J. Phys. Chem. 1980, 84, 2605-2607
and equals ca. -0.5 kcal; i.e., it is very close to that value found from the more general eq 10. It seems, however, that our results indicate the considerable differences in the value of Po going from oxides through sulfides to selenides. This sequence seems to be understandable, as the resonance integral should change in the same direction. Finally, it has to be emphasized that the value of Wois very low as compared with that found for analogous complexes with nitrogen bases.1° Presumably it is connected with a large contribution of the polarization effect to the stabilization of the complex which compensates almost completely for a considerable repulsion during shortening of the X-.I distance.
h \
3Q ‘2
2
9
2605
IO-
8 -
6 -
4 -
Acknowledgment. The authors are indebted to Dr. Therese Zeegers-Huyskens for helpful discussion. References and Notes
Figure 3. The plot of -AHagainst [(pN- p,,)I(p, - k ~ ~ ) ]for ” ~oxides (0),sulfides 0,and selenides A;some data for sulfides taken additionally
from ref
9.
Only after considerable simplifications may one attain simple correlations such as those proposed by Guryanova or Ratajczak and Orville-Thomas. However, any extrapolation down to Aii = 0 may be valid for weak complexes. Consequently the assumption that the overlapping integral is small seemri to be justified in such cases, and eq 10 turns into the form
This equation, similar to that proposed by Ratajczak and Orville-Thomas, we used to compare qualitatively the behavior of oxides, sulfides, and selenides. The results shown in Figure 3 indicate that for all donors considered W, extrapolated down to Aii = 0 is approximately the same
(1) G. Briegleb, “Elektron-Donor-Acceptor Komplexe”, Springer-Verlag, Berlin, 1961. (2) R. S. Mulliken, W. B. Person, “Molecular Complexes”, Wiley, New York, 1969. (3) J. Yarwood, Ed., ”Spectroscopy and Structure of Molecular Complexes”, Clerum, London and New York, 1973. (4) R. S. Mulllken, J . Am. Chem. SOC.,74, 811 (1952). (5) I. G. Arzamanova and E. N. Guryanova, Dokl. Akad. Nauk SSSR, 157, 375 (1964). (6) I. G. Arzamanova and E. N. Guryanova, Zh. Obshch. Khim., 36, 1157 (1966). (7) R. K. Chan and S. C. Laio, Can. J . Chem., 48, 299 (1970). (8) A. F. Foubert and P. L. Huyskens, Can. J . Chem., 54, 610 (1976). (9) E. N. Guryanova, I. P. Goldstein, and J. P. Romm, “Donorno-Akceptornaya Svyaz, Khimya”, Moskva, 1973. (10) H. Ratajczakand W. J.Orville-Thomas, J. Mol. Stnrct., 14, 149(1972). (11) M. S. Sambhl and S. K. Khoo, J . Phys. Chem., 79, 666 (1975). (12) J. P. Hawranek and L. Sobczyk, Acta Phys. Poi. A , 39, 651 (1971). (13) A. Benes1 and J. Hildebrand, J . Am. Chem. Soc., 71, 2705 (1945). (14) F, Lux, R. Paetzdd,J. Danel,and L. Sobczyk, J. Chem. Soc., Faraday Trans. 7, 71, 1610 (1975). (15) L. Sobczyk and J. Danel, J . Chem. SOC.,Faraday Trans. 1 , 68, 1544 (1972). (16) J. W. Smith, “Electric Dipole Moments”, Butterworth, London, 1955. (17) J. Applequist, J. C. Carl, and Kwok-Keung Fung, J. Am. Chem. Soc., 94, 2952 (1972). (18) C. K. Prout and B. Kamenar in “Molecular Complexes”, R. Foster, Ed., P. Elek, London, 1973. (19) C. Dorval and Th. Zeegers-Huyskens, Ann. Chem., 10, 5 (1975). (20) A. E. Pekary, J . Phys. Chem., 78, 1744 (1974).
Partition Coefficient of Naphthalene between Water and Cetyltrimethylammonium Bromide Micelles Elsa Abuln and Eduardo Llssl’ Departamento de Qdmica, Facultad de Ciencia, UniversMad T6cnica del Estado, Santiago, Chi/e (Received: Juiy 30, 1979; In Final Form: May 27, 1980)
Ni(I1) salts were found to be effective quenchers of naphthalene fluorescence in water solution. Employing Ni(II)S04as a selective quencher of the naphthalene present in the water solution, it is possible to determine the partition of naphtlhalene between water and cetyltrimethlammoniumbromide micelles. Contrary to previous determinations, it is found that the micelles dissolve nearly three times more naphthalene than expected from its solubility in normal hydrocarbons. Introduction Fluorescence measurements are extensively employed to study the properties of micellar systems. In particular, the fluorescent behavior of a probe (canbe used to determine its partition between the micellar and aqueous phases. This method has been employed in three recent publications ito measure the partition of naphthalene between water and CTAB (cetyltrimet hylammonium brom0022-36514/8012084-2605$01.0010
ide) mi~elles.l-~The resul s obtained by using a timecorrelated fluorescence techn que1v2and those obtained by measuring the fluorescence intensity as a function of CTAB concentration were quite similar3and indicated that the partition of the probe between water and the micellar phase was nearly identical with that predicted from its partition between water and n-hexane.lV2 This result implies that, regarding the solubility of naphthalene, the
1
0 1980 American Chemical Society
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The Journal of Physical Chemistry, Vol. 84, No. 20, 1980
micelle can be considered as a normal hydrocarbon solvent. In the present work we have applied a different experimental technique to determine the probe solubility in the micelle, and the results obtained are in disagreement with the above conclusion. Experimental Section Fluorescence measurements were carried out in a 204-S Hitachi-Perkin-Elmer spectrofluorometer with an excitation wavelength beam of 2700 A. Quenching experiments of naphthalene fluorescence by Ni(I1) salts were carried out by employing Ni(II)S04or Ni(II)C12. Addition of ZnS04 to maintain the sulfate concentration constant at 0.1 M in the experiments using Ni(II)S04 as quencher did not appreciably modify the results. The partition of naphthalene between water and nheptane was determined by measuring the amount taken up by the water phase when a measured amount of water was shaken with a M solution of naphthalene in nheptane. The extraction was carried out with either water or a 0.1 M solution of ZII(SO)~.The relative amount of naphthalene extracted was determined from the fluorescence intensities of the solutions. The partition coefficients obtained were 2.5 X lo3between n-heptane and water and 3.0 X lo3 between n-heptane and the 0.1 M Zn(SO), solution. These results are similar to that reported by Hautala et al.' (2.5 X lo3 between n-hexane and water) and would indicate that the presence of the salt does not significantly modify the partition of naphthalene between water and n-heptane. Water-naphthalene-CTAB solutions M in CTAB) were prepared by adding a concentrated CTAB solution to a napthalenewater solution (naphthalene concentration M). Quenching of the naphthalene less than 2 X fluorescence by Ni(II)S04in the presence of a fixed concentration of CTAB was studied by using a CTAB/ naphthalene ratio larger than 100. The total concentration of salt was kept constant by addition of ZnS04. Ni(II)SO4-6H20(Merck, p.a.), Ni(II)Cl2.6H20(Merck, p.a.), Zn(S04).7H20(Baker analyzed), and n-heptane (Merck, p.a.) were employed without purification. CTAB (Merck) was purified by recry~tallization.~ Naphthalene (Hopkins & Williams, purified) was employed without purification. The absorption and fluorescence spectra of the naphthalene were identical with those reported in the literat~re.~?~ Results and Discussion The critical micelle concentration in the presence of 0.1 M sulfate was determined from the change in the fluorescence intensity of an aqueous solution of naphthalene as a function of added CTAB. The value obtained was, as e ~ p e c t e dconsiderably ,~ smaller than the 0.8 X M value reported in the absence of salts.* The amount of free CTAB can then be disregarded under the experimental conditions employed in the present work. The quenching of naphthalene fluorescence in water by Ni(I1) salts was found to be the same when Ni(II)C12or Ni(II)S04were employed. Furthermore, the results were not significantly modified by the addition of ZnS04. The data obtained gave a linear Stern-Volmer plot with a slope of 14 M-l. Since the lifetime of singlet naphthalene in aereated water solutions is 39 ns,l the value of the quenching rate constant can be estimated as 0.35 X lo9 M-' S-1.
Since the Ni(1I) ions will not closely approach the cationic micelle, it can be assumed that they will not affect the behavior of naphthalene inside the Se-
Abuin and Lissi
lective quenching of the emission arising from the water phase can then be employed to evaluate the probe dist r i b ~ t i o n .The ~ method used in the present work to carry out this determination was a modification of that given by Quina and Toscanog that does not require the assumption of equal absorptivity of the probe in both phase^.^ The change in emission intensity as a function of Ni(I1) concentration at a given CTAB concentration will be given by eq 1, where Io is the intensity at zero Ni(I1) (Io- O/Z0 = al4[Ni]/(1 + 14[Ni]) (1) concentration, I is the intensity at a given Ni(I1) concentration, a is the fraction of light emitted from the water phase at zero Ni(I1) concentration, and 14 is the value of kQ7 measured in M-l. Plotting the left-hand side of eq 1 against 14[Ni]/(1+ 14[Ni]), we found that, at CTAB M, 15 f 4% of the emission in the absence of Ni(I1) arises from the water phase. This value is nearly equal to that previously determined from fluorescence decay curve analysis.'V2 The fraction of probe in the water phase can be obtained by using eq 2, where Zwois the emission in% in the water phase = 100aZo/Zwo (2) tensity of equimolecular sample of naphthalene in water. Since Zo/Zwo was found to be 0.26 under our experimental conditions, it can be concluded that only -4% of the probe remains in the water phase. The amount of naphthalene in the water phase at low occupancy of the micelles has been estimated previously, and considerably larger values were 0btained.l" Algrem et aL3have estimated the amount of probe inside the micelles with the use of eq 3.
( I - Zwo)/(Zt - Zwo) = 1 + (K,,CTABm)-l
(3)
I , Zwo,and Zt are the relative emission intensity readings at 100% solubilization, without surfactant present, and at intermediate amounts of surfactant, respectively. CTAB, is the surfactant concentration in micellar form. Equation 3 requires that the fluorescence quantum yield inside the micelle and in the water phase be independent of the surfactant concentration. Nevertheless, the data of Hautala et al. indicate that the lifetime of the probe both in water and in the micelle decrease when the CTAB concentration increases.' This dependence, which can be related to quenching of the probe by bromide ions, invalidates the application of eq 3. Hautala et al.' and Van Bockstaele et ala2used eq 4 to determine the percentage of probe inside the micellar phase, where Z is the emission intensity from a given phase,
zmiccw7w6FW 6
3'
zwcmic7mi~6F~'~
70 in micellar phase =
(4)
is the extinction coefficient, 7 is the singlet lifetime, and
6 F is the fluorescence quantum yield. Equation 4, which was used to correct the results for differences in +F and 7 between the two environments,l seems to be wrong. The relative intensity from both phases is given by (5)
which rearranges to % in micellar phase =
[+ 1
zwemic6FmiC Imicewd'FW
I-'
(6)
Comparison of eq 4 and 6 shows that singlet lifetimes must not be included in the evaluation of the probe distribution. If eq 6 is applied instead of eq 4, the results of Van Bockstaele, et a1.2 indicate that, at M CTAB, only
J. Phys. Chem. 1980, 84, 2607-2611
about 5% of the probe remains in the water phase. This result, which is significantly smaller than the reported 1 5 7 0 , ~is similar to that obtained in the present work. The partition coefficient between a normal hydrocarbon and water has been determined by Hautala and Turro, who have reported a value of 2500.l We have obtained a similar value, and we have shown that addlition of 0.1 M ZnS04 only modifies this value in approximately 20%. The analysis carried out previously1i2indicates that nearly 15% of the probe would remain in the water phase at M CTAB if the micelle behavior were similar to a hydrocarbon solvent. Comparison of this value with those found in the present work shows that there exists an excess of probe incorporated to the micelle (by a factor of nearly 3) relative to that expected if only the dissolution in the core were considered. The partition between CTAB micelles and water have also been measured by the total solubilization meth~d.~JO The proportion of naphthalene remaining in the water phase was, at M CTAB, 7% (ref 3) and nearly 10% (ref lo), and the average number of probe molecules by micelle was estimated as 3 and 20.3J0 Under these conditions, the distribution constant can be different from that estimated at occupancies below 1,but the results obtained also indicate an excess of probe inside the micelle that can be attributed to adsorption on the ~urface.~ Algrem et ala3 have estimated that at least 20% of the naphthalene molecules sohbilized in the micelles are located at the surface. The value that can be estimated from our results, as well as from the reevaluation of the data of Van Bockstaele et al., would indicate that at low occupancies the proportion of probe in the surface must be considerably larger. This result is similar to that observed by Mukerjee and Cardinal when benzene is used iis probe” and can be
2007
explained in terms of a larger proportion of probe adsorbed in the surface at low occupancies. We can conclude then that under all of the conditions so far considered (low occupancies both with and without added electrolytes, and at the saturation point) the amount in the micelle is larger than that expected from its partition between normal hydrocarbons and water. The present results can then be explained in terms of the two-state model of solubilization and the interfacial activity of aromatic solutes at hydrocarbon-water interfaces.11J2
References and Notes R. R. Hautala, N. E. Schore, and N. J. Turro, J. Am. Chem. SOC., 05, 5508 (1973). M. Van Bockstaele, J. Gelan, H. Martens, J. Put, J. C. Deberen, N. Boens, and F. C. de Schrijver, Chem. fhys. Lett., 58, 211 (1978). M. Algrem, F. Grieser, and J. K. Thomas, J . Am. Chem. Soc., 101, 279 (1979). C. A. Bunton, F. Rambez, and L. SepGlveda, J . Org. Chem., 43, 166 (1978). R. A. Friedel and M. Orchin in “Ultraviolet Spectra in Aromatic Compounds”, Wiley, New York, 1973. The fluorescence spectra of naphthalene were similar In all of the solvents considered (ref 1) and identical with that reported in related systems (Le., in ref 3). Ch. Tamford, ”The Hydrophobic Effect: Formation of Micelles and Bblcgical Membranes”, Wiley-Interscience, New York, 1973, Chapter 7. E. J. Fendler and J. H. Fend&, Adv. phys. Org. Chem., 8, 271 (1970); P. Mukerjee, K. J. Mysels, “Crltical Micelle Concentration of Aqueous Surfactants Systems”, NSRDS-36, National Bureau of Standards, Washington, D.C., 1971. F. H. Quina and V. G. Toscano, J . fhys. Chem., 81, 1750 (1977). L. Sepilveda and R. Soto, Makromol. Chem., 179, 765 (1978). P. Mukerjee and J. R. Cardinal, J . fhys. Chem., 82, 1620 (1978). P. Mukerjee, J. R. Cardinal, and N. R. Dessai in “Micellization, Solubilization and Microemulsions”, Vol. 1, K. L. Mitall, Ed., Plenum Press, New York, 1977, p 241; J. C. Eriksson and G. Gillberg, Acfa Chem. Sand., 20, 2019 (1966); J. Ulmius, B. Llndman, G. Lindblom, and T. Drakenburg, J. Colloid Interface Sci., 85, 88 (1978).
Reaction of p-Nitrophenyl Diphenyl Phosphate in Cetyltrimethylammonium Fluoride. Apparent Failure of the Pseudophase Model for Kinetics Clifford A. Bunton,” Jane Frankson,’ and Laurence S. Romsted Department of Chemistry, 1Jniversity of California,Santa Barbara, California 93 106 (Received:February 25, 1980)
The reaction of F- with p-nitrophenyl diphenyl phosphate (pNPDPP) is very rapid in aqueous solutions of cetyltrimethylammoniumfluoride (CTAF). However, the results do not conform to the pseudophase ion-exchange model because the rate constant does not become constant when the substrate is fully micellar bound, but continues to increase with increasing [CTAF]or with addition of NaF. Added Br- as NaBr or CTABr inhibits reaction showing that Br- displaces F- from the micelle. Reasons for the apparent failure of the pseudophase model are considered.
Rate surfactant profiles for micellar-catalyzed bimolecular reactions of nonionic reactants in water can be explained unambiguously in terms of reactant distribution between aqueous and micellar pseudi~phases.~-~ The situation is more complex for ionic reactions where the distribution of ions between the two pseudophases generally cannot be measured directly. Micellar catalysis of ionic bimolecular reactions is almost always retarded by added salts which contribute counterions that compete with reactive ions for the micelle, and the inhibition increases with 0022-3654/80/2084-2607$0 1.OO/O
decreasing hydrophilicity of the added ion^.^^^ However, it has also been suggested that added ions reduce micellar catalysis by reducing the surface potential of the micelle.718 Some workers have treated the phenomena in terms of one or other of these effects; others have combined them in the treatment.2JB Micellar effects on acid-base equilibria have also been discussed in terms of these m o d e l ~ . ~ J ~ One method of exploring the different approaches is to eliminate the complications caused by a mixture of counterions by using reactive counterion surfactants whose 0 1980 American
Chemical Society
