n'ov., 1962
KINETICS O F THE
HYDROLYSIS O F MONOALKYL SCLFATES
and i ia an effective degree of ionization of the polyelectrolyte, while rn3' is the molarity of simple electrolyte, The test plots of (a6 - a*)/P1's against l/ma' for LiPP-1, LiPP-2, and NaPP2 are shown in Fig. 4. I n accordance with the theory, {,he points for the two LiPP samples fall on the same line. The slopes of these plots are 0.043 and 0,0213for LiPP and NaPP, respectively. Substituting 6.3 and 6.5 A. for (Eo2/P)'/zin eq. 11 for NaPP and LiPP, respectively, gives 1.29 and 1.42 for their respective CI' values. To reach agreement with eq. 10, the values of i would have to be 0.13 for LiPP and 0.10 for KaPP. These i values a,re slightly lower than their respective values, 0.17 for LiPP and 0.14 for KaPP, obtained from membrane equilibrium experiments by extrapolation to zero ionic strength, but the differences are not large considering that i is really the result of electrostatic interactions which may affeet the two types of polyelectrolyte plroperties differently. The intercept J of eq. 10, which can be considered a, measure of the excluded volume of the hypothetical uncharged polymer, is -0.026 for
2239
LiPP and -0.068 for NaPP. Two conclusions may be drawn from this result. First, the fact that these values are negative indicates that the solvent is incompatible with the uncharged polymers. Only when the contribution of the electrical charges, represented by the positive last term in eq. 10, approaches (or exceeds) J , will the polymer go into solution. Second, the difference between the two values of J indicates that the uncharged sodium polymer has a much higher solvent incompatibility than the uncharged lithium polymer. This difference has been postulated previously to account for the differences in both intrinsic viscosities and phase-separation behavior of these polymers, and has been ascribed to the greater ability of the bigger unhydrated sodium ion to cross-link two phosphate groupsSaa Acknowledgment.-The authors wish to thank Mrs. Jean W. Day for performing many of the viscosity rneiisurements. (32) U. P. Btrauas and P. D. Ross, J . A m Chem S a c , 81, 5295 (1 959).
EFFECTS OF MICELLIZATION ON THE KINETICS OF THE HYDROLYSIS OF MONOALKYL SULFATES BY JOSEPH L. KURZ Central Basic Research Laboratory of Esso Research and Engineering Company, Linden, New Jersey Received June 7,186%
Data are given for the kinetics of the hydrolysis of primary straight chain sodium alkyl sulfates t o the corresponding alcohols in acidic, basic, and neutral aqueous solution. Both micellar and non-micellar esters are included. The hydrogen ion-catalyzed rate of hydrolysis is strongly accelerated by the aggregation of the ester anions into micelles, which lowers the enthalpy of activation but has little effect on the entropy. The corresponding h droxide ion-catalyzed rate is strongly suppressed and the uncatalyzed rate appears to be unohanged. Both the lack of an egect on the uncatalyzed rate and the magnitude of the electrostatic potential of the micelles (calculated from the acid-catalyzed rates) are in agreement with the rough model for micelles. Salt effects on the hydrolysis rate are larger for the micellar evters. They appear t o be due to an increase in double layer shielding with increasing salt concentration rather than t o any specific competition among counterions for sites on the micelle. The variation in the rate of hydrolysis of sodium decyl Nulfate with concentration is uaed t o calculate the critical micelle concentration, and values from conductance measurements are given t o provide an independent standard for comparison.
The presence of micellar electrolytes recently has been r e p ~ r t e d l -to ~ give rise to changes in the rates of various ionic reactions which are far in excess of and far more specific than those expected from an ordinary kinetic salt effect. Duynstee and Grunwaldl have studied the changes in the rates of the alkaline fading of dyes of various charge types which accompany the partition of the dye molecules into micelles, and have emphasized the dominant role of electrostatic interactions in those effects. The rate of the proton-catalyzed hydrolysis of N-benzyljdeneaniline hap been shown t o be strongly retarded in the presence of the cationic detergent , n-hexadecyltrimethylammonium bromide, by van Senden and Koningsberger.2 Large rate factors arising from the presence of ionic micelles also have been reported by Lowe and (1) E. F. J Duynstee and E. Grunuald, J . A m . Chem. Soc., 81,4540, 4542 (1959) ( 2 ) K. G. van Senden and C. Koningsberger, Tetrahedron Letters, 1, 7 (1960). (3) M.B. Lowe and J. N. Phillips, Natura, 190, 262 (1961).
Phillipsa for the incorporation of cupric ion into the porphyrin ring. Since these rate effects were assumed by their investigators to arise from incorporation of the reactants into (or onto) the micelles, it appeared to be of interest to examine a reaction in which the micellar salt itself is directly involved. The hydrolysis of straight chain alkyl sulfate salts to the corresponding alcohols ROSOa-
+ H20
-
ROH
+ HSOd-
(1)
was chosen for investigation. A single homologous series contains both micellar and non-micella1 esters, and since the micelles are constructed directly from the reactant ions, their structure is unperturbed by the preeence of foreign molecules during the initial few per cent of reaction. The hydrolysis is known t o proceed by three kinetically distinct paths--hydrogen ion catalysis, hydroxide ion catalysis, and uncatalyzed solvolysis-whose
JOSEPH L. RURZ
2240
Val. 66
mechanisms have been e l ~ c i d a t e d , ~thus - ~ provid- extraction from the aqueous solution into 1-butanol according three closely related reactions of different charge ing t o the method of Dreger.’* Each salt was recrystallled at least three times from methanol-isopropyl alcohol rnixtypes. Since only univalent ions are involved in tures, and was dried over phosphorus pentoxide. Immethese reactions, the inter retation of kinetic effects diately before meighing, each sample of salt w m redried over should be simplified. Tphe observation that long P Q Oa~t 80’ under vacuum. The salts were analygedia by chain alkyl nulfates appear t o undergo more rapid potentiometric titration of the acid liberated when a sample passed through a column of Amberlite IIZ 120 (H’) acid-catalyzed hydrolysis than do the short chain was Titrant was compared t o potassium acid phthalate as the esters has been reported,gJ* but no discussion of primary standard. It was necessary t o run the ion-exthe factors which might have given rise to such an change column a t an elevated temperature in the analysis of the three longeat chain salts, since their ICrafft polnts lie effect was presented. room temperature. It is likely that the high assays Both the spontaneous6 and the hydroxide ion abovc obtained for the octadecyl ester arose from partial hydrolysis catalyzed8 routes proceed with carbon-oxygen on the column due t o the particularly high temperature path yields (55-60”)necessary t o keep this compound In solution. The cleavage, while the acid-~atalyzed5~’~8 sulfur-oxygen cleavage. Calhoun and Burwell4 analytkal results are expressed beluw as percentages of the have presented evidence supporting the contention theoretical titer. that the hydroxide ion-catalyzed reaction proceeds Salt Assay (%I by direct nucleophilic displacement, while the #odium methyl sulfate 100 0; 100 2 uncatalyzed path is intermediate in character Sodium ethyl sulfate 99.7; 99.9 ~ 8 ~ 2 . The existence of apprebetween S N and Sodium amyl sulfate 100.0; 99 R ciable binding of water to carbon in the transition Sodium decyl sulfate 100 1; 100 0 state for the uncatalyzed hydrolysis is shown to be Sodium dodecyl sulfate 100 0; 100,3 probable by the observation that the uncatalyzed Sodium tetradecyl sulfate 100.0; 100 3 hydrolysis of see-butyl eulfate proceeds with inSodium hexadecyl sulfate 100 2; 100 2 versions6 The acid-catalyzed hydrolysis presumSodium octadecyl sulfate 100.7; 100 9 ably proceeds through equilibrium protonation of sodium hydroxide solutions were prepared the sulfate moiety followed by attack of water on byCarbonate-free dilution of a filtered 50% solution. Stock solutions of sulfur .6 perchloric acid, acetic acid, and sodium tetraborate were
R801-
RSOIH
+ HZ0 --+
+ H+ ROB
+
RSOdH
HSO4-
+ H+
(2)
(3)
Since an ionic micelle in aqueous solution may be regarded as a region of the solution composed of oriented molecules and possessing both a low polarity and a high electrostatic potential with respect to the bulk solution,ll the study of the effects of micellar electrolytes on the rates of chemical reactions is of interest from several viewpoints, It should be possible to exploit the structure of the micelle to change the rate and course of reactions, and the observed rate effects should be useful in exploring the details of this structure, In addition, the similarity of a micellar solution to systems of biological importance renders such kinetic data of potential biochemical interest.
Experimental Materials.-Sodium methyl sulfate and sodium ethyl sulfate were prepared by artial hydrolvsis of the corresponding dia& 1 esters ?Matheson, Cbleman & Bell) with aqueous so&xn hydroxide. The longer chain salts were prepared by reaction of the normal alcohol (Eastman Kodak, dodecanol; Matheson, Coleman %I Bell, others) with chlorosulfonic acid. This sulfation was carried out in ethereal solution and, after neutralization with sodium hydroxide, the product was isolated either by evaporation t o dryness and extraction with hot methanol or by direct (4) G. M. Calhoun and R. L. Burwell, Jr., J . Am. Chem. Soc., 77, 6441 (1955). (5) R. L. Burwell, Jr.,
