Article pubs.acs.org/JPCA
Rotational Diffusion of Coumarin 153 in Nanoscopic Micellar Environments of n‑Dodecyl-β‑D‑maltoside and n‑Dodecylhexaethylene-glycol Mixtures J. M. Hierrezuelo and C. Carnero Ruiz* Grupo de Fluidos Estructurados y Sistemas Anfifílicos, Departamento de Física Aplicada II, Escuela de Ingenierías, Universidad de Málaga, Málaga, Spain S Supporting Information *
ABSTRACT: The microstructure of mixed micelles containing n-dodecyl-β-Dmaltoside and n-dodecyl-hexaethylene-glycol, two nonionic surfactants belonging to the alkyl polyglucoside and polyoxyethyelene alkyl ether families, respectively, has been investigated. With the aim of understanding how the micellar composition affects the microenvironmental properties of micelles, we have examined the photophysics and dynamics of the neutral probe coumarin 153 in the binary mixtures of the surfactants across the entire composition range. We present data on the steady-state absorption and emission spectra of the probe, as well as fluorescence lifetimes and both steady-state and time-resolved fluorescence anisotropies. These data indicate that the participation of the ethoxylated surfactant in the mixed micelle induces an increasing hydration in the palisade layer of the micelle, which forces the probe to migrate toward the inner micellar region, where it senses a slightly less polar environment. The time-resolved fluorescence anisotropy data were analyzed on the basis of the two-step and wobbling-in-cone model. The average reorientation time of the probe molecule was found to decrease with the presence of the ethoxylated surfactant, in good agreement with steady-state fluorescence anisotropy data, suggesting a reduction of the microviscosity in the solubilization site of the probe. The behavior of all diffusion reorientation parameters was analyzed on the basis of two factors: the micellar hydration and the headgroup flexibility of both surfactants. It was concluded that the increasing participation of the ethoxylated surfactant induces a greater hydration in the micellar palisade layer, producing the formation of a less compact microenvironment where the probe experiences a faster rotational reorientation.
1. INTRODUCTION Under the right conditions of concentration and temperature, amphiphilic molecules or surface-active agents (surfactants) self-assemble in solution to form thermodynamically stable nanometer-sized structures called micelles. Micelles can be envisaged as amphiphilic colloids composed of two distinct regions with opposite affinities toward the solvent system.1 Because of their unique characteristics, surfactant-based systems are frequently used in many technical applications. For instance, properties such as colloidal stability or micellar size have strong implications in the performance of many home and personal care products.2 In addition, micellar systems can act as nanoreactors that may dramatically modify not only the velocity but also the mechanism of a determined reaction.3 Moreover, micelles are currently emerging as nanotherapeutic systems in the pharmaceutical industry; they can provide a useful model system in the study of fundamental interactions between the carrier, the drug, and the cell, the results of which can subsequently be transferred to more complex systems.4−7 In many of these applications, mixed surfactant systems are often preferred because they show numerous advantages compared with those comprising a single surfactant. As a consequence, considerable efforts have been made to elucidate their behavior © 2012 American Chemical Society
in solution and the different structural aspects in mixed systems composed of two or more surfactants of a different nature.8−12 Nonionic surfactant-based systems are often preferred over ionic ones because of their greater tolerance to the presence of electrolytes or changes in pH. They are also being included in both liquid and lyophilized formulations due to their favorable behavior in the stabilization of proteins and reduced toxicity profiles.13 In recent years, mixed surfactant systems formed by the combination of conventional ethoxylated and sugar-based surfactants have been the subject of considerable attention from both fundamental and applied viewpoints.14−25 Although, generally, these systems mix ideally, they show some interesting properties that can be regulated by varying the system composition; this can be advantageous in certain applications. The mixed system formed by n-dodecyl-β-D-maltoside (βC12G2) and n-dodecyl-hexaethylene-glycol (C12E6) is a representative example that has recently been studied from different perspectives.20−22 Stubenrauch and co-workers have examined this mixed system with regard to surface properties, Received: August 23, 2012 Revised: December 5, 2012 Published: December 6, 2012 12476
dx.doi.org/10.1021/jp308379j | J. Phys. Chem. A 2012, 116, 12476−12485
The Journal of Physical Chemistry A
Article
bulk properties, foam films, and foams.20,21 They reported that most of their experimental observations could be explained qualitatively by the different hydration and headgroup flexibility properties plus the higher surface activity of the ethoxylated surfactant.21 Moreover, Bäverbäck et al.22 have carried out a detailed structural study on the same mixed system. Although their static light scattering data indicate that β-C12G2 forms small globular micelles, while both pure C12E6 and the mixtures form elongated micelles of much higher molar mass, SANS and SAXS data for these latter systems require more complex models, including the coexistence of spherical and spherocylindrical micelles, to describe the experimental results. This article is intended to offer a complementary picture of the same system. With this purpose, we have carried out an investigation focused on the effect of the system composition on the microenvironmental properties of micelles. The motivation of our study relies on the fact that both surfactants have structurally different head groups (see Chart 1); maltoside
2. EXPERIMENTAL SECTION 2.1. Materials. The surfactants n-dodecyl-β-D-maltoside (βC12G2) and n-dodecyl-hexaethylene-glycol (C 12E6) were obtained from Calbiochem (Ultrol, 99.7%) and Sigma (BioXtra ≥99%), respectively, and used without further purification. Stock solutions of both surfactants were prepared by dissolving a known mass in ultra-pure water. The ultra-pure water, with a resistivity ∼18 MΩ·cm, used to prepare the aqueous micellar solutions was obtained by passing pure water from a Millipore Elix system thru an ultra-high quality Millipore Synergy purification system. The experimental solutions, with a total surfactant concentration of 20 mM, were prepared daily by diluting the stock solutions. The bulk composition of each mixed system is expressed in terms of mole fraction of the ethoxylated surfactant, α2, which is defined by the relationship [C12E6]/[(β-C12G2] + [ C12E6]), where [β-C12G2] and [C12E6] are the molar concentrations of β-C 12 G 2 and C 12 E 6 , respectively. Laser grade coumarin 153 (C153) dye was purchased from Exciton and also used as received. A 1 mM stock solution of C153 was prepared in methanol and stored at 4 °C. Measurement samples of 7 μM in C153 were prepared by adding appropriate (small) volumes of the methanolic probe solution to the aqueous micellar solutions. 2.2. Methods. Absorption spectra were recorded using a Cecil 2021 UV−vis spectrophotometer and 1 × 1 cm quartz cells. Steady-state fluorescence measurements were made using a FluoroMax-4 (Horiba Jobin Yvon) spectrofluorometer in “S” mode, with a 1 cm path-length quartz cuvette. This apparatus is equipped with a Peltier drive that allowed the temperature to be controlled to ±0.01 °C. Fluorescence anisotropy measurements were recorded in the same apparatus provided with a polarization accessory, which uses the L-format instrumental configuration46 and an automatic interchangeable wheel with Glan−Thompson polarizers. The steady-state fluorescence anisotropy values, rss, were determined using the equation
Chart 1. Structures of the Surfactants and Fluorescence Probe Molecule Used in the Present Investigation
being bulky and rigid, while the ethoxylated headgroup is flexible and polymer-like, and their hydration behavior is also different.26 These differences will result in changes of the local structure of the micellar palisade layer with the system composition. To explore the effect of the system composition on the micellar microstructure, we have examined the photophysics and dynamics of a hydrophobic probe, coumarin 153 (see Chart 1), solubilized in the micellar pseudophase. Coumarins are a large and important family of hydrophobic dyes, widely occurring in nature, which exhibit notable photophysical properties. In fact, these molecules have been shown to be excellent probes of solvation dynamics and local friction of different complex environments.27−30 The ability to probe the local environment around coumarin 153 and therefore examine the local polarity and viscosity of micelles has previously been exploited by several groups.31−45 Herein, steady-state and time-resolved fluorescence techniques, including time-resolved anisotropy measurements, have been used to investigate the microenvironmental properties of pure and mixed micelles of β-C12G2 and C12E6 as a function of the system composition. The main objective of this study is to investigate the possibility of using these mixed systems as potential nanocarriers of hydrophobic drugs. This is very important for comparison purposes. By gaining information on the nature of the local environment inside each nanocarrier, the properties of different drug delivery systems as encapsulation vehicles for determined hydrophobic drugs can be compared with one another.
rss =
IVV − GIVH IVV + 2GIVH
(1)
where the subscripts of the fluorescence intensity values (I) refer to vertical (V) and horizontal (H) polarizer orientation. The G factor, required for the L-format configuration, was experimentally determined by using a methanolic solution of C153, thus ensuring a very fast rotational relaxation of the probe. The anisotropy values were averaged over an integration time of 20 s, and a minimum of three measurements were made for each sample. The anisotropy values of the probes in micellar media presented in this work are the mean value of three individual determinations. All steady-state fluorescence measurements were performed at 25.00 ± 0.01 °C. Time-resolved fluorescence measurements were carried out with a LifeSpec II luminescence spectrometer (Edinburgh Instruments, Ltd.). This is a diode-laser-based spectrometer, which uses the time-correlated single photon counting technique. In the present work, a picosecond pulsed diode laser at 405 nm (Edinburgh Instruments, Ltd.) at a repetition rate of 20 MHz was used as the excitation source, with the emission being recorded at 530 nm. To optimize the signal-tonoise ratio, 104 photon counts were collected in the peak channel. The instrumental response function (IRF) was regularly obtained by measuring the scattering of a Ludox solution. For this setup, the IRF was about 230 ps at fwhm. Fluorescence decays were recorded by keeping the emission 12477
dx.doi.org/10.1021/jp308379j | J. Phys. Chem. A 2012, 116, 12476−12485
The Journal of Physical Chemistry A
Article
polarizer at the magic angle (54.7°) with respect to the vertically polarized excitation beam. The decay parameters were determined by reconvolution and fitting of the decay curves with the help of the FAST software package from Edinburgh Instruments. The intensity decay curves for all lifetime measurements were fitted as a sum of exponential terms: I (t ) =
⎛t ⎞ ⎟ ⎝ τi ⎠
∑ Ai exp⎜ i
(2)
where Ai is a pre-exponential factor of the component i with a lifetime τi. In these experiments, the temperature was maintained at the desired value (25 °C) using a Peltier system with an accuracy of ±0.1 °C. Time-resolved fluorescence anisotropy measurements were performed on the same apparatus, which is fitted with an automatic set of polarizers. These experiments were carried out by measuring the fluorescence decays for parallel, IVV(t), and perpendicular, IVH(t), polarizations with respect to the vertically polarized excitation light. The anisotropy decay, r(t), was obtained using the relationship46 r (t ) =
IVV(t ) − GIVH(t ) IVV(t ) + 2GIVH(t )
Figure 1. Absorption spectra of C153 in pure and mixed micelles of βC12G2 and C12E6 at different compositions.
Table 1. Absorption, (λabs)max, and Emission Maxima, (λem)max, and Lifetimes, τ, of C153 in β-C12G2/C12E6 Mixed Micelles As a Function of the Mole Fraction of C12E6 in the Bulk (α2)
(3)
where the G factor was also experimentally determined using a solution of C153 in methanol. In all cases, a double-exponential decay was required for the best fit of the anisotropy decays. In the analysis of either fluorescence or anisotropy decays, the quality of the fits was evaluated by the reduced χ2 values and the distribution of the weighted residuals among the data channels. The statistical criteria determining the level of fit was a reduced χ2 value of
