Porous structure of rigid and swelling adsorbents according to pulsed NMR data on the mobility of adsorbed water and benzene molecules

Potentialities of pulsed NMR technique to estimate parameters of the porous structure of microporous adsorbents according to the data on nuclear magnetic relaxation and self-diffusion of adsorbed molecules are analyzed. Main attention was focused on the adsorbed systems studied by the adsorption and NMR techniques. Measurements of nuclear magnetic relaxation of adsorbed molecules make it possible to analyze the porous structure at the scale of pore sizes; data on self-diffusion, at the scale of different-porosity zones. Specific features of the translational mobility of water and benzene molecules with different mechanisms of adsorption in active carbons having relatively rigid structure and polymer adsorbents capable of swelling are discussed. In general, data obtained by the NMR technique agree with and supplement the results of adsorption measurements.     

Specific Features of the Adsorption and Nuclear Magnetic Relaxation of the Water Molecules in Active Carbons: 2. The State of Water in Active Carbon with Relatively Large Pores According to the NMR Relaxation Data

The state of water upon adsorption on FAS-3 active carbon with relatively large micropores is studied by the NMR relaxation method. The dependences of the times of spin–lattice (T ₁) and spin–spin (T ₂) NMR relaxation of adsorbed water molecules on the adsorption value are established. The character of the dependences of T ₁ and T ₂ on the number of adsorbed water molecules per primary adsorption site reflects the specific features of the volume filling of micropores and the formation of a continuous adsorption layer on the mesopore surface due to cluster coalescence on the one wall of a pore. The results obtained are compared with the data for typical microporous active carbons, as well as with the data obtained by the adsorption method.     

Complex-Emulsion synthesis of organo-inorganic amphiphilic sorbents with specific affinity for glucose

Granular organo-inorganic amphiphilic sorbents with specific affinity for glucose were synthesized with the aim of developing selective hemosorbents for efferent therapy of hyperglycemia. The sorbents were prepared in oil/water/oil complex emulsions by copolymerization of 2-hydroxyethyl methacrylate and ethylene glycol dimethacrylate on the surface of selenium nanoparticles stabilized with polyvinylpyrrolidone. The phases were stabilized with excess selenium/polyvinylpyrrolidone nanocomplexes. Optimum synthesis conditions ensuring formation of hybrid sorbents with the physicochemical properties required for performing efficient preparative hemosorption and plasma adsorption processes were found. The specific affinity for glucose was reached by modification of these sorbents via imprinting with template glucose molecules in the surface polymer layer and via introduction of boric acid as an affine ligand into the hybrid matrices.     

Specific Features of the Adsorption and Nuclear Magnetic Relaxation of Water Molecules in Active Carbons. 1. Relation between the Spin-Spin Relaxation of Adsorbed Water Molecules and Structural Parameters of Microporous Active Carbons

Relation between the spin-spin nuclear magnetic relaxation time T ₂ of adsorbed water molecules and parameters of microporous structure of carbon adsorbents is disclosed. The pattern of dependences of T ₂ on the relative pressure and the number of water molecules per one primary adsorption site (PAS) is governed by the pore sizes and the number and nature of PASs. At a complete micropore filling, the T ₂ value depends on the volume density of PASs in active carbons. In the absence of PASs in the micropores, T ₂ is equal approximately to 21 ms. The larger the volume density of PASs, the smaller the number of water molecules per one PAS at the complete filling of micropores; i.e., the looser the packing of water molecules. The results of studying active carbons by the pulsed ¹H NMR method agree well with the data of the adsorption method.     

Adsorption and mobility of molecules of water and organic compounds in carbon adsorbents

The mobility of molecules of water and fluorine-containing organic compounds adsorbed in a mixture on active carbons has been studied by the NMR pulsed field gradient technique (¹H and¹⁹F resonances). Samples with various H₂O/C₆F₆ and H₂O/C₂Cl₃F₃ ratios have been examined. The mobility of components at total fill factors > 0.8 has been shown to decrease in comparison with the adsorption of pure substances while the diffusion activation energy increases. The results are interpreted on the basis of adsorption mechanisms of water and organic compounds on active carbons.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 56–61, January, 1993.In conclusion the authors would like to express their thanks to the German Research Society for the financial support of this work.     

Surface modification of poly(oligoethylene oxide methacrylate) for resisting protein adsorption

We prepare poly(2-methoxyethyl-, 2-(2-methoxyethoxy)ethyl-, 2-[2-(2-methoxyethoxy)ethoxy]ethyl methacrylate) (p(nEOMA), n=1, 2, and 3) brushed surfaces with varying the polymer density by surface initiated polymerization. The amount of bovine serum albumin (BSA) adsorbed on the surfaces is investigated. The mobility of the polymer chain in the polymer/water interfaces and the structure of adsorbed water on the surfaces are characterized by Electron Spin Resonance (ESR) and transmission-Fourier transform infrared (FT-IR) spectroscopy, respectively. This work reports the relationship between these surface properties and albumin adsorption. As a result, the surface having both a high molecular mobility and bulk-like water found to be very effective in preventing albumin adsorption.     

Interaction of Water and Methanol Molecules with the Surface of Carbonaceous Materials According to Gas Chromatographic Data

We have used gas chromatography to study the interaction of water and methanol molecules with active hydrophilic centers on the surface of thermally expanded graphite and graphitized thermal carbon black. We have determined the concentration of carboxyl and phenol hydroxyl groups on the surface of the sorbents and the heats of adsorption for adsorption of the studied substances on these groups. We have shown that water molecules are adsorbed as clusters on the hydrophilic centers of the studied sorbents at very low relative pressures, with n = 2 molecules in the cluster.     

Mobility and orientation of semi-rigid polyesters

Mobility and orientational behavior of semi-flexible polyesters from diphenyl-dicarbonic acid and some branched propyl-and butyl spacers have been investigated using static NMR techniques. Increasing molar mass of the polymers was found to influence the mobility of the rigid parts of the polymer chain while the spacer remains almost unaffected by the degree of polymerization. The mobility of the spacer carbons depends, however, on the length of the side chains. Even a magnetic field of 7 Tesla was not able to cause a reasonable macroscopic orientation in most of the samples, probably hindered by the high viscosity of the melts.     

Direct probing of sorbent-solvent interactions for amylose tris(3,5-dimethylphenylcarbamate) using infrared spectroscopy, X-ray diffraction, solid-state NMR, and DFT modeling

The sorbent-solvent interactions for amylose tris(3, 5-dimethylphenylcarbamate) (ADMPC) with five commonly used solvents, hexane, methanol, ethanol, 2-propanol (IPA), and acetonitrile (ACN), are studied using attenuated total reflection infrared spectroscopy (ATR-IR) of thin sorbent films, X-ray diffraction (XRD) of thin films, (13)C cross polarization/magic angle spinning (CP/MAS) and MAS solid state NMR of polymer-coated silica beads (commercially termed "Chiralpak AD"), and DFT modeling. The ADMPC-polymer-coated silica beads are used commercially for analytical and preparative scale separations of chiral enantiomers. The polymer forms helical rods with intra- and inter-rod hydrogen bonds (H-bonds). There are various nm-sized cavities formed between the polymer side-chains and rods. The changes in the H-bonding states of the C=O and NH groups of the polymer upon absorption of each of the five solvents at 25 degrees C are determined with ATR-IR. The IR wavenumbers, the H-bonding interaction energies, and the H-bonding distances of the polymer side-chains with each of the solvent molecules are predicted using the DFT/B3LYP/6-311+g(d,p) level of theory. The changes in the polymer crystallinity upon absorption of each solvent are characterized with XRD. The changes in the polymer crystallinity and the H-bonding states of C=O groups are also probed with (13)C CP/MAS solid-state NMR. The changes in the polymer side-chain mobility are detected using (13)C MAS solid-state NMR. The H-bonding states of the polymer change upon absorption of each polar solvent and usually result in an increase in the polymer crystallinity and the side-chain mobility. The polymer rods are reorganized upon solvent absorption, and the distance between the rods increases with the increase in the solvent molecular size. These results have implications for understanding the role of the solvent in modifying the structure and behavior of the polymer sorbents.     

Cluster-Mediated Water Adsorption on Carbon Nanopores

The adsorption isotherms of water at 303 K and N2 at 77 K on various kinds of porous carbons were compared with each other. The saturated amounts of water adsorbed on carbons almost coincided with amounts of N2 adsorption in micropores. Although carbon aerogel samples have mesopores of the great pore volume, the saturated amount of adsorbed water was close to the micropore volume which is much small than the mesopore volume. These adsorption data on carbon aerogels indicated that the water molecules are not adsorbed in mesopores, but in micropores only. The adsorption isotherms of water on activated carbon having micropores of smaller than 0.7 nm in width had no clear adsorption hysteresis, while the water adsorption isotherms on micropores of greater than 0.7 nm had a remarkable adsorption hysteresis above P/P0 = 0.5. The disappearance of the clear hysteresis for smaller micropores suggested that the cluster of water molecules of about 0.7 nm in size gives rise to the water adsorption on the hydrophobic micropores; the formation and the structure of clusters of water molecules were associated with the adsorption mechanism. The cluster-mediated pore filling mechanism was proposed with a special relevance to the evidence on the formation of the ordered water molecular assembly in the carbon micropores by in situ X-ray diffraction.     

Determination of standard isotherms of the sorption of some vapors with cellulose

Standard isotherms of the sorption of water, methanol, and benzene vapors on cellulose using a cellulose standard are determined. The standard, namely, mesoporous cellulose with specific surface of up to 350 m2/g, is obtained by the method of exchanging water in swollen cellulose with organic solvents. A comparison of the experimental sorption isotherm with the standard isotherm makes it possible to determine the specific surface of celluloses accessible a the given sorbate and, in combination with the Brunauer-Emmett-Teller adsorption equation, to characterize their surface properties. The identity of the sorption properties of the initial and dewatered (porous) celluloses relative to active vapors is shown, which evidences the assumed mechanism of swelling as the sorbent's division into morphological structures with the formation of new surface. A comparative analysis of the sorption properties of cellulose and silica, whose nature of active sorption centers is similar (weak acid hydroxyl groups), has been made. The affinity of the standard isotherms and close values of the cross-sectional area of different sorbates on both sorbents testify the similarity in their sorption behavior. Thus, the processes of sorption with rigid and swelling sorbents can be regarded in a unified context. Therefore, the adsorption models developed for rigid sorbents can be applied to cellulose sorbents to analyze their sorption properties.     

Features of the sorption of water vapor and nitrogen on cellulose

Molecular-kinetic parameters of adsorptives, i.e., water (at 300 K) and nitrogen (at 77 K) vapors, are calculated and compared at the initial steps of their adsorption by cellulose. The role of the dipole structure of water molecules is considered upon their interaction with active centers of cellulose, forming heterogeneous electric fields in its pores. The effect of the temperature of the adsorptive and the sizes of its molecules on activation penetration through narrowings of the micropores dominant in absolutely dry cellulose due to the mobility of its structure is determined. The development of a porous system upon water adsorption is demonstrated according to ¹H NMR. It is concluded that low-temperature nitrogen adsorption on cellulose yields rather limited information on its structure and adsorption properties.     

Preparation and investigation of active carbons based on furfural copolymer

Active carbons were synthesized from a furfural copolymer using a polyester resin as an activator. The influence of the temperature of the thermal treatment of the carbon sorbents on the weights and ash contents of the obtained materials was established. The molecular structures of the carbons were studied by NMR spectroscopy.     

Features of nuclear magnetic relaxation of water and benzene molecules during absorption on activated carbons and estimation of pore size distribution in adsorbents

Nuclear magnetic relaxation in activated carbon—water and activated carbon—benzene adsorption systems was studied by pulse NMR methods. Activated carbons characterized by different porous structures and chemical state of the surface were used. The application of the three-pulse Goldman—Shen sequence to the adsorption system generates a dipole echo caused by the dipole-dipole coupling of structural protons, which is not averaged due to their mobility during experiment. The non-exponential character of relaxation attenuations of the transverse and longitudinal nuclear magnetizations of physically adsorbed molecules in activated carbon pores is a result of differencies in pore sizes. The pore sizes in activated carbon and the size distribution were determined from the data of nuclear magnetic relaxation with allowance for the contribution from the structural protons.     

Fourier-transform Carr-Purcell-Meiboom-Gill NMR experiments on polymers in colloidal dispersions: how many polymer molecules per particle?

Fourier transform relaxation NMR has been used to study how the mobility of poly(ethylene oxide) is affected by its adsorption onto colloidal silica particles of various sizes. Novel results have been obtained which illustrate the unexploited potential of this method for the study of interfacial species in complex systems. The results quantify how polymer mobility varies along an adsorption isotherm. When the particles are in excess, the polymer is strongly adsorbed and hence has a large spin-spin magnetic relaxation rate constant, R(2). The value of R(2) in this region increases with particle size, because the associated reduction in particle surface curvature results in a reduction in the mobility of the adsorbed polymer. This is accompanied by a reduction in the signal intensity, as a higher fraction of the polymer is adsorbed in the form of train segments too immobile to detect using the Carr-Purcell-Meiboom-Gill pulse sequence. When the polymer concentration reaches approximately 0.5 mg m(-2), the initial region of high affinity adsorption ends and so the polymer solution concentration increases. This is accompanied by a reduction in R(2), which then approaches the value for a simple polymer solution in the absence of particles. The results are corroborated by comparison with rheological measurements and molecular dynamics simulations of an analogous particle-polymer system.     

Complexation of polycations to anionic liposomes: composition and structure of the interfacial complexes

Poly(N-ethyl-4-vinylpyridinium bromide) (a polycation with a degree of polymerization of 1100) was adsorbed onto liposomes composed of egg lecithin with a 0.05-0.20 molar fraction (nu) of anionic headgroups provided by cardiolipin (a doubly anionic lipid). According to electrophoretic mobility data, this led to total charge neutralization of the liposomes, whereupon the liposomes adopted a positive charge as additional polymer continued to adsorb. Although the liposomes aggregated at the charge-neutralization point, they disassembled into individual liposomes after becoming positively charged. The degree of polymer adsorption was shown to reach a limit. Thus, by measuring the free polymer content in a liposome suspension, it was possible to determine the polymer concentration at which the liposome surface became saturated with polymer. Beyond this point, an electrostatic/steric barrier at the surface suppressed further adsorption. Dynamic light scattering studies of liposomes with and without adsorbed polymer allowed calculation of the polymer film thickness which ranged from 22 to 35 nm as the molar fraction of cardiolipin (nu) increased from 0.05 to 0.20. The greater the content on the anionic lipid in the bilayer, the thicker the polymer film. The maximum number of polymer molecules adsorbed onto the liposomes was estimated: 1-2 molecules for nu = 0.05; 3 molecules for nu = 0.1; 4- molecules for nu = 0.15; and 6 molecules for nu = 0.2. The polymer appears to lie on the liposome surface, rather than embedding into the bilayer, because addition of NaCl easily dislodges the polymer from the liposome into the bulk water.     

Water-compatible molecularly imprinted polymers obtained via high-throughput synthesis and experimental design

A technique allowing high-throughput synthesis and evaluation of molecularly imprinted polymer sorbents at a reduced scale (mini-MIPs) was developed and used for the optimization of MIPs for use in pure aqueous environments. The technique incorporated a 4-port liquid-handling robot for the rapid dispensing of monomers, templates, solvents and initiator into the reaction vessels of a 96-well plate. A library of 80 polymers, each ca. 50 mg, could thus be prepared in 24 h. The MIP rebinding capacity and selectivity could be rapidly assessed in the batch mode by quantifying nonbound fractions in parallel using a UV monochromator plate reader. This allowed a complete evaluation of the binding characteristics of an 80 polymer library in approximately 1 week. With the objective of optimizing a polymer imprinted with the local anaesthetic Bupivacaine for use in pure aqueous systems, a polymer library was prepared by varying the original poly(MAA-co-EDMA) MIP composition. The variable factors were the added amount of the hydrophilic comonomer, 2-hydroxyethyl methacrylate (HEMA), the cross-linking ratio, and the porogen. This optimization resulted in polymers showing high imprinting factors (IF = K(MIP)/K(NIP)) in water as a result, mainly, of reduced binding to the nonimprinted polymer. Normal scale batches of these materials showed strong retention of the template and low nonspecific binding when assessed as chromatographic stationary phases using pure phosphate buffer, pH 7.4, as mobile phase, by equilibrium batch rebinding experiments and as sorbents for extractions of the analyte from blood plasma samples.     

An experimental and molecular simulation study of the adsorption of carbon dioxide and methane in nanoporous carbons in the presence of water

The adsorption of carbon dioxide and methane in nanoporous carbons in the presence of water is studied using experiments and molecular simulations. For all amounts of adsorbed water molecules, the adsorption isotherms for carbon dioxide and methane resemble those obtained for pure fluids. The pore filling mechanism does not seem to be affected by the presence of the water molecules. Moreover, the pressure at which the maximum adsorbed amount of methane or carbon dioxide is reached is nearly insensitive to the loading of preadsorbed water molecules. In contrast, the adsorbed amount of methane or carbon dioxide decreases linearly with the number of guest water molecules. Typical molecular configurations obtained using molecular simulation indicate that the water molecules form isolated clusters within the host porous carbon due to the nonfavorable interaction between carbon dioxide or methane and water.     

NMR study of interphase structure in layered polymer morphologies with mobility contrast: disorder and confinement effects vs. dynamic heterogeneities

Nanostructured multiphase polymers with mobility contrast, such as semicrystalline polymers or block copolymers with two separate glass transitions, are usually characterized by the presence of an interphase material with in-between mobility. This interphase is often assumed to form a contiguous layer between the adjacent main phases, possibly exhibiting a mobility gradient. Here, we present evidence from proton low-field NMR experiments based upon the spin diffusion effect that suggests less trivial possible arrangements. A numerical analysis of the NMR data based upon a 2D lattice model demonstrates that a part of the mobile phase must be in rather direct contact with the rigid phase. Tentatively, we assume an island-like distribution of the interphase, or its location within the rigid phase, with sizes on the scale of a few nanometers. We observe qualitatively the same phenomenon in a semicrystalline polymer, poly(ε-caprolactone), and in a lamellar poly(styrene)-poly(butadiene) block copolymer, suggesting that the phenomenon has some degree of universality. We hypothesize that the non-trivial location of the interphase results from either a higher than one-dimensional constraint imposed by the surrounding rigid phase and disorder effects arising from local roughness or thickness distributions or from the intrinsic dynamic heterogeneity length scale of material close to the glass transition.