Solution of telechelic ionomers in water/AOT/oil (w/o) microemulsions: a static and dynamic light scattering study

Static and quasielastic light-scattering measurements of endsulfonated polyisoprene in a water in oil (w/o) microemulsions were used to characterize the structure and diffusion properties of this complex system. The hydrophilic end groups of the polymer stick to the surfactant covered oil/water interface, thus bridging the water droplets. This structure formation decreases the mobility of the aqueous nanodroplets and polymer molecules. At interdroplet distances larger than the end-to-end distance of the ionomer chain a decrease of the osmotic modulus is observed. It can be explained by a depletion force of free ionomer chains acting on the nanodroplets. With increasing polymer concentration structure formation of the microemulsion is observed at nanodroplet concentrations where the ionomer chains just fit the average separation of two nanodroplets.     

Polymeric nanocapsules containing an antiseptic agent obtained by controlled nanoprecipitation onto water-in-oil miniemulsion droplets

The modified nanoprecipitation of polymers onto stable nanodroplets has been successfully applied to prepare well-defined nanocapsules whose core is composing of an antiseptic agent, i.e., chlorhexidine digluconate aqueous solution. The stable nanodroplets were obtained by inverse miniemulsions with an aqueous antiseptic solution dispersed in an organic medium of solvent/nonsolvent mixture containing an oil-soluble surfactant and the polymer for the shell formation. The change of gradient of the solvent/nonsolvent mixture of dichloromethane/cyclohexane, obtained by heating at 50 degrees C, led to the precipitation of the polymer in the organic continuous phase and deposition onto the large interface of the aqueous miniemulsion droplets. The monodisperse polymer nanocapsules with the size range of 240-80 nm were achieved as a function of the amount of surfactant. Using various polymer contents, molecular weights and types, an encapsulation efficiency of 20-100% was obtained as detected by proton-nuclear magnetic resonance spectroscopy ((1)H NMR) measurements. The nanocapsules could be easily transferred into water as continuous phase resulting in aqueous dispersions with nanocapsules containing an aqueous core with the antiseptic agent. The encapsulated amount of the antiseptic agent was evaluated to indicate the durability of the nanocapsule's wall. In addition, the use of different types of polymers having glass transition temperatures (T(g)) ranging from 10 to 100 degrees C in this process has been also successful.     

A spectroscopic study of atmospherically relevant concentrated aqueous nitrate solutions

Concentrated aqueous nitrate aerosols are present in the Earth's atmosphere as a result of heterogeneous reactions of sea salt and mineral dust aerosol with nitrogen oxides (e.g., NO2, NO3, HNO3 and N2O5). Because the water content of these aerosols depends on relative humidity (RH), the composition and nitrate ion concentration will also depend on RH. Unlike the original aerosols, aqueous nitrate aerosols are photochemically active at solar wavelengths. To gain a better understanding of the nitrate ion chromophore in concentrated aqueous nitrate aerosols, we have measured the ATR-FTIR and UV/vis spectra of concentrated nitrate solutions over a large concentration range. Both ATR-FTIR and UV/vis spectroscopy show changes in the nitrate ion spectra with increasing concentration. Ab initio calculations are used to aid in the assignment and interpretation of these spectra. From these data, we predict that the photoreactivity of aqueous nitrate aerosols will strongly depend on relative humidity as the molecular and electronic structure of the nitrate ion becomes increasingly perturbed from that of the isolated ion in highly concentrated atmospherically relevant solutions.     

Generation of metal oxide nanoparticles in optimised microemulsions

The phase behavior and structure of aqueous-in-n-heptane microemulsions, stabilized by surfactant mixtures of di-n-didodecyldimethylammonium bromide, DDAB, and Brij(R)35 were studied by small angle (neutron or X-ray) scattering techniques. The aqueous nanodroplets contain either a precursor reactive salt or a precipitating agent, so that simple mixing induces nanoparticle formation. These formulated microemulsions display good phase stability against added polar additives such as monovalent, divalent, trivalent metal ions, ammonia solution, tetrabutylammonium hydroxide, and their mixtures. Nanoparticle formation was demonstrated via precipitation of metal oxides inside the water nanodroplets, affording control over the resulting particle size. Nanoparticle characteristic size (XRD- and HR-TEM derived sizes) and specific surface areas (S(BET) (m(2)g(-1))) for iron oxide and CeO(2) prepared in these mixed microemulsions, are compared with those stabilized by single surfactants DDAB and Pure AOT.     

Molecular dynamic simulation of dicarboxylic acid coated aqueous aerosol: structure and processing of water vapor

Organic monolayers at the surfaces of aqueous aerosols play an important role in determining the mass, heat transfer rate and surface reactivity of atmospheric aerosols. They can potentially contribute to the formation of cloud condensation nuclei (CCN) and are involved in a series of chemical reactions occurring in atmosphere. Recent studies even suggest that organic-coated interfaces could have played some role in prebiotic biochemistry and the origin of life. However, creating reproducible, well-characterized aqueous aerosol particles coated with organic films is an experimental challenge. This opens the opportunity for computer simulations and modeling of these complex structures. In this work, molecular dynamics simulation was used to probe the structure and the interfacial properties of the dicarboxylic acid coated aqueous aerosol. Low molecular weight dicarboxylic acids of various chain lengths and water solubility were chosen to coat a water droplet consisting of 2440 water molecules. For malonic acid coated aerosol, the surface acid molecules dissolved into the water core and formed an ordered structure due to the hydrophobic interactions. The acid and the water are separated inside the aerosol. For other nanoaerosols coated with low solubility acids, phase separation between water and acid molecules was observed on the surface of the particle. To study the water processing of the coated aerosols, the water vapor accommodation factors were calculated.     

Nanoinclusions in cryogenically quenched nanoemulsions

Nanodroplets containing mixtures of silicone oil and squalene are dispersed in a simple aqueous surfactant solution, quenched in liquid ethane, and examined using cryogenic transmission electron microscopy (CTEM). Depending on the phase of ice that forms around the nanodroplets and on the composition of the oil mixture, nanoinclusions can be observed inside oil nanodroplets, independent of surfactant type. Our observations suggest that these nanoinclusions arise from nucleation of vapor cavities as the water freezes and expands while the oil remains liquid during the quench.     

Structural Transitions of Self-Nanoemulsifying Drug Delivery System of Glibenclamide,Carvedilol, and Lovastatin upon Progressive Aqueous Dilution

The dynamics of morphological transition in amphiphillic systems such as self-nanoemulsifying drug delivery system (SNEDDS) has become an increasingly active field of research in colloidal science. The present contribution deals with the morphological transition of selected optimized SNEDDS formulations of glibenclamide, carvedilol, and lovastatin on progressive aqueous dilution using transmission electron microscopy. The study emphasizes the structural aspects of the systems and stresses the effect of aqueous dilution under which the systems transform from water-in-oil (L₂) phase into bicontinuous structure and, finally, in oil-in-water (L₁) nanodroplets.     

Global inorganic source of atmospheric bromine

A few bromine molecules per trillion (ppt) causes the complete destruction of ozone in the lower troposphere during polar spring and about half of the losses associated with the "ozone hole" in the stratosphere. Recent field and aerial measurements of the proxy BrO in the free troposphere suggest an even more pervasive global role for bromine. Models, which quantify ozone trends by assuming atmospheric inorganic bromine (Bry) stems exclusively from long-lived bromoalkane gases, significantly underpredict BrO measurements. This discrepancy effectively implies a ubiquitous tropospheric background level of approximately 4 ppt Bry of unknown origin. Here, we report that I- efficiently catalyzes the oxidation of Br- and Cl- in aqueous nanodroplets exposed to ozone, the everpresent atmospheric oxidizer, under conditions resembling those encountered in marine aerosols. Br- and Cl-, which are rather unreactive toward O3 and were previously deemed unlikely direct precursors of atmospheric halogens, are readily converted into IBr2- and ICl2- en route to Br2(g) and Cl2(g) in the presence of I-. Fine sea salt aerosol particles, which are predictably and demonstrably enriched in I- and Br-, are thus expected to globally release photoactive halogen compounds into the atmosphere, even in the absence of sunlight.     

A computer simulation study of the formation of liquid crystal nanodroplets from a homogeneous solution

The aggregation of liquid crystal nanodroplets from a homogeneous solution is an important but not well understood step in the preparation of various advanced photonic materials. Here, the authors performed molecular dynamics computer simulations of the formation of liquid crystalline nanodroplets, starting from an isotropic and uniform binary solution of spherical Lennard-Jones (solvent) and elongated ellipsoidal Gay-Berne (solute) rigid particles in low (<10%) concentration. They studied the dynamics of demixing and the mesogen ordering process and characterized the resulting nanodroplets assessing the effect of temperature, composition, and specific solute-solvent interaction on the morphology, structure, and anisotropy. They find that the specific solute-solvent interaction, composition, and temperature can be adjusted to tune the nanodroplet growth and size.     

A structural phase diagram for model aqueous organic nanodroplets

Our density functional theory calculations predict that model aqueous organic nanodroplets have either well mixed or core-shell structures, depending on the state of the metastable binary vapor and that, furthermore, there is a broad transition region in the phase diagram where both structures can occur at the same vapor state.     

The structure and energetics of 3He and 4He nanodroplets doped with alkaline earth atoms

We present systematic results, based on density functional calculations, for the structure and energetics of 3He and 4He nanodroplets doped with alkaline earth atoms. We predict that alkaline earth atoms from Mg to Ba go to the center of 3He drops, whereas Ca, Sr, and Ba reside in a deep dimple at the surface of 4He drops, and Mg is at their center. For Ca and Sr, the structure of the dimples is shown to be very sensitive to the He-alkaline earth pair potentials used in the calculations. The 5s5p <-- 5s2 transition of strontium atoms attached to helium nanodroplets of either isotope has been probed in absorption experiments. The spectra show that strontium is solvated inside 3He nanodroplets, supporting the calculations. In the light of our findings, we emphasize the relevance of the heavier alkaline earth atoms for analyzing mixed 3He-4He nanodroplets, and in particular, we suggest their use to experimentally probe the 3He-4He interface.     

Nonplanarity of adenine: vibrational transition moment angle studies in helium nanodroplets

Mid-infrared spectra are reported for adenine monomer in helium nanodroplets. We show that there is only one tautomer of adenine, the global minimum structure, observed in helium nanodroplets and characterized by using ab initio calculations and the measurement of vibrational transition moment angles (VTMAs) for the various vibrational modes of the adenine monomer. On the basis of the VTMA analysis on the amino group of the global minimum tautomer, which gives insights into its nonplanarity, a detailed VTMA study of three lowest-energy amino tautomers of adenine is discussed in this study.     

On the reactive uptake of gaseous compounds by organic-coated aqueous aerosols: theoretical analysis and application to the heterogeneous hydrolysis of N2O5

The presence of organic coatings on aerosols may have important consequences to the atmospheric chemistry, in particular to the N2O5 heterogeneous hydrolysis. This is demonstrated by recent experiments which show that the uptake of N2O5 by aqueous aerosols is slowed considerably when an organic coating consisting of monoterpene oxidation products is added on the particles. To treat the mechanisms behind the suppression, an extension of the resistor model, which has been widely applied in investigation of the heterogeneous uptake by aerosols, was derived. The extension accounts for dissolution, diffusion, and chemical reactions in a multilayered organic coating, and it provides a parametrization for the heterogeneous uptake by organic-coated aerosols that can be applied in large-scale models. Moreover, the framework was applied to interpret the findings regarding the decreased uptake of N2O5 by the organic-coated aerosols. Performed calculations suggested that the reaction rate constant of N2O5 in the coating is decreased by 3-5 orders of magnitude, in addition to which the product of the solubility of N2O5 and its diffusion coefficient in the coating is reduced more than an order of magnitude compared to the corresponding value for the aqueous phase. The results suggest also that the accommodation coefficient of N2O5 to such coatings is no more than a factor of 2 smaller than that to pure water surfaces. Finally, the relevance of the results to the atmospheric N2O5 heterogeneous hydrolysis is discussed and implications to planning further laboratory studies focusing on secondary organic aerosol formation are pointed out.     

Temperature-controlled change of charge transport mechanisms in nonionic water-in-oil microemulsions

 The temperature-controlled transition from the Stokes charge transport of aqueous nanodroplets to the intrinsic conduction of nanodroplet clusters in nonionic microemulsions was studied. Two different charge transport processes apparent from a minimum value of the conductance have been simulated based on straightforward physical models. Their predictions compare favourably with the observations. Received: 4 September 2001 Accepted: 20 September 2001     

Non-linear vibrational sum frequency spectroscopy of atmospherically relevant molecules at aqueous solution surfaces

Surface vibrational sum frequency spectroscopy has been shown to be a powerful surface probe of molecules adsorbed at solid and liquid surfaces. Studies described herein apply this method to studying heterogeneous air/aqueous solution interfaces to understand surface adsorption and structure of several solute molecules adsorbed at aqueous surfaces. The molecules examined at aqueous solution surfaces include Dimethyl sulfoxide (DMSO), methane sulfonic acid (MSA) and acetone. These results reveal that small soluble molecules such as these organize in different ways at the surface of aqueous solutions. This surface organization has implications for atmospheric chemical processes since adsorption at the surface of atmospheric aerosols affects bulk chemical concentrations.     

Interfacial synthesis of dendritic platinum nanoshells templated on benzene nanodroplets stabilized in water by a photocatalytic lipoporphyrin

Nanoscale metal shells have many potential uses and in some applications offer significant advantages over nanoparticles. The synthesis of platinum nanoshells using stabilized nanodroplets of benzene in water as growth templates is described; the nanodroplets are stabilized by a surfactant-like tin(IV)-porphyrin complex localized at the benzene-water interface. The porphyrin also acts as a photocatalyst that reduces the platinum complex and deposits metal onto the nanodroplets to form dendritic metal nanoshells. Below the solubility limit of benzene in water, the lipoporphyrin-stabilized nanodroplets have a reproducible number, size distribution, and surface area, which allows the thickness of the platinum shell walls to be controlled by changing the amount of platinum complex. Nanoscale platinum shells with magnetic interiors can be made by dispersing Fe3O4 nanoparticles in the benzene nanodroplets.     

Blockcopolymers in microemulsions differential heats of solution and shift of the percolation temperature in W/O microemulsions

Solutions of blockcopolymers (POE-b-PI-b-POE) in fluids of interacting aqueous nanodroplets (W/O microemulsions) are studied. The interaction strength between the (pseudo) two components is measured by the shift of the percolation temperature relative to that of the pure microemulsion. A quantitative measure of the interaction, the differential heat of solution, is thermodynamically related to the slopes of the equilibrium temperature of the system with varying monomeric nanodroplet concentration and the experimental percolation line in the presence of copolymer.     

Spatial Separation of Individual Substances in Effloresced Crystals of Ternary Ammonium Sulphate/Dicarboxylic Acid/Water Aerosols

This work examines the crystals resulting from the efflorescence of internally mixed aqueous aerosols comprising ammonium sulphate and different dicarboxylic acids. Most studies on the deliquescence of aerosols use previously effloresced aerosols in their experiments. However, during efflorescence a highly supersaturated solution crystallises in a kinetically controlled way unlike the case of thermodynamically controlled crystallisation. Herein the distribution of individual substances within the effloresced crystals is investigated using Raman scanning experiments. The data presented show an intriguingly complex behaviour of these ternary and quarternary aerosols. A spatial separation of substances in the crystals resulting from the efflorescence of previously internally mixed ternary salt/dicarboxylic acid/water aerosol droplets is demonstrated and mechanistic aspects are discussed.     

What is so special about aerosol-OT? Part IV. Phenyl-tipped surfactants

Properties are reported for new phenyl-tipped anionic surfactants, which are aromatic chain relatives of the normal aliphatic aerosol-OT (AOT, sodium bis(2-ethyl-1-hexyl)sulfosuccinate). Variations in chain length and branching with these aromatic surfactants have important effects on aqueous and water-in-oil (w/o) microemulsion phase properties. In dilute aqueous systems, chain structure affects the cmc and surface tension behavior: compared to linear chain analogues, the branched-chain surfactants display lower surface tensions but also reduced packing as measured by molecular area at the cmc a(cmc). Owing to the phenyl-tipped structure, water-in-oil microemulsions were stabilized with aromatic toluene as an oil but not with aliphatic heptane; the latter is commonly used with normal AOT. Contrast variation small-angle neutron scattering (SANS) was used to characterize the microemulsion aggregates and adsorbed films. These SANS data show that water-in-toluene microemulsions stabilized by aromatic-AOTs contain mildly polydisperse spherical nanodroplets of similar structure to those found in systems containing normal AOT. Molecular areas at the air-water and toluene-water interfaces are found to be of similar magnitude and follow a trend that correlates with variations in surfactant chain structure. The new results with aromatic surfactants build on extensive studies of aliphatic AOT analogues (Nave, S.; Eastoe, J.; Penfold, J. Langmuir 2000, 16, 8733. Nave, S.; Eastoe, J.; Heenan, R. K.; Steytler, D.; Grillo, I. Langmuir 2002, 16, 8741. Nave, S.; Eastoe, J.; Heenan, R. K.; Steytler, D.; Grillo, I. 2002, 18, 1505), suggesting that the versatility of normal AOT originates from an optimized head and chain spacer group rather than from any specific effects of the 2-ethyhexyl chain structure.     

Green fluorescent protein in inertially injected aqueous nanodroplets

We inertially inject and study the contents of optically trappable aqueous nanodroplets (hydrosomes) emulsified in a perfluorinated matrix. A new piezoelectric actuated device for production of single hydrosomes on demand is introduced. Hydrosomes containing enhanced green fluorescent protein (EGFP) were injected, optically trapped, and held at the focus of an excitation laser in a confocal microscope, and single-molecule photobleaching events were observed. The rotational diffusion time of EGFP in trapped hydrosomes was measured using time-resolved fluorescence anisotropy. In free solution, the mean rotational diffusion time was determined to be 13.8 +/- 0.1 ns at 3 microM and 14.0 +/- 0.2 ns at 10 microM. In hydrosomes, the mean rotational diffusion time was similar and determined to be 12.6 +/- 1.0 ns at 3 microM and 15.5 +/- 1.6 ns at 10 microM. We conclude that the rotational motion inside the nanodroplets is consistent with rotation in free solution and that the protein therefore does not aggregate at the water-oil interface. Protein can be confined in hydrosomes with high efficiency using this technique, which provides an alternative to surface attachment or lipid encapsulation and opens up new avenues of research using single molecules contained in fluid nanovolumes.