Water-surfactant Interface Research Articles

A chromogenic anthraimidazoldione probe for ratiometric analysis of Hg2+ exclusively at mesoscopic interface: Screening of real-life biological samples

In the present work, we have synthesized amphiphilic anthraimidazoldione derivative that can form nanoassembly in the aqueous medium. The π-conjugated backbone with facile intramolecular charge transfer from the substituted aryl residue to anthraquinone unit attributed to the profound orange color. Despite having the metal ion chelating aminoethyl acetamide unit, the compound showed no interaction with metal ions in the aqueous medium. However, a highly selective color-changing response, from orange to yellow (Δλ ∼52 nm), was observed upon addition of Hg2+ ion at micellar interface. Also, a turn-on fluorescence response (∼5.2-fold) was witnessed with Hg2+. It was observed that in addition to hydrophobic microenviroment, the orientation of molecules at water-surfactant interface also responsible for the chromogenic response towards Hg2+ ion. The mechanistic investigation indicated that Hg2+ ion coordinates with the aminoethyl acetamide unit, resulting restriction in the extent of intramolecular charge transfer. Considering such highly ratiometric response, the present system has been employed for analysis of Hg2+ ion in diluted urine and blood serum samples. The high recovery values (95.4 % to 104.3 %) with relatively low standard deviations (< 4.2 %) indicated that the present system is fairly successful in screening of complex real-life samples. Finally, we have designed low-cost, reusable, chemically modified paper strips for on-location, rapid analysis of mercury ions.

On-water surface synthesis of crystalline, few-layer two-dimensional polymers assisted by surfactant monolayers.

Despite rapid progress in recent years, it has remained challenging to prepare crystalline two-dimensional polymers. Here, we report the controlled synthesis of few-layer two-dimensional polyimide crystals on the surface of water through reaction between amine and anhydride monomers, assisted by surfactant monolayers. We obtained polymers with high crystallinity, thickness of ~2 nm and an average crystal domain size of ~3.5 μm2. The molecular structure of the materials, their grain boundaries and their edge structures were characterized using X-ray scattering and transmission electron microscopy techniques. These characterizations were supported by computations. The formation of crystalline polymers is attributed to the pre-organization of monomers at the water-surfactant interface. The surfactant, depending on its polar head, promoted the arrangement of the monomers-and in turn their polymerization-either horizontally or vertically with respect to the water surface. The latter was observed with a surfactant bearing a carboxylic acid group, which anchored amine monomers vertically through a condensation reaction. In both instances, micrometre-sized, few-layer two-dimensional polyamide crystals were grown.

Electrical Double Layer Probed by Surface-Specific Vibrational Technique

In this issue of Chem, Paesani and co-workers present a computational analysis of sum-frequency generation (SFG) spectra of water to disentangle the interfacial χ(2) and bulk χ(3) contributions. SFG line shapes are in good agreement with experimental results. The analysis reveals the molecular response of interfacial water, which is usually masked by the bulk water spectra at the water-charged surfactant interface.

Coarse-grained modeling of the oil–water–surfactant interface through the local definition of the pressure tensor and interfacial tension

Mesoscopic simulations are conducted to study the dodecane–water interface in the presence of an increasing quantity of sodium dodecyl sulfate. Dissipative Particle Dynamics and Molecular Dynamics using the MARTINI force field were used to model the dodecane–SDS–water system through a coarse-grained description. We have used a full description of the water and oil phases to study the structure of the interface. We have calculated the interfacial tension at the dodecane–water interface and established the accuracy of the calculation by checking the mechanical equilibrium and the stability of the interface. The local components of the pressure tensor are calculated along the interface by comparing both atomistic and molecular definitions. Both coarse-grained models give a good description of the system structure, with SDS molecules located at the interfaces. Increasing the number of SDS results in a drastic decrease in the interfacial tension and a saturation of the interface with SDS molecules. Higher SDS concentrations do not affect the interface structure, as it is saturated. The limiting value of the interfacial tension is very low and corresponds to the formation of a SDS film between the dodecane and water phases. Both coarse-grained potentials studied in this work are suitable for the study of surfactants in equilibrium with an oil–water interface. They provide a good description of the dodecane–SDS–water system and quantitative results for interfacial tension and interfacial concentrations. © 2017, Springer-Verlag Berlin Heidelberg.

ChemInform Abstract: “On‐Water” Multicomponent Reaction for the Diastereoselective Synthesis of Functionalized Tetrahydropyridines and Mechanistic Insight.

An ecofriendly approach for the synthesis of highly substituted tetrahydropyridines by an ‘on-water’ multicomponent reaction has been demonstrated. The use of water as the reaction medium is essential under the catalytic influence of a surfactant. The use of a variety of anionic, cationic, and non-ionic surfactants in water was examined and the reaction was successfully catalyzed by anionic surfactants sodium dioctyl sulfosuccinate (SDOSS) and sodium dodecyl sulfate (SDS), with the former being superior. The use of an organic solvent together with a catalytic amount of sodium dioctyl sulfosuccinate to form homogeneous conditions afforded inferior yields and highlighted the specific role of water through the creation of microreactors at the water surfactant interface. A mechanistic insight for the five-component reaction leading to the formation of tetrahydropyridines is provided invoking a tandem inter- and intramolecular Mannich reaction pathway.

‘On-Water’ Multicomponent Reaction for the Diastereoselective Synthesis of Functionalized Tetrahydropyridines and Mechanistic Insight

An ecofriendly approach for the synthesis of highly substituted tetrahydropyridines by an ‘on-water’ multicomponent reaction has been demonstrated. The use of water as the reaction medium is essential under the catalytic influence of a surfactant. The use of a variety of anionic, cationic, and non-ionic surfactants in water was examined and the reaction was successfully catalyzed by anionic surfactants sodium dioctyl sulfosuccinate (SDOSS) and sodium dodecyl sulfate (SDS), with the former being superior. The use of an organic solvent together with a catalytic amount of sodium dioctyl sulfosuccinate to form homogeneous conditions afforded inferior yields and highlighted the specific role of water through the creation of microreactors at the water surfactant interface. A mechanistic insight for the five-component reaction leading to the formation of tetrahydropyridines is provided invoking a tandem inter- and intramolecular Mannich reaction pathway.

Two Types of Water at the Water-Surfactant Interface Revealed by Time-Resolved Vibrational Spectroscopy.

The surfactant sodium dodecyl sulfate (SDS) is widely used as a detergent for both domestic and industrial applications. It forms a self-assembled monolayer on the surface of water. We report a microscopic model for the interaction between the surfactant and water and between water molecules at the interface, revealed using static and time-resolved two-dimensional sum frequency generation spectroscopy. Two distinct subensembles of water in the presence of this negatively charged SDS surfactant have been identified: those close to the SDS headgroup having fairly isolated O-H groups, i.e., localized O-H stretch vibrations, and those whose O-H stretch vibrations are delocalized, i.e., shared between multiple O-H bonds. The two subensembles are coupled, with subpicosecond energy transfer occurring between them. This is markedly different from O-H bonds at the air-water interface, which are less heterogeneous, and indicates that the water molecules that interact with the surfactant headgroups have hydrogen-bonding properties different from those of water molecules interacting with the other water molecules.

Proton transfer in ionic and neutral reverse micelles.

Proton-transfer kinetics in both ionic and neutral reverse micelles were studied by time-correlated single-photon counting investigations of the fluorescent photoacid 8-hydroxypyrene-1,3,6-trisulfonate (HPTS). Orientational dynamics of dissolved probe molecules in the water pools of the reverse micelles were also investigated by time-dependent fluorescence anisotropy measurements of MPTS, the methoxy derivative of HPTS. These experiments were compared to the same experiments in bulk water. It was found that in ionic reverse micelles (surfactant Aerosol OT, AOT), orientational motion (fluorescence anisotropy decay) of MPTS was relatively unhindered, consistent with MPTS being located in the water core of the reverse micelle away from the water-surfactant interface. In nonionic reverse micelles (surfactant Igepal CO-520, Igepal), however, orientational anisotropy displayed a slow multiexponential decay consistent with wobbling-in-a-cone behavior, indicating MPTS is located at the water-surfactant interface. HPTS proton transfer in ionic reverse micelles followed kinetics qualitatively like those in bulk water, albeit slower, with the long-time power law time dependence associated with recombination of the proton with the dissociated photoacid, suggesting a modified diffusion-controlled process. However, the power law exponents in the ionic reverse micelles are smaller (∼ -0.55) than that in bulk water (-1.1). In neutral reverse micelles, proton-transfer kinetics did not show discernible power law behavior and were best represented by a two-component model with one relatively waterlike population and a population with a faster fluorescence lifetime and negligible proton transfer. We explain the Igepal results on the basis of close association between the probe and the neutral water-surfactant interface, with the probe experiencing a distribution of more and less waterlike environments. In addition, the observation in bulk water of a power law t(-1.1) for diffusion-controlled recombination is in contrast to the theoretical prediction of t(-1.5) and previously reported observations. The difference from prior experimental results is discussed.

Role of Charge and Solvation in the Structure and Dynamics of Alanine-Rich Peptide AKA2 in AOT Reverse Micelles.

The propensity of peptides to form α-helices has been intensely studied using theory, computation, and experiment. Important model peptides for the study of the coil-to-helix transition have been alanine–lysine (AKA) peptides in which the lysine residues are placed on opposite sides of the helix avoiding charge repulsion while enhancing solubility. In this study, the effects of capped versus zwitterionic peptide termini on the secondary structure of alanine-rich peptides in reverse micelles are explored. The reverse micelles are found to undergo substantial shape fluctuations, a property observed in previous studies of AOT reverse micelles in the absence of solvated peptide. The peptides are observed to interact with water, as well as the AOT surfactant, including interactions between the nonpolar residues and the aliphatic surfactant tails. Computation of IR spectra for the amide I band of the peptide allows for direct comparison with experimental spectra. The results demonstrate that capped AKA2 peptides form more stable α helices than zwitterionic AKA2 peptides in reverse micelles. The rotational anisotropy decay of water is found to be distinctly different in the presence or absence of peptide within the reverse micelle, suggesting that the introduction of peptide significantly alters the number of free waters within the reverse micelle nanopool. However, neither the nature of the peptide termini (capped or charged) nor the degree of peptide helicity is found to significantly alter the balance of interactions between the peptides and the environment. Observed changes in the degree of helicity in AKA2 peptides in bulk solution and in reverse micelle environments result from changes in peptide confinement and hydration as well as direct nonpolar and polar interactions with the water–surfactant interface.

Location of anti-TB drugs and microstructural changes in organized surfactant media using optical properties

Microstructure elucidation is a critical step in the understanding of organized surfactant media such as microemulsions. In this work, a systematic approach for revelation of microstructure changes of Tween 80 microemulsion has been carried out using photophysical properties. Microemulsion structural changes have been monitored using pyrene and different solvatochromic probes for the presence of oil-rich, water-rich, and bicontinuous microstructures. The hydrophobic optical probe, Nile red (NR), has been used to report from the side of the oil–surfactant interface and to complement this, a hydrophilic optical probe tris(2,2′-bipyridine)ruthenium(II) dichloride (RC) from the side of the water–surfactant interface. The results show the partitioning of probes into different microenvironments within the system. NR occupies the interface toward the apolar side and RC toward the polar. The analysis also indicates conspicuous dependence of the emission/absorbance behavior of optical probes on the water:surfactant molar ratio ( ω). Quantitative estimates have been obtained for the polarities (ET(30)) of solubilization sites of the probes in different regions of microemulsions and are well correlated with a Stokes shift. Lastly, these probes of different solubilities have been used to locate the position of anti-tuberculosis drugs, i.e., rifampicin (hydrophobic), pyrazinamide, and isoniazid (hydrophilic) of variable solubilities loaded in Tween 80 systems.

Protons in Non-ionic Aqueous Reverse Micelles

Using molecular dynamics techniques, we investigate the solvation of an excess proton within an aqueous reverse micelle in vacuo, with the neutral surfactant diethylene glycol monodecyl ether [CH3(CH2)11(OC2H4)2OH]. The simulation experiments were performed using a multistate empirical valence bond Hamiltonian model. Our results show that the stable solvation environments for the excess proton are located in the water-surfactant interface and that its first solvation shell is composed exclusively by water molecules. The relative prevalence of Eigen- versus Zundel-like solvation structures is investigated; compared to bulk results, Zundel-like structures in micelles become somewhat more stable. Characteristic times for the proton translocation jumps have been computed using population relaxation time correlation functions. The micellar rate for proton transfer is approximately 40x smaller than that found in bulk water at ambient conditions. Differences in the computed rates are examined in terms of the hydrogen-bond connectivity involving the first solvation shell of the excess charge with the rest of the micellar environment. Simulation results would indicate that proton transfers are correlated with rare episodes during which the HB connectivity between the first and second solvation shells suffers profound modifications.

Water Mobility in Reverse Micelles Studied by Quasielastic Neutron Scattering and Molecular Dynamics Simulation

AbstractReverse micelles (RMs) are aggregates in which nanoscale droplets of a polar liquid, usually water, are surrounded by a surfactant layer in a nonpolar continuous phase. They are widely used as media for reactions in which the extent of confinement or the presence of a surfactant interface play a central role. We have used molecular dynamics (MD) computer simulation and quasielastic neutron scattering (QENS) and to investigate the mobility of water molecules in reverse micelles. The contribution of water to the QENS signal is enhanced by deuterating the surfactant and the nonpolar phase. Our studies of water mobility have focused on the effects of water pool size, determined by the water/surfactant mole ratios w0, as well as on the properties of the water-surfactant interface. Specifically, we have examined the effects of varying w0 and of substituting other alkali ions for the usual Na+ counterion of the anionic surfactant AOT (bis (2-ethylhexyl) sulfosuccinate)). We find good agreement between the QENS signal and its prediction from MD simulation. This allows us to obtain additional insight into water mobility by analyzing the MD self-intermediate scattering function (ISF) of water hydrogens in terms of contributions from molecular rotation and translation and from molecules in different interfacial layers. MD data indicate that the translational ISF decays nonexponentially due to lower water mobility close to the interface and to confinement-induced restrictions on the range of translational displacements. Rotational relaxation also exhibits nonexponential decay. However, rotational mobility of O-H bond vectors in the interfacial region remains fairly high due to the lower density of water-water hydrogen bonds in the vicinity of the interface.

Interparticle interactions in water-in-oil microemulsions: Dielectric and ir investigations

The high-frequency dielectric response of sodium bis(2-ethylhexyl)sulfosuccinate (AOT) reverse micelles in n-heptane has been measured in the range 0.01--20 GHz as a function of volume fraction \ensuremath{\varphi} of a dispersed phase at a fixed value of the water to surfactant molar ratio $W=11.$ A relaxation phenomenon has been observed that is connected to the AOT ion pair diffusion at the water-surfactant interface. This dielectric relaxation has been found to be markedly affected by the interparticle interactions that have been modulated by adding different amounts of NaCl. In order to reveal the connections between attractive interactions among the droplets and the stereochemical configurations of AOT molecules, an ir study has been performed on the carbonyl stretching band of AOT molecules in the micellar aggregates. A comparison with dielectric data shows that a correlation exists between the intensity of attractive interactions and the composition of AOT rotamers in the microemulsion system. These results support the hypothesis that attractive intermicellar interactions are favored by a particular orientation and packing of nonpolar tails of AOT molecules in the micellar microaggregate.

Alcohol partition in a water-in-oil microemulsion from small-angle neutron scattering

It is often necessary to add a cosurfactant, typically an alcohol of medium chain length, to water oil, and surfactant for a microemulsion to form. To gain information on the role of alcohols, we have measured small-angle neutron scattering (SANS) from microemulsions comprised of n-hexadecane, potassium oleate, and water, plus several concentrations of 1-pentanol, 1-hexanol, and 1-octanol, in two regions of the phase diagram. Several models, including polydisperse spheres interpreted by recently developed equations for the structure function, were tested. Experimental data could be fit successfully with the core-plus-shell monodisperse-oblate-ellipsoids model previously used for similar compositions. Variation of the alcohol-to-surfactant molar ratios, particle dimensions increase with alcohol chain length in both regions of the phase diagrams. The water phase volume can reasonably by assumed constant, and estimates of the amount of alcohol at the water-surfactant interface were made from the parameters obtained from the fits. At fixed alcohol-to-surfactant molar ratios, we found a lower fraction of larger alcohols at the interface, in agreement with the increase of hydrophobicity. For a given alcohol, increasing the alcohol-to-surfactant ratio increased the amount of alcohol at the interface, although the fraction of total alcohol in the particles decreases. 32 refs., 14 figs., 9 tabs.

ENDOR studies of alkyl substituted p-Benzosemiquinones in reversed micelles and in 2-propanol

Various substituted p-benzosemiquinone radical anions, inter alia ubisemiquinone and derivatives, have been investigated in 2-propanol and in reversed micelles by EPR and ENDOR spectroscopy. Unsymmetrical semiquinones, with respect to the oxygen atoms, experience remarkable hyperfine shifts depending on the medium. This effect even allows differentiation between stereoisomers. Immobilization of the semiquinone molecules at the water-surfactant interface in reversed micelles gives rise to pronounced asymmetric linewidth effects. In the case of 2-cyclohexyl-3-methyl-1,4-benzosemiquinones, mixtures of two species (conformers) have been observed.