Langmuir 1993,9,2814-2819
2814
Effects of Short-Chained Alcohols on the Intramicellar Fluorescence Quenching of 2,3-Dimethylnaphthaleneby the Counterion in Mixed Micelles of Alcohol and Decyltrimethylammonium Bromide D.J. Jobe and R. E.Verrall' Department of Chemistry, University of Saskatchewan, Saskatoon, Saskatchewan S7N 0 WO, Canada
B. D. Skalski Faculty of Chemistry, A. Mickiewicz University, 60- 780 Poznan, Poland Received January 19,1992. I n Final Form: August 9,199P Time-resolved fluorescence intensity measurements of 2,3-dimethylnaphthalene (2,3-DMN) in decyltrimethylammonium bromide (CloTAB) micellar systems in the absence and presence of 1-propanol, 1-butanol,1-pentanol,and 2-(n-butoxy)ethanolhave been carried out at 25 O C . Fluorescence quenching of the fluorophore by both cetylpyridiniumbromide (CpyB) and the bromide (Br)counterions of the micelles has been observed. The quenching kineticsof the B r counterionwere estimated by f i t studying the fluorescence quenching of 2,3-DMN by CpyB in sodium dodecyl sulfate micellar systems where it is assumed that there is little or no quenching by the counterion. In the absence of alcohols,the fluorescence quenching of 2,3-DMN by B r shows that 2,3-DMN becomes more accessible to BF as the hydrocarbon chain length increases in the surfactant series CloTAB, C12TAB, and C1tjTAB. This is possibly due to a decrease in the thickness of the Stern layer with increasing surfactant hydrocarbon chain length. The addition of alcohols to the micellar phase of CloTAB increases the accessibilityof B r to 2,3-DMN, and this accessibility increases with increasing hydrocarbon chain length of the alcohol. This result may be due to decreased solvation of the head groups of the micellized surfactants as the alcohol chain length increases.
Introduction The properties of mixed micelles of surfactant and alcohol are usually very different from those of the original surfactant micelle. Recently, we have reported ultrasonic relaxation studies of several alcohols in micelles of decyltrimethylammonium bromide.lP2 To assist in our interpretation of the ultrasonic data using the Aniansson theory, mean aggregation numbers (n) and the fraction of counterions dissociated from the surface of the micelle (8) were obtained from fluorescence and conductivity measurements, respectively. The results of a number of studies1-l1of mixed micelles have shown that these two micellar properties appear to be most affected by the presence of alcohol. For many mixed alcohol-micelle systems, 8 usually increases as the mole fraction of alcohol in the micellar phase increases. The variation in n is often found to depend on the alcohol-micelle system; however, n generally decreases with increasing alcohol content in the micellar phase. e Abstract Dublished in Advance ACS Abstracts. October 1.1993. (1) Ai&, E.; Jobe, D. J.; Skalski, B.; Verrall, R. E. J.Phys. Chem. 1992,96, 2348. (2) Jobe, D. J.; Verrall, R. E.; Skalski, B.; Aicart, E. J. Phys. Chem. 1992,96, 6811. (3) Zana, R.; Yiv, S.;Strazielle, S.;Lianos, P. J . Colloid Interface Sci. 1981, 80, 208. (4) Lianos, P.; Zana, R. Chem. Phys. Lett. 1980, 76, 62. (5) Almgren, M.; Swamp, S. J. Colloid Interface Sci. 1983, 91, 256. (6) Gritzel, M.; Thomas,J. K. J. Am. Chem. SOC.1973,95,6885. (7) Almgren, M.; Ufroth, J.-E. J. Colloid Interface Sci. 1981,81,486. (8) Gettins, J.; Hall, D.; Jobling, P. L.; W i n g , J. E.; Wyn-Jones, E. J. Chem. Soc., Faraday Trans. 2 1978, 74, 1967. (9)Lianos, P.; Zana, R. J. Colloid Interfuce Sci. 1984, 101, 587. (10) Muto, Y.; Yoda, K.; Yoshida, N.; Esumi, K.; Meguro, K.; BinanaLimbele. W.: h a , R. J. Colloid Interface Sci. 1989. 130. 166. (11)Lianos, P.; Lang, J.; h a , R. J. Phys. Chem.' 1982,86, 4809.
0743-7463/93/2409-2814$04.00/0
Quina et al.12J3have studied the role of counterions in micelle formation and their abilityto exchangewith various other ions a t the surface of the micelle. One interesting result of these studies was the observation of counterion quenching of the fluorescence from micelle-solubilized aromatic hydrocarbons. Some counterions, such as B r and I-, strongly quench the fluorescence from aromatic hydrocarbons solubilized in these systems. Their studies have shown that the exchange of these counterions is very rapid and that the fluorescence from the micelle-bound fluorophore is dynamically quenched by thq,counterions. If it is assumed that probe-probe interactions are negligible, that the fluorophore is completely s o l u b i l i i by the micelle, and that only counterion quenching of the fluorophore occurs, the inverse fluorescence lifetime (rl) of the fluorophore is given as12 =
+ K,e
(1)
where 70 is the fluorescence lifetime of a micelle-bound fluorophore in the absence of any quenching counterion, k, is the pseudo-fit-order quenchingrate constant when there is unit coverage of the surface by the quenching counterion, and 0 is the fraction of the surface covered by the quenching counterion. For micelles containing only one type of counterion
e=i-p (2) The quenching constant R, is related to the bimolecular quenching constantin homogeneous solution (koq)by12J4J6 (12) Abuin, E.; Lissi, E.; Bianchi, N.; Miola, L.; Quina,F. H. J. Phys. Chem. 1983,87,6166. (13) Quina,F. H.; Chaimovich, H. J. Phys. Chem. 1979,83,1844. (14) Almgren, M.; Grieser, F.; Thomas,J. K. J. Am. Chem. SOC.1979, 101,2021.
0 1993 American Chemical Society
Intramicellar Fluorescence Quenching 2,3-DMN
Langmuir, Vol. 9, No.11, 1993 2815 (3)
vq
The fluorescence quenching of micelle-bound fluorophores by quenchers (Q) other than the counterion can also provide information about the micelle. Such methods have long been used to calculate micelle aggregation numbers1618 where, for a strongly bound quenchep fluorophore pair, the time (t) resolved fluorescence intensity (I(t)) from the quenched fluorophore is given by
where is the effective volume per mole of micellized surfactant, Fa, is a factor which takes into account any changes in the accessibility of the fluorophore by the quencher due to the presence of the micellar environment, and F m is a factor which takes into account any intrinsic changes in the mechanism of the quenching due to the presence of the micelle environment. I(t) = A, exp{-A2t - A&- exp(-A,t)I) (8) For a typical quencher-probe pair, the separation of the factors J" and Fa,has been rather a r b i t r ~ r y , ~and ~ J ~ J ~ where A1 is the fluorescence intensity from the fluorophore just after excitation (Le., at t = 01, A2 is the inverse they are usually assigned the values of l/3 and l/2, fluorescence lifetime of the fluorophore in the absence of respectively. However, other approaches4J6have looked Q (Le., A2 = 1/71, A3 is the ratio of the quencher at the quenching process as being diffusion controlled, concentration to the micelle (MI concentration (CQ/&), having components which are both radial and tangential and A4 isthe pseudo-fmborderrate constant for quenching tothe micelle surface. Thisis especiallytrue for fluorescent of the fluorophore when only one quencher is present in probes which are preferentially solubilized in the surface the micelle. These curves generally have a "double" region. In the case of quenchingby counterion,quenching exponential character where the fast initial fluorescence due to tangential diffusion of the probe is proportional decay is usually attributed to the contribution of the A3 only to the fraction of counterions present at the micelle and A4 terms, while the longer lived fluorescence comes surface. Therefore, the factors that contribute most to predominately from the A2 term. quenching by counterions are the radial diffusion of the Thus, if a fluorophore is contained in a micelle which probe acrose the micelle and the fraction of counterions contains a quencher Q and there are nearby counterions, covering the surface. both of which are able to quench the fluorophore, the Indeed, this is what was found by Gratzel et ala6for the resulting time-dependent (as well as time-averaged) fluquenching of l-bromonaphthalene by the bromide counorescence intensity will be a convolution of these two terions in cetyltrimethylammonium bromide (CleTAB) quenching processes and the analysis can become commicelles. By adding excess bromide ions and using Fick's plicated. However, for surfactants which contain B r as diffusion theory, the fluorescent lifetime of the probe was the counterion, the fluorescence from the micelle-bound found to correspond to the maximum time necessary for fluorophoresis usually single exponential in nature. Under the probe to diffuse the radius of the micelle and encounter such conditions, we can assume that the quenching a counterion at the surface of the micelle. Therefore, for contribution from the B r in eq 8 is contained in the term counterion quenching, rather than arbitrarily assigning A2, while the quenchingfrom the micelle-bound quencher values to the constants F m and Fac, they should be is contained in the A3 and A4 terms. Therefore, the value interpreted in terms of the radial diffusion of the probe of T obtained from A2 can be used to evaluate the in the micelle. This interpretation also implies that the fluorescence quenching by B r in the mixed surfactantquenching volume encompasses the whole micelle (Vm) alcohol micelle using eq 7. and that V, can be derived from this quantity. In order to evaluate the photophysical characteristics Substitution of eqs 2 and 3 into eq 1yields of counterion quenching in mixed alcohol-surfactant micelles, the coefficient for the distribution of the alcohol 7 - l 7 ~i 1 ko,Fa$"(l - @)/Vq (4) between the micelle and the bulk phase must also be obtained. Recently,lg electrical conductivity measureFor a micelle-bound fluorophore quenched by the counments have been shown to be an effective method for terion, increasing the mole fraction of alcohol in the obtainingthe binding constanta for various alcohols mixed micellar phase will strongly affect the resulting fluoreswith alkyltrimethylammoniumbromide micelles. This cence,as many of the micellar parameters in eq 4 are altered method has the added advantage of yielding a value for by the presence of the alcohol. Aside from the expected @ for these systems. increase in 8, V, will also change due to a change in n and In this paper, studies of the fluorescence emission from V,. For a mixed micelle of surfactant and alcohol, Vm can 2,3-dimethylnaphthalene (2,3-DMN) quenched by both be approximated by6 cetylpyridinium bromide (CpyB) and the B r counterion Vm = nu, + aVa (5) are reported. These studies were made in both normal micelles and mixed micelles of CloTAB containing l-prowhere Vsand Va are the partial molar volumes of the pan01 (Pr), 1-butanol (Bu), l-pentanol (Pe), or 2-(nsurfactant and alcohol, respectively, and a is the number butoxy)ethanol (BE). The aggregation numbers for the of alcohol molecules per micelle. Therefore, for a mixed surfactant and the alcohol have been determined previalcohol-surfactant micelle, U, can be approximated by ously.1~2In this study the quenching kinetics of the B r counterion are evaluated. V, = vm/n= (nVs+ aVa)/n (6) Experimental Section and substitution of = V d n into eq 4 yields The surfactants decyltrimethylammoniumbromide (CloTAB) (Kodak-Eaetman, 99%), dodecyltrimethylammonium bromide (7) (C12TAB)(Sigma, 99% ), hexadecyltrimethylammoniumbromide (ClaTAB)(Sigma, 99%), and sodium dodecyl sulfate (SDS) Therefore, a plot of r1versus 0 - @In/Vm should be linear with a slope equal to koqFa$m and an intercept equal to (16) Malliaris, A.; Lang, J.; Zana, R. J. Chem. SOC.,Faraday Tram. the inverse fluorescence lifetime for the fluorophore in 1 1986,82, 109. the micelle in the absence of any quencher. (17) Yekta, A.; Aikawa, M.; T w o , N. J. Chem. Phys. Lett. 1979,63,
+
vq
543. (15) Dederen, J. C.; Van der Auweraer,M.; De Schryver, F.C . J. Phys. Chem. 1981,85,1198.
(18) Tachiya, M. Chem. Phys. Lett. 1975,33,289. (19) Abu-Hamdiyyah, M.; Kumari, K. J. Phys. Chem. 1990,94,2518.
2816 Langmuir, Vol. 9, No.11, 1993
Jobe et al.
Table I. Values of Coefficients in Equation 8, cmc, 8, and II for Aqueous C. (T = 25 "C) surfactant C., mol dm-3 CQ,mmol dm-3 cmc, mmol dm4 CioTAB 0.120 3.2 65 CioTAB 0.130 1.1 65
CizTAB
Cl6TAB SDS SPFO SPFO
SPFO
0.155 0.058 0.095
0.060 0.090 0.090
3.0 0.42 3.0 1.0 2.0 4.0
15
0.80
Systems Containing Surfactants of Concentration
B
n
0.35 0.35 0.29 0.16
39 37 50 83 65 9 10 10
8.0 32 32 32
(BDH,99%)were twice recrystallizedfrom an acetone/methanol mixture (90:lOv/v). Sodium perfluorooctanoate (SPFO) (PCR Inc., 99%) was used as supplied. The fluorophore 2,3-DMN (Lancaster Synthesis, 99%) was purified using TLC, and a micellar stock solution was prepared from the purified fraction. The alcohols1-propanol(Pr) (Fisher,>99%) and 1-butanol(Bu) (Fisher, >99%) were used as supplied, while the alcohols 2-(nbutoxy)ethanol (BE)(Aldrich,99%)and 1-pentanol(Pe) (Terrochem, 99%) were purified using vacuum distillation. The quencher CpyB (Sigma,99%) was recrystallizedfrom hot water. The proceduresfor canying out the fluorescenceand conductivity measurements as well as the analysis of the data have been previously described.lP2 The distribution coefficients of the alcoholswere obtained from an analysis of the conductivitydata and have the values 0.49,1.2,1.6, and 4.9 dm3mol-' for Pr, BE, Bu, and Pe, respectively, and are very close to the values for these alcohols in other surfadant systems.8.20
Results and Discussion Surfactant Systems. Due to the low solubility of 2,3DMN in water, it was difficult to obtain time-resolved fluorescence data for DMN in water. Similar difficulties were observed when trying to obtain this measurement for ~,EP-DMN.~~ However, 2,3-DMN dissolves readily in aqueous micellar surfactant solutions. Table I shows the values of the coefficients in eq 8 obtained by fitting the time-resolved fluorescence data for those systems containing only surfactant of concentration C,. Most of this data has not been previously reported. The values for the critical micelle concentration (cmc),8, and n are also given. The values of A2 for 2,3-DMN in the alkyltrimethylammonium bromide micelles are much larger than the value obtained for 2,3-DMN in SDS, 0.023 ns-l. This is consistent with the view that 2,3-DMN is quenched by the B r counterion in the alkyltrimethylammoniumbromide micelles. In the SDS micelle, it is assumed that there is little or no quenchingby the counterion as the negative surface of the SDS micelle will repel the counterions of the quenching agent but not the C,,+ cation. A large decrease in the fluorescence lifetime of 2,3-DMN in SPFO micelles compared to the value for 2,3-DMN in a SDS micelle is also observed. The shorter fluorescencelifetime for 2,3-DMN in SPFO must be due to the presence of the fluorocarbon environment of the SPFO micelle as Na+ does not quench the fluorescence. If it is assumed that the reciprocal of the A2 value for 2,3-DMN in a SDS micelle is the fluorescence lifetime of 2,3-DMN in a micelle in the absence of any quenching counterions, then TO = 43 ns and the product F&'ac can be calculated for B r quenching in the alkyltrimethylammonium bromide micelles from FmFaC= [ ( ~ d -d1)V$k',~~(l- 8) (9) where 7 is the inverse of A2 for 2,3-DMN solubilized in the alkyltrimethylammoniumbromide micelles. Because of (20) Marangoni, D. G.; Kwak, J. C. T. Longmuir 1991, 7,2083. (21)flkawa, M.; Yekta, A.; Liu, J.-M.; Turro, N. J. Photochem. Photobrol. 1980, 32, 291.
Az, ne-' 0.055
As
0.054 0.068
2.06 0.599 1.09
0.069
0.609
0.023 0.058 0.056 0.065
2.28 0.315 0.340 0.667
Ad, 0.112 0.104 0.170 0.172 0.064
0.409 0.252
0.409
Table 11. Values of Micelle Viscosity, %, and Factors Fm and . F of Equation 4 for the Homologous Surfactant Series CioTAB to CiaTAB ( T =25 "C)
surfactant CioTB
C12TAB
V,,dmSmol-l
qm,cP
0.262 0.296 0.360
14 18 22
F, 0.40 0.52
0.70 0.78
0.54
0.80
F.e
the low solubility limit of 2,3-DMN in water, the value of Izo, = 4.8 X lo7 M-' s-l for B r quenching of naphthalene in water22was used in the calculation of F&'ac. As noted previously, the separation of the productF a m is rather arbitrary and the calculated values depend on the model chosen. Since the model used here depends only on the degree of counterion binding and the effect of the micelle on the radial diffusion of the probe, the intrinsic factor, Fm, will reflect the effect of the micelle interior on the diffusion of the probe (e.g., the micelle microviscosity, etc.). It has been shown that, although aromatic molecules are preferentially solubilized at the surface of the micelle, they can be distributed throughout the m i ~ e l l e . ~ Therefore, ~ * ~ ~ radial diffusion of the probe could occur over the radius of the micelle volume, V m ,and the intrinsic factor, Fm, may be approximated by
F, = tm/tw where t , is the time necessary for the fluorophoreto diffuse through the micelle to the surface and be quenchedby Br and t, is the time necessary to cover the same distance in water. If it is assumed that quenching occurs at the micelle surface, then the maximum distance that 2,3-DMN may have to diffuse is the radius of the micelle @) in order for it to be quenched by Br.6 According to Fick's diffusion theory, t , and t , can be approximated by t, = (2?r?/rNz2)/(k0, x
io3)
t , = (3?d27,r~)/(kT x
lo6)
(11)
and (12) where N is Avogadro's number, r r is ~ the radius of a 2,3DMN molecule, k is the Boltzmann constant, T is the absolute temperature, and 7, is the viscosity in the core of the micelle. Therefore, division of eq 12by eq 11yields
F, = (3v,k0,)/(2000NkT) = O.0291vm (13) where qm is given in centipoise. The values of vm,Fm, and Fac for each of the surfactants are given in Table 11. The values of Fac appear to increase with increasing hydrocarbon chain length of the surfactant, indicatingthat 2,3-DMN is becoming more accessible with increasing chain length. "his may be due to a decrease in the thickness of the Stem layer that occurs when the (22) Shizuka,H.; Nakamura, M.; Morita, T. J.Phye. Chem. 1980,84, 989.
(23) Mukerjee, P.; Cardinal, J. R. J. Phys. Chem. 1978,82, 1620. (24) J o b , D. J.; Verrd, R. E. Langmuir 1990,6, 1750.
Langmuir, Vol. 9, No. 11, 1993 2817
Intramicellar Fluorescence Quenching 2,3-DMN
Table 111. Values of Coefficients in Equation 8, cmc, B, and n for Mixed Cl0TAB-2-(n-Butox~)ethanolSystemfi of Different Concentrations
CS,mol dm3 0.120 0.120 0.120 0.120 0.120 0.100 0.080 0.050 0.035
CBE,mol dm3 0.200 0.381 0.562 0.724 0.830 0.830 0.830 0.830 0.830
cmc, mol dm4 0.048 0.039 0.030 0.024 0.020 0.020 0.020 0.020 0.020
1@cQ,mol dm4 3.4 3.4 3.4 3.4 3.4 3.0 3.0 3.0 3.0
0.060
0.060
0.050
0.050
0.040
0.040
0.030
0.030
B
n
a
0.37 0.42 0.64 0.67 0.81 0.81 0.81 0.81 0.81
31 30 28 26 26 20 19 13 10
9 20 35 49 63 51 66 66 58
Az, ne-' 0.046 0.041 0.037 0.033 0.032 0.026 0.026 0.026 0.024
Aa 1.76 1.40 0.98
0.86 0.66 0.59 0.70 0.84
0.87
Ad, ne-' 0.118 0.121 0.122 0.139 0.136 0.131 0.126 0.119 0.102
1 1.00 2.00 3.00
0.020 0.00
n(l-B)/vm 0.060 :
0.050
0.040 :
0.030
0.030 1
0.020 0.00
1.00
2.00
j/
3.00
n( 1 -B ) /Vm
7 1 versus n(1- b)/V, for those solutions containing (a, top left) CloTAB + Pr (01, (b, top right) CioTAB + BE (a),(c, bottom left) CloTAB + Bu (01, and (d, bottom right) CmTAB + Pe (W.
Figure 1.
surfactant chain length is This would allow the Br- to be closer to the surface of the micelle and therefore be more accessible to the solubilizedfluorescence probe. Mixed CloTAB-Alcohol Systems. For the systems containing CloTAB alcohol, the best fit parameters of the time-resolved fluorescence data to eq 8 for the alcohols Pr, Bu, and Pe have been previously reported.1*2However, the data for the system containing CloTAB BE are given in Table I11 as these data have not been previously reported. In all systems,at a fixed quencher concentration, the value of As decreases as the ratio of alcohol to CloTAB increases. This indicates that the meanmicelle aggregation number decreases as the fraction of alcohol in the micelle increases. The value of A2 is also affected by the presence of alcohol. It decreases as the amount of alcohol in the micelle increases. This is consistent with the observed increase in 8, as the amount of quenching by the counterion will decrease when 8 increases.
+
+
(25)Ben, S.S.J. Phys. Chem. 1987,91,4760. (26)Ben, S.;Jones, R. R. M.; Johnson, J. S. J. Phys. Chem. 1992,96, 5611.
Figure 1 shows plots of r1versus (1 - O)n/Vmfor the four alcohol-CloTAB systems studied. The lines drawn through the points are the least-squares fits of the data. These plots appear linear, yielding correlation coefficienta between 0.92 and 0.98. The values of used to calculate Vmwere 0.262,0.072,0.092,0.102, and 0.122 dm3mol-' for CIOTAB,Pr, Bu, Pe, and BE, respe~tively?~-~~ The intercepts of these plots yield an average value of TO = 46 f 3 ns. This value is in agreement with the value of 43 ns for 2,3-DMN in a SDS micelle and is close to the values of 36 and 43 ns for naphthalene in water and SDS micelles, respe~tively.~~ Assuming the value of ko, is the value for the bimolecular quenching constant of naphthalene by Br-, the slopes of these plots yield the factor F a a c . These values were found to be 0.26,0.26,0.28, and 0.31for Pr, BE, Bu, and Pe, respectively. The value of the
v
(27)DeLisi, R.; Milioto, S.;Triolo, R.J. Solution Chem. 1988,17,673. (28)De Lisi, R.;Ostiguy, C.; Perron, G.; Desnoyers, J. E. J. Colloid Interface Sei. 1979,71, 147. (29)De Lisi, R.;Milioto,S.;Triolo,R. J. Solution Chem. 1989,18,906. (30)Van Bockstaele, M.; Gelan, J.; Martens,H.; Put, J.; Dederen, J. C.; Boens, N.; De Schryver, F. C. Chem. Phys. Lett. 1978,68,211.
Jobe et al.
2818 Langmuir, Vol. 9, No. 11, 1993
product F a s m in these systems appears to have a slight dependence on the hydrocarbon chain length of the alcohol. In their study of the effects of various alcohols on qm of SDS micelles, Lianos et al.ll have shown that the initial addition of 1-alcohols decreases the qm rapidly, ita value falling to ca. 9-11 cP.11*31This value was found to be almost independent of the chain length of the alcohol. As with pure micellar systems, if radial diffusion is assumed and qm is taken to be this limiting value, then F m = 0.32 and Fa, ranges in magnitude between 0.75 and 0.95. It should be noted that these values are model dependent and should be interpreted as such. It is noteworthythat Fa, increases with the chain length of the alcohol and that the accessibility of the fluorophore in the mixed micelle systemsis greater than in the absence of alcohol. Relative to regular CloTAB micelles, the systems containing Bu and Pe are 119% and 14 % ,respectively, more effective in making 2,3-DMN accessible to Br- quenching whereas BE and Pr only increase the accessibility of 2,3-DMN by 7 % The dependence of the surface properties of the mixed micelle on the hydrocarbon chain length of the alcohol, at least for Pr and the longer chain alcohols, has been observed by othersS3 The increase in Fa, with alcohol hydrocarbon chain length may arise if the distance between 2,3-DMN and B r decreases due to the presence of the alcohol. This could result if the binding of the alcohol causes either a decrease in the thickness of the Stern layer or a shift in the principal solubilization site of 2,3-DMN within the micelle. The first possibility seems unlikely as the addition of alcohols increases /3 and the (potential of the micelles decreases. Consequently, the Stern layer thickness increases and becomes more d i f f ~ s e . ~Since . ~ the binding constant for the alcohol increases with ita hydrocarbon chain length, longer chain alcohols would bind to the micelle more strongly and decrease the ( potential more effe~tively.~ The second possibility seems more likely; solvent penetration into the micelle decreases with increasing hydrocarbon chain length of the alcohol. This decrease in solvation moves the micellelwater interface closer to the head groups, and 2,3-DMN will eventually move closer to the surface of the micelle and become more accessible to B r . Yet a third possible explanation is that a decrease in the dielectric permittivity occurs at the surface of the micelle due to the displacement of water molecules by This may cause more 2,3-DMN molecules to be located at the surface and, hence, to be more accessible to the counterions. The second and third possibilities are supported by the behavior of the dependence of the charge density at the micelle surface on the mole fraction of alcohol in the micelle, Xam (Figure 2). The charge density, q, at the surface can be approximated by6
0.6
0.4
0.2
0.0
I 0.40 0.80 x,
0.00
Figure 2. qle versus X,mforthose solutionscontaining CloTAB Pr (01, CloTAB + BE (01,CloTAB + Bu (01, and CIOTAB+ Pe (m).
+
.
qle = /3nl4rRm2
(14) where R m is the radius of a sphere having a volume Vm. The curves in Figure 2 show that the change in qle at the surface is strongly dependent on the composition of alcohol in the micellar phase as well as the hydrocarbon chain length of the alcohol. For all of the alcohols, there is a sharp decrease in qle at higher alcohol compositions in the micelle. Furthermore, the values of Xamwhere these changes occur appear to correlate with the binding strength of the alcohols; i.e., the least strongly bound alcohol requires more alcohol in the micellar phase to induce a change. (31)Lianos, P.;Lang,J.;Strazielle, C.; Zana, R. J. Phys. Chem. 1982, 86, 1019.
1.0
0.8
0.6
The change in solvation of the micelle head groups with the variation in the hydrocarbon chain length of the alcohol is also reflected in the fractional coverage of the micelle surface by alcohol and surfactant molecules. The average surface areas of an alcohol (gal and surfactant (aa)head group are ca. 0.20and 0.80 nm2,r e s p e c t i ~ e l y Therefore, .~~~ the fraction of the micelle's surface (y)that is covered by the surfadant and alcohol head groups is given by
+
y = (nu8 aaa)/4~Rm2
(15)
Figure 3 shows a plot of y versus Xem.For a fixed mole fraction of alcohol in the micelle phase, the fraction of the total surface area covered by surfactant and alcohol head groups is smaller for the Pr or BE mixed micelles. Thie observation and the behavior of the surface charge density indicate more solvent penetration occurs in the Pr and BE mixed micelles and supports the view that head group solvation decreases as the hydrocarbon chain length of the alcohol increases. The sharp change in slope for both the y and q/e plots at ca. Xam= 0.7 is interesting. Thisdecrease may indicate the concentration where the micelle starta to break down to form loosely packed or "hydrated" surfactant-alcohol fragments.14*32* Flu~rescence~~ and ultrasonic relaxation s p e c t r ~ a c o p y ~studies * ~ * ~ of mixed surfactant-alcohol micelles suggest that the formation of these fragments occurs at a high alcohol concentration. However, whether (32)Malliarie, A.;Laug, J.; Sturm, J.; Zana, R.J. Phys. Chem. 1987, 91, 1475.
(33) Kato, S.; Job, D.J.; Rao,N.P.;Ho,C. H.;Verrall, R.E.J. Phys. Chem. 1986,90,4167.
Intramicellar Fluorescence Quenching 2,3-DMN
these “fragments” break away from the micelle or are formed in the aqueous phase is uncertain.
Conclusion The B r counterion is found to quench the fluorescence lifetime of 2,3-DMN in CloTAB micelles, and the degree of quenchingis related to the fraction of counterion bound to the micelle surface as well as the effective volume per mole of surfactant. For alkyltrimethylammoniumbromide micelles in the absence of alcohol, the accessibility of 2,3DMN to quenching by B r is found to increase with the hydrocarbon chain length of the surfactant, possibly due to a decrease in the thickness of the Stern layer. The addition of alcohols to the micellar phase of CloTAB aqueous systems appears to increase the accessibility of
Langmuir, Vol. 9, No. 11,1993 2819
2,3-DMN to B r . Furthermore, the accessibility appears to increase with the hydrocarbon chain length of the alcohol. This may be due to decreased solvation of the head groups of the surfactants within the micelle which may lead to a lowering of the dielectric permittivity and/ or a decrease in the distance between the micelle interface and the B r , either of which brings the B r ions and 2,3DMN closer together.
Acknowledgment. We thank the Natural Sciencesand Engineering Research Council of Canada for financial support. B.D.S. thanks the President’s NSERC fund of the University of Saskatchewan for partial funding of a postdoctoral fellowship.
