Investigation on extremely dilute solution of PEO by PFG-NMR

The technique of pulsed-field gradient nuclear magnetic resonance (PFG-NMR) was applied to study the solution properties of a series of low molecular weight poly(ethylene oxide). The self-diffusion coefficients of solutions from semi-concentrated to extremely diluted were measured, leading to a critical concentration. When the concentration of solution is higher than the value of critical concentration, the diffusion coefficient of the solute decreases as the concentration increases and remains the same when the concentration is lower than it. This critical concentration agrees well with the definition of dynamic contact concentration (C _s) and confirms indirectly the Flory's scaling law between the molecular weight and D ₀. In addition, the influences of molecular weight and terminal groups on C _s were discussed. All the diffusion coefficients determined at extremely dilute condition were equivalent to the diffusion coefficients at infinite concentration (D ₀), from which the polymer coil size was estimated.     

Self-diffusion and molecular association of acetylsalicylic acid and methyl salicylate in methanol-<Emphasis Type="Italic">d</Emphasis>4 in the temperature range 278–318 K

The effect of concentration on the self-diffusion coefficients of acetylsalicylic acid and methyl salicylate in methanol-d4 is investigated in the temperature range of 278–318 K using NMR. It is found that the self-diffusion coefficients increase along with temperature and fall as concentration rises. Within the limit of an infinitely dilute solution, the effective radii of solute molecules, calculated using the Stokes–Einstein equation shrink as the temperature grows. It is shown that the observed reduction of effective radii is associated with an increase in the fraction of solute monomers as the temperature rises. The physicochemical parameters of heteroassociation of acetylsalicylic acid and methyl salicylate with methanol are determined.     

Partition Equilibria for Alcohols in Reverse Micellar AOT-Oil-Water Systems Studied by PGSE-FT NMR. A Comparison Between AOT-containing and the Corresponding AOT-free Systems

Microemulsions of the reverse micellar type were investigated by determining the self-diffusion coefficients of the components using the Pulsed Field Gradient Spin Echo – Fourier Transform NMR method (PGSE-FT NMR). The microemulsions were composed of the surfactant AOT, water and an oil (either benzene or cyclohexane), forming a water core in an oil continuum. The primary alcohols ranging from methanol to 1-decanol were added to the microemulsions as a fourth component. The degree of binding, p, of the alcohol to the micelles was determined from the measured self-diffusion coefficients for this component. Partition equilibrium constants were calculated from the values of p. Thermodynamic partition equilibrium constants, K _c , calculated from the partition coefficients are presented and compared with values based on literature data for AOT-free systems. Similarities and differences between these cases are revealed and interpreted.     

Formation of diethylene glycol as a side reaction during production of polyethylene terephthalate

Polymers of poly(ethylene terephthalate) (PET) always contain a certain amount of incorporated diethylene glycol (DEG), substituting the incorporated glycol. DEG is formed in a side reaction during the ester interchange of dimethyl terephthalate (DMT) with ethylene glycol or during direct esterification of terephthalic acid with ethylene glycol, and to a smaller extent during the polycondensation of the low-molecular material. DEG is formed via an unusual type of reaction: ester + alcohol → ether + acid. Some evidence of this type of reaction is given by the formation of dioxane in low molecular PET and of methyl Cellosolve and methyl carbitol during the ester interchange of DMT with ethylene glycol and diethylene glycol, respectively. The strongest support for this type of reaction, however, was obtained from kinetic data. Polyesters of low molecular weight with OH group contents ranging from 3 to 0.5 mole/kg were heated at 270°C in sealed tubes for 1–7 hr. The kinetic equation for the proposed reaction is: d[DEG]/dt = k[OH] [ester]. With the aid of one rate constant the formation of DEG in all esters could be described.     

Self-diffusion and sedimentation of dextran in concentrated solutions

Frictional coefficients for dextran in water have been evaluated from (i) self-diffusion coefficients determined by pulsed-field-gradient NMR, and (ii) sedimentation coefficients in concentrated solutions. The results show that these frictional coefficients are only equal at infinite dilution and that f_s increases more rapidly than f_D^* as the concentration increases.     

氧气在碱溶液中结构与扩散系数的分子动力学模拟

采用分子动力学方法对不同温度(25-120 ℃)及碱浓度(1:100-1:5, 摩尔比)下NaOH和KOH溶液中的氧气进行了模拟. 本文考察了NaOH及KOH溶液中溶剂-溶剂、氧气-溶剂及氧气-溶质的径向分布函数, 并采用爱因斯坦方程从均方位移曲线中计算得到了氧气及溶质离子的扩散系数. 结果显示随着碱浓度的升高, 氧气扩散系数逐渐减少; 在相同条件下, 氧气在NaOH溶液中扩散系数小于在KOH溶液中的扩散系数; 溶质离子扩散系数的变化规律与氧气一致. 通过与现有实验结果对比, 发现了分子动力学方法的可靠性及用于研究实验受限领域的优越性.     

Self-diffusion of lithium cations and ionic conductivity in polymer electrolytes based on polyesterdiacrylate

The processes of ionic conductivity are studied in a polymer gel electrolyte synthesized based on polyesterdiacrylate and a low-molecular solvent ethylene carbonate. The self-diffusion coefficients of solvent molecules and Li⁺ cations are measured by the NMR with the pulsed magnetic field gradient. The Li⁺ self-diffusion coefficients increase with the increase in the solvent content and are independent of the diffusion time in the interval from 10 to 1600 ms. The latter values imply the absence of limitations for the translational mobility of lithium ions in the spatial range from 10⁻⁷ to 10⁻⁵ m. Based on the Nernst-Einstein equation, the ionic conductivities are calculated and compared with the experimental conductivities measured by the impedance method. These values coincide for high contents of solvent; for low ethylene carbonate concentrations, the calculated conductivities much exceed the experimental values.     

Composition of Coexisting Liquid Phases Determined by Rayleigh Interferometry

Rayleigh interferometry is a precise macroscopic gradient technique that has been utilized for the determination of multicomponent diffusion coefficients. Because concentration gradients in multicomponent systems drive a diffusion-based partial separation of different solutes, this interferometric technique may be potentially used for the determination of solute concentrations. We have therefore theoretically examined how Rayleigh interferometry can be applied for the determination of composition of ternary aqueous mixtures. The effect of cross-term diffusion coefficients on the accuracy of this method is also discussed. Furthermore, since the poly(vinyl)alcohol+poly(ethylene)glycol+water system undergoes liquid–liquid phase separation (LLPS), we have experimentally characterized its LLPS boundary at 25 °C. The corresponding tie-lines were characterized by determining the composition of the two coexisting liquid phases using Rayleigh interferometry.     

Volumetric and viscometric study of aqueous binary mixtures of some glycol ethers at T = (275.15 and 283.15) K

The experimental data for the density (ρ) and viscosity (η) are reported for aqueous binary mixtures of different glycol ethers, namely ethylene glycol monomethyl ether (EGMME), ethylene glycol monoethyl ether (EGMEE), diethylene glycol monomethyl ether (DEGMME), and diethylene glycol monoethyl ether (DEGMEE), at different temperatures (T = 275.15 K and 283.15 K) within the concentration range 0 mol · kg⁻¹ to 0.1 mol · kg⁻¹. The values of density (ρ) and viscosity (η) of the solutions were used to compute different derived parameters, such as apparent molar volume (?_V) of the solute, excess molar volume (V^E) of the solution, viscosity B and D coefficients of solution and temperature coefficient of viscosity B-coefficient (dB/dT) of solution. The limiting apparent molar volume of the solutes () have been obtained for aqueous binary mixtures of these glycol ethers by smooth extrapolation of ?_V–m curves to zero concentration. By using the values of , the limiting excess partial molar volumes () have also been calculated. The results are interpreted in term of various interactions such as solute–solvent interactions and hydrogen bonding.     

Quantitative determination of diethylene glycol contamination in pharmaceutical products

A simple, rapid, and reliable method was developed for the quantitative determination of diethylene glycol (DEG) in pharmaceutical products using capillary gas chromatography with flame ionization detection. The method uses ethylene glycol as internal standard and allows for the separation of propylene glycol and DEG. The assay was linear in a DEG concentration range between 1.0 and 10.00 mg/mL, with coefficients of variation of 2.3-4.4% for the tested concentrations. Quantitation and detection limits, respectively, were 1.0 mg/mL and 0.15 mg/mL diethylene glycol. The method was used to analyze 3 pharmaceutical products possibly contaminated with diethylene glycol, of which one was suspected of causing intoxication and death in children. Infrared spectroscopy was used to confirm the identity of diethylene glycol. This analytical methodology is proposed for evaluation of pharmaceutical products containing glycols to prevent intoxication and for security level verification.     

Temperature and concentration dependence of diffusion coefficients in ethylene–propylene-diene copolymers

Self-diffusion and partition coefficients were measured for two commercial ethylene–propylene-diene copolymers (EPDM) and five solvents at infinite dilution using inverse gas chromatography. Mutual diffusion coefficients for solvents in EPDM also were measured for finite concentration using gravimetric sorption for three of the solvents. From the inverse gas chromatography experimental values for self-diffusion coefficients were obtained. Free-volume parameters were obtained through regression of the self-diffusion coefficient as a function of temperature. Mutual diffusion coefficients as a function of concentration were predicted using free volume theory and compared with experimental data obtained using gravimetric sorption. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1713–1719, 1998     

The effect of polymers on the phase behavior of balanced microemulsions: diblock-copolymer and comb-polymers

The effect of some amphipilic diblock-copolymers and comb-polymers on a balanced Winsor III microemulsion system is investigated with the quaternary system n-octyl-β-d-glucoside/1-octanol/n-octane/D₂O as basis system. The diblock-copolymers are polyethyleneoxide-co-polydodecenoxide (PEO _x PEDODO _y ) and polyethyleneoxide-co-polybutyleneoxide (PEO _x PEBU _y ), constituted of a straight chain hydrophilic part and a bulky hydrophobic part. Addition of the diblock-copolymer leads to an enhancement of the swelling of the middle phase by uptake of water and oil; a maximum boosting factor of 6 was obtained for PEO₁₁₁PEDODO₂₅. Nuclear magnetic resonance diffusometry yields the self-diffusion coefficients of all the components in the system. The diffusion experiments provide information on how the microstructure of the bicontinuous microemulsion changes upon addition of the polymers. The reduced self-diffusion coefficients of water and oil are sensitive to the type of polymer that is incorporated in the film. For the diblock-copolymers, as mainly used here, the reduced self-diffusion coefficient of oil and water will respond to how the polymer bends the film. When the film bends away from water, the reduced self-diffusion of the water will increase, whereas the oil diffusion will decrease due to the film acting as a barrier, hindering free diffusion. The self-diffusion coefficient of the polymer and surfactant are similar in magnitude and both decrease slightly with increasing polymer concentration.     

Interaction of simple acids with nylon. Part 2: Diffusion of simple acids

The paper deals with the diffusion of two mineral acids, hydrobromic and sulfuric acids, and two simple dye acids, NOG (C. I. Acid Orange 7) and SY (C.I. Food Yellow 3), in water-swollen nylon 66. Anion self-diffusion coefficients were obtained by radiotracer techniques. The bromide ion and the SY anion self-diffusion coefficients show very little variation with concentration in the amino-dyeing region, whereas the H₂SO₄ and NOG anion diffusion coefficients are concentration-dependent. The variation of the H₂SO₄ anion diffusion coefficient with concentration is consistent with the formation of small quantities of the highly mobile bisulfate ion. The low SO₄ diffusion coefficient may be explained by the interaction of this ion with single, fixed sites in the polymer. The variation of the NOG anion diffusion coefficient with concentration does not follow a simple D = D₀[1/(1 ? θ)] relationship at intermediate concentrations but the rapid increase observed as the available sites became saturated, i.e., as θ → 1, is consistent with a site saturation model.     

The influence of polymer molecular-weight distributions on pulsed field gradient nuclear magnetic resonance self-diffusion experiments

 The influence of polymer molecular-weight distributions on the outcome of pulsed field gradient (PFG) NMR self-diffusion experiments has been considered. The self-diffusion coefficient, D, of monodisperse poly(ethylene oxide) (PEO) polymers has been determined in order to accurately determine the scaling behavior of D both with molecular weight and concentration. In order to investigate the influence of polydispersity on the PFG NMR signal, a model system consisting of ten reasonably monodisperse PEO polymers was made, and the PFG NMR signal intensities were recorded at a low total concentration. The data were analyzed using both inverse Laplace transformation and nonlinear least-squares fitting to a prescribed distribution function of D. Finally, the molecular-weight distribution was obtained by use of the values of the scaling parameters. We also present some model calculations used to investigate the sensitivity of the degree of polydispersity on the NMR signal decays. Received: 27 May 1999 Accepted in revised form: 19 October 1999     

Self-diffusion in solutions of a 20 base pair oligonucleotide: effects of concentration and ionic strength

The long-time self-diffusion coefficients of a 20 base pair duplex oligonucleotide are measured as functions of 20-mer and added NaCl salt concentrations. The self-diffusion coefficients decrease monotonically with increasing 20-mer concentrations for the high-added salt sample and display non-monotonically decreasing 20-mer concentration dependences at lower added salt concentrations. The non-monotonic behavior is attributed to the opposing effects of the tendency to increase the interactions between 20-mers as the concentration is increased and to a decrease in the extent of the Coulomb forces as counterions from the 20-mer increasingly screen them. Attempts to account for the effect of the Coulomb forces on the self-diffusion coefficients by using effective dimensions in the hard rod theory give good agreement with experiment at the highest salt concentration studied. For the lower salt concentrations there appear to be two scaling regimes--one at low polyion concentration in which the high salt scaling of the rod dimensions by adding the Debye screening to the length and diameter of the rod is appropriate and one at high polyion concentrations where the scaling of the dimensions is the addition of 1/2 the Debye screening length. Estimates of the "overlap" concentration C*=1/L(eff) indicate that the non-monotonic decrease occurs at concentrations lower than C*. Finally, the fluorescence correlation spectroscopy self-diffusion coefficients measured here are compared with the mutual diffusion coefficients measured by dynamic light scattering.     

Calculations of the distribution of trace impurities in cylindrical pores

The equilibrium distribution of a trace impurity and the self-diffusion coefficients of molecules of the base component and the trace impurity in narrow cylindrical pores were calculated using the lattice-gas model. Two types of lattice structures with six and eight closest neighbors were considered. The sizes of the base component and impurity molecules were taken to be identical. Lateral interactions were taken into account in the quasi-chemical approximation. The equilibrium distributions of the trace impurity across a pore section in the gas and liquid phases of the base component and at the interface for the case of capillary condensation were considered. The probability of existence of isolated dimeric clusters was estimated and the self-diffusion coefficients of the base component and trace impurity for a single-phase distribution of the base component were calculated. The effects of the energy of interaction of impurities with the pore walls and the concentration of the base component on the diffusion mobility of the impurities were analyzed. The concentration dependences of the partition coefficient for the trace impurity between the pore center and the pore wall and the concentration dependences of the self-diffusion coefficients for the trace impurity molecules become nonmonotonic with an increase in the base component concentration. These effects are due to the displacement of the impurity from the near-surface area to the bulk of a pore following an increase in the pore coverage by the base component and to higher mobility of the impurity in the free bulk of the pore. Further filling of the pore bulk reduces the mobility of all molecules. The energetics of intermolecular interactions also plays a certain role. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 605–615, April, 2000.     

Configurational properties of aromatic polyesters: Dipole moments of poly(diethylene terephthalate) chains

Dielectric constants have been determined for a fraction of poly(diethylene terephthalate) in benzene at several temperatures. The data indicate that the dipole moment ratio 〈μ²〉/Nm² is somewhat higher than that of poly(ethylene oxide), and its temperature coefficient is in the vicinity of zero. Both the dipole ratio and its temperature coefficient are in very good agreement with those predicted by the rotational isomeric state theory. Using this theory, the unperturbed dimensions of poly(diethylene terephthalate) were calculated and it was found that (〈r²〉/M)_∞ = 0.80 Ų (g mol wt)^?1, a value intermediate between those of poly(ethylene oxide) (0.57) and poly(ethylene terephthalate) (1.05).     

Self-Diffusion of PEO-Modified Paclitaxel in Aqueous Solution: Hydrodynamic Properties

The anti-cancer drug Paclitaxel has been hydrophilized by coupling with poly(ethylene oxide) through a self-immolating linker to the polymer. The mobility of the functionalized polymer and of the amphiphilic drug-modified polymer in D₂O was studied by the temperature and concentration dependence of the long-time self-diffusion coefficients of the components. The technique applied was pulse-gradient field NMR spectroscopy. The diffusivity of the molecules can be described in terms of the Einstein-Sutherland and the Stokes-Einstein equations. First results indicate that combining the drug with the polymer rather influences the rigidity parameter and the pair-interaction in solution than the shape and the hydrodynamic radius of the polymer coil. Unexpectedly, micelle-formation, although highly probable, was not observed within the concentration range investigated here.     

Thermodynamic and optical studies of some ethylene glycol ethers in aqueous solutions at T = 298.15 K

Experimental results of density (ρ), speed of sound (u), and refractive index (n_D) have been obtained for aqueous solutions of ethylene glycol monomethyl ether (EGMME), ethylene glycol monoethyl ether (EGMEE), diethylene glycol monomethyl ether (DEGMME), and diethylene glycol monoethyl ether (DEGMEE) over the entire concentration range at T = 298.15 K. From these measurements, the derived parameters, apparent molar volume of solute (?_V), excess molar volume (V^E), isentropic compressibility of solution (β_S), apparent molar isentropic compressibility of solute (?_KS), deviation in isentropic compressibility (Δβ_S), molar refraction [R]_1,2 and deviation in refractive index of solution (Δn_D) have been calculated. The Redlich–Kister equation has been fitted to the calculated values of V^E, Δβ_S and Δn_D for the solution. The results obtained are interpreted in terms of hydrogen bonding and various interactions among solute and solvent molecules.