3748
J. Phys. Chem. 1985,89, 3748-3152
Fluorescence Study on Characterization of Llquld Domains Formed in a Frozen Acetone-Water Mixture Koji Kano,* Bin Zhou, and Shizunobu Hashimoto Department of Applied Chemistry, Faculty of Engineering, Doshisha University, Kamikyo- ku. Kyoto 602, Japan (Received: January 30, 1985; In Final Form: April 18, 1985)
The steady-state and time-resolved fluorescence measurements of pyrene and 1,3-di-l-pyrenyIpropane (P(3)P) in rapidly frozen water containing 0.5 or 5% (v/v) acetone indicated that liquid acetone domains were formed between ice crystals and that the fluorescent probe molecules were concentrated in these domains. The fluorescence spectra of l-anilino-8napthalenesulfonate (ANS) suggested that the amounts of water in these liquid domains decreased as the freezing temperature decreased. The concentration effect resulting from the freezing process resulted in an acceleration of the formation of the pyrene excimer and of the fluorescence quenching of pyrene by N,N-dimethylaniline.
Introduction
Recently, Kano et al. have found that the solubilization of hydrophobic molecules such as 1,3-di-l-pyrenylpropane (P(3)P) in ionic surfactant micelles and 5,10,15,20-tetra-p-tolylporphine is drastically accelerated by freezing and thawing the micellar solutions.’,* Concentration of both micelles and solute molecules in liquid domains in ice crystals during freezing has been postulated as one of the mechanisms for the accelerated solubilization.2 Such concentration effects during freezing have been observed for molecular complex formation3v4and bimolecular reactions5-’ in water. Szent-Gyorgyi and his co-workers measured the chargetransfer complex formation of benzoquinone-hydroquinone, serotonin-flavin mononucleotide (FMN), serotonin-diphosphopyridine nucleotide (DPN), tryptophan-FMN, etc., by freezing aqueous solutions of these system^.^^^^^ Solute molecules seem to be excluded from growing ice crystals during freezing and concentrated between ice crystals leading to aggregates.1° Freezing may force the solute molecules to gather at the same place to form heterogeneous aggregates of electron donor and acceptor molecules. When the aqueous solution is completely frozen, mobilities of the solute molecules should be severely restricted. Dynamic bimolecular processes in frozen system are, therefore, expected to be decelerated compared with those in the fluid solution system. It is surprising that the rate of the imidazole-catalyzed hydrolysis of 6-aminopenicillanic acid in frozen water at -78 OC is larger than that in water at 22 OC.s It is assumed that the freezing effects on hydrolysis reactions cannot be explained only by the concentration of both catalysis and substrate. The mechanism may be more complex as suggested by Bruice and Butler.” If an aqueous solution consists of water and small amounts of organic solvent such as glycerol or methanol, freezing may also concentrate the solvent. When the temperature is above the freezing point of the organic solvent, liquid domains of organic solvent may be formed between ice crystals. Wang suggested (1) Kano, K.; Ishibashi, T.; Ogawa, T. J. Phys. Chem. 1983, 87, 3010. (2) Kano, K.; Ueno, Y.; Umakoshi, K.; Hashimoto, S.; Ishibashi, T.; Ogawa, T. J. Phys. Chem. 1984,88, 5087. (3) Szent-GyBrgi, A. ‘Introduction to a Submolecular Biology”; Academic Press: New York, 1960; Chapter 7. (4) Montenay-Garestier, T.; HClhe, C. Biochemistry 1971, 10, 300. ( 5 ) Grant, N. H.; Clark, D. E.; Alburn, H. A. J. Am. Chem. SOC.1961, 83, 4476. (6) Prusoff, W. Biochim. Biophys. Acta 1963, 68, 302. (7) Butler, A. R.; Bruice, T. C. J . Am. Chem. SOC.1964, 86, 313. (8) Isenberg, I.; Szent-Gyorgyi, A. Proc. Natl. Acad. Sci. U.S.A. 1958,44, 857. (9) Isenberg, I.; Szent-Gyorgyi, A. Proc. Natl. Acad. Sci. U.S.A. 1959.45, 1229. (10) Korber, C.; Wollhover, K.; Scheiwe, M. W. Sci. Tech. Froid 1981, 161. (11) Bruice, T. C.; Butler, A. R. J. Am. Chem. SOC.1964, 86, 4104.
0022-3654/85/2089-3748$01.50/0
“puddle” formation by freezing water containing 2% methanol.’* Irradiation of 1,3-dimethyIuracil in frozen water containing methanol gave 6-methoxy- 1,3-dimethylhydrouracil (a methanol adduct of 1,3-dimethyluracii) while photodimers are produced in frozen water without methanol. Wang assumed that the aggregates of 1,3-dimethyluracil formed by freezing aqueous solution are the precursors of the photodimers and the photochemical addition of methanol to 1,3-dimethyluracil occurs in “puddles” formed in the frozen aqueous methanol. Despite these reports concerning interesting phenomena in frozen aqueous systems, no detailed study of the microscopic environments in frozen water and frozen water containing small amounts of organic solvent has been undertaken. In connection with freeze-thaw effect studies on solubilization in micelles: we tried to characterize the microscopic environment in frozen water containing small amounts of acetone by using a fluorescent probe method. Acetone can be regarded as a substitute for micelles. Experimental Section
Materials. P(3)P and A N S employed were previously described.2 Pyrene (Nakarai) was purified by silica gel column chromatography with cyclohexane. N,N-Dimethylaniline (DMA) was purified by distillation. Sample Preparation. The aqueous solution of P(3)P was prepared by injecting an appropriate volume of the stock solution of P(3)P (1 X M) in acetone into 50 mL of water, sonicating the solution for 2 min, removing acetone completely by evaporating in vacuo a t 60 OC, and making up with water to adjust [P(3)P] = 5 X lo4 M. This method provided the aqueous P(3)P dispersion which was visually transparent. The acetonewater mixed solution of P(3)P was prepared by adding an appropriate volume of acetone into the aqueous P(3)P solution. Spectral Measurements. Fluorescence emission and excitation spectra were taken on a Shimadzu RF-540 spectrofluorometer. The sample in a Pyrex tube with a 4-mm i.d. was quickly frozen by plunging it into dry ice-methanol or -diethyl ether in a Dewar vessel equipped with a quartz cell. After the whole sample was frozen, the fluorescence spectra were measured. The temperatures of methanol or diethyl ether cooled by dry ice were measured by a Takeda Riken digital multithermometer (TR2114). The fluorescence decay curves were taken by using an ORTEC-PRA single-photon-counting apparatus. All measurements were carried out under aerobic conditions. Results and Discussion
Fluorescence of P(3)P in Frozen Media. Since the first observation by Hirayama,I3 many intramolecular excimer systems (12) Wang, S . Y. Nature (London) 1961, 190, 690. (13) Hirayama, F. J. Chem. Phys. 1965,42, 3163.
0 1985 American Chemical Society
Liquid Domains Formed in a Frozen Acetone-Water Mixture
The Journal of Physical Chemistry, Vol. 89, No. 17, 1985 3149
' -
-
t
3701 3501
200
300
400
500
600
330
Wavelength, nm
have been de~eloped.'"'~ A homogeneous solution of P(3)P in acetone showed emissions from both locally excited (M*) and intramolecular excimer states (E*). The fluorescence maximum of the intramolecular excimer of P(3)P in acetone was 485 nm.
P(3)P hL)M
-
P(3)P
I
I
I
1
I
-60
-40
-20
0
20
40
Temperature, 'C
Figure 2. Changes in the fluorescence excitation (Xu,,) and emission maxima (hem,,) of P(3)P and the ratio of the intensity of fluorescence from the locally excited state to that from the excimer state of P(3)P &/IB) in water containing 0.5% (v/v) acetone on freezing at various temperatures. The wavelengths set for measuring the emission and excitation spectra were the same as those shown in Figure 1.
0
Py-Py
p(3)p hl'E +
p(3)p
Decreasing the temperature (ca. -60 "C) caused a decrease in the relative intensity of the fluorescence from E* and a marked increase in that from M*. This temperature effect can be well interpreted on the assumption that the conformational change of P(3)P from an all-trans to eclipsed form is restricted due to the viscous nature of acetone at low temperature. Of course, one can observe rise and subsequent decay of the fluorescence intensity of the intramolecular excimer of P(3)P at temperatures above the freezing point of acetone (-94.8 "C). On the other hand, the P(3)P molecules dispersed in water and water containing 0.5 or 5% (v/v) acetone showed only excimer-like fluorescence with a maximum at 476 nm. In both water and water containing small amounts of acetone, the decay of the excimer-like fluorescence of P(3)P was multiexponential and no rise of the excimer emission was observed at room temperature. These fluorescence phenomena were ascribed to the formation of the P(3)P aggregates in these aqueous solutions.2 Presumably, the pyrenyl moieties of the P(3)P aggregates orient randomly with each other to form various kinds of intermolecular excimer states, which may lead to the multiexponential fluorescence decay. Since the pyrenyl moieties in the aggregates cannot move freely, the mean energy level of the excimer states of the P(3)P aggregates should be higher than that of intramolecular excimer state of P(3)P in homogeneous solution. It seems, therefore, probable that the fluorescence maximum of the excimer-like emission in water (AF, = 476 nm) should appear at a shorter wavelength compared with that of the intramolecular excimer in acetone (485 nm). The fluorescence behavior of P(3)P in water essentially did not change Chandross, E. A.; Dempter, C. J. J. Am. Chem. Soc. 1970,92,3586. Johnson, G. E. J. Chem. Phys. 1974, 61, 3002. Zachariasse, K.; KIihnle, W. Z . Phys. Chem. (Munich) 1976, IO!, (17) Wang, Y.-C.; Morawetz, H. J . Am. Chem. Soc. 1976, 98, 3611. (18) Hayashi, T.; Mataga, N.; Sakata, Y.; Misumi, S.; Morita, M.; Tanaka, J. J. Am. Chem. SOC.1976, 98, 5910. (19) Yamamoto; M.; Goshiki, K.; Kanaya, T.; Nishijima, Y. Chem. Phys.
Left. 1978, 56, 333.
I
-80
I
-200
Figure 1. Fluorescence spectra of P(3)P (5 X lod M) in water containing 0.5%(v/v) acetone at various temperatures. The solutions were rapidly frozen at the indicated temperatures, and the fluorescence excitation and emission spectra were measured at the same temperatures. The emission spectra were taken by exciting P(3)P at 347 nm, and the excitation spectra were followed at 480 nm.
Pp-Py
, u /
700
-32'C
I
I
I
I
I
0
100
200
300
400
Time,
ne
Figure 3. Fluorescence decay curves of the excimer of P(3)P in water containing 0.5%(v/v) acetone at various temperatures.
when the solution was rapidly frozen at -62 O C ; i.e., only broad excimer-like emission was observed at around 476 nm, and its decay was multiexponential in frozen water. Freezing the solution of P(3)P in 0.5% (v/v) acetone-99.5% (v/v) water, however, caused dramatic changes in the fluorescence phenomena. The temperature-dependent fluorescence emission and excitation spectral changes of P(3)P in the aqueous acetone are shown in Figure 1. The excitation spectra were followed at 480 nm. When the aqueous acetone solution of P(3)P (5 X 10" M) was frozen at temperatures above -40 OC, only excimer-like emission and the relatively broad excitation spectra due to the P(3)P aggregates were observed. The fluorescence emission and excitation spectra of P(3)P dramatically changed when the solution was rapidly frozen a t temperatures below -40 OC. Namely, the intense fluorescence bands due to M* appeared, the fluorescence maximum of the excimer shifted from 476 to 486 nm, and each band in the excitation spectrum was sharpened and shifted to shorter wavelength. The effect of altering the freezing temperature on the fluorescence emission and excitation maxima and the ratio
3750
The Journal of Physical Chemistry, Vol. 89, No. 17, 1985
Kano et al. PR-70 DECRY ( B I N 1 i 0/12/e4 11/19/e4 FIT FROM 43 T O 511 CHRNNEL 0 . ~ 3 ~+-~ s0.00655 R 1 R z = -.i57es +- 0.00579 R 3 0.0~72s +- 0.00541
-
.": G
.
E -
200
300
400
500
.
--
0.77920 NSEC/CHRNNEL CHISPR 1.04699 i 1 = i e a . 1 ~ 7+- Z.S~ZES
5 8 . 4 1 ~+-~ 2 . 1 3 2 4 ~ T 2 T 3 = 2.3~068+- 0.24282
I
l HCR
CHRNNEL NUMBER
600
Wavelength, nm
Figure 4. Fluorescence spectra of pyrene ( 5 X 10" M) in water containing 5% (v/v) acetone frozen at -32 and -78 OC. The fluorescence spectra were measured by exciting pyrene a t 336 nm. The excitation spectrum was measured for the sample frozen at -78 O C by following the excimer fluorescence at 480 nm.
of the intensity of the fluorescence from M* to that from E* (IM/IE) is shown in Figure 2. Figure 2 indicates that the physical properties of the environment, where the P(3)P molecules are located, are markedly varied by freezing at temperatures below -40 "C. Figure 3 shows the fluorescence decay curves of the excimer-like emission in water containing 0.5% (v/v) acetone frozen at various temperatures. At -32 O C , the decay curve is similar to that for the P(3)P dispersion at room temperature. When the solution is frozen at lower temperature, however, the dynamic formation of the excimer state of P(3)P becomes detectable by means of the single-photon-counting time-resolved fluorometer. As Figure 3 shows, the rise and decay of the excimer fluorescence were clearly observed at -78 OC. All fluorescence phenomena of P( 3)P clearly indicate that the aggregates of P(3)P dissociate into the monomeric state when the solution is quickly frozen at temperatures below -40 "C. The dissociation can be interpreted as that both P(3)P aggregates and acetone are excluded from the aqueous phase during growth of ice crystals to provide the acetone domains (or "puddles") in which P(3)P is soluble. Since the freezing point of acetone is -94.8 O C , the acetone domain should be a liquid phase at temperatures above -80 O C . As Figures 1 and 2 show, it is necessary to freeze the solution at the temperatures below -40 O C before the P(3)P aggregates dissolve in the acetone domains. This may be due to the fact that the water content of the acetone domain formed by freezing at temperatures above -40 OC is too high to solubilize the P(3)P molecules. Fluorescence of Pyrene in Frozen Media. If the rapid freezing of water containing small amounts of acetone favors formation of acetone domains, organic solute molecules coexisting in the system may be concentrated in these organic solvent domains. This assumption was confirmed by fluorescence experiments using pyrene. A 5 X 10" M solution of pyrene in water containing 5% (v/v) acetone showed only monomer fluorescence around 400 nm, suggesting that pyrene is soluble in water at very low pyrene concentrations and no collision of excited pyrene with another pyrene in the ground state occurs within the lifetime of the pyrene fluorescence (1 34 ns) under these conditions. When the solution was frozen at -78 "C, fluorescence due to pyrene excimer appeared at 480 nm along with greatly enhanced monomer emission (Figure 4). A rise and subsequent decay curve was obtained for the pyrene excimer fluorescence in water containing 5% (v/v) acetone frozen at -78 O C (Figure 5), indicating that the excimer formation in the frozen aqueous acetone proceeds via a dynamic process. A nonlinear least-squares deconvolution method was used to analyze the decay data in Figure 5 and yielded three separate lifetimes: 2.4 f 0.2 (the preexponential factor al = 0.0873 f 0.0054), 58.4 & 2.1 (a2= -0.1579 f 0.0058), and 189.2 f 2.9 ns (a3= 0.2369
1 oo 0.0
200
100 TIME
/
300
NRNO SECONDS
Figure 5. Fluorescence decay curve of the pyrene excimer in water containing 5% (v/v) acetone frozen at -78 OC. f 0.0066). The component having the shortest lifetime (2.4 ns) may be ascribed to the light scatter, and the preexponential factor suggests that the contribution of this component for whole decay curve is negligibly small. It can be concluded, therefore, that the decay curve of pyrene in the frozen water containing 5% (v/v) acetone consists of two factors, Le., formation and subsequent decay of the pyrene excimer. This means that an excited pyrene molecule associates with a pyrene molecule in the ground state via a dynamic process to yield the excimer in the acetone domain formed in the frozen water and that there is no aggregate of pyrene under these conditions. The fluorescence of pyrene proved the formation of the fluid acetone domains in the frozen aqueous acetone in which the solute molecules are concentrated. Fluorescence of ANS in Frozen Media. It is well-known that the fluorescence of ANS is very sensitive to solvent polarity. The fluorescence maximum of ANS shifts to shorter wavelength while its fluorescence intensity increases markedly with decreasing solvent polarity. We used A N S to estimate the microscopic polarity of the acetone domain in frozen aqueous acetone. Figure 6 shows the changes in the wavelength of the fluorescence maxof ANS when the 1 X M ANS solutions were imum (AF,,,) frozen at various temperatures. In water, AFmax suddenly shifted to shorter wavelength on freezing at around 0 O C , which can be understood as that the solute-solvent interaction is dramatically reduced when the solvent is frozen and the solute molecules are, therefore, apparently located in a very hydrophobic environment. Decreasing the freezing temperature caused a gradual blue shift of XFmax, which may be ascribed to the gradual dehydration of ANS molecules. The XFmax in acetone (freezing point of acetone = -94.8 "C) simply shifted to longer wavelength with decreasing temperature. This can be interpreted in terms of the increase of the dielectric constant of liquid acetone at lower temperature. When water containing 0.5 or 5% (v/v) acetone was frozen at -5 to -35 O C , AFmax of A N S appeared at longer wavelength compared with that in frozen water, suggesting that the ANS molecules move from an aqueous phase to the acetone domain
Liquid Domains Formed in a Frozen Acetone-Water Mixture
The Journal of Physical Chemistry, Vol. 89, No. 17, 1985 3751
TABLE I: Fluorescence Lifetimes of Pyrene in the Absence of Quencher ( T ~ and ) Apparent Rate Constants for Fluorescence Quenching of Pyrene by DMA (k.) in Acetone and Water Containing 0.5 and 5% (v/v) Acetone at 25, -32, and -78 OC" TO, ns 10-9k,,M-' s-l temv. "C 0.5% 5% 100% 0.5% 5% 100% 16 6.7 f 0.5 25 134 15.5 0.6 -32 45 43 42 1530 f 250 410 f 130 4.8 0.3 -78 45 44 44 4700 f 610 700 60 3.8 f 0.3
* *
*
"The 5 X conditions.
lo-'
M solutions of pyrene were used for these experiments. No pyrene excimer was observed on freezing under these experimental
520
I
1.2
1.o
0.8 1-
0.6 -80
-60
-40
-20
0
20 0.4
Temperature, *C
Figure 6. Changes in the fluorescence maximum of ANS (1 X M) in water (0)and water containing 0.5 (0) and 5% (v/v) (e)acetone on
freezing at various temperatures. The fluorescence spectra were measured by exciting ANS at 376 nm. The spectra of ANS in acetone (@) were also measured under similar conditions.
on freezing. The liquid acetone domain should be more polar than the frozen aqueous domain. Between -5 and -35 O C , XFmax of A N S shifted to shorter wavelength with decreasing freezing temperature. The water content of the acetone domain may decrease with decreasing freezing temperature. The fluorescence maximum of A N S in aqueous acetone frozen at -78 OC v a s intermediate that between those of frozen water and acetone at -78 O C . Although it is very difficult to determine the water content in the acetone domain, the XFmax of A N S suggests that the polarity of the acetone domain is more polar than that of pure acetone. A considerable amount of water may be included in the acetone domain. Accelerated Fluorescence Quenching in Frozen Media. It was expected that any photochemical bimolecular reactions should be accelerated by the concentration effect of freezing. We studied the effect of freezing on the fluorescence quenching of pyrene by DMA in aqueous acetone. The results were analyzed by a Stern-Volmer equation z o / I = 1 + kq.o[QI
(2)
where Io and I are the fluorescence intensities of the pyrene monomer in the presence and absence of quencher, respectively, kq is the rate constant for fluorescence quenching, T~ is the fluorescence lifetime of pyrene in the absence of quencher, and [Q] is the bulk concentration of quencher. The results are summarized in Table I. The fluorescence quenching in acetone at various temperatures (25, -32, and -78 "C) obeyed the SternVolmer linear relationship, and the k, value at 25 OC ((1.55 f 0.06) X 1O1O M-'s-') obtained from the slope of the plot of Zo/Z vs. [DMA] was in good agreement with the diffusion-controlled rate constant of acetone (kdiff = 2.1 X 10'' M-' s-'). It is well established that the fluorescence quenching of pyrene by DMA is a diffusion-controlled process.20 From the equation of kdir = 8RT/3000q, the viscosities ( 9 ) of acetone at -32 and -78 OC were calculated to be about 1 cP. The fluorescence quenching of pyrene (20) 2750.
Okada, T.; Oohari, H.;Mataga, N. Bull. Chem. Soc. Jpn. 1970,43,
0.2 0
1
2
3
4
5
~ O ~ E D M AMI ,
Figure 7. Plot of r' vs. [DMA] for fluorescence quenching of pyrene (5 X IO-' M) by DMA in frozen water containing 5% (v/v) acetone at -78 OC.
by DMA was markedly accelerated on freezing water containing 0.5 or 5% (v/v) acetone. The inhomogeneity of cracked ice caused considerably large experimental error as shown in Figure 7 . Consequently, we analyzed a lot of data using standard statistical treatment. At -78 OC, the apparent kq values in water containing 0.5 and 5% (v/v) acetone frozen at -78 OC were 1200 and 190 times larger than those in acetone at -78 OC, respectively. In aqueous acetone, there was a large difference in the k, values between -78 and -32 OC (see Table I). As the P(3)P fluorescence indicates, the formation of the acetone domains becomes remarkable when the sample is frozen at temperatures below -40 OC. Above this temperature, formation of the acetone domains may be insufficient so that the quenching efficiency is much lower than that at temperatures below -40 OC. If the acetone domain is completely isolated from ice crystals, the solute molecules are concentrated maximally by factors of 200 for 0.5% (v/v) acetone-water and of 20 for 5% (v/v) acetone-water upon freezing. The degree of the concentration by freezing, therefore, cannot be simply calculated. From the fluorescence quenching results, one may imagine that the volume of the liquid domain formed by freezing of the water-acetone mixture is much smaller than the expected value because of the unobvious iceacetone interface. The formation of the acetone domain was also strongly supported from the measurements of the fluorescence lifetimes ( T ~ ) of pyrene (Table I). Under the air-saturated conditions, T~ in acetone was much smaller than that in water containing 0.5% (v/v) acetone because of the higher oxygen solubility and the larger diffusion rate in acetone. Cooling the acetone solution of pyrene caused an increase of T ~ which , is an ordinary phenomenon in photochemistry. Contrary to this, T~ decreased markedly and became the same as that in acetone at the same temperature on freezing the aqueous pyrene solution containing 0.5% (v/v) acetone. The decrease in T~ can be explained as that the pyrene molecules translocate from the aqueous phase to the acetone
J . Phys. Chem. 1985,89, 3152-3151
3752 puddles when the solution is frozen. Concluding Remarks
The present study revealed that both solute and organic solvent molecules are excluded from growing ice crystals during freezing to form the "puddles" of organic solvent in which the solute molecules are concentrated, when the aqueous solution containing small amounts of organic solvent is quickly frozen. In previous studies, we found that the freeze-thaw cycles can effect the solubilization of hydrophobic solute molecules in ionic surfactant micelles and assumed that the accelerated solubilization observed on freezing and thawing is due to a concentration of both micelles and solute molecules in the liquid domain remaining between the ice crystals and dehydration of micelles leading to formation of more hydrophobic micel1es.'v2 The present system can be regarded as a model for investigating this concentration effect on freezing in the micellar system where micelles have been replaced by
acetone. The results obtained in this study strongly support the belief that the concentration effect of freezing contributes significantly to the acceleration of solubilization in micelles by freezing and thawing. Since the solute molecules can diffuse freely in these puddles, photochemical bimolecular processes, such as pyrene-excimer formation and the fluorescence quenching of pyrene by DMA, are drastically accelerated by freezing. Such a freezing effect is expected to be very useful in photochemical reactions in which two or more molecules participate.
Acknowledgment. We are grateful to Professor S. Hirayama for his help in analyzing the fluorescence decay curves. The support of the Asahi Glass Foundation for Industrial Technology is gratefully acknowledged. Registry No. P(3)P, 61549-24-4; ANS, 82-76-8; DMA, 121-69-7; pyrene, 129-00-0: acetone, 67-64-1; water, 7732-18-5.
Complexation of Alkali Metal and Barlum Cations by sym-Dibenzo-16-crown-5-oxyacetic Acld in 80 % Methanol-Water. Dependence by Calorimetric mratlon
Determination of pH
Raymond J. Adamic, Edward M. Eyring,* Department of Chemistry, University of Utah, Salt Lake City, Utah 84112
Sergio Petrucci, Department of Chemistry, Polytechnic Institute of New York, Farmingdale, New York I I735
and Richard A. Bartsch Department of Chemistry, Texas Tech University, Lubbock, Texas 79409 (Received: February 4, 1985; In Final Form: April 15, 1985)
The acid dissociation constant, K,, for sym-dibenzo-16-crown-5-oxyaceticacid, 1, in 80% methanol-water (w/w) has been determined potentiometrically. Dependence of K,,on NaI concentration was also determined. The thermodynamic parameters AH, AG, and TAS and stability constants have been measured by thermometric titration for complexation of alkali metal and BaZ+cations by 1 in 80% methanol-water (w/w) at 25 OC. Dependence on pH and Na+ concentration was investigated. A modest increase in complex stability (A log K = 0.92) was observed for the acid crown as the pH increases: this enhanced binding is attributed to the anionic participation of the ionizable side group in metal ion coordination. An equation relating the free energy changes of complexation in terms of contributions from the polyether ring and the carboxylate group is presented and discussed.
Recent advances in the synthesis of new ionophores capable of complexing alkali metal ions have included crown ethers specially designed to serve as the counterion to the metal ion they are One particular class of crown compounds, the so-called 'acid crowns", contains, in addition to the polyether ring, a pendant carboxylic acid group. These ionizable crown ethers have been shown to have higher extraction efficiencies and selectivities for alkali metal and alkaline earth metal cations over their nonacidic c o ~ n t e r p a r t s . ~Stability ,~ constant measurements of cation complexation by 15-crown-5 and 18-crown-6 macrocycles which bear pendant carboxylate groups have shown the complexes formed by the ionized acid crown in water and 90% (v/v)
methanol-water to have greater stability than those produced with the corresponding protonated f ~ r m . ~This ? ~increase in stability has been attributed to an electrostatic interaction of the charged side group(s) with the complexed metal ion. Proximity of the carboxylate moiety to the metal ion and the charge of the cation can both be expected to influence the strength of the interaction. Increasing the pH to favor the anionic form should, in principle, enhance the overall stability of the complex. Thus, just as cavity size is an important factor in determining cation selectivity of neutral crown ethers, pH, ionic charge, and degree of electrostatic interaction should also have important bearing on the stability of complexes formed from crown ethers with pendant ionizable side groups. To enhance understanding of the relative importance of these factors, we have conducted a pH dependence study of
(1) Bartsch, R. A.; Heo, G. S.;Kang, S. I.; Liu, Y.; Strzelbicki, J. J. Org. Chem. 1982, 47, 457. (2) Czech, B.; Kang, S.I.; Bartsch, R. A. Tetrahedron Lett. 1983,24,457. ( 3 ) Strzelbicki, J.; Bartsch, R. A. Anal. Chem. 1981, 53, 2247. (4) Strzelbicki, J.; Bartsch, R. A. Anal. Chem. 1981, 53, 2251.
( 5 ) Behr, J. P.; Lehn, J.-M.; Vierling, P. J.Chem. Soc., Chem. Commun. 1976, 621. ( 6 ) Frederick, L. A.; Fyles, T. M.; Gurprasad, N. P.; Whitfield, D. M. Can. J . Chem. 1981, 12, 1724.
Introduction
0022-3654/85/2089-3752$01 SO10 0 1985 American Chemical Society
