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.     

Mechanism of Proton Relaxation for Enzyme‐Manipulated,Multicomponent Gold–Magnetic Nanoparticle Chains

Longitudinal and transverse relaxation times of multicomponent nanoparticle (NP) chains are investigated for their potential use as multifunctional imaging agents in magnetic resonance imaging (MRI). Gold NPs (ca. 5 nm) are arranged linearly along double‐stranded DNA, creating gold NP chains. After cutting gold NP chains with restriction enzymes (EcoRI or BamHI), multicomponent NP chains are formed through a ligation reaction with enzyme‐cut, superparamagnetic NP chains. We evaluate the changes in relaxation times for different constructs of gold–iron oxide NP chains and gold–cobalt iron oxide NP chains using 300 MHz ¹H NMR. In addition, the mechanism of proton relaxation for multicomponent NP chains is examined. The results indicate that relaxation times are dependent on the one‐dimensional structure and the amount of superparamagnetic NP chains present in the multicomponent constructs. Multicomponent NP chains arranged on double‐stranded DNA provide a feasible method for fabrication of multifunctional imaging agents that improve relaxation times effectively for MRI applications.     

Sorption and transport behavior of gasoline components in polyethylene glycol membranes

Desulphurization mechanism of polyethylene glycol (PEG) membranes has been investigated by the study of solubility and diffusion behavior of typical gasoline components through PEG membranes with various crosslinking degrees. The sorption, diffusion and permeation coefficients were calculated by the systematic studies of dynamic sorption curves of gasoline components such as thiophene, n-heptane, cyclohexane, cyclohexene and toluene in PEG membranes. Furthermore, the temperature dependence of diffusion and solubility coefficients and the influence of crosslinking degree on sorption and diffusion behaviors were conducted to elucidate the mass-transfer mechanism. According to the discussions on dynamic sorption curves, transport mode, activation energy and thermodynamic parameters, thiophene species were the preferential permeation components. Crosslinking is an effective modification way to improve the overall performance of PEG membranes applied in gasoline desulphurization. The pervaporation (PV) and gas chromatography (GC) experiments results corresponded to the conclusions. All these investigations will provide helpful suggestions for the newly emerged membrane desulphurization technology and complex organic mixture separation by pervaporation.     

On measurement of thermal diffusion coefficients in multicomponent mixtures

We investigate the steady-state separation of the individual components of an incompressible multicomponent liquid mixture in a narrow two-dimensional thermogravitational column. Analytic working equations for measuring thermal diffusion coefficients analogous to the existing equations for a binary mixture are derived. Similar to the binary results, we find that when compositional variation has negligible effect on fluid density and vertical diffusive flux can be ignored, molecular diffusion does not affect steady-state separation. However, when compositional effects on density are taken into account, molecular diffusion does affect the bulk convective flow and the steady-state separation of the components. There may be also two distinct trends in the velocity and separation profiles. With one or more negative thermal diffusion coefficients, there may be more than one convection cell resulting in oscillatory behavior of separation. The working equations presented can be used to measure thermal diffusion coefficients of multicomponent mixtures. Such measurements have not yet been reported in the literature.     

Transient separation of multicomponent liquid mixtures in thermogravitational columns

Transient separation of the individual components in a multicomponent liquid mixture in a thermogravitational column can be used to determine the thermal and molecular diffusion coefficients. Two models of the transient behavior are developed. First, a classical model where density only depends on temperature. Second, a general model where the compositional effect on density is also taken into account. Diffusion coefficients can be determined by fitting experimental data to either model. The procedure is demonstrated for a ternary liquid mixture. The results reveal that the classical model is very unreliable even though composition variation only contributes to the total buoyancy by one-third. Diffusion coefficients can be obtained reliably from the general model provided the experimental noise does not exceed +/-1% of steady-state separation. This level of accuracy in composition measurements is achievable.     

Sorption and transport of linear alkane hydrocarbons in biaxially oriented polyethylene terephthalate

Equilibrium sorption and uptake kinetics of n‐butane and n‐pentane in uniform, biaxially oriented, semicrystalline polyethylene terephthalate films were examined at 35 °C and for pressures ranging from 0 to approximately 76 cmHg. Sorption isotherms were well described by the dual‐mode sorption model. Sorption kinetics were described either by Fickian diffusion or a two‐stage model incorporating Fickian diffusion at short times and protracted polymer structural relaxation at long times. Diffusion coefficients increased with increasing penetrant concentration. n‐Butane solubility was lower than that of n‐pentane, consistent with the more condensable nature of n‐pentane. However, n‐butane diffusion coefficients were higher than those of n‐pentane. Infinite‐dilution, estimated amorphous phase diffusion and solubility coefficients were well correlated with penetrant critical volume and critical temperature, respectively. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 1160–1172, 2001     

Interaction effects in multicomponent separation by reverse osmosis

Separation of multicomponent mixtures, in particular ternary systems, was investigated theoretically and experimentally. The interactions in various multicomponent mixtures were determined. Using data from literature as well as the limited data obtained in our laboratory, we identified the situations where the multicomponent effects are strong. A correlation between the coupled permeability coefficients and straight permeability coefficients and concentration of components in ternary systems was obtained. Capabilities of thermodynamic models in describing the electrolyte–nonelectrolyte systems consisting of a weak acid were also investigated. The results of this work show that the multicomponent effects can be used in improving the separation properties of membranes. The findings of this study may be applied to several processes including recovery of small amounts of metals from wastes, separation of ionic compounds from organic, and separation of weak and strong electrolytes.     

Separation of phenol‐containing pyrolysis products using comprehensive two‐dimensional chromatography with columns based on pyridinium ionic liquids

We describe the application of columns with highly polar stationary liquid phases based on pyridinium ionic liquids for the two‐dimensional chromatography separation of bio‐oil and product of coal pyrolysis. By using inverse combination columns—a first ionic liquid column and a second nonpolar column—good separation results have been obtained. In the analysis of coal pyrolysis products, the suggested approach provides a much better resolution between components in comparison with a less polar first‐dimension column (based on polyethylene glycol). A good selectivity for the peaks of phenols is observed, and the group of phenols is well detached and separated from the group of diaromatics. A good separation picture was obtained also for bio‐oil, the groups of phenols and guaiacol derivatives are distinguished with good resolution of substances within each group.     

NMR spectroscopy and free volume analysis of the effects of copolymer composition on the swelling kinetics and chain dynamics of highly crosslinked copolymers of acrylic acid with PEG‐containing multiacrylates

Novel ionizable polymer networks were prepared from oligo(ethylene glycol) (OEG) multiacrylates and acrylic acid (AA), employing bulk radical photopolymerization techniques. The properties of these materials exhibited a complex dependence on the network structure and composition. Penetrant sorption experiments demonstrated that the crosslinked structure of the copolymers depended very strongly on the AA content as well as the number of ethylene glycol groups. The impact of varying the AA content and the oligo(ethylene glycol) chain length on the polymer chain dynamics was examined using diffusion and ¹³C NMR relaxation studies. The penetrant uptake studies indicated a coupling of Fickian and relaxation‐driven contributions to the swelling behavior. The effect of increasing the AA content on the characteristic chain relaxation time was reversed as the oligo(ethylene glycol) chain length was varied, indicating that chain relaxation is controlled by structural considerations, for shorter oligo(ethylene glycol) chains, and by compositional considerations, for longer oligo(ethylene glycol) chains. Measured compositional effects on solid state ¹³C NMR relaxation times supported these conclusions. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1953–1968, 1999     

Rapid 1H[13C]-resolved diffusion and spin-relaxation measurements by NMR spectroscopy

Hadamard-encoded heteronuclear-resolved NMR diffusion and relaxation measurements allow overlapping signal decays to be resolved with substantially shorter measuring times than are generally associated with 2D heteronuclear cross-correlation experiments. Overall measuring time requirements can be reduced by approximately an order of magnitude, compared to typical 2D heteronuclear single-quantum correlation-resolved diffusion or relaxation measurements. Specifically, in cases where chemical shift correlation information provides enhanced spectral resolution, the use of Hadamard encoding can be used to overcome uniqueness challenges that are associated with the analysis of concurrent dynamic processes and the extraction of time constants from overlapping exponential signal decays. This leads to substantially improved resolution of similar time constants than can be achieved solely through the use of post-acquisition processing techniques. In the ideal case of complete spectral separation of the signal decays, the usual constraint that time constants must be sufficiently different to resolve by exponential analysis can be circumvented entirely. Hadamard-based pulse sequences have been used to determine 1H[13C]-resolved diffusion coefficients and spin-relaxation time constants for the chemically similar components of an aqueous solution of ethanol, glycerol, and poly(ethylene glycol), and a dye-containing block-copolymer solution, which exhibit significant spectral overlap in their 1H NMR spectra.     

极性和非极性溶剂在聚乙烯中的无限稀释扩散系数的测定

 气相法研究小分子溶剂与高分子聚合物材料之间的相互作用是一个快速、准确、方便的方法 ,可以测量多种小分子溶剂在聚合物中的溶解、扩散参数。通过气相法测定了 5种小分子溶剂 (正己烷、正庚烷、正癸烷、乙醇和水 )在固定液聚乙烯中的保留时间和半峰宽 ,运用vanDeemter模型进行数据处理 ,得到上述 5种小分子在聚乙烯中的无限稀释扩散系数 ,获得了十分有意义的结果。     

Separation of polyethylene glycols and maleimide‐terminated polyethylene glycols by reversed‐phase liquid chromatography under critical conditions

The separation of polyethylene glycols and maleimide‐substituted polyethylene glycol derivatives based on the number of maleimide end‐groups under critical liquid chromatography conditions has been investigated on a reversed‐phase column. The critical solvent compositions for nonfunctional polyethylene glycols and bifunctional maleimide‐substituted polyethylene glycols were determined to be identical at about 40% acetonitrile in water on a reversed‐phase octadecyl carbon chain‐bonded silica column using mixtures of acetonitrile and water of varying composition as the mobile phase at 25°C. The maleimide‐functionalized polyethylene glycols were successfully separated according to maleimide functionality (with zero, one, two, or three maleimide end‐groups, respectively) under the critical isocratic elution conditions without obvious effect of molar mass. The separation was mainly due to the hydrophobic interaction between the maleimide end‐groups and the column packing. Off‐line matrix‐assisted laser desorption/ionization time of flight mass spectrometry was used to identify the repeating units and, especially, the end‐groups of the maleimide‐substituted polyethylene glycols. Liquid chromatography analysis at critical conditions could provide useful information to optimize the synthesis of functional polyethylene glycols. To our knowledge, this is the first report of the baseline separation of maleimide‐functionalized polyethylene glycols based on the functionality independent of the molar mass without derivatization by isocratic elution.     

Diffusion des additifs du polyethylene—I: Influence de la nature du diffusant

Chemical efficiency and stabilizer migration are the most important factors to be considered in connection with the ageing of polyethylene. This paper gives diffusion coefficients and solubilities for various industrial additives in low density polyethylene. The results, together with activation energies for diffusion, allow study of the diffusion process. It seems that it is necessary to consider deformations of the diffusing molecules and the polymer molecules. Additives of low molecular weight diffuse as gases, those of high molecular weight behave like polymer chains in self-diffusion.     

ITQ-3分子筛中二元混合分子扩散性质的模拟

采用平衡分子动力学方法先是模拟研究了纯组分Ar、SF_6以及CF4在ITQ-3分子筛中的扩散行为,结果表明,在窄孔道中Ar的扩散系数随负载的增加呈现先增加再减小的变化趋势,在宽孔道中则随负载逐渐减小。纯组分的大分子SF_6以及CF4只在y方向扩散,扩散系数与小分子在z方向的变化规律一致。随后,又模拟了二元混合组分SF_6和CF4在ITQ-3分子筛中的扩散行为,模拟结果与各自纯组分SF_6和CF4进行了对比,发现二元混合的SF_6和CF4整体变化趋势与其单组分保持一致,在y方向的扩散系数都是随着负载的增加而先增大后减小,但是两者之间还存在一种相互作用,使得在整个负载范围内扩散较快的CF4的扩散系数比纯组分的扩散系数相对小一些,而纯组分时扩散相对缓慢的SF_6在二元混合中的扩散系数则相对增大。     

Diffusion relaxation times of nonequilibrium isolated small bodies and their solid phase ensembles to equilibrium states

The possibility of obtaining analytical estimates in a diffusion approximation of the times needed by nonequilibrium small bodies to relax to their equilibrium states based on knowledge of the mass transfer coefficient is considered. This coefficient is expressed as the product of the self-diffusion coefficient and the thermodynamic factor. A set of equations for the diffusion transport of mixture components is formulated, characteristic scales of the size of microheterogeneous phases are identified, and effective mass transfer coefficients are constructed for them. Allowing for the developed interface of coexisting and immiscible phases along with the porosity of solid phases is discussed. This approach can be applied to the diffusion equalization of concentrations of solid mixture components in many physicochemical systems: the mutual diffusion of components in multicomponent systems (alloys, semiconductors, solid mixtures of inert gases) and the mass transfer of an absorbed mobile component in the voids of a matrix consisting of slow components or a mixed composition of mobile and slow components (e.g., hydrogen in metals, oxygen in oxides, and the transfer of molecules through membranes of different natures, including polymeric).     

Multicomponent effective medium-correlated random walk theory for the diffusion of fluid mixtures through porous media

Molecular transport in nanoconfined spaces plays a key role in many emerging technologies for gas separation and storage, as well as in nanofluidics. The infiltration of fluid mixtures into the voids of porous frameworks having complex topologies is common place to these technologies, and optimizing their performance entails developing a deeper understanding of how the flow of these mixtures is affected by the morphology of the pore space, particularly its pore size distribution and pore connectivity. Although several techniques have been developed for the estimation of the effective diffusivity characterizing the transport of single fluids through porous materials, this is not the case for fluid mixtures, where the only alternatives rely on a time-consuming solution of the pore network equations or adaptations of the single fluid theories which are useful for a limited type of systems. In this paper, a hybrid multicomponent effective medium-correlated random walk theory for the calculation of the effective transport coefficients matrix of fluid mixtures diffusing through porous materials is developed. The theory is suitable for those systems in which component fluxes at the single pore level can be related to the potential gradients of the different species through linear flux laws and corresponds to a generalization of the classical single fluid effective medium theory for the analysis of random resistor networks. Comparison with simulation of the diffusion of binary CO(2)/H(2)S and ternary CO(2)/H(2)S/C(3)H(8) gas mixtures in membranes modeled as large networks of randomly oriented pores with both continuous and discrete pore size distributions demonstrates the power of the theory, which was tested using the well-known generalized Maxwell-Stefan model for surface diffusion at the single pore level.     

Effect of macromolecular polydispersity on diffusion coefficients measured by Rayleigh interferometry

Rayleigh interferometry has been extensively used for the precise determination of diffusion coefficients for binary and ternary liquid mixtures. For ternary mixtures, the 2x2 matrix of multicomponent diffusion coefficients is obtained. Polydispersity adds complexity to the meaning of these measured diffusion coefficients. Here we discuss three important issues of polydispersity regarding the diffusion measurements extracted from this interferometric technique. First, we report novel equations for the extraction of diffusion moments from the Rayleigh interferometric pattern. These moments are used to define polydispersity parameters for macromolecular systems. We have experimentally determined mean diffusion coefficients and polydispersity parameters for aqueous solutions of poly(ethylene glycol) and poly(vinyl alcohol) at 25 degrees C. Aqueous solutions of poly(ethylene glycol) mixtures were used to examine the accuracy of the polydispersity parameters. Second, we compare Rayleigh interferometry to dynamic light scattering. Specifically, we have performed diffusion measurements on the same system using both techniques. To our knowledge, no direct experimental comparison between dynamic light scattering and classical methods for the measurements of diffusion coefficients has been previously reported in relation to polydispersity. We find that substantial discrepancies (i.e., 1 order of magnitude) between the mean diffusion coefficients obtained from these two different techniques can be observed when polydispersity is large. Third, for two-solute mixtures with one polydisperse solute, we report a novel corrective procedure for extracting accurate ternary diffusion coefficients from Rayleigh interferometry. Computer simulations were used to examine the accuracy of the extracted ternary diffusion coefficients.     

Capillary electrophoresis of small solutes in linear polymer solutions: relation between ionic mobility, diffusion coefficient and viscosity

Electrophoretic mobilities, mu, and diffusion coefficients, D, of a small ion (molecular weight 579) were determined in dependence on the viscosity, eta, of aqueous buffer solutions containing ethylene glycol, or polyethylene glycol (PEG) with average molecular weights of 400, 20000, 100000 or 2000000, respectively, as additives. The values for mu and D are inversely proportional to the viscosity for the solutions with small-sized additives (ethylene glycol and PEG400), in accordance to Walden's rule. In contrast, for the longest polymers the mobilities and the diffusion coefficients approximate the values observed for pure water, and are nearly independent of the viscosity. This result agrees with the model of fractional free volume and the obstruction theory. For solutions with equal viscosity, three ranges can be differentiated for mu and D in relation to the size of the additive: for small additives, on the one hand, and the long-chained polymers, on the other hand, the values for mu and D are nearly independent of the size of the additive. In contrast, a pronounced increase of mu and D is found with increasing polymer size in the molecular weight range between 20000 and 100000. The ratio mu/D, occurring in a number of expressions for the plate height contributions, exhibits a remarkably small change over the entire polymer size and viscosity range (between 1 and 7 cP) under consideration. Consequently, the separation efficiency, expressed by the plate number, is found to be nearly constant, and is independent of viscosity.     

Evaluation of protein 15N relaxation times by inverse Laplace transformation

Relaxation times (T1, T2, T1rho) are usually evaluated from exponential decay data by least-squares fitting methods. For this procedure, the integrals or amplitudes of signals must be determined, which can be laborious with large data sets. Moreover, the fitting requires a priori knowledge of the number of exponential components responsible for the decay. We have adapted inverse Laplace transformation (ILT) for the analysis of relaxation data. Exponential components are resolved with ILT to reciprocal space on their corresponding relaxation rate values. The ILT approach was applied to 3D linewidth-resolved 15N HSQC experiments to evaluate 15N T1 and T2 relaxation times of ubiquitin. The resulting spectrum is a true 3D spectrum, where the signals are separated by their 1H and 15N chemical shifts (HSQC correlations) and by their relaxation rate values (R1 or R2). From this spectrum, the relaxation times can be obtained directly with a simple peak-picking procedure.     

Resolving natural product epimer spectra by matrix-assisted DOSY

High resolution diffusion-ordered NMR spectroscopy allows the separation of signals from different species based on their diffusion coefficients. In general this requires that the NMR spectra of the components do not have overlapping signals, and that the diffusion coefficients are significantly different. Modifying the solvent matrix in which a sample is dissolved can change the diffusion coefficients observed, allowing resolution ("matrix-assisted DOSY"). We show here that dissolving the two naturally-occurring epimers of naringin in an aqueous solution of β-cyclodextrin causes both shift and diffusion changes, allowing the signals of the epimers to be distinguished. Chiral matrix-assisted DOSY has the potential to allow simple resolution and assignment of the spectra of epimers and enantiomers, without the need for derivatisation or for titration with a shift reagent.