1280
S.
Dy, the complexation isotope ratio is close to 1, but there is a large decrease in the dissociation rate constant upon switching to D20. Thus, one pattern of results occurs for the light rare earths, a second for the heavy rare earths, and the changeover occurs near Gd. This trend is typical in systematic studies of lanthanides and has often beeu interpreted in terms of different coordination number species existing for the light and heavy lanthanides.lSJ9 A detailed interpretation of the significance of the isotope ratio for each individual lanthanide is not possible. However, the data cannot be explained by a single process occurring in step 111, but must be interpreted in terms of at least two steps. This is the case with an Eigen-type mechanism where this step consists of cation-solvent exchange followed by cation-ligand bond formation. The large isotope differences observed for the association and dissociation steps is not consistent with the rate-determining cation-solvent, as explained previously.1 The kinetic isotope data for each lanthanide in this study is considerably different than that found with anthranilate. The big difference occurs with Gd where a
Friberg, L. Rydhag, and G . Lindblom
large complexation isotope effect occurs with anthranilate but not with sulfate. A possible explanation arises in that both sets of studies were carried out at different temperatures, and that each of the systems in each solvent has considerably different activation parameters. This explanation cannot be resolved in this study. One of the questions which this study set out to answer was whether or not similar deuterium isotope effects occur for a given lanthanide cation with different ligands. The experimental answer is that the deuterium isotope effects are not a function of solvated cation alone, but instead are a function of the solvated cation-ligand complex. Acknowledgments. The authors thank Drs. Vitullo and Pollack for several helpful discussions. Partial support for this research came from the Frederick Gardner Cottrell Program of the Research Corporation. (18) J. H. Spedding, et a/., J. Phys. Chem., 70, 2423,2430, 2440,2450
(1966). (19) T. Moeller, D. F . Martin, L. C. Thompson, R. Ferris, G . R. Feistel, and W. J. Randall, Chern. Rev., 65,1 (1965).
Catalysis of p-Nitrophenol Laurate Hydrolysis in Solution Showing Transition from Reversed to Normal Micelles S. Friberg," L. Rydhag, The Swedish Institute for Surface Chemistry, Stockholm. Sweden
and G. Lindblom University of Lund, Chemical Center, Lund, Sweden (Received December 12, 1972) Publication costs assisted by the Swedish Board for Technical Development
Catalysis of the hydrolysis of p-nitrophenol laurate by normal and reversed micelles was investigated in their common solution area in the system water-cetyltrimethyltetraammonium bromide-water. Maximal reaction rates were found at the critical micellization concentrations for both kinds of micelles. In the range where transformation of one kind of micelle into the other could be expected, the reaction rates were minimal. The enhancement factor in the aqueous part was 1500, which is higher than has been reported earlier for such systems.
Catalysis by surfactant micelles has been investigated for more than a decade following the pioneering work by Duynstee and Grunwald in 1959.1 Extensive reviews2-4 of progress in the field have recently been published. The investigations prior to 1970 were, however, concerned exclusively with the effect of micelles in aqueous solutions which show the well-known association structure with the hydrophobic part of the surfactant forming the central part of the micelle and the polar parts oriented toward the surrounding aqueous solution. In amphiphilic liquids such as hexanol reversed micelles may also be formed when water and an ionic surfactant are added in sufficient quantities.5 The first report on The Journaiof PhysicalChemistry, Vol. 77, No. 10, 1973
the catalytic influence of such reversed micelles on the hydrolysis of a phenol ester was published in 19716 with reference to reversed micelles of water and cetyltrimethylammonium bromide (CTAB) in hexanol. Corresponding (1) E. F . J. Duynstee and E. Grunwald, J. Amer. Chem. SOC.,81, 4540 (1959). (2) E. H. Cordes and R. B. Dunlop, Accounts Chem. Res., 2, 329 (1969). (3) H. Morawetz, Advan. Catai., 20, 341 (1969). (4) E. J. Fendler and J. H . Fendler, Advan. Phys. Org. Chem., 8, 271 (1969). (5) P. Ekwali, L. Mandell, and P. Soiyorn, J. Colloid lnterface Sci., 35,
266 (1971). (6) S.Friberg and S. I. Ahmad, J. Phys. Chem., 75,2001 (1971).
1281
Catalysis in Transition from Reversed to Normal Micelles investigations of other reactions in nonaqueous systems? showed a more pronounced catalytic effect. Later the system water-hexanol-CTAB was also used to demonstrate hydrolysis catalysis in lyotropic liquid crystals9 for the first time. Earlier results10 of ester hydrolysis in a liquid crystalline phase showed reduced reaction rates when the surfactant was nonionic. Thus, considering that catalysis by both normall and reversed6 micelles as well as by liquid crystalline phases9 had been displayed, we found it worthwhile to investigate the rate of hydrolysis in a solution exhibiting a transition from normal to reversed micelles, such as, the isotropic liquid solution region in the system water-CTAB-butanol.11 Materials and Methods Materials. Analytical reagents, p-nitrophenyl laurate (Schuchardt, Munchen), 1-butanol (Fluka), and cetyltrimethylammonium bromide (CTAB) (Merck), were used. The water was distilled twice and buffered to pH 11.5 by means of a 0.01 M sodium phosphate buffer. Phase Region. The region of the isotropic solution in the system was determined by the addition of water and CTAB to homogeneous samples until1 turbidity was observable. The temperature was kept constant at 30 f 0.1" during the determination. Light Scattering. The intensity of scattered light a t an angle of 90" was determined by means of a Sophica photodiffusogoniometer 42000. The cells were cleaned with chromosulfuric acid and rinsed several times by distilled water. Final rinsing was effected by condensation of water vapor in the inverted cell. The cells were filled and rotated in an ultracentrifuge at 30,000 rpm for 4 hr to sediment dust particles. Reaction Rates. Kinetic measurements were performed by determining, at various times, the amount of p-nitrophenolate ion from the extinction at 400 mp by means of a Zeiss PMQII spectrophotometer. The initial concentration of p-nitrophenyl laurate ester was 2.4 X 10-5 M in all determinations, Nmr Measurements. The 79Br and 81Br nmr measurements were performed with a Varian V-4200 nmr spectrometer equipped with a 12-in. V-3603 magnet and a Varian Mark I1 Fieldial as described previously.12 The sample temperature was 30 2" measured with a thermocouple before and after recording a series of spectra. The reported line widths are the arithmetical means of twofour spectra, and the individual measurements are usually within 5% of the mean.
*
Results
Properties of the Solution. The solution region of the system formed a broad continuous area extending from the bulfer corner to the butanol corner (Figure 1). The limit against the butanol-CTAB axis was a straight line with an approximate CTAB-water molar ratio of 1:6. The width of the region toward the water corner was about 20% (w/w). The compositions of the samples for the kinetic determinations are indicated in the figure. The intensity of the scattered light at a right angle and the Br line width for series 3 are indicated as a function of the water content in Figure 2. At a water concentration below 40% (w/w) the intensity of the scattered light was low (Figure 2) and the AB values were too high to be conveniently determined. In the 40-60% (w/w) water range a sharp reduction in the line width took place and was ac-
Butanol
Figure 1. The solution region in t h e system cetyltrimethyltetraammonium bromide-butanol-water buffered at pH 11.5 with phosphate buffer. The composition of samples for which reaction rates were determined is indicated in the figure.
water,weight % Figure 2. Light scattering and 81Br line width as a function of water content for series 3 according to Figure 1. companied by a pronounced rise in the intensity of the scattered light. Between 60 and 70% (w/w) water both factors were fairly constant. A sharp reduction between 70 and 80% (w/w) was followed by a less sharp decrease up to about 95% (w/w). Solutions with water content in excess of 95% were not investigated owing to numerous determinations of the properties of solutions in this range.13 Reaction Rates. The reaction rate for a first-order reaction is given for a different series in Figure 3. At low water contents the reaction rate was enhanced by an increasing water content. For series 3 and 4 the rates passed a low maximum at 40 and 60% water. A shallow minimum was observed for these series in the 70-75% water range followed by a pronounced maximum a t about 90% water. The position of the low maximum was increased to higher percentages of water when the hexanol/CTAB ratio was decreased. The position of the pronounced maximum in the 90-95% water range was not determined with corresponding accuracy. Discussion The interpretation of the results from light scattering and nmr determinations both indicated that association into micellar aggregates took place in a concentration range of a few per cent around 40% water (w/w) for the (7) E. J. Fendjler, J. H. Fendler, R . T. Medacy, and W. A. Woods, Chem. Commun., 1497 (1971). (8) J. H. Fendler, Chem. Commun., 642 (1972). (9) S.I. Ahmad and S. Friberg, J. Amer. Chem. SOC.,94, 5196 (1972). (10) K. S. Murthy and E. G. Rippie, J. Pharm. Sci., 59, 459 (1970). (11) G. Gillberg, H. Lehtinen, and S. Friberg, J. Colloid lnterface Sci., 33, 40 (1970). (12) G. Lindblom. H. Wennerstrtjrn, and 6. Lindman, Chem. Phys. Lett., 8,489 (1971). (13) P. Ekwall, L. Mandell, and K. Fontell, J. Colloid lnterface Sci., 29, 9 (1969). The Journal of Physical Chemistry, Vol. 77,
No. 10, 1973
S.Friberg, L. Rydhag, and G. Lindblom
la82
M e r ,weight per cent
Figure 3. Reaction rates vs. water content for different series according to Figure 1: series 1, A ;series 2 , 0; series 3, Y ; series4, +.
series with a butanol/CTAB ratio of 2. The drop of line width to a level of 4 G at the association concentration was in accordance with the results from investigations of the hexanol solution in the system water-CTAB-hexanol.I4 A comparison with these results indicated that addition of the water gave rise to micelles of a reversed structure in a continuous medium of butanol with dissolved water and CTAB. The premise about a formation ofreversed micelles was supported by a calculation of the amounts of the totally included water (40% w/w) which were solubilized and molecularly dispersed. Pure butanol dissolved 20% (w/w) water in a molecularly dispersed form. The added CTAB 20% (w/w) required some further water in order to form ion pairs. Maximum 15% (w/w) CTAB was dissolved in pure butanol (Figure 1, right part). The necessary water to bring the remaining 5% (w/w) of CTAB into solution could be estimated from the slope of the right-hand limit of the solution area in Figure 1. A reasonable estimation was 5% (w/w) of the water being needed in order to dissolve the remaining CTAB. The content of butanol and of CTAB could safely be estimated to account for 20% (w/w) of the water as molecularly dispersed. The remaining 20% (w/w) can not reasonably be sufficient to form a continuous aqueous phase containing normal micelles. Since the micelles in the aqueous corner are normal micelles and since no phase transition could be observed the conclusion of the results must be an evidence of a transformation from reverse to normal micelles in a one-phase region of an isotropic liquid. This conclusion draws attention to the structure of reversed micelles and their transition into normal micelles. Reversed micelles have been postulated to be spherical15 but other forms are also probable. It has recently been shownle by low-angle X-ray diffraction investigations that reversed micelles of monoglycerides in triglyceride solution actually possessed a lamellar structure containing several layers. In the present case a preliminary determination of the low-angle X-ray diffraction pattern showed no signs of multilayer structures in any water range. This result gave evidence that the structures did not contain a sufficient number of ordered, sequential layers for the X-rays to give the multireflection patterns which are charaderistic of a liquid crystalline phase. A liquid crystalline phase should furthermore have given rise to a pronounced widening of the nmr lines. This was not found and the formation of a multilayer structure similar to a liquid crystalline phase in the The Journalof Physical Chemistry, Vol. 77, No. IO, 1973
transition region between reversed and normal micelles must be ruled out for the present solution. In need of more precise experimental investigations of the structure in the transformation region, the present results give sufficient basis for the following interpretation. The formation of reversed micelles was initiated when about 40% (w/w) of the water was added in series 3 (Figure 1). A transition region to normal micelles was then observed between 60 and 70% (w/w) water. Normal micelles of various structures account for water percentages in excess of the last value. The variations in reaction rates.us. the water content with two rate maxima at 45 and 95% (w/w) of water corresponding to two kinds of micelles were in accordance with this interpretation. The rise in the reaction rate was less sudden and the maximum rate for the reversed micelles showed slightly lower values compared with the results in hexanol.6 The less sudden increase could be ascribed to the formation of premicellar aggregates, which would also have some catalytic effect on the hydrolysis. The formation of premicellar aggregates has been observed in aqueous solutions of sodium carboxylates with a chain length of f0ur.l' The reduction of the maximum reaction rate when the cmc is reduced in aqueous systems due to shorter chain lengths has been observed recently18 in the catalytic effect of alkyl sulfates on the Hg2+ induced aquation of CO(NH&CP+. The influence of the substrate on the rnicellization was not observable; the concentration of the ester was only 2.4 X 10-5 M . The localization of the substrate was not determined; it is obvious that at least part of the differences between the rates in the aqueous and nonaqueous part of the solution might occur due to differences in localization of the substrate. For both the series which contained normal and reversed micelles a minimum was observed between the two maxima at critical concentrations for reversed and normal micelles. These results gave evidence that no structural conditions with specific catalytic effects existed in the concentration range when a transformation from reversed to normal micelles took place. A simple consideration of reaction rates suggests an inverse relation between water content in excess of cmc and reaction rate. Such an assumption is in accordance with a minimum between the two maxima. An inverse relation is not followed exactly, but this is no surprise since deviations appear to be the rule.18 It has been demonstrated earlier that changes in reaction rates are sensitive to changes in micellar structure.9 The pronounced maximum at high water content displayed an enhancement factor of about 1500 for normal micelles, which is considerably higher than the values of 200-500 which have earlier been reported in aqueous systems.2-4 The specific cause for this increase in reaction rates was not investigated, as it was beyond the original scope of the study. The influence of nonionic19 as well as ionic20 substances has been determined and a reasonable pattern of the combined effects on charge density and (14) G. Lindblom, B. Lindman, and L. Mandell, J. Colloid lnterface Sci., 34, 262 (1970). (15) P. Ekwall, J. Colloid lnterface Sci., 29, 16 (1969). (16) K. Larsson, Chem. Phys. Lipids, 9, 181 (1972). (17) I. Danielsson and P. Stenius, J, Colloid lnterface Sci., 37, 264 (1971). (18) J.-R. Cho and H. Morawetz, J. Amer. Chem. Soc., 94, 375 (1972). (19) D. S. Kemp and K. Paul, J . Amer. Chem. Soc., 92, 2553 (1970). (20) C, A. Bunton, M. Minch, and L. Sepulvede, J. Phys. Chem., 75, 2707 (1972).
Equilibrium and Kinetics of Several Hydroxyalkyl Radicals
I 283
electric double layer now appears to have been determined.21 With regard to these factors we would only want to point out that the enhancement factor in the present case is higher than those found earlier in aqueous systems; in completely nonaqueous systems larger tors have been found.738
Acknowledgment. The experimental assistance of Mrs.
B. M. Hultin is gratefully acknowledged. The investigations were financed by Swedish Board for Technical Development. (21) C. A. Bunton, Proc. Amer. Chem. Soc., Conf. React. Kinet. Mic. Membr., 73 (1972).
Equilibrium and Kinetics of the Acid Dissociation of Several Hydroxyalkyl Radicals' ,* Gary P. Laroff and Richard W. Fessenden" Radiation Research Laboratories, Center for Special Studies and Department of Chemistry, Mellon institute of Science, Carnegie-Melion University, Pittsburgh, Pennsylvania 15213 (Received December 22, 19721 Publication costs assisted by Carnegie-Melion University and the U. S. Atomic Energy Commission
The equilibrium constants for the dissociation of the hydroxyl proton of several hydroxyalkyl radicals in aqueous solutions a t about 20" have been determined by esr. The values expressed as pKa are for CHzOH, 10.7; for CH&HOH, 11.5; and for (CH&COH, 12.0. These values are in good agreement with those determined previously in pulse radiolysis experiments. The radical (CF3)zCOH is found to be a strong acid with pKa = 1.7. In all cases the pK is 4-8 units lower than for the corresponding alcohol. The dynamics of the equilibrium ROH OH- e RO- + HOH has been investigated for the radicals CH&HOH and (CH3)COH by means of the esr line broadening associated with the dissociation. Values for the forward and reverse rate constants are, for CH&HOH, 7 x lo9 and 4 x lo5, and, for (CH3)2COH, 9 X 109 and 1.8 X 106 (M-1 sec-1). The values for the forward reactions are clearly diffusion controlled. The rates for the reverse reactions (protonation by water) are seen to be so fast as to be accessible to direct pulse experiments 0-nly in the nsec region. Depending upon the pK and the forward rate constant a number of patterns of esr line broadening as a function of pH are possible. The various behaviors are discussed and examples given of most of them. Of particular importance is the conclusion that exchange of protons with high pK values (>16) will be readily observable in basic solution.
+
Introduction The esr spectra of hydroxyalkyl radicals derived from alcohols show changes as the solution pH is varied in the approximate range 10-13.3 These changes can be illustrated by the behavior of the radical (CH3)zCOH. In neutral water the spectrum is the well-known septet split by the OH hyperfine splitting of -0.5 G. As the pH is raised the lines broaden and then ultimately narrow to give a new septet with a somewhat smaller hyperfine constant for the CH3 protons and no splitting by an OH proton. This latter spectrum is attributed to the dissociated or ketyl form of the radical, (CH&CO-. In the intermediate region the central line of the septet is narrow but the lines with nonzero magnitudes of Iz for the six CH3 protons are progressively wider away from the center of the spectrum and the line separation is intermediate between that of the two limiting forms. The line broadening effects must be caused by the dynamics of the dissociation equilibrium
+
kOH r 'k0H+ (1) and the fact that the esr parameters are different for the two forms of the radical. Because the time scale involved
with esr line broadening is -10-8-10-7 sec it is immediately evident that the reverse of reaction 1 cannot be important because of the low H+ concentration and that the actual process is most likely kOH OHk HOH (2)
io- +
+
kr
Two types of information are likely to come from a detailed study of this phenomenon, namely, the equilibrium constant for the dissociation and the kinetics of the protonation-deprotonation. The equilibrium constants for a number of the simpler aliphatic alcohols have already been determined by pulse radiolysis4 but a determination by a totally different method is desirable because the optical method has, in at least one case, led to an incorrect conclusion.5 The esr method, in cases where rapid exSupported in part by the U. S. Atomic Energy Commission. Based on portions of a dissertation submitted by G. P. Laroff in partial fulfillment of the requirements for the degree of Doctor of Philosophy at Carnegie-Meilon University. K. Eiben and R. W. Fessenden,J. Phys. Chem., 75,1186 (1971). K.-D. Asmus, A. Hengiein. A. Wigger, and G. Beck, Eer. Bunsenges. Phys. Chem., 70, 756 (1966). The Journal of Physical Chemistry, Voi. 77, No. 10, 1973
