IR investigations of water structure in Aerosol OT reverse micellar aggregates

Onori, G.; Santucci, A.

doi:10.1021/j100122a040

Cited by 294 publications

(351 citation statements)

References 3 publications

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“…For pure water, subensembles of water molecules with more and/or stronger hydrogen bonds have red-shifted absorptions, whereas subensembles with fewer and/or weaker hydrogen bonds are blueshifted. In some other water systems, the blue-shift of the entire water spectrum has been interpreted as reflecting an overall weaker distribution of the hydrogen bonds (25,28). However, this is not necessarily true.…”

Section: Resultsmentioning

confidence: 99%

Hydrogen bond dynamics in aqueous NaBr solutions

Park

Fayer

2007

Proc. Natl. Acad. Sci. U.S.A.

293

403

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Hydrogen bond dynamics of water in NaBr solutions are studied by using ultrafast 2D IR vibrational echo spectroscopy and polarization-selective IR pump-probe experiments. The hydrogen bond structural dynamics are observed by measuring spectral diffusion of the OD stretching mode of dilute HOD in H2O in a series of high concentration aqueous NaBr solutions with 2D IR vibrational echo spectroscopy. The time evolution of the 2D IR spectra yields frequency-frequency correlation functions, which permit quantitative comparisons of the influence of NaBr concentration on the hydrogen bond dynamics. The results show that the global rearrangement of the hydrogen bond structure, which is represented by the slowest component of the spectral diffusion, slows, and its time constant increases from 1.7 to 4.8 ps as the NaBr concentration increases from pure water to Ϸ6 M NaBr. Orientational relaxation is analyzed with a wobbling-in-a-cone model describing restricted orientational diffusion that is followed by complete orientational randomization described as jump reorientation. The slowest component of the orientational relaxation increases from 2.6 ps (pure water) to 6.7 ps (Ϸ6 M NaBr). Vibrational population relaxation of the OD stretch also slows significantly as the NaBr concentration increases.ultrafast 2D IR spectroscopy ͉ water dynamics in ionic solutions W ater plays an important role in chemical and biological processes. In aqueous solutions, water molecules dissolve ionic compounds, charged chemical species, and biomolecules by forming hydration shells (layers) around them. Pure water undergoes rapid structural evolution of the hydrogen bond network that is responsible for water's unique properties (1). A question of fundamental importance is how the dynamics of water in the immediate vicinity of an ion or ionic group differ from those of pure water. For monatomic ions, molecular ions, charged groups of large molecules, or charged amino acids on the surfaces of proteins, the basic structure of hydration shells is determined by ion-dipole interactions between water molecules and the charged group (2, 3). Such ion-dipole interactions will influence both the structure and dynamics of water in the proximity of ions. Water dynamics in ion hydration shells play a significant role in the nature of systems such as proteins and micelles (3) and in processes such as ion transport through transmembrane proteins (4).Over the last several years, the application of ultrafast IR vibrational echo spectroscopy (5, 6), particularly 2D IR vibrational echo experiments, (7, 8) combined with molecular dynamics (MD) simulations (9-12) have greatly enhanced understanding of the hydrogen bond dynamics in pure water. These experiments directly examine the dynamics of water rather than study the indirect influence of water dynamics on a probe molecule (13). The ultrafast 2D IR vibrational echo experiments on water (7, 8) and other hydrogen-bonded systems (14) build upon earlier IR pump-probe experiments that have been extensively used to study ...

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Section: Resultsmentioning

confidence: 99%

Hydrogen bond dynamics in aqueous NaBr solutions

Park

Fayer

2007

Proc. Natl. Acad. Sci. U.S.A.

293

403

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“…[1][2][3][4] The linear absorption is measured over the whole ensemble of molecules, thus the line broadening obscures any information about subspecies. These changes might reflect a homogeneous weakening of the hydrogen bond network throughout the whole droplet due to reduced cooperative effects, as implied by the homogeneous droplet model.…”

Section: Decomposition Of Ftir Reverse Micelles Spectramentioning

confidence: 99%

Ultrafast Energy Transfer in Water−AOT Reverse Micelles

et al. 2007

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A spectroscopic investigation of the vibrational dynamics of water in a geometrically confined environment is presented. Reverse micelles of the ternary microemulsion H2O/AOT/n-octane (AOT = bis-2-ethylhexyl sulfosuccinate or aerosol-OT) with diameters ranging from 1 to 10 nm are used as a model system for nanoscopic water droplets surrounded by a soft-matter boundary. Femtosecond nonlinear infrared spectroscopy in the OH-stretching region of H2O fully confirms the core/shell model, in which the entrapped water molecules partition onto two molecular subensembles: a bulk-like water core and a hydration layer near the ionic surfactant headgroups. These two distinct water species display different relaxation kinetics, as they do not exchange vibrational energy. The observed spectrotemporal ultrafast response exhibits a local character, indicating that the spatial confinement influences approximately one molecular layer located near the water−amphiphile boundary. The core of the encapsulated water droplet is similar in its spectroscopic properties to the bulk phase of liquid water, i.e., it does not display any true confinement effects such as droplet-size-dependent vibrational lifetimes or rotational correlation times. Unlike in bulk water, no intermolecular transfer of OH-stretching quanta occurs among the interfacial water molecules or from the hydration shell to the bulk-like core, indicating that the hydrogen bond network near the H2O/AOT interface is strongly disrupted.

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“…Ultimately, it is the intermolecular forces between neighboring molecules that govern surfactant packing at the interface, and this depends on its molecular structure (10), the organic phase (14), and the electrostatics of the interface (15). The studies described herein are aided by prior work at neat oil-water interfaces as examined by vibrational sumfrequency spectroscopy (VSFS) and computational studies on planar interfaces (16)(17)(18).…”

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confidence: 99%

Molecular characterization of water and surfactant AOT at nanoemulsion surfaces

Hensel

Carpenter

Ciszewski

et al. 2017

Proc. Natl. Acad. Sci. U.S.A.

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Nanoemulsions and microemulsions are environments where oil and water can be solubilized in one another to provide a unique platform for many different biological and industrial applications. Nanoemulsions, unlike microemulsions, have seen little work done to characterize molecular interactions at their surfaces. This study provides a detailed investigation of the near-surface molecular structure of regular (oil in water) and reverse (water in oil) nanoemulsions stabilized with the surfactant dioctyl sodium sulfosuccinate (AOT). Vibrational sum-frequency scattering spectroscopy (VSFSS) is used to measure the vibrational spectroscopy of these AOT stabilized regular and reverse nanoemulsions. Complementary studies of AOT adsorbed at the planar oil-water interface are conducted with vibrational sum-frequency spectroscopy (VSFS). Jointly, these give comparative insights into the orientation of interfacial water and the molecular characterization of the hydrophobic and hydrophilic regions of AOT at the different oil-water interfaces. Whereas the polar region of AOT and surrounding interfacial water molecules display nearly identical behavior at both the planar and droplet interface, there is a clear difference in hydrophobic chain ordering even when possible surface concentration differences are taken into account. This chain ordering is found to be invariant as the nanodroplets grow by Ostwald ripening and also with substitution of different counterions (Na:AOT, K:AOT, and Mg:AOT) that consequently also result in different sized nanoparticles. The results paint a compelling picture of surfactant assembly at these relatively large nanoemulsion surfaces and allow for an important comparison of AOT at smaller micellar (curved) and planar oil-water interfaces.nanoemulsions | oil-water interfaces | vibrational sum-frequency scattering spectroscopy | surface spectroscopy | surfactants W e are all familiar with the adage that "oil and water do not mix," but of course, it depends upon the definition of "mix." Emulsions are an important special case, where the oil is dispersed as tiny droplets in the aqueous phase, taxonomically called a regular emulsion, or where the water is dispersed as tiny droplets throughout the oil phase, called a reverse emulsion. Because both emulsions are thermodynamically unstable, overcoming this requires an emulsifying agent such as a surfactant. Recently, there has been interest in surfactant-stabilized emulsions with droplet diameters in the nanoscale range for unique applications in drug delivery (1, 2) and oil recovery (3, 4) and as nanoreactors to produce materials ranging from polymers to quantum dots (5). Regular or reverse emulsions with droplet diameters in the range of 10-1,000 nm are called nanoemulsions. Little is known about the processes or molecular structures that result in their stability via surfactants. Even less is known about the structure-function relationship, which is crucial to determine the best surfactant for a given nanoemulsion application. Their utility hinges on a ...

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IR investigations of water structure in Aerosol OT reverse micellar aggregates

Cited by 294 publications

References 3 publications

Hydrogen bond dynamics in aqueous NaBr solutions

Hydrogen bond dynamics in aqueous NaBr solutions

Ultrafast Energy Transfer in Water−AOT Reverse Micelles

Molecular characterization of water and surfactant AOT at nanoemulsion surfaces

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