Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-1061
Takahiko Kurosawa and Manabu Igawa Department of Applied Chemistry, Faculty of Engineering, Kanagawa University, Kanagawa-ku, Yokohama 221,Japan
Selectlve transport of lead( 1I ) over cadmium( I I ) chloride complexes through a bulk chloroform membrane that contalns tetraheptylammonlum Ions as the carrier for metal chlorlde complexes Is found to resutt from slow release of the Cd( I I ) complex from the organlc membrane Into the aqueous receiving phase. For an emulsion llquld membrane transport system In whlch dlmethyldloctylammonlumbromide functlons not only as the llquld anion exchanger but also as the surfactant for emulslon stabillration, good permeatlon selectivity for lead( I I ) chlorlde complex over that of Cd( I I ) and high permeation selectlvlty over Cu( I I ) , Fe( I I I), Ni( I I), and Zn( I I ) were achieved.
INTRODUCTION Selective separation of toxic heavy metal ions from waste solutions is frequently required in hydrometallurgical processing (I).In ion-association extraction, certain heavy metal ions form anionic metal halide complexes in the presence of excess halide ions, which may be extracted into organic solutions containing liquid anion exchangers, such as lipophilic quaternary ammonium salts (2,3).Since Pb(II), Cd(II), and Hg(I1) are strongly complexed by halide ions ( 4 ) , selective extraction systems for the separation of these heavy metal ions can be designed (5-8). 0003-2700/91/0363-1023$02.50/0
Metal ion separation by selective permeation of liquid membranes has several advantages over solvent extraction (9). Several different types of liquid membrane systems (i.e., bulk, emulsion, and polymer-supported liquid membranes) have been developed. The objective of this study is to determine the feasibility of selective heavy metal ion separation by liquid membrane transport processes in which lipophilic tetraalkylammonium ions serve as carriers for heavy metal chloride complexes.
EXPERIMENTAL SECTION Reagents. Sources of reagents include Kanto Chemical Company [tetraheptylammonium bromide (THAB), sodium picrate monohydrate, CdC12,PdC12,and ZnC12], Wako Chemical Iwai Kagaku Company [Cd(N03)2.4H20 and Zn(N03)2*6H20], Company [Pb(N03)2]and Sogo Pharmaceutical Company [dimethyldioctadecylammonium bromide (DDAB)]. Other inorganic and organic compounds were reagent grade and were used as received from commercial suppliers. Chloroform solutions (0.10 M) of tetraheptylammonium chloride (THAC) or nitrate (THAN) were prepared by shaking an aqueous solution (250 mL) of 1.0 M KCl or KNOBwith a 0.10 M chloroform solution (50 mL) of THAB for 20 min with a mechanical shaker (Taitec Co., Model SR-11).After phase separation, the chloroform solution was collected for use as a stock solution. The concentration of tetraheptylammoniumion (THA') in the stock solution was determined by picrate extraction as follows. Aqueous solutions (10.0 mL) of 0.010 M sodium picrate 0 1991 American Chemical Society
