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Aerosol-OT reverse micelles represent a chemical construct where surfactant molecules self-assemble to stabilize water nanodroplets ~1-10 nm in diameter. Although commonly assumed to adopt a spherical shape, all-atom molecular dynamics simulations and some experimental studies predict a non-spherical shape. If these aggregates are not spherical, then what shape do they take? Because the tools needed to evaluate the shape of something that lacks regular structure, order, or symmetry are not well developed, we present a set of three intuitive metrics- coordinate-pair eccentricity, convexity, and the curvature distribution- that estimate the shape of an amorphous object and we demonstrate their use on a simulated Aerosol-OT reverse micelle. These metrics are all well-established methods and principles in mathematics, and each provides unique information about the shape. Together, these metrics provide intuitive descriptions of amorphous shapes, facilitate ways to quantify those shapes, and follow their changes over time.

In bulk aqueous environments, the exchange of protons between labile hydroxyl groups typically occurs easily and quickly. Nanoconfinement can dramatically change this normally facile process. Through exchange spectroscopy (EXSY) NMR measurements, we observe that nanoconfinement of glucose and water within AOT (sodium bis(2-ethylhexyl) sulfosuccinate) reverse micelles raises the energy barrier to labile hydrogen exchange, which suggests a disruption of the hydrogen bond network. Near room temperature, we measure barriers high enough to slow the process by as much as 2 orders of magnitude. Although exchange rates slow with decreasing temperatures in these nanoconfined environments, the barrier we measure below ∼285 K is 3-5 times lower than the barrier measured at room temperature, indicating a change in mechanism for the process. These findings suggest the possibility of hydrogen tunneling at a surprisingly high-temperature threshold. Furthermore, differences in exchange rates depend on the hydroxyl group position on the glucose pyranose ring and suggest a net orientation of glucose at the reverse micelle interface.

Aside from its prominent role in the excretory system, urea is also a known protein denaturant. Here, we characterize urea as it behaves in confined spaces of AOT (sodium bis(2-ethylhexyl) sulfosuccinate) reverse micelles as a model of tight, confined spaces found at the subcellular level. Dynamic light scattering revealed that low temperatures (275 K) caused the smallest of the reverse micelle sizes

The light-induced photolysis of [Zn(NTAdeCage)]- generates a temporally controlled burst of Zn2+, which is rapidly chelated in situ by the free ligand Zincon2-. The [Zn(Zincon)]2- coordination progress is monitored using absorption spectroscopy in bulk aqueous buffer and reverse micelle environments. The [Zn(NTAdeCage)]- photocage and free ligand Zincon2- have different reverse micelle locations that affect the [Zn(Zincon)]2- formation at the nanoscale compared to the bulk aqueous buffer. The formation of [Zn(Zincon)]2- in a bulk aqueous buffer is more efficient despite the released Zn2+ and Zincon2- being physically closer within reverse micelles. The observed reduction of complex formation is attributed to the interfacial partitioning of Zincon2-, distinct from the Zn2+ photocage in the water pool, requiring diffusion for the species to meet to form [Zn(Zincon)]2-. This work introduces a proof-of-concept methodology to experimentally measure fast chelation reactions in confined spaces and thus provides an approach to exploring cellular responses.

The research presented here reports the surprising observation that adding glucose and other carbohydrate osmolytes to the polar phase of water-containing reverse micelles causes the particles to shrink. This apparent change in reverse micelle size is attributed to two factors: an increase in the surface area per surfactant molecule induced by the presence of carbohydrate and changes in the particle shape eccentricity. The studies reported here not only focus on glucose but also explore other carbohydrate osmolytes, specifically ethylene glycol, glycerol, erythritol, xylitol, sorbitol, myo-inositol, and trehalose, in the nanoconfined environments of reverse micelles. Through two-dimensional proton nuclear Overhauser enhancement nuclear magnetic resonance spectroscopy, the osmolytes were determined to reside solvated in the aqueous interior of the reverse micelles. This paper reports the loading limit of carbohydrates into AOT [sodium bis(2-ethylhexyl)sulfosuccinate] reverse micelles, demonstrates the location of the carbohydrates in the reverse micelles, and shows an unexpected effect where the carbohydrates add to the reverse micelle volume without causing an apparent increase in the reverse micelle diameter.

The light-induced photolysis of [Zn(NTAdeCage)]

Cryoprotectants allow cells to be frozen in liquid nitrogen and cryopreserved for years by minimizing the damage that occurs in cooling and warming processes. Unfortunately, how the specific cryoprotectants keep the cells viable through the cryopreservation process is not entirely evident. This contributes to the arduous process of optimizing cryoprotectant formulations for each new cell line or species that is conserved. Coherent anti-Stokes Raman scattering microscopy facilitates the visualization of deuterated cryoprotectants within living cells. Using this technique, we directly imaged the location of fully deuterated dimethyl sulfoxide (d6-DMSO), the deuterated form of a commonly used cryoprotectant, DMSO, within rice suspension cells. This work showed that d6-DMSO does not uniformly distribute throughout the cells, rather it enters the cell and sequesters within organelles, changing our understanding of how DMSO concentration varies within the cellular compartments. Variations in cryoprotectant concentration within different cells and tissues will likely lead to differing protection from liquid nitrogen exposure. Expanding this work to include different cryoprotectants and mixtures of cryoprotectants is vital to create a robust understanding of how the distributions of these molecules change when different cryoprotectants are used.

Glucose nanoconfined by solubilization in water-containing AOT (sodium bis(2-ethylhexyl) sulfosuccinate) reverse micelles has been investigated using 1H NMR. NMR spectra reveal well-defined signals for the glucose hydroxyl groups that suggest slow chemical exchange between them and the water hydroxyl groups. Using the EXSY (ZZ-exchange) method, the chemical exchange rate from water to glucose hydroxyl groups was measured for glucose in reverse micelles as a function of size (water pool diameter of ∼1–5 nm) at 25 °C. The chemical exchange rates observed in the nanoconfined interior are dramatically slower (5–20 times) than those observed for glucose in bulk aqueous solution at the same concentration as the micelle interior. Exchange rate constants are calculated via a mechanism that accounts for these observations, and implications of these results are presented and discussed.

Polyoxometalates can serve as versatile catalysts in aqueous media. Solvation of these compounds is critical to their catalytic properties. In the studies reported here, we employ 51V NMR spectroscopy, dynamic light scattering and continuous shape measure analysis to probe the solvation of two polyoxometalates, decavanadate and monomolybdononavanadate, encapsulated in reverse micelles. The 51V NMR chemical shift reports on the protonation state of the oxometalate while its linewidth reveals the local environment sensed by the oxometalate in the reverse micelle. We have shown that placement of decavanadate in several protonation states in a microemulsion results in spectroscopic observation of the deprotonated [V10O28]6− (V10) molecule (Baruah, et al., J. Am. Chem. Soc., 2006, 128(39), 12758–12765). Previous studies have shown that the oxometalate requires at least 2–3 layers of water to stabilize it in the reverse micelles and present work shows that no differences are observed whether oxometalate has a charge of −5 or −6. The dynamic light scattering studies demonstrate that the size of the reverse micelles containing polyoxometalate does not change significantly upon loading. Finally, continuous shape measure (CShM) further describes the structural perturbation of V10 as one of the V-atoms is replaced with a heteroatom. CShM analysis shows minor structural perturbation of the oxovanadate core with heteroatom replacements even though the charge and thus overall polarity of the anion changes. Similarly, the continuous shape measure analysis of protonated forms of V10 show that the variation related to protonation of the V10 anion does not significantly change the shapes of the different classes of vanadium atoms within the compact oxoanion even though the charge and thus overall polarity of the anion changes. Thus, the change in shape and steric interactions are very minor because any change in geometry is counteracted by changes elsewhere in the structure. This study represents the first application of shape measures to oxometalates. Because size and shape could both be critical for interaction with interfaces, two differently charged oxometalates are placed in a nanosized water droplet to investigate how they are solvated. Because little change was observed in the 51VNMR spectra and smaller changes were found in the size, we conclude that the shape was most important for this interaction and that changes in charge appeared to have a smaller impact on the system.

Many students have misconceptions about how chemistry knowledge is generated. These misconceptions can be addressed by highlighting how chemistry knowledge has developed over time. To effectively accomplish this, science educators must be well-versed in the history of chemistry. In this paper, we describe topics that could be covered in a continuing education course for secondary level science educators that centers on the history of chemistry. We hope to stimulate discussions across chemistry departments to encourage our colleagues to offer such courses to in-service secondary science teachers. A history of chemistry course will allow teachers to meet the learning needs of their students. When teachers can explain how chemists made discoveries, overcame obstacles, collaborated with colleagues to answer complex questions, and built upon one another’s ideas, students are presented with a realistic picture of scientific research, hence strengthening their understanding of the nature of science.

To succeed in physical chemistry courses, students need competence with a significant amount of mathematics, including ordinary and partial differential equations and linear operators. This can present a barrier to student success because many of these topics are not taught in the traditional prerequisite calculus sequence. Through a collaboration of the Departments of Chemistry and Mathematics, Colorado State University has developed and implemented a two-semester sequence of courses, Applied Mathematics for Chemists, aimed specifically at providing students with the mathematical tools necessary for success in physical chemistry. This report summarizes one of the invited papers to the ConfChem online conference on Mathematics in Undergraduate Chemistry Instruction, held from October 23 to November 27, 2017, and hosted by the ACS DivCHED Committee on Computers in Chemical Education (CCCE).

In aqueous environments, acidity is arguably the most important property dictating the chemical, physical, and biological processes that can occur. However, in a variety of environments where the minuscule size limits the number of water molecules, the conventional macroscopic description of pH is no longer valid. This situation arises for any and all nanoscopically confined water including cavities in minerals, porous solids, zeolites, atmospheric aerosols, enzyme active sites, membrane channels, and biological cells and organelles. To understand pH in these confined spaces, we have explored reverse micelles as a model system that confines water to nanoscale droplets. At the appropriate concentrations, reverse micelles form in ternary or higher order solutions of nonpolar solvent, polar solvent (usually water), and amphipathic molecules, usually surfactants or lipids. Measuring the acidity, or local density of protons, commonly known as pH, of these nanoscopic water pools in reverse micelles is challenging. First, because the volume of the water in these reverse micelles is so minute, we cannot probe its proton concentration using traditional pH meters. Second, the traditional concept of pH breaks down in a nanosystem that includes fewer than 10(7) water molecules. Third, the interpretation of results from studies attempting to measure acidity or pH in these environments is nontrivial because the conditions fall outside the accepted IUPAC definition for pH. Researchers have developed experimental methods to measure acidity indirectly using various spectroscopic probe molecules. Most measurements of intramicellar pH have employed optical spectroscopy of organic probe molecules containing at least one labile proton coupled to electronic transitions to track pH changes in the environment. These indirect measurements of the pH reflect the local environment sensed by the probe and are complicated by the probe location within the sample and how that location affects properties such as pK(a). Thus, interpretation of the measurement in the highly heterogeneous reverse micellar environment can be challenging. Organic pH probes can often produce ambiguous acidity measurements, because the probes can readily associate with or penetrate the micellar interface. Protonation can also dramatically change the polarity of the probe and shift the probe's location within the system. As a result, researchers have developed highly charged pH-sensitive probes such as hydroxypyrene trisulfonate, vanadate or phosphate that reside in the water pool both before and after protonation. For inorganic probes researchers have used multinuclear NMR spectroscopy to directly measure conditions in the water droplet. Regardless of the probe and method employed, reverse micellar studies include many implicit assumptions. All reported pH measurements comprise averages of molecular ensembles rather than the response of a single molecule. Experiments also represent averages of the dynamic reverse micelles over the time of the experiments. Thus the experiments report results from an average molecular position, pK(a), ionic strength, viscosity, etc. Although the exact meaning of pH in nanosized waterpools challenges scientific intuition and experimental data are non-trivial to interpret, continued experimental studies are critical to improve understanding of these nanoscopic water pools. Experimental data will allow theorists the tools to develop the models that further explore the meaning of pH in nanosized environments.

Cyanine-3 (Cy3) fluorescent dye molecules confined in sodium di-2-ethylhexyl sulfosuccinate (AOT) reverse micelles were examined using steady-state absorption and emission as well as time-resolved fluorescence spectroscopy to understand the effect of confinement on the spectroscopic properties of the dye. This study explored a wide range of reverse micelle sizes, with hydrodynamic radii ranging from ∼1.7 to ∼5 nm. The relative concentrations of Cy3 and AOT reverse micelles were such that, on average, one dye molecule was present for every 2 × 10(4) to 9 × 10(5) reverse micelles. In the smallest reverse micelles examined, observed changes in the absorption and emission spectra and fluorescence lifetime of the dye molecules indicated H-aggregation of Cy3 into side-by-side dimers. It is hypothesized that this dimerization is governed by the high local concentrations that result from the confinement of the Cy3 in the reverse micelles. What is notable about this study is that this dimer occurs even at overall dye concentrations in the nanomolar range. Such concentrations are too low for aggregation to occur in bulk solution. Hence, the reverse micelles serve as nanocatalysts for this aggregation process.

Microemulsions form in mix- tures of polar, nonpolar, and amphi- philic molecules. Typical microemul- sions employ water as the polar phase. However, microemulsions can form with a polar phase other than water, which hold promise to diversify the range of properties, and hence utility, of microemulsions. Here microemul- sions formed by using a room-tempera- ture ionic liquid (RTIL) as the polar phase were created and characterized by using multinuclear NMR spectros- copy. 1 H, 11 B, and 19 F NMR spectrosco- py was applied to explore differences between microemulsions formed by using 1-butyl-3-methylimidazolium tet- rafluoroborate ((bmim)A4)) as the polar phase with a cationic surfactant, benzylhexadecyldimethylammonium chloride (BHDC), and a nonionic sur- factant, Triton X-100 (TX-100). NMR spectroscopy showed distinct differen- ces in the behavior of the RTIL as the charge of the surfactant head group varies in the different microemulsion environments. Minor changes in the chemical shifts were observed for (bmim) + and (BF4) in the presence of TX-100 suggesting that the surfactant and the ionic liquid are separated in the microemulsion. The large changes in spectroscopic parameters observed are consistent with microstructure for- mation with layering of (bmim) + and (BF4) and migration of Cl within the BHDC microemulsions. Comparisons with NMR results for related ionic compounds in organic and aqueous en- vironments as well as literature studies assisted the development of a simple organizational model for these micro- structures.

The versatility of the heterogeneous environment in microemulsions and its impact on coordination chemistry is illustrated by the simple reactions of oxovanadates in aerosol-OT (AOT)/isooctane reverse micelles (RMs). In reverse micelles, vanadate dimerization at neutral and basic pH increases compared to bulk aqueous solution, consistent with localizing in the water pool and an increase in the proton concentration. Formation of tetrameric and pentameric oxovanadate also increases at neutral and basic pH in micellar environments, but at some pH values this system achieves oxovanadate mixtures not accessible in aqueous solutions. Proton transfer reactions monitored using decavanadate suggest that the interior of water pools have proton concentration approximating neutral pH values. These results lead to the proposal that a proton gradient from the RM interior toward the periphery is established in these systems, when the initial aqueous solution is acidic prior to RM formation. In addition to solvation, the effect of these RM structures on such simple systems (although less conventional coordination complexes), suggest the potential for application of these systems to direct the coordination chemistry of metals and metal complexes for preparative purposes. For example, in the emerging field of nanochemistry RMs are used as a template for the synthesis of nanostructures.

Recent advances in ultrafast laser technology have spurred investigations of microheterogeneous solutions. In particular, researchers have explored details of reverse micelles (RMs), which present isolated droplets of polar solvent sequestered from a continuous nonpolar phase by a surfactant layer. This review explores recent studies utilizing a variety of ultrafast laser techniques to uncover details about structure and dynamics in various RMs. Using ultrafast vibrational spectroscopy, researchers have probed hydrogen-bond dynamics and vibrational energy relaxation in RMs. These studies have developed our understanding of reverse micellar structure, identifying varying water environments in the RMs. In a plethora of experiments employing probe molecules, researchers have explored the confined environment presented by RMs and their impact on a range of chemical reactions. These studies have shown that confinement, rather than the specific interactions with surfactants, is an important factor determining the impact of the reverse micellar environment on the chemistry.

The dynamics of water confined in two different types of reverse micelles are studied using ultrafast infrared pump-probe spectroscopy of the hydroxyl OD stretch of HOD in H2O. Reverse micelles of the surfactant Aerosol-OT (ionic head group) in isooctane and the surfactant Igepal CO 520 (non-ionic head group) in 50/50 weight percent cyclohexane/hexane are prepared to have the same diameter water nanopools. Measurements of the IR spectra and vibrational lifetimes show that the identity of the surfactant head groups affects the local environment experienced by the water molecules inside the reverse micelles. The orientational dynamics (time dependent anisotropy), which is a measure of the hydrogen bond network rearrangement, is very similar for the confined water in the two types of reverse micelles. The results demonstrate that confinement by an interface to form a nanoscopic water pool is a primary factor governing the dynamics of nanoscopic water rather than the presence of charged groups at the interface.

A wide variety of probe molecules have been used to characterize the environment found within reverse micelles. This manuscript reports on results from steady-state and time-resolved spectroscopies of the probe molecule Coumarin 343. The steady-state spectra for the probe molecule in ternary and quaternary systems are nearly identical. However, the time-resolved studies clearly indicate substantial differences in the probe sensed by the probe molecule. Implications of these results for the interpretation of spectroscopy of probe molecules in confined media are discussed.

Thermally induced gauche–trans isomerization and direct solute–solvent interactions of the solute, ethyl isocyanate (EIC), in the solvent, 2-methylpentane (2MP), are investigated using ultrafast infrared vibrational echo experiments and linear vibrational absorption spectroscopy of the isocyanate (N=C=O) antisymmetric stretching mode (2278 cm−1). Both the EIC vibrational echo measured pure vibrational dephasing and the absorption spectra show complex behavior as a function of temperature from room temperature to 8 K. The EIC data are compared to absorption experiments on the same mode of isocyanic acid (HNCO), which cannot undergo isomerization. To describe the observations, a model is presented that involves both intramolecular dynamics and intermolecular dynamical interactions. At room temperature, gauche–trans isomerization is very fast, and the isomerization dynamics contribution to the vibrational echo decay and the absorption line shape is small because it is motionally narrowed. The dominant contribution to both the vibrational echo decay and the absorption spectrum is from direct dynamical interactions of the solute with the solvent. As the temperature is lowered, the direct contribution to vibrational dephasing decreases rapidly, but the contribution from isomerization increases because the extent of motional narrowing diminishes. The combined effect is a very gradual decrease of the rate of pure dephasing as the temperature is initially lowered from room temperature. At very low temperature, below the 2MP glass transition, isomerization cannot occur. The absorption spectrum displays two peaks, interpreted as the distinct gauche and trans absorption bands. Even at 8 K, the pure dephasing is surprisingly fast. The direct solvent-induced dephasing is negligible. The dephasing is caused by motions of the ethyl group without isomerization occuring. At intermediate temperatures (150 K>T>100 K), isomerization takes place, but its contribution to the pure dephasing is not motionally narrowed. The absorption spectral shapes are complex. Dephasing arising from direct interaction with the solvent is small. Both isomerization and fluctuations on the gauche–trans surface contribute to the absorption line shape. The model that is used to describe the results involves a NMR type exchange calculation with additional contributions from the direct solvent interactions that are obtained from the temperature-dependent HNCO IR spectra. From the temperature dependence of the isomerization “jump” rate, the barrier height for the isomerization is found to be ∼400 cm−1.

Vibrational relaxation and hydrogen bond dynamics in methanol-d dissolved in CCl4 have been measured with ultrafast infrared pump−probe spectroscopy. We excited the subensemble of methanol-d molecules both accepting and donating hydrogen bonds at ∼2500 cm-1. Following vibrational relaxation with a ∼500 fs lifetime, the signal does not decay to zero. Rather, the signal increases to a second maximum at ∼4 ps. The decay from the second maximum occurs on two time scales. We propose a model in which hydrogen bond dissociation, following vibrational relaxation, decreases the concentration of methanol-d molecules that accept and donate hydrogen bonds and produce the observed long-lived bleach of the absorption signal. Using a set of coupled kinetic equations, the time constants for hydrogen bond dissociation and reformation have been determined. Hydrogen bond breaking occurs with ∼200 fs and ∼2 ps time constants. We attribute the fast rate to a direct breaking mechanism wherein the excited hydroxyl stretch decay...

While dynamics of polar solvation have been tabulated for a wide range of pure polar solvents, substantially less is known about the dynamic response of solvent mixtures. Here, results for polar solvation dynamics are presented for the nonassociating mixture of a dipolar solvent, acetonitrile, and a quadrupolar solvent, benzene. The solvation response observed is sensitive to the mixing of the pure solvents, affecting both the inertial and diffusive components of the solvation response function. Addition of acetonitrile to benzene increases the amplitude of the inertial response. At high benzene mole fractions, the diffusive relaxation reveals a slow component attributed to translational diffusion of the acetonitrile.

As a beginning graduate student, well before I even met the man I would eventually marry, I recall considering my future. I was training to become a scientist, but would I find a partner? Would my career path allow me to have a family? Although these basic personal choices had always seemed inevitable to me as a child, in the frenetic schedule of a chemical physics graduate student they were anything but given. At that point, studies showing the impact of advanced education on women’s personal lives had yet to appear [1]. Still, it seemed clear; the likelihood of finding a partner while spending almost all my waking hours working on science was probably pretty small. After many musings, I made an active decision: the rich career afforded by my advanced degree would fulfill me whether I married or not, or had a family. This decision played a role in my success as a graduate student. It allowed me to focus my energy on science and leave my personal life to chance. I thrived in graduate school, both academically and personally. About 2 years later, I met Pete, the man who would become my best friend, my husband, the father of my children, and the person who made it possible for me to succeed in my career as a professor and as a mother.

Vibrational relaxation of methanol-d (MeOD) in carbon tetrachloride has been investigated via ultrafast infrared pump–probe experiments. Exciting at 2690 cm−1, only the free O-D (where the D is not H-bonded) stretching mode is initially populated. For MeOD mole fractions ⩽0.025, a 2.15 ps single exponential decay is observed. At mole fractions ⩾0.0375, the signal decays (2.15 ps decay time) below zero (increased absorption) and then recovers on time scales of 22 ps and ≫300 ps. The increased absorption indicates the formation of additional free ODs caused by the breaking of H-bonds that are not directly coupled to the initially excited vibration. The two-time scale recovery of this signal arises from geminate and nongeminate recombination. The data are fit with a set of kinetic equations that accurately reproduce the data. The results suggest that vibrational relaxation of the initially excited free OD stretch into intramolecular modes of the methanol leads to H-bond breaking. This contrasts studies that suggest direct relaxation of a vibrationally excited OH stretch into an H-bond stretch is responsible for H-bond breaking.

The effect of confinement on solvation dynamics has been explored in Brij-30/cyclohexane and Triton X-100/cyclohexane nonionic reverse micelles. Inside the reverse micelles, the polar solvation dynamics become slower and show additional slow relaxation modes not observed for bulk water. The solvation dynamics inside the Triton X-100 reverse micelles is slower than the dynamics inside Brij-30 reverse micelles. The results for solvation dynamics in the reverse micelles contrast solvation dynamics in aqueous tri(ethylene glycol) monoethyl ether solutions comparable to the reverse micellar interiors, which show significantly faster response. Measurements in the nonionic reverse micelles are also compared to previous work on ionic reverse micelles. Results reported here show that the interactions of water with the polyoxyethylene ether, as well as the micellar confinement inside these reverse micelles, effectively immobilize the water in the micellar interiors.

The continuous nonpolar phase can change the morphology of lecithin reverse micelles dramatically. In alkane solvents, increasing the hydration leads to gelation as isolated water droplets transform into an entangled tube network. In contrast, increasing the hydration for lecithin in benzene maintains isolated, largely spherical traditional reverse micelles. Solvation dynamics experiments have been carried out in the two different micellar environments. These measurements show that solvation dynamics are considerably more restricted in the tubular micelles than in the spherical droplets. This is interpreted as evidence for water pool formation and disorder at the interface in the spherical micelles and supports the hypothesis of substantial water incorporation into the tubular micelles. To perform solvation dynamics experiments in the benzene/lecithin/water reverse micelles, we have synthesized a new headgroup-labeled probe molecule. This synthesis, coupling the standard solvation dynamics probe, coumarin...

Recently techniques of solvation dynamics have been applied to reverse micelles, microemulsions and vesicular systems. The dynamical data obtained provides a complement to steady-state experiments and builds an understanding of the details of these systems. Recent molecular dynamics simulations show the power of combined experiments and simulations.

Aerosol OT/cyclohexane reverse micelles solubilizing DMSO and DMSO/water mixtures have been investigated utilizing dynamic light scattering, IR spectroscopy and UV/vis absorption by a polarity probe. Without water, ∼2.5 DMSO molecules/AOT molecule are solubilized inside the micelles. Adding water to the reverse micelles significantly increases the DMSO solubilized. Reverse micelles solubilizing DMSO are larger than corresponding micelles containing an equal amount of water. IR spectra reveal water interacts both with DMSO and the surfactant head group. The polarity of DMSO increases inside the micelles, while the intramicellar DMSO/water polarity depends both on the ratio of the solvents and on micellar size.

Time-resolved fluorescence-upconversion spectroscopy has been used to study the polar solvation dynamics of H{sub 2}O and D{sub 2}O at the surface of zirconia (ZrO{sub 2}) nanoparticles. While an isotope effect is observed for the solvation dynamics of bulk D{sub 2}O, there is no isotope effect on the interfacial solvation dynamics. The interfacial solvation dynamics are the same for H{sub 2}O and D{sub 2}O and are faster than the bulk solutions. The bulk isotope effect is due to stronger hydrogen bonding in D{sub 2}O compared to H{sub 2}O, slowing the reorientation of the excited-state dipoles in the bulk D{sub 2}O. The lack of isotope effect for the interfacial dynamics is explained in terms of the solvent interacting with the ZrO{sub 2} surface.

Polar solvation dynamics of water sequestered inside Aerosol OT (AOT) reverse micelles have been investigated as a function of the surfactant countercation, specifically replacing Na+ for K+ and Ca2+. For Ca-AOT reverse micelles, the solvation dynamics for the smallest micelles probed occurs on a subnanosecond time scale. The K-AOT reverse micelles display an additional ultrafast component that is attributable to bulklike water motion. As previously reported for Na-AOT reverse micelles [Riter, Willard, and Levinger, J. Phys. Chem. B 102, 2705 (1998)], solvent mobility increases with increasing micellar size for both Ca-AOT and K-AOT reverse micelles. The solvation dynamics in strongly ionic aqueous solutions of Ca2+ and K+ have also been investigated. The 10 M electrolyte solutions display water motion on significantly shorter time scales with substantial ultrafast components. These results show that the micellar interfacial structure plays a significant role in immobilizing intramicellar water and that solvent immobilization in the reverse micelles is not merely a result of solvent–ion interactions.

Polar solvation dynamics of a 95:5% (v/v) water/acetone mixture have been measured at the ZrO 2 nanoparticle surface by time-resolved fluorescence of a probe molecule adsorbed to the particle surface. The interfacial solvent response displays two sub-picosecond diffusive components with the same time constants as bulk solution. However, the relative amplitudes for the individual relaxation components are significantly different, leading to a faster average solvation response for molecules at the ZrO 2 surface. Furthermore, the overall fluorescence Stokes shift is approximately three times smaller for dye molecules adsorbed to the nanoparticle surface. Implications for electron injection into semiconductor nanoparticles are discussed.

We have measured the dynamics of water solvation in two different reverse micellar environments with low water content, w0 = 1.7. In the water/Aerosol OT (AOT)/isooctane reverse micelles, the chara...

The solvation dynamics of water in lecithin/cyclohexane reverse micelles have been determined via ultrafast time-resolved fluorescence studies. At hydration levels w0 ≤ 4.8, the reverse micellar samples are nonviscous. Here a single relaxation time is observed that is much longer than the response of free or bulk water. In contrast, small additions of water to the samples produces a viscous gel, referred to by others as an organogel or “living polymer”. At hydration levels of w0 ≥ 5.8, three relaxation times are observed with approximate time constants of 0.5, 15, and 200 ps, the shortest of which correlates to free water motion. The dynamics reveal no evidence of micelle crossover or branch points associated with gel formation. In comparison to Aerosol OT reverse micelles of similar hydration, the water in the lecithin reverse micelles is significantly more restricted. It is proposed that lecithin sequesters significantly more water than previously predicted precluding formation of distinct core water po...

The mobility of water in water/Aerosol OT (AOT)/isooctane reverse micelles has been investigated via polar solvation dynamics measurements using ultrafast, time-resolved fluorescence-upconversion spectroscopy. For the smallest reverse micelles studied, w0 = 1.1, no solvation dynamics are observed on the time scale of the instrument, suggesting that the water is essentially frozen. As w0 increases, the mobility of water increases, and the time-correlation functions exhibit multiexponential decays. A subpicosecond component is observed on a time scale similar to that of diffusive bulk-water motion.1 The other components decay on time scales of a few picoseconds to hundreds of picoseconds. The amplitude of the solvent response increases as w0 increases, which is attributed to increased mobility of the water inside the reverse micelles. Because the micellar interior is highly ionic, the solvation dynamics of coumarin 343 in a 1 M Na+ (Na2SO4) solution were also investigated to compare the dynamics of water in...

Reverse microemulsions have been characterized in isooctane and decane using the surfactant Aerosol-OT (AOT) and the polar solvents formamide, ethylene glycol, acetonitrile, methanol, N,N-dimethylformamide, and 1,2-propanediol. Dynamic light scattering (DLS) experiments show that reverse micelles form in these solutions. The DLS data and steady-state absorption spectra of Coumarin 343 in these solutions reveal that the character of these reverse micelles depends partially upon the solubility of the polar solvent in the hydrocarbon. For formamide and ethylene glycol, which are highly immiscible in the hydrocarbon solvents, changes in micellar size occur for variations in the volume fraction φ of the polar solvent and surfactant in the continuous phase as well as for the traditional changes in w. These reverse micelles with w = 1.1 and φ = 0.006−0.051 exhibit diameters ranging from 5.0 to 26.1 nm. For the solvents acetonitrile, methanol, N,N-dimethylformamide, and 1,2-propanediol, which are slightly miscibl...

We have used a white light continuum generated with ultrashort laser pulses from a Ti:sapphire laser system as the radiation source for second-harmonic generation measurements. The white light continuum provides easily tuned radiation for experiments requiring a range of wavelengths. Despite the small coherence length of the radiation, parametric processes, such as second-harmonic generation, are possible with this source. In particular, surface second-harmonic generation using the white light continuum is reasonable because the extent of the interface is much smaller than the coherence length of the radiation. We demonstrate second-harmonic generation from a gold surface and show that surface second-harmonic generation using the white light continuum can be used to measure absorption spectroscopy of molecules adsorbed to surfaces via resonance enhancement of the surface second-harmonic signals.

The dramatic impact of differing environments on proton transfer dynamics of the photoacid HPTS prompted us to investigate these systems with two highly complementary methods: ultrafast time-resolved transient absorption and two-dimensional NMR spectroscopies. Both ultrafast time-resolved transient absorption spectroscopy and time-resolved anisotropy decays demonstrate the proton transfer dynamics depend intimately on the specific reverse micellar system. For w(0) = 10 reverse micelles formed with anionic AOT surfactant, the HPTS proton transfer dynamics are similar to dynamics in bulk aqueous solution, and the corresponding (1)H 2D NOESY NMR spectra display no cross peaks between HPTS and AOT consistent with the HPTS residing well hydrated by water in the interior of the reverse micelle water pool. In contrast, ultrafast transient absorption experiments show no evidence for HPTS photoinduced proton transfer reaction in reverse micelles formed with the cationic CTAB surfactant. In CTAB reverse micelles, clear cross peaks between HPTS and CTAB in the 2D NMR spectra show that HPTS embeds in the interface. These results indicate that the environment strongly impacts the proton transfer reaction and that complementary experimental techniques develop understanding of how location critically affects molecular responses.