KINETICS OF CATALYSIS BY REVERSED MICELLES are l-propanol and l-hexanol using CoClz a t concentrations ranging from about 0.1 M to about 2.7 M . By taking into account that the reported work on propanol was a t 30 MHz, it is found that the chemical shift for the protons on the a carbon of propanol agrees rather well, as to absolute value, with the observed chemical shift of the methylene protons of ethanol in the present study; however, the shifts for the propanol and the ethanol are recorded as having opposite field shift directions. It is not clear from information at hand concerning the propanol and hexanol studies whether they were made a t constant frequency-variable field, or a t constant field-variable frequency. If it was the former the disagreement as to direction of chemical shift is only apparent, for a + A H value in this case would represent an excess a electronic spin for the proton under observation. There is no agreement for the propanol and hexanol as to absolute magnitude or direction for the 0carbon protons, or for the hydroxyl group proton, or for the y, 6, etc., carbon proton chemical shifts and the shifts observed for the hydroxyl and methyl group protons of ethanol. There have been many contributions to the understanding of observed hyperfine interactions and most of these have been reviewed by Eaton and Phillips.22a It appears, however, that none of these will fully explain the results reported in Table VI and possibly the sim-
2001 plest view would be that there is polarization of the u electronic system of the ethanol by the unpaired electron(s) of the paramagnetic ions. Judging from the magnitude of this effect as recently reported by Pople and Beveridge26for a methyl group proton in a plane perpendicular to the 2pn orbital of the methylene carbon in the ethyl radical, the polarization effect will well accommodate the spin densities reported in Table VI. The attenuation in relative spin density in going from the methylene to the methyl group seems consistent with the spin polarization being transmitted through the bonding system of the ethanol. It is interesting to note that the relative spin densities calculated by eq 9-12 show a decrease of the total spin density on the ethanol protons as the number of unpaired electrons on the cations increases. It is also of interest to note that the calculated spin densities should give rise t o hyperfine coupling constants of 0.5-1.5 G, a range of couplings near those found for some transition metals with the protons of metallocenesZ6and ~helates.~’ (25) J. A. Pople and D. L. Beveridge, “Approximate Molecular Orbital Theory,” McGraw-Hill, New York, N. Y.,1970,p 144. (26) H. M.McConnell and C. H. Holm, J . Chem. Phys., 27, 314 (1957);27, 749 (1957). (27) (a) D.R.Eaton, A. D. Josey, W. D. Phillips, and R. E. Benson, Discuss. Faraday SOC.,34, 77 (1962); (b) R. S. Milner and L. Pratt, ibid., 34, 88 (1962); (c) D.R. Eaton, J . Amer. Chem. SOC.,87, 3097 (1965).
Kinetics of the Catalysis by Reversed Micelles of Cetyltrimethylammonium Bromide in Hexanol by S. Friberg* and S. I. Ahmad The Swedish Institute for Surface Chemistry, S-114 28 Stockholm, Sweden
(Received January 96,1971)
Publication costs assisted by the Swedish Board for Technical Development
The influence of reversed micelles on the hydrolysis rate of p-nitrophenyl laurate has been investigated. The rate constant for hydrolysis was determined in hexanol solutions of water and cetyltrimethylammonium bromide of different compositions. The rate constant showed a pronounced increase when reversed micelles were formed in the system, giving evidence of a catalytic effect of reversed micelles. Catalysis in micelles by cationic species has been treated in several articles following the pioneering work of Duynstee and Grunwald in 1959.l Cordes and Dunlap2 have recently published an extensive review of the field. They point to two factors as being responsible for enhanced reaction rates : first, electrostatic and second, hydrophobic interactions of reactants, transi-
tion complex, and reaction products with the micellar phase. The existence of a charged transition state when a neutral molecule is hydrolyzed by hydroxide (1) E. F. J. Duynstee and E. Grunwald, J . Amer. Chem. SOC.,81, 4540 (1959). (2) E. H . Cordes and R. B. Dunlap, Accounts Chem. Res., 2 , 329 (1969).
The Journal of Physical Chemistry, VoE. 76, No. 19, 1971
S. FRIBERG AND S. I. AHMAD
2002 ions shows the importance of electrostatic interactions.394 Structural effects in reactant species are reflected in accentuated hydrophobic bonding which gives rise to a higher concentrations in the micelles.5 The majority of these investigations has been carried out in aqueous solutions where the poorly soluble reactant is solubilized a t the surface of normal micelles, while the second reactant, commonly the hydroxide ion, is soluble in the aqueous part. Exceptions are recent investigations on the kinetics of catalysis a t the heptane-water interface6 and on catalysis in nonhydroxylic solvents.' However, in none of these investigations is the possible influence of reversed micelles treated. Electrostatic interactions have a dominant effect on reactions in aqueous micellar solutions. However, since reversed micelles present in nonaqueous solutions of surfactants have received increased attention in recent yearss-12 we considered it worthwhile to investigate the possible catalytic activity of reversed micelles, In view of the numerous treatments of hydrolysis of pnitrophenyl esters catalyzed by cationic surfactants we chose p-nitrophenyl laurate and hexadecyltrimethylammonium bromide for the present investigation.
Experimental Section Materials. Analytical reagents p-nitrophenyl lau-
rate (Schuchardt, Munchen), 1-hexanol (Fluka) , and cetyltrimethylammonium bromide (Merck), CTAB, were used. Doubly distilled water, buffered to pH 11.5 by means of a 0.01 M sodium phosphate buffer, was used. Reaction Rates. Kinetic measurements were performed by determination, at various times, of the amount of p-nitrophenolate ion from the extinction at 400 mp b y means of a Zeiss PMQ I1 spectrophotometer. Initial concentration of p-nitrophenyl laurate ester was 2.4 X M . First-order rate constants were determined from the half-life, measured from the actual and final concentration of the phenolate ion. The determinations were made on seven series of solutions, each with a constant ratio of hexanol :surface active substance and with varying amounts of water.
HEXANOL A 0 0
1
WATER
0
C.T.A.B.
Figure 1. The hexanol solution area in the system water-CTAB-hexanol and the compositions used in the investigation. The numbers refer to the different series.
dent on the water content. At higher content of the surface active substance ab series 4,an increase of rate constants occurred when the water content was increased, and the rate constant reached values of between 0.1 and 0.4. This increase in rate constant took place rapidly over a narrow range of concentration.
Discussion
Results
The results show a well-defined trend; the reaction rate constant increases rapidly over narrow concentration regions. This increase is observed when the alcohol content is reduced to below 70% (w/w) and when the water content is increased a few per cent above the minimum level where micellar solutions exist. I n order to explain this, the association conditions of the solutions in the region studied must be considered. The system water-hexanol-CTAB has been investigated by Lawrence,13who found the liquid alcohol phase. Recently Lindblom," in a study of the counterion binding using nuclear magnetic relaxation of 81Br, found a pronounced decrease of relative line width, AB/ ABo, as the alcohol concentration was reduced below about 70 wt %. Below this value the line width was con-
The reaction rates, expressed as rate constants for a first-order reaction in the ester, were determined for the seven series according to Figure 1. The first three series with alcohol contents above 70% were chosen in the narrow region where the solubility of water is increased only slightly over that in pure hexanol. Increasing amounts of water were present in the next three series reaching a maximum of 55% in series No. 6. Series No. 7 was chosen with a higher content of CTAB and smaller amounts of water. The results are shown in Figure 2, with Figure 3 giving a "three dimensional" picture of the variation. The first three series gave low rate constant values between 0 and 0.03 min-l which were not markedly depen-
(3) J. L. Kurz, J.Phys. Chem., 66, 2239 (1962). (4) R. B. Dunlap and E. H. Cordes, {bad., 73, 361 (1969) ( 5 ) C. A . Benton, L. Robinson, and L. Sepulveda, J . Org. Chem., 35, 108 (1970). (6) F. M. Menger, J . Amer. Chem. SOC.,92, 5965 (1970). (7) R. L. Snell, W.-K. Kwok, and Y. Kim, ibid., 89, 6728 (1967). (8) S,Friberg, L. Mandell, and P. Ekwall, Acta Chern. Scand., 20, 2632 (1966). (9) G. Gillberg, H. Lehtinen, and S. Friberg, J . Colloid Interface S C ~ . , 33, 40 (1970). (10) G. Soderlund and S. Friberg, 2. Phys. Chem., 70, 39 (1970). (11) G. Lindblom, B , Lindman, and L. Mandell, J . Colloid Interface Sci., 34, 262 (1970). (12) P. Ekwall, L. Mandell, and P. Solyom, ibid., 35, 266 (1971). (13) A . J. Hyde, D. M. Langbridge, and A. S. C. Lawrence, Discuss. Faraday Soc., 18, 239 (1954).
The Journal o j Physical Chemistry, Vol. 76, N o . 13, 1071
KINETICS OF CATALYSIS BY REVERSED MICELLES
2003
12 ' 10. Y
4
0 c3
T*
H
s
z O.' 0 u a
L L
c
O.' 0
10
20
30
40
50
WATER,WEIG HT % Figure 2. The rate constants as function of the water content in the different series: (sign, series), 0, 1; 0, 2 ; . , 3; Y, 4; A, 5; X, 6; 0, 7.
C.T.A.B.,WEIGHT '10 Figure 3. Overall view of the rate constants as function of the composition of the solution.
stant. After comparison with the X-ray and viscosity data of Ekwall, l 2 Lindblom concluded that the three substances formed micelles a t alcohol concentrations below 70%. At higher alcohol concentrations, the formation of ion pair complexes, solvated by water and hexanol, was suggested. At the ratio 1:3.5 of CTAB to hexanol, the solutions have a maximum water solubilizing capacity. When the water content is decreased in this solution, an increase in the line width, AB/Bo, a t 20-300/, water was found." It was concluded that the bromide ions are more firmly bound to the micellar surface in the region with water concentrations below this value. According to Ekwall12 both the viscosity of and the Bragg
-
8-
Lr.
6.
4l 2
0 10
15
20
30
40
WATER, W E I GHT "h
Figure 4. Change in intensity of light scattering with concentration of water.
spacing from the solutions increase considerably when the water content is in excess of 30% (w/w). As the water content is decreased from this value, the viscosity decreases linearly while the Bragg spacing increases. Our light scattering determinations (Figure 4), expressed as a plot of log AI against log concentration according to Malik, l4 show approximately two straight lines with an intersection a t a water content of 25%. The increased Bragg spacing a t low water content is not necessarily due to the formation of micelles. However, on comparison of the results from several different investigations, micellixation appears highly probable at water contents above 30% (w/w). The present results therefore strongly indicate a correlation between the formation of reversed micelles and a sudden increase of the reaction rate. For all water concentrations studied, on reduction of the concentration of hexanol, an increase of reaction rate begins a t the concentration where micelles begin to form. It appears evident that hydrolysis is catalyzed to a greater extent when the water is associated into micelles, as compared when it serves as solvation for ion pairs a t high alcohol concentrations. In this latter case the solvated water will probably not show the same hydroxide ion activity and the effect of the buffer in alcohol solutions cannot be directly compared with its effect in aqueous solutions. The primary cause of the increased catalytic influence when micelles are present could consequently be but is not necessarily found in these phenomena. The rate constant referred to a first-order reaction is of the same magnitude as from investigations on normal micelles containing CTABlb showing a maximum value of K o b s d = 0.26 min-l. This shows that this method of solubilizing the hydrolyzing agent gives a (14) W. U. Malik, A . K. Jain, and 0. P. Jhamb, Int. J. A p p l . Radkt. Isotop., 21, 564 (1970). (15) L.R.Ramsted and E. H. Cordes, J . Amer. Chem. Soc., 90,4404 (1968). The Journal of Physical Chemistry, Vol. 76, No. 18, 1971
S. FRIBERG AND S. I. AHMAD
2004 similar efficiency to that obtained when the reactant is solubilized. The calculation of a rate constant that referred to a first-order reaction is justified since the buffered aqueous solution gives a constant pH value, so that the hydroxide ion is present in large excess over the reactant even in the most water-poor solutions. The total amount of water is, however, not constant in the different solutions. Dividing by the concentration of water and by the maximum value of the rate constant gives the relative “approximate second-order rate constant” according to Figure 5 . This presentation of the results shows the effect of the different activity of water in the different solutions. The sudden increase of the rate constant a t micellization can formally be referred to a change of the hydroxyl ion activity. Furthermore, in all the series where a sudden increase of the reaction constant is found, a maximum value of the rate constant is observed immediately after the sudden increase. The decrease of the value of the rate constant after this maximum may be explained by assuming an increased micellar size with higher water content. Since the hydrolysis reaction must occur in a surface layer a t the micelle, an increased micelle diameter will give less catalytic efficiency due to a decreased micellar surface area per volume of water. Both the results of Ekwall12 and our light scattering data can be interpreted as originating from increased micellar size with increased water content. This would explain the decrease of the second-order rate constant with increased water content.
The Journal of Physical Chemistry, Vol. 76,No. 18, 1971
i
z
22
2n I
3
z 0
V W !-
a,.
‘z
g2 a ‘T 0
gC?l
o x 0 2 w Y, W
>
it
a
4
w K
WATER, W E I G HT *la Figure 5. Relative second-order rate constants : (sign, series), 0, 1; 0, 2; M, 3; Y, 4; A, 5 ; X, 6; 0, 7.
The investigations will be continued with the reactions in surfactants under different association conditions.
Acknowledgment. The authors are grateful to the Swedish Board for Technical Development for financial support. Dr. E(. Roberts kindly revised the English text.
