SANS contrast matching for the unambiguous localization of anionic dye in cationic surfactant micelles

Contrast variation in small-angle neutron scattering (SANS) was successfully applied to localize the anionic azo dye Blue in co-assemblies with the cationic surfactant dodecyltrimethylammoniumbromide (DTAB). For this purpose, the scattering contrast between DTAB and the aqueous solvent was eliminated by SANS contrast matching, leaving only the scattering signal from Blue to be detected. Results obtained by contrast matching were confirmed by NOESY NMR-spectroscopy, showing that Blue interacts with the positively charged DTAB head groups and with up to the 4th neighbouring methylene group of the DTAB C12-alkyl chain. Its localization in the outer layer of the Blue-DTAB co-assembly explains the uniaxial growth of spheroidal DTAB micelles to wormlike micelles with increasing [Blue] : [DTAB] ratio from 0 : 1 to 1 : 3. This is in line with the concept of the packing parameter for amphiphilic substances.

19F NMR Characterization of the Encapsulation of Emerging Perfluoroethercarboxylic Acids by Cyclodextrins

Legacy perfluoroalkyl substances (PFASs) are known environmental pollutants with serious adverse health effects. Perfluoroethercarboxylic acids (PFECAs), emerging PFASs now being substituted for legacy PFASs, have recently been detected in the environment. Cyclodextrins (CDs) have been proposed as agents for the remediation of problematic pollutants, including legacy PFASs. The current study uses ¹⁹F NMR spectroscopy to measure the complexation of mono-, di-, and triether PFECAs by CDs for eventual environmental applications. Eight PFECAs were characterized by ¹⁹F and ¹³C NMR. The change in chemical shift of individual fluorines upon complexation of CDs at various stoichiometric ratios was used to determine the host-guest association constants. All studied PFECAs were most strongly encapsulated by β-CD, with association constants from 10²-10⁵ M^-1 depending on chain length and number of ether functionalities. ¹⁹F-¹H heteronuclear Overhauser effect spectroscopy (HOESY) NMR experiments were performed for the β-CD complexes of two branched monoethers, PFPrOPrA ("GenX") and PFDMMOBA, to elucidate the structural details of the complexes, determine the specific orientation, and position of β-CD along the PFAS chain, and assess the roles of hydrogen-bonding and PFECA branching on the host-guest interactions. The results give new understanding into the fundamental nature of the host-guest complex between cyclodextrins and perfluorinated surfactants.

Preparation and Characterization of a Polymer-Based "Molecular Accordion"

A urethane-based polymer material, denoted HDI-1, was obtained from the addition reaction of β-cyclodextrin (β-CD) with 1,6-hexamethylene diisocyanate (HDI) at the 1:1 mole ratio. In aqueous solution and ambient temperature conditions, HDI-1 adopts a compact (coiled) morphology where the cross-linker units become coiled and are partially self-included in the annular hydroxyl (interstitial) region of β-CD. As the temperature is raised or as p-nitrophenol (PNP) was included within the β-CD cavity and the noninclusion sites of the polymer, an extended (uncoiled) morphology was adopted. The equilibrium distribution between the extended and the compact forms of HDI-1 is thermally and chemically switchable, in accordance with the hydration properties and host-guest chemistry of this responsive polymer system. The molecular structure of this water-soluble urethane polymer and its host-guest complexes with PNP were investigated using spectroscopic (Raman, (1)H NMR, induced circular dichroism), dynamic light scattering (DLS), and calorimetric (DSC) methods in aqueous solution at ambient pH, and compared with native β-CD. This study reports on the unique supramolecular properties of a polymer that resembles a thermally and chemically responsive "molecular accordion".

A volumetric and NMR study of cyclodextrin-inhalation anesthetic complexes in aqueous solutions

The apparent molar volumes (Vϕ) of two anesthetics (halothane and forane) have been determined in water and in binary solvent (H2O + cyclodextrin) systems at 25 °C. The results show that the magnitudes of Vϕ are greater in ternary solutions than in the binary aqueous systems. The apparent molar volumes at infinite dilution (Vϕo) of halothane in ternary solutions are observed to depend on the following factors: (i) the magnitude of the binding constant (K1:1), (ii) the lipophilicity of the anesthetic, (iii) the mole ratio of the host/halothane system, and (iv) the topology (i.e., facial vs. inclusion) of the host/guest complex. The volumetric properties of the ternary systems have been analyzed in terms of the complexed and uncomplexed species by application of Young’s rule. The formation of 1:1 CD–halothane complexes was successfully modeled using a two-state model. The binding affinity of the various cyclodextrins toward halothane is listed in descending order as follows: DM-β-CD > HP-β-CD > β-CD > α-CD > TM-β-CD.