Effect of temperature on the organized self-assembly of SDS/β-Cyclodextrin aqueous solution by dielectric relaxation behavior

The temperature dependence of microstructures of SDS/β-cyclodextrin aqueous solution was investigated by impedance analyzer at the temperature range of 15°C to 55°C. The dielectric relaxation behaviors were observed from 15°C to 35°C, which was attributed to the Maxwell-Wagner interfacial polarization. The microstructure transition of the SDS/β-cyclodextrin aqueous solution from micro-tube to vesicle and then to monomer with increasing temperature was confirmed by conjointly analyzing dielectric parameters, confocal laser scanning microscopy, and infrared spectra. Furthermore, the dielectric analysis was proved to be useful to study surfactant-based organized self-assembly.     

Hairpin folding behavior of mixed α/β-peptides in aqueous solution

The invention of new strategies for the design of protein-mimetic oligomers that manifest the folding encoded in natural amino acid sequences is a significant challenge. In contrast to the α-helix, mimicry of protein β-sheets is less understood. We report here the aqueous folding behavior of a prototype α-peptide hairpin model sequence varied at cross-strand positions by incorporation of 16 different β-amino acid monomers. Our results provide a folding propensity scale for β-residues in a protein β-sheet context as well as high-resolution structures of several mixed-backbone α/β-peptide hairpins in water.     

Aggregation behavior of hydrophobically modified β-cyclodextrins in aqueous solution

Three amphiphilic -cyclodextrins in aqueous solution were investigated upon their aggregation behavior. Surface methods, conductivity and scattering methods were used to determine the cmc and radius of gyration of the aggregates. Depending on the cotenside it was possible to shift the cmc to lower concentrations. Above the cmc small spherelike aggregates existed.     

Dielectric relaxation study of aqueous α-amylase using time domain reflectometry technique

Time domain reflectometry technique has been used to study the complex permittivity spectra of aqueous α-amylase solution at different pH. Static dielectric constant and relaxation time were obtained from the complex permittivity spectra using nonlinear least square fit method. The dielectric relaxation parameter increases with an increase in molar concentration of α-amylase in water due to the formation of hydrogen bonding. The hydration numbers were also determined from the static dielectric constant.     

Discontinuous behavior of Rayleigh relaxation in supersaturated aqueous NH_4H_2PO_4 Solution

The algorithm of Rayleigh intensity correlation is employed for the elucidation of relaxa-tion time constants of the aqueous NH_4H_2PO_4 solution with various concentrations.The resultshows that there is a discontinuous(abrupt)increases in the relaxation time constant as the super-saturation state is reached.This quantity,the relaxation time constant,therefore,can be an orderparameter.Also discussed is the dependence of the relaxation time constant on the aggregationnumber n in(H_2PO_4~-)_n.This dependence is complicated and may be crucial in understanding theaggregation process in the supersaturation state.     

Micellar behavior of acrylamide– octylphenylpoly(oxyethylene) acrylate copolymer in aqueous solution

The copolymer of acrylamide and octylphenylpoly(oxyethylene) acrylate macromonomer (AM-C₈PhEO₇Ac) was synthesized and characterized by IR and NMR spectroscopy. The molecular weight of the copolymer was determined to be 1.21 × 10⁵ by static light scattering. The weight contents of AM and macromonomer were determined to be 67.8 and 32.2%, respectively, by elemental analysis. The micellar behavior of the copolymer in aqueous solution was studied by UV spectroscopy and atomic force microscopy (AFM). The molecules of AM-C₈PhEO₇Ac copolymer form monomolecular micelles in the concentration range 4 × 10⁻⁶ – 3 × 10⁻⁵ g/ml and polymolecular micelles at concentrations above 3 × 10⁻⁵ g/ml according to the UV analysis. The AFM images indicate that the monomolecular micelles are globular with diameters of 70 nm and have a narrow size distribution. The polymolecular micelles can be globular or cylindrical depending on the concentration, and have a wide size distribution. Received: 10 February 1999 Accepted in revised form: 28 June 1999     

Triphenylphosphine-catalyzed isomerization of alkynyl ketones in aqueous solution

An efficient isomerization of alkynyl ketones to(E,E)-diene ketones was developed.The reaction catalyzed by 20 mol% triphenylphosphine in aqueous media gave the desired products in good yields.     

Structural mimicry of the α-helix in aqueous solution with an isoatomic α/β/γ-peptide backbone

Artificial mimicry of α-helices offers a basis for development of protein-protein interaction antagonists. Here we report a new type of unnatural peptidic backbone, containing α-, β-, and γ-amino acid residues in an αγααβα repeat pattern, for this purpose. This unnatural hexad has the same number of backbone atoms as a heptad of α residues. Two-dimensional NMR data clearly establish the formation of an α-helix-like conformation in aqueous solution. The helix formed by our 12-mer α/β/γ-peptide is considerably more stable than the α-helix formed by an analogous 14-mer α-peptide, presumably because of the preorganized β and γ residues employed.     

β-Cyclodextrin polymer promoted green synthesis of cinnamaldehyde to natural benzaldehyde in aqueous solution

An environmentally benign synthesis of natural benzaldehyde from cinnamaldehyde under mild conditions has been investigated with sodium hypochlorite as oxidant and β-cyclodextrin polymer as phase-transfer catalyst. The polymer showed excellent catalytic activity exhibiting 92% conversion and 62% selectivity to benzaldehyde at ambient pressure and at 70°C. This catalyst could be recovered and reused six times, and the catalyst efficiency remained unchanged, which suggests that the catalyst is an efficient and green catalyst for oxidation of cinnamaldehyde. The results reported herein may be a promising method in industry for the synthesis of natural benzaldehyde.     

Hydration numbers of α-alanine in an aqueous urea solution

Literature data on the apparent molar volumes ϕ of alanine in water and aqueous urea solutions at 298 K are analyzed. It is shown that the slope of the ϕ dependence on the alanine concentration is not dependent on the urea concentration. The standard partial volume of alanine increases linearly with the increase in the urea concentration (wt.%). The structural characteristics of hydrated complexes of alanine (hydration number, molar volume of water inside and outside the hydration sphere, and proper volume of alanine in solution) are given. The hydration number of alanine decreases by a factor of two in passing from water to a saturated (20m) urea solution. The effects of urea additions on the hydration numbers of alanine and glycine are compared.     

Dielectric analysis on phase transition and micelle shape of polyoxyethylene trisiloxane surfactant in dilute aqueous solution

The cloudy Silwet L-77 aqueous solution on the concentration range from 0.5%to 50%was investigated by dielectric relaxation spectroscopy.The concentration dependence of phase microstructure was confirmed by means of analyzing the dielectric parameters of bulk solution.The relaxation behavior was assigned to the interfacial polarization between the micelle and the medium,and the relaxation distribution parameter was used to figure the shape transition from sphere to ellipsoid with the concentration increasing.The synchronous reduction of permittivity and conductivity indicated the formation of the lamellar phase. As compensation,the quantity of the surfactant liquid phase gradually decreased,whose shape constantly kept ellipsoidal.     

Dielectric relaxation pattern of dilute colloidal suspensions

An immediate method of analysis of the relaxation characteristics of a colloidal suspension, like of any dielectric, is based on the so-called Cole-Cole representation (imaginary part versus real part) of its complex dielectric constant in a wide frequency range. In this work, we show theoretical plots calculated according to the models developed by DeLacey and White (J Chem Soc Faraday Trans 2 77:2007–2039), and by Rosen et al. (J Chem Phys 98: 4183–4194; this model uses the dynamic Stern layer theory). Both theoretical approaches to the dielectric relaxation pattern of a colloidal suspension are compared to each other, and to experimental data obtained on polystyrene suspensions. Although no significant differences are found between the theoretical predictions of the relaxation patterns (except for the values of the dielectric constant; the DSL model yields higher polarizabilities of the suspensions), none of the models can exactly reproduce the frequency dependence of the dielectric constant of a colloidal system. We propose a modification of DeLacey and White's model to include the possibility that the ionic drag coefficients depend on the ion position in the double layer. The final results show that the general trends of the frequency dependence of the quantities involved are not modified, irregardless of the changes in ionic drag coefficients.     

Phases confirmation of cloudy Silwet L-77 aqueous solution by dielectric relaxation spectroscopy

<正>The phase transition of Silwet L-77(a kind of siloxane surfactant) dilute aqueous solution with temperature was investigated by dielectric relaxation spectroscopy.In the initial heating process,a dielectric relaxation was found at about 10~6 Hz,which was considered as the interface polarization ascribed to the interface between water and liquid phases(W and L_1).With the temperature increasing further,a new dielectric relaxation was observed at about 10~4 Hz,which was assigned to the appearance of liquid crystal phase(L_α).According to the dielectric parameters fitted by Cole-Cole equation,the coexistence temperature of W,L_1 and L_αwas determined at about 37.0℃.     

Fluorometric determination of trace selenium in β-CD/SDS aqueous solution

A proper mixture of β-cyclodextrin and sodium dodecylsulfate in aqueous solution was found to show a synergistic enhancement of the fluorescence intensity of 4,5-benzopiaselenol. The determination of trace selenium can be carried out directly in aqueous solution.     

Dielectric behavior of some aromatic polyimide films

Aromatic polyimides were prepared by polycondensation reaction of two aromatic diamines, such as 4,4′-diaminodiphenylmethane (DDM) and 3,3′-dimethyl-4,4′-diaminodiphenylmethane (MDDM), with aromatic dianhydrides, such as 4,4′-isopropylidene-diphenoxy-bis(phthalic anhydride) (6HDA), benzophenonetetracarboxylic dianhydride (BTDA) and hexafluoroisopropylidene-bis (phthalic anhydride) (6FDA). These polymers are soluble in polar aprotic solvents and can be cast into thin films from such solutions. The polyimides show high thermal stability, with decomposition temperature being above 430 °C in air, and high glass transition temperature being in the range of 200–287 °C. The free standing films, having the thickness of tens of micrometers, exhibited good mechanical and electrical insulating properties. The dielectric constant, molecular mobility and AC conductivity of thin films prepared from these polymers were investigated in detailed. The study of their dielectric behavior evidenced low dielectric constant values, in the range of 2.88–3.48 at 1 Hz at room temperature, and three relaxation processes (γ,β₁ and β₂) were observed at sub-glass temperatures for polyimides based on 6HDA and 6FDA and only two (γ and β) relaxations were detected for polyimides based on BTDA. The cooperativity of the molecular motions associated with the relaxation processes was discussed.     

Dielectric and mechanical relaxation processes in methyl acrylate/tri-ethyleneglycol dimethacrylate copolymer networks

The dielectric properties of methylacrylate (MA)/tri-ethyleneglycol dimethacrylate (TrEGDMA) copolymers at different compositions, ranging from 0 to 100, were measured between −120 and 150 °C over the frequency range 0.1 Hz-1 MHz. In the given frequency range, three relaxation processes were detected by dielectric relaxation spectroscopy in homo poly-TrEGDMA and copolymers: the α process associated with the glass transition, and two secondary processes due to localized mobility. In PMA only one secondary process was observed besides the alpha relaxation process. The influence of copolymerization going from PMA, monofunctional softer component with a glass transition determined calorimetrically as 284 K, to poly-TrEGDMA, higher glass transition component, bifunctional, that also forms a dense network due to cross linking, reflects mainly in the alpha process that shifts to higher temperatures and becomes broader. The raise and broadening in the glass transition with TrEGDMA increase was also observed by dynamic mechanical thermal analysis and differential scanning calorimetry. The glass transition temperature of poly-TrEGDMA was not detected calorimetrically but a value of 429 K was estimated from the best fit of the Fox equation. In what concerns the secondary relaxation process detected in poly-TrEGDMA and copolymers at the lowest temperatures, it is related with local twisting motions of ethyleneglycol moieties, being designated as γ relaxation, while the process detected in the medium temperature range is associated with the rotation of the carboxylic groups as in poly(alkyl methacrylates), designated as β relaxation. This process is detected at much lower temperatures in homo PMA in the same temperature region than the above mentioned γ relaxation. The copolymerization influences mainly the α process while the γ process remains almost unaffected in copolymers relative to homo poly-TrEGDMA. The β process is largely determined by the presence o the tri-ethylene glycol dimethacrylate monomeric units even in copolymers with the lowest TrEGDMA content.     

Dielectric relaxation of poly(ester-ether-carbonate) multiblock terpolymers

The dielectric properties of a series of poly(ester-ether-carbonate) multiblock terpolymers have been investigated as a function of ether and carbonate composition in the frequency range of 10³–10⁶ Hz. The degree of polymerization of the samples was determined by viscosimetry measurements. The weight fraction, degree of crystallinity, and melting temperatures were characterized by means of x-ray diffraction and DSC methods. Dielectric behavior has been discussed in terms of Havriliak-Negami formulation. The variation of the dielectric properties with temperature has been associated with two relaxation processes: a) the -relaxation process observed at low temperature, which is associated to local motion of polar groups attached to both the soft and the hard segments, and b) the process assigned to long-range molecular motions above the glass transition temperature.     

Dielectric relaxation in a new double perovskite oxide Ho2MgZrO6

A new double perovskite oxide holmium magnesium zirconate Ho₂MgZrO₆ (HMZ) was prepared by solid state reaction technique. The crystal structure has been determined by powder X-ray diffraction which shows monoclinic phase at room temperature with cell parameters a = 9.3028 ± 0.0030 Å, b = 5.2293 ± 0.0008 Å, c = 4.4009 ± 0.0009 Å, β = 103.3746 ± 0.0166°. An analysis of complex permittivity with frequency was carried out assuming a distribution of relaxation times. The frequency dependent electrical data are analyzed in the framework of conductivity and electric modulus formalisms. At the high temperature range, conductivity data satisfy the variable range hopping (VRH) model. In this regime, the conductivity of sample obeys Mott’s T^1/4 law, characteristic of VRH. High temperature data indicates the formation of thermally activated small polarons. The scaling behaviour of imaginary part of electric modulus suggests that the relaxation describes the same mechanism at various temperatures.