A visual approach to stationary phase selectivity classification based on the Snyder–Dolan Hydrophobic-Subtraction Model

A novel type of stationary phase selectivity classification “triangle” has been developed based on the Snyder–Dolan (S–D) Hydrophobic-Subtraction Model, wherein the apices of a set of four triangles represent the relative contributions of steric hindrance (χ_S), hydrogen-bonding acidity (χ_A), hydrogen-bonding basicity (χ_B), cation-exchange capacity (χ_C) to selectivity. We found that “effective selectivity” of a stationary phase is mathematically given by the ratio of system dependent interaction coefficients but not their absolute values. Thus by normalizing the S*, A, B and C terms of the S–D model by H, we were able to obtain four parameters which fully define the chromatographic selectivity of the stationary phases. By examining the parameters in groups of three, we can represent all the result in a set of four “selectivity triangles”. The distinctive feature of this approach compared to the S–D phase classification scheme is that it allows the visualization of column selectivity by plotting three-dimensional data in a two-dimensional space. Moreover, it very clearly shows that the RPLC columns thus far characterized cover only a small fraction of separation selectivity space leaving a great deal of room for researchers to develop novel RPC materials. Various applications of these “selectivity triangles” will be discussed in this paper.     

Modeling of the three-phase equilibrium in systems of the type water + nonionic surfactant + alkane

Liquid–liquid equilibria of systems water (A) + C_iE_j surfactant (B) + n-alkane (C) have been modeled by a mass-action law model previously developed and so far successfully applied to a series of binary water + C_iE_j systems and to the ternary system water + C₄E₁ + n-dodecane. These calculations provide the basis for the presented modeling. The aqueous systems give information about the association constants and the χ_AB-parameter of the Flory–Huggins theory and the ternary C₄E₁-system provides universal temperature functions for the χ_AC- and the χ_BC-parameter. The three-phase equilibrium for seven ternary C_iE_j systems (i = 6–12, j = 3–6) has been calculated by fitting one additional parameter for each of both temperature functions to the characteristic “fish-tail” point. The agreement with the experimental data is reasonably well. For systems with very small three-phase areas the results can considerably be improved by individual temperature functions that incorporate the experimental temperature maximum of the “fish” into the parameter fit. Based on the parameters of the system water + C₈E₄ + n-C₈H₁₈ the “fish-shaped” phase diagram of the system water + C₈E₄ + n-C₁₄H₃₀ was predicted reasonably well.     

Use of potentiometric detection in (ultra) high performance liquid chromatography and modelling with adsorption/desorption binding kinetics

Observation of a potentiometric sensor's response behaviour after injection in flow injection analysis at different concentrations allowed studying “on” and “off” kinetics of the analyte's adsorption/diffusion behaviour. The alkaloid metergoline was mostly used as an example. k_on and k_off rate constant values were measured, and the association constant K_ass, and ΔG values of the analyte–surface interaction were calculated with an adsorption-based model which proved to be fully applicable. k_on increased by decreasing the sensor dimensions, while k_off was unaffected by miniaturization. Increasing acetonitrile concentrations in the running buffer increased k_off, while k_on was unaffected. The experimentally determined ΔG values of the analyte–surface interaction showed a linear relation to the response of the sensor, in mV. This knowledge was applied to optimize the potentiometric detection of plant alkaloids in (U)HPLC. Sub-micromolar detection limits were obtained with the potentiometric detector/(U)HPLC combination. This is the first time that the response rates and the response itself can be modelled accurately for coated wire potentiometric sensors, and it is the first application of a potentiometric detector in UPLC.     

Vagaries of artificial bilayers and gating modes of the SCl channel from the sarcoplasmic reticulum of skeletal muscle

The elucidation of the structure-function relation of reconstituted ion channel proteins into artificial planar bilayers is expected to be greatly influenced by the physical properties of these bilayers. The dependence of the gating mode of a voltage- and calcium-dependent small chloride channel (SCl channel) on the bilayer properties, as deduced from the specific bilayer capacitance, C_b, was investigated. In bilayers having a C_b value of 0.42 μF/cm² the SCl channel was in the “burst mode”. At a C_b value of 0.28 μF/cm² the “burst mode” gave way to a “flickering mode”. At C_b values between 0.24 and 0.28 μF/cm² channel bursts were often infrequent and brief. No ion channel activities were observed at C_b values lower than 0.24 μF/cm². These results suggest that changes in bilayer thickness, mediated via n-decane, contribute to changes in channel gating mode, i.e. “gear shift”. Standardizing C_b and its time-independence are important in comparative and detailed investigations of ion channel characteristics.     

Solubility parameter used to predict the effectiveness of monolithic in-needle extraction (MINE) device for the direct analysis of liquid samples

The sorbent/eluent systems combined from three macroporous poly(styrene–divinylbenzene) (PS-DVB) monoliths and four solvents as eluents were used for the extraction of phenol, 4-chlorophenol and p-benzochinon from water samples. Monolithic in-needle extraction (MINE) devices were used in the preparation of a series of test water samples for chromatographic analysis. The extraction of phenolic compounds from water samples was carried out by pumping liquid samples through the MINE device. Solubility parameter concept was applied for estimation of effectiveness of MINE. Solubility parameters for (PS-DVB) monoliths were determined according to Small, considering different molar fraction of the monomers used for synthesis. Effectiveness of these systems was estimated according to difference of solubility parameter value in analyte/sorbent, sorbent/eluent, analyte/eluent pairs. The procedure enabling easy prediction of, e.g. the strength of the interactions between the analyte and sorbent, eluent efficiency or the extraction efficiency in MINE system was proposed.     

Mechanochemical synthesis and physical–chemical properties of carbon–fluorocarbon nanocomposition materials. A review

There are the described novel class of porous carbon–fluorocarbon nanocomposition materials “C–CF_1+x”, prepared via the mechanochemical activation (MA) in the heterogeneous mixed systems “nano-C–nano-CF_1+x”, having an atomic ratio C:F as 1.14–4.0. As nano-C it was used thermally expanded graphite TEG and mesoporous carbon material NUMS (free porosity 45–95% and specific area 25–400 m²/g). As nano-CF_1+x these were used a superstoichiometric fluorocarbons FS and FT (CF_1.18–1.25) having the coherent diffraction area (CDA) ∼20–25 Å, free porosity 60–70%, sum O + H₂O in mixtures ∼0.1–0.5 wt.%, and metals sum <0.01 wt.%.Prepared nanocomposites “C–CF_1+x” were studied by FTIR, Raman, XPS C1s, O1s, F1s, X-ray diffraction and by chemical C, H, F-analyses. It was shown, that decrease of weight in systems “C–CF_1+x” does not exceed 0.5 wt.% and stated two main features of the temporary dynamics in changes for all MA-products. These are monotonous changes in bulk properties, such as decrease of C-nanophase relative amounts, confirmed with XRD. Simultaneously, a decrease of sp³-C–F and sp³-CF₂-groups at 1200 and 1320 cm⁻¹ and an origination of sp³-C–F-groups at 1080–1120 cm⁻¹, typical for C₂F-like structures are observed. Decrease of specific surface is corresponding to decrease in CDA sizes and dencity for all MA-products.O-containing admixtures in starting materials have a key influence to interactions in nano-“C–CF_1+x” systems during MA-processing, despite to their low content. The main O-contained participant is H₂O and it is interaction with sp³-C–F–bonds is leading to primary hydrolytic substitution of F onto OH with the origin of surface sp³-C–OH-bonds and their subsequent transformations into edged sp²>CO or/and into ester bridges C–O–C among basal and edged nano-C and nano-fluorocarbon blocks. The presence of basal sp³-C–OH and edged sp²>CO (sp²>COOH)-groups is confirmed by FTIR and XPS C1s and O1s spectra for all MA-nanocomposites in “C–CF_1+x” systems. Changes in the surface properties of prepared MA-“C–CF_1+x” nanocomposites are corresponded to the origin of extrema in properties for MA-time 6–10 min, such as content of surface C-nanophases and surface F with the simultaneous appearance of extrema in the specific electro conductivities and capacitances. The nature of observed phenomena is explained by the origin of chemical carbon nanosized contacts on particle surface and it is possible to use for practical applications. The main difference among MA-“C–CF_1+x” nanocomposites is that the “FS–TEG” systems are dominated by sp²-C–sp³-C–F electroconductive bridges, whereas in “NUMS–FT” systems the character of conductivity is determined by contribution of ester bridges C–O–C.     

Rapid magnetic-mediated solid-phase extraction and pre-concentration of selected endocrine disrupting chemicals in natural waters by poly(divinylbenzene-co-methacrylic acid) coated Fe3O4 core-shell magnetite microspheres for their liquid chromatography–tandem mass spectrometry determination

A new Fe₃O₄/poly(divinylbenzene-co-methacrylic acid) core-shell magnetite microspheric material have been successfully developed as magnetic-mediated solid-phase extraction micro-particle sorbent in dispersion mode (MM-SPE-MP) for the determination of selected estrogenic endocrine disrupting chemicals (EDCs), namely: estrone (E1), 17β-estradiol (E2), estriol (E3), 17α-ethynylestradiol (EE2) and bisphenol-A (BPA), in natural water, via quantification by HPLC tandem mass spectrometry. The magnetite Fe₃O₄ core of this MM-SPE-MP sorbent was fabricated by a solvothermal approach and the thin layer of amphipolar poly(divinylbenzene-co-methacrylic acid) (pDVB-MAA) coating was established via suspension polymerization. The resultant core-shell MM-SPE-MP sorbent material was characterized by electron microscopy, X-ray diffraction and Fourier-transformed infrared spectroscopy. Particle size distribution of the core-shell microspheres was within the range 300–700 nm in diameter and the thickness of the pDVB-MAA coating was ca. 10 nm. This magnetite microspheric material can be easily dispersed in aqueous samples and retrieved by the application of external magnetic field via a small piece of permanent magnet. The MM-SPE-MP process for the selected estrogenic EDCs involved the dispersion of the core-shell microspheric sorbent in water samples with sonication, followed by magnetic aided retrieval of the sorbent and solvent (methanol) desorption of extracted EDCs for LC–MS/MS analysis. Partition equilibrium for all the selected EDCs onto this MM-SPE-MP sorbent was achieved within 15 min. Recoveries of the EDCs were in ranges of 56–111%. Analytes with smaller K_OW value showed relatively lower recovery (and relatively longer equilibration time for partitioning). Method detection limits achieved were found to be 1–36 pg ml⁻¹ (n = 3), while the repeatability was 6–34% (p < 0.05, n = 3). This work demonstrates the usefulness of MM-SPE-MP in the rapid and highly sensitive monitoring of trace organic contaminants in natural waters.     

Optimising resolution for a preparative separation of Chinese herbal medicine using a surrogate model sample system

This paper builds on previous modelling research with short single layer columns to develop rapid methods for optimising high-performance counter-current chromatography at constant stationary phase retention. Benzyl alcohol and p-cresol are used as model compounds to rapidly optimise first flow and then rotational speed operating conditions at a preparative scale with long columns for a given phase system using a Dynamic Extractions Midi-DE centrifuge. The transfer to a high value extract such as the crude ethanol extract of Chinese herbal medicine Millettia pachycarpa Benth. is then demonstrated and validated using the same phase system. The results show that constant stationary phase modelling of flow and speed with long multilayer columns works well as a cheap, quick and effective method of optimising operating conditions for the chosen phase system—hexane–ethyl acetate–methanol–water (1:0.8:1:0.6, v/v). Optimum conditions for resolution were a flow of 20 ml/min and speed of 1200 rpm, but for throughput were 80 ml/min at the same speed. The results show that 80 ml/min gave the best throughputs for tephrosin (518 mg/h), pyranoisoflavone (47.2 mg/h) and dehydrodeguelin (10.4 mg/h), whereas for deguelin (100.5 mg/h), the best flow rate was 40 ml/min.     

A 4-hydroxy-N′-[(E)-(2-hydroxyphenyl)methylidene]benzohydrazide-based sorbent material for the extraction-HPLC determination of biogenic amines in food samples

A sorbent material based on a newly synthesized hydrazone ligand, 4-hydroxy-N′-[(E)-(2-hydroxyphenyl)methylidene]benzohydrazide was prepared by immobilizing the ligand into a silica sol-gel matrix. The capability of the sorbent material for the extraction of seven biogenic amines (BAs), i.e., tryptamine (TRY), β-phenylethylamine (PEA), putrescine (PUT), cadaverine (CAD), histamine (HIS), tyramine (TYR), and spermidine (SPD) was studied. Under the adopted conditions, the sorbent showed good selectivity towards PUT, CAD, HIS and SPD (% extraction (%E) > 96) while %E for TYR, TRY and PEA were 82.0, 78.9 and 46.4%, respectively. The sorbent could be used up to six extraction cycles for SPD, CAD and PUT and was applied to the determination of food samples (“budu”, ketchup, orange juice, soy sauce) that were spiked with 20 mg L⁻¹ of the BAs. The extracted analytes were derivatized with dansyl chloride before the HPLC determination. With the exception of HIS and TYR in “budu” sample, reasonable recoveries were found for the other analytes in all the tested food samples.     

Fluorescent recognition of deoxyribonucleic acids by a quantum dot/meso-tetrakis(N-methylpyridinium-4-yl)porphyrin complex based on a photo induced electron-transfer mechanism

We have developed a new fluorescent probe of thioglycolic acid (TGA)-capped CdTe quantum dots (QDs) complexed with a model drug, meso-tetrakis(N-methylpyridinium-4-yl)porphyrin (TMPyP) for detecting deoxyribonucleic acids (DNAs). This probe operates with an “Off–On” mode: TMPyP quenches the photoluminescence (PL) of QDs via a photo induced electron-transfer (PIET) process; the presence of DNA can break the QD/TMPyP complexation, interrupting the PIET process, and switch on the PL of QDs. Sensitive detection of DNA with the detection limit of 0.16 nM and a linear detection range of 0.25–6.0 nM are achieved. Importantly, this probe can be used to distinguish the binding modes of DNA–TMPyP interactions, exhibiting the DNA sequence-dependent PL recovery behaviors. The obtained binding constant for poly(dA)·poly(dT) is ∼3.30 × 10⁷ L mol⁻¹, which is approximately one order of magnitude larger than those for native DNAs and poly(dG)·poly(dC). Furthermore, the thymine bases preferential of the TMPyP–DNA interaction is proved by this probe.     

Polarized IR spectra of the hydrogen bond in acetic acid crystals

The paper presents the results of our investigations of the polarized IR spectra of the hydrogen bond in crystals of acetic acid, CH₃COOH, as well as in crystals of three deuterium isotopomers of the compound: CH₃COOD, CD₃COOH and CD₃COOD. The spectra were measured at 283 K and at 77 K by a transmission method using polarized light. Theoretical analysis of the results concerned the linear dichroic effects, together with the H/D isotopic and temperature effects observed in the solid-state IR spectra of the hydrogen and of the deuterium bond at the frequency ranges of the ν_O–H and the ν_O–D bands, respectively. Basic spectral properties of the crystals can be interpreted satisfactorily in terms of one of the quantitative theories of the IR spectra of the hydrogen bond, i.e. the “strong-coupling” theory or the “relaxation” theory when a hydrogen bond dimer model is used. From the spectra obtained it resulted that the strongest exciton coupling involved the closely spaced hydrogen bonds, belonging to different chains of associated acetic acid molecules. These results contradict the former explanation of the spectra within a model, which assumed a strong vibrational exciton coupling between four hydrogen bonds in a unit cell. On analyzing the spectra of isotopically diluted crystalline samples of acetic acid it has been proved that a non-random distribution of the protons and deuterons takes place in the hydrogen bond lattices. This non-conventional isotopic effect is a result of dynamical co-operative interactions involving hydrogen bonds in the system. Simultaneously it has been also found that in an individual hydrogen bonded chain in the crystals, distribution of the hydrogen isotope atoms H and D was fully random. The H/D isotopic “self-organization” mechanism most probably involves a pair of hydrogen bonds from each unit cell where each hydrogen bond belongs to a different chain.     

Hydrophobic polymer monoliths as novel phase separators: Application in continuous liquid–liquid extraction systems

Hydrophobic macroporous polymer monoliths are shown to be interesting materials for the construction of “selective solvent gates”. With the appropriate surface chemistry and porous properties the monoliths can be made permeable only for apolar organic solvents and not for water. Different poly(butyl methacrylate-co-ethylene dimethacrylate) (BMA-EDMA) and poly(styrene-co-divinylbenzene) (PS-DVB) monoliths prepared with tailored chemistries and porosities were evaluated for this purpose. After extensive characterization, the PS-DVB monoliths were selected due to their higher hydrophobicity and their more suitable flow characteristics. BMA-EDMA monoliths are preferred for mid-polarity solvents such as ethyl acetate, for which they provide efficient separation from water. Breakthrough experiments confirmed that the pressures necessary to generate flow of organic solvents through PS-DVB monoliths were substantially lower than for water. A phase separator was constructed using the monoliths as the flow selector. This device was successfully coupled on-line with a chip-based continuous liquid–liquid-extraction (LLE) system with segmented flow. Efficient separation of different solvents was obtained across a wide range of flow rates (0.5–4.0 mL min⁻¹) and aqueous-to-organic flow ratios (β = 1–10). Good robustness and long life-time were also confirmed. The suitability of the device to perform simple, cheap, and reliable phase separation in a continuous LLE system prior to gas-chromatographic analysis was proven for some selected real-life applications.     

Low-Temperature Phase Formation of Sn-Doped LaMnO3+δ Perovskite

The incorporation of Sn into LaMnO₃ perovskite and its influence on magnetotransport properties were studied in samples synthesized at low temperature. Single-phase materials for two series of samples with La/(Sn+Mn)=1 and La/(Sn+Mn)<1 ratios were obtained by substitution of up to 10% of the Mn ions by Sn⁴⁺. The effect of Sn substitution was monitored through measurements of thermal, “M(T)”, and magnetic field, “M(H)”, dependences of magnetization, as well as of resistivity, “ρ(T)”, at 0 and 70 kOe. These showed that this effect depends strongly on the perovskite cation site ratio (A/B). For La/(Sn+Mn)=1, M and T_C were depressed as Sn content was increased. The magnetization data suggest the presence of magnetic clusters with superparamagnetic behavior. No evidence of magnetoresistance (MR) was found. For La/(Sn+Mn)<1 ratio, the samples showed ferromagnetic behavior and MR and both M and T_C raised as Sn content increased. The results are discussed in terms of A site vacancies.     

“Small-particle limit” in the second harmonic generation from noble metal nanoparticles

In this paper, we have probed the origin of SHG in copper nanoparticles by polarization-resolved hyper-Rayleigh scattering (HRS). Results obtained with various sizes of copper nanoparticles at four different wavelengths covering the wavelength range 738–1907 nm reveal that the origin of second harmonic generation (SHG) in these particles is purely dipolar in nature as long as the size (d) of the particles remains smaller compared to the wavelength (λ) of light (“small-particle limit”). However, contribution of the higher order multipoles coupled with retardation effect becomes apparent with an increase in the d/λ ratio. We have identified the “small-particle limit” in the second harmonic generation from noble metal nanoparticles by evaluating the critical d/λ ratio at which the retardation effect sets in the noble metal nanoparticles. We have found that the second-order nonlinear optical property of copper nanoparticles closely resembles that of gold, but not that of silver.     

FTIR study on the nature of water sorbed in polypropylene (PP)/ethylene alcohol vinyl (EVOH) films

FTIR transmission spectroscopy, along with gravimetric analysis and scanning electron microscopy (SEM), have been used to investigate the structure of water sorbed in PP/EVOH films. Aims of this investigation were to determine water content spectroscopically, to elucidate the different types of interaction that water molecules form with the macromolecular network and their change as a function of blend composition, morphology and the state (gaseous or liquid) of the water contacting with the PP/EVOH system at equilibrium.The extinction coefficients (ε) of the water sensitive bands (ν_OH, δ_OH, 2150 and 700 cm⁻¹) are proportional to the EVOH content. These results are explained by the phase separation between PP and EVOH and the increasing size of the dispersed EVOH spheres with the EVOH content. Moreover, the ε values suggest that when the films are in contact with water vapor, most of the water sorbed locates around the hydrogen bonding sites of the polymer although the effective hydration region around the EVOH residues should be much larger than the primary hydration region. Besides, “bulk water”, generally categorized as “free water”, is lacking in the polymer blends under our experimental conditions. When they contact with liquid water both the water uptake and the ε values are changed, but these variations are not large enough to assume the formation of “bulk water”. Band decomposition of the O-H stretching vibration (ν_OH) on the basis of the four-state model supports the assumption of the absence of “water-rich domains”, regardless of the blend composition or whether the PP/EVOH film contacts with liquid water or water vapor. Besides, either in contact with water vapor or liquid water, the more hydrophobic mixtures (90/10) show the greatest effect of perturbation with respect to pure water for the “weaker hydrogen bonded” water. Conversely, the interactions between the “strongly bound” water and OH groups in the EVOH chains become stronger with increasing hydrophilicity of the blends and this type of water has a greater plasticising efficiency. Furthermore, these structural changes are greater for liquid water than for water vapor and this trend is equally observed for mixtures of different hydrophilicity.     

Electrochemistry of fluorine production

The electronic properties of carbon-fluorine films (denoted C-F) formed on carbon electrodes in KF-2HF during fluorine evolution reaction were investigated in aqueous solution containing a redox couple and in mercury. It was shown that the passivating C-F films behave as electronic conductors. STM measurements have shown composition heterogeneities at the surface of fluorinated HOPG (conducting ans insulating areas). The influence of the amount of insulating graphite fluorides on the surface of the electrodes was demonstrated. Thus, the high anodic overvoltage observed during fluorine evolution on C/C-F anodes in KF-2HF is mainly attributed to the poor wettability of the electrodes by the melt, which results in a small electroactive area. A new model was proposed for representing the electrode/electrolyte interface; it includes the presence of a “fluidized” layer between the surface C-F film and the fluorine gas film. The “fluidized” layer is composed of liquid KF-2HF melt and dissolved fluorine gas. The influence of the mass transfer phenomenon occurring in that layer was pointed out mainly by impedance measurements. Finally, the contributions of the C-F film, η_C-F, and of the “fluidized” layer, η_fluid, to the total anodic overvoltage, η_T, were studied using a numerical calculation method. Both contributions must be taken into account for a global understanding of the fluorine evolution process.     

Exploiting in situ hydride trapping in tungsten coil atomizer for Se and As determination in biological and water samples

A flow injection hydride manifold was coupled to a 150 W tungsten coil electrothermal atomizer for in situ hydride collection followed by selenium and arsenic determination by ET AAS. Rhodium (200 μg), thermally reduced over the double layer tungsten atomizer, was very efficient at collecting selenium or arsenic hydrides. Prior to analysis, biological samples were digested in closed-vessels microwave digestion system. Prior to the hydride formation, both selenium and arsenic were reduced to valence state (IV) and (III), respectively. The detection limit was 35 ng L⁻¹ for selenium and 110 ng L⁻¹ for arsenic. Sample throughput was 70 h⁻¹ using 30 s of hydride trapping time. Method accuracy was evaluated by analyzing biological-certified reference materials from the National Institute of Standard and Technology (SRM-1577a and SRM-1577b “bovine liver” and RM-8414 “bovine muscle powder”) and from the International Agency for Energy Atomic (A-13 “animal blood”) and one water-certified reference material from the National Institute of Standard and Technology (SRM-1640 trace elements in natural water). By applying a t-test, there was no significant difference at the 95% probability level between the results obtained with the proposed method and those certified values.     

Asymmetric flow field flow fractionation of aqueous C60 nanoparticles with size determination by dynamic light scattering and quantification by liquid chromatography atmospheric pressure photo-ionization mass spectrometry

A size separation method was developed for aqueous C₆₀ fullerene aggregates (aqu/C₆₀) using asymmetric flow field flow fractionation (AF4) coupled to a dynamic light scattering detector in flow through mode. Surfactants, which are commonly used in AF4, were avoided as they may alter suspension characteristics. Aqu/C₆₀ aggregates generated by sonication in deionized water ranged in size from 80 to 260 nm in hydrodynamic diameter (D_h) as determined by DLS in flow through mode, which was corroborated by analysis of fractions by DLS in batch mode and by TEM. The mass of C₆₀ in each fraction was determined by LC–APPI–MS. Only 5.2 ± 6.7% of the total aqu/C₆₀ mass had D_h less than 80 nm, while 58 ± 32% of the total aqu/C₆₀ mass had D_h between 80 and 150 nm and 14 ± 9.2% of the total aqu/C₆₀ were between 150 and 260 nm in D_h. With the optimal fractionation parameters, 77 ± 5.8% of the aqu/C₆₀ mass eluted from the AF4 channel, indicating deposition on the AF4 membrane had occurred during fractionation; use of alternative membranes did not reduce deposition. Channel flow splitting increased detector response although channel split ratios greater than 80% of the channel flow led to decreased detector response. This is the first report on the use of AF4 for fractionating a colloidal suspension of aqu/C₆₀.     

Classical and recent free-volume models for polymer solutions: A comparative evaluation

In this work, two “classical” (UNIFAC-FV, Entropic-FV) and two “recent” free-volume (FV) models (Kannan-FV, Freed-FV) are comparatively evaluated for polymer–solvent vapor–liquid equilibria including both aqueous and non-aqueous solutions. Moreover, some further developments are presented here to improve the performance of a recent model, the so-called Freed-FV. First, we propose a modification of the Freed-FV model accounting for the anomalous free-volume behavior of aqueous systems (unlike the other solvents, water has a lower free-volume percentage than polymers). The results predicted by the modified Freed-FV model for athermal and non-athermal polymer systems are compared to other “recent” and “classical” FV models, indicating an improvement for the modified Freed-FV model for aqueous polymer solutions. Second, for the original Freed-FV model, new UNIFAC group energy parameters are regressed for aqueous and alcohol solutions, based on the physical values of the van der Waals volume and surface areas for both FV-combinatorial and residual contributions. The prediction results of both “recent” and “classical” FV models using the new regressed energy parameters are significantly better, compared to using the classical UNIFAC parameters, for VLE of aqueous and alcohol polymer systems.