J . Phys. Chem. 1986, 90, 5258-5265
5258
Effect of Alcohols and Oils on the Kinetics of Micelle Formation-Breakdown in Aqueous Solutions of Ionic Surfactants Jacques Lang* and Raoul Zana Institut Charles Sadron (CRM-EAHP) and Greco Microemulsions, CNRS, F-67000 Strasbourg, France (Received: February 12, 1986; In Final Form: April 25, 1986)
The temperature-jump method has been used for an extensive study of the dependence of the relaxation time 7 2 characterizing the process of micelle formation-breakdown on the surfactant concentration C and upon additions of alcohols and oils to moderately concentrated surfactant solutions. The vs. Ccurves have been found to be V-shaped, as in previous studies. For the four surfactants investigated, the increase of 1 / 7 2 at high C has been interpreted in terms of rapid coagulationfragmentation processes postulated by Kahlweit (Kahlweit, M. Pwe Appl. Chem. 1981, 53, 2060. J . Colloid Interface Sci. 1982,90,92). The changes of 72 have been found to be very sensitive to the nature, chain length, and concentration of both the added alcohol and oil. Additions of medium chain length alcohols (up to 1-hexanol) increase dramatically l/rz by up to a factor of IO4. Additions of oils to mixed surfactant + alcohol micellar solutions result in the opposite effect. Longer chain alcohols behave like medium chain length alcohols at low alcohol concentration and like oils at high alcohol concentration, when they become solubilized in the micelle core. Additional measurements by means of time-resolved fluorescence probing and quasi-elastic Pght scattering have been performed in order to obtain information on the micelle aggregation number and the micelle size and polydispersity of some of the investigated systems. The observed changes of I/rz upon addition of alcohols and oils have been interpreted in terms of variations of the concentration of the species around the minimum of the size distribution curve and, in turn, in terms of the polydispersity of the systems and its effect on the micelle breakdown-formation, in agreement with the quasi-elastic light-scattering results. The relationship between the present chemical relaxation results on micellar kinetics and those obtained recently by time-resolved fluorescence probing is discussed.
Introduction The kinetics of micelle formation has been extensively studied in pure surfactant solutions by means of chemical relaxation Kinetic results, although in fewer number, have also been reported for more complex systems as, for instance, alcohol-swollen r n i ~ e l l e sor~ oil ~ ~ in water (O/W) microemulsions.6 Recall that in relatively dilute solutions of pure surfactants two relaxation processes are usually observed which have been first accounted for by Aniansson and Wall.' The fast process is due to the exchange of surfactants between micelles and surrounding solution and the slow one to the reaction of micelle breakdownformation. Strictly, the theory of Aniansson and Wall applies to nonionic surfactants and is based on the assumption that the formation or breakdown of micelles occurs by successive uptakes or releases of one surfactant at a time. This theory has been later modified by Hall* and Lessner et aL9 to take into account the reaction of association/dissociation of counterions to/from micelles. This modification, however, led to minor changes in the expressions of the relaxation times given by Aniansson and Wall. A more important extension of the theory was presented by Lessner et al.,I&l2who took into account the reversible coagulation ~
~~~
~~
~
(1) Hoffmann, H. Ber. Bunsen-Ges. Phys. Chem. 1978, 82, 988. (2) Kahlweit, M.; Teubner, M. Adu. Colloid Interface Sri. 1980, 13, 1. (3) Aniansson, E. A. G.; Wall, S. N.; Almgren, M.; Hoffmann, H.; Kielmann, I.; Ulbricht, W.; Zana, R.; Lang, J.; Tondre, C. J . Phys. Chem. 1976, 80, 905. (4) Yiv, S.; Zana, R.; Ulbricht, W.; Hoffmann, H. J . Colloid Interface Sci. 1981, 80, 224. (5) Uehara. H . J . Sci. Hiroshima C'nic..,Ser. A: Phys. Chem. 1976, 40, 305. (6) Lang, J. Proceedings of the 5th International Symposium on Surfactants in Solution, Bordeaux, 1984; Mittal, K. L., Bothorel, P., Eds.: Plenum: New York, in press. (7) Aniansson, E. A. G.; Wall, S . N. J . Phys. Chem. 1974, 78, 1024; 1975, 79, 857. In Chemical and Biological Applications of Relaxation Spectrometry; Gettins, W. J., Wyn-Jones, E., Eds.; Reidel: Dordrecht, Holland, 1975; p 223. (8) Hall, D. G. J . Chem. S o t . , Faraday Trans. 2 1981, 77, 1973. (9) Lessner, E.; Teubner, M.; Kahlweit, M. Ber. Bunsen-Ges. Phys. Chem. 1981, 82, 1529. (10) Lessner, E.; Teubner, M.; Kahlweit, M. J . Phys. Chem. 1981, 8 5 , 3167.
(1 1 ) Kahlweit, M. (a) Pure Appl. Chem. 1981, 53. 2060. (b) J . Colloid Interface Sci. 1982, 90, 92. (12) Kahlweit, M. In Physics of Amphiphiles, Micelles, Vesicles and Microemulsions; Degiorgio, V., Corti, M.. Eds.; North-Holland: Amsterdam, 1985; p 212.
0022-3654/86/2090-5258$01.50/0
of submicellar aggregates in the calculation of the relaxation time 72 associated with the slow process. The expression of 7 2 derived by these authors is very different from the one given by Aniansson and Wall and appears to be particularly appropriate for interpreting the data reported for nonionic surfactant solutions even at concentrations close to the cmc and for ionic surfactant solutions at high concentration or high ionic strength." In the present work we have carried out a systematic study of the effect of additives on 7 2 for concentrated micellar solutions of ionic surfactants. The additives were alcohols and/or oils, leading in this last case to the formation of O / W microemulsions. Indeed, the relatively high surfactant concentrations used in this work allowed the dissolution of appreciable amounts of alcohol and oil. Some of the systems contained up to 6% alcohol and 8.5% oil in volume. Recall that microemulsions have attracted much interest in the past decade due to their actual or potential applications in fields as different as paints, cosmetics, pharmacology, oil recovery, and cleaning and also for theoretical reasons.I3 Many studies at equilibrium have been undertaken on these systems but only little is known about their dynamical properties. The main purpose of the present work was to fill this gap by obtaining kinetic data concerning the progressive generation of O/ W microemulsions, starting from a micellar solution. The results are given in the order of increasing complexity: (1) effect of the surfactant nature and concentration on T ~ (2) ; variation of T~ upon addition of an increasing amount of alcohol to concentrated surfactant solutions; ( 3 ) variation of 7 2 upon oil additions to some of the preceding mixed surfactant alcohol micellar solutions thereby generating O/W microemulsions; and (4) effect of sodium chloride additions on 7 2 for pure micellar solutions, mixed surfactant + alcohol micellar solutions, and O/ W microemulsions. Some values of the surfactant aggregation number in selected surfactant solutions containing alcohol and/or oil have been determined and are used in the Discussion section.
+
Experimental Section Materials. Tetradecyl- and hexadecylpyridinium bromides (TPyB and HPyB) were obtained by reacting pyridine with bromo- 1-tetradecane or bromo- 1-hexadecane in dry ethanol for 24 h at 135 O C in an autoclave. They were purified by two crystallizations in ethyl acetate and two crystallizations in acetone. (13) Widom, B. J . Chem. Phys. 1984, 81, 1030.
0 1986 American Chemical Society
Effect of Alcohols and Oils on Micelle Kinetics Tetradecyl- and hexadecylpyridinium chlorides and hexadecyltrimethylammonium chloride (TPyC, HPyC, and HTAC) were obtained by exchanging the bromide ions of TPyB, HPyB, and hexadecyltrimethylammonium bromide (Merck) for chloride ions with an ion-exchange resin (Merck 111). TPyC was purified by three crystallizations in ethyl acetate. HPyC was purified by two crystallizations in ethyl acetate and one crystallization in a water-acetone mixture. HTAC was purified by three crystallizations in ethanol-ethyl acetate mixtures. The purity of all of the surfactants was checked by elementary analysis of their carbon, hydrogen, nitrogen, and bromine or chlorine contents. All other products, alcohols (ethanol to 1-octanol, 1-decanol, and benzyl alcohol), alkanes (n-hexane to n-decane, n-dodecane, n-tetradecane, cyclohexane), toluene, and butylbenzene, were of various origins but always of high purity. Methods. The relaxation times were measured with the Joule heating temperature-jump technique using optical d e t e ~ t i o n . ' ~ The relaxation transients were detected on the UV absorption band of the surfactant for wavelengths between 280 and 295 nm in the case of TPyC and HPyC and between 290 and 325 nm in the case of TPyB and HPyB. A small quantity of HPyC or HCNPyI (1-hexadecyl-4-cyanopyridinium iodide) was added to the solutions of HTAC in order to detect the relaxation process since HTAC does not bear any chromophore group in the UV or visible range. Indeed, it has been shown that dyes can be used for the measurement of the long relaxation time r2 provided that the [dye] / [surfactant] molar concentration ratio is kept sufficiently N o variation of 72 was observed in our systems when the HPyC or HCNPyI to HTAC molar ratio was increased from In addition, we have found that when the anionic dye to eosine was added to pure micellar solutions, mixed micellar solutions or O /W microemulsions made of alkylpyridinium surfactants, the detection of the relaxation transients on the absorption band of eosine (in the visible range) or on the absorption band of the surfactant (in the UV range) yielded identical values of 7 2 , within the experimental error, for [eosine]/ [surfactant] molar This result shows that whatever ratios between 0 and 3 X the micellar systems under study the addition of small quantities of dye does not perturb the relaxation process. It also gives an argument in favor of the assignment of the observed relaxation process to the reaction of micelle breakdown-formation. Indeed, if it were due to the surfactant exchange between micelles and bulk, the relaxation times measured in the UV range (for the surfactant) and in the visible range (for eosine) would certainly have been different owing to the difference between these two types of molecules.I6 Other arguments given in the Discussion section confirm the fact that the observed relaxation transients can be attributed to the slow process. For the HTAC systems, relaxation signals were detectable only on the absorption band of the added HPyC or HCNPyI. Nevertheless, the corresponding 7 2 values lie very close to the ones found for HPyC systems and, therefore, can be attributed to the same process. All the values of T~ are plotted in the figures as decimal logarithms of 1 / 7 2 with 7 2 in seconds. The magnitude AT of the temperature jump applied to the solutions was dependent on the system investigated. AT was kept as low as possible while still allowing a precise determination of 7 2 . In most cases AT was less then 1.8 O C . In the few cases where the relaxation amplitudes were small, AT as high as 4.1 O C was applied to the solutions. The final temperature Tf was 25 OC in most of the studies. Lower temperatures (15 "C) were used in order t o increase 7 2 when the relaxation signal at 25 OC was too fast to be resolved with our equipment. Higher temperatures (30 ~
(14) Sturm, J. D. Sc. Thesis, University Louis Pasteur, Strasbourg, France, 1974. See also: Methods in Enzymology; Kustin, K., Ed.; Academic: New York, 1969; Vol. 16. (1 5 ) Tondre, C.; Lang, J.; Zana, R. J . Colloid Interface Sci. 1975,52, 372. (16) See, for instance, in ref 3 the variation of the fast relaxation time with the surfactant alkyl chain length or in ref 4 the difference between the fast relaxation times associated with the exchange of 1-pentanol and hexadecyltrimethylammonium bromide (HTAB) from HTAB micelles.
The Journal of Physical Chemistry, Vol. 90, No. 21, 1986 5259
Figure 1. Variation of 1 / with ~ the ~ surfactant concentration for solutions of (1) HPyC (Tf = 25 OC, (+) without dye, with eosine, (0)on the absorption band of the surfactant, (A)on the absorption band of eosine, (V)from ref 18), (2) TPyC ( T , = 25 "C ( O ) ) , (3) TPyB ( T I = 25 OC (0)and Tf= 30 OC (W)), and (4) H T A C (Tf = 25 OC ('I)).
Figure 2. Effect of the concentration of added alcohol on 1 / for ~ a 0.3 M HPyC solution; (X) ethanol; (A) 1-propanol; (+) 1-butanol; (I') benzyl alcohol; (0)1-pentanol. Tf = 25 OC.
I
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/
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Alcohol
0.b5
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Figure 3. Effect of the concentration of added alcohol on 1 / for ~ a ~0.3 M HPyC solution: (X) 1-hexanol; ( 0 ) 1-heptanol; (0)1-octanol; (A) 1-decanol. Tf = 25 OC.
"C) were used when the Krafft point of the surfactant was high. The surfactant aggregation numbers were measured by the same fluorescent probing method as in other studies.I7
Results 1. Pure Micellar Solutions. Figure 1 shows that 1/r2increases with the surfactant concentration C, in the range investigated, for all of the surfactants except HPyC below 0.2 M. It is known that the plot of 1/r2vs. C has a rather complicated shape: it usually goes through a broad maximum at C close to the cmc18-21 (17) Malliaris, A.; Lang, J.; Zana, R. J . Phys. Chem. 1986, 90, 655. (18) Hoffmann, H.; Nagel, R.; Platz, G.; Ulbricht, W. Colloid Polym. Sci. 1976, 254, 812. (19) Inoue, T.; Tashiro, R.; Shibuya, Y.; Shimozawa, R. J . Colloid Inrerface Sci. 1980, 73, 105.
5260 The Journal of Physical Chemistry, Vol. 90, No. 21, 1986
Atcohol
1
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Lang and Zana
0 4
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Figure 4. Effect of the concentration of added alcohol on 1 /rZfor a 0.3 M TPyC solution: (0)1-pentanol; ( X ) 1-hexanol; ( 0 ) 1-heptanol. T f = 25 OC.
011 I Y I
I
0
0.3
0.2
0.1
Figure 6. Effect of the Concentration of added oil to the mixed micellar solution 0.3 M HPyC + 0.2 M 1-pentanol: (V)n-hexane; ( 0 )n-heptane; (0)n-octane; ( X ) n-nonane; (R) n-decane; (0)n-dodecane; (V)n-tetradecane; (+) cyclohexane; (A)toluene; ( 0 )n-butylbenzene. T, = 25 OC.
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0 3
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I
01
02
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Figure 5. Effect of the concentration of added alcohol on 1/72 for a 0.3 M H T A C solution: (+) 1-butanol; (0)1-pentanol; (X) 1-hexanol; ( 0 ) 1-heptanol with 3 X M HCNPyI as a probe for the detection of the M HPyC as relaxation signals; (R) results for 1-pentanol with 6 X a probe. The results for pentanol additions are thus seen to be independent both of the nature and concentration (when sufficiently low) of the probe. T, = 25 OC.
Figure 7. Effect of the counterion nature. Variations of l / r 2 upon additions of dodecane to ( 0 )0.3 M HPyC 0.2 M 1-pentanol, (0)0.3 M HPyB 0.2 M I-pentanol, (R) 0.4 M HPyC + 0.4 M I-pentanol, and (0)0.4 M HPyB 0.4 M 1-pentanol. Tf = 25 O C . 4D is the volume fraction of dodecane.
+
+
+
0.02
and through a minimum at higher C.10,11,22 A minimum is indeed seen in the case of HPyC. It is not observed for the other surfactants, probably because the measurements start at too high C. Notice also the large effect of the counterions, Br- or C1-, on the 1 values for the tetradecylpyridinium surfactants. 2. Variations of r2 upon Additions of Alcohols to Pure Micellar Solutions. The variation of 1/r2 upon additions of alcohols to 0.3 M solutions of HPyC, TPyC, and HTAC are shown in Figures 2-5. It is seen that short-chain alcohols, up to 1-pentanol, induce a monotonic increase of 1/r2. Longer chain alcohols show a more complex behavior, with 1 /r2 going through a maximum at an alcohol concentration CAwhich decreases when the alcohol chain length increases. Nevertheless, here also the effect of alcohol ~ . slope of the log 1 / VS.~ C,~ at low CA is to increase 1 1 ~ The plots at CA 0 increases by a factor of 35 in going from ethanol to 1-decanol. The overall increase of 1 / r 2 upon alcohol addition can be very large: it reaches a factor of lo4 for 1-pentanol additions to HPyC (see Figure 2 ) and a factor of lo3 for TPyC. 1-Hexanol also gives rise to a large increase of 1 / ~ the : plot shows an inflection (Figure 3, HPyC) or levels off (Figure 4, TPyC). Finally, Figure 2 shows that benzyl alcohol gives an effect intermediate between those for I-butanol and 1-pentanol. Recall that the average micelle lifetime ( t ) increases with 72.23 Thus the results of Figures 2-5 indicate a monotonous decrease of ( t ) upon formation of mixed micelles of surfactant and short-chain alcohols. In other words the pure micelles break down much more slowly than the mixed micelles (see Discussion). The results presented in the next paragraph will help in the under-
104
(20) Inoue, T.; Tashiro, R.; Shibuya, Y.; Shimozawa, R. J . Phys. Chem. (21) Folger, R.; Hoffmann, H.; Ulbricht, W. Ber. Bunsen-Ges. Phys. Chem. 1974, 78, 986. (22) Lang, J.; Tondre, C.; Zana, R.; Bauer, R.; Hoffmann, H.; Ulbricht, W. J . Phys. Chem. 1915, 79, 276. (23) Aniansson, E. A. G . Prog. Colloid Polym. Sci. 1985, 70, 2.
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1978, 82, 2037.
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Figure 8. Effect of the surfactant and alcohol concentration. Variations of upon additions of dodecane to (0)0.1 M TPyB + 0.1 M 1pentanol, Tf = 25 "C; ( 0 )0.3 M TPyB + 0.1 M 1-pentanol, T, = 15 OC; (+ and m) 0.3 M TPyB 0.2 M I-pentanol, Tf = 15 'C (+) and 25 OC (R); (A)0.4 M TPyB 0.3 M I-pentanol, Tf = 15 OC;(V)0.4 M TPyB 0.4 M 1-pentanol, Tf = 15 O C ; (0)0.4 M TPyB + 0.5 M 1-pentanol, Tf = 15 OC; ( X ) 0.5 M TPyB 0.3 M 1-pentanol, 7-f = 15 "C.
+
+
+
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1
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is
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i
o
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Jo s.1 02 03 0L 05 Figure 9. Effect of the concentration of added toluene ( 0 )and dodecane (0)on I / T ~for 0.3 M HPyC + 0.2 M I-butanol. Tf = 25 "C.
Effect of Alcohols and Oils on Micelle Kinetics
The Journal of Physical Chemistry, Vol. 90, No. 21, 1986 5261
standing of the behavior of 1/T2 upon addition of long-chain alcohols. 3. Variations of T2 upon Additions of Oils to Mixed Surfactant 3 Alcohol Micellar Solutions (Generation of O / W Microemulsions). Some typical results are shown in Figures 6-9. Figure 6 shows the effect of additions of seven linear alkanes, cyclohexane, butylbenzene, and toluene to a 0.3 M HPyC + 0.2 M 1-pentanol solution. For all of the oils investigated decreases upon oil additions, except for toluene where 1/72 first increases, goes through a maximum, and then decreases. This decrease becomes more and more important as the n-alkane chain length increases. The same behavior is observed if one plots the N a C l (VI oil concentration in weight or volume fraction rather than in moles per liter. This indicates that 7 2 depends not only on the oil Ob dl 012 0'3 O l i 0.5 concentration but also on the nature and chain length of the oil. Figure 10. Effect of NaCI. Variation of 1 / upon ~ ~NaCl additions to The decrease of 1/ r 2upon oil addition appears to be a general 0.3 M HPyC, Tf = 25 OC (0) and 30 OC (e);to 0.3 M HPyC + 0.2 M effect (see Figures 6-8). It must be mentioned that a decrease 1-pentanol, Tf = 25 OC (X); and to 0.3 M HPyC + 0.2 M 1-pentanol of 1 / has ~ also ~ been observed upon addition of octane (0.06 M) + 0.0412 M decane, Tf = 25 OC (A). to a micellar solution of HPyC at C = 0.6 M,6 that is, above the concentration which corresponds to the minimum in the plot 1 / ~ ~ 0 0.1 0.2 0.3 i vs. C. For some of the more concentrated systems, however, additions of small amounts of oil may affect 1/r2only little (see Figures 7 and 8). It is only for the addition of a sufficient amount 3 of oil that a rapid decrease of 1 / is~observed. ~ It is noteworthy that the values of 1/T2 at a given C a r e higher for the bromide than for the chloride surfactant both in solutions of the pure surfactants (Figure 1) and in the presence of 1-pentanol 2 and dodecane (Figure 7 ) . Also, the increase of both C and C, shifts the 1 / curve ~ ~ upward (Figure 8 ) . Moreover, the comparison of Figures 6 and 9 shows that when 1-pentanol is replaced l - O c t a n o i , O e c a n e (MI I 1 by 1-butanol in the system 0.3 M HPyC + 0.2 M alcohol, qualitatively the same variations of 1 / are ~ obtained upon addition of toluene or dodecane. Differences appear, however, in the values of l / T 2 , which are lower with 1-butanol than with 1-pentanol. Finally, Figure 8 shows that the increase of temperature also symbols m, 0 , and X show ;he changes of 1 / for ~ the ~ s'ame systems at results in an upward shift of the curve. Tf 30 OC. The results obtained upon oil addition suggest the following possible explanation of the behavior of 1/ T at ~ high concentration ' N ' 180 of long-chain alcohols. Recall that alkanes are known to be solubilized in the interior of micelle^^^^*' or mixed surfactant + alcohol micelles. Both fluorescence probing26 and quasi-elastic light-scattering2' studies (see Discussion) have shown that this solubilization affects the micelles which tend to become less anisotropic and less polydisperse. These changes are probably re, the relationship sponsible for the associated variations of I / T ~and between these two effects is further discussed below. The behavior of long-chain alcohols can then be explained by assuming that at low C, they are solubilized in the palisade layer with their head ,Alcohol , 011 , [MI 0 group anchored at the micelle surface, whereas at high CA,the 0 0 1 02 0 3 O i 05 0 6 palisade layer being saturated, they solubilize in the micelle inFigure 12. Effect of the concentration of added 1-butanol (X, +) and terior.2s Their effect is then similar to that of oils. The effect 1-pentanol (m,0) on the micellar aggregation number N (X, m) and of toluene (Figures 6 and 9), which is known to first solubilize values in the palisade layer like alcohol before going in the i n t e r i ~ r , ~ ~ . ~ ~ (+, 0)of 0.3 M HTAC. Effect of the concentration of added ~ of~ 0.3 M HTAC + 0.2 M 1-pentanol. Tf decane on N (A)and 1 / (A) agrees with this interpretation. For the shorter chain alcohols, = 25 "C. which are partitioned between the micelles and the bulk, the solubilization occurs only in the palisade with a resulting increase locates in the micelle palisade layer brings about an increase of ~ solubilization in the micelle interior would require of 1/ T because 1/T2, whereas its penetration in the micelle interior results in a C, values well above the range investigated in the present work. ~ this . interpretation of our results proves to decrease of 1 / ~ If The effect of short-chain alcohols on 1/72 is discussed further be correct, the measurement of r2 would permit a very rapid below. In summary, on the basis of the above a solubilizate that determination of the preferential site of solubilization of additives in micellar solutions. 4. Effect of NaCI on T~ Figure 10 shows that 1 / T 2 increases (24) Ericksson, J.; Gilbert, G. Acta Chem. Scand. 1966, 20, 2019. monotonously or increases and levels off upon additions of NaCl (25) Ericksson, J.; Henriksson, U.; Klason, T.; Odberg, L. In Solution to pure micellar solutions, mixed surfactant + alcohol micellar Behavior of Surfactants; Mittal, K. L., Fendler, E., Eds.; Plenum: New York, 1982; Vol. 2, p 907. solutions, and O / W microemulsions. Moreover, qualitatively (26) Lianos, P.; Lang, J.; Zana, R. J . Phys. Chem. 1982, 86, 1019 and similar variations of 1 / with ~ ~CA (for 1-pentanol and I-octanol) 4809. and with the concentration of added dodecane are obtained both (27) Hirsch, E.; Candau, S. J.; Zana, R. Proceedings of the 5th Internain the absence and in the presence of NaCl (Figure 11). tional Symposium on Surfactants in Solution, Bordeaux, 1984; Mittal, K. L., Bothorel, P., Eds.; Plenum: New York, in press. 5 . Surfactant Aggregation Number. The variations of the (28) Hayter, B.; Hayoun, M.; Zemb, T. Colloid Polym. Sci. 1984, 262, surfactant aggregation number N and the corresponding variations 798. of 1 / upon ~ ~addition of various alcohols to 0.3 M HTAC solutions (29) Gonzalez, M.; Vera, J.; Abuin, E.; Lissi, E. J . Colloid Interface Sci. and of decane to a mixed surfactant alcohol micellar solution 1984, 98, 152.
+
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+
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5262 The Journal of Physical Chemistry, Vol. 90, No. 21, 1986
I
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i
0
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0’
C’5
C2
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Figure 13. Effect of the concentration of added I-hexanol (+, X ) and I-heptanol(0,O) on the micellar aggregation number N (+, 0) and l/r, values (X, 0 ) of 0.3 M HTAC at Tf= 25 OC.
are given in Figures 12 and 13. These results show that N is not the only parameter which determines the changes of 1/i2. Thus, Figure 12 shows that upon addition of 1-butanol and 1-pentanol 1 / i 2 increases, whereas Ndecreases (1-butanol) or levels off and then increases (1-pentanol). Upon 1-hexanol and 1-heptanol additions (Figure 13), the steepest increase of 1 / i 2 occurs in the range where N increases slowly with C,, then 1 / i 2 decreases when N starts increasing faster. At high C, the behavior of these long-chain alcohols is thus again qualitatively similar to that of oil (see the effect of decane in Figure 12).
Discussion 1, Attribution of the Observed Relaxation Process. Some evidence that the relaxation process investigated in the present work was associated with the process of micelle breakdownformation has been given in the Experimental Section. Additional evidences are the following. First, the relaxation time i1 associated with the surfactant exchange has been measured for TPyB, TPyC, and HPyC at low C.I8 The reported values are at least 10 times smaller than those determined in the present work. As T~ decreases with increasing C,7the process that we observed is thus characterized by a relaxation time much larger than the surfactant exchange and cannot be due to this process. Moreover, i2 has been measured at low C, close to the cmc, where the 1/T2 vs. Cplot goes through a smooth maximum.18 The two extreme values found by Hoffmann et a1.I8 for HPyC are vs. C curve for HPyC plotted in Figure 1. It is seen that the 1/i2 is V-shaped with a minimum at CminN 0.2 M. The same shape is found for the plots corresponding to TPyB and TPyC when the reported data at low CISare taken into account. Similar V-shaped curves have been reported for the variations of 1 / with ~ ~C for dodecylpyridinium bromide,22TPyB,I9 and sodium dodecyl sulfate (SDS)lo,l’in contrast to the linear or close to linear increase of the reciprocal of the relaxation time with C characterizing the exchange p r o ~ e s s . The ~ increases of 1 / r 2 with C obtained for dodecylpyridinium iodide at C between 0.06 and 0.14 M22and for tetradecylpyridinium bromide at C between 0.1 and 0.2 M4 have been also attributed to the micelle breakdown-formation, These facts favor the assignment of the observed relaxation process to the micelle breakdown-formation. In the following a semiquantitative interpretation of our data will be attempted in terms of the theory developed by Kahlweit et and which involves coagulation-fragmentation reactions. We were led to use this theory because, first, Kahlweit et al. showed that it fully accounts for their data and, second, it is the only presently available theoretical treatment which predicts the observed V-shaped curves. 2. Pure Micellar Solutions. We first briefly recall the main features of the theoretical treatment proposed by Kahlweit et a1.+I2 for the micelle These authors showed that the relaxation time i2 breakdown-formation can be written under the form
Lang and Zana
Figure 14. Schematicrepresentation of a micellar distribution curve. A , represents the concentration of the aggregates with an aggregation number s. A , is the concentration of free surfactant, and A , is the concentration of the species at the minimum of the distribution curve. Nand N , are the mean aggregation number and the aggregation number at the minimum of the micellar distribution curve, respectively. Region 1 , oligomers: region 2, submicellar aggregates; and region 3, micelles
proper. 7 2 1 is the relaxation time associated with the process of micelle breakdown-formation through a series of reactions (see reaction 2) where one surfactant AI at a time associates to, and dissociates
A, + A I +
O’,+I
- jN,
+
A,+’
(2)
from, the surfactant aggregates. The decreasing branch of the V-shaped plots is associated with these reactions.”’ In reactions 2, A, and A,+’ refer to aggregates containing s and s + 1 surfactants; j , and j,+, are the numbers of undissociated monomers in the aggregates A, and A,+’, respectively, and A, represents a counterion. Reactions 2 take into account the contribution of the counterions in the makeup of the aggregates and the incomplete micellar ionization. These facts were not considered in the original treatment by Aniansson and Wall.’ 722is the relaxation time associated with the reversible coagulation-fragmentation reactions 3, which become predominant A,
+ A,
A,
(3)
with respect to reactions 2 at high C, above Cmin,and are responsible for the increase of 1/ T 2 with C in this range. In reactions 3 Ak and Aj denote two classes of submicellar aggregates and A, refers to a micelle proper.I0-l2 Thus micelles proper and submicellar aggregates are in direct dynamic equilibrium. Kahlweit et aL9J2have derived the following expressions of 1/i21 and 1/T22for the experimental conditions that prevail in our studies (C >> A I = cmc)
_1 ~ 2 1
1 + (1
+ p)(a2/N)a
with
and
In eq 4-9, A , is the free surfactant concentration; u is the width of the size distribution curve; N is the mean micellar aggregation number (see Figure 14); A, is the free counterion concentration; Nr is the aggregation number of the species at the minimum of the distribution curve (see Figure 14); a, and a, are the effective degrees of dissociation of the aggregates in the region of the micelles proper and at the minimum of the distribution curve, respectively; M is a quantity independent of C30A,, is the counterion concentration at the onset of rapid coagulation; Po represents a complex average dissociation rate constant, which (30) In eq 4 M is proportional to . W - p , where p Cml,.lo 3. Srrrfactant Alcohol Systems. In this section we attempt to explain the large increase of l / r 2 observed upon addition of alcohols to the surfactant solutions investigated. Contrary to the effect of C, the solubilization of alcohols by micellar solutions and the resulting penetration of alcohol into micelles are known to modify significantly most micellar parameters: aggregation number N (and thus radius R ) ,ionization degree (and thus surface potential), shape, size distribution, etc. As these quantities determine to a large extent the relaxation time 72 given by eq l , the interpretation of the variations of T~ will be very complex and cannot be done quantitatively as the changes of many of the above parameters with the alcohol concentration are not known. Therefore, in this section we shall only try to determine which are the parameters the changes of which are most important in explaining the variations of 1/72 with CA. At the outset it must be pointed out that most of our studies were performed at C values well above Cminfor which 1/72 is minimum in the absence of alcohol. Therefore, these C values likely correspond to the range where micelle breakdown-formation takes place mainly through reactions 3. Indeed, Kahlweit has shown that Cminis shifted to lower C upon addition of alcohols.Ilb Nevertheless, one must also consider the change of the contribution
+
of reactions 2 to the increase of upon additions of alcohols. An examination of eq 4 shows that 1 / is ~proportional ~ ~ to N2.30 However, the magnitude of the changes of N with CAis much too small to account for the increase of 1 / (see ~ Figures ~ 12 and 13 for HTAC3I). On the other hand, 1 / is~proportional ~ to the concentration A , of the species a t the minimum of the size distribution curve (Figure 14) and to the rate constant k; for the dissociation of one surfactant from the aggregate A,. However, in view of previous results it appears that k; probably depends only little on the concentration of added alcoh01.~Nevertheless, one cannot exclude a very rapid increase of A , with CA,reflecting an increased stability of the small aggregates upon incorporation of alcohols. This explanation was discussed at length in a previous study of the effect of alcohol on micelle kinetics4 and is further considered below. A consideration of eq 9 suggests that the increase of l / s 2 upon additions of alcohol may be due to an increase of Po and/or A, or to a decrease of A,, or q'. We consider the example of additions of 0.5 M 1-pentanol to 0.3 M solutions of HPyC or HTAC. In both instances 1 / increases ~ ~ by about a factor of lo4. All of our attempts to explain this increase in terms of changes of A,, Ago, and q'failed. Thus an increase of A, by a factor of 3 upon addition of a l ~ o h o lproduced ~ ~ , ~ ~ a 50-fold increase of 1/ 7 2 , much smaller than the experimental change. Likewise, a decrease of A,, by a factor of 2 produced a 10-fold increase of 1/72 when eq 9 was used with q' = 3.5.34 Finally, the change of q'through increased micelle radius (about 25%)35and decreased micelle surface potential 4, (by a factor of 2)36J7upon pentanol addition produced a 300-fold increase of 1/72, again much smaller than the experimental one (this last calculation also took into account the change of A,). In the case where A,, A ,, and q' all vary as indicated above, a largely o v e r e ~ t i m a t e d ~upper ~ - ~ bound ~ , ~ ~ value of lo3 is obtained for the factor by which l / r 2 is increased, still lower than experimentally observed. We are therefore led to the conclusion that the average dissociation rate constant Po must also largely contribute to the observed changes of 1 / ~ In ~ the . theory developed by Kahlweit et al. Po is an average rate constant, the average being taken over all micelles of different aggregation numbers, on the assumption that no potential barrier exists for coagulation-fragmentation. Thus our results suggest that this average dissociation rate constant increases rapidly when alcohol is added to the micellar solutions investigated. (Note that this conclusion is not inconsistent with the fact that a rate constant should be a constant. In the present case, Po varies because the system is progressively modified by the addition of alcohol.) An increase of Po should, of course, result in an increase of the relative proportion of submicellar aggregates and thus of the micelle polydispersity. This prediction was confirmed by the results of quasi-elastic light-scattering measurements performed on some (31) HTAC is the only surfactant for which N could be determined by the fluorescence probing method." Indeed, the pyridinium group of the other surfactants investigated is a very efficient quencher of the fluorescence of the probe used in the present work, i s . , pyrene. (32) Alcohol additions are known to increase the micelle ionization degree a,.33 Equation 10 then predicts an increase of A at constant C upon increasing CA. We have assumed that a, is increased from 0.3 to 0.9, the latter value being certainly overestimated, when CA increased to 0.45 M. The change of 1 / ~ was * calculated by using q'= 3.5 (see section 2 in the Discussion section). (33) Zana, R.; Yiv, S.; Strazielle, C.; Lianos, P. J. Colloid Interface Sci. 1981, 80,208. Jain, A,; Singh, R. J. Colloid Interface Sci. 1981, 81, 536. (34) Results reported by KahlweitiIb appear to indicate a decrease of A,, for SDS in the presence of 1-pentanol. (35) If all the added 1-pentanol is assumed to be solubilized in the micelles, the values of N in the presence or absence of 1-pentanol indicate a 25% increase of the micelle radius R. This increase of R should result in a small decrease of l/rt. (36) The addition of 0.5 M 1-pentanol to 0.3 M HTAC solution produces about a 2-fold increase of the surface area SAper surfactant head group3' and therefore a 2-fold decrease of the surface charge density p if the micelle ionization degree remains constant. This decrease of p gives in turn a decrease of micelle surface potential by a factor of 2, on which our calculations were based. This factor, however, is certainly largely overestimated as the incorporation of the alcohol in the micelles increases a,.323 This effect largely compensates the changes of SA. (37) Almgren, M.; Swarup, S. J. Colloid Interface Sci. 1983, 91, 256.
5264
Lang and Zana
The Journal of Physical Chemistry, Vol. 90, No. 21, 1986
1
,
ilecoie (PI
~
3
Figure 15. Variations of 1/7, (0, Tf = 25 "C), D (+), and v (A)with the concentration of added 1-pentanolto 0.3 M HPyC 0.2 M NaCl at 25 "C.
+
O 05 l-cC'OlO
I
C
hl
I
0 05
i l
Figure 16. Variations of 1/r2(0, Tf = 25 "C), D (+), and u (A) with the concentration of added 1-octanol to 0.3 M HPyC + 0.2 M NaCl at 25 'C.
of the above micellar systems as part of this work. Such measurements yield the micelle collective diffusion coefficient D, which is inversely proportional to the micelle hydrodynamic radius, and the variance u of the decay of the autocorrelation function of the intensity of the scattered light which reflects the polydispersity of the system: the larger v, the larger the p o l y d i ~ p e r s i t y . ~ ~ , ~ ~ upon Figures 15 and 16 show the changes of D,u, and addition of 1-pentanol and 1-octanol, respectively, to a 0.3 M HPyC 0.2 M NaCl solution (salt was added in order to reduce the electrostatic repulsions between micelles which affect the values of D and 0). In both instances, the initial addition of alcohol which produces a steep increase of 1 / also ~ results ~ in an increase of u, that is, of the micelle size polydispersity. Also, it can be seen that at CAabove 0.1 M 1-pentanol or above 0.04 M I-octanol, where 1 , ' ~ ~increases much less rapidly or decreases, L' and thus the polydispersity decrease. On the other hand, D decreases (that ~ u~ increase, is, the size increases) in the C , range where 1 / and ~ ~ whereas D nearly levels off when u decreases and 1 / increases less steeply or decreases. The changes of D are in agreement with the variations of N caused by addition of 1-pentanol and longer alcohols to HTAC solutions (see Figures 12 and 13). It is known that spherical or spheroidal micelles are characterized by a low polydispersity, whereas large elongated micelles have usually a large variance.38 The results of Figure 15 and 16 suggest that the HPyC micelles are initially spheroidal and then grow and become elongated and more polydisperse upon addition of alcohol which then probably remains mostly located in the palisade layer.28 Upon further alcohol addition, the alcohol penetrates in the micelle interior.28 This tends to make the micelles spherical again with a decrease of polydispersity (and 1 / T ~ ) . The near constancy of D is then the result of two opposite effects: the swelling of the micelle by the alcohol (Ddecreases3') and the change of shape from more to less elongated ( D increase^^^).
+
(38) Mazer, N.; Benedek, G.; Carey, C. J . Phys. Chew. 1976,80, 1075. (39) Hirsch, E.; Candau, S. J.; Zana, R. J. Colloid Interface Sci. 1984, 97, 3 1 8 .
(40) De Lisi, R.; Genova, C.; Testa, R.; Turco Liveri, V. J . Solufion Chem. 1984. 13, 121.
Effect of Alcohols and Oils on Micelle Kinetics
The Journal of Physical Chemistry, Vol. 90, No. 21, 1986 5265
associated with the micelle breakdown-formation. The concentration of the small aggregates decreases upon solubilization of oil probably because alcohol and surfactant are then required to constitute an interfacial film between water and the added oil. The small aggregates are used for this purpose. It should be noted that the decrease of micelle size evidenced in Figure 17 by the increase of D upon increasing decane concentration confirms the conclusion of our previous investigation by means of time-resolved fluorescence probing.26 The effect of long-chain alcohols on 1 / r 2 can now be examined in a more complete manner in view of the preceding results. At low CA,as long as the alcohol is solubilized in the palisade layer, all alcohols behave similarly and in the solution large surfactant-rich aggregates and small alcohol-rich submicellar aggregates are in dynamic equilibrium. However, at higher C Athe alcohol starts dissolving in the micelle interior. The micelle shape then becomes less anisotropic, and the small aggregates are used to make up the interface between the alcohol solubilized in the micelle interior and the bulk water. As a result the polydispersity and, in turn, 1/r2 decrease. Our results suggest that the CArange where the large micelles proper and the small alcohol-rich micelles may coexist decreases rapidly as the alcohol chain length increases, becoming close to that of the surfactant. This conclusion from our kinetic study is the same as that reached in classical studies of mixed surfactant systems: two surfactants form mixed micelles only when their chain lengths are not too different, at least at low c o n ~ e n t r a t i o n . ~Mixed ~ micelle formation is facilitated at high concentration. 5. Comparison between the Results of Time-Resolved Fluorescence Probing and Chemical Relaxation Studies Concerning Micelle Dynamics. The main conclusion of the present investigation is that at high surfactant concentration or in the presence of alcohols, reactions of coagulation-fragmentation can take place in micellar systems. Recent time-resolved fluorescence probing studies have provided direct evidence for these processes by showing that water-insoluble probes and/or quenchers can migrate from micelle to micelle probably via submicellar aggregates which act as carriers for the probes or quenchers.17 This migration is observed in those systems where chemical relaxation studies show a large increase of 1 / r 2 with C and/or C A but generally a t much larger C and/or C A (see below). The fluorescence probing studies permit a direct measurement of an average rate constant of dissociation of a small aggregate from a micelle proper, the values of which were all found to be in the 105-106 s-I range," that is, much larger than those found for pure micellar SDS solutions: 10 s-l.Io Such a large difference is difficult to explain. We note, however, that coagulation-fragmentation is observed by fluorescence probing always at much larger C values than by chemical relaxation: 1 M vs. 0.1 M in the case of SDS. No process is observed with 1 M HTAC solution by fl~orescence,'~ whereas the process is already observed at 0.2 M by chemical relaxation. The same is true for systems containing alcohols.43 Along this line, recall that the micelle polydispersity is larger at high C than at low C for most surfactants including SDS, but not for HTAC.42 On the contrary, a t a given C the screening of electrostatic intermicellar repulsions should be nearly the same for the two surfactants. These facts suggest that the aggregates at the left-hand side of reactions 3 detected by the two methods may not be the same, although reactions 3 would be formally written in the same manner. In chemical relaxation, reactions 3 would involve two submicellar aggregates from region 2 in Figure , ~ would ~ ~ ~ then be fairly slow 14, as suggested by K a h l ~ e i tand
owing to the low concentrations of these species. In fluorescence probing only those reactions between a micelle proper (region 3 in Figure 14) and a submicellar aggregate (region 2) would be seen as they are much more rapid (see above) and may be characterized by a time falling in the fluorescence time scale." Another way to express the above is to assume that there is a distribution of dissociation rate constants and that each method measures an average over only one part of the spectrum: chemical relaxation would measure the low rate constant part, while fluorescence probing would measure the large rate constant part. The shape of the spectrum of dissociation rate constants would be shifted upward upon increasing C or alcohol addition.
Conclusions
In this paper we have reported new data concerning the dynamic behavior of ionic micelles in the presence of alcohols and oils. The results show a considerable sensitivity of the relaxation time characterizing the process of micelle breakdown-formation to the surfactant concentration and to the nature, chain length, and concentration of the added alcohol and oil. Micelle breakdownformation can occur via successive dissociation-addition of surfactant monomers and fragmentation-coagulation involving submicellar aggregates. The contribution of the second process is predominant at high surfactant concentration C in pure ionic surfactant solutions or at lower C in the presence of medium chain length alcohols. This contribution tends to vanish upon oil addition to mixed surfactant alcohol micellar solutions (formation of O / W microemulsions). The results for the alcohol-containing systems suggest that the submicellar aggregates may be small alcohol-rich micelles in dynamic equilibrium with the micelles proper, the composition of which is close to the stoichiometric composition. The fragmentation-coagulation process evidenced in the present study by chemical relaxation was also observed for the same systems by time-resolved fluorescence probing but at much higher surfactant and/or alcohol concentration. The average rate constants measured by these two techniques differ largely in the case of pure ionic surfactants. This result may reflect the fact that the two techniques measure different averages, involving reactions between different aggregates, that is, between two submicellar aggregates in chemical relaxation and between a micelle proper and a submicellar aggregate in fluorescence probing. Another conclusion of this work is that a small amount of additive (alcohol or oil) and thus of an impurity of this nature may have a very large effect on r2,that is, on the micelle lifetime. This fact may have some bearing when micelles are used as media for studies of rates of chemical reactions: the solubilized reactants may, depending on their nature, significantly increase or decrease the micelle lifetime. This effect may in turn significantly affect the rate of the reaction.
+
Acknowledgment. We thank very much Dr. S. J. Candau for performing the quasi-elastic light-scattering measurements reported in Figures 15-17. We also acknowledge the financial support of PIRSEM (CNRS) under Grant AIP No. 2201. Registry No. TPyC, 2785-54-8; HPyC, 123-03-5; HTAC, 112-02-7; TPyB, 1155-74-4; HPyB, 140-72-7; HTAB, 57-09-0; HCNPyI, 9957310-1; ethanol, 64-17-5; I-propanol, 71-23-8; 1-butanol, 71-36-3; I-pentanol, 71-41-0; 1-hexanol, 11 1-27-3; I-heptanol, 11 1-70-6; 1-octanol, 111-87-5; 1-decanol, 112-30-1; benzyl alcohol, 100-51-6; n-hexane, 11054-3; n-decane, 124-18-5; n-dodecane, 1 12-40-3; n-tetradecane, 629-59-4; cyclohexane, 110-82-7; toluene, 108-88-3; butylbenzene, 104-51-8; ethyl acetate, 141-78-6; Merck 111, 78321-05-8; eosin, 17372-87-1. ~
(41) De Waerd, R.; De Haan, J.; Van De Ven, L.; Achten, M.; Buck, H. J. Phys. Chem. 1982, 86, 2523. Zana, R.; Muto, Y . , unpublished results. (42) Porte, G.; Poggi, Y . ;Appell, J.; Maret, G . J . Phys. Chem. 1984, 88, 5713. (43) Malliaris, A.; Lang, J.; Zana, R., submitted for publication.
~
~~~
~
(44) Contrary to the statement that "the submicellar aggregates are aggregates on the left-hand side of the minimum of the size distribution1Ib"we believe that the submicellar aggregates in reactions 3, which give rise to the chemical relaxation process, can be on both sides of the minimum. This is easily realized by evaluating the relative concentrations of the aggregates and assuming a Gaussian distribution curve.
