Molecular motion of alcohols adsorbed in ACF hydrophobic nanoslits as studied by solid-state NMR

• Published in: Adsorption : journal of the International Adsorption Society 2015-05, Vol.21 (4), p.273-282
• Main Authors: Omichi, Hiroaki, Ueda, Takahiro, Eguchi, Taro
• Format: Article
• Language: English
• Subjects: Alcohols; Chain dynamics; Chemistry; Chemistry and Materials Science; Engineering Thermodynamics; Heat and Mass Transfer; Industrial Chemistry/Chemical Engineering; Liquids; Molecular motion; Molecular structure; Nuclear magnetic resonance; Slits; Spectra; Surfaces and Interfaces; Thin Films
• ISSN: 0929-5607; 1572-8757
• DOI: 10.1007/s10450-015-9669-5

The molecular motion and local structure of methanol-d 1 (CH 3 OD) and ethanol-d 1 (C 2 H 5 OD) in activated carbon fiber (ACF) with a slit width of 0.7 and 1.1 nm have been investigated by solid-state 1 H and 2 H NMR. 2 H NMR spectra for alcohols confined in ACF gave a so-called Pake doublet below 140 K, which were characterized by the 2 H quadrupole coupling constant (QCC) of 185 kHz and the asymmetric parameter of the electric-field-gradient tensor ( η ) of 0.1. The QCC value of the deuteron was indicative of the hydrogen bond formation with the O···O distance of ca. 0.27 nm, suggesting the solid-like feature of alcohols in ACFs. The quadrupole broadening vanished on heating and the single isotropic resonance line was observed. Alcohol molecules were undergoing a rapid motion like as in the bulk liquid, indicating a transition from solid to liquid in ACFs. Temperature-dependent 1 H NMR spectra were used for evaluating the E a value for reorientation in solid-like alcohols, whereas the 2 H NMR spectra were used for obtaining the translation accompanying reorientation in liquid-like alcohols. Alcohols with a bilayered structure in ACF with a slit width of 1.1 nm gave similar E a values, whereas C 2 H 5 OD in ACF with a slit width of 0.7 nm, in which a monolayered structure is expected, exhibited a cross-over of two activation processes at 172 K. The variation in E a was probably caused by the structural relaxation concerning with the hydrogen bonding formation and/or excitation of the large amplitude local motion. The competition of the directionality of hydrogen bonds and a freedom of molecular orientation plays an important role to characterize the intermolecular structure as well as the physicochemical properties in amphipathic molecules in confinement.