Langmuir 2002, 18, 7759-7760
7759
Comments Comments on Fluorescence Study of Premicellar Aggregation in Cationic Gemini Surfactants
Gemini (dimeric) surfactants are made up of two amphiphilic moieties connected at the level of, or close to, the headgroups by a spacer group.1,2 It has been shown that in aqueous solution gemini surfactants start to selfassociate at concentrations below the critical micellization concentration (cmc) if the surfactant alkyl chain is long enough.2-9 Recently, Mathias et al. investigated the aggregation behavior of four series of cationic gemini surfactants with particular emphasis to the premicellar range of concentration.8 The surfactant aggregation number N was determined using time-resolved fluorescence spectroscopy.10-13 The authors reported values of N close to 2 in submicellar surfactant solutions, at concentrations as low as 0.1 µM.8 However because of experimental constraints discussed below, time-resolved fluorescence spectroscopy cannot be used at such low concentrations and/or for determining surfactant aggregation numbers that are, say, below 10. Thus, the results reported in ref 8 that concern the submicellar range of the investigated gemini surfactants have little meaning. Time-resolved fluorescence spectroscopy has been much used to determine the aggregation number of surfactants in solution.10-13 It uses a fluorescent probe molecule and a quencher of the probe fluorescence. An important condition for meaningful measurements of aggregation number is that the probe and quencher are mainly solubilized within the surfactant aggregates. Studies of aqueous surfactant solutions often used the fluorescent probe pyrene that has a very low solubility in water and an excited state with a long lifetime.10-13 Measurements of surfactant aggregation number use pyrene and one of its quenchers, such as an alkylpyridinium chloride, or pyrene alone at a sufficient concentration.10,12 Indeed, a pyrene molecule in the excited state can bind a pyrene molecule in the ground state, forming an excimer, a process that is analogous to a quenching process, the quencher being the ground-state pyrene molecule.10 A first advantage of the use of pyrene excimer over that of a pyrene/ quencher pair is that the former permits measurements at lower surfactant concentrations. Indeed a low surfactant concentration means a low concentration of quencher and difficulties associated to quencher partitioning between aggregates and aqueous phase. The very low solubility of pyrene in water, as compared to that of usual quenchers (1) Zana, R.; Benrraou, M.; Rueff, R. Langmuir 1991, 7, 1072. (2) Menger, F. M.; Littau, C. A. J. Am. Chem. Soc. 1993, 115, 10083. (3) Rosen, M. J.; Liu, L. J. Am. Oil. Chem. Soc. 1996, 76, 885. (4) Song, L.; Rosen, M. J. Langmuir 1996, 12, 1149. (5) Rosen, M. J.; Mathias, J. H.; Davenport, L. Langmuir 1999, 15, 7340. (6) Pinazo, A.; Wen, X.; Perez, L.; Infante, M.-L.; Franses, E. Langmuir 1999, 15, 3134. (7) Menger, F. M.; Keiper, J. S.; Azov, V. Langmuir 2000, 16, 2062. (8) Mathias, J. H.; Rosen, M. J.; Davenport, L. Langmuir 2001, 17, 6148. (9) Zana, R. J. Colloid Interface Sci. 2002, 246, 182. (10) Atik, S.; Nam, M.; Singer, L. Chem. Phys. Lett. 1979, 67, 75. (11) Infelta, P. Chem. Phys. Lett. 1979, 61, 88. (12) Zana, R. In Surfactant Solutions. New Methods of Investigation; Zana, R., Ed.; Marcel Dekker: New York, 1987; p 256. (13) Almgren, M. Adv. Colloid Interface Sci. 1992, 41, 9.
of the pyrene fluorescence, much reduces partitioning and associated problems. A second advantage of the pyrene excimer method is that it uses a pyrene concentration that is about N-fold larger than that used with a pyrene/ quencher pair. This results in a much larger signal/noise ratio in the decay experiments. Those are the two main reasons for using the pyrene excimer method. However even this method has limits in terms of surfactant concentration and value of the aggregation number that can be investigated. We now show that these limits have been exceeded in the study reported in ref 8. At the outset we note that Mathias et al. tested the good operating condition of their time-resolved fluorescence spectrometer by measuring correct values of the surfactant aggregation number for conventional surfactant systems.8 However, this does not mean that correct aggregation numbers would be measured under any experimental condition. Large errors may still be made or the measured values may have little meaning when the surfactant concentration is too low or the aggregation number too small, as is the case in the premicellar range of concentration of the investigated long-chain gemini surfactants.8 The lower bound value of the surfactant aggregation number, N, that can be measured by time-resolved fluorescence spectroscopy is discussed first. A value of the molar concentration ratio n ) [probe]/[aggregate] around 1 is recommended for accurate measurements of N.10-13 Thus, the average molar ratio pyrene (or quencher) over surfactant in aggregates would be 1/10 ) 0.10 if N ) 10, and 1/2 ) 0.50 if N ) 2. On the basis of Poisson’s distribution of the pyrene among the micelles10-13 about one-third of the micelles contains an amount of pyrene twice as large. Such a high content of pyrene in the aggregates certainly perturbs the state of aggregation of the surfactant, to an extent that can be extremely large and difficult to estimate. Measured values of the aggregation number around 10 may retain a semiquantitative meaning. Measured values of N around 2 have probably very little meaning, the investigated aggregates being mixed pyrene/surfactant aggregates and not surfactant aggregates that have solubilized pyrene. Consider now the lower bound value of the surfactant concentration that can be investigated by time-resolved fluorescence spectroscopy. It is easy to show that the method cannot be used at too low surfactant concentration. For instance with N ) 10 and n ) 1, the required concentration of pyrene (or quencher) is 0.001 mM or 0.0001 mM at surfactant concentration of 0.01 or 0.001 mM, respectively. With N ) 2 the required concentration of pyrene would be 5 times larger but still very low. At such a low concentration pyrene is partitioned between water and the intermicellar solution and the results must be corrected for this partition. Besides, the decay plot then includes a fast component associated to the pyrene solubilized in the intermicellar solution, and that adds to the component corresponding to the excimer quenching. This can introduce very large errors if not properly accounted for. These errors are not avoided by the procedure used by Mathias et al.,8 that is the extrapolation
10.1021/la0200360 CCC: $22.00 © 2002 American Chemical Society Published on Web 08/23/2002
7760
Langmuir, Vol. 18, No. 20, 2002
of the long time part of the decay curve to time zero, and can be particularly large if the measurements are performed on systems that have not been deoxygenated. These aspects are not discussed by Mathias et al. and the investigated solutions do not appear to have been deoxygenated.8 Many of the reported results refer to concentrations in the range 0.01-0.001 mM.8 One surfactant was even investigated down to 0.0001 mM. The corresponding values of the aggregation numbers are therefore affected by the errors just discussed. These errors can be so large that the reported N values become meaningless. The values of N reported for the investigated long-chain gemini surfactants are all close to 2,8 suggesting that only dimers are formed as premicellar aggregates, whereas larger premicellar aggregates are also expected to occur as the surfactant concentration comes closer to the cmc. Also, for the (C18N)2Ar surfactant the measured aggregation number goes through a sharp maximum in the concentration range between 1 and 10 µM (see Figure 13 in ref 8). Some element of explanation can be given for this last result. When the concentration of a surfactant in solution approaches the cmc, the presence in the solution of a very hydrophobic molecule such as pyrene can result in the formation of mixed pyrene/surfactant aggregates via hydrophobic interactions, even if the surfactant alone would not self-associate. The formation of such mixed aggregates results in a decrease of the pyrene intensity ratio I1/I3 at a concentration that is lower than the cmc. Such decreases have been reported for conventional surfactants that have a cmc value in the 0.1-10 mM range.14-16 The mixed pyrene/surfactant aggregates are expected to occur at concentrations that are increasingly smaller than the cmc as the surfactant hydrophobicity increases. For the very hydrophobic gemini surfactant (C18N)2Ar that has a very low cmc,8 the tendency to form mixed aggregates is very strong. The mixed pyrene/ surfactant aggregates will give rise to an excimer emission. A maximum of the excimer emission intensity is very often seen close to the cmc.17-19 Indeed the amount of pyrene (14) Zana, R.; Le´vy, H. Colloids Surf. 1997, 127, 229. (15) Zana, R.; Le´vy, H.; Kwetkat, K. J. Colloid Interface Sci. 1998, 197, 370. (16) Alargova, R. G.; Kochijasshky, I. I.; Sierra, M. L.; Kwetkat, K.; Zana, R. J. Colloid Interface Sci. 2001, 235, 119. (17) Kim, J.-H.; Domach, M. M.; Tilton, R. D. J. Phys. Chem. B 1999, 103, 10582. (18) Kim, J.-H.; Domach, M. M.; Tilton, R. D. Langmuir 2000, 16, 10037. (19) Zana, R. Unpublished results on the nonionic surfactants dodecylmaltoside and Montanox 80 (from Seppic, France). These two surfactants have cmc values in the 10-20 µM range.
Comments
involved in mixed pyrene/surfactant aggregates increases with the surfactant concentration. A maximum of the intensity of excimer emission is expected when the value of the molar concentration ratio [pyrene]/[surfactant] in the mixed aggregates is a maximum. This occurs at a concentration close to the cmc. At concentration close to but above the cmc the [pyrene]/[surfactant] ratio decreases upon increasing surfactant concentration because the amount of surfactant in the mixed aggregates increases faster than the amount of pyrene. Time-resolved fluorescence experiments performed in such conditions will show a maximum of the measured aggregation number at a surfactant concentration very close to that where the pyrene excimer emission is a maximum. Note that at the maximum in the N vs [surfactant] plot in Figure 13 of ref 8 the reported aggregation number of 60 corresponds to a pyrene concentration of about 10-5/60, that is 0.16 µM, for n close to 1. This concentration is well below the solubility of pyrene in water and a significant amount of pyrene is thus expected to be partitioned in the intermicellar solution, affecting the value of the measured aggregation number. In conclusion, Mathias et al.8 are certainly right as regards the formation of premicellar aggregates in the aqueous solutions of the investigated long-chain gemini surfactants. Also the values of the aggregation numbers that they report for solutions with sufficiently high surfactant concentration are most likely correct. However in view of the arguments presented above, the N values close to 2 reported in the premicellar range, at concentration at and below 0.01 mM, are likely to be affected by such large errors that they have little meaning. Besides, that sharp maximum of N value observed at surfactant concentrations below the cmc, between 1 and 10 µM, does not reflect a true change of aggregation number. It is due to the formation of mixed pyrene/surfactant aggregates. The state of aggregation of long-chain gemini surfactants in the premicellar range can probably be investigated by methods that are sensitive to the number of species present in the solutions or by electrical conductivity. The later has been already used by Pinazo et al.6 for such a study. Raoul Zana
Institut C. Sadron (CRM-EAHP), CNRS-ULP, 6, rue Boussingault, 67083 Strasbourg cedex, France Received January 10, 2002 In Final Form: June 12, 2002 LA0200360
