Poly(ethylene-co-acrylic acid) stationary phases for the separation of shape-constrained isomers

Separations by Shape: Molecular Shape Recognition in Liquid Chromatography

Molecular shape can provide a basis for chromatographic separations that is distinct from other interaction mechanisms often considered in liquid chromatography. Molecular shape recognition, or shape selectivity, is most evident for the separation of isomeric compounds that have constrained molecular structures, such as polycyclic aromatic hydrocarbons. A central feature of shape-selective columns is conformational order within the stationary phase; this aspect of stationary phase morphology is revealed through spectroscopic studies and molecular dynamics simulations, and is correlated with chromatographic performance.

Tetrapolymer Network Hydrogels via Gum Ghatti-Grafted and N-H/C-H-Activated Allocation of Monomers for Composition-Dependent Superadsorption of Metal Ions

Herein, gum ghatti (GGTI)-g-[sodium acrylate (SA)-co-3-(N-(4-(4-methyl pentanoate))acrylamido)propanoate (NMPAP)-co-4-(acrylamido)-4-methyl pentanoate (AMP)-co-N-isopropylacrylamide (NIPA)] (i.e., GGTI-g-TetraP), a novel interpenetrating tetrapolymer network-based sustainable hydrogel, possessing extraordinary physicochemical properties and excellent recyclability, has been synthesized via grafting of GGTI and in situ strategic protrusion of NMPAP and AMP during the solution polymerization of SA and NIPA, through systematic multistage optimization of ingredients and temperature, for ligand-selective superadsorption of hazardous metal ions (M(II)), such as Sr(II), Hg(II), and Cu(II). The in situ allocation of NMPAP and AMP via N-H and C-H activations, grafting of GGTI into the SA-co-NMPAP-co-AMP-co-NIPA (TetraP) matrix, the effect of comonomer compositions on ligand-selective adsorption, crystallinity, thermal stabilities, surface properties, swellability, adsorption capacities (ACs), mechanical properties, and the superadsorption mechanism have been apprehended via extensive microstructural analyses of unloaded and/or loaded GGTI-g-TetraP1 and GGTI-g-TetraP2 bearing SA/NIPA in 8:1 and 2:1 ratios, respectively, using Fourier transform infrared (FTIR), 1H/13C/DEPT-135 NMR, X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis, differential scanning calorimetry, X-ray diffraction, field emission scanning electron microscopy, rheological analysis, and energy-dispersive X-ray spectrometry, along with measuring % gel content, pH at point of zero charge (pHPZC), and % graft ratio. The thermodynamically spontaneous chemisorption has been inferred from FTIR, XPS, fitting of kinetics data to pseudo-second-order model, and activation energies. The chemisorption data have exhibited excellent fitting to the Langmuir isotherm model. For Sr(II), Hg(II), and Cu(II), ACs were 1940.24/1748.36, 1759.50/1848.03, and 1903.64/1781.63 mg g-1, respectively, at 293 K, 0.02 g of GGTI-g-TetraP1/2, and initial concentration of M(II) = 500-1000 ppm.

¹³C SPE MAS measurement of ligand concentration in compressible chromatographic beads

A method for measuring the ligand concentration in heterogeneous materials like chromatography media is described. In this method, (13)C single pulse excitation magic angle spinning NMR experiment with broadband (1)H decoupling is used to determine the peak integrals for a butyl ligand in the spectrum of a dried chromatography medium. Within a carefully controlled protocol, those integrals compared with that of the internal reference compound dimethyl sulfone provide the required volume concentration with an accuracy of ca 2%. The effects of temperature, degree of hydration, and other experimental parameters are discussed.

Silica nanoparticles functionalized via click chemistry and ATRP for enrichment of Pb(II) ion

Silica nanoparticles have been functionalized by click chemistry and atom transfer radical polymerization (ATRP) simultaneously. First, the silanized silica nanoparticles were modified with bromine end group, and then the azide group was grafted onto the surface via covalent coupling. 3-Bromopropyl propiolate was synthesized, and then the synthesized materials were used to react with azide-modified silica nanoparticles via copper-mediated click chemistry and bromine surface-initiated ATRP. Transmission electron microscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and thermogravimetric analysis were performed to characterize the functionalized silica nanoparticles. We investigated the enrichment efficiency of bare silica and poly(ethylene glycol) methacrylate (PEGMA)-functionalized silica nanoparticles in Pb(II) aqueous solution. The results demonstrated that PEGMA-functionalized silica nanoparticles can enrich Pb(II) more quickly than pristine silica nanoparticles within 1 h.

Molecular shape recognition through self-assembled molecular ordering: evaluation with determining architecture and dynamics

The relationship between molecular gel-forming compound-based double-alkylated L-glutamide-derived functional group-integrated organic phase (Sil-FIP) structure and chromatographic performance is investigated and compared with widely used alkyl phases (C(30), polymeric and monomeric C(18)) as references. The functional group-integrated molecular gel on silica is chemically designed newly in a way that the weak interaction sites are integrated with high orientation and high selectivity can be realized by multiple interactions with the solutes. Its functions can be emphasized by being immobilizable with a terminal carboxyl group and the fact that five amide bonds including β-alanine subunit are integrated per molecule. Furthermore, its self-assembling function can be detected by monitoring of the chiroptical property. Temperature-dependent circular dichroism (CD) intensity was determined as an indicator of chirality for the gel forming compounds. (13)C cross-polarization magic angle spinning (CP/MAS) NMR spectra of the Sil-FIP phase indicate that predominance of gauche conformations exists at higher temperature (above 30 °C). (29)Si CP/MAS NMR were carried out to investigate the degree of cross-linking of the silane and silane functionality of the modified silica. Temperature-dependent (13)C CP/MAS NMR and suspended-state (1)H NMR measurements of the Sil-FIP phase exhibit the dynamic behavior of the alkyl chains. To correlate the NMR and CD results with temperature-dependent chromatographic studies, standard reference materials (SRM 869b and SRM 1647e), column selectivity test mixture for liquid chromatography was employed. Additional shape selectivity text mixtures were also used to clarify the mechanism of shape selectivity performance of Sil-FIP compared with commercially available columns. The evaluation with the spectroscopic and chromatographic analyses presents very important information on the surface morphology of the new organic phase and the molecular recognition process. Integrated and ordered functional groups were investigated to be the main driving force for very high molecular shape selectivity of the Sil-FIP phase.

Molecular-shape selectivity by molecular gel-forming compounds: bioactive and shape-constrained isomers through the integration and orientation of weak interaction sites

A molecular gel system was assembled on carrier particles and the integrated effect of weak interaction sites enabled highly efficient separation of the bioactive and shape-constrained isomers of tocopherols, β-carotene, and polycyclic aromatic hydrocarbons (PAHs) by multiple interaction mechanisms.

Molecular shape selectivity through multiple carbonyl-pi interactions with noncrystalline solid phase for RP-HPLC

A new approach for the synthesis of double-alkylated L-glutamide-derived stationary phases to use in RP-HPLC is described. TEM observation of lipid distearylglutamide (DSG) showed the formation of fibrous aggregates in methanol or in chloroform through intermolecular hydrogen bonding among the amide moieties while dibutylglutamide (DBG) cannot aggregate in aqueous or organic media due to its lower order of short alkyl chain. DSG and DBG were covalently bonded to silica via amino-propyl linkages. Lipid membrane analogues (e.g., DSG) attached to the silica surface have been found in noncrystalline and solid states and can form supramolecular assemblies with specific properties based on their highly ordered structures in aqueous and organic media. 13C CP/MAS NMR and suspension (in methanol)-state 1H NMR, elemental analysis, and DSC measurements were used to characterize Sil-DSG and were compared with the three other octadecyl phases, i.e., monomeric C18, polymeric C18, and silica grafted poly(octadecyl acrylate) Sil-ODA25. The chromatographic behavior of the new RP material was investigated using detailed retention studies of planar and nonplanar polyaromatic hydrocarbons (PAHs) and nonpolar aromatic positional isomers. Aspects of shape selectivity were also evaluated with Standard Reference Materials 869a, Column Selectivity Test Mixture for Liquid Chromatography. Detailed chromatographic study revealed that Sil-DSG showed extremely enhanced molecular shape selectivity compared with the other phases investigated. The higher molecular shape selectivity obtained by Sil-DSG can be explained by a carbonyl pi (present in lipid-grafted stationary phases)-benzene pi (present in guest PAHs) interaction mechanism, and these interactions are more effective for ordered carbonyl groups.

Conformational temperature dependence of a poly(ethylene-co-acrylic acid) stationary phase investigated by nuclear magnetic resonance spectroscopy and liquid chromatography

A polymer-based RP sorbent was prepared by immobilizing a poly(ethylene-co-acrylic acid) copolymer with an acid mass fraction of 5% on silica by using a 3-glycidoxypropyl linkage. 13C cross-polarization/magic angle spinning NMR spectroscopy of the sorbent, either in the dry state or suspended in the mobile phase, showed an increase in mobility at elevated temperatures. Alkyl chain segments with gauche conformations were more mobile than chain segments with trans conformations. The strength of the 13C-1H dipolar couplings in the alkyl chains was measured using the constant time dipolar and chemical shift pulse sequence, revealing less molecular motion for the trans conformation. Non-linear van't Hoff plots were observed for separations of shape-constrained solutes (such as geometric beta-carotene isomers and polycyclic aromatic hydrocarbons). At higher temperatures, the retention behavior was similar to that of monomeric C18 sorbents, whereas at ambient and lower temperatures, enhanced shape-selective properties were exhibited similar to those of polymeric C30 sorbents.

Influence of synthetic routes on the conformational order and mobility of C18 and C30 stationary phases

Silica gels modified with n-alkyl chains (n = 18, 30) are prepared by two different synthetic routes and are examined by variable temperature FTIR and solid-state NMR spectroscopy. HPLC measurements of SRM 869, cis/trans ss-carotene isomers and xanthophylls isomers confirm the dependence of the separation mechanism on the alkyl chain length and the synthetic routes. The determination of the silane functionality and degree of cross-linking of silane ligands on the silica surface is achieved by 29Si CP/MAS NMR measurements. The structural order and mobility of the alkyl chains are investigated by means of variable temperature 13C CP/MAS NMR measurements. Variable temperature FTIR studies are performed where conformational order and flexibility of the alkyl chains in C18 and C30 phases are monitored through conformational sensitive CH2 symmetric, anti-symmetric stretching and wagging modes. In addition, the chromatographic properties of the C18 and C30 phases are determined. The results derived from the FTIR, NMR and HPLC measurements are discussed in the context of the applied synthetic routes and alkyl chain lengths.

Assembly of coordination nanostructures via ligand derivatization of oxide surfaces

A scheme is presented for the construction of coordination nanostructures on oxide surfaces (glass, Si/SiO2, quartz), based on application of epoxy-terminated monolayers as anchors for covalent grafting of ligands. Two ligands bearing amine groups were reacted with epoxysilane monolayers on oxide surfaces, providing ligand-terminated substrates. The ligands employed were (i) a pyridine moiety, used for subsequent binding of cobalt tetraphenylporphine (CoTPP), and (ii) deferoxamine (DFX), which contains hydroxamic acid moieties, used for subsequent construction of various Zr4+-based coordination layers. The results suggest that a dense ligand layer was obtained in both cases, allowing the formation of coordination overlayers on the oxide surfaces. The growth of coordinated layers was similar to analogous overlayers assembled on Au substrates, indicating that high ligand coverage is achieved by the epoxy-amine surface reaction. Epoxy-based functionalization of oxide substrates is a mild and efficient method for preparing high-quality coordination overlayers. Moreover, the method makes use of commercially available silane and amine reactants, providing the basis for wide application.

Contact-angle, ellipsometric, and spin-diffusion solid-state nuclear magnetic resonance spectroscopic investigations of copolymeric stationary phases immobilized on SiO2 surfaces

SiO2 surfaces-silica gel particles and silica wafers-were modified by covalently immobilizing three poly(ethylene-co-acrylic acid) copolymers, (-CH2CH2-)x[CH2CH/(CO2H)-]y, with different chain lengths and mass fractions of acrylic acid. 13C solid-state NMR spectroscopy on the modified silica gel particles revealed both mobile gauche and rigid trans aligned alkyl chains in the copolymers. For copolymers attached to silica wafers via a 3-aminopropyltriethoxysilane spacer molecule, ellipsometric measurements revealed a mean value of the layer thickness distribution of 6.5 and 4.3 nm, respectively, for the more acidic and the shorter copolymers with mobile alkyl chains mostly in the gauche conformation. For the longest and least acidic copolymer with more rigid trans ordered alkyl chains, however, a mean phase thickness of 10.6 nm was found. When this copolymer was immobilized via a 3-glycidoxypropyltrimethoxysilane spacer molecule we measured a mean layer thickness of 9.9 nm. A model of the surface morphology of this immobilization strategy was derived using spin-diffusion 13C NMR measurements on the corresponding modified silica. It was thereby proven that the trans and gauche-aligned alkyl chains occur in distinct domains of certain sizes on the silica surface. The surface polarity of all modified silica wafers was also investigated by measurement of contact-angle.

Order and disorder in alkyl stationary phases

Covalently modified surfaces represent a unique state of matter that is not well described by liquid or solid phase models. The chemical bond in tethered alkanes imparts order to the surface in the form of anisotropic properties that are evident in chromatographic and spectroscopic studies. An understanding of the structure, conformation, and organization of alkyl-modified surfaces is requisite to the design of improved materials and the optimal utilization of existing materials. In recent years, the study of alkyl-modified surfaces has benefited from advances in modern analytical instrumentation. Aspects of alkyl chain conformation and motion have been investigated through the use of nuclear magnetic resonance spectroscopy, Fourier transform infrared spectroscopy, Raman spectroscopy, fluorescence spectroscopy, and neutron scattering studies. Chromatography provides complementary evidence of alkyl chain organization through interactions with solute probes. Computational simulations offer insights into the structure of covalently modified surfaces that may not be apparent through empirical observation. This manuscript reviews progress achieved in the study of the architecture of alkyl-modified surfaces.

Selectivity of long chain stationary phases in reversed phase liquid chromatography

A series of commercial monomeric and polymeric C(18), C(27), and C(30) stationary phases were compared with immobilized poly(ethylene-co-acrylic acid) stationary phases synthesized in-house. The columns were characterized on the basis of methylene selectivity, silanol activity, metal activity, pore size, shape selectivity, and the ability to separate tocopherol isomers and carotenoid isomers. Monomeric and polymeric C(30) phases were shown to yield excellent separations of the tocopherol isomers while the polymeric C(30) and polyethylene phases were more appropriate to the separation of carotenoids.

Nuclear magnetic resonance and high-performance liquid chromatographic evaluation of polymer-based stationary phases immobilized on silica

Three poly(ethylene-co-acrylic) acid copolymers (-CH(2)CH(2)-)(x)[CH(2)CH(CO(2)H)-](y) with different chain lengths and mass fractions of acrylic acid were covalently immobilized as stationary phases on silica via two variants of spacer molecules (3-aminopropyltriethoxysilane and 3-glycidoxypropyltrimethoxysilane). Different mobilities of the alkyl chains in the stationary phases were observed using (13)C solid-state NMR spectroscopy. The stationary phases with more rigid trans-ordered alkyl chains had better selectivity for geometric beta-carotene and xanthophyll isomers (provitamin A derivatives). Also, all the separations of the analytes were affected by polar interactions with the chromatographic sorbent. This was further proved by separating more polar cis/trans retinoic acid isomers (vitamin A derivatives). (13)C high-resolution/magic-angle spinning (HR/MAS) NMR measurements of the chromatographic sorbents suspended in the mobile phase confirmed a dependence of molecular shape recognition ability on alkyl chain conformation.

Thermodynamics and kinetics of solute transfer in reversed-phase liquid chromatography

A series of four-ring polycyclic aromatic hydrocarbons (PAHs) with varying annelation structure was studied by reversed-phase liquid chromatography. Using a polymeric octadecylsilica stationary phase over a temperature range from 273 to 303 K and an average pressure range from 585 to 3585 psi (1 psi = 6894.76 Pa), the thermodynamic and kinetic aspects of the retention mechanism were examined. Thermodynamic behavior was characterized by the retention factor, together with the associated changes in molar enthalpy and molar volume, whereas kinetic behavior was characterized by the rate constants, together with the associated activation enthalpies and activation volumes. The data indicate that pyrene, with a more condensed annelation structure, exhibits smaller changes in molar enthalpy and molar volume (delta Hsm = -4.4 kcal/mol, delta Vsm = -1.9 ml/mol; 1 cal = 4.184J) than PAHs with a more linear structure such as chrysene (delta Hsm = -8.2 kcal/mol, delta Vsm = - 11.7 ml/mol). The kinetic data indicate that pyrene undergoes faster rates of transport than chrysene (k(ms) = 313 and 14 s(-1), respectively), but the non-planar benzo[c]phenanthrene undergoes the fastest transport (k(ms) = 330 s(-1)). The activation enthalpies and activation volumes are similarly affected by the annelation structure. It is noteworthy that deviations from the exponentially modified Gaussian (EMG) model are observed for some PAH zone profiles at the lowest temperature, which suggests a possible change in retention mechanism. In order to characterize these deviations, the non-linear chromatography (NLC) model and a new bi-exponentially modified Gaussian (E2MG) model were examined. The regression results indicate that neither the NLC nor E2MG model offer significant improvements in the statistical quality of fit or provide a better description of the observed retention behavior.

Studies on a poly(styrene-divinylbenzene)-encapsulated sub-micrometer sized organic–inorganic hybrid silica packing, its synthesis, some of its characteristics and applications

The hydrolyzed product of a mixture of tetraethoxysilane (TEOS) and vinyltriethoxysilane (VTEOS) was used to produce organic-inorganic hybrid silica by sol-gel processing. The product was a kind of composite with vinyl groups incorporated into the structure of the final particles through C-Si bonds. These hybrid silica particles were spherical and the average diameter was about 350 nm as measured by scanning electron microscopy (SEM). The particles were non-porous as proved by BET method. The hybrid silica particles were subsequently encapsulated with a layer of PS-DVB, a polymeric mixture of styrene and divinylbenzene, with the vinyl groups on the particle surface as anchors of the polymer chains. The encapsulated packing was used as stationary phases in CEC, and the chromatographic behaviors were studied. Experimental results showed that, as expected, the smaller diameter stationary phase resulted in higher column efficiencies. In comparison with the columns packed with 5 microm diameter encapsulated-silica stationary phases of the same kind and other sub-micron stationary phases reported, it can be concluded that the EOF velocity is independent of the particle diameter when using 350 nm phase, but determined by the surface nature of the stationary phase.