J. Phys. Chem. 1993,97, 1243-1248
1243
Effects of Surfactant/&Cyclodextrin Complex Formation on the Surfactant Monomer-Micelle Exchange Rate in Aqueous Solutions of Decyltrimethylammonium Bromide D. J. Jobe and R. E. VerraU’ Department of Chemistry, University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N 0 WO
E. Junquera and E. Aicart Departamento de Quimica Fisica, Facultad de Ciencias Quimicas, Universidad de Complutense de Madrid, 28040 Madrid, Spain Received: August 5. I992
Solutions of decyltrimethylammonium bromide (DTAB) and 8-cyclodextrin (8-CD) were studied using both ultrasonic relaxation techniques and electrical conductivity. Multiple ultrasonic relaxation processes were found in all solutions, independent of the presence of micelles. These relaxations are believed to be due to the presence of the surfactant monomer-micelle exchange process and/or the molecular dynamics of the cyclodextrinsurfactant complex. In order to analyze the surfactant monomer-micelle exchange process using the Aniansson model, the sound absorption due to the presence of the cyclodextrin-surfactant complex was subtracted from the =mixed” spectrum and the resulting data were analyzed using a single relaxation expression. The analysis indicates that the kinetics of the surfactant monomer-micelle exchange process are unaffected by the presence of the 1: 1 complex of DTAB and 8-CD. Furthermore, the presence of the inclusate, DTAB, appears to induce ultrasonic relaxation processes which are related to the cyclodextrin structure and are normally absent in solutions containing only @CD. Furthermore, the sound absorption for the 1:l complex is very similar to that for modified 8-cyclodextrin, indicating that the intramolecular hydrogen-bonding network within the 8-CD structure may be disrupted upon its initial encounter with the surfactant.
Introduction Ultrasonic relaxation has been shown to be a useful technique in the study of reaction kinetics occurring in the microsecond to nanosecond timedomain.’+ It hasbeenused tostudythedynamic processes of micelles in an aqueous mediumI4 and more recently has been shown to provide important new information concerning dynamic processes in aqueous solutions of cyClodextrins and modified cyclodextrins as well as the inclusion complexes formed between cyclodextrins and various inclusate molecule^.^^ The absorption of ultrasound in micellar surfactant solutions is due to the exchangeof a surfactant monomer between the bulk phase and the micelle. The slower relaxation process associated with the formation/dissociation of the micelles occurs at lower frequencies, far outside the ultrasonic range. The characteristic relaxation frequency of the monomer-micelle surfactant exchange is dependent on both the surfacatnt chain length and the concentration of micelles. Although two models’OJlhave been proposed to describe the dependence of the relaxation time ( T= ~ 1 1 2 ~ 5on ) micelle concentration, the Aniansson and Walllo model has been the most widely used. It is statistically based and assumes the occurrence of a normal distribution of micelle sizes around a mean aggregation number (n) having a variance uzand has been shown to be very effectivein characterizingthemonomermicelle surfactant exchange process in nonionic micelles. However, use of this model for ionic surfactants is limited to those surfactants with a low degree of micelle ionization (8). More recently, Hall and Wyn-Jones4J1have presented a phenomenological model which corrects for the ionic nature of the surfactant in the determination of the kinetic rateconstants for the exchange process. It should also be noted that the equation derived by Hall simplifies to the Aniansson and Wall equation when j3 = 0, Le., for a nonionic surfactant. In addition to the kinetic information, ultrasonic relaxation studies can provide thermodynamic information about the relaxation process. The strength or amplitude ( A ) of the ultrasonic relaxation is dependent upon many
vi)
0022-3654/93/2091- 1243304.00/0
parameters, but for aqueous surfactant solutions, the average volume change ( A n associated with the relaxation appears to be the principal contributing factor. Aqueous solutions of cyclodextrinsand modified cyclodextrins also absorb ultrasound in the 0.8-210-MHz frequency range. Kat0 et a1.6 and Rauh and Kn&he7 have shown that aqueous solutions of a- and y-cyclodextrin (a-and 6-CD) have weak ultrasonic absorption, while little or no absorption is observed in the case of @-CD.This is due, in part, to the low, limiting solubility of 8-CD in water. For a-and y-CD, multiple relaxation processes were observed; however, the interpretation of these data and the assignment of the relaxation processes differed in the two reported studies. More recently? we have used ultrasonic relaxation techniques to examine aqueous solutions of modified 8-cyclodextrins, which have a much higher solubility than their parent compound. It was found that these systems absorb ultrasound more strongly than their parent compound, having multiple relaxation frequencies similar to those observed for solutions of a- and y-CD. In the case of 2,6-0-dimethyl-fl-cyclodextrin (DMCD), the ultrasonic absorption in the 0.8-21-MHz range was found to be one of the largest reported for an aqueous solution. These absorption data were interpreted in terms of three relaxation processes: the lowest frequency relaxation was assigned to an intramolecular “wigwag” motion of the pyranose ring; a midrange relaxation was assigned to the resulting exchange of water with the cavity, and the highest relaxation frequency was assigned to the bond rotation at the CS-C6 bond of the pyranose ring. The third relaxation was not observed in solutions of either a- and y-CD or 2,3,6-0-trimethyl-8-cyclodextrin (TMCD). Cyclodextrins are also known to strongly complex surfactant molecules in either the absence12J3or p r e s e n ~ e ~of~micelles. J~ It is generally believed that surfactants form 1:l complexes with 8-CD. However, not much is known about these complexes or how their presence may affect the surfactant monomer-micelle exchange process in the postmicellar region. It is also generally believed that, if enough cyclodextrin is added to a micellar 0 1993 American Chemical Societv
Jobe et al.
1244 The Journal of Physical Chemistry, Vol. 97, No. 6, 1993
TABLE I: Ultrasonic Relaxation Best Fit Parameters (Cf. Equation 1) CO-CD,
C,,
C,,
moldm-3
moldm-3
moldm-3 0.058 0.179 0.077 0.054 0.047 0.037 0.026 0.021 0.006 0.000 0.000 0.000
0.000 0.017 0.017
0.017 0.012 0.021 0.032 0.017 0.052 0.061 0.089 0.017 0.017 0.060 0.120
0.120 0.258 0.156 0.133 0.120 0.120 0.120 0.100 0.120 0.120 0. I20 0.075 0,017 0.060 0.120
AI,Nps2 cm-’ X 10” 1 I67 1033 1817 3360 2529 2856 4013 6990 9900 10243
frl,
MHz 1.36 0.92 0.90 0.69 0.74 0.69 0.52 0.28 0.18 0.15
A23 NP s2 cm-l X IO”
0.000 0.000 0.000
surfactant solution, all of the surfactant will eventually be complexed, resulting in the total breakdown of the micelles.14Js The object of this study is to examine the effect of &CD, and the resulting 1: 1 complex, on the surfactant monomer-micelle exchange rate for decyltrimethylammonium bromide (DTAB) micelles. In our previous study,8 the addition of nonmicellar concentrationsof DTAB to DMCD solutionsresulted in a decrease in the sound absorption related to the CD’s internal motion; however, no studies were carried out using concentrations of DTAB > cmc. In this paper, we present theresultsof an ultrasonic relaxation study of solutions containing micellar concentrations of DTAB in the presence of &CD. Since previous studies have shown that 8-CD has little or no anomalous sound absorption in the 0.8-2 10-MHz range, it was expected that the effect of 8-CD on the surfactant monomermicelle exchange process for DTAB could be readily studied. As well, if any other relaxation phenomena were observed, such as those that may occur due to the presence of the 1:1 complex, new kinetic information concerning those processes would also be available.
Experimental Section Materials. 8-CD (Aldrich, >99%) was used without further purification. Thesurfactant, DTAB (Eastman-Kodak),was twice recrystallized from an acetone/methanol mixture (90/ 10 v/v) and dried under vacuum. All solutions were prepared by using Super Q Millipore water. Ultrasonic Absorption Measurements. Ultrasonic absorption measurementswere made at 25.00 “C using a previously described apparatus.8J6 Both the cylindrical resonator (0.8-5-MHz) and the pulsed (10-190-MHz) methods were employed. Sound velocity measurements were made using a MAPCO Nusonic (Model 6080) sound velocimeter which operates on the principle of the “singaround” method. Although the temperaturestability of the bath was fO.O1 OC, localized heating may have occurred during the cylindrical resonator measurements. However, due to the low voltage applied to the crystal (10 V), this effect was minimal. The ultrasonic data were fitted to the expression
where a is the absorption coefficient at frequencyf, z is the number of relaxations, and B is the background or classical sound absorption for the solution (typically 23 X lo-’’ Np s2cm-I for water). The computer program for fitting the data was based on the Marquardt algorithm and fitted the data by minimizing the x2 value. A plot of the residuals was also produced to assist in the determination of the number of relaxation processes.
55 57 339 107 178 1290 2055
fr29
MHz
11 5.6 1.8 0.52 0.85 0.63 0.82
A39 NPs2 cm-1 X IO’’ 483 267 80 35 36 62 86 65 79 97 25 32 36 133
fr39
MHz 6.9 6.O 13 21 29 18 17 22 19 17 16
6.3 24 22
B, Np s2 cm-I X IO1’ 25 29 27 29 27 25 27 26 30 28 31 26 23 24 32
x2
0.23 1.37 1.37 1.32 1.40 1.71 1.68 1.37 1.44 2.31 1.25 0.84 2.89 1.59 1.37
ConductivityMeasurements. Conductance measurementswere made at 25.00 f 0.01 “C using a Wayne Kerr Precision Component Analyzer 6425 and an immersion conductivity cell having a cell constant of 1.1573 cm-I. Measurements were made using a titrametric dilution method.
Results and Discussion Table I contains the best fit parameters Ai,fii, and B as well as the x2 values that result from fitting the ultrasonic absorption data toeq 1. For those data containing more than one relaxation, the value of z was increased by one until the value of x2 was minimized and the residual difference between the experimental and calculated values of a / f appeared to be sufficiently random. The lines drawn through the data points in Figures 1-4 are the fitted values of a / f according to eq 1 using the “best fit” values in Table I. It should also be noted that some of the calculated relaxation frequencies are outside the experimental frequency range studied and therefore are expected to have a higher uncertainty associated with them. Figure 1shows thevariation in ultrasonicabsorption,expressed as a / f 2, with the applied frequency for solutions in which the concentration of DTAB remained fixed at 0.12 M and the cyclodextrin concentration was varied as 0.00 < C6-c~< 0.052 M. It can be readily seen that there is a steady increase in the ultrasonic absorption in the 10-60-MHz frequency range as the concentration of 8-CD is increased. Furthermore, for those solutions containing < 0.052 M 8-CD, the absorption in the range 0.8-10 MHz is much higher than for the solution containing 0.052 M 8-CD. Figure 2 shows the variation of a / f for those solutions containing 0.12 M DTAB, except that the 8-CD concentration varied as 0.062 < CP-CD < 0.12 M. It can be seen that, as C,-CD increases,the sound absorption in the range 10-60 MHz continues to increase. As well, the sound absorption in the 0.8-10-MHz range also increases. This is in contrast to the absorptionchanges observed for this same frequency range when C,.CD< 0.052 M; the sound absorption was found to vary only slightly under the latter conditions. This may indicate that there is a change in the relaxation mechanism responsible for this absorption when 0.042 < C ~ C Cf + CIand we can assume that also means that, for micellar DTAB solutions in the presence of micelles are present. Therefore, the sound absorption in the , given by C, = C, - CI - 0.062. W D , the value of C,,is frequency range 0.8-20 MHz is due to the surfactant monomer-
+
+
The Journal of Physical Chemistry, Vol. 97, No. 6, 1993 1247
Effects of SurfactantlB-CD Complex Formation
2.5
2.0
20
1
clmu
x 10'
15
L
0
3
/6
0.5
lw0l
C,/cmc Figure 7. Plot of 2 ~ vs5 C,/cmc for solutions where C,
> 0.
TABLE II: Ultrasonic Relaxation Best Fit Parameters after Sound Absorption from the 1:l Complex Has Been Subtracted
Figure 8. Plot of lmar vs r/(r
+ 1) for solutions where C, > 0.
by (2) a slow step where the inclusate makes a full insertion into the cavity. For micellar DTABIB-CD systems, these processes CU-CD, ct, Cm 4 N p s 2 53 B,Nps2 will occur in additionto the surfactant monomer-micelle exchange mol dm-3 mol dm-3 mol dm-3 cm-I X lOI7 MHz cm-l X lOI7 x 2 process. The similarity in the values of k/&, k / n , and AV in 0.021 1747 0.54 28 4.6 0.017 0.100 the absence and presence of 8-CD suggests that the surfactant 0.96 30 4.4 0.037 1594 0.021 0.120 monomer-micelle exchange process is unaffected by the presence 29 1.6 1828 0.97 0.047 0.012 0.120 of the cyclodextrin. It has been shown recently that,20plotting 1.2 29 3.6 0.054 1405 0.017 0.133 the sound velocity vs C, for solutions containing various con2.3 29 1.5 0.077 939 0.017 0.156 centrationsof 8-CD in the postmicellarregion, u is almost invariant 3.7 29 1.7 0.179 730 0.017 0.258 with DTAB concentration and the slopes are parallel, regardless of the C0-c~.This observation also supports the view that DTAB micelleexchangeprocess. Using the Aniansson and Wall mode1,lO micelles are unaffected by the 1:1 complex, as these slopes, and one can then write for this relaxation process hence the compressibilities, would be expected to change. 2n-L = l / r = k-/a2 (k-/n)(C,/cmc) (6) The ultrasonic absorption for the 1:l complex is noteworthy (Figure 4). As mentioned earlier, there are no micelles present and under these conditions, and therefore, no exchange process will pmax ( A K U ) / ~ [ ( ~ A ~ ~ C ~ C ) / ( ~ R T+Kr) , ) I(7) ~ / ( ~ occur. Also, aqueous solutions of 8-CD have almost no excess ultrasonic absorption. Furthermore, these ultrasonic curves are where r is the relaxation time for the exchange of a surfactant very similar to those obtained for solutions of modified B-CD's. monomer with the micelle, pmsx is the absorption maximum per If it is assumed that, during the first step of the formation of the wavelength, u is the speed of sound, K, is the adiabatic com1:l complex, there is some disruption of the internal 0 2 - 0 3 pressibility of the solution and is assumed to be approximately hydrogen bonds! fr2 in these curves could be assigned to the the value of the solvent, AV is the average volume change resulting wigwag of the pyranose ring, whilef,3 would be the associated with the exchange,'I = (u2/n)(Cm/cmc), and R and exchange of the water with the cavity. Surfactants are not the T have their usual meaning. A recent study by Okubu et al." only inclusates that induce an ultrasonic absorption in solutions has shown that the kinetics for the inclusion of a surfactant into of 8-CD. Ultrasonic relaxations have been found for inclusion the CD cavity is much slower than that the surfactant monomercomplexes of 8-CD with dyes25 and with various inorganic anions.5 micelle exchange rate; therefore, during the relaxation process It appears that the absorption associated with the process is related the fraction of included DTAB is considered unavailable to to the cyclodextrin and not to the inclusate; however, further exchange with the micelle and hence the cmc is assumed to be ultrasonic absorption studies of these 1:l complexes are required Cf 0.062 M. in order to better understand the dynamics of these processes. Using this value and the ultrasonic data in Table 11, a plot of These observations also imply that the forward rate constant 2zf, vs C,/cmc (Figure 7) yields values of k / n = (7.4 f 0.8) for the formation of the 1:l complex is slower than the exit rate X lo6 s-I and &/a2 = (2.8 f 1.7) X lo6 s-I. These values are of the monomer from the micelle. If the opposite were true, the comparable to the values of k / n = 7.9 X 106 s-1 and k / u 2 = exit rate of the monomer would have to increase or else C r would 3.0 X 106 s-I reported by Kat0 et al.1 for DTAB in the absence decrease. Furthermore, even though the formation of the complex of D-CD. Figure 8 shows a plot of p,,, (eq 7) vs I'/(I' + 1) for is slower than the monomer-micelle exchange, the formation of the surfactant monomer-micelle exchange process. The slope of the 1:l complex results in the two fast processes which absorb this line yields AV = 5.6 0.6 cm3 mol-1, and this value is also ultrasound in the 1-50-MHz frequency range, the possible origins comparable to the value of AV = 4.4 cm3 mol-' also reported by of which were discussed above. Kat0 et a1.l Okubo et a1.17 have reported a forward rate constant for the inclusion of surfactants by 8-CD using the stopped flow method. For most inclusates, it is generally believed that inclusion into Their results indicate that, for sodium decanesulfonate, the the CD cavity proceeds via two steps:5J7-21-24 (1) a rapid step forward rate constant is 3.3 X 103 M-1 s-1, considerably slower where the inclusate and the CD form a uloose" complex followed
+
vr),
1248 The Journal of Physical Chemistry, Vol. 97, No. 6, 1993
than the exit rate of the surfactant from the micelle. However, Robinson et a1.21have attempted to measure the forward rate constant for the inclusion of various surfactants using a Joule heating T-jump apparatus and have found that the forward rate is at least as fast as the forward rate of inclusion forp-nitrophenol into 8-CD, ca.4.7 X lo7 M-I s-I. This would be in agreement with the forward rate of inclusion reported by Turro et aLZ6for a series of cationic phosphorescent-labeled detergents, where the forward and backward rate constants for the formation/ dissociation of the complex were found to be ca. 2 X lo7 M-I s-) and 5 X IO4 s-I, respectively. Therefore, it is possible that the forward rate of complexation between the surfactant and 8-CD that Okubo et al. have measured may be the second step in the inclusion process, where the surfactant is fully included into the 8-CD cavity. If it is assumed that the above conclusions hold, the following mechanism evolves for the inclusion of surfactants monomers by 8-CD in the presence of micelles. The surfactant molecules in the micelle exchange with the monomers in the bulk solvent at a rate which is faster than the forward rate of inclusion of the monomer by gCD. The inclusion of the monomer proceeds via two steps, a fast initial step forming some type of loose complex between the surfactant and &CD followed by a slow full inclusion of the monomer within the cavity of the cyclodextrin. The fast initial step probably leads toa breakdown in the hydrogen-bonding structure at the mouth of the cavity, resulting in more wigwag motion in the cyclodextrin's pyranose rings and, as well, the explusion of water from the cavity. However, any further discussion about this mechanism would be speculative without additional studies.
Conclusion The ultrasonic study of DTAB micelles in the presence of 8-CD shows that the surfactant monomer-micelle exchange process for DTAB micelles appears unaffected by the presence of @-CD/ DTAB complexes. Ultrasonic relaxations were also found in the absence of micelles, due to the presence of the 1:1 complex. These relaxation processes are similar to those found in modified cyclodextrins and are believed to becaused by the internal motions of the CD structure that can arise from the interaction of 8-CD with the surfactant when the inclusion complex is formed.
J o b et al. Acknowledgment. Funding from the Natural Sciences and Engineering Research Council of Canada is gratefully acknowledged. E. J. thanks the M.E.C. (Spain) for financial assistance in the form of a travel grant. References and Notes (1) Kato, S.; Nomura, H.; Honda, H.; Zielhski, R.; Ikeda, S. J . Phys. Chem. 1988, 92,2305. (2) Rassina. J.: Sams. P. J.: Wvn-Jones. E. Faradav Trans. 1978.1247. (3) Yiv, S.:Zana, R.: Ulbrichi, W.; Hoffmann, H:J. Colloid InreGace Sci. 1981, 80,224. (4) Wan-Badhi, W. A.; Palepu, R.; Bloor, D. M.; Hall, D. G.; WynJones. E. J . Phvs. Chem. 1991.95.6642. ( 5 ) Rohrbach, R. P.; Rodriguez, L. J.; Eyring, E. M.; Wojcik, F. J. J . Phys. Chem. 1977,81,944-8. (6)Kato, S.;Nomura, H.; Miyahara, Y. J. Phys. Chem. 1985,89,5417. (7) Rauh, S.;Knkhe, W. J . Chem. Soc., Faraday Trans. I 1985,81, 255 1 . (8) Jobe, D. J.; Verrall, R. E.; Reinsborough, V. C. Can. J . Chem. 1990, 68,2131. (9) Hall,D.; Bloor, D.;Khalid,T.; Wyn-Jones,E. J . Chem.Soc.,Faraday Trans. I 1986,82,2 1 11. (IO) Aniansson, E.A.G.:Wal1.S.N. ChemicalandBiologicalApplications of Relaxation Specfromerry; Wyn-Jones, E., Ed.;D. Reidel Publishing: Dordrecht, Holland, 1975;pp 223-238. (1 1) Hall, D. G. J. Chem. Soc., Faraday Trans. I 1981, 77, 1973. (12) Satake, I.; Ikenoue, T.; Takeshita, K.; Hayakawa, K.; Maeda, T. Bull. Chem. Soc. Jpn. 1985,58,2746. (13) Palepu, R.; Richardson, J. E.; Reinsborough, V. C. Longmuir 1989, 5,218. (14) Palepu, R.; Reinsborough, V. C. Can. J. Chem. 1988,66,325. (15) Palepu, R.; Reinsborough, V. C. Can. J . Chem. 1989,67,1550. (16) Verrall, R. E.; Nomura, H. J . S o h . Chem. 1977,6, 1 . (17)Okubo, T.; Maeda, Y.; Kitano, H. J . Phys. Chem. 1989,93,3721. ( 1 8 ) Elvingson, C. J . Phys. Chem. 1987,91, 1455. (19) Elvingson, C.; Wall, W. J. Colloid Interface Sci. 1988,121,414. (20) Junauera. E.: Aicart. E.:Tardaios.G. J. Phvs. Chem.1992.96.4533. (21) Herby, A.; Robinson, B. H.; Kelly, H. C:J. Chem. Soc.; Faraday Trans. I 1986,82,1271. (22) Hersey, A.; Robinson, B. H. J . Chem.Soc., Faraday Trans. I 1986,
80.2039. (23) Clarke, R. J.; Coates, J. H.; Lincoln, S. F. Adv. Carbohydrate Chem. 1988,46,205. (24) Jobe, D. J.; Verrall, R. E.; Palepu, R.; Reinsborough, V. C. J. Phys. Chem. 1988.92. 3582. (25)JOG,D.'J.;Holzwarth, J. F.; Verrall, R. E. Minutes: 5th International Symposium on Cyclodexrrins; DuchCne, D., Ed.; Editions de Santt: Paris, 1990;pp 309-313. (26) Turro, N . J.; Okubo, T.; Chung, C. J . Am. Chem. Soc. 1982,104, 1789.
