Bulk Aqueous Phase Research Articles

Encapsulation of Anilinonaphthalenesulfonates in Carboxylate-Terminated PAMAM Dendrimer at the Polarized Water|1,2-Dichloroethane Interface

Molecular encapsulation of water-soluble anionic fluorescent dye molecules, 8-anilino-1-naphthalenesulfonic acid (ANS), and its bimolecular derivative (bis-ANS), in the generation 3.5 polyamidoamine (G3.5 PAMAM) dendrimer was investigated in the bulk aqueous phase and at the polarized water|1,2-dichloroethane interface. ANS(-) was electrostatically incorporated in the dendrimer, and the fluorescence enhancement with a blue shift of the emission maximum was observed at pH values <6, where the interior of the dendrimer was positively charged. The fluorescence enhancement of ANS was maximized around pH 3 and then decreased under more acidic conditions. The potential dependences of the molecular encapsulation and the interfacial mechanism were studied in detail by means of potential modulated fluorescence (PMF) spectroscopy. Under acidic conditions, the dendrimer incorporated ANS(-) at the positively polarized interface as well as in the aqueous phase. ANS(-) was released from the dendrimer at the intrinsic transfer potential and independently transferred across the interface. Bis-ANS exhibited relatively strong interaction with the dendrimer over a wide pH range (1 < pH < 8), and a negative shift of the transfer potential was observed under the corresponding pH condition. The PMF analysis clearly demonstrated that the interfacial mechanism of the dendrimer involves transfer and adsorption processes depending on the pH condition and the Galvani potential difference.

Ionic Disruption of the Liquid−Liquid Interface

The interface formed between an aqueous salt solution and a hydrophobic liquid has been investigated using molecular dynamics simulations. The salt solutions of NaCl, NaNO3, and Na2SO4 have been studied to determine their presence and distribution in the interfacial region and their effect on interfacial water molecules. Density and orientation profiles reveal the formation of ionic double layers with widths that vary with the respective anions’ surface affinities and effects on the geometry of interfacial water molecules. The NO3− anion shows enhanced surface concentration above that of the bulk aqueous phase, whereas the Cl− and SO42− anions exhibit similar characteristics as are found for corresponding air−water interfaces. Sum frequency spectra were calculated for the OH vibrational modes of water to show the effect of the various ions on the hydrogen-bonding network strength of interfacial water. These calculated spectra show good agreement with the conclusions and observations of our recent spectros...

ChemInform Abstract: Stable β-Phosphorylated Cyclic Aminoxyl Radicals in SDS Micelles.

The behaviour of a series of stable cyclic aminoxyl radicals in the presence of SDS micelles was studied by EPR spectroscopy. Six different compounds, i.e. two non-phosphorylated and four β-phosphorylated, were investigated. Except in the case of the strongly hydrophilic 3-CP 1, which always remained in the bulk aqueous phase, all the radicals were found to exchange between water and micelles. Their partition coefficients were evaluated from computer simulations of the EPR spectra and in the case of two aminoxyls TOMER-Et 5 and TOMER-Pri 6, the variation of the rate constant with temperature allowed us to estimate the exchange activation energy.

Modeling the environmental fate of perfluorooctanoic acid and perfluorooctanoate: An investigation of the role of individual species partitioning

A multimedia multi-species environmental fate model was developed for the conjugate pair perfluorooctanoic acid (PFOA):perfluorooctanoate (PFO). The model allows assessment of the relative contribution of each individual species, in equilibrium with each other, to the overall environmental movement of the pair. The Lake Ontario (Canada/USA) watershed system was selected for this investigation and is simulated in a single-region, seven-compartment model, including a water surface microlayer, and aqueous aerosol generation and redeposition. Results indicate that in the equilibrated presence of both PFOA and PFO, the environmental fate of the pair can be accomplished by consideration of the physical properties of the neutral acid, which govern the intermedia distribution of the pair, coupled with processes of media advection, such as air or water flow. The role of the anion, while the most populous species in the aqueous phase, appears merely to be as a source of the neutral acid for subsequent partitioning. Thus, when only the bulk aqueous phase anion concentrations are of interest a multimedia fate model is not required because these concentrations are largely predictable from the magnitude of emissions to and the advection of the phase. With neutral species partitioning, all local field measurement concentrations of the conjugate pair, PFO(A), are explained by the model to within approximately an order of magnitude, with the exception of lake sediment solids. Model results indicate that bulk aqueous phase PFO acts as a net source for PFOA to the atmosphere, where it may be subject to long-range transport (LRT). Initial calculations suggest an atmospheric LRT potential for PFO(A) of thousands of kilometers, rendering it comparable to hexachlorobenzene.

Radiowave Dielectric Properties of Sodium Maleate Copolymers in Aqueous Solutions in Light of a Scaling Approach

The concentration dependence of the dielectric properties of aqueous solutions of sodium maleate copolymers with comonomers of different hydrophobicities have been investigated by means of frequency domain dielectric spectroscopy in the frequency range from 1 kHz to 2 GHz. The dielectric relaxation falling between the process at low frequencies (polarization involving the whole polymer chain) and the one at high frequencies (polarization of the bulk aqueous phase) has been analyzed in light of the scaling properties of polyelectrolyte solutions. Within this framework, the dielectric properties are governed by two characteristic lengths, the distance R(cm) between chains in the dilute regime and the correlation length xi in the semidilute regime. We find that, for both these regimes, the exponents of the scaling laws, which describe the dielectric increment Deltaepsilon and the relaxation time tau(ion) are in a reasonably good agreement with the ones experimentally observed. This analysis gives further support to the scaling approach of the dynamic behavior of polyelectrolytes, appearing very suitable to modeling our dielectric results.

Model Studies of Membrane Disruption by Photogenerated Oxidative Assault

We have investigated the response of solid-supported phospholipid bilayers to short doses of photogenerated oxidative stress to characterize physical membrane changes during early phases of membrane oxidation. The low-dose oxidative stress is generated by uniformly exposing the bilayer samples using short-wavelength UV radiation (184-257 nm) for short periods (approximately 3 min) and resulting membrane morphological transformations characterized using a combination of wide-field epifluorescence microscopy and imaging ellipsometry measurements. Our results establish that the early phase of membrane oxidation is characterized by the nucleation and growth of discrete microscopic voids within the bilayer. The locations of the voids are randomly distributed throughout the sample surface, despite the uniform illumination. Over longer time scales, the voids continue to grow after the termination of the UV radiation. We also find that the voids heal as sample temperature is raised and that the supported bilayers consisting of fully saturated lipids are less susceptible to the mild oxidation conditions used, regardless of phase state. Analyzing these results in terms of (1) reactive-oxygen species mediated oxidative attack, (2) in situ generation of membrane oxidation products, and (3) their reequilibration between the membrane and the bulk aqueous phase explains the membrane morphological changes observed and provides insights into membrane perturbations following oxidative assault. Specifically, molecular properties of oxidation products (e.g., intrinsic curvature) account for formation and stabilization of voids within contiguous bilayers, and the long-term structural evolution is consistent with slow kinetics of the desorption of these oxidation products from the bilayer into bulk solution. A corollary benefit from our study is that the thermal properties of voids appear to offer a useful means to measure the thermal expansivity of supported membranes.

Studies of Triton X-165–β-cyclodextrin interactions using both extrinsic and intrinsic fluorescence

The interaction of β-cyclodextrin with the non-ionic micelle-forming surfactant Triton X-165 (TX-165) has been studied using steady state fluorescence and fluorescence anisotropy techniques. Both extrinsic and intrinsic fluorescence have been exploited for the purpose. Phenosafranin (PSF), a cationic phenazinium dye, has been used as the extrinsic probe while fluorescence of TX-165 has served as the intrinsic one. PSF shows discernible interactions with both TX-165 and β-CD. The experimental results reveal that the extent of interaction of PSF with TX-165 is greater than with β-CD. However, addition of β-CD to a micellar solution of TX-165 containing PSF leads to a disruption of the micelles whereby the fluorophore is released from the micellar environment to the bulk aqueous phase. It has been substantiated that an inclusion complex is formed between the non-ionic surfactant and the cyclodextrin. A 1:1 stoichiometry of the TX-165–β-CD inclusion complex has been proposed. Such a complexation between TX-165 and β-CD results in an inhibition in the micellization process of TX-165 leading to an enhancement in the apparent CMC value. The inferences are drawn from a series of experiments, viz., binding studies, determination of micropolarity, heavy-ion quenching studies and steady state fluorescence anisotropy experiments monitoring both extrinsic and intrinsic fluorescences.

Partitioning of Phenylalkanols between Micelles and Water Studied by Limiting Interdiffusion Coefficients in Water and Tetradecyltrimethylammonium Bromide Solutions

The Taylor dispersion method was used to measure limiting interdiffusion coefficients for benzyl alcohol, 2-phenylethanol, 3-phenylpropan-1-ol, 4-phenylbutan-1-ol, and 5-phenylpentan-1-ol in water and tetradecyltrimethylammonium bromide micellar solutions at 298.2 K. These data were used to obtain the degree of binding of the phenylalkanols to the micelles. Partition coefficients of the phenylalkanols between the micellar pseudophase and the bulk aqueous phase and values of the standard Gibbs energy for transfer from the bulk aqueous phase to the micellar pseudophase were calculated. The results are compared with literature values for the sodium dodecyl sulfate and hexadecyltrimethylammonium micellar systems.

An electrochemical study of H 2O 2 decomposition on single-phase γ-FeOOH films

The electrochemical reduction and oxidation kinetics of hydrogen peroxide on γ-FeOOH films chemically deposited on indium tin oxide substrates were studied over the pH range of 9.2–12.6 and the H 2O 2 concentration range of 10 −4 to 10 −2 mol dm −3. The Tafel slopes for H 2O 2 reduction and oxidation obtained from polarization measurements are 106 ± 4 and 93 ± 15 mV dec −1, respectively, independent of pH and the concentration of H 2O 2. Both the reduction and oxidation of H 2O 2 on γ-FeOOH have a first-order dependence on the concentration of molecular H 2O 2. However, for the pH dependence, the reduction has an inverse first-order dependence, whereas the oxidation has a first-order dependence, on the concentration of OH −. For both cases the electroactive species is the molecular H 2O 2, not its base form, HO 2 −. Based on these observations, reaction kinetic mechanisms are proposed which involve adsorbed radical intermediates; HO OH − and HO for the reduction, and HO 2 H +, HO 2 , and O 2 − for the oxidation. These intermediates are assumed to be in linear adsorption equilibria with OH − and H + in the bulk aqueous phase, respectively, giving the observed pH dependences. The rate-determining step is the reduction or oxidation of the adsorbed H 2O 2 to the corresponding intermediates, a reaction step which involves the use of Fe III/Fe II sites in the γ-FeOOH surface as an electron donor–acceptor relay. The rate constant for the H 2O 2 decomposition on γ-FeOOH determined from the oxidation and reduction of Tafel lines is very low, indicating that the γ-FeOOH surface is a very poor catalyst for H 2O 2 decomposition.

Functional interaction structures of the photochromic retinal protein rhodopsin

We studied functional interaction structures of the vertebrate membrane photoreceptor rhodopsin containing retinal as a chromophore. Using time-resolved fluorescence depolarization we analyzed real-time dynamics and conformational changes of the cytoplasmic helix 8 (H8) preceding the long C-terminal tail of rhodopsin. H8 runs parallel to the membrane surface and extends from transmembrane helix 7 whose highly conserved NPxxY(x)F motif connects that region of rhodopsin with the retinal binding pocket. Our measurements indicate that photo-induced retinal isomerization from 11-cis to all-trans provokes conformational changes of H8, including slower motion and reduced flexibility, that are specific for the active metarhodopsin-II photo-intermediate. These conformational changes are absent in the retinal-devoid state opsin and in the phosphorylated metarhodopsin-II state upon receptor deactivation. Furthermore we show that membrane rim effects can influence interfacial reactions at the cytoplasmic rhodopsin surface such as proton transfer reactions between surface and aqueous bulk phase or binding of the signaling protein transducin visualized with single-molecule widefield microscopy. These findings are important for an understanding of the effects of membrane structure on the photo-transduction mechanism.

Fat-crystal stabilised w/o emulsions for controlled salt release

The potential of fat-crystal stabilised w/o emulsions to control the release of salt contained within their aqueous phase has been investigated. A mixture of mono- and triglyceride crystals was used to obtain Pickering-stabilised 60:40 w/o emulsions. Salt release was monitored by placing small amounts of these emulsions into a bulk aqueous phase (this was either pure water or glucose aqueous solutions in order to vary osmotic pressure gradients) and measuring changes in conductivity over time. Less than 5% of total salt was released after 1 month in emulsions containing a mixture of mono- and triglycerides. Emulsions containing only monoglycerides released more than 40% salt in this period. SEM pictures showed that crystals have sintered at the interface to give total surface coverage with no or very few defects. Stability measurements at varying temperatures and Differential Scanning Calorimetry (DSC) indicate that any release is the result of partial melting of the crystals. Finally, varying the osmotic pressure gradient has no impact on salt release.

An acid induced mixed-micelle mediated cloud point extraction for the separation and pre-concentration of platinum from road dust and determination by inductively coupled plasma mass spectrometry

An acid induced mixed-micelle mediated cloud point extraction procedure is developed for the separation and pre-concentration of platinum from digested road dust matrices. The mixed-micelle is formed from the micelle–micelle interaction of cetyltrimethylammonium bromide (CTAB) (cationic) and Triton X-100 (nonionic) surfactants. Nitric acid induces the clouding as well as phase separation in these mixed-micelles. In the presence of mixed acids (HCl and HNO3), platinum forms stable and extractable anionic species compared to palladium and rhodium, which provides the quantitative separation of platinum, from palladium and rhodium, by forming the hydrophobic ion-associated complex with quaternary ammonium head groups of mixed-micelles. This hydrophobic complex is subsequently pre-concentrated from a bulk aqueous phase into a small mixed-micelles rich phase, avoiding the addition of an external chelating agent. The potential oxide interference of Hf is eliminated effectively up to 1 mg L−1 level, enabling interference free determination of 195Pt using inductively coupled plasma mass spectrometry (ICP-MS). The parameters affecting the extraction process are optimized. Under the optimized conditions, the pre-concentration factor and detection limit are 13 and 10 pg g−1, respectively. The recoveries are in the range 92–99% at 100–200 pg mL−1 with relative standard deviation 2.0–3.5%. The accuracy of the procedure is evaluated by platinum determination in certified reference materials such as CCRMP PTC-1a Copper-Nickel sulfide concentrate and BCR 723 Road dust and the results agreed with the certified values with 95% confidence level, as well as being applied to road dust samples.

Application of PARAFAC to determination of distribution constants and spectra of fluorescent solutes in micellar solutions

Steady state fluorescence excitation emission matrices (EEMs) and time-resolved fluorescence of biphenyl (BP) and 2,5-diphenyloxazole (PPO) dissolved in aqueous solutions of sodium dodecyl sulfate (SDS) were measured as a function of micelle concentration. Parallel factor analysis (PARAFAC) was applied for the first time to the determination of the distribution of solutes between the bulk aqueous phase and the SDS micellar phase, as well as their excitation and emission spectra in both phases. The raw scores of the PARAFAC analysis, which contain concentration information, are also dependent on photophysical parameters which can change upon micellization of the solute; we have developed a method of isolating accurate concentration values from the scores. The distribution constant of PPO, previously unreported, was found to be (8.8 +/- 0.7) x 10(5) M(-1), while the distribution constant of BP, for which various values have been reported, was found to be (4.5 +/- 0.2) x 10(4) M(-1). BP and PPO both showed weakly phase-dependent excitation and emission spectra. The method was validated by determining distribution constants for anthracene, phenanthrene, naphthalene, and pyrene, all of which showed good agreement with consensus values available in the literature.

Cylindrical Nanopore Electrode and Its Application to the Study of Electrochemical Reaction in Several Hundred Attoliter Volume

A method to fabricate cylindrical nanopore electrodes is presented. The volume of the cavity formed in the cylindrical nanopore electrode can be as small as several hundred attoliters. It has been characterized by using scanning electron microscopy and electrochemical methods. Our results show that the radius of the cavity can affect the diffusion coefficient of a redox species in the cavity. The cylindrical nanopore electrode has also been used to study charge transfer across the interface between an aqueous phase of several hundred attoliters in volume and a bulk chloroform phase. Compared with the same charge-transfer reaction across the interface between a bulk aqueous phase and a bulk chloroform phase, the potential of the charge-transfer reaction has a negative shift. The effect of the phase ratio on the distribution of the supporting electrolyte in the aqueous and organic phases has been used to explain the shift.

Modeling desorption kinetics of a persistent organic pollutant from field aged sediment using a bi-disperse particle size distribution

With the predicted climate change, it is expected that the chances of river flooding increase. During flood events, sediments will resuspend and when sediments are polluted, contaminants can be transferred to the surrounding water. In this paper we discuss a numerical intraparticle diffusion model that simulates desorption of dieldrin from a suspension of contaminated porous sediment particles with a well-characterized particle size distribution. The objective of this study was to understand the desorption rate (flux) of dieldrin from a suspension of field-aged sediment at different hydraulic retention times (HRT) of the aqueous phase and to elaborate the effect of particle-size distribution on mass transfer. Desorption kinetics of dieldrin, a persistent organic pollutant (POP), were experimentally measured and described in a separate paper using field-contaminated sediment. A radial diffusion model, accommodating intraparticle reversible sorption kinetics, aqueous phase pore diffusion, and a sink term for bulk aqueous phase refreshment was used to describe the experimental data. We observed rapid equilibrium of contaminants between small particles (10 μm) and the surrounding water even though the sorption affinity of dieldrin towards organic matter was high. On the contrary, for the larger particles (84 μm), calculations show that desorption was limited by intraparticle diffusion. Combining small and larger particles in our radial diffusion model resulted in the biphasic desorption behavior often observed even when using a linear isotherm. Flood events will result in an increase of desorption rate of POPs from sediments to the surrounding water. HRT and the particle-size distribution determine the desorption rate. We conclude that nonstationary diffusion within organic matter is the main process of mass transfer. Particle size distributions are very valuable to understand the phenomenology related to mass transfer limitations often described as limited bioavailability and can be used as basis to develop engineering options to limit contaminant mass fluxes into the environment.

Effect of iron chelates on oil–water interface, stabilized by milk proteins: The role of phosphate groups and pH. Prediction of iron transfer from aqueous phase toward fat globule surface by changes of interfacial properties

Iron incorporated into food systems induces oxidation and precipitation. The consequences are reduced bioavailability and a functional modification of other food components such as proteins. The iron-chelates such as ferrous bisglycinate represent a possibility to avoid side effects, since the iron is protected. The aim of this study is to investigate the effects of iron-chelates compounds on the properties of an oil/water interface stabilized by caseinate or β-lacotoglobulin, under environmental conditions at 20 °C. Analyses were performed using dynamic drop tensiometry during 5000 s. The aqueous bulk phase is an imidazole/acetate buffer (0.1 M), containing 0.4 × 10 −6 M protein, and 0.2 × 10 -6 9 M iron-chelates compounds. The results indicate that, under neutral conditions, the addition of some irons salts (NaFe-EDTA or Fe-bisglycinate) do not change the structure of the interface stabilized by a protein containing no phosphate groups (β-lactoglobulin). In the case of caseinate, NaFe-EDTA addition increases the lowering rate of surface tension at pH 6.5. On the contrary, the lowering rate of surface tension with caseinate is inhibited by Fe-bisglycinate at pH 6.5. Such an effect is not observed with β-lactoglobulin. The low transfer of irons ions from the bulk to the interface stabilized by β-lactoglobulin is confirmed by zetameter and FTIR measurements. These results indicate an effective strategy to follow for controlling the physical and chemical stability of an emulsion stabilized with proteins.

X-ray absorption fine structure of bromide ions attracted by cationic surfactants at the heptane-water interface

The total-reflection X-ray spectroscopic technique was applied to an interfacial species at the liquid-liquid interface. The XAFS spectrum of bromide ions at the heptane-water interface was successfully obtained in the fluorescence mode. A slight change in the spectra was observed in the presence of the cationic surfactants, dimethyldilaurylammonium and stearyltrimethylammonium ions. This suggested that the hydrated structure of bromide ions attracted by the cationic surfactants at the heptane-water interface is different from neat interfaces or bulk aqueous phases.

Model and cell membrane partitioning of perfluorooctanesulfonate is independent of the lipid chain length

Perfluorooctanesulfonic acid (PFOS) is a persistent environmental pollutant that may cause adverse health effects in humans and animals by interacting with and disturbing of the normal properties of biological lipid assemblies. To gain further insights into these interactions, we investigated the effect of PFOS potassium salt on dimyristoyl- (DMPC), dipalmitoyl- (DPPC) and distearoylphosphatidylcholine (DSPC) model membranes using fluorescence anisotropy measurements and differential scanning calorimetry (DSC) and on the cell membrane of HL-60 human leukemia cells and freshly isolated rat alveolar macrophages using fluorescence anisotropy measurements. PFOS produced a concentration-dependent decrease of the main phase transition temperature ( T m) and an increased peak width (Δ T w) in both the fluorescence anisotropy and the DSC experiments, with a rank order DMPC > DPPC > DSPC. PFOS caused a fluidization of the gel phase of all phosphatidylcholines investigated, but had the opposite effect on the liquid-crystalline phase. The apparent partition coefficients of PFOS between the phosphatidylcholine bilayer and the bulk aqueous phase were largely independent of the phosphatidylcholine chain length and ranged from 4.4 × 10 4 to 8.8 × 10 4. PFOS also significantly increased the fluidity of membranes of cells. These findings suggest that PFOS readily partitions into lipid assemblies, independent of their composition, and may cause adverse biological effects by altering their fluidity in a manner that depends on the membrane cooperativity and state (e.g., gel versus liquid-crystalline phase) of the lipid assembly.

Carbon nanotube clusters as universal bacterial adsorbents and magnetic separation agents

The magnetic susceptibility and high bacterial affinity of carbon nanotube (CNT) clusters highlight their great potential as a magnetic bio-separation agent. This article reports the CNT clusters' capability as "universal" bacterial adsorbents and magnetic separation agents by designing and testing a multiwalled carbon nanotube (MWNT) cluster-based process for bacterial capturing and separation. The reaction system consisted of large clusters of MWNTs for bacterial capture and an external magnet for bio-separation. The designed system was tested and optimized using Escherichia coli as a model bacterium, and further generalized by testing the process with other representative strains of both gram-positive and gram-negative bacteria. For all strains tested, bacterial adsorption to MWNT clusters occurred spontaneously, and the estimated MWNT clusters' adsorption capacities were nearly the same regardless of the types of strains. The bacteria-bound MWNT clusters also responded almost instantaneously to the magnetic field by a rare-earth magnet (0.68 Tesla), and completely separated from the bulk aqueous phase and retained in the system. The results clearly demonstrate their excellent potential as highly effective "universal" bacterial adsorbents for the spontaneous adsorption of any types of bacteria to the clusters and as paramagnetic complexes for the rapid and highly effective magnetic separations.

Spectral modulation of charge transfer fluorescence probe encapsulated inside aqueous and non-aqueous β-cyclodextrin nanocavities

In this work, we report the spectral modulations of a intramolecular charge transfer (ICT) molecule ethyl ester of N, N-dimethylaminonaphthyl-(acrylic)-acid (EDMANA) when encapsulated in the water and N, N-dimethylformamide (DMF) solution of β-CD nanocavities. From the nature of the Benesi–Hildebrand (B–H) plots, the stoichiometry of the host guest inclusion complexes are found to be 1:1 in water β-CD solution and both 1:1 and 1:2 in DMF β-CD solution. The preferential location and difference in orientation of EDMANA molecule inside the β-CD cavity has been accessed by analysis of the effect of acid and metal cation Ni 2+ on the spectral characteristics in both the media. In case of 1:1 complex, the polar donor group prefers to expose to bulk aqueous phase capable of binding with H + and Ni 2+ ions and the acceptor to the hydrophobic interior. On the other hand, the acceptor group remains exposed to the non-polar bulk phase and the donor group is orientated preferentially inside the non-polar core in 1:2 inclusion complexes.