J . Phys. Chem. 1984, 88, 1909-1913
1909
Dynamics of Excited-State Reactions in Reversed Micelles. 1. Proton Transfer Involving a Hydrophilic Fluorescent Probe Elisabeth Bardez, Bich-Thuy Goguillon, Erlend Keh: and Bernard Valeur * Laboratoire de Chimie GPndrale. Conseruatoire National des Arts et Mdtiers, 75003 Paris, France (Received: July 26, 1983; In Final Form: September 29, 1983)
The efficiency of proton transfer within the water pool of reversed micelles of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) is investigated by using pyranine as a fluorescent probe. The rates of deprotonation and reprotonation of pyranine are measured by phase fluorometry as a function of the water content of the reversed micelles. The rate of deprotonation increases and the rate of reprotonation decreases as the water content w = [HzO]/[AOT]increases. Both rates reach values comparable to those observed in bulk water for w 12. The results are interpreted in terms of water structure with special attention to proton hydration in conjunction with the salt effect. Values of water activity can be derived from the kinetic constant of proton ejection. Proton-transfer efficiency in reversed micelles offers a new insight into the acidity of the aqueous core and appears to be one of the interesting aspects of the relation between structure and reactivity.
-
Introduction Reversed micelles can solubilize large amounts of water, forming aqueous cores in which various reactions can be achieved.’ The reactivity in any reaction medium depends not only on its chemical nature but also on its structure. One of the interesting features of the relation between structure and reactivity is relevant to the efficiency of proton transfer. Proton transfer is indeed implied in various reactions such as acid-base reactions, catalysis by acids or bases, hydrolysis, etc. Futhermore, proton transfer is closely related to acidity. When the water pool of a reversed micelle is used as a reaction medium for enzymes, one has to face the question of the acidity in the aqueous core.2 The acidity within the water pool can be investigated by injecting into reversed micelles organic dyes whose absorption spectrum depends on pH2-9 or by measuring the pH-dependent 31Pchemical shifts of injected phosphate buffem2J All the investigations show that no absolute determination of pH within the water pool is possible because water of the aqueous core has peculiar properties as regards to polarity, viscosity, structure, etc. which are different from those of bulk water.’@l6 Therefore, an acidity scale in reversed micelles can only be defined in an empirical way. An excellent discussion is presented in the paper of Smith and Luisi,2 who emphasize that the question of the acidity in the aqueous core is relevant to the more general question of the structure and properties of water. W e agree with this statement, and in addition, we believe that acidity should be considered in terms of proton transfer which depends on the solvent structure. Information on proton-transfer efficiency and thus on acidity within the water pool can be obtained from the behavior of pyranine as a fluorescent probe: Na’ -O,S*OH
Na‘ -
0
,
S
~
S
acidic form (ROH)
O basic form (RO-)
The emissive properties of this molecule as a function of pH have been extensively studied by staticI7 or dynamic’”’ measurements. Pyranine has already been used as a probe of vesicles:2-26 reversed micelle^,'^,^^^^^ and binding sites of protein^.^^,^^ While the present work was in progress,31 Kondo and coworkers2s published a paper devoted to fluorescence probing of Laboratoire de Physico-chimie des Systtmes Polyphasts, CNRS, B.P. 5051, 34033 Montpellier Ctdex, France.
0022-3654/84/2088-1909$01.50/0
reversed micelles by pyranine. The stationary experiments (emission spectra and polarization) were interpreted by these
(1) J. H. Fendler and E. J. Fendler in “Catalysis in Micellar and Macromolecular Systems“, Academic Press, New York, 1975. (2) R. E. Smith and P. L. Luisi, Helu. Chim. Acta, 63, 2302 (1980). (3) F. M. Menger and G. Saito, J. A m . Chem. Soc., 100, 4376 (1978). (4) A. V. Levashov and V. I. Pantin, Colloid J., 41, 380 (1979). (5) H. Fujii, T. Kawai, and H. Nishikawa, Bull. Chem. SOC.Jpn., 52, 2051 (1979). ‘ ( 6 j A . T. Terpko, R. J. Serafin, and M. L. Bucholtz, J . Colloid Interface Sci., 84, 202 (1981). (7) 0. A. El Seoud, A. M. Chinelatto, and M. R. Shimizu, J . Colloid Interface Sci.. 88. 420 (1982). (i) H. Fuji, T.’ Kawai, H. Nishikawa, and G. Ebert, Colloid Polymer Sci., 260, 697 (1982). (9) 0. A. El Seoud and M. R. Shimizu, Colloid Polymer Sci., 260, 794
~ m . . (10) M. Wong, J. K. Thomas, and M. Gratzel, J . Am. Chem. SOC.,98,
2391 (1976). (1 1) M. Wong, J. K. Thomas, and T. Nowak, J . Am. Chem. Soc., 99,4730 (1977). (12) F. M. Menger, G. Saito, G. V. Sanzero, and J. R. Dodd, J . Am. Chem. SOC.,97, 909 (1975). (13) F. M. Menger. J. A. Donohue. and R. F. Williams. J . Am. Chem. Soc., 95, 286 (1973j. (14) G. D. Correll, R. N. Cheser, F. Nome, and J. H. Fendler, J . Am. Chem. SOC.,100, 1254 (1978). (15) (a) B. Valeur and E. Keh, J . Phys. Chem., 83, 3305 (1979). (b) E. Keh and B. Valeur, J . Colloid Interface Sci., 79, 465 (1981). (16) P. E. Zinsli, J. phys. Chem., 83, 3223 (1979). (17) (a) A. Weller, Z . Phys. Chem. (Wiesbaden), 17, 224 (1958). (b) A. Weller, Prog. React. Kinet., 1, 187 (1961). (18) T. H. Forster and S. Volker, Chem. Phys. Lett., 34, 1 (1975). (19) K. K. Smith, K. J. Kaufmann, D. Huppert, and M. Gutman, Chem. Phys. Lett., 64, 522 (1979). (20) M. Gutman, D. Huppert, and E. Pines, J . Am. Chem. Soc., 103, 3709 (1981). (21) D. Huppert and E. Kolodney, Chem. Phys., 63, 401 (1981). (22) K. Kano and J. H. Fendler, Biochim. Biophys. Acta, 509,289 (1978). (23) K. Kano and J. H. Fendler, Chem. Phys. Lipids, 23, 189 (1979). (24) N. R. Clement and J. M. Gould, Arch. Biochem. Biophys., 202,650 (1980). (25) N. R. Clement and J. M. Gould, Biochemistry, 20 1534 (1981). (26) T. Nomura, H. Kondo, and J. Sunamoto, Bull. Chem. SOC.Jpn., 54, 1239 (1981). (27) U. Klein and M. Hauser, Z . Phys. Chem. (Wiesbaden), 90, 215 (1974). (28) H. Kondo, I. Miwa, and J. Sunamoto, J . Phys. Chem., 86, 4826 (1 9.82). (29) M. Gutman, D. Huppert, and E. Nachliel, Eur. J . Biochem., 121,637 (1982). (30) M. Gutman, E. Nachliel, and D. Huppert, Eur. J . Biochem., 125, 175 (1982). (31) Part of this work has been presented at the meeting “Micelles et Microtmulsions: Structure et Rtactivitt” held in Saclay (France) in May 1983 (abstract submitted in Oct 1982). ~
0 1984 American Chemical Society
1910 The Journal of Physical Chemistry, Vol. 88, No. 9, 1984
authors in terms of biphasic structure of the water pool.16 The aim of the present work is to measure, by means of a dynamic method (phase fluorometry), the rates of deprotonation and reprotonation of pyranine within the aqueous core of AOT reversed micelles as a function of water content. The results allow us to examine the relation between structure and reactivity regarding proton transfer around the center of the water pool. Localization around the center results from the electrostatic repulsion between the anionic heads of the surfactant and the negative charges of pyranine.22~24~28 This localization is similar to that of perylene sodium tetra~arboxy1ate.l~
Experimental Section Materials. Sodium bis(2-ethylhexyl) sulfosuccinate (AOTJ was obtained from Fluka A.G. and further purified by the procedure described in ref 32 using redistilled n-pentane as the extracting solvent. Trisodium 8-hydroxy- 1,3,6-pyrene trisulfonate (pyranine) was purchased from Eastman and purified by chromatography with an alumina column. Impurities were first eluted by a 1propanol-water mixture (3:l v/v). Pure pyranine was then eluted by a 1-propanol-water mixture (1:3 v/v). Measurements. The fluorescence spectra were recorded with an Aminco S P F 500 specfrofluorometer connected to a Kontron PSI 80 microcomputer for storage, correction, and data analysis. The dynamic studies $ere carried out by means of a SLM 4800 phase fluorometer equipped with a R 928 photomultiplier and operating at a modulation frequency of 30 MHz. Fluorescence of the acidic form of pyranine was selected thanks to a combination of a Balzers interferential filter (maximum at 433 nm, width at half-maximum of 8 nm) and a 7-59 Corning filter. With such a combination of filters (Fl), negligible fluorescence from the basic form was detected., A 2-63 Corning filter (F,) was used for the observation of the basic form fluorescence. Parasitic effects due to polarization were avoided thanks to a polarizer interposed in the excitation beam and oriented at 35’ to the vertical.33 All the experimenjs were performed at 25 “C using freshly prepared 0.1 M surfactant solutions. The concentration of pyranine was kept below M. Consequently, less than 1 out of 100 micelles contains a probe molecule. Methods, The behavior of pyranine is described by the usual kinetic scheme:
I4
ROH
+
H20
Kd,
RO- + H,O+
k , is the rate constant of proton transfer to HzO, k-, is the recombination rate constant of the excited basic form and a proton, k, and k,, are the radiative and nonradiative rate constants of ROH*, respectively, and k’, and k‘,,, are the corresponding values for RO-*. The’appropriate coupled differential equations describing the appearance and disappearance of the excited species can be solved to yield expressions for the steady-state fluorescence intensities,” the &pulse responses,34and the harmonic responses35which can be used in stationary experiments, pulse fluorometry, and phase fluorometry, respectively. With use of a phase fluorometer (Le. with a sinusoidally modulated exciting light it has long been that, when (32) J. Rogers and P. A. Winsor, J. Colloid Interface Sci., 30, 247 (1969). (33) R. D. Spencer and G. Weber, J. Chern. Phys., 52, 1654 (1970). (34) W. P. Laws and L. Brand J . Phys. Chern., 83, 795 (1979). (35) J. R. Lakowicz and A. Balter, Biophys. Chem., 16, 99 (1982). (36) N. S. Bazilevskaya, L. A. Limareva, A. S. Cherkasov, and V. I. Shirokov, Opt. Spectrosc. (Engl. Transl.), 19, 39 (1965). (37) T. V. Veselova, A. J. Cherkasov, and V. I. Shirokov, Opt. Spectrosc. (Engl. Transl.), 42, 39 (1977). (38) G . Weber in “Excited States of Biological Molecules’’, J. B. Birks, Ed., Wiley, New York, 1976, p 363.
Bardez et al.
t
400 500 600 700 Figure 1. Corrected fluorescence spectra of pyranine in aqueous solutions (A,, = 390 nm): 1, 7.5 N H2S04;2, 1 N H2S04;3, HC1-glycine buffer (pH 1); 4, HCI-citrate buffer (pH 2); 5 , HC1-citrate buffer (pH 4); 6,
citrate buffer (pH 6).
the initially excited fluorescent centers A form new emitting centers B during the lifetime of their excited states, the phase difference C # J ~- $A represents the decay time of centers B. In the present study, the acidic form R O H is the only form that exists in the ground state and the phase shift A$ = $RO- - 4ROH is in fact given by a simple expression only involving the rate constants for the disappearance of RO-*: tan A$ = UT&,+,=
w
k-l[H30+]
+1/~’~
(1)
where T*,+, is an apparent differential lifetime, T ’ ~is the lifetime ~ of RO-* in the absence of excitedktate reaction ( l / ~=’ k’, k’,,), and w is the circular frequency of the exciting light. T&,+, can be measured directly by moving back and forth the appropriate filters FI and FZ,selecting the emission from ROH* and RO-*, respectively. The measurement of T*,+, and T ’ ~allows one to calculate k-, [H30+]by means of eq 1. Furthermore, thanks to RO-* additional measurement of the apparent lifetime T’ of RO-* (tan 4Ro-= W T ’ ) , we have shown that the rate constant kl can be determined by using the following expression:
+
where T~ is the lifetime of ROH* in the absence of excited-state reaction. Details are provided in the supplementary material. (See paragraph at end of text regarding supplementary material.)
Results Emission Spectra in Aqueous Solutions. The pK of pyranine in the ground state (pK = 7.2) is much higher than in the excited state (pK* = 0.5). Therefore, at pH values smaller than 6, the absorption spectrum is characteristic of the acidic form R O H alone. At higher pH values, an absorption band appears at 445 nm, revealing the presence of the basic form RO- in the ground state. The basic form solely exists in the ground state at pH values higher than 8. The emission spectra of pyranine excited at 390 nm at various pH are given in Figure 1. In a highly acidic solution (7.5 N H2S04),the emission spectrum exhibits a single band around 450 (39) J. R. Lakowicz and A. Balter, Biophys. Chem., 16, 117 (1982). (40) J. R. Lakowicz and A. Balter, Chem. Phys. Lett., 92, 117 (1982).
The Journal of Physical Chemistry, Vol. 88, No. 9, 1984 1911
Excited-State Reactions in Reversed Micelles
Z (nul
400
500
600
760
/KAQ
2 4
1 - 1
9
0 1
l o8 ]
c 2
4
8
6
10
1 2 W
Figure 3. Variations in apparent lifetime of RO-* ( T ’ ) and differential lifetime (ram) vs. water content w. ‘9
‘i”
d‘
1
’1
10’ ,
400
500
; 600
700
Figure 2. Corrected fluorescence spectra of pyranine in AOT reversed micelles at various water contents w = [H,O]/[AOT] (Aexc = 390 nm): (A) 1, w = 0.35; 2, w = 0.93; 3, w = 1.4; 4, w = 2.0. (B) 5, w = 2.5; 6, w = 3.1;7, w = 3.6; 8, w = 4.2;9, w = 4.7; 10, w = 6.1; 11, w = 8.8; 12, w = 11.5.
nm arising from the blue fluorescence of the acidic form. As the pH increases (by using various buffered solutions), the excited R O H molecules can be partially converted into excited ROmolecules before returning to the ground state; thus, an additional emission band at 510 nm, characteristic of the green fluorescence of RO-*, appears. An isoemissive point is observed at 490 nm. Emission Spectra in Reversed Micelles. The behavior of pyranine in water containing AOT reversed micelles in heptane has been investigated a t various water contents w = [H,O]/[AOT]. The absorption spectra show that only the acidic form R O H is present in the ground state whatever the water content. The emission spectra as a function of w are shown in Figure 2A,B. At w 5 2.5, the spectra are blue-shifted and do not go through the isoemissive point observed at 480 nm for higher water contents. This means that a minimum of 2.5 water molecules/ AOT molecule is necessary for the probe to be in an actual aqueous environment. Further addition of water causes the ROH* emission band to decrease with a concomitant increase in the RO-* emission band (Figure 2B). These results are in good agreement with those obtained by Kondo et a1.28 In addition to these observations it is worth noting that the emission spectrum remains unchanged beyond w 12. Futhermore, the gap of 10 nm between the isoemissive points observed in aqueous solutions and in micellar solutions is due to the difference in the emissive properties of pyranine in these two media, as confirmed by lifetime experiments (see below). The evolution of the emission spectra reveals that the conversion of ROH* into RO-* in the excited state becomes more and more competitive with fluorescence emission as the water content increases. For an unambiguous interpretation of this behavior, determination of the rate constants involved in the kinetic scheme is required.
-
lo8
I 10
0
2
4
6
8
10
12
W
Figure 4. Variations in rate constants of deprotonation ( k , ) and backrecombination (kLJ vs. water content w.
Determination of Lifetimes and Rate Constants by Phase Fluorometry. The intrinsic lifetimes of ROH* and RO-* (Le. in the absence of excited-state reaction) in aqueous solutions can be measured in the following way. In 5 M HC104,the acidic form ROH is solely present in the ground and excited states, and upon excitation at 390 nm, the lifetime T~ is found to be 4.10 ns. Upon excitation at 450 nm in a basic solution (NaOH at pH 1 l ) , the basic form RO- has a lifetime of T’,, = 5.45 ns. The same principle of determination applies to reversed micelles in which 5 M HC104 or N a O H (pH 11) solutions are injected. The respective values are T~ = 3.70 ns for ROH* and T ’ ~= 5.00 ns for RO-* whatever the water content (w 1 2.5). The constancy of lifetime is consistent with the presence of an isoemissive point, which implies no change in the decay rate constants k,, k,,, krr, and kfnr. Following the procedure described in the Experimental Section, the differential lifetime T&+ and the apparent lifetime T’ of RO-* are then measured as a function of water content in the range w = 2.5-12 (Figure 3) with an accuracy of 0.03-0.05 ns. From these experimental values together with the values of T~ and T ’ ~ ,the rate constants for proton transfer and back-recombination can
1912 The Journal of Physical Chemistry, Vol. 88, No. 9, 1984
Bardez et al.
TABLE I: Apparent Lifetime of RO-*, Differential Lifetime, and Rate Constants of Pyranine in AOT Reversed Micelles and in Bulk Waterc IY
r' ,a ns
2.5 3.1 3.6 4.2 4.7 6.1 7.4 8.8 11.5
6.21 6.17 5.85 5.77 5.70 5.50 5.28 5.23 5.21
H2O
5.63
TAQ'a
____
ns 2.15 2.53 2.16 3.09 3.33 3.74 4.05 4.29 4.5 1 4.67
-
0 -
--
I O - ~ ~ ,1 ,0 ~- ~ k ' . , , ~ SKI
S-'
0.73 0.96 1.6
27 20 16 12.5 10 6.7 4.7 3.2 2.2 3 .O
1.9 2.2
3.4 6.3 1.2 9.7 10.4
Accuracy: 20.03-0.05 ns. Accuracy: see error bars in Figure 4 . Modulation ffequency 3 0 MHz. a
be calculated (eq 1 and 2). The latter is best represented by kCl instead of k_,[H30+]sirice the concentration in H30+has no meaning in reversed micelles. The values are reported in Table I, and the variations in k , and kLl are shown in Figure 4.
Discussion Kinetics of Proton Dissociation of Excited Pyranine. As the water content of the fnicelles is increased, the rate constant kl of proton dissociation of the acidic form ROH* increases and reaches, when the H 2 0 / A O T ratio is about 12, a value in the region of 1 O I o s-I, as in pure water. This numerical value is in agreement with recent measurements carried out by Huppert and Kolodney2' and by Smith.41 As amounts of solubiliied water are decreased, the aggregation number decreases in such a way that the Na+ concentration of the water pool increases (the number of water molecules per sodium ion is evidently equal to the H 2 0 / A O T ratio). Consequently, if the theory of controlled diffusion reactions in the presence of Coulomb f o r ~ e s ~were ~ J ~to~apply ' ~ here, the increase of electrostatic shielding of the ionic atmosphere of RO-* caused by the increasing number of Na+ ions would lead to an increase of the rate of deprotonation. On the contrary, the rate of deprotonation decreases as it does in concentrated solutions of strong electrolytes, in water-alcohol solutions,21or furthermore as it is the case when the excited pyranine is bound to bovine serum albumin29or to a p o m y ~ g l o b i n . ~ ~ The decrease of the rate constant of proton ejection depicts the decrease of probability of proton transfer from the proton donor ROH* to a proton trap which is an aggregate of water molecules interconnected by hydrogen bonds. The proton transfer leads to the formation of a cluster H30+(H20),, the stabilization of the hydrated proton increasing with its hydration number ( n I10 in liquid ~ a t e r ) . ~Under ~ , ~ ~conditions where the number of available molecules of water is reduced and/or where the structure of water is partially broken (for instance, by addition of an alcohol), the probability of transfer is reduced. Hence, in AOT reversed micelles, the decrease of excited pyranine reactivity at lower water contents is directly related to the structure of the aqueous core. As a matter of fact, it has been established by N M R measurements that the water included in micelles at low water contents contains less hydrogen bonding than normal bulk water11*45,46 and that the mobility of the oxygen atoms it contains is greatly red~ced.~'Furthermore, fluorescence probing by measurements of the lifetimes of xanthene dyes confirms the changes in the hydrogen-bonding character of the water molecule~.~~ (41) K. K. Smith, Ph.D. Dissertation, University of Illinois at Urbana, Urbana. - ._ IL. 1980 (42) P.-Debye, Trans. Electrochem. SOC., 82, 265 (1942). (43) M. D. Newton and S. Ehrenson, J. Am. Chem. Soc., 93,4971 (1971). (44) M. D. Newton, J . Chem. Phys., 67, 5535 (1977). (45) A. Llor, Personnal communication. (46) J. Rouviere, J. M. Couret, M. Lindheimer, J. L. Dejardin, and R. Marrony, J . Chim. Phys. Phys.-Chim. Biol., 76, 289 (1979).
t 10
20
w
*
Figure 5. Activjty of water as a function of water content: theoretical function (solid line) and vapor pressure determinations (0) from ref 49; values of the present work ( 0 ) .
Water encased in reversed micelles is a peculiar solvent consisting essentially of oriented water molecules around the hydrated sodium and sulfonate ions and inadequate to hydrate the proton; the proton transfer in such a medium is hampered. It is only upon complete solvation of the Na+ and sulfonate ions ( w I12-15) that additional water is able to exhibit the behavior of ordinary water as proton acceptor. This work has to be compared with the studies of electron solvation in reversed micelles, studies which demonstrate that the absorption spectra and the lifetime of the hydrated electron are dependent on the pool size and are almost identical with those in bulk water only for w > 15-20.48 Gutman et al.29have established that the probability of protpn transfer in aqueous solutions is a direct function of the chemical activity of water, the rate constant k l is given by k , = k10(aA20)n, k o , being the rate constant in pure water; for pyranine in concentrated electrolyte solutions, n is independent of the electrolyte (LiBr, LiC1, KCl, MgC12) and was empirically determined as being 6.9. Considering the analogy between the addition of water in a reversed micelle and the dilution of a concentrated electrolyte solution, we have used this equation for the calculation of water activity around the center of the aqueous core from our kinetic measurements. The water activity as a function of the solubilized amount of water is shown in Figure 5. One should notice the good agreement of this activity plot with both the activities yielded by measurements of partial pressure of water vapor 4q and those calculated from the theofetical electrostatic model of Jonsson and Wennerstrom.50 The rate of deprotonation of excited acid pyranine not only depicts the ability of surroundiqg water molecules to accept a proton with regard to structural considerations but may also be interpreted in terms of the activity equivalent to that of an electrolytic homogeneous medium. The decrease of water activity in the center of the water pool reflects the progressive concentration of the aqueous core with sodium ions, constituting the environment of ROH*. From these considerations, it does not seem necessary to explain the behavior of pyranine in an AOT reversed micelle by introducing a distribution of the probe between the two aqueous "phases" of Zinsli's biphasic modelI6 as is proposed by Kondo c~-workers.~*~~~ Kinetics of Proton Recombination. The rate of proton recombination in the excited state kCl is reduced when increasing 12, kCl reaches a plateau the water content of micelles. At w whose value (3 X lo7 8)is in accordance with that obtained in 10-12, the rebulk water (pH -5-6). Therefore, up to w combination rate is higher than the diffusion-controlled rate" of
-
-
(47) M. A. J. Rodgers, J . Phys. Chem., 85, 3372 (1981). (48) M. P. Pileni, B. Hickel, C. Ferradipi, and J. Pucheault, Chem. Phys. Lett., 92, 308 (1982). (49) R Kubik, H. F. Eicke, and B. Jonsson, Helu. Chim. Acta, 65, 170 ( 1982). (50) B. Jonsson and H. Wennerstrom, J . Colloid Interface SEI'., 80, 428 (1981). ( 5 1) Besides, considering the changes in the lifetimes of ROH* and RO'* as a function of w, the measurements of the probe mobility carried out by these
authors using fluorescence polarization are questionable.
J . Phys. Chem. 1984, 88, 1913-1916
+
the bimolecular reaction RO-* H30+,the probability of which being practically nil during the lifetime of the excited state. The rate that is measured in water is possibly the rate of geminate recombination from a small fraction of the ion pairs (RO-*,H30+), while the proton is still within the Coulomb r a d i u ~ ;for ~ ~most ,~~ of the ion pairs, the proton diffusion leads to complete dissociation and then the recombination takes place in the ground state, after the fluorescence decay of R0-*.l8 In reversed micelles (w 5 12), the proton recombination is faster than in bulk water, owing to the higher probability of geminate recombination with respect to the dissociation of the ion pair. As a matter of fact, the dissociation is hampered because of the peculiar structure of the aqueous core (little amount of hydrogen bonding, lower mobility of the oxygen atoms of water molecules, and relatively high viscosity of the medium). It is also of interest to consider the size parameters involved in this system: pyranine has a hydrodynamic radius of 4.7 A;54the Debye radius42is 28 A18 in water and less than 10 A in a 0.1 M salt solution;21 the radius of the water pool is about 9 A at w = 3 and about 20 A at w = 11.5.’5 All these considerations lead us to think that, during the lifetime of the excited state, the ejected proton remains more or less in the vicinity of RO-* at low water contents. This cavity (52) M. Hauser, H. P. Haar, and U. K. A. Klein, Ber. Bunsenges. Phys. Chem., 81, 27 (1977). (53) H. P. Haar, U. K. A. Klein, and M. Hauser, Chem. Phys. Lett., 58, 525 (1978). (54) H. P. Haar, U. K. A. Klein, F. W. Hafner, and M. Hauser, Chem. Phys. Lett., 49, 563 (1977).
1913
effect is to be compared with the observations of Gutman et aL30 concerning the behavior of pyranine in the apomyoglobin binding site.
Conclusion Kinetics of proton transfer in reversed micelles brings interesting information on the relation between structure and reactivity and permits a better understanding of the acidity in the aqueous core and an evaluation of the water activity. In the present work, fluorescence probing around the center of the water pool reveals that the proton-transfer efficiency and the water activity become similar to those in bulk water beyond a water content of w 12. However, at a closer distance to the interface and even more at the interface, proton transfer is expected to be less efficient because water molecules close to the interface participate in the hydration of sodium ions and ionic heads of surfactant. Appropriate probes will be used in the future for this study. Experiments are also in progress in our laboratory for investigating the effects of injected buffered solutions and basic or acid solutions on proton-transfer dynamics in the water pool of reversed micelles.
-
Acknowledgment. We are grateful to Dr. M. T. Le Bris for her help in the difficult purification of pyranine. Registry No. AOT, 577-1 1-7; pyranine, 6358-69-6. Supplementary Material Available: Derivation of the equations used in differential phase fluorometry in the case of reversible and irreversible proton transfer (2 pages). Ordering information is available on any current masthead page.
Enthalpies, Free Energies, and Entropies of Transfer of Phenols from Nonpolar Solvents to Water Paul Haberfield,* Juris Kivuls, Michael Haddad, and Thomas Rizzo Department of Chemistry, Brooklyn College of the City University of New York, Brooklyn, New York 11210 (Received: September 27, 1983)
The enthalpies of transfer of seven phenols from 1-octanol to water and from toluene to water were determined by calorimetry. In the case of two phenols, whose rate of solution in water was found to be too slow for measurement by the usual heat of solution method, a new two-phase titration method was employed. The free energies of transfer between these solvents were determined by measuring the appropriate partition coefficients. The nature of the nonpolar solvent (toluene or 1-octanol) was found to cause large changes in the average values of the thermodynamic parameters of transfer into water, as well as changes in the ordering of the phenols in the series with respect to these transfer parameters. A curious correlation was observed between the octanol-water partition coefficients and the toluene water entropies of transfer.
-
Introduction The importance of hydrophobic interactions for the understanding of a variety of biological problems such as the stabilization of protein structures has long been recognized and is the subject of continuing study.’” The very nature of the hydrophobic effect is at present a subject of active investigation7-I0as is the associated (1) Frank, H. S.; Evans, M. W. J . Chem. Phys. 1945, 13, 507. (2) Kauzmann, W. Adu. Protein Chem. 1959, 14, 1. (3) Franks, F. Ed. “Water, a Comprehensive Treatise”; Plenum Press: New York, 1972-1982; Vol. 1-7. (4) Hopfinger, A. J. “Intermolecular Interactions and Biomolecular Organization”; Wiley: New York, 1977. (5) Tanford, C. “The Hydrophobic Effect: Formation of Micelles & Biological Membranes”; Wiley: New York, 1980. (6) Nemethy, G.; Peer, W. J.; Sheraga, H. A. Ann. Reu. Biophys. Bioeng. 1981, 10, 459. (7) Abraham, M. H. J . A m . Chem. Sor. 1982, 104, 2085.
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question of the water structure enhancing capacity of nonpolar solutes in water”J* and the consequences this may have for chemical13 and b i ~ l o g i c a l phenomena. ’~ One of the most interesting and useful demonstrations of the importance of the hydrophobic effect is to be found in the Hansch a n a l y s i ~of’ ~drug ~~~ (8) Mirejovsky, D.; Arnett, E. M. J . A m . Chem. SOC.1983, 105, 1112. (9) Wertz, D. H. J . A m . Chem. SOC.1980, 102, 5316. (10) Stillfinger, F. H. Science 1980, 209, 451. (11) Savaminathan, S.; Beveridge, D. L. J . A m . Chem. SOC.1979, 101, 5832. (12) Jorgensen, W. L. J . Chem. Phys. 1982, 77, 5757. (13) Engbersen, J. F. J.; Engberts, J. B. F. N. J . Am. Chem. SOC.1975, 97, 1563. (14) Haberfield, P.; Kivuls, J. J . Med. Chem. 1973, 16, 942. (!5) Hansch, C.; Muir, R. M.; Fujita, T.; Maloney, P. P.; Geiger, F.; Stretch, M. J . A m . Chem. SOC.1963, 85, 2817. (16) Hansch, C.; Leo, A. “Substituent Constants for Correlation Analysis in Chemistry and Biology”; Wiley: New York, 1979.
0 1984 American Chemical Society
