Published: January 19, 2011 r2011 American Chemical Society 958 dx.doi.org/10.1021/jp110737q | J. Phys. Chem. B 2011, 115, 958–963 ARTICLE pubs.acs.org/JPCB Thermal Expansivity of Ionic Clathrate Hydrates Including Gaseous Guest Molecules Kyuchul Shin, †,§ Wonhee Lee, † Minjun Cha, † Dong-Yeun Koh, † Yong Nam Choi, ‡ Heeju Lee, ‡ Bae Soon Son, ‡ Seongsu Lee, ‡ and Huen Lee* ,† † Department of Chemical and Biomolecular Engineering (BK21 Program) and Graduate School of EEWS, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea ‡ Korea Atomic Energy Research Institute, P.O. Box 105, Yuseong-gu, Daejeon 305-600, Republic of Korea b S Supporting Information ABSTRACT: Although thermal expansion is a key factor in relation to the host-guest interaction of clathrate hydrates, few studies have investigated the thermal behavior of ionic clathrate hydrates. The existence of ionic species in these hydrates creates a unique host-guest interaction compared to that of nonionic clathrate hydrates. It was revealed that X-ray diffraction cannot be used for research of tetramethy- lammonium hydroxide clathrate hydrates due to damage of the cations by the X-ray, which results in abnormal thermal expansion of the ionic clathrate hydrates. Hence, in the pres- ent work, the thermal expansivities of binary sII Me 4 NOD 3 16D 2 O and sI DClO 4 3 5.5D 2 O were measured by neutron powder dif- fraction (NPD) in order to shed light on their thermal behavior. General correlations for the thermal behaviors of given structures were established and lattice expansions depending on the guests were compared between ionic and nonionic clathrate hydrates. The peculiar change in the thermal expansivity of binary DClO 4 3 5.5D 2 O was also considered in relation to the host-guest configuration. 1. INTRODUCTION Ionic clathrate hydrates are known to form various crystal structures by encaging hydrophobic cations or anions into confined cavities and incorporating counterions into the water host lattice. 1,2 Due to the ionic interaction between the ionic guest and host, ionic clathrate hydrates exhibit discrete magnetic behavior, metal ion encagement, cohost inclusion, and excellent thermal stability and proton conductivity. 3-8 The thermal ex- pansivity of both nonionic and ionic clathrate hydrates and a close comparison between these two cases can provide valuable information concerning the interaction between the guest and host lattice. Diffraction studies are essential for calculation of the lattice parameter and determination of the volume change of hydrates depending on temperature. A variety of nonionic clathrate hydrates have been explored using the X-ray diffraction techni- que to reveal their thermal properties. Tse et al. reported that the larger thermal expansion of the sI ethylene oxide hydrate relative to that of hexagonal ice is profoundly affected by the greater anharmonicity of the clathrate lattice. 9 Takeya et al. revealed that the lattice parameter and volume expansion of sI, sII, and sH mixed gas hydrates are strongly affected by the physicochemical properties of the guest molecules. 10,11 Natural gas and air clathrate hydrates from sediments and ice cores were also studied to elucidate their thermal properties. 12,13 Recently, Hester et al. measured the thermal expansivity of sI and sII gas hydrates at various gas concentrations and constructed general correlations for thermal expansivity. 14 While diffraction studies have yielded the aforementioned findings, it is very difficult to identify light atoms in the presence of heavy atoms by X-ray diffraction, because X-ray scattering originates from the interaction of X-ray photons with electron clouds. Furthermore, X-ray diffraction may not be appropriate for some types of ionic clathrate hydrates in that, in the case of peralkylammonium hydroxide clathrate hydrates, X-irradiation splits the tetraalkylammonium cations into alkyl radicals. 15,16 Hence, X-ray scattering can only be used for determination of the crystal structure for peralkylammonium hydroxide hydrates, while measurement of the cell parameters or thermal properties of these compounds with X-ray diffraction technique leads to unreliable results. In contrast, the neutron diffraction technique shows high sen- sitivity to light atoms such as D due to interaction of the neutrons with nuclei instead of with electron clouds. Furthermore, neu- tron scattering does not damage peralkylammonium cations, and consequently measurement results of the lattice parameter and thermal properties of ionic clathrate hydrates are more reliable. Despite this advantage, the neutron approach has not been used Received: November 10, 2010 Revised: January 3, 2011