1868
J. Phys. Chem. 1980, 84, 1868-1869
NlzL2-
+ c i
5-
$ N I L ( C N I + NI2+(aqi
NiL(CNI6- + CN-
& NiL(CN)2s-
NiL(CN)26-+ CNNiL(CN)?-
k
(5)
(fast)
NiL(CN)?-
References and Notes (fast)
(6)
(rds)
(7)
k-3
+ CN- 2Ni(CN)42-+ L6-
(fast) (8)
The important step in the scheme is the cyanide-assisted dissociation of Ni2L to give NiL(CNI5- and Ni2+(aq)both of which react with cyanide to form Ni(CN)42-. The last three steps are in line with the reactions of other mono(aminocarboxylato)nickel(II)complexes investigated earlierS7-l3The second-order dependence in cyanide at higher concentration is interpreted to mean that NiL(CN) reacts with two cyanides to produce the intermediate NiL(CN)3 in the rate-determining step rather than the formation of Ni2L(CN)2as assumed by Stara and Kopanica.15 The mechanism suggested above is supported by the following additional facts: (I) Addition of dimethylglyoxime to a solution of NizL in the presence of a low concentration of cyanide gives the characteristic precipitate of the Ni(dmg), chelate. No such change is observed in absence of CN- even on long standing. This shows the presence of Ni2+(aq)as a consequence of dissociation according to eq 5. (2) Addition of CN- to about a tenfold excess of NizL gives the same product as is obtained by similar addition of CN- to NiL. This product is NiL(CN) and its stoichiometry and stability constants were established by the mole ratio method.12 (3) A large absorbance change is observed after mixing the reactants (Le., Ni2L and cyanide) as a result of formation of Ni(CN)t- not from displacement of TTHA from Ni2TTHA but from cyanide-assisted dissociation of NizTTHA according to eq 5 and rapid formation of Ni(CN),2- from Ni2+(aq). Thereafter, the reaction follows steps 6-8. The observed absorbance jump equals that expected from Beer's law and stoichiometric conversion according to Ni2L4-n+ 5CN-
-
dissociation of Ni,TTHA to NiTTHA and Ni2+(aq)followed by subsequent steps. (7) Preliminary investigations on the reaction of Ni2DTPA17 (where DTPA = diethylenetriaminepentaacetic acid) with cyanide point to similar conclusions as arrived at for the Ni2TTHA reaction.
NiL(CN)5- + Ni(CN)42-
(9)
(4)The rather small activation energy (E, = 6.3 kcal mol-l, determined from a temperature dependence study) of the foward reaction where cyanide dependence is second order shows an associative mechanism to be operative in which bond breaking and bond making are taking place simultaneously. This value is comparable to that of other mono(aminocarboxylato)nickel(II) reactions with CN- investigated earlier.l*12 Compared to this, the activation energy for dissociation according to eq 4 is 14.82 kcal mol-l. ( 5 ) Kinetic investigations on the reactions of two bis complexes, viz., Ni(IDA)2and Ni(MIDA)2 (where IDA is iminodiacetic acid and MIDA is N-methyliminodiacetic acid), with cyanide8 showed that the bis complexes must first dissociate to give mixed complexes of the type NiL(CN) which react further with excess cyanide to produce Ni (CN)42-. (6) In their study of the substitution reaction (eq lo),
(1) Pearson, R. G.; Muir, M. M. J. Am. Chem. SOC.1968, 88, 2163. (2) Muir, M. M.; Cancio, E. M. Inorg. Chim. Acta 1970, 4, 565. (3) Muir, M. M.; Cancio, E. M. Inorg. Cbim. Acta 1970, 4, 568. (4) McMane, D. G.; Martin, Jr., D. S. Inorg. Cbem. 1966, 7, 1169. (5) Teggins, J. E.; Gano, D. R.; Tucker, M. A.; Martin, Jr., D. S. Inorg. Cbem. 1967, 6, 69. (6) Teggins, J. E.; Martin, Jr., D. S. Inorg. Cbem. 1967, 6 , 1003. (7) Margerum, D. W.; Bydaiek, T. J.; Bishop, J. J. J. Am. Cbem. SOC. 1961, 83, 1761. (8) Coombs, L. C.; Margerum, D. W. Inorg. Cbem. 1970, 9, 1711. (9) Coombs, L. C.; Margerum, D. W.; Nigam, P.C. Inorg. Cbem. 1970 9,2081. (10) Kumar, K.; Nigam, P. C.; Pandey, G. S. J. Phys. Chem. 1978, 82, 1955. (1 1) Kumar, K.;Nigam, P. C. J. Pbys. Cbem. 1979, 83, 2090, (12) Kumar, K.; Nigam, P. C. J. Pbys. Cbem. 1980, 84, 140. (13) Pagenkopf, G. K. J. Coord. Cbem. 1972, 2, 129. (14) Kumar, K.; Nlgam, P.C. J. Coord. Cbem. 1979, 9, 139. (15) Stara, V.; Kopanica, M. Collect. Czech. Cbem. Commun. 1972, 37, 2882. (16) Stara, V.; Kopanica, M. Collect. Czech. Cbem. Commun. 1972, 37, 80. (17) Kumar, K.; Bajaj, H. C.; Nigam, P. C. J. Phys. Chem. Submitted for publication. Department of Chemistry Indian Institute of Technology Kanpur-2080 16, U.P., India
Received August 15, 1979: Revlsed Manuscript Received Marcb 7, 1980
Volume Increase on Comlcellization of Fluorocarbon and Hydrocarbon Surfactants as Evidence for the Mutual Phobicity in Comicelles
Sir: Two or more surfactants spontaneously form mixed micelles above the critical micellization concentration (cmc). This phenomenon may be important in relation to the formation of biological membranes, which are composed mainly of lipids and proteins.' Micelles have different kinds of properties: some can be explained best in terms of aggregation whereas others can be explained best in terms of a micellar phase. A recent theory2 which deals with a micelle as a small system3 may be the most rigorous.1 Fluorocarbons are oleophobic as well as hydrophobic, as exhibited by properties such as vapor pressure, solubility, heat of mixing, and volume ~ h a n g e . This ~ ! ~ property of bulk fluorocarbons might be observed in small systems such as micelles of fluorocarbon surfactants; mixed micelles of fluorocarbon and hydrocarbon surfactants have a limited mutual solubility and higher cmc values than those expected from ideal m i ~ i n g . ~ ! ~ In this work, we report volume expansion on comicellization of fluorocarbon and hydrocarbon surfactants as direct evidence for mutual phobicity in micelles. The chemical structure of Neos Ftergent (NF) used is CCF3)2CF,
0022-3654/80/2084-1868$01 .OO/O
,CF3
(cF3 ) 2 c ~ =c c \ o ~ 3N~a o
NizTTHA + 2Cu2++ CuzTTHA + 2Ni2+ (10) Stara and KopanicalG have themselves postulated the
K. Kumar P. C. Nlgam"
The apparent molal volume 4 of a mixture of N F and 0 1980 American
Chemical Society
The Journal of Physical Chemistry, Vol. 84,
Communications to the Editor
TABLE I: Partial M o l a l V o l u m e s of M i x e d NF a n d STS Micelles in a 110 mmol/L S o d i u m C h l o r i d e S o l u t i o n a t 30 a C x,STS
cmc, mmol/L
3tbSTS
0
0
0.065 0.113 0.470
0.113 19.113 0.113
1
'1
0.555 0.617 0.617 0.617 0.619
mL/mol
__
obsd
calcda
341.6 f 0.4 340.8 i 0.4 338.0 f 0.4
318.0
k
0.5
337.8 335.0 314.2
283.4 t 0.3
a Calculated from the a d d i t i v i t y of t h e m o l a l volumes of p u r e NF a n d STS micelles.
I
3
E
5 I>
320d
1 300
5 m(mmol/K.g)
0
10
Flgure 1. Partial molal volumes of mixed NF and STS plotted against the total molality at different mole fractions: (a) xrnSTS = 0.065: (b) xSTS = 0.1 13; (C:I xrnSTS = 0.470; (d) xSTS =: 0.470.
sodium tetradecyl sulfate (STS) W ~ Lmeasured B in a 10 mmol/L sodium chloride solution at 30.00 f 0.01 "C8The partial molal volume u of this mixture of NF and STS was calculated by wing the expression fl =
4
.+. m(d4/dm)T,p,xm
(1)
When determining the mean molal volume of a mixed micelle of two surfactants, one must measure the densities of a series of solutions in which the micellar composition x , is kept constant but not the overall composition x . In general, the x , and xb values depend on the total surfactant concentration as well as on the x ~alue.~JO From surface tension measurement^.,^ the relation of the micellar composition to the concentrations of monomeric N F and STS hais been determined for the NF-STS system in a 10 mmol/L sodium chloride solution a t 30 "C. At xrnSTS < 0.065 and xrnSTS > 0.470, NF-rich and STS-rich micelles are present, respectively. A t 0.065 < Z,STS < 0.470, two kinds of mixed micelles with compositions of xS , TS = 0.065 and 0.470 coexist in a solution in which cnic = 0.617 mmol/L and XbSTS = 0.113, as shown in Table I. In Figure 1,the partial molal volumes of mixtures of NF and STS are plotted against the total molality of NF and STS. In curvezi b and d, the overall molar ratio is fixed whereas in curves a and c, the micellar composition is kept constant. The total concentration for each mixture is changed from a considerably higher concentration than the cmc (see Table I) to the solubility limit. In curve d, asl the total concentration is increased, the micellar composition and the monomeic concentrations of N F and STS are altered sim~ltaneously;~ x,STS decreases from 0.508 to 0.485, C b N F increases from 0.513 to 0.525 mmol/L, and ChsTs decreases from 0.304 to 0.186 mmol/L. Therefore, the partial molal volume depends on the total surfactant concentration. In curve b, x,STS = XbSTS = x = 0.113 and cmc = 0.617 mmol/L, regardless of the total surfactant concentration. In curves a and c, xrnsTS= 0.065
No.
14, 1980 1869
and 0.470, respectively, cmc = 0.617 mmol/L, and XbSTS = 0,113. Thus, from curves a, b, and c, we can determine the partial molal volume of micelles for each x, value, as shown in Table I. As Table I shows, the partial molal volumes of mixed NF and STS micelles are higher than those expected from the additivity of the molal volumes of pure N F and STS micelles. That is, when the micelles of pure NF and STS mix, the volume of the comicelles formed increases. This result directly shows the mutual phobicity between fluorocarbon and hydrocarbon chains in small systems such as comicelles. Similar increases in volume were reported in bulk fluorocarbon and hydrocarbon mixture^.^?^ These results agree with the conclusions based on surface tension measurement^.^ That is, comicelles at x,~w = 0.065 and 0.470 are in equilibrium with a solution in which cmc = 0.617 mmol/L and XbSm = 0.113. Thus, these two comicelles can coexist as different species in the same solution. As described above, micelles with a composition of xdm = 0.113 are composed of a mixture of two comicelles with the compositions xrnSTS = 0.065 and 0.470. The partial = 0.113, calculated molal volume of the micelles at xS, TS from the additivity of volumes of these two comicelles, is 338.1 mL/mol,ll and this value agrees well with the observed one 338.0 mL/mol. The molal volume of mixed NF and STS micelles increased upon comicellization. This behavior is similar to the volume increase observed when bulk fluorocarbons and hydrocarbons are mixeda4p5This finding demonstrates the existence of the mutual phobicity between fluorocarbons and hydrocarbon chains in small systems such as micelles. Furthermore, the present density measurements support the conclusion, derived by use of surface tension meas u r e m e n t ~that , ~ two kinds of mixed micelles with the compositions xdTs = 0.065 and 0.470 coexist in a solution. Acknowledgment. Thanks are due to Dr. T. Mizuno of Neos Co. for the gift of samples of NF. We appreciate the reviewers for helpful comments and Dr. Ramachandran for his help with the manuscript. References and Notes (1) Tanford, C. "The Hydrophobic Effect: Formation of Micelles and Biological Membranes"; Wiley: New York, 1973. Tanford, C. Science 1978, 200, 1012. (2) Hall, D. G.; Pethica, 6.A. In "Nonionic Surfactants"; Schick, M. J., Ed.; Marcel Dekker: New York, 1967; p 516. (3) Hill, T. L. "Thermodynamics of Small Systems", Vol. 1, 2; Benjamin: New York, 1963, 1964. (4) Hildebrand, J. H.; Prausnitz, J. M.; Scott, R. L. "Regular and Related Solutions"; Van Norstrand: New York, 1970; Chapters 2, 10. (5) Hildebrand, J. H.; Dymond, J. J. Chem. Phys. 1967, 46, 624. (6) Mukerjee, P.; Yang, A. Y. S. J. Phys. Chem. 1976, 80, 1388. Mukerjee, P.; Mysels, K. J. ACS Symp. Ser. 1975, No. 9, 239. Funasaki, N.; Hada, S. Chem. Lett. 1979, 717. (7) Funasaki, N.; Hada, S. J. Phys. Chem. 1980, 84, 736. (8) The density dof all solutions was measured wlth a 20-mL Ostwald pycnometer and 4 was calculated from = 1000(do - d)/mdod, where dois the density of a 10 mmol/L sodium chloride solution and rn is the total molality of NF and STS. (9) Funasaki, N.; Hada, S. J. Phys. Chem. 1979, 83, 2471. (10) Nomenclature: C = molarity of surfactant; C,, = total NF concentration; CsTs= total STS concentration; C,= CF Csrs; xSTs =C /,;, CBTS= concentration of STS in monomeric form (simlhrty for NF); XbSTS = cbSTS/(cbtiF + CbSTS); XmSTS = ( C t X S T S cmc)x,,,,)/(C, - cmc). (11) [(0.113 - 0.065) X 318.0 (0.470 - 0.113) X 340.8]/(0.470 - 0.065) = 338.1.
+
+
Kyoto College of Pharmacy Yamashina-ku, Kyoto 607 Japan
Received February 26, 1980
Norlaki Funasakl" Sakae Hada
