The chameleon effect in zwitterionic micelles: Binding of anions and cations and use as nanoparticle stabilizing agents

Zwitterionic surfactants are relatively overlooked and some of their properties resemble those of ionic and non-ionic surfactants, but others are unique. There is unimpeachable evidence that aqueous solutions of zwitterionic micelles interact specifically with anions, forming “anionoid” micelles, which concentrate cations in the surfaces. Thus, unlike ionic micelles, both the cation and anion of the electrolyte solution can be concentrated at zwitterionic interfaces. This unique effect, known as the “Chameleon Effect”, can be used to catalyze a variety of simple reactions, as the attraction of ions to the micelle brings the reactants together. Furthermore, zwitterionic surfactants stabilize metallic nanoparticles and the magnetically stirred two-phase system could be reused 3 more times in the hydrogenation of cyclohexene (Pd:cyclohexene ratio of 1:18300), with very little loss in activity, and an average turn-over frequency of 1000 h^? 1.     

On a dual function of anions in crystallogenesis of compounds: Structure-directing and stabilizing

The crystallogenesis of three groups of structures of fluoride-orthoborates (and their analogs) with large cations is studied from the point of view of the current cation-framework representations. Their cation frameworks are sterically templated by rigid borate anions and are built as antizeolite structures. Cavities of channel or layered types are filled with components different their composition and configuration with the main function as charge compensators. An example of different functionalities of the identical anions are Ba₃(BO₃)₂ and La₃Ca₃(BO₃)₅ structures. In these structures, two (BO₃) types generate a cation framework, and the third type stabilizes it in ready channels (!).     

Ultraviolet-curable epoxy acrylate dispersions: effect of urethane acrylate anionomers on stabilizing and film properties

 Epoxy acrylate dispersions stabilized using urethane acrylate anionomers were prepared for an application of ultraviolet (UV) curing. By observing the optical microscopy and colloidal stability for the epoxy acrylate dispersions, it was confirmed that the urethane acrylate anionomers incorporated have an interfacial activity in the interface between the epoxy acrylate oil and the water/ ethanol mixture (80/20, w/w). This was possible by the structurally designed urethane acrylate anionomers, containing a hydrophobic soft segment and two hydrophilic ionic sites in their molecules. In addition, when ultraviolet (UV)-cured, the urethane acrylate anionomers agglomerated to form the rubber domains in the epoxy acrylate film, which were induced by the ionic interaction. Consequently, this agglomerated rubber domains improved the final film properties. Received: 4 April 1998 Accepted: 1 July 1998     

Interfacial properties of zwitterionic surfactants: N-dodecylbetaine

The interfacial properties and adsorption of a zwitterionic surfactant, n-dodecylbetaine, were investigated in interfacial tension and calorimetric studies. The enthalpy of micellization was directly obtained from calorimetric curve of surfactant dilution measurements and changed as expected. Adsorption onto silica gel showed mechanisms dependent on amphiphile concentration and temperature.     

The alkali metal cation effect on the surface-enhanced Raman spectra of phosphate anions adsorbed at silver electrodes

The interaction of adsorbed phosphate anions with alkali metal cations at the Ag|aqueous solution interface has been investigated by surface-enhanced Raman spectroscopy (SERS). Formation of ion pairs at the interface was evident from the cation-induced perturbations in the SER spectra of anions. The frequency of the external vibration, silver–oxygen (Ag---O′), was not sensitive to the nature of cation, while the relative intensity of this mode was cation-dependent and was explored as a sensitive probe for the monitoring of coadsorption of ions at the interface. From the internal phosphate vibrations, both asymmetric modes, δ_as(PO) and ν_as(PO), were found to be the most sensitive to the nature of the cation. At a relatively positive potential (0.00 V vs. Ag | AgCl) the spectral parameters for the Cs⁺ and K⁺ cations were very similar indicating the same bonding type with anions. A more inhomogeneous chemical environment for the phosphate oxygen atoms was detected in the case of Na⁺ and Li⁺ cations. An increase in ν_as(PO) frequency by ca. 10 cm⁻¹ was the characteristic spectral signature for the interaction of phosphates with Li⁺. The formation of water-shared ion pairs at the interface was suggested based on the absence of splitting in the ν_as(PO) mode and the previously observed frequency sensitivity of this band to solvent H₂O substitution by D₂O. At negative potential (−0.80 V), a stabilization effect of Cs⁺ on the phosphate adlayer was detected based on the twofold increase in intensity of the ν(Ag---O′) mode compared with Li⁺. Splitting of the ν_as(PO) mode suggested the contact interaction of anions with specifically adsorbed Cs⁺ cations.     

Base and phosphate electron detachment energies of deoxyribonucleotide anions

Photoelectron spectra of deoxyribonucleotide anions are interpreted with ab initio, electron propagator calculations. Ground-state structures display hydrogen bonds which are not present in less stable minima that resemble Watson-Crick fragment geometries. For the adenosine and thymidine anions, there are two vertical electron detachment energies (VEDEs) within 0.1 eV of each other that correspond to phosphate- and base-centered Dyson orbitals (DOs). The first VEDE of the cytidine anion belongs to a phosphate-centered DO. The anomalously low VEDE of the guanosine anion is assigned to a base-centered, pi DO. Higher VEDEs of all four anions also are assigned.     

Kinetics of hydrated electron reactions with phosphate anions: a laser photolysis study

The decay kinetics of hydrated electron (e_aq ⁻) formed upon photolysis of aqueous solutions of sodium pyrene-1,3,6,8-tetrasulfonate at λ = 337 nm in the presence of phosphate anions (up to 2 mol L⁻¹) was studied by nanosecond laser-pulse photolysis in a wide range of pH (3.5–10) and ionic strength (I, up to 2 mol L⁻¹) values. At high pH values, where the HPO₄ ²⁻ ions dominate, the e_aq ⁻ decay kinetics depends only slightly on phosphate concentration (rate constant for the reaction is at most 2·10⁵ L mol⁻¹ s⁻¹). The H₂PO₄ ⁻ ions react with e_aq ⁻ at a rate constant of 2.8·10⁶ L mol⁻¹ s⁻¹ (I = 0), which increases linearly with the parameter in accordance with the Debye-Hückel theory. The rate constant for quenching of e_aq ⁻ by H₃PO₄ at pH ≤ 4 decreases linearly with the parameter due to the secondary salt effect and equals 1.6·10⁹ L mol⁻¹ s⁻¹ at I = 0. The logarithm of the rate constant for quenching of e_aq ⁻ by phosphates is linearly related to the number of the O-H bonds in the phosphate molecule. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1277–1280, July, 2007.     

Interfacial properties of heat-treated ovalbumin

The interfacial properties (kinetics of adsorption at the air/water interface, rheology of the interfacial layer) of ovalbumin molecules, unheated or previously heat-denatured in solution (10 g L(-1), pH 7, NaCl 50 mM) under controlled conditions (up to 40 min at 80 degrees C), were investigated. Heat treatments induced the formation of covalent aggregates which surface exhibits a higher hydrophobicity and an increased exposition of sulfhydryl groups when compared to native ovalbumin (unheated). Although they have a larger hydrodynamic size, aggregates adsorb as fast as native ovalbumin at the air/water interface. However, aggregates are able to established rapid contacts in the interfacial layer as shown by the fast increase of both surface pressure and shear elastic constant. In contrast, native ovalbumin needs longer time to developed intermolecular contacts and exhibits lower foam stability even if the shear elastic constant on aging reached higher value than for ovalbumin aggregates.     

Interfacial properties of colloid-polymer mixtures

Density functional theory and a virial approach are used to calculate the surface tension and bending rigidity of the interface between demixed fluid phases for a colloid-polymer mixture. The calculated surface tension compares well with results from computers simulations and experiments. The bending rigidity obtained from both theoretical approaches is negative (approximately equal to -0.1k(B)T), its magnitude increases away from the critical point and it is in reasonable agreement with computer simulations.     

NMR enantiodifferentiation of triphenylphosphonium salts by chiral hexacoordinated phosphate anions

BINPHAT anion—rather than TRISPHAT—is an efficient NMR chiral shift reagent for triphenylphosphonium salts containing stereogenic centers on the aliphatic side-chain.     

Imino- and aminomethylphenylboronic acids: stabilizing effect of hydrogen bonds

ortho-Iminomethylphenylboronic acids were synthesized from the reaction of 2-formyl–phenylboronic acid with primary aromatic amines. Reduction of these compounds yielded the corresponding aminomethylphenylboronic acids. For both types of the compounds, the crystal structure was determined by single crystal X-ray diffraction method. Hydrogen-bonded dimers with an additional intramolecular B–O–H…N hydrogen bond have been observed. Calculations at MP2/6–31+G** level proved that the most stable form is that with the above-mentioned intramolecular hydrogen bond while the form with dative N→B bond is less favoured. Since the calculated energy difference is small, the competition between possible forms was analyzed in terms of substituent effect stabilization energy (SESE). In the case of p-iminomethylphenylboronic acid, both hydroxyl groups are engaged in intermolecular O–H…O interactions resulting in a supramolecular ribbon motif.     

Interfacial dynamic properties of polymer films

Many vinyl homopolymers are surface active by virtue of their side chains being hydrophilic while backbone hydrocarbon structure being hydrophobic, thus they form remarkably stable monolayers. We report here the studies of collective dynamics of these monolayers and thin films with the technique of surface light scattering by the spontaneous capillary waves on the air/water and oil/water interfaces. The dynamics are represented by the viscoelastic parameters of the lateral dilational mode, which couples efficiently with the transverse mode in the case of air/water interface.     

Interfacial properties of acidified skim milk

The purpose of this study is to investigate the tension properties and dilatational viscoelastic modulus of various skim milk proteins (whole milk, EDTA-treated milk, beta-casein, and beta-lactoglobulin) at an oil/water interface at 20 degrees C. Measurements are performed using a dynamic drop tensiometer for 15,000 s. The aqueous bulk phase is a skim milk simulated ultrafiltrate containing 11 x 10(-3) g L(-1) milk protein. At pH 6.7, beta-casein appears as the best to decrease the interfacial tension, whereas beta-lactoglobulin leads to the highest interfacial viscoelastic modulus value. Whole milk was almost as surface-active as individual beta-casein in terms of the final (steady-state) lowering of the interfacial tension, but the rate of tension lowering was smaller. EDTA treatment improved the rate of tension lowering of whole milk. The acidification of milk, from previous measurements, would lead to the enhancement of surface activity. At t=15,000 s, the order of effectiveness is pH 4.3 > pH 5.3 = pH 5.6 > pH 6.7 whole milk, suggesting that pH 4.3 whole milk is the best surface active. As compared to pH 6.7 whole milk, the use of pH 5.3 and pH 5.6 milk as surface active would result in the use of milk containing more free beta-casein born of pH-dissociated casein micelles.     

Interfacial properties of pulmonary surfactant layers

The composition of the pulmonary surfactant and the border conditions of normal human breathing are relevant to characterize the interfacial behavior of pulmonary layers. Based on experimental data methods are reviewed to investigate interfacial properties of artificial pulmonary layers and to explain the behavior and interfacial structures of the main components during compression and expansion of the layers observed by epifluorescence and scanning force microscopy. Terms like over-compression, collapse, and formation of the surfactant reservoir are discussed. Consequences for the viscoelastic surface rheological behavior of such layers are elucidated by surface pressure relaxation and harmonic oscillation experiments. Based on a generalized Volmer isotherm the interfacial phase transition is discussed for the hydrophobic surfactant proteins, SP-B and SP-C, as well as for the mixtures of dipalmitoylphosphatidylcholine (DPPC) with these proteins. The behavior of the layers depends on both the oligomerisation state and the secondary structure of the hydrophobic surfactant proteins, which are controlled by the preparation of the proteins. An example for the surface properties of bronchoalveolar porcine lung washings of uninjured, injured, and Curosurf treated lavage is discussed in the light of surface behavior. An outlook summarizes the present knowledge and the main future development in this field of surface science.     

Interfacial properties of compressible polymer solutions

A chemomechanical model for the interfacial concentration and density in compressible polymer solutions is formulated using variational principles. The nonlinear model with boundary conditions obtained from phase equilibrium calculations gives the coupled concentration and density profiles. The couplings between chemical and mechanical balances are identified and efficient ways to calculate the interfacial structure is identified. A specific model appropriate to high‐pressure processing of the polyolefins is developed using the modified Sanchez Lacombe equation of state. Bakker's formula for the interfacial tension is adapted to compressible polymer solutions. The structure and tension of a flat interface is characterized using the developed model and material properties of three molecular weight hydrogenated polybutadiene; the main variables of interest were the pressure, polymer molecular weight, and temperature. The relation between the pressure profile across the interface and the interfacial tension is characterized. Scaling power laws for interfacial tension and interfacial thickness as a function of pressure are obtained and contrasted with the corresponding laws observed and predicted for incompressible polymer solutions. It is found that the modified Sanchez Lacombe‐based power law prediction predictions for compressible solutions in terms of pressure quenches are similar to those from those obtained by the Flory‐Huggins incompressible model for temperature quenches. The present results provide the basis for the future study of the kinetics of pressure‐induced phase separation in compressible polymer solutions. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 640–654, 2009     

Adsorption of phosphate and nitrate anions on ammonium-functionalized MCM-48: effects of experimental conditions

In this study, ammonium-functionalized MCM-48 (Mobil Composite Material No. 48) was used as an adsorbent to remove nitrate (NO(-)(3)) and monobasic phosphate (H(2)PO(-)(4)) anions from aqueous solutions. The effects of operating conditions such as temperature, adsorbent loading, initial anion concentration, pH, and the presence of competitive ions on the adsorption performances were examined. Results showed that adsorption capacity decreased with increasing temperature. The adsorption capacity increased with adsorbent loading and initial anion concentration. The removal of nitrate was maximum at pH<8, while phosphate removal was maximized at pH 5. The adsorption was almost unaffected by the presence of competitive ions in the case of phosphate anions. However, their presence adversely affected nitrate adsorption. Desorption of both anions was rapidly achieved within 10 min using NaOH at 0.01 M. Regeneration tests showed that the adsorbent retained its capacity after 5 adsorption-desorption cycles.     

Tensor magneto-optical properties of some carboxylate anions

The Cotton—Mouton effect was used for the study of benzoate, phthalate. isophthalate, and terephthalate anions. The additive tensor characteristics of optical polarizability and magnetic susceptibility ellipsoids have been calculated for C(Ar)-COO^– and . The spatial structures of mono- and deprotonated phthalate anions have been discussed.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 874–876, May, 1993.     

Interfacial properties of cyclic hydrocarbons: a Monte Carlo study

The Monte Carlo technique is used to study the vapor-liquid interface of cyclopentane, cyclohexane, and benzene. The OPLS and TraPPE potential fields are compared in the temperature range from 298.15 to 348.15 K (273.15-298.15 K for C5H10). A new method for the treatment of the long-range interactions in inhomogeneous simulations is used. When this new method is employed, the obtained values of saturated liquid density and of enthalpy of vaporization are equal to those obtained using the bulk isothermal-isobaric Monte Carlo technique. The values of surface tension become independent of the cutoff distance and they are significantly larger than those when only simple spherical truncation of intermolecular interactions is used.     

Contrasting the denaturing effect of guanidinium chloride with the stabilizing effect of guanidinium sulfate

Guanidinium chloride, GdmCl, is a strong denaturing agent of globular proteins, whereas guanidinium sulfate, Gdm(2)SO(4), is a stabilizing agent of globular proteins. The stabilizing activity of Gdm(2)SO(4) is unexpected because the denaturant capability of GdmCl is due to direct interactions of Gdm(+) ions with protein surface groups. It is shown that the statistical thermodynamic approach devised to explain the molecular origin of cold denaturation [G. Graziano, Phys. Chem. Chem. Phys., 2010, 12, 14245-14252] can provide a rationalization of the different behaviour of GdmCl and Gdm(2)SO(4) towards globular proteins. The fundamental quantity is the reversible work to create in the aqueous solution a cavity suitable to host the D-state and a cavity suitable to host the N-state. In aqueous GdmCl solutions, this contribution is not large enough to overwhelm the conformational entropy gain upon unfolding and the direct attractions between Gdm(+) ions and protein surface groups; in aqueous Gdm(2)SO(4) solutions, it is so large that it overwhelms the two destabilizing contributions. Sulfate ions, due to their high charge density, interact strongly with water molecules producing a number density increase, that, in turn, renders the cavity creation process very costly, reversing the denaturing power of Gdm(+) ions and stabilizing the N-state of globular proteins.