3590
Langmuir 1997, 13, 3590-3590
Additions and Corrections Aggregation Number for Sodium Deoxycholate from Steady-State and Time-Resolved Fluorescence A. Jover, F. Meijide, E. Rodrı´guez Nu´n˜ez, J. Va´zquez Tato,* and M. Mosquera Langmuir 1997, 13, 161. In the final editing of this manuscript, the following reference was omitted: Ueno, M.; Asano, H.; Gotoh, N.; Uchida, S.; Sasamoto, H. Colloids Surf. 1992, 67, 257. This reference should be written as 5d. In last paragraphs of pages 161 and 164, we affirmed that Zana and co-workers (refs 4 and 5a)† have used a static fluorescence method to determine the aggregation numbers of bile salt micelles, which is not actually the case. In both paragraphs the cited references have to be substituted by this missed ref 5d. The numerical value of N ) 18 corresponding to the measured aggregation number for sodium cholate also comes from this reference. From the analysis of the fluorescence decay curves (eq 1), Zana and Gu¨veli (ref 4) obtained the aggregation number for NaDC aggregates at different surfactant and NaCl concentrations and the aggregation number of mixed micelles of NaC and nonionic surfactants:5a
I(t) ) I(0) exp{-kt - n[1 - exp(-kEt)]}
(1)
In this equation I(t) and I(0) are the intensities of fluorescence at t and 0 times, kE is the rate constant for the intramolecular excimer formation, k is the decay rate constant of the probe in the aggregate, and the average occupancy number, n, is related to the aggregation number, N, through equation
N)
n(C - Cbulk) [P]M
where C is the total bile salt concentration, Cbulk its concentration in the bulk (usually taken as constant and equal to cmc), and [P]M the concentration of probes in aggregates or micelles. †Reference
numbers correspond to the original paper.
S0743-7463(97)00407-1 CCC: $14.00
Obviously, the analysis of the fluorescence decay curves allows the determination of N at every single bile salt concentration. In our paper, the aggregation number was obtained as a fitting parameter and therefore was considered constant with bile salt concentration. When the aggregation number increases with surfactant concentration, this could lead to erroneous values and conclusions. This is a very important question because it is related to the old controversies concerning the associative behavior of this type of surfactant. In a recent paper, Nagarajan16 has showed that polydispersity and the increase of the aggregation number with both the total surfactant concentration and inert added salt concentration are closely related: if micelles are all of a single size, their size cannot increase significantly with either increasing surfactant or inert added salt concentrations. But if the micelles are polydisperse, the weight average aggregation number must necessarily increase with total surfactant concentration. For NaDC, Zana and Gu¨veli4 have found that “within the experimental error, the values of N are independent of the bile salt and NaCl concentration...” At NaCl 0.4-0.6 M, the average value was N ) 11 ( 2. On the other hand, Kratohvil et al.,16 from static light scattering experiments, have published values of N equal to 8 and 11.6 at 0.15 and 0.6 M NaCl, respectively. Obviously, both experimental techniques give exactly the same aggregation numbers. Furthermore, from Kratohvil et al.16 results, a value of 6 is obtained by linear extrapolation at zero NaCl concentration. Such a value is in close agreement with published results for NaDC from freezing point measurements11 ()5.8, zero NaCl) and the one obtained in the our commented paper ()8 ( 2). As a resume, those quite different experimental techniques lead to aggregation numbers which are in a very close agreement. The final consequence is that bile salt aggregates are not highly polydisperse and that our value of N ) 8 can be considered correct as far as bile salt aggregates do not increase with surfactant concentration very much. LA970407Q
© 1997 American Chemical Society
