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J. Phys. Chem. B 2004, 108, 19246-19254
Role of Micellar Size and Hydration on Solvation Dynamics: A Temperature Dependent Study in Triton-X-100 and Brij-35 Micelles Manoj Kumbhakar, Teena Goel, Tulsi Mukherjee, and Haridas Pal* Radiation Chemistry & Chemical Dynamics DiVision, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India ReceiVed: July 20, 2004; In Final Form: September 15, 2004
The temperature effect on solvation dynamics has been investigated in two neutral micelles, namely, Triton X-100 (TX100) and Brij-35 (BJ-35), using dynamic fluorescence Stokes’ shift method, to explore the role of micellar size and hydration on the solvation process. In TX-100, the temperature effect on C(t) is not only very strong but shows an unusual inversion around 298 K. On the contrary, for the BJ-35 micelle, the temperature effect is not that significant. Present results on solvation dynamics in the two micelles have been rationalized on the basis of the temperature dependent changes in micellar size and hydration, which are reported to be very large for TX-100 but very marginal for BJ-35. Observed inversion in the solvation rate in TX-100 around 298 K is inferred to be arising due to the interplay of two factors, one is the largely reduced micellar size at lower temperature, which causes the bulk water to come reasonably closer to the probe and thus contribute to the solvation process, and the other is the largely increased hydration at higher temperature that makes the micellar structure loose and consequent enhancement in the solvation rates. With intermediate micellar size and hydration, the solvation dynamics is slowest at around 298 K. For the BJ-35 micelle, as the micellar size and hydration does not change to any significant extent, there is almost no temperature effect on the solvation dynamics for this micelle.
1. Introduction The solvation dynamics in different heterogeneous media, such as micelles,1-5 microemulsions,6-9 lipids,10,11 proteins,12-15 DNA,16 cyclodextrin,17,18 etc., has been the subject of extensive research in about the last decade. It has been observed that the solvation process is 2-3 orders of magnitude slower in the confined media in comparison to that in bulk water.1-21 Over the years, efforts have been made to understand the details of the responses of the water molecules around the probe in heterogeneous media, both experimentally and theoretically.19-21 Understanding the water dynamics in confined media is having a direct relevance to the biological systems.22 Micelles are pictured as a simplified version of the molecular aggregates, which resemble some of the characteristics of the rather complex and subtle biological systems like proteins, lipids, and DNA environments.19-22 Micellar systems also have a wide variety of scientific, engineering, and technological applications.23 Properties of micelles are largely dependent on their size, shape, composition, etc.3,24-32 In some cases the behaviors of the micelles critically depend on the surrounding environments. It is thus important to study the solvation dynamics in micellar media under different external conditions to understand how the changes in the microenvironments affect the dynamics of the solvation process in these confined systems. For micelles, it is possible to change their internal as well as external environments by changing the temperature or pressure of the solution,3,28-31 or by adding a suitable electrolyte.26,27,32 Recently, Hara et al.3 have observed substantial changes in the solvation dynamics in the TX-100 micelle on changing the applied pressure and explained their results in terms of the * Corresponding author. E-mail:
[email protected]. Fax: 9122-25505151 and 25519613.
changes in the degree of hydration of the micelle and the changes in the strength of the intermolecular hydrogen bonding for the confined water molecules. For the TX-100 micelle, it is reported that the micellar size and hydration increase substantially with increasing temperature.30,31 In a recent article, Sen et al.5 have reported the temperature effect on solvation dynamics in the TX-100 micelle using 4-aminophthalimide (4AP) as the fluorescence probe. They have observed a gradual increase in the solvation rate with an increase in the temperature. These authors have explained this observation simply on the basis of the enhancement in the rates of the activation-controlled exchange process between the free and bound water molecules in the micellar phase with temperature. According to these authors,5 the temperature dependent changes in the micellar size and hydration do not cause any significant effect on the observed solvation process.5 Because the changes in the micellar size and hydration with temperature are quite large for TX-100,30,31 it is difficult to accept the inference of Sen et al.5 that these changes have no effect on the observed solvation dynamic. Moreover, if the micellar size and hydration was really not playing any significant role in determining the dynamics of the confined water molecules, the solvation process in different micelles was expected to occur almost in the similar time scale. Literature reports, however, indicate a completely different picture. Solvation dynamics in the cetyltrimethylammonium bromide (CTAB) micelle is seen to be substantially slower than that in sodium dodecyl sulfate (SDS) micelle, even though the Stern layer thicknesses are comparable for both the micelles.1,19,20 It is suggested that the slower solvation process in CTAB than in SDS is due to the comparatively dry Stern layer for the former micelle than the latter. For a neutral micelle like TX-100, the solvation rate is much slower than in ionic micelles such as SDS and CTAB and is attributed to the much larger Palisade
10.1021/jp0468004 CCC: $27.50 © 2004 American Chemical Society Published on Web 11/17/2004
Role of Micellar Size and Hydration on Solvation Dynamics CHART 1
TABLE 1: Micellar Parameters for TX-100 and BJ-35 Micelles at Different Temperaturesa rmicelle (Å) temp, (K)
TX-100
BJ-35
288 298 308
37 44 54
44
CMC (mM) b
Nagg b
TX-100
BJ-35
TX-100
BJ-35b
0.334 0.319 0.294
0.10
48 86 150
40
a The parameters have been obtained from refs 28-31. b For BJ-35 there is no significant change in micellar parameters with temperature.
layer thickness (∼25 Å) for the former micelle than the Stern layer thickness (∼9 Å) of the latter two micelles.1,19,20 Due to the very thick Palisade layer, most of the fluorescence probes in the TX-100 micelle reside quite deeply into the micelle, where the water structure is not as loose as that in the Stern layer of CTAB and SDS micelles having substantially thinner micellar Stern layers. Accordingly, the response of the water molecules around the probe in the TX-100 micelle is slower in comparison to that in CTAB and SDS micelles. From the literature reports it is thus indicated that the micellar size and hydration play a significant role in determining the solvation dynamics in micellar media. The reports also indicate that the locations of the probe in the micelle have a significant role in determining the rate of the solvation process. To resolve the paradox whether the temperature effect on the solvation dynamics in micellar solution is mainly due to the enhancement in the exchange process for the free and bound water molecules or the changes in the micellar size and hydration also play a role, in the present work we have carried out temperature dependent studies on the solvation dynamics in TX100 and Brij-35 (BJ-35) micelles, using coumarin-153 (C153) as the fluorescence probe. The chemical structures of the surfactants, TX-100 and BJ-35, and that of the probe, C153, are shown in Chart 1. Both TX-100 and BJ-35 are reported to form neutral spherical micelles. Due to large differences in the number of the hydrophilic oxyethylene units in TX-100 and BJ-35, their micellar characteristics are, however, quite different. Important micellar parameters for TX-100 and BJ-35 micelles are listed in Table 1. As reported in the literature,28-31 the size and hydration of the TX-100 micelle increases largely with temperature, but for BJ-35, there is not much of a temperature effect on the micellar size and hydration. Thus, it was expected that the temperature effect on the solvation dynamics in TX100 and BJ-35 micelles will help us answer whether the changes in the micellar size and hydration with temperature do have some effect or not on the observed changes in the solvation dynamics with temperature. 2. Experimental Section TX-100 was obtained from Sigma and BJ-35 was obtained from Pierce Chemical Co., and they were used without further purification. Laser grade C153 was obtained from Exiton and used as received. Nanopure water, having a conductivity of ∼0.1 µS cm-1, was obtained by passing distilled water through a
J. Phys. Chem. B, Vol. 108, No. 50, 2004 19247 Barnstead Nanopure Water System and used for the preparation of the micellar solutions. In the experimental solutions the concentrations of TX-100 and BJ-35 surfactants were such that the concentration of the micelles remains in the range of ∼1 mM. For all the experiments, the concentrations of the probe C153 were kept quite low, only in the range of ∼10 µM. With this experimental condition, where the probe concentration is ∼100 times lower than that of the micelles, it is expected that none of the micelles contains more than one probe molecule in it. Because the dye C153 is almost insoluble in water,33 and because the dye solubility increases substantially in micellar solutions, it is expected that the dye C153 mostly solubilized in the micellar phase. Steady-state absorption spectra were recorded using a JASCO (Tokyo, Japan) model V530 spectrophotometer. Fluorescence spectra were recorded using a Hitachi (Tokyo, Japan) model F-4010 spectrofluorometer. Time-resolved fluorescence measurements were carried out using a diode laser based spectrofluorometer from IBH. The instrument works on the principle of time-correlated single-photon counting (TCSPC).34 In the present work, a 408 nm diode laser (
