- Email: [email protected]
The micelle-water monomer exchange process in solutions of ionic surfactants measured by transient fluorescence quenching
Volume 155, number 6
CHEMICAL PHYSICS LETTERS
17March 1989
THE MICELLE-WATER MONOMER EXCHANGE PROCESS IN SOLUTIONS OF IONIC SURFACTANTS MEASURED BY TRANSIENT FLUORESCENCE QUENCHING Angelos MALLIARIS =, Nijel BOENS b, Hongwen LUO b, Mark VAN DER AUWERAER Frans C. DE SCHRYVER b and Steven REEKMANS b
b,
a Nuclear Research Centre ‘Demokritos”, Athens 153 10, Greece b Department of Chemistry, Kafholieke Universiteit Leuven, 3030 Heverlee. Belgium Received 4 November 1988; in final form 27 January 1989
The effect of various structural factors on the dissociation rate constant k- of a quencher surfactant from a host micelle has been examined by the time-resolved fluorescence method.
1. Introduction
The kinetics of micelle formation has been investigated mostly by chemical relaxation methods [ 11, and two main relaxation processes, a fast one and a slow one, have been identified in dilute solutions of pure micelles [ 21. From the theoretical point of view, the general treatment of Aniansson and Wall [ 3] provides adequate explanation for the appearence of these two relaxation times. Accordingly, the slow process, characterized by the lifetime rz, is related to the micelle breakdown/formation, while the fast one, characterized by r,, corresponds to the exchange of monomeric surfactants between micelles and the intermicellar solution [ 2 1. As measured by chemical relaxation techniques, the monomer-micelle dissociation rate constant k- for typical ionic micelles has values in the range 104- 1O6 s- * [ 21, depending on the length of the aliphatic chain. On the other hand, it is known that the dynamic intramicellar quenching of the fluorescence of aromatic molecules proceeds at rates of the order of 1O7 s- ’ or lower [ 4 1, It is possible therefore to employ the quenching of the long-lived fluorescence of a micelle-bound fluorophor (e.g. pyrene or its derivatives) [ 51 to study the exit rate from a micelle of a solubilized quencher. In the present work our main objective was to study the dissociation of fluorescence quenching surfactant ions, called the quencher surfactants, from various ionic host micelles made of amphiphiles differ0 009-2614/89/$ ( North-Holland
03.50 Q Elsevier Science Publishers Physics Publishing Division )
ent from the probe surfactants and called the host surfactants. More specifically, we have used the class of the effective fluorescence quenchers n-alkylpyridinium ions (n = 12-16), as the quencher surfactants, and a number of common anionic, cationic and zwitterionic micelle-forming detergents as the host surfactants. The strongly lipophilic pyrene and lmethylpyrene were used as fluorophors. Our study involved the systematic investigation of the effect of the aliphatic chain and the ionic head, of both the quencher and the host surfactant, on the rate constant k-.
2. Experimental All chemicals used in this study (sodium dodecyl sulfate and sodium tetradecyl sulfate (SDS and STS; Merk fur biochemische Zwecke) [6], dodecyltrimethylammonium chloride and tetradecyltrimethylammonium chloride (DDTAC and ‘ITAC; Tokyo Chemical Industries) [ 61, cetyltrimethylammonium chloride (CTAC; Kodak, 98%) [ 71, 3sulfonate (dimethyldodecylammonio)propane (DDAPS; Fluka) [ 7 1, decyl-, dodecyl-, tetradecyland hexadecyl-pyridinium chloride ( ClOPyC1, C,,PyCl, C1,PyCl, C,,PyCh Henkel) [6], pyrene (Aldrich, 99%) [7] and 1-methylpyrene [S]) were purified according to previously described methods. The single-photon timing method [9] was emB.V.
587
Volume 15 5, number 6
CHEMICAL
PHYSICSLETTERS
17 March 1989
ployed for the accumulation of the fluorescence decay data. Two different fluorescent probes were used, i.e. l-methylpyrene with fluorescence decay rate constant Zq,= 5 x lo6 s-‘, and pyrene with k0=3.0x106 s-l. In the general case the fluorescence decay obeys Z,=ZO exp{-A,t-&[l-exp(-AJ)]},
(1)
where lo and Z, represent the fluorescence intensities at zero time and at a later time t respectively. Al, AS and A, are time-independent fitting parameters [ 1O121. For the case of ionic micelles (negligible intermicellar collisions [ 13 ] ) , and mobile quencher (the fluorophor is always immobile), parameters AZ, A3 and A4 are expressed by
>
J~~=[QIW/A:(~+~[W)
A,=k,+k-.
(MI
Fig. 1. Plots of A2 versus quencher concentration [Q] for mobile M DDTAC, a, and immobile quenchers. 4, C,$y+/O.l C,,Py+/O.l MTTAC; O,C,,Py+/O.l MSDS.
(3)
(4)
All symbols have their usual meaning [ 141. Clearly when the quencher is immobile, i.e. k- =O, eqs. (2)-(4) simplify to Az=k,,
J%=([Csl-CMC)/Wl,
(9)
where [C.] and [M] are the total surfactant and micellar concentrations respectively, and CMC the critical micelle concentration.
(5) ,
A4=kq.
(6)
3. Results and discussion
(7)
The main factors expected to influence the magnitude of k- are the mutual polarity of the probe and host surfactants and the length of their aliphatic chains [ 21. The effect of each of these factors will be examined separately on the basis of our experimental results which are listed in table 1.
Therefore a very convenient criterion for quencher mobility is the value of A2 with respect to k,,, thus when Az = ko the quencher is immobile, while when AZ> k0 it is mobile. This is graphically depicted in fig. 1, where a plot of the quencher concentration [ Q ] versus A2 is shown for the case of the C&Py+ quencher surfactant in micelles of SDS, DDTAC and TTAC. Note also that eqs. (2)- (4) produce k,=&W(A,-k,+A&
>
(8)
which allows determination of the intramicellar fluorescence quenching rate constant, k,, from the experimentally accessible parameters b, AZ, A3 and Aq_ Evidently, the value of k, is totally independent of the mobility of the quencher. The other rate constant k -, for the micelle-monomer exchange, can always be estimated from eqs. (8) and (4). Finally, the mean micellar aggregation number ZV,can be measured when the quencher is immobile from 588
IIQlxlO'
(2)
A~=~o+[QI~~~+/A~(~+KIMI),
.43= [Ql/Wl
-II
3.1. Effect of the charge of the head group Table 1 shows that for any combination of oppositely charged host and probe surfactants AZ= k,,, i.e. k- is zero on the time scale of the fluorescence quenching. Thus, no micelle-water exchange of C,Py+ (n = 10-l 6) was detected when these cationic surfactant quenchers were solubilized in anionic micelles of SDS or STS. Evidently, in these cases the length of the aliphatic chain, either of the host or of the quencher surfactant, has no detectable effect on the magnitude of k-. This indicates that the electrostatic attraction between the oppositely charged quencher and host ionic heads is strong enough to
Volume 155, number 6 Table 1 Micellar parameters
[Surf1 (M) SDS ” 0.1 0.1 0.1
CHEMICAL
for ionic surfactants
in aqueous solutions,
PHYSICS LETTERS
obtained
17 March 1989
from fluorescence
quenching
measurements
a)
k,
k+
k-
(106s-1)
(109M-‘s-l)
(IO’s_‘)
[Ql
kl
A2
(lo-‘M)
(106s-1)
(106s-1)
5.00 5.00 5.00
5.02 5.05 5
67 69 71
39.7 38.5 31.4
immobile immobile immobile
GOPY
N,
+
4 6 8
0.1 0.1
%PY+ 4 8
5.0 5.0
5 4.95
67 72
40.6 31.1
immobile immobile
0.278
C,zY+ 59.1
2.85
2.85
111
33.4
immobile
5.0
4.97
121
19
immobile
5.0
4.91
125
lS.9
immobile
5.34 5.34 5.34
5.64 5.95 6.36
54 h, 54h’ 54 h)
41.9 39.6 43.0
7.5 8.6 8.9
5.27 5.27
5.44 5.64
54h’ 54”’
40.7 41.6
2.5 2.9
3.00
3.03
54
45.9
5.2 5.2 5.2
5.71 6.36 6.94
70”’ 70”’ 70”’
21.6 17.9 22.1
6.9 8.2 8.0
5.30 5.30
5.36 5.45
70 hJ 70”’
23.4 22.6
2.1 1.7
5.2 5.2
5.15 5.07
74 67
23 22
STS ” 0.1
C,aPY+ s +
Cl4PY
0.1 DDTAC d, 0.1 0.1 0.1 0.1 0.1 0.195 T=l-AC c, 0.1 0.1 0.1
8 GaPY 4 8 12
+
c12py+ 8
16 C&’ 58.9 GOPY 8 16 24
22.5 24.2 30.9 4.08 4.92 immobile
+ 20.8 23.1 24.2
0.085 0.085
c**PY+ 8 16 + CtrPY 4 12
CTAC f’ 0.361
C,LlPY+ 50.7
3.01
5.74
122 ‘)
IS.0
5.8
18.1
0.2 0.46
C,,pY’ 12 65
3.01 3.01
3.17 3.44
119” 135”
11.4 9.0
1.4 0.7
2.5 2.7
0.361
C16PY 58.5
3.01
3.03
122
14.3
DDAPS B’ 0.214
ClOPY+ 54.4
3.07
6.28
78 ”
25.7
0.214
C14PYf 37
3.07
3.1
i9
25.2
immobile
0.214
C,sPY+ 59
3.07
3.07
77
24.6
immobile
0.1 0.1
1.82 1.7 immobile immobile
+
a) Thefluorophor iseither pyrene (&3x b, CMCz0.0083 M. ‘) CMC=0.00205 h, N, from immobile quencher. ‘) Ref.
lo6 s-l), or I-methylpyrene (b-5X 106s-I). M. M. d’ CMCzO.016 M. e, CMCz0.0045 [ 151.
immobile
6.4
‘) CMC=0.00138
19.2
M.
rJ CMC=0.0036
M.
589
Volume 15 5, number 6
CHEMICALPHYSICSLETTERS
minimize the free energy of the quencher at the micelle-water interface, independently of the magnitude of the hydrophobic interaction. This result seems to contradict the conclusion drawn from therrnodynamic calculations that the electrostatic part of the change of the total dissociation free energy is negligible compared to its hydrophobic part [ 16,17 ] _ On the other hand, when the cationic quencher surfactants C,Py+ are solubilized in cationic micelles the magnitude of k - is strongly dependent on the hydrophobic interaction, and it can assume values varying from the very small, outside the range of the fluorescence quenching method, up to the order of lo6 s-l, depending on the length of the aliphatic chain (see section 3.2). Interestingly, cationic quencher surfactants solubilized in the zwitterionic micelles of DDAPS demonstrate kinetic behavior quite similar to the one observed in cationic micelles. Thus, the rate of dissociation of CloPyc from the micelles of DDAPS was found to be approximately equal to its corresponding rate of dissociation from the cationic micelles. Similarly, C4Pyc and C16Py+ in micelles of DDAPS were shown to behave as immobile quenchers (k- < 1OSs-l), in exactly the same way they behave when they are bound to cationic micelles. In view of the fact that in the case of zwitterionic surfactants there is no electric field outside the micelle, unlike the case of the cationic micelles, the similarity of the k - values in DDAPS and DDTAC is surprising. Ordinarily one would expect to find lower kvalues for positive quencher surfactants solubilized in zwitterionic than in cationic micelles. A possible rationalization of these results can be obtained in terms of the structure of the zwitterionic micelle. Thus, the DDAPS surfactant has two charges, one positive quatemary ammonium group and a negative sulfonate, with the latter further away from the micellar center than the former. Therefore, in spite of the absence of an electric field outside the micelle, an electric field exists inside the Stern layer. Due to this electric field the positive head group of the quencher is expected to be attracted towards the negative outer part of the Stem layer. Consequently, some of the CHI groups of the aliphatic chain of the quencher, adjacent to the pyridinium head, will be exposed to the water, which is known to permeate strongly the Stern layer of the DDAPS micelle [ 181. 590
17 March 1989
As a result, the hydrophobic component of the free energy of the binding of the quencher to the micelle will be lower compared to the hydrophobic free energy of C,Py+ in DDTAC micelles where the entire aliphatic chain of the quencher is imbedded in the host micelle. A second explanation could be that the electrostatic contribution to the free energy of activation on k- does not change in a symmetrical way when the charge of the micelle is changed. Although this effect will make the free energy of activation larger for a hydrophobic counterion compared to a neutral hydrophobic molecule, it does not make it smaller for a co-ion compared to a neutral molecule. This argument is supported by the fact that the relaxation experiments indicate that k- for a surfactant ion does not decrease significantly when the ionic strength is increased [ 191. Actually, increase of the ionic strength leads to a decrease of the micellar potential, making it resemble more closely a neutral micelle. This argument is also supported by the fact that the distribution of the neutral molecules, not bound to the micelle, will resemble more closely that of co-ions than that of “free” counterions [ 20,2 11. 3.2. Effect of the length of the aliphatic chain The data for the cationic micelles DDTAC, TTAC and CTAC (table 1) , clearly demonstrate the effect of the aliphatic chain length, n, of the C!,Py+ quencher surfactant, on the rate of its dissociation from a similarly charged cationic host micelle. More specifically, k- decreases by about one order of magnitude as n increases from 10 to 12. Thus for CloPy+ the mean k- values in DDTAC, TTAC and CTAC micelles are 2.6~ 106, 2.2~ lo6 and 1.8~ 106 s-’ respectively, whereas for C12Py+ k- is 0.4 x lo6 s- ’in DDTAC, 0.18x lo6 s-’ in TTAC and 0.26x IO6 s-l in CTAC. Further increase of n reduces the value of k- below the sensitivity of the fluorescence method, and therefore &Py+ and ClaPy+ behave as immobile quenchers. Note that these results conform to the theoretical prediction that log k- should decrease linearly with increasing chain length. Indeed, as shown in fig. 2, when the average k- value of C,2Py+ with respect to DDTAC micelles (table 1) is placed on a plot of log k- versus n, it is in excellent agreement with similar k- values previously obtained from relaxation experiments [ 22 I. Further-
Volume 155, number 6
CHEMICAL PHYSICS LETTERS
Fig. 2. Dependence of the logarithm of k- on the number of carbon atoms (n) in the aliphatic chain of the exchanging surfactant: 0, C,PyBr; 0, C,NH,CI; +, this study. Data for pure C.Py+Br and C.NH&l micelles from ref. [ 19 1,Values from this study represent average numbers for the three cationic micelles (table 1).
more, the log k- for the exit of C,,Py+ from micelles of DDTAC has a value in between the corresponding values for the exit rates of a monomer from pure C,*PyBr and C,,NN,Cl micelles. Similar conclusions are drawn for the log k- value of C,,,Py+, as shown in fig. 2. Finally, the effect of the length of the aliphatic chain of the host surfactant on the monomer exchange rate constant can be examined by comparing k- for Cl,,Py+ in DDTAC, TTAC and CTAC. It is seen in table 1 that as the length of the chain of the host surfactant increases from 12 to 16 carbon atoms, k- decreases only slightly (x 30%). This change is small and its origin is probably the difference in hydration of the quaternary ammonium heads in the three micelles. Such a difference is expected to affect the repulsive electrostatic forces between the escaping ion and the micellar surface. A similar small decrease was also reported in a phosphorescence study of micelles of alkyl sulfates [ 23 1. The above results for cationic micelles clearly demonstrate the importance of the hydrophobic interactions as far as the association of a surfactant ion to a micelle with the same charge is concerned. The hydrophobic interaction in this case is expressed in terms of the interaction of the aliphatic chain of the particular pyridinium ion with the aliphatic chains of the surrounding surfactants which make up the micelle. It appears from our results that the chain
17 March I989
length of the micelle-forming surfactant, i.e. DDTAC, TTAC and CTAC, does not affect the dissociation rate of the pyridinium surfactant. This is understood in terms of the fact that the aliphatic chain of the Czy’ is embedded inside the hydrocarbon-like micellar core and therefore it always has a total hydrophobic interaction with the environment proportional to n, the number of carbon atoms of its chain, independently of the length of the chain of the host surfactant. It should be emphasized at this point that the values of k - obtained by means of fluorescence quenching turn out to be approximately two orders of magnitude lower than the corresponding values obtained from chemical relaxation studies [ 2 1. The reason is that in chemical relaxation experiments all the surfactants making up a micelle are exchanged, and therefore they all contribute to the measurement. On the other hand, in fluorescence studies, although again all the surfactants can be exchanged, only the exchange of the quencher surfactant is sensed by the experimental method. Consequently, kfrom chemical relaxation must be divided by the mean aggregation number N,, before it is compared to k- obtained from fluorescence probing. Since N, is x 100, the difference of approximately two orders of magnitude between the two values is accounted for. Another kinetic parameter of interest in micellar systems is the association rate constant k+ (1 M- ’ s- ’), i.e. the rate constant of the entrance of a monomer in the micelle. This reaction is known to be diffusion-controlled [ 21 with a magnitude in the range between 10’ and 10’‘. It can be estimated from eq. (3), where all other variables are known except k+. The values thus obtained are also listed in table 1. It can be seen that k+ is indeed diffusion-controlled and it decreases with increasing length of the diffusing surfactant. Finally, the rate constant k, for the pseudo-firstorder intramicellar fluorescence quenching is effectively independent of the head group charge of the host and probe surfactants. Moreover, k, is also independent of the length of the aliphatic chain of the probe surfactant. Therefore its value is not related either to the Coulombic or to the hydrophobic interactions. It turns out that under the present conditions the only micellar parameter upon which k, 591
Volume 155, number 6
CHEMICAL
PHYSICS LETTERS
depends is the length of the aliphatic chain of the host surfactant, that is, the micellar size.
4. Conclusions The main conclusions of the present study are the following: (a) The rate constant k- of a quencher surfactant with na 10 is negligible (below the time scale of fluorescence quenching), when the quencher and the host surfactants have opposite charges. (b ) When the two surfactants have the same charge, kdecreases with increasing length of the aliphatic chain of the quencher surfactant, but it is independent of the length of the chain of the host surfactant. (c) The zwitterionic micelle DDAPS behaves as the corresponding cationic micelle with a 12-carbon ahphatic chain. (d) The rate constant of the monomer micelle association is diffusion-controlled and decreases with the chain length of the diffusing monomer. (e) The intramicellar fluorescence quenching constant kq is independent of the head charge and chain length of the probe monomer but it decreases with increasing length of the host surfactant, i.e. with increasing micellar size.
References [ 1 ] R. Zana, in: Surfactants
in solution, Vol. 4, eds. K.L. Mittal
and P. Bothorel (Plenum Press, New York, 1986) p. 115, and references therein. [2] E.A.G. Aniansson, S.N. Wall, M. Almgren, H. Hoffmann, I. Kielmann, W. Ulbricht, R. Zana, J. Lang and C. Tondrc, J. Phys. Chem. 80 (1976) 905.
592
17 March 1989
[ 31 E.A.C. Aniansson and S.N. Wall, J. Phys. Chem. 78 ( 1974) 1024;79 (1975) 857. [4] J.K Thomas, J. Phys. Chem. 91 (1987) 267. [5] P.P. Infelta, Chem. Fhys. Letters 61 (1979) 88. [6] H. Luo, N. Boens, M. Van der Auweraer, F.C. De Schryver and A. Malliaris, J. Phys. Chem., in press. [7] A. Malliaris, J. Phys. Chem. 91 (1987) 6511. [ 81 N. Boens, H. Luo, M. Van der Auweraer, S. Reekmans, F.C. De Schryver and A. Malliaris, Chem. Phys. Letters 146 (1988) 337. [9] D.V. O’Connor and D. Phillips, Time-correlated single photon counting (Academic Press, New York, 1984). [lo] P.P. Infelta, M. Gratzel and J.K. Thomas, J. Phys. Chem. 78 (1974) 190. [ 111 L.A. Singer, in: Solution behavior of surfactants, Vol. 1, eds. K.L. Mittal and E.J. Fendler (Plenum Press, New York, 1982) p. 73. [ 121 M. Van der Auweraer, J.C. Dederen, E. Gelade and F.C. De Schryver, J. Chem. Phys. 74 (1981) 1140. [ 131 A. Malliaris, J. Langand R. Zana, J. Chem. Phys. 90 ( 1986) 655. 1141 M. Van der Auweraer, C. Dederen, C. PalmansWindels and F.C. De Schryver, J. Am. Chem. Sot. 104 (1982) 1800. [ 15 ] A. Malliaris, J. Lang and R. Zana, J. Chem. Sot. Faraday Trans. I 82 (1986) 109. [16]C.Tanford,Proc.Natl.Acad.Sci.US71 (1974) 1811. [ 171 C. Tanford, J. Phys. Chem. 78 (1074) 2469. [ 18 ] P. Lianos and R. Zana, J. Colloid Interface Sci. 84 ( 198 1)
100. [19]R.ZanaandS.Yiv,Can. J.Chem.58 (1980) 1763. [20] B. Lindmann and H. Wennerstrijm, Topics Current Chem. 87 (1979) 1. [2 I ] M. Almgren, P. Linse, M. Van der Auweraer, F.C. De Schryver, E. Gelade and Y. Croonen, J. Phys. Chem. 88 (1984) 289. [22] H. Hoffmann, Ber. Bunsenges. Physik. Chem. 82 (1978) 988. [23] J.D. Bolt and N.J. Turro, J. Phys. Chem. 85 (1981) 4029.
CHEMICAL PHYSICS LETTERS
17March 1989
THE MICELLE-WATER MONOMER EXCHANGE PROCESS IN SOLUTIONS OF IONIC SURFACTANTS MEASURED BY TRANSIENT FLUORESCENCE QUENCHING Angelos MALLIARIS =, Nijel BOENS b, Hongwen LUO b, Mark VAN DER AUWERAER Frans C. DE SCHRYVER b and Steven REEKMANS b
b,
a Nuclear Research Centre ‘Demokritos”, Athens 153 10, Greece b Department of Chemistry, Kafholieke Universiteit Leuven, 3030 Heverlee. Belgium Received 4 November 1988; in final form 27 January 1989
The effect of various structural factors on the dissociation rate constant k- of a quencher surfactant from a host micelle has been examined by the time-resolved fluorescence method.
1. Introduction
The kinetics of micelle formation has been investigated mostly by chemical relaxation methods [ 11, and two main relaxation processes, a fast one and a slow one, have been identified in dilute solutions of pure micelles [ 21. From the theoretical point of view, the general treatment of Aniansson and Wall [ 3] provides adequate explanation for the appearence of these two relaxation times. Accordingly, the slow process, characterized by the lifetime rz, is related to the micelle breakdown/formation, while the fast one, characterized by r,, corresponds to the exchange of monomeric surfactants between micelles and the intermicellar solution [ 2 1. As measured by chemical relaxation techniques, the monomer-micelle dissociation rate constant k- for typical ionic micelles has values in the range 104- 1O6 s- * [ 21, depending on the length of the aliphatic chain. On the other hand, it is known that the dynamic intramicellar quenching of the fluorescence of aromatic molecules proceeds at rates of the order of 1O7 s- ’ or lower [ 4 1, It is possible therefore to employ the quenching of the long-lived fluorescence of a micelle-bound fluorophor (e.g. pyrene or its derivatives) [ 51 to study the exit rate from a micelle of a solubilized quencher. In the present work our main objective was to study the dissociation of fluorescence quenching surfactant ions, called the quencher surfactants, from various ionic host micelles made of amphiphiles differ0 009-2614/89/$ ( North-Holland
03.50 Q Elsevier Science Publishers Physics Publishing Division )
ent from the probe surfactants and called the host surfactants. More specifically, we have used the class of the effective fluorescence quenchers n-alkylpyridinium ions (n = 12-16), as the quencher surfactants, and a number of common anionic, cationic and zwitterionic micelle-forming detergents as the host surfactants. The strongly lipophilic pyrene and lmethylpyrene were used as fluorophors. Our study involved the systematic investigation of the effect of the aliphatic chain and the ionic head, of both the quencher and the host surfactant, on the rate constant k-.
2. Experimental All chemicals used in this study (sodium dodecyl sulfate and sodium tetradecyl sulfate (SDS and STS; Merk fur biochemische Zwecke) [6], dodecyltrimethylammonium chloride and tetradecyltrimethylammonium chloride (DDTAC and ‘ITAC; Tokyo Chemical Industries) [ 61, cetyltrimethylammonium chloride (CTAC; Kodak, 98%) [ 71, 3sulfonate (dimethyldodecylammonio)propane (DDAPS; Fluka) [ 7 1, decyl-, dodecyl-, tetradecyland hexadecyl-pyridinium chloride ( ClOPyC1, C,,PyCl, C1,PyCl, C,,PyCh Henkel) [6], pyrene (Aldrich, 99%) [7] and 1-methylpyrene [S]) were purified according to previously described methods. The single-photon timing method [9] was emB.V.
587
Volume 15 5, number 6
CHEMICAL
PHYSICSLETTERS
17 March 1989
ployed for the accumulation of the fluorescence decay data. Two different fluorescent probes were used, i.e. l-methylpyrene with fluorescence decay rate constant Zq,= 5 x lo6 s-‘, and pyrene with k0=3.0x106 s-l. In the general case the fluorescence decay obeys Z,=ZO exp{-A,t-&[l-exp(-AJ)]},
(1)
where lo and Z, represent the fluorescence intensities at zero time and at a later time t respectively. Al, AS and A, are time-independent fitting parameters [ 1O121. For the case of ionic micelles (negligible intermicellar collisions [ 13 ] ) , and mobile quencher (the fluorophor is always immobile), parameters AZ, A3 and A4 are expressed by
>
J~~=[QIW/A:(~+~[W)
A,=k,+k-.
(MI
Fig. 1. Plots of A2 versus quencher concentration [Q] for mobile M DDTAC, a, and immobile quenchers. 4, C,$y+/O.l C,,Py+/O.l MTTAC; O,C,,Py+/O.l MSDS.
(3)
(4)
All symbols have their usual meaning [ 141. Clearly when the quencher is immobile, i.e. k- =O, eqs. (2)-(4) simplify to Az=k,,
J%=([Csl-CMC)/Wl,
(9)
where [C.] and [M] are the total surfactant and micellar concentrations respectively, and CMC the critical micelle concentration.
(5) ,
A4=kq.
(6)
3. Results and discussion
(7)
The main factors expected to influence the magnitude of k- are the mutual polarity of the probe and host surfactants and the length of their aliphatic chains [ 21. The effect of each of these factors will be examined separately on the basis of our experimental results which are listed in table 1.
Therefore a very convenient criterion for quencher mobility is the value of A2 with respect to k,,, thus when Az = ko the quencher is immobile, while when AZ> k0 it is mobile. This is graphically depicted in fig. 1, where a plot of the quencher concentration [ Q ] versus A2 is shown for the case of the C&Py+ quencher surfactant in micelles of SDS, DDTAC and TTAC. Note also that eqs. (2)- (4) produce k,=&W(A,-k,+A&
>
(8)
which allows determination of the intramicellar fluorescence quenching rate constant, k,, from the experimentally accessible parameters b, AZ, A3 and Aq_ Evidently, the value of k, is totally independent of the mobility of the quencher. The other rate constant k -, for the micelle-monomer exchange, can always be estimated from eqs. (8) and (4). Finally, the mean micellar aggregation number ZV,can be measured when the quencher is immobile from 588
IIQlxlO'
(2)
A~=~o+[QI~~~+/A~(~+KIMI),
.43= [Ql/Wl
-II
3.1. Effect of the charge of the head group Table 1 shows that for any combination of oppositely charged host and probe surfactants AZ= k,,, i.e. k- is zero on the time scale of the fluorescence quenching. Thus, no micelle-water exchange of C,Py+ (n = 10-l 6) was detected when these cationic surfactant quenchers were solubilized in anionic micelles of SDS or STS. Evidently, in these cases the length of the aliphatic chain, either of the host or of the quencher surfactant, has no detectable effect on the magnitude of k-. This indicates that the electrostatic attraction between the oppositely charged quencher and host ionic heads is strong enough to
Volume 155, number 6 Table 1 Micellar parameters
[Surf1 (M) SDS ” 0.1 0.1 0.1
CHEMICAL
for ionic surfactants
in aqueous solutions,
PHYSICS LETTERS
obtained
17 March 1989
from fluorescence
quenching
measurements
a)
k,
k+
k-
(106s-1)
(109M-‘s-l)
(IO’s_‘)
[Ql
kl
A2
(lo-‘M)
(106s-1)
(106s-1)
5.00 5.00 5.00
5.02 5.05 5
67 69 71
39.7 38.5 31.4
immobile immobile immobile
GOPY
N,
+
4 6 8
0.1 0.1
%PY+ 4 8
5.0 5.0
5 4.95
67 72
40.6 31.1
immobile immobile
0.278
C,zY+ 59.1
2.85
2.85
111
33.4
immobile
5.0
4.97
121
19
immobile
5.0
4.91
125
lS.9
immobile
5.34 5.34 5.34
5.64 5.95 6.36
54 h, 54h’ 54 h)
41.9 39.6 43.0
7.5 8.6 8.9
5.27 5.27
5.44 5.64
54h’ 54”’
40.7 41.6
2.5 2.9
3.00
3.03
54
45.9
5.2 5.2 5.2
5.71 6.36 6.94
70”’ 70”’ 70”’
21.6 17.9 22.1
6.9 8.2 8.0
5.30 5.30
5.36 5.45
70 hJ 70”’
23.4 22.6
2.1 1.7
5.2 5.2
5.15 5.07
74 67
23 22
STS ” 0.1
C,aPY+ s +
Cl4PY
0.1 DDTAC d, 0.1 0.1 0.1 0.1 0.1 0.195 T=l-AC c, 0.1 0.1 0.1
8 GaPY 4 8 12
+
c12py+ 8
16 C&’ 58.9 GOPY 8 16 24
22.5 24.2 30.9 4.08 4.92 immobile
+ 20.8 23.1 24.2
0.085 0.085
c**PY+ 8 16 + CtrPY 4 12
CTAC f’ 0.361
C,LlPY+ 50.7
3.01
5.74
122 ‘)
IS.0
5.8
18.1
0.2 0.46
C,,pY’ 12 65
3.01 3.01
3.17 3.44
119” 135”
11.4 9.0
1.4 0.7
2.5 2.7
0.361
C16PY 58.5
3.01
3.03
122
14.3
DDAPS B’ 0.214
ClOPY+ 54.4
3.07
6.28
78 ”
25.7
0.214
C14PYf 37
3.07
3.1
i9
25.2
immobile
0.214
C,sPY+ 59
3.07
3.07
77
24.6
immobile
0.1 0.1
1.82 1.7 immobile immobile
+
a) Thefluorophor iseither pyrene (&3x b, CMCz0.0083 M. ‘) CMC=0.00205 h, N, from immobile quencher. ‘) Ref.
lo6 s-l), or I-methylpyrene (b-5X 106s-I). M. M. d’ CMCzO.016 M. e, CMCz0.0045 [ 151.
immobile
6.4
‘) CMC=0.00138
19.2
M.
rJ CMC=0.0036
M.
589
Volume 15 5, number 6
CHEMICALPHYSICSLETTERS
minimize the free energy of the quencher at the micelle-water interface, independently of the magnitude of the hydrophobic interaction. This result seems to contradict the conclusion drawn from therrnodynamic calculations that the electrostatic part of the change of the total dissociation free energy is negligible compared to its hydrophobic part [ 16,17 ] _ On the other hand, when the cationic quencher surfactants C,Py+ are solubilized in cationic micelles the magnitude of k - is strongly dependent on the hydrophobic interaction, and it can assume values varying from the very small, outside the range of the fluorescence quenching method, up to the order of lo6 s-l, depending on the length of the aliphatic chain (see section 3.2). Interestingly, cationic quencher surfactants solubilized in the zwitterionic micelles of DDAPS demonstrate kinetic behavior quite similar to the one observed in cationic micelles. Thus, the rate of dissociation of CloPyc from the micelles of DDAPS was found to be approximately equal to its corresponding rate of dissociation from the cationic micelles. Similarly, C4Pyc and C16Py+ in micelles of DDAPS were shown to behave as immobile quenchers (k- < 1OSs-l), in exactly the same way they behave when they are bound to cationic micelles. In view of the fact that in the case of zwitterionic surfactants there is no electric field outside the micelle, unlike the case of the cationic micelles, the similarity of the k - values in DDAPS and DDTAC is surprising. Ordinarily one would expect to find lower kvalues for positive quencher surfactants solubilized in zwitterionic than in cationic micelles. A possible rationalization of these results can be obtained in terms of the structure of the zwitterionic micelle. Thus, the DDAPS surfactant has two charges, one positive quatemary ammonium group and a negative sulfonate, with the latter further away from the micellar center than the former. Therefore, in spite of the absence of an electric field outside the micelle, an electric field exists inside the Stern layer. Due to this electric field the positive head group of the quencher is expected to be attracted towards the negative outer part of the Stem layer. Consequently, some of the CHI groups of the aliphatic chain of the quencher, adjacent to the pyridinium head, will be exposed to the water, which is known to permeate strongly the Stern layer of the DDAPS micelle [ 181. 590
17 March 1989
As a result, the hydrophobic component of the free energy of the binding of the quencher to the micelle will be lower compared to the hydrophobic free energy of C,Py+ in DDTAC micelles where the entire aliphatic chain of the quencher is imbedded in the host micelle. A second explanation could be that the electrostatic contribution to the free energy of activation on k- does not change in a symmetrical way when the charge of the micelle is changed. Although this effect will make the free energy of activation larger for a hydrophobic counterion compared to a neutral hydrophobic molecule, it does not make it smaller for a co-ion compared to a neutral molecule. This argument is supported by the fact that the relaxation experiments indicate that k- for a surfactant ion does not decrease significantly when the ionic strength is increased [ 191. Actually, increase of the ionic strength leads to a decrease of the micellar potential, making it resemble more closely a neutral micelle. This argument is also supported by the fact that the distribution of the neutral molecules, not bound to the micelle, will resemble more closely that of co-ions than that of “free” counterions [ 20,2 11. 3.2. Effect of the length of the aliphatic chain The data for the cationic micelles DDTAC, TTAC and CTAC (table 1) , clearly demonstrate the effect of the aliphatic chain length, n, of the C!,Py+ quencher surfactant, on the rate of its dissociation from a similarly charged cationic host micelle. More specifically, k- decreases by about one order of magnitude as n increases from 10 to 12. Thus for CloPy+ the mean k- values in DDTAC, TTAC and CTAC micelles are 2.6~ 106, 2.2~ lo6 and 1.8~ 106 s-’ respectively, whereas for C12Py+ k- is 0.4 x lo6 s- ’in DDTAC, 0.18x lo6 s-’ in TTAC and 0.26x IO6 s-l in CTAC. Further increase of n reduces the value of k- below the sensitivity of the fluorescence method, and therefore &Py+ and ClaPy+ behave as immobile quenchers. Note that these results conform to the theoretical prediction that log k- should decrease linearly with increasing chain length. Indeed, as shown in fig. 2, when the average k- value of C,2Py+ with respect to DDTAC micelles (table 1) is placed on a plot of log k- versus n, it is in excellent agreement with similar k- values previously obtained from relaxation experiments [ 22 I. Further-
Volume 155, number 6
CHEMICAL PHYSICS LETTERS
Fig. 2. Dependence of the logarithm of k- on the number of carbon atoms (n) in the aliphatic chain of the exchanging surfactant: 0, C,PyBr; 0, C,NH,CI; +, this study. Data for pure C.Py+Br and C.NH&l micelles from ref. [ 19 1,Values from this study represent average numbers for the three cationic micelles (table 1).
more, the log k- for the exit of C,,Py+ from micelles of DDTAC has a value in between the corresponding values for the exit rates of a monomer from pure C,*PyBr and C,,NN,Cl micelles. Similar conclusions are drawn for the log k- value of C,,,Py+, as shown in fig. 2. Finally, the effect of the length of the aliphatic chain of the host surfactant on the monomer exchange rate constant can be examined by comparing k- for Cl,,Py+ in DDTAC, TTAC and CTAC. It is seen in table 1 that as the length of the chain of the host surfactant increases from 12 to 16 carbon atoms, k- decreases only slightly (x 30%). This change is small and its origin is probably the difference in hydration of the quaternary ammonium heads in the three micelles. Such a difference is expected to affect the repulsive electrostatic forces between the escaping ion and the micellar surface. A similar small decrease was also reported in a phosphorescence study of micelles of alkyl sulfates [ 23 1. The above results for cationic micelles clearly demonstrate the importance of the hydrophobic interactions as far as the association of a surfactant ion to a micelle with the same charge is concerned. The hydrophobic interaction in this case is expressed in terms of the interaction of the aliphatic chain of the particular pyridinium ion with the aliphatic chains of the surrounding surfactants which make up the micelle. It appears from our results that the chain
17 March I989
length of the micelle-forming surfactant, i.e. DDTAC, TTAC and CTAC, does not affect the dissociation rate of the pyridinium surfactant. This is understood in terms of the fact that the aliphatic chain of the Czy’ is embedded inside the hydrocarbon-like micellar core and therefore it always has a total hydrophobic interaction with the environment proportional to n, the number of carbon atoms of its chain, independently of the length of the chain of the host surfactant. It should be emphasized at this point that the values of k - obtained by means of fluorescence quenching turn out to be approximately two orders of magnitude lower than the corresponding values obtained from chemical relaxation studies [ 2 1. The reason is that in chemical relaxation experiments all the surfactants making up a micelle are exchanged, and therefore they all contribute to the measurement. On the other hand, in fluorescence studies, although again all the surfactants can be exchanged, only the exchange of the quencher surfactant is sensed by the experimental method. Consequently, kfrom chemical relaxation must be divided by the mean aggregation number N,, before it is compared to k- obtained from fluorescence probing. Since N, is x 100, the difference of approximately two orders of magnitude between the two values is accounted for. Another kinetic parameter of interest in micellar systems is the association rate constant k+ (1 M- ’ s- ’), i.e. the rate constant of the entrance of a monomer in the micelle. This reaction is known to be diffusion-controlled [ 21 with a magnitude in the range between 10’ and 10’‘. It can be estimated from eq. (3), where all other variables are known except k+. The values thus obtained are also listed in table 1. It can be seen that k+ is indeed diffusion-controlled and it decreases with increasing length of the diffusing surfactant. Finally, the rate constant k, for the pseudo-firstorder intramicellar fluorescence quenching is effectively independent of the head group charge of the host and probe surfactants. Moreover, k, is also independent of the length of the aliphatic chain of the probe surfactant. Therefore its value is not related either to the Coulombic or to the hydrophobic interactions. It turns out that under the present conditions the only micellar parameter upon which k, 591
Volume 155, number 6
CHEMICAL
PHYSICS LETTERS
depends is the length of the aliphatic chain of the host surfactant, that is, the micellar size.
4. Conclusions The main conclusions of the present study are the following: (a) The rate constant k- of a quencher surfactant with na 10 is negligible (below the time scale of fluorescence quenching), when the quencher and the host surfactants have opposite charges. (b ) When the two surfactants have the same charge, kdecreases with increasing length of the aliphatic chain of the quencher surfactant, but it is independent of the length of the chain of the host surfactant. (c) The zwitterionic micelle DDAPS behaves as the corresponding cationic micelle with a 12-carbon ahphatic chain. (d) The rate constant of the monomer micelle association is diffusion-controlled and decreases with the chain length of the diffusing monomer. (e) The intramicellar fluorescence quenching constant kq is independent of the head charge and chain length of the probe monomer but it decreases with increasing length of the host surfactant, i.e. with increasing micellar size.
References [ 1 ] R. Zana, in: Surfactants
in solution, Vol. 4, eds. K.L. Mittal
and P. Bothorel (Plenum Press, New York, 1986) p. 115, and references therein. [2] E.A.G. Aniansson, S.N. Wall, M. Almgren, H. Hoffmann, I. Kielmann, W. Ulbricht, R. Zana, J. Lang and C. Tondrc, J. Phys. Chem. 80 (1976) 905.
592
17 March 1989
[ 31 E.A.C. Aniansson and S.N. Wall, J. Phys. Chem. 78 ( 1974) 1024;79 (1975) 857. [4] J.K Thomas, J. Phys. Chem. 91 (1987) 267. [5] P.P. Infelta, Chem. Fhys. Letters 61 (1979) 88. [6] H. Luo, N. Boens, M. Van der Auweraer, F.C. De Schryver and A. Malliaris, J. Phys. Chem., in press. [7] A. Malliaris, J. Phys. Chem. 91 (1987) 6511. [ 81 N. Boens, H. Luo, M. Van der Auweraer, S. Reekmans, F.C. De Schryver and A. Malliaris, Chem. Phys. Letters 146 (1988) 337. [9] D.V. O’Connor and D. Phillips, Time-correlated single photon counting (Academic Press, New York, 1984). [lo] P.P. Infelta, M. Gratzel and J.K. Thomas, J. Phys. Chem. 78 (1974) 190. [ 111 L.A. Singer, in: Solution behavior of surfactants, Vol. 1, eds. K.L. Mittal and E.J. Fendler (Plenum Press, New York, 1982) p. 73. [ 121 M. Van der Auweraer, J.C. Dederen, E. Gelade and F.C. De Schryver, J. Chem. Phys. 74 (1981) 1140. [ 131 A. Malliaris, J. Langand R. Zana, J. Chem. Phys. 90 ( 1986) 655. 1141 M. Van der Auweraer, C. Dederen, C. PalmansWindels and F.C. De Schryver, J. Am. Chem. Sot. 104 (1982) 1800. [ 15 ] A. Malliaris, J. Lang and R. Zana, J. Chem. Sot. Faraday Trans. I 82 (1986) 109. [16]C.Tanford,Proc.Natl.Acad.Sci.US71 (1974) 1811. [ 171 C. Tanford, J. Phys. Chem. 78 (1074) 2469. [ 18 ] P. Lianos and R. Zana, J. Colloid Interface Sci. 84 ( 198 1)
100. [19]R.ZanaandS.Yiv,Can. J.Chem.58 (1980) 1763. [20] B. Lindmann and H. Wennerstrijm, Topics Current Chem. 87 (1979) 1. [2 I ] M. Almgren, P. Linse, M. Van der Auweraer, F.C. De Schryver, E. Gelade and Y. Croonen, J. Phys. Chem. 88 (1984) 289. [22] H. Hoffmann, Ber. Bunsenges. Physik. Chem. 82 (1978) 988. [23] J.D. Bolt and N.J. Turro, J. Phys. Chem. 85 (1981) 4029.