Lanthanide Ion Interaction with a Crown Ether Methacrylic Polymer, Poly(1,4,7,10-tetraoxacyclododecan-2-ylmethyl methacrylate), as Seen by Spectroscopic, Calorimetric, and Theoretical Studies

Free Energy Calculations and Conformational Analysis of Dibenzo-30-crown-10 with Sm2+, Eu2+, and Three Halide Salts in THF Using the AMOEBA Force Field

Crown ether complexes have been tailored for use in industrial separations of lanthanides (Ln) as a part of rare earth mining and refining. Dibenzo-30-crown-10 (DB30C10) is one of the most efficient complexants for the separation of rare earth mixtures based on the cation size. To understand the origin of this complexation, molecular dynamics (MD) simulations of DB30C10 have been performed using different combinations of divalent Sm and Eu and three halide salts Cl^-, Br^-, and I^- in tetrahydrofuran (THF) solvent. DB30C10 was parameterized here for the polarizable atomic multipole optimized energetics for biomolecular simulation (AMOEBA) force field, and the existing parameters of THF, Sm²⁺, and Eu²⁺ were employed from our previous efforts. The large conformational fluctuations present in the DB30C10 systems were found to be dependent both on the identity of the lanthanide and halide complexes. For Cl^- and Br^- systems, there were no observed conformational changes at 200 ns, while in I^- systems, there were two conformational changes with Sm²⁺ and one with Eu²⁺ within that same timeframe. In SmI₂-DB30C10, there were three stages of conformational changes. In the first stage, the molecule is unfolded, in the second stage, the molecule is partly folded, and finally, in the third stage, the molecule is completely folded. Lastly, the Gibbs binding free energies of DB30C10 with SmBr₂ and EuBr₂ have been computed, which resulted in nearly identical ΔG_comp values for each lanthanide with Sm²⁺ being slightly more favorable. Considering the folding mechanism of the SmI₂ system with DB30C10, the Gibbs binding free energies of DB30C10 and dicyclohexano-18-crown-6 (DCH18C6) with SmI₂ were calculated separately and compared to probe their complexation affinities, in which the former was found to be more favorable.

Molecular Dynamics and Free Energy Calculations of Dicyclohexano-18-crown-6 Diastereoisomers with Sm2+, Eu2+, Dy2+, Yb2+, Cf2+, and Three Halide Salts in Tetrahydrofuran and Acetonitrile Using the AMOEBA Force Field

With the continual development of lanthanides (Ln) in current technological devices, an efficient separation process is needed that can recover greater amounts of these rare elements. Dicyclohexano-18-crown-6 (DCH18C6) is a crown ether that may be a promising candidate for Ln separation, but additional research is required. As such, molecular dynamics (MD) simulations have been performed on four divalent lanthanide halide salts (Sm2+, Eu2+, Dy2+, and Yb2+) and one divalent actinide halide salt (Cf2+) bound to three diastereoisomers of DCH18C6. Dy2+, Yb2+, Cf2+, DCH18C6, and tetrahydrofuran (THF) solvent were parameterized for the AMOEBA polarizable force field for the first time, whereas existing parameters for Sm2+ and Eu2+ were utilized from our previous efforts. A coordination number (CN) of six for Ln2+/An2+-O solvated in THF indicated that the cations interacted almost entirely with the oxygens of the polyether ring. A CN of one for Ln2+/An2+-N solvated in acetonitrile for systems containing iodide suggested that the N atom of acetonitrile was competitive with I- for cation interactions. Fluctuation between five and six CNs for Dy2+ and Yb2+ suggested that although the cations remained in the polyether ring, the size of the ring may not be an ideal fit as these cations possess comparatively smaller ionic radii. Gibbs binding free energies of Sm2+ in all DCH18C6 diastereoisomers solvated in THF were calculated. The binding free energy of the cis-syn-cis diastereoisomer was the most favorable, followed by cis-anti-cis, and then trans-anti-trans. Finally, two major types of conformation were observed for each diastereoisomer that were related to the electrostatic interactions and charge density of the cations.

Nanostructuring with Surfactants: The Self-Assembly of a New Poly(thiophene-phenylene) Conjugated Polymer Bearing Azacrown Ether Pendant Groups

We report the synthesis of a new conjugated polymer bearing crown ether moieties, poly[(N(1-aza-[18]crown-6)carbamido)thiophene-2,5-diyl-alt-1,4-phenylene] (BG2). In water, BG2 forms a dispersion with a slightly cloudy appearance. We have studied the effect of adding surfactants, with different polar head groups, on these polymer-polymer aggregates. Special attention is given to the system with the anionic surfactant, sodium dodecyl sulfate (SDS). The combination of photophysical techniques with electrical conductivity, NMR (¹H, ¹³C, and ²⁷Na), DFT calculations, molecular dynamics simulations, and small-angle neutron scattering (SANS) provides a detailed picture on the behavior of the SDS/BG2 system in aqueous solution and in thin films. NMR, electric conductivity, and DFT results suggest that hydrophilic interactions occur between the polar headgroup of the surfactant (OSO₃^- Na⁺) and the aza-[18]-crown-6 moiety. DFT calculations confirmed the capability of BG2 to form stable complexes with the Na⁺ cations, where the cation can be either inside the azacrown cavity or sandwiched between the cavity and the polymer chain, which seem to determine the position of the surfactant hydrocarbon chain and, therefore, be responsible for the disruption of the BG2 aggregates and subsequent increase in the photoluminescence quantum yields. SANS measurements, made with hydrogenated and deuterated SDS in D₂O, clearly show how micron-sized aggregates of BG2 are broken down by SDS and then how BG2 becomes preferentially incorporated within joint colloidal particles of BG2 and SDS with increasing [SDS]/[BG2] molar ratio.

The formation of host-guest complexes between surfactants and cyclodextrins

Cyclodextrins are able to act as host molecules in supramolecular chemistry with applications ranging from pharmaceutics to detergency. Among guest molecules surfactants play an important role with both fundamental and practical applications. The formation of cyclodextrin/surfactant host-guest compounds leads to an increase in the critical micelle concentration and in the solubility of surfactants. The possibility of changing the balance between several intermolecular forces, and thus allowing the study of, e.g., dehydration and steric hindrance effects upon association, makes surfactants ideal guest molecules for fundamental studies. Therefore, these systems allow for obtaining a deep insight into the host-guest association mechanism. In this paper, we review the influence on the thermodynamic properties of CD-surfactant association by highlighting the effect of different surfactant architectures (single tail, double-tailed, gemini and bolaform), with special emphasis on cationic surfactants. This is complemented with an assessment of the most common analytical techniques used to follow the association process. The applied methods for computation of the association stoichiometry and stability constants are also reviewed and discussed; this is an important point since there are significant discrepancies and scattered data for similar systems in the literature. In general, the surfactant-cyclodextrin association is treated without reference to the kinetics of the process. However, there are several examples where the kinetics of the process can be investigated, in particular those where volumes of the CD cavity and surfactant (either the tail or in special cases the head group) are similar in magnitude. This will also be critically reviewed.

The role of L-arginine in inclusion complexes of omeprazole with cyclodextrins

In this study, we investigate how the effect of L-arginine (ARG) and cyclodextrins upon omeprazole (OME) stability and solubility. The effect of the presence of ARG on the apparent stability constants (K(1:1)) of the inclusion complexes formed between OME and each cyclodextrin, beta-cyclodextrin (betaCD), and methyl-beta-cyclodextrin (MbetaCD) is studied by phase solubility diagrams and nuclear magnetic resonance (NMR) spectroscopy. The interaction of OME with those cyclodextrins, in the presence of ARG, is characterized using NMR spectroscopy and molecular dynamics simulations. ARG significantly increases the drug solubility and complex stability, in comparison to inclusion complexes formed in its absence. The effect is more pronounced for the OME:betaCD complex. ARG also contributes to a larger stability of OME when free in aqueous solution. The combination of ARG with cyclodextrins can represent an important tool to develop stable drug formulations.

Solid-state characterization and dissolution profiles of the inclusion complexes of omeprazole with native and chemically modified beta-cyclodextrin

The aim of this work was to investigate the formation of the inclusion complex between omeprazole (OME), a benzimidazolic derivative and a methylated cyclodextrin, methyl-beta-cyclodextrin (MbetaCD), with an average degree of substitution of 0.5. Inclusion complex between OME and beta-cyclodextrin (betaCD), a natural cyclodextrin, was used as reference. In aqueous media, apparent stability constants (K(s)), which describe the extent of formation of the complexes, have been determined by UV spectroscopy and 1H NMR experiments. The stoichiometry of the complexes was found to be 1:1 mol:mol OME:cyclodextrin (CD) and the value of K(s) was higher for OME:MbetaCD than for OME:betaCD inclusion complexes. Solid binary systems of OME and CDs were prepared by different techniques, namely kneading, spray-drying and freeze-drying. The formation and physicochemical characterization of solid inclusion complexes were investigated by differential scanning calorimetry (DSC), Fourier transform-infrared (FTIR), X-ray diffractometry (XRD) and scanning electron microscopy (SEM). The results show that freeze-drying method produces true inclusion complexes between OME and both CDs. In contrast, crystalline drug was detectable in kneaded and spray-drying products. The dissolution of OME from the binary systems was studied to select the most appropriate system for the development of a buccal drug delivery formulation. It was concluded that the preparation technique played an important role in the dissolution behaviour of the drug and the inclusion complex between OME and MbetaCD obtained by spray-drying and freeze-drying allowed better performances.

Stability constants: comparative study of fitting methods. Determination of second-order complexation constants by (23)Na and (7)Li NMR chemical shift titration

NMR chemical shift titration has been widely used as a method for the determination of stability constants. Systems involving metal-ligand complexation have been investigated using a number of methodologies. There are significant differences in the values reported for stability constants obtained by different experimental methods, such as calorimetry and ion selective electrode (ISE) titrations; nor has NMR chemical shift titration always yielded consistent results. Different researchers have obtained different results for the same system with results differing by as much as an order of magnitude. The chemical shift data are generally plotted against the concentration ratio of the metal and ligand for a set of solutions. A nonlinear least squares fitting method using an analytical solution of the cubic equation for the equilibrium concentration of the free ligand is used in this study and compared with methods used in the literature. Second-order association constants for the LiClO(4):12-crown-4 system in acetonitrile and the NaClO(4):12-crown-4 system in methanol are reported. Formation of both 1:1 and 1:2 metal-ligand complexes are considered. The LiClO(4):12-crown-4 acetonitrile system had been investigated previously by NMR titration but only 1:1 complexation was considered in that study. This study provides convincing evidence that both 1:1 and 1:2 complexes are important, at least, in the lithium system. A Monte Carlo investigation of the propagation of errors from the chemical shifts to the stability constants shows that the choice of data analysis methods may, in part, contribute to discrepancies and that the nonlinear nature of the model can dramatically affect the error limits on the stability constants.