J. Phys. Chem. 1980, 84, 361-365

Ion Exchange in Micellar Solutions. 4.’” “Buffered” Systems

361

Frank H. Quina, *,lb Mario J. Politl,lb Ioianda M. Cuccovia,lb Eiisa 6aumgarten,lb Sandra M. Martins-Franchetti,Ib and Hernan Chaimovich”

Group for Interfacial Studies (GIST), Instituto de QGmica, Universidade de SFjo Paulo. C.P. 20780, SFjo Paulo, Brazil (Received June 4, 1979)

The alkaline hydrolysis of the positive micelle-excluded N-methyl-Ccyano- and N-methyl-2-cyanopyridinium ions (4-MCP and 2-MCP) and of the efficiently micelle-incorporated neutral substrate p-nitrophenyl octanoate (NPO) has been investigated in buffered micellar solutions of N-hexadecyl-Nflfl-trimethylammonium bromide (CTAB). The observed rate constants for hydrolysis of 4-MCP (pH 9.80) and 2-MCP (pH 9.50) are independent of CTAB, clearly demonstrating that the intermicellar pH is maintained constant by the buffer system employed (0.020 M borate) up to at least 0.10 M detergent. In contrast, the observed rate constants for hydrolysis of NPO (pH 9.50 and 9.80) under the same conditions vary with CTAB above the cmc, reflecting the variation of the local concentration of bound OH- [ m b ] in the micellar pseudophase with detergent Concentration. These results unambiguously demonstrate that buffering of the intermicellar aqueous phase does not imply unique “buffering” of the micellar pseudophase. The kinetic results for NPO and for the alkaline hydrolysis of p-nitrophenyl acetate are analyzed quantitatively within the framework of ion exchange in micellar solutions. This analysis points to several potential complexities in the interpretation of reactivity patterns in buffered micellar solutions and leads to criteria for appropriate buffering conditions.

Introduction The distribution of H+ or OH- ions between the micellar

and aqueous phases is a sensitive function of the analytical concentration of strong acid or base present, the nature and total concentration of detergent (CT), and the ionic content of the m e d i ~ m . ~ , ~ It has recently been possible to verify experimentally the fraction of H+ bound to micellar sodium dodecyl sulfate (SDS)2a*c and of OH- bound to micellar N-hexadecyl-N,N,N-trimethylammonium bromide (CTAB)3b in the absence of buffer as a function of CT and added salts.

Although one might intuitively expect that the use of a buffer would simplify the system, particularly with regard to kinetic studies of micelle-modified reactions involving H+ or OH-, this has not proved to be the case.4 Several fundamental questions contribute to the complexity of buffered micellar systems: (1) To what extent do the components of the buffer interact with the micellar pseudophase, and to what extent does the presence of micelles affect the buffering equilibria5 or the buffer ca- pacity? (2) What is the relationship between the pH in the aqueous phase and the local concentration of bound ions, [Hb] or [ @ & I , in the micellar phase? (3) Are these local concentrations maintained constant by the buffer under changing condition^?^^

If the buffer components do not interact with the mi- cellar phase and the buffer capacity is not exceeded, it is evident that the intermicellar pH should be maintained constant under changing conditions (e.g., CT, added salts). Given such an “appropriate” buffer system and the se- lectivity coefficient for ion exchange of H+ or OH-, we have recently shown that one can calculate the dependence of the concentration of “bound” exchangeable H+ or OH- ions, Hb or OHb, on CT and thus analyze the resultant kinetic behavior of reactions involving these ions in buffered micellar solutions of ionic detergents3a (e.g., OH- in CTAB).

In the present work, we describe a series of simple ex- periments ,in aqueous micellar solutions of CTAB that unambiguously demonstrate that buffering of the inter- micellar aqueous phase does not imply unique “buffering” a t the micelle surface. Thus, even under conditions where

it can be directly demonstrated that the buffer maintains the intermicellar pH invariant with CT, the local concen- tration of “bound” OH- in the micellar pseudophase [ m b ] varies with CT above the critical micelle concentration (cmc). These results and their quantitative analysis are in accord with the predictions of ion exchange in micellar solutions.3a

Experimental Section Materials. N-Methyl-2-cyanopyridinium chloride (2-

MCP) and N-methyl-4-cyanopyridinium chloride (4-MCP) were prepared from the corresponding ( k n ~ w n ) ~ iodide salts by exchange with AgCl in slightly acidic (HC1) aqueous solution at room temperature. After filtration and lyophilization, the salts were purified by extraction with CH3CN (Aldrich spectroquality) and dried in vacuo. The molar extinction coefficients were in excellent agreement with the data of Kosower and Patton’ for the corre- sponding perchlorates, and chloride titrations* gave the expected equivalent weights. CTAB (Merck, p.a.) was purified as previously described.3b p-Nitrophenyl acetate (NPA, Sigma Chemical Co.) was used as received; a pure sample of p-nitrophenyl octanoate (NPO) was kindly furnished by Dr. Omar A. E. Seoud (Instituto de Quimica, Universidade de Ssio Paulo). Tris(bipyridine)ruthenium- (11) dichloride hexahydrate ( R ~ ( b p y ) , ~ + , G. Frederick Smith) was used as received. All inorganic reagents were analytical reagent grade or superior, and all solutions were prepared in freshly boiled deionized water, doubly distilled in glass, which had been allowed to cool under an argon atmosphere.

Methods. Kinetic measurements were carried out a t 30.0 “C with a Beckman M25 kinetic spectrophotometer. The alkaline hydrolysis of 4-MCP was followed by absorbance (at 260 nm) as previously de~cribed.3~9~ The corresponding reaction of 2-MCP was followed in a similar manner by monitoring the increase in absorbance at 297 nm due to the appearance of N-methyl-2-pyridone. The alkaline hydrolyses of NPA and NPO were followed by monitoring the appearance of the p-nitrophenoxide ion a t 405 nm. Rate constants were calculated as previously describedgJO from log (D, - Dt) vs. time plots, which were linear for a t

1980 American Chemical Society 0022-3654/80/2084-036 1$01 .OO/O

362 The Journal of Physical Chemistty, Vol. 84, No. 4, 1980

least four half-lives in all cases. pH was measured with a semimicrocombination electrode (Beckman Inc.) and a Metrohm Herisau E 388 compensator calibrated against standard reference buffers (Beckman Inc.). Critical micelle concentrations (cmc) were determined with an A. Kruss Model 8551 de Nouy tensiometer equipped with a Pt ring. Emission measurements were performed on a Hitachi- Perkin-Elmer MPF-4 spectrofluorimeter operated in the ratio mode. Aliquots (3.00 mL) of air-equilibrated solu- tions containing Ru(bpy),C1,.6H20 (3.5 X 10-5 M), KBr (0.10 M), and CTAB (M.040 M), thermostatted at 30 "C, were excited at 450 nm, and the emission spectrum was recorded. Successive aliquots of a concentrated stock so- lution of 2-MCP (0.192 M) or 4-MCP (0.154 M) were added, and the emission spectrum was recorded following each addition. The concentrations of the stock solutions were verified by absorption, and the total addition did not exceed 40 pL. Emission data were treated in Stern-Volmer fashion (6 Oem/t$ em vs. quencher concentration) to obtain the Stern-Volmer quenching constant (Ksv).1','2 For both 2-MCP and 4-MCP the pyridone/amide product ratios were determined by absorptiong and by determination of CN,', using calibration curves corresponding to our con- ditions. Stock solutions of CTAB were prepared by weight, and the final concentration was checked by Br- titration8 in all cases.

Results We have employed N-alkyl-4-cyanopyridinium ions as

substrates to study micellar effects on product distribu- t i ~ n , ~ premicellar aggregat i~n, '~ the partitioning of mo- nomer-like substrates with the micellar phase12 and to quantify the binding of OH- to CTAB micelles.3b These studies have taken advantage of the fact that these ions quench the emission of positive-micelle-excluded probes such as the R ~ ( b p y ) , ~ + ion,12 the fact that they undergo alkaline hydrolysis to give the N-alkyl-4-carbamido- pyridinium ion (A) and the corresponding N-alkyl-4- pyridone (P) in a pH- and medium-dependent r a t i ~ , ~ , ~ and the complete exclusion of the 4-MCP ion from the micellar phase of positive detergents such as CTAB.3b,'2 By analogy with the 4-MCP ion, it is reasonable to assume that the 2-MCP ion is also excluded from the micellar phase of CTAB. That this is indeed the case is demonstrated by the fact that the Stern-Volmer constants (Ksv) for the quenching of the emission of the Ru(bpy)32+ ion by both 4-MCP (1040 f 20 M-l) and 2-MCP (870 f 10 M-I) are independent of CTAB (0-0.04 M) within experimental errorlj in the presence of 0.10 M KBr.

In contrast to our previous results in the absence of buffer,3b CTAB is without effect on the observed rate constant ( h ,(MCP)) for alkaline hydrolysis of 4-MCP or 2-MCP in the presence of 0.020 M borate buffer (Figure 1A). On the other hand, under the same conditions the observed rate constants (K,(NPO)) for the alkaline hy- drolysis of NPO, a substrate that should be efficiently incorporated into the micellar pseudophase,16 decrease sharply with increasing concentration of CTAB above 1 x lo-, M (Figure 1B). Finally, the kinetic profile for the alkaline hydrolysis of NPA,lg a substrate which partitions between the aqueous and micellar p h a ~ e ~ , ~ ~ , ~ ~ , ~ ~ ~ , ~ ~ exhibits a broad maximum with a gradual decrease in k,(NPA) at high CTAB (Figure 2).

Discussion The present results (1) demonstrate that, in the pH

range investigated, the intermicellar concentration of free OH- ion ([OH,]) is maintained constant by 0.020 M borate buffer up to at least 0.10 M CTAB, (2) show that the local

Quina et al.

1 2 3 4 5 6

B [CTAB]. M x IO'

I 2 3 4 5 6 7 8 9 IO

[CTAB]. M x IO2

Figure 1. Effect of CTAB on the observed rate constants for alkaline hydrolysis in the presence of 0.020 M borate buffer: (A) 2-MCP at pH 9.50 (0) and 4-MCP at pH 9.80 (0); (B) NPO at pH 9.50 (0) and pH 9.80 (0). Data at low [CTAB] are shown in expanded scale in the inset. The curves are calculated (see text).

O'I 6 1 1 1 1 1 I I 1 1 I 1

0 5 I O

[CTAB], M x IO2 Figure 2. Effect of CTAB on the observed rate constants for alkaline hydrolysis of NPA at pH 9.50, maintained with 0.020 M borate buffer. The curve is calculated (see text).

concentration of OH- ion in the micellar phase ([mb]) decreases under these conditions with increasing concen- tration of micellized detergent, (3) imply that there may be a small interaction of borate ion with the micellar phase, an interaction which, however, is insufficient to exceed the buffer capacity, and (4) point to several potential com- plexities in the quantitative analysis of reactivity in buffered micellar systems. Each of these points will be discussed separately.

I s [OHf] Constant in a n Appropriately Buf fered Mi- cellar Sys tem? The alkaline hydrolysis of the positive micelle-excluded N-methylcyanopyridinium ions (2-MCP and 4-MCP) is a sensitive probe of the concentration of OHp3b The observed rate constant for hydrolysis (k+-

Ion Exchange in Micellar Solutions

(MCP)) is relatable to the intermicellar pH by eq 1. Since

(1) k JMCP) exhibits no significant variation for either ion (i.e., 2-MCP at pH 9.5 or 4-MCP at pH 9.8, Figure 1A) with increasing CTAB concentration,22 we are led to conclude that the intermicellar pH is indeed maintained constant by 0.020 M borate buffer up to (at least) 0.10 M CTAB. - Is There an Amphiphile Concentration Dependence of

in the Presence of Buffer? For a totally micelle-in- corporated substrate, the observed rate constant for al- kaline hydrolysis would be given by3" eq 2, where kzm is

k,(MCP) = k20 antilog (pH - pK,)

k$ = k2m [OH,] (2) the second-order rate constant in the micellar pseudophase. From an estimated incorporation constant of 1.5 X lo4 M-' for NP0,16 the limit of total incorporation (Le., >95%) is reached at detergent concentrations only slightly above (ca. 3 times) the cmc of CTAB under these conditions (5 X M). Thus, the kinetic data for NPO should directly reflect [OHb] above ca. 2 X M CTAB. The fact that k,(NPO) is not constant and decreases with increasing detergent concentration is thus a direct indication that [OH,] de- creases in a similar manner. Such a decrease implies that 0.020 M borate buffer, which maintains [OH,] constant, does not maintain [ m b ] constant with changing amphi- phile concentration.

Quantitative Analysis of Micelle-Modified Alkaline Hydrolysis in the Presence of Buffer. We have recently d e m ~ n s t r a t e d ~ ~ that, in the presence of a buffer which maintains [OH,] constant, the appropriate expression for the observed rate constant (k,) for a bimolecular reaction between an uncharged substrate such as NPA or NPO and a reactive univalent ion such as OH- in a micellar solution of CTAB is eq 3. In this equation, Ks is the distribution

._

The Journal of Physical Chemistry, Vol. 84, No. 4, 1980 303

and CY is the degree of ionization of the micelle (=0.2 f 0.0524-26).

When the experimental values for Ks (54 M-' for NPA and 1.5 X lo4 M-' for NPO), KOHpr (0.08), cmc ( 5 X M), and k2[OHf] (0.050 min-' for NPA at pH 9.50,0.027 min-' for NPO at pH 9.50, and 0.048 min-I for NPO at pH 9.80 in the absence of CTAB), the values of Q = 0.37 M-' and CY = 0.2, and eq 3-7 are used, no value of k2, which satisfactorily simulated the kinetic data for NPA or NPO could be found. Simultaneous variation of a and k2, also resulted in poor simulation of the kinetic data, the devi- ation between the experimental and calculated behavior being somewhat diminished by the use of very small values of a (50.1).

Indeed, it is perhaps overly simplistic to presume, as do eq 4-7, that the buffer used in the present study is entirely ideal. Thus, if the borate ion (B) binds at all to the positive micellar phase of CTAB, the appropriate expressions for [Bib], [Br,], and [OH,] become

[Brb] = (1 - ~ ) C D - [OH,] - [Bbl (8) [Br,] = ~ C D cmc + [OH,] + [Bb] (9)

+ [OHbl2(A2 + KOH/B[OHfl + [BTI) + [OH~][OH~~(KOH/BAS -k KOH/BABT - (1 - ~ ) C D ) ) -

K0H/Br[oHfl2(1 - a)C&OH/B = 0 (10)

where A2 is given by eq 7. In eq 10, we have made use of the borate-hydroxide ion exchange selectivity coefficient (KOHIB), defined as eq 11 and related to the borate-

constant of the neutral substrate (54 M-' for NPA21 and 1.5 X lo4 M-' for NP016); k: and kzm are the second-order rate constants for the alkaline hydrolysis in the aqueous and micellar phases, respectively. P is the volume per mole of micellized detergent of the region surrounding the micelle and within which the exchangeable ions may be said to be "bound"; Q has been taken to be roughly equivalent to the partial molar volume of the micellized

selectivity coefficient for OH/Br exchange at the micelle surface (=0.083b), and CD is the analytical concentration of micellized detergent, taken to be equal to the total detergent concentration (C,) minus the critical micelle concentration measured under the reaction conditions.

In the presence of an ideal buffer, Le., a buffer whose components do not interact ut all with the micellar phase, the appropriate expressions for the analytical concentra- tions of bound ([Brb]) and free ([Br,]) bromide ion and for bound hydroxide ion ([OHb]) are3a

[BI'b] = (1 - CY)CD - [OHb] (4) [Br,] = CYCD + cmc [OHb] ( 5 )

(6)

detergent (ca. 0.37 L/mol for CTAB).l8vZ3 K OH/Br is the

A2 + [ ( A z ) ~ + ~ K o H / B ~ [ O H ~ I ( ~ - a)cDl'/2

2 [OHbl =

where A2 = (YCD + cmc KoH/B~[OH~] (7)

bromide selectivity coefficient by eq 12, where [BT] = [Bf]

+ [Bb] is the total concentration of the B(OH)L ion at the experimental pH.

Two independent lines of experimental evidence imply that borate/bromide exchange is highly unfavorable. Substantial borate ion binding would be expected to shift the buffer equilibrium, resulting in variations in [OH,] in the intermicellar aqueous phase. That no significant shift of the buffer equilibrium occurs is apparent from the constancy of [OH,] up to 0.1 M CTAB (vide supra). Moreover, 0.020 M borate buffer only diminishes the cmc of CTAB from 9.4 X lo4 M to 5.0 X lo4 M.n Nonetheless, a small KBpr does not necessarily imply that borate binding will not affect the dilution of [OH,]. In this context, the key factors become the unfavorable binding of OH- to the CTAB micelle and the low OHf concentra- tion (pH 9.5-9.8). As a consequence, any binding a t all of the B(OH)4- ion, whose concentration is orders of magnitude higher than that of OHf, will exert a significant influence on the amount of OH- bound to the CTAB micelle, especially a t low CD.

If borate binding is highly unfavorable and the buffer equilibrium not significantly perturbed, we may approx- imate [BT] as eq 13, where [BH,] is the total concentration [BT] = [Bf] = [BHT]I1 -+ antilog (pK, -pH)I-' (13) of buffer species present and the value of the pKa of boric acid (9.2229) was taken to be constant. When eq 3 and 8-13 and the parameter values above (with CY = 0.2) were used, simulations were performed varying only kzm and KOHIF, with the additional restriction that KOH B be the same in all cases. The best-fit values of k2m/k{ = 0.37 for NPA

364

a t pH 9.50, k2,/k: = 0.29 for NPO at pH 9.50 and 9-80, and KOHIB = 2.3 gave excellent simulations for all three systems over the entire CTAB concentration range. The resultant value of KB,Br N 0.035 is quite satisfying, cor- responding to very inefficient binding of the borate ion to the CTAB micelle as required by the considerations dis- cussed aboveSz7

It should be noted that the values of kzm/k,O derived from our analysis have no absolute significance to the extent that they depend on the value chosen for P. Nonetheless, it is unlikely that the value of P is much higher than that utilized in this work. Thus, our values of k2 , /k2 are consistent with other indications in the literaturem of an apparent reduced reactivity of OH- in the micellar phase of CTAB. In this regard, we emphasize, however, that even an absolute value of kzm (unlike kzo) will reflect a t best only an apparent reactivity in the micellar phase. Thus, kz , is necessarily a composite of the true reactivity in the volume element related to 9, within which the ionic nucleophile may be said to be “bound”, times the probability of encountering the mi- celle-incorporated substrate within this same volume ele- ment. More specifically, if the micellar reaction zone were to encompass a volume element which corresponded es- sentially only to the Stern layer, i.e., the region where the highest local concentration of “bound” exchangeable OH- ions should be encountered, then the value of k2, should reflect the true reactivity attenuated by the intramicellar distribution of the incorporated NPA or NPO between nonreactive (e.g., core) and reactive (e.g., Stern layer) zones. Since NPO is certainly more liposoluble than NPA, the model recently proposed by Almgren et would suggest a difference in the intramicellar distribution of these two substrates in the direction favoring the lower apparent reactivity of NPO with OH- in the present case (Le,, kzm-

General Remarks Regarding “Buffering” in Micellar Solutions. One of the fundamental problems is quanti- tative analysis of reactivity in buffered micellar systems has been the proper manner in which to calculate the total concentration of a “buffered” reactive species at any given surfactant con~ent ra t ion .~ ,~~ There now exist experimental methods for evaluating the selectivity constant for H+zaic and OH-3b ion exchange with the micellar counterion in the absence of buffer. The equations utilized in this paper, coupled with the demonstration that a buffer can maintain’ [OH,] constant, permit one, at least formally,38 to calculate the quantitative behavior of [mb] (or by analogy [Rb]) as a function of CD from a knowledge of the selectivity coefficient determined in the absence of buffer.

Nonetheless, it should be emphasized that the constancy of [OH,] in the system studied in the present work will not necessarily be true for all presumably buffered micellar systems. Several factors may contribute, sometimes si- multaneously, to variations in [OH,], even in the presence of a buffer system, as the detergent concentration is in- creased. For example, a t high pH values, the extent of binding of OH- to the micellar phase of a cationic detergent such as CTAB becomes appreciable in relation to the buffer capacity. Indeed, ignoring binding of the buffer species, one can estimate that roughly M OH- will be bound to the micellar phase of CTAB at pH 11.5 in the limit of high CTAB.32 Thus, even for an “ideal” buffer, the buffer capacity could be exceeded at this or higher pH values unless one used a relatively high concentration of a buffer system near the pKa of the acidic component and measurements were restricted to the low CD concentration range. In addition, ion exchange involving the buffer

The Journal of Physical Chemistty, Vol. 84, No. 4, 1980

(NPO)/kz, (NPA) N 0.4).

Quina et al.

species potentially presents rather severe complications in terms of the effect of such exchange on both the buffer capacity and the mb dilution profile. These two effects may depend not only on the chemical nature but also on the relative proportions and total concentrations of the buffer species present.33

From a conceptual point of view, especially with regard to quantitative analysis, perhaps the most appropriate buffer systems for micellar solutions would be those whose buffer species are highly hydrophilic and have the same charge as the surfactant monomer and whose counterions are identical with those of the detergent employed. Under these conditions, the influence of added buffer is poten- tially analyzable as an added common salt effect.34

Acknowledgment. This work was supported by grants from FAPESP (77/028p to F.H.Q. and 76/0401 to H.C.), the CNPq (7186/75 to H.C.), and the PNUD/UNESCO (RLA 78/024 to F.H.Q. and H.C.).

References and Notes (1) (a) Part 3: J. B. S. Bonilha, H. Chaimovich, V. G. Toscano, and F.

H. Quina, J. Pbys. Cbem., 83, 2463 (1979). (b) F.H.Q. is a senior research fellow of the Conselho Nacional de Desenvolvimento Cienthco e TecnoEgiio (CNPq). M.J.P., I.M.C., and E.B. are gaduate feiiows of the FundacHo de Amparo I Pesquisa do Estado de SHo Pauio (FAPESP), and S.M.M-F. is a graduate fellow of the CNPq.

(2) (a) C. A. Bunton and 8. Wolfe, J. Am. Chem. Soc., 96, 7747 (1974); (b) C. A. Bunton, F. Rivera, and L. Sepuiveda, J . Org. Chem., 43, 1166 (1976); (c) C. A. Bunton, L. S. Romsted, and G. Saveili, J. Am. Cbem. Soc., 101, 1253 (1979).

(3) (a) F. H. Quina and H. Chaimvlch. J. phvs. Chern.. 83. 1844 (1979k . I . I

(b) H. Chalmovich. M. J. Politi, J. B. S. &nilha, and F . H. Qui&, !bid:; 83 1951 119791. - _ , - - \ - - ,

(4) See, for example, E. H. Cordes, Pure Appl. Chem., 50, 617 (1978). (5) The perturbation of buffer equilibria by CTAB has been clearly

(6) C. A. Bunton and M. J. Minch, J. Phys. Chem., 78, 1490 (1974). (7) E. M. Kosower and J. W. Patton, Tetrahedron, 22, 2081 (1966). (8) 0. Schales and S. S. Schales, J. Bo/. Chem., 140, 875 (1941). (9) M. Politi, I. M. Cuccovia, H. Chaimovlch, M. L. C. de Almeida, J. B.

S. Boniiha, and F. H. Quina, Tetrahedron Lett., 115 (1978). 10) I. M. Cuccovia, E. H. Schroter, P. M. Montelro, and H. Chaimovlch,

J. Org. Chem., 43, 2248 (1978). 1 1 ) J. N. Demas and J. W. Addington, J. Am. Cbem. Soc., 98, 5600

(1976). 12) F. H. Quina, Tese de L i v r W n c i a , Institute de Quimica, Universklade

de SHo Paulo, SHo Pauio, Brazil, 1977. 13) S. Sparlin, W. Hinze, and D. W. Armstrong, Anal. Lett., 10,997 (1977). 14) H. Chaimvich, F. H. Quina, R. B. Abakerli, S. Schreier, J. R. Ernandes,

and M. J. Politi, “Abstracts of Papers”, 175th National Meeting of the Amerlcan Chemical Society, Los Angeles, CA, 1978.

15) At detergent concentrations of the order of 0.1 M, there is a slight increase in Ksy even in the presence of 0.1 M KBr, due principally to an increase in the intermicellar ionic strength w b increasing CTAB. The incorporation into the micellar phase of long-chain analogues of CMCP, such as the Ndodecyl-4-cyanopyrMinlum Ion, Increases markedly with increasing KBr and CTAB, resulting in a significant decrease in Ksv with increasing CTAB or KBr.“

16) The association constant of NPO was calculated to be ca. 1.5 X IO4 M’ by exVapolath of literature for the lower homologues (NPA through p-nitrophenyl heptanoate).

17) C. Gitler and A. Ochoa-Solano, J. Am. Cbem. Soc., 90,5004 (1968). 16) A. K. Yatsimirski, K. Martinek, and I. V. Berezin, Tetrahedron, 27,

2855 (1971). 19) The study of the effect of detergents on the alkaline hydrolysls of

NPA is perhaps one of the most widely investigated systems in the fieid of micellar Notwithstanding the popularity of this system, quantitative analysis of the effect of CTAB on this reaction has only been recently attemptd2’

20) (a) L. R. Romsted and E. H. Cordes, J. Am. Cbem. SOC., 90, 4404 (1968); (b) M. T. A. Behme, J. G. Fullingtm, R. Noel, and E. H. Cordes, ibid,, 87, 266 (1965); (c) G. Meyer, Tetrahedron Left., 4581 (1972); (d) M. Chevion, J. Katzhendler, and S. Sarel, Isr. J. Cbem., 10, 975

(21) N. Funasaki, J. Pbys. Cbem., 83, 237 (1979). (22) The PIA product ratio is constant (0.05 f 0.01) for 4-MCP at pH

9.8 over the entire CTAB concentration range (0-0.1 M). The PIA product ratio for 2-MCP at pH 9.5 is constant (2.4 f 0.2) from 0 to 0.04 M CTAB. Above 0.04 M CTAB, the PIA ratio for 2-MCP increases gradually up to a value of about 4.4 at 0.1 M CTAB. In view of the results for CMCP and the CTAB independence of k@CP) for 2-MCP, we suspect that this increase in PIA at high CTAB may reflect a micehenhanced pyr i ine formations invoking entry of the

demonstrated for carboxylate-carboxylic acid buffers.’

(1972).

J. Phys. Chem. 1980, 84, 365-369 365

neutral cyanohydrin intermediate into the micellar phase, following ratedeterminina' OH- attack, rather than a variation of OH, itself.

(23) J. M. Corkill, J. p. Goodman, and T. Walker, Trans. Faraday SOC.,

(24) L. R. Romsled, PhD. Thesis, Indiana University, Bloomington, IN, 63, 768 (1967).

1975. (25) N. Funasaki, J . Colloid Interface Sci., 64, 461 (1978). (26) D. H. Smith, J . Colloid Interface Sci., 68, 70 (1979). (27) Our experimental values of the cmc of CTAB in water (9.4 X lo-'

M) and 0.020 M borate buffer pH 10.10 (5.0 X lO-'M) may be used to estimate K, on the basis of a charpd-phase separation treatment of micellization.28 Thus, for the equilibrium:

mm, + m(1 - a' - P)[Br,'] + mP[B,] F=

micelle (separate pseudophase)

it can be readily shown that

log m , = C , - (1 - a') log [Br,'] -I- P(log [Br;] - log [B,])

where C, is nominally equal to -(log K,)IN. At the cmc in the presence of borate buffer, m , cmc; thus log cmc = C, -(1 - a') log cmc + &log cmc - log [B,]) and

[BrF]

Since the literature values" of the constants C, = -4.84 and (1 - a') = 0.60 agree with our cmc in pure water and [By] can be calculated from eq 13, these equations permit one to estimate that P 0.28 and KBI, 0.025 at the cmc. Obviously, the above equations are at best approximate since C , will certainly vary as 6 N (1 - a'); nonetheless, they cleariy indicate that an unfavorable selectivity coefficient for the added ionic species implies a small effect on the cmc and vice versa.

(28) T. Sasaki, M. Hattori, J. Sasaki, and K. Nukina, Bull. Chem. SOC. Jpn., 48, 1397 (1975), and references cited therein.

(29) I. H. Kolthoff, "Treatise on Analytical Chemistry", Interscience, New York, 1959.

(30) C. A. Bunton, N. Camsco, S. K. Huang, C. H. Paik, and L. S. Romsted, J. Am. Chem. Sm., 100, 5420 (1978), and references cited therein.

(31) M. Almgren, F. Grieser, and J. K. Thomas, J. Am. Chem. Soc., 101, 279 (1979).

(32) For an "ideal" buffer, the limit of [OH,], the analytical concentration of "bound" hydroxide ion at "infinite" CT, is given bya

KOH/Br(l- a ) c lsLoHb] = a [OH,]

Thus, with K,,, = 0.08 and a = 0.2, an intermicellar pH of 11.5 ([OH, = 3.2 X M at 25 OC) corresponds to a limiting value of [OH, 1' = 1.0 X M, assuming that the buffer system is capable of maintaining the intermicellar pH at 11.5. I n this regard, it is interesting to note that the CTAB micelle is in certain respects a weakly acidic species; however, unlike a conventional weak acid, its "acidky" (binding of OH') varies with the pH and the ionic content of the intermicellar aqueous phase.

(33) Evidence for possible additional kinetic complications arising from the interaction of bnlc buffer species with the micellar phase of CTAB has been reported by Funasaki."

(34) An interesting case in point is SDS in the presence of sodium phosphate buffer at alkallne pH, where the influence of added buffer is treatable as an added commn salt effect (M. PolRi, I. M. Cuccovla, F. H. Quina, and H. Chaimovich, unpublished results). While this manuscript was in preparation, Funasaki*' reported that, among several systems, a buffer system compatible with our criteria [2- amim2methyC1,3-propanedii (AmptHBr] appeared to be the nwst suitable for studies of alkaline hydrolysis In CTAB. In fact, the data of Funasaki2' for the alkaline hydrolysis of NPA in the presence of 0.01 M Amp-HBr buffer are quite nicely reproduced by eq 3-7 (including the additional terms for the added common salt In eq 5 and 6") using the same parameter values empbyed In our calculation (Le., Ks = 54 W', k,Ik: = 0.37, a = 0.20, V = 0.37 Llmol, and KOHp = 0.08) and his value of k:[OH,] = 0.049 min-' at pH 9.59 (25 C).

Miscibility of Fluorocarbon and Hydrocarbon Surfactants in Micelles and Liquid Mixtures. Basic Studies of Oil Repellent and Fire Extinguishing Agents

KBzO Shinoda" and Toshio Nomure

Department of Applied Chemistry, Faculty of Engineering, Yokohama National University, Hodogaya-ku, Yokohama 240, Japan (Recelved May 16, 1979; Revised Manuscript Received October 11, 1979)

Publication costs assisted by Yokohama National University

The mutual solubility of C,F2,+lCOOH (n = 7-12) with C,H2,+lCOOH (rn = 7-17) and of C,F2,+1CH2CH20H (n = 8-10) with C,Hh+lOH (rn = 11-18) has been studied. From the solubility and critical solution temperatures, it is concluded that a carbon chain longer than 8 carbons is necessary to cause the phase separation in liquid-liquid mixtures of these fluorocarbon and hydrocarbon surface-active substances. Theoretical equations for the partial and total critical micelle concentrations (cmc) of surfactant mixtures have been derived in the case when the energy of mixing between surfactants has to be taken into account. Miscibilities of fluorocarbon and hydrocarbon surfactants at the interface or in micelles have been examined by the comparison of the theoretical and the cmc values of surfactant mixtures. It is concluded that C,FI5COONa will mix with CloH2,S04Na in all com- positions but that C8F17COO"4 will only partially mix with C12H25S04NH4 at 25 "C. The importance of the immiscibility of fluorocarbon and hydrocarbon surfactants for oil repellent and fire extinguishing properties is discussed.

Introduction The critical solution temperature of heptane and per-

fluoroheptane is 50 "C, above which two liquids mix over all compositions.' If one end of the respective molecules is converted into the same group, such as COOH or OH, both components will readily mix a t 25 "C because the enthalpy of mixing is considerably depressed. Actually, a 1:3 volume mixture of C7F15COOH and C7H15COOH and an equivolume mixture of C8FI7COOH and CllH2J!OOH

0022-3654l8Ol2084-0365$0 1 .OO/O

are one liquid phase above 19 and 47 OC, respectively, below which crystals of respective perfluoroalkanoic acids precipitate. This direct observation implies the miscibility of two surfactants, C7F15COONa and C7H15COONa, in micelles, because a micelle is a pseudophase in a liquid state. When the chain lengths of both surfactants are sufficiently long, however, two types of micelles, one rich in fluorocarbon surfactant and the other rich in hydro- carbon surfactant, may be formed. If this happens, two

0 1980 American Chemical Society