Femtosecond water dynamics in reverse-micellar nanodroplets

Vibrational energy relaxation and ultrafast thermalization following impulsive excitation of the OH-stretching band of water nanodroplets confined to reverse micelles is studied by infrared pump–probe spectroscopy with sub-100 fs time resolution. The self-consistent analysis of experimental data for micelles diameters ranging from 1 to 10 nm as well as for bulk water reveals distinctly different vibrational lifetimes for the water molecules in the bulk-like core (270 fs) and in the surfactant vicinity (800 fs), which is a direct proof of a strongly disturbed hydrogen-bond network. 2005 Elsevier B.V. All rights reserved. The geometric confinement of liquid water is a central issue for a wide variety of research areas ranging from the materials to the life sciences [1,2]. In particular, aqueous droplets embedded in nanoporous hosts serve as perfectly size-adapted reaction media for the heterogeneous synthesis of novel semi-conducting colloidal materials with promising applications in optoelectronics [3]. Nanometer-dimensioned water inclusions such as water wires and pockets are key ingredients that determine the tertiary structure and, consequently, the function of proteins [2]. The quasi-two-dimensional confinement of water at membrane interfaces [4] is considered crucial to the dynamics of biological charge and mass transfer and hence, for the communication between cells or cell compartments [5]. As a model system of water under geometrically confined conditions, aqueous reverse micelles have recently raised considerable interest [6]. The reverse micellar L2-phase of the ternary oil-surfactant–water mixtures is composed of nearly spherical 0009-2614/$ see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.04.020 * Corresponding author. E-mail addresses: g.d.cringus@rug.nl (D. Cringus), m.s.pchenitchnikov@rug.nl (M.S. Pshenichnikov). nanometer-sized water droplets covered by a monolayer of amphiphilic surfactant, thereby immersing the otherwise immiscible pair of liquids. MD simulations show [7] that the structure of water in reverse micelles with radii larger than 1 nm can be discussed in terms of bulk-like and AOT-bound water molecules (frequently called free and bound molecules, respectively, which might be misleading). Bulk-like water, found in the center of the micelle, has a hydrogen-bond network similar to bulk phase water. AOT-bound water, which forms a hydration layer of the polar surfactant head groups, is characterized by a strongly perturbed hydrogen-bond network as compared to the bulk. Aqueous reverse micelles have been under extensive experimental scrutiny through a variety of experimental methods, e.g., nuclear magnetic resonance [8], THz spectroscopy [9], neutron scattering [10], linear infrared [11] and nonlinear ultrafast spectroscopy [12–19]. To date, most of the femtosecond time-resolved studies have concentrated on dynamics of a solvated chromophore such as a dye molecule [12,17], a small inorganic anion [13], a hydrated electron [14], or an HODmolecule in aqueous environment [20]. These experiments have confirmed the earlier conclusion that with a gradually increasing size of Fig. 1. OH-stretching band of bulk water (pink diamonds and shaded contour) and of water trapped in reverse micelles of sizes corresponding to l = 35 (blue triangles), l = 10 (green circles) and l = 1 (red squares). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) D. Cringus et al. / Chemical Physics Letters 408 (2005) 162–168 163 the micelle the properties of the confined water approach those of bulk water. Recently, ultrafast dynamics of H2O in reverse micelles have been investigated by picosecond infrared [15,16] and by IR/anti-Stokes Raman [18] spectroscopy following direct excitation of the OH-stretching mode of water molecules. These experiments (especially the latter ones) revealed a number of fascinating and detailed insights into the process of heat transfer from the water droplet to the micelle exterior. However, they did not address water dynamics within the nanodroplets as the picosecond time resolution was insufficient to unravel such dynamics. In this Letter, we report results of pump–probe experiments on vibrational energy relaxation in aqueous reverse micellar nanodroplets following an OH-stretching mode excitation with 70-fs mid-IR pulses. The high temporal resolution allows us to disentangle vibrational population relaxation, energy equilibration within the entrapped water pool and finally, heat flux away from the micelles into the surrounding oil phase. On the basis of the developed model we show that for large micelles enclosing tens of thousands of water molecules, the population dynamics are similar to those in bulk water. In the smallest micelles, which contain only tens of water molecules, the OH-stretch lifetime is considerably longer than in pure water but is still much shorter than that of highly diluted water of liquid binary mixtures [21–23]. Dynamics in intermediate-size micelles can be decomposed into independent contributions of these two limiting cases, which strongly suggests that the OH-lifetime is predominantly sensitive to interactions with nearest neighboring molecules. Femtosecond IR pulses with a central wavelength of 3 lm were generated in a standard chain of optical parametric amplifiers (OPA) pumped by a 1-kHz Ti:Sapphire system. The OPA output was split into an intense pump and a weak probe beam, both of which were then focused in the sample and subsequently dispersed by a monochromator to probe the dynamics at a selected frequency. The spectral content of the 70-fs pulses allowed for measuring pump–probe signals over a wide range of detection frequencies without tuning of the 3400 cm 1 central frequency of the pulses. For the experiments on bulk water the pump and probe pulses were derived from two independent OPAs to further enhance the spectral coverage. In this case, the pump duration was somewhat longer ( 150 fs). The rotational-free signals were either measured directly under a magic angle (54.7 ) geometry or recalculated from two orthogonally polarized probe scans according to Ref. [24]. The details of the anisotropy measurements are discussed elsewhere [19]. The micelles samples were prepared by shaking for several minutes mixtures of bis(2-ethylhexyl) sulfosuccinate sodium salt (AOT, SigmaUltra 99%), 2,2,4-trimethylpentane (isooctane, Aldrich 99.8%) and water (Aldrich, HPLC grade). Aqueous nanodroplets in the ternary mixture isooctane–H2O–AOT are commonly characterized by the molar water-to-AOT ratio, l = [H2O]/[AOT], and the water mass fraction, a = mwater/mtotal [6]. The former quantity defines the size of the entrapped water droplet whereas the latter determines the reverse micelles concentration. By adjusting the molar water-to-AOT ratio the properties of water can be continuously tuned from an aqueous nanometer-sized object to the aqueous bulk phase. The water mass fraction was a 0.01, resulting in optical density (OD) within 0.5–1.2. To avoid accumulation of thermal effects, the samples at room temperature were contained in a 100 lm thick free-standing jet; for bulk water a rotating cell with a sample thickness of 2 lm was used. We investigated the influence of a geometrical confinement on the vibrational dynamics of water by studying aqueous nanodroplets with 1 6 l 6 35 corresponding to diameters ranging from 1 to 10 nm. For each reverse micelle size, the complete data set consists of pump–probe transients measured as a function of delay between pump and probe pulses up to 25 ps, and monochromator settings from 3100 to 3650 cm 1 with a step of 50 cm . For small micelles (l = 1–2), the amount of free bulk-like water is thought to vanish, for medium sizes (l = 7–10) there is an approximately equal number of bound and free water molecules in the nanodroplet, while for the largest micelles (l = 35) most of the water molecules belong to the aqueous bulk-like core [7]. The linear absorption spectra clearly illustrate this trend (Fig. 1). For very large micelles (Fig. 1, blue triangles), the absorption spectrum is almost identical to that of pure water (Fig. 1, pink diamonds and shaded contour). Decreasing the micelle diameter causes a blue shift of the spectrum (Fig. 1, green circles) indicating a strong perturbation of the hydrogen-bond network of 164 D. Cringus et al. / Chemical Physics Letters 408 (2005) 162–168 water molecules. The effect is even more pronounced when water is confined in the smallest micelles (Fig. 1, red squares) with yet even weaker hydrogen bonds formed presumably between water and AOT molecules. Breaking and weakening of the hydrogen bonds result in a distinct blue shift ( 100 cm ) of the latter spectrum compared to that of the largest micelles. Fig. 2a shows transient spectra for three representative micelle sizes at a pump–probe delay of 100 fs. These spectra are mostly related to the excited state, |1æ, of the OH-stretching mode because at such a short delay, its depopulation is negligible. All three signals are characterized by induced bleaching and stimulated emission of the |0æ ! |1æ vibrational transition (maximum around 3500 cm ), and induced absorption of the |1æ ! |2æ vibrational transition whose center frequency is shifted to 3200 cm 1 due to anharmonicity. However, a Fig. 2. Transient spectra at 100 fs delay (a) and pump–probe transients at the wavelength of 3450 cm 1 (b) for samples with l = 1 (red squares), l = 10 (green circles) and l = 35 (blue triangles), and bulk water (pink diamonds). The peak and the dip in (a) correspond to bleaching/emission at the |0æ ! |1æ transition and induced absorption at the |1æ ! |2æ transition, respectively, as schematically shown by diagrams. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) straightforwa