J. Phys. Chem. 1980, 84,2281-2287
228 1
M. R. Tavares, MSc. Dissertation, Instituto de Flsica, Universidade de Sao Paulo, 1978. D. M. Chen, L. W. Reeves, A. S. Tracey, and M. M. Tracey, J. Am. Chem. Soc., 96, 5349 (1974). L. W. Reeves and A. S. Tracey, J. Am. Chem. Soc., 97,5729(1975). D. M. Chen, F. Y. Fujiwara, and L. W. Reeves, Can. J. Chem., 55,
the first four carbons, 0.931 for the next three, and 1.09, 1.19, and 1.52 for the last three carbons of the 1-decanol chain. The ratio of quadrupole splittings between mesophases CsC12 and KC10 for the eight segments which can be compared (carbons 2-8 for dodecanoate and decanoate ions) emphasizes the direct effect of the longer chain in preserving higher degrees of order. The ratios are 0.98 for the segments 1-5,l.lO for segment 6, and 1.37 for segments 7 and 8. The ratio of the quadrupole splittings in the region of uniform oirder to the last CD2group in the chain is not greatly different for CsC12 (l.62) and KClO (1.54) in the carboxylate host amphiphile. Supplementary data of the quadrupole splittings measured for all mphiphiles may be obtained in tabular form by writing to the corresponding author.
2404 (1977). K. Radley, L. W. Reeves, and A. S. Tracey, J. Phys. Chem., 80,
174 (1976). D. M. Chen, F. Y. Fujiwara, and L. W. Reeves, Can. J. Chem., 55,
2396 (1977). P. G.de Gennes, “The Physics of Liquid Crystals”, Clarendon Press, Oxford, 1974. Ng Dinh-Nguyen, A. Raal, and E. Stenhagen, Chep. Scr., 2, 171
(1972). L. W. Reeves, A. S,Tracey, and M. M. Tracey, Can. J. Chem., 57,
747 (1979). F. Y. Fujiwara and L. W. Reeves, Can. J. Chem., 56,2178 (1978). J. Seelig and A. Seellg, Biochemistry, 13, 4839 (1974). F. Y. Fujiwara and L. W. Reeves, J. Am. Chem. SOC.,98, 6790
(1976).
Acknowledgment. This work has been made possible by operating grants made available to L.W.R. by the National Research Council of Canada.
F. Y. Fujiwara, L. W. Reeves, A. S. Tracey, and L. A. Wilson, J. Am. Chem. SOC.,96, 5249 (1974). F. Y. Fujiwara, L. W. Reeves, M. Suzuki, and J. A. Vanin In “Solution Chemistry of Surfactants”, Vol. 1, “Proceedings of the National Colloid Symposium, Knoxville, Tenn., 1978”,K. L. Mlttal, Ed., Plenum Press, New York. 1979. B. J. Forrest, F. Y. Fujiwara, and L. W. Reeves, J. Phys. Chem.,
References and Nlotes (1) F. Y. Fujlwara and L. W. Reeves, J. Phys. Chem., 84, 653 (1980). (2) L. Queiror do Amaral, C. Pimentel, M. R. Tavares, and J. A. Vanin, J. Chem. Phys., 71, 2940 (1979).
84,662 (1980). K. D. Lawson and T. J. Flautt, J. Am. Chem. Soc., 97,5729(1975).
Rapid Excitation Quenching by Host Micelles. Observations from Hexadecylpyridinium Chloride and Rrij-35 Micellar Systems Containing Pyrene Scintillator A. V. Sapre, K. V. S. Rama Rao, and K. N. Rao* Cherriistry Division, Bhabha Atomic Research Centre, Bombay 400085, India (Received: May 24, 1978; In Final Form: Apri/ 16, 1980)
It was found that a cationic surfactant hexadecylpyridinium chloride (CPC) quenches the fluorescence emission from pyrene. Below the critical micellar concentration (cmc) of CPC, the quenching rate is diffusion controlled. Above the cmc, the quenching rate becomes faster and finally becomes independent of the surfactant concentration. The observations were interpreted on the basis of partitioning of pyrene between the aqueous and micellar phases. A quantitative treatment gives the binding constant K, = 5 X lo5 dm3 mol-’. For pyrene solubilized in the micellar core the estimated emission lifetime of C0.4 ns is in sharp departure from a diffusion-controlled quenching mechanism requiring a lifetime of >lo0 ns. Quenching of pyrene was also studied in the Brij-35-CPC mixed micelle system. The intermicellar quenching rate constant, Le., pyrene and CPC on different micelles, is estimated to be -5 X lo9 dm3mol-’ 8. Intramicellar quenching becomes perceptible even in micelles having just one or four CPC molecules. Such fast energy transfer processes open up exciting possibilities for channeling photoexcitation energies to reaction site situated elsewhere. As an example, photolysis of menadione in CPC-sodium lauryl sulfate micelles and methanol is discussed.
Introduction Micellar systems and micellar catalysis have of late been recognized as good models for biomembranes and enzyme catalysis.’ Studies on the dynamics of solutes solubilized in a microscopic micellar phase gives significant information on the structurad and related aspects of the micellar phase. It is conceivable that some typical micelles not only offer solubilization sites to water-insoluble solutes but also play a vital role in promoting energy or electron transfer from or to the micelle interior. As a preliminary to investigating such phenomena, we have selected hexadecylpyridinium chloride (CPC) as a surfactant to study the quenching of fluorescence from pyrene. CPC is known to quench the fluorescence of dansylglycine,2 and a recent investigation reports quenching of pyrene fluorescence solubilized in sodium lauryl sulfate micelles by CPC.3 We observed a remarkable enhancement of the quenching rate 0022-365418012084-228 1$01 .OO/O
as the CPC monomer aggregates to form micelles, and this can open up exciting possibilities of channeling photoexcitation energies to reaction sites situated elsewhere.
Experimental Section Materials. Pyrene supplied by L. Light was recrystallized from ethanol. N-Hexadecylpyridinium chloride (CPC, E. Merck), hexadecyltrimethylammonium bromide (CTAB, Hopkins and Williams),Brij-35 (poly(oxyethy1ene) lauryl ether, C12H23(OCH2CHz)z,0H, Pierce Chemical Co.), sodium lauryl sulfate (Fluka), and menadione (Fluka Purris) were used as received. Triple distilled water was used in preparing solutions for optical absorption and fluorescence measurements. Other solvents used were of spectroscopic grade. Methods. Absorption spectral measurements were carried out on a Hitachi 200-10 spectrophotometer. Pyrene 0 1980 American Chemical Society
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The Journal of Physical Chemistry, Vol. 84, No. 18, 1980
Sapre, Rao, and Rao
1
I
6ool
500
1
2
3
4 [CPC]
5
MX
io3
--. 6
7 CCPCI
Mx
oi 4,
Figure 1. Quenching of pyrene emission by CPC in degassed solution.
Figure 2. Stern-Volmer plot.
concentrations were estimated by employing c = 5 X IO4 dm3 mol-' cm-' at 337 nm with 5-cm cells. An Aminco Bowman spectrofluorimeter was used for fluorescence measurements. Pyrene emission was measured at 390 nm when excited at 337 nm. Experiments were carried out in aerated, degassed, and argon-flushed solutions. The argon-flushed detergent solutions of pyrene gave irreproducible emission values, probably due to some carry-over of pyrene in the foam. Hence oxygen-freesolutions were prepared by degassing 25 cm3 of pyrene solution in water on a vacuum line by freeze-pump-thaw cycles. Some aliquots ( cmc. As (faq)i 0, Ii
-+
Im.Thus the difference
(VIU = [Icmc - I M l ( f i - fref) where Zi and Iref are observed fluorescence intensities and fi and frefare functions of pyrene in the aqueous phase at CPC concentrations [ CPCIi and [CPCIref,respectively. If [CPClrefis kept constant, fi varies with [CPCIi since fref remains constant. For convenience we may select [CPC],,f at the beginning of the CD region (Figure 1)and vary [CPCIi from the cmc through concentrations a little over the cmc: (Ii-- zref)
Zi - l r e f
= [Icmc - ZmI(1
+ Ka[MIil-'
-
[Zcmc - I M I+~ KaIMIrefl-' ~ (VW The second term on the right-hand side of eq VI11 is also constant and can be denoted as A:
+
Thus a plot of [Ii- Iref A]-l vs. [CPCIi should be a linear - Z,) can be estimated by a plot. A, cmc, K,, and (Icmc successive approximation method. As a first approximation, we considered that lines AB and BC in Figure 1 intersect at [CPCI cmc, Iref I,, and Ka is at least lo3. From the approximate A so computed [Ii - Zref + A1-l is plotted vs. [CPCIi. For Iivalues close to the cmc, the plot is fairly linear with A having only a slight influence. From the deviation of the other points from this straight line better values of A were obtained for drawing the best straight line shown in Figure 4. Figure 4 also contains data points for [CPCIi < cmc for which
-
(cf. Stern-Volmer eq 1). Thus the intersection of the lines A'B' and B'C' (Figure 4) gives a cmc on the X coordinate and [ICMC- 1,I-l on the Y coordinate. From eq IX we therefore calculate Ka = 5 X lo6 dm3 mol-' (Table 11). The cmc value of -8.2 X mol dm-3 so obtained is in agreement with that obtained from relative surface tension measurements. In cases where solubilization involves one solute per micelle binding, as in eq a, solubilization data can be utilized to obtain binding constant^.^ The fraction of solutes in the micelles ( a )and in the aqueous phase (1- a ) can be related bylo . a
1 - C Y =Ka(
[CPC] - cmc
)
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Sapre, Rao, and Rao
TABLE 111: Comparison of Absorption and Fluorescence Intensities of Vibronic Bands in Electronic Spectra in Different Mediaa no,
solvent
E , dm3 mol-' cm-'
I / I ~ ~ ~
1 2
cyclohexane 138 0.6 methanol 245 1.015 3 water 1.5 4 NaLs 132 0.906 5 Brij-3 5 230 1.00 6 CTAB 310 1.04 l CPC 305 a Reference 25. Ratio of fluorescence intensities of solvent-sensitive ( 3 7 2 nm) to solvent-insensitive (386 nm) bands. 40 .01
T I 301 ,005
201
Figure 3. Concentration dependence of pyrene emission: (0) vacuum, no CPC; (A)air, no CPC; (0) vacuum, with CPC; (a)air, with CPC.
-
[PVRENE] 30
5 4 X 15' mot /dm3
A =0.022
-
d
i=z
101
--
9
T 2 0 -
LCPCI M x lo3 Figure 5. Solubilization of pyrene In CPC micelles.
In the next series of experiments we attempted to determine the binding constant (K,) by determining the OD of pyrene saturated in various concentrations of CPC solutions. It was found from Table I that, below the cmc, the solubility of pyrene remains unchanged thus ruling out premicellar aggregates. At [CPC] > cmc, the solubility of pyrene increased greatly to the extent of two to three molecules per micelle. For this kind of situation we can write 1
2
I
I
I
3
4
5
[CPC] mol /dm3 X
KJ1'
Pa, + M ePM
lo3
Figure 4.
and
TABLE 11: Binding Constants and cmc Values Obtained from Fluorescence Quenching Experiments in Deaerated Micellar Solutions of CPC
-- -
no.
1 2 3 mean
a
lo6[pyrene 1, mol dm-3
10-5K,
0.38 0.44 0.56
5.6 4.4 5.2 5.0 * 0.6
104cmc, mol dm-3
9.0 8.0 7.5 8.2 6.5a Obtained from relative surface tension experiments.
1-a'
z[Pmicellarl [Pa,] [KJ1)[P,,]
KJ*'
PM + P e[PJMetc.
+ K,(1)K,(2)[Pa,2]+ ...I [MI
(XIU
Paql
[Pa,] and [PM] represent the amount of unbound and bound pyrene, respectively. Since the aqueous phase is always kept saturated in these solubility experiments, a/(1 - a ) vs. [CPC] could be a straight line as shown in Figure 5. From the slope, we obtain [K,(l) + K,'l)K,"[Paq] + ...1
Rapid Excitatioii Quenching by Host Micelles
= 3.6 X lo6. That the higher terms in eq XI1 contribute to the slope is evident by comparing the above with the value of K, obtained from the previous section which, in our opinion, truely represents one solute per micelle binding constant. The order of magnitude of K, obtained is of same order as reported for aromatic solutes in detergent micelles.lOJ1 A distribution coefficient for sodium pyrenyl butyrate between water and NaLS micelles of 2.34 X lo5 in comparative units has been obtained by Quina and Toscano.12 If the greater hydrophobicity of pyrene as compared to sodium pyrenyl butyrate is considered, these K, values can be understandable. Let us consider the concept of a distribution coefficient where an otherwise homogeneous detergent solution in water consists of a large number of microscopic pseudophases formed by the aggregation of detergent molecules. Hall and Pethica13 have applied the small-system thermodynamics developed by Hill14 and shown that the distribution coefficient of a solute between the solvent and micellar phase is a function of the mole fraction of the micelles. However, it was argued15 that in the case of nonmicelle forming solutes the distribution coefficient will be independent of the micelle mole fraction. We have deliberately chosen very low pyrene concentrations in this work such that each micelle would at best have one pyrene molecule. The very fact that not all the micelles contain exactly the same number of solute molecules irrespective of the total solute concentrations makes it impossible to speak of a microscopic single-micelle-water equilibrium or distribution coefficient. The existence of another micelle with a different number or more solute molecules incorporated in its interior disturbs the above equilibrium. Nevertheless, a dynamic solute-micelle binding equilibrium seems to have been established, since in spite of the availability of solute-free micelles at higher CPC concentrations a certain amount of solute remains in the aqueous phase. The fugacity of such an ensemble of micelles with varying amount of solute molecules (0 or 1 in the present case) would be a statistical average in the sense that f , = fa,:fmi
The Journal of Physical Chemistry, Vol. 84, No. 18, 1980 2285
It may be pointed out that this is the highest rate constant so far reported for quenching of emitters solubilized in the interior of micelles. For example, quenching of P* by CTAB micelles has a rate constant of 1.1X lo7 s-l and presumably occurs in the diffusion of P* from the micellar interior to micellar exterior where the quenching Brcounterions are situated. A similar process involving P* diffusion to the Stern layer where pyridinium groups are situated would take time t given by6"
(XW
t = 3f2?rqr/kT
Taking the viscosity (a) of the micellar interior as 30 CP and f , the average distance travelled by the solute, as 2.2 nm?'j the one-dimensional random walk diffusion time would be 106 ns, far too large to agree with the experimentally observed k5. The anomalously high k5 may be due to a very efficient excitation quenching via the hydrocarbon chain containing the end pyridinium group. Such very fast energy transfer is reminiscent of exciton migration in crystals (viz., anthracene). We could assign a migration coefficient (A) given by17 = (2A~)l/~ (XV) and using 7 = l/k5, we obtain A = 5.95 X cm2 s-', a value comparable to exciton migration coefficients in organic crystals.17 Whether one should treat the micellar core as a crystalline solidlike system containing a scintillator impurity is not known at present. For the present we consider that it is enough for P* to be within an encounter distance from the hydrocarbon end of the CPC molecule for it to interact with the pyridinium group and get quenched. An exciplex formation on encounter P* + CPC (P.CPC)* P + CPC f
-
-
may be suggested to explain the observed quenching. An electron transfer from P* to CPC may also be possible. The site of solubilization of pyrene in CPC micelles may also influence the quenching rate. It has been observedls that the absorption or emission intensities of forbidden vibronic bands of weak electronic transitions are enhanced # faq. in polar media. Table I11 shows some of our results on Quenching within the Micelle. The concentration-inpyrene in different solvents and in 0.1 mol dm-3 solutions dependent quenching in the CD region can be expressed of surfactants in water. These data suggest that pyrene by may have been solubilized in a somewhat polar region akin (IO/~M) = 7aqTrn-' (XIII) to methanol in CPC and CTAB micelles. where T , ~ and T, are the lifetimes of P* in the aqueous and A broadening of NMR peaks due to (CH,),at 6 = 1.14 micellar phases, respectively. and CH3 at 6 = 0.74 was observed on the incorporation of Figure 3 shows a plot of the fluorescence intensities for pyrene in CPC. The CH3peak was also shifted downfield different pyrene concentrations in pure water and in 5 X by -5 Hz. Neither shift nor broadening was observed for mol dm-3 CPC solutions in both aerated and deaerated any pyridinium group proton peaks at 6 8.15 or 8.27. water. At this high [CPC], Iobgd = I,, since the amount These results indicate that pyrene is located within the of pyrene remaining in aqueous phase is extremely small cetyl core of the CPC micelles.6b Any chemical shifts in (-0.25%). From the slopes of Figure 3, we obtain (Io/Im)& pyrene resonance peaks at 6 = 8.1 and 8.18 due to so= 700 and ( I o / I ~ ) v a=c560. Taking T~~ = 126 and 227 ns lute-micelle binding could not be detected because of the in the presence and absence of air,5we evaluate, from eq presence of large pyridinium signals at 6 = 8.15 and 8.27. ~ X lo9and 2.76 X lo9 s-l as the rates of XIII, T ~ =- 5.5 The spectral data (Table 111) may indicate higher decay of excited pyrene in CPC micelles in the presence permeability of cationic micelles like CPC and CTAB to and absence of air. Allthough the presence of air somewhat water as compared to anionic NaLS and nonionic Brij-35 enhances the decay rate, we feel that the experimental data micelles and may not indicate the solubilization site.lg In at these low fluorescence intensities are not sufficiently view of the fact that both CTAB and CPC offer similar accurate to contribute to the discussion on any additional solubilization environments to pyrene but differ vastly in quenching due to oxygen in micelles and we consider k5 their quenching efficiencies, we conclude that, when a = Tm-l = 2.76 x 109 8-1. quencher group is part of the micelle, a very fast quenching We attribute the rapid decay of P* in oxygen-free CPC prevails either by exciplex formation or by electron/energy micelles to a high quenching rate, k5, by host micelles. In transfer to the end pyridinium group. nonquencher micelles such as NaLS, Brij-35, etc., lifetimes Quenching by CPC in Brij-35 Micelles. In this section of P* are as high as 300-350 119.~9~~ we shall examine the possibility of a similar fast quenching
-
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The Journal of Physical Chemistry, Vol. 84, No. 18, 1980
Sapre, Rao, and Rao
TABLE IV: Quenching by CPC in Brij-35 Micellesa no. 1 2 3 4 5 6 7 8 9
b
P
Iobsd
I/I,
0 0.04 0.08 0.16 0.32 0.4 0.8 1.6 10
78.7 74.7 71.7 67.7 56.2 49.7 34.7 17.7 1.02
1
1
0.9492 0.9111 0.8602 0.7141 0.6315 0.4409 0.2249 0.01296
0.9608 0.923 0.8521 0.7261 0.6903 0.4493 0.2019 0.00004 5
pfo?
b
P(1)
P(1)
0.0384 0.0738 0.1363 0.232 0.2681 0.3395 0.323 0.000045
0.00077 0.0029 0,0109 0.0372 0.0536 0.1438 0.2584 0.00227
0.00008 0.00058 0.00396 0.00715 0.0383 0.1378 0.007566
a Total pyrene concentrations = 6 X mol dm-3. Total Brij-35 concentration = 1 X lo-* mol dm-3. Concentration of Brij-35 micelles assuming an aggregation of 4 0 molecules per micelle = 2.5 X mol dm-3. Average occupancy ( p ) = (conFraction of Brij-35 micelles containing 0, 1, 2, . . ., etc. centration of CPC molecules)/(concentration of Brij-35 micelles). CPC molecules. p ( z )is calculated from the equation p ( z )= p Z e - p / Z ! For p = 10, p ( 4 )= 0,0189 and p ( 3 )= 0.0378.
process in the mixed micellar system Brij-35 and CPC. By varying the relative concentrations of CPC and Brij-35, we may be able to determine the minimum number of CPC molecules per host micelle which can effect the quenching of pyrene. In Table IV, the observed fluorescence intensities are summarized for pyrene solubilized in Brij-35 at different added CPC concentrations. At the concentrations employed, we will have host micelle containing no solute, only pyrene, only CPC, one molecule each of pyrene and CPC, and one molecule each of pyrene and two or more molecules of CPC. The probable fraction of Brij-35~micelles containing zero, one, two, etc. molecules of CPC are estimated by a Poisson distribution functionz0and are summarized in the same table. In the absence of CPC, the observed lifetime of pyrene in Brij-35 was taken as 350 In view of the above distribution, pyrene would have different decay constants depending on the character of the host micelle. The observed intensities are therefore the results of these decay constants. I t can be presumed that pyrene in a quencher-free host micelle is also susceptible for quenching by a diffusion-controlled encounter with a CPC-containing micelle. In principle, it is possible to distinguish this trivial phenomenon from a far more rapid and interesting intramicellar quenching and assign limits to the capability of a quencher containing micelle. Rewriting Iobsd as a sum of contributions from different types of micelles, we have Iobsd
= I ( 0 ) + I(1)+ I(2) + * * *
(XVI)
where I(o),I(l), I ( z ) ,... are contributions to fluorescence intensities from host micelles containing 0, 1,2, ... molecules of CPC. In absence of any CPC, Io = h O h p ,with T = 350 ns:
where P!,),P(l),... are the probabilities for the occurrence of CPC in host micelles (cf. Table IV) and ~ Q [ Qis] the concentration-dependent quenching rate function for diffusion-encounter quenching of pyrene in quencher free micelles and Iql),k(z),... are intramicellar quenching rate constants in micelles containing one, two, ... molecules of CPC. It is obvious from Table IV that when p(o)is 0.69, I,,M/Io is only 0.63 and trivial intermicellar quenching does occur (the first term in eq XVII). From the data for p = 10 it is also obvious that four CPC molecules per micelle are adequate to induce rapid quenching. By successive approximation of the data points kQ was found to be -5 X lo9 dm3 m-l s-l and k(z)was at least -10' s-l where z is a small number which could even be one. The data were not sufficiently accurate t o determine h(,)'s which must
have lager values with increasing n. Further, it was found that the contribution from the intermicelle quenching ] not necessarily linear with [Q], especially function k ~ [ & is at high [Q]. It is understandable that it should be so since we conceive that the intermicelle quenching mechanism should involve two steps: (i) a diffusion-controlled encounter between pyrene-containing and CPC-containing micelles and (ii) transmission of pyrene to the quencher micelle. At nominally high concentrations, the second process will be rate determining. Allowing for these considerations, we consider Iq1)is significant and just one or four CPC molecules in a host micelle can induce intramicellar quenching. The lower limit for Iq1)N lo7s-l signifies that quenching does occur even when pyrene is still within the micelle, the mean traversal time in Brij-35 micelle (eq XIV, f = 3.5 nm)z2 being 207 ns. Thus the net lifetime of pyrene in a few CPC molecules-Brij-35 micelle is less than 80 ns, in sharp contrast with 350 ns in Brij-35 alone. The very fact that, in pure CPC micelle containing 80 CPC molecules, the quenching rate constant is as high as -2.46 X lo9s-l indicates that intramicellar quenching is concentration dependent and the interaction distance is essentially determined by a collision probability between the freely moving pyrene in the micellar interior and more or less fixed CPC molecule along the length of its hydrocarbon chain. Rodgers and Wheeler3carried out quenching of pyrene fluorescence incorporated in NaLS micelles in the presence of cationic quenchers including CPC, using the laser flash photolysis technique. They observe one fast and one slow component. The slow component is attributed to the natural decay of pyrene and the fast one is due to the presence of quenchers. In presence of two to three CPC molecules per NaLS micelle, they obtain a rate constant of -2.5 x lo7s-l for this fast quenching, which agrees well with our estimated Iqz)value. However, it is felt that the system chosen by us is much cleaner, because of the interaction of cationic CPC with anionic NaLS detergent which leads to precipitation at higher concentrations of both. During the revision of this paper two other studies of intramicellar quenching were reported. Henglein and S ~ h e e r ehave r ~ ~ studied the quenching of fluorescence of 2-(4-aminophenyl)-3-methylbenzothiazol by duroquinone and Atik and Singer24have studied the quenching of cationic fluorophors [1-pyr[CH2],N(CH3)3]+C1- by cationic surfactant nitroxyl, [CH3(CH2),,N(CH3),PhNO]+C1-, both in NaLS micelles (see also ref 26). Quenching of a Photochemical Reaction. Such a fast energy or electron transfer process holds exciting possibilities in photosynthesis and other photo- and radiation chemical processes in biosystems. A few experiments were
2287
J. Phys. Chem. 1980, 8 4 , 2287-2291
conducted with this in view choosing menadione (MD)vitamin K3as a solute. On photolysis in aerated methanol, this compound produces a product which is fluorescent (A,, = 330 nm, A,, = 455 nm). Photolysis a t -350 nm of MD after solubilization in NaLS and CPC micelles show marked differences. MD solutions in NaLS show product fluorescence after photolysis a t 455 nm whereas CPC solutions do not. The absorption spectra on photolysis show a correspondingly little change a t h = 330 nm in CPC solutions but a large change in methanol and NaLS solutions. Change in [MD] on photolysis was measured by the cysteine method4 arid typically decomposition of MD is -3.5 times higher in NaLS and n ~ 2 . 5times higher in methanol as compared with CPC solutions. This observation clearly points out the influence of host micelle in governing photoreactions in typical biologically important moleculea. Conversely, we suggest that hostmicelle-sensitized photodecomposition of a solubilized solute may also be possible and plays an important role in photobiology.
E. W. Anacker, J. Phys. Chem., 62, 41 (1958). The authors are very thankful to one of the referees for suggestlng this. Reference 7, pp 90 and 92. M. E. L. McBain and E. Hutchinson, “Solubilization and Related Phenomena”, Academic Press, New York, 1955, pp 75-77. F. H. Quina and R. G. Toscano, J . Phys. Chem., 81, 1750 (1977). D. G. Hall and B. A. Pethlca in “Non-ionic Surfactants”, M. J. Schick, Ed., Marcel Dekker, New York, 1967. T. L. Hill, “Thermodynamlcs of Small Systems”, Vols. 1 and 2, W. A. Benjamin, New York, 1963. P. Mukherjee, J . Pharm. Sci., 60, 1531 (1971). Rwas taken as (radius of micelle) - (radius of pyrene), Le., 2.5nm - 0.3 nm = 2.2 nm. J. B. Blrks, “Photophysics of Aromatic Molecules”, Wiley-Interscience, New York, 1970, pp 532 and 604. (a) A. Nakajima, Bull. Chem. SOC.Jpn., 44, 3272 (1971); (b) Specfrochim.Acta, Part A, 30, 860 (1974); (c) Bull. Chem. Soc. Jpn., 50, 2473 (1977). B. B. Craig, J. Kirk, and M. A. J. Rodgers, Chem. Phys. Lett., 49,
References and Notes
S. S. Atik and L. A. Singer, Chem. Phys. Lett., 59, 519 (1978);86,
437 (1977). U. Khuanga, B. K. Selinger, and R. McDonald, Aust. J . Chem., 29,
1 (1976).
S. C. Wallace and J. K. Thomas, Radiat. Res., 54, 49 (1973). P. Becker and M. Arai, J . Colloid Interface Sci., 27, 634 (1968). A. Henglein and R. Scheerer, Ber. Bunsenges. Phys. Chem., 82,
1107 (1978). 234 (1979).
-
Pyrene (- lo3 mol/dm3) was sonicated in respective surfactant solutions (0.1 mol/dm3)for 1 h, allowed to settle, and filtered, and then absorption spectra were recorded. To determine exact concentrations, we diluted the surfactant solutions 100 times in ethanol and measuredthe OD at 337 nm. From these OD values, assuming cg3, = 5 X lo4 dm3 mol-‘ cm-I, we calculated the pyrene concentrations. A recent paper, A. Yekta, M. Aikawa, and N. J. Turro, Chem. Phys. Lett., 63, 543 (1979),describes Poisson statisticaltreatment of limiting cases of luminescence quenching which allow an estimation of Ka, n , and the substrate exit and entry rates. Our experiments on P’ quenching by CPC in Brij-35 micelles may be viewed as case I of their treatment, Le., static quenching where the quencher is totally micellized.
(1) (a) E. J. Fendler and J. H. Fendler, Adv, Phys. Org. Chem., 8,271 (1970); (b) E. H. Cordes and C. Gitler, Prog. Bioorg. Chem., 2, 1 (1973). (2) G. A. Davis, J. Am. Chem. SOC.,94, 5089 (1972). (3) M. A. J. Rodgersand M. F. daSilva E. Wheeler, Chem. Phys. Left., 53, 165 (1977). (4) F. D. Snell and L. S. Ettre, Ed., “Encyclopedia of Industrial Chemical Analysis”, Vol. IO, Interscience, New York, 1970, p 196. (5) N. J. Turro and M. W Geiger, Phofochem. Photobioi., 22, 273 (1975). (6) (a) M. Grakel and J. k:. Thomas, J. Am. Chem. Soc., 95, 6885 (1973); (b) M. Gratzel, K. Kalyansundaram,and J. K. Thomas, J. Am. Chem. SOC.,06, 7869 (1974). (7) J. H. Fendler and E. J. Fendler, “Catalysls in Micellar Macromolecular Systems”, Academlc Press, New York, 1975, p 88.
Krafft Points of Anionic Surfactants and Their Mixtures with Special Attention to Their Applicability in IHard Water Kaorui Tsujii, * Naoyukl Saito, and Takashi Takeuchi Tochigi Research Laboratories, Kao Soap Company, Ichikai-machi, Haga-gun, Tochigi 32 1-34, Japan (Received: October 29, 1979)
The Krafft points of the sodium and calcium salts of typical anionic surfactants and their mixtures have been measured to examine their applicability in hard water. The pure model compounds of the linear alkylbenzene sulfonates, a-olefin sulfonates, and alkylpoly(oxyethy1ene) sulfates were synthesized and used for Krafft-point measurements. Among the above three types of surfactant, the alkylpoly(oxyethy1ene)sulfates are shown to be the best surfactant for their practical uses in hard water, since their sodium and calcium salts as well as their mixtures are readily soluble at room temperature. The Krafft point vs. composition curves observed in binary surfactant mixtures have been classified into two groups. In group I, there exists a minimum in the Krafft point at a certain composition, whereas the Krafft point varies monotonously with the composition change in group 11. It is found from the composition analysis of the solid phase that both components are immiscible in group I but are completely miscible even in the solid phase in group 11. The thermodynamic theory for freezing-point depression has been favorably applied to the Krafft point vs. composition curves in group I. Theoretical calculations for the Krafft point vs. composition curve (liquiduscurve) and the corresponding solidus curve in group I1 have also been made, assuming the ideal solutions in both liquid (micellar) and solid phases. The calculated curves are in poor agreement with the observed ones probably because of the nonideality of the solution especially in the solid phase.
Introduction The Krafft points of calcium salts of ordinary anionic surfactants are generally higher than ambient temperature, and they cannot be used in hard water without any sequestering agents. The phosphate builders are well-known to be the most commonly used sequestering agents in
detergent formulations but are now implicated in eutrophication problems in some developed countries. It is then of great importance for practical uses of surfactants to find the agent which is applicable in hard water. In the present work, the Krafft points of the sodium and calcium salts of anionic surfactants and their mixtures have
0022-3654/80/2084-2287$01 .OO/O 0 1980 American Chemical Society
