Thermal Properties and Mixing State of Ethylene Glycol-Water Binary Solutions by Calorimetry, Large-Angle X-ray Scattering, and Small-Angle Neutron Scattering Masaru Matsugami, ² Toshiyuki Takamuku,* ,‡ Toshiya Otomo, § and Toshio Yamaguchi | Department of Functional Molecular Science, The Graduate UniVersity for AdVanced Studies, Myodaiji, Okazaki 444-8585, Japan, Department of Chemistry and Applied Chemistry, Faculty of Science and Engineering, Saga UniVersity, Honjo-machi, Saga 840-8502, Japan, Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), Oho, Tsukuba 305-0801, Japan, and AdVanced Materials Institute and Department of Chemistry, Faculty of Science, Fukuoka UniVersity, Nanakuma, Jonan-ku, Fukuoka 814-0180, Japan ReceiVed: March 9, 2006; In Final Form: April 29, 2006 Thermal properties and mixing states of ethylene glycol (EG)-water binary solutions in the entire mole fraction range of EG, 0 e x EG e 1, have been clarified by using differential scanning calorimetry (DSC), large-angle X-ray scattering (LAXS), and small-angle neutron scattering (SANS) techniques. The DSC curves obtained have shown that the EG-water solutions over the range of EG mole fraction 0.3 e x EG e 0.5 are kept in the supercooling state until ∼100 K, and those in the range of 0.6 e x EG e 0.8 are vitrified, and those in the ranges of 0 < x EG e 0.2 and 0.9 e x EG < 1 are crystallized. The radial distribution function (RDF) for pure EG obtained from the LAXS measurements has suggested that a gauche conformation of an EG molecule is favorable in the liquid. The RDFs for the EG-water solutions have shown that the structure of the binary solutions moderately changes from the inherent structure of EG to the tetrahedral-like structure of water when the water content increases. The SANS intensities for deuterated ethylene glycol (HOCD 2 CD 2 OH) (EGd 4 )-water solutions at x EG ) 0.4 and 0.6 have not been significantly observed in the temperature range from 298 to 173 K, showing that EG and water molecules are homogeneously mixed. On the other hand, the SANS intensities at x EG ) 0.2 and 0.9 have been strengthened when the temperature decreases due to crystallization of the solutions. On the basis of all the present results, a relation between thermal properties of EG-water binary solutions and their mixing states clarified by the LAXS and SANS measurements has been discussed at the molecular level. Introduction Ethylene glycol (EG) is mixed with water at any ratio, and addition of EG to water and aqueous solutions can easily prevent crystallization with decreasing temperature. Hence, EG is often used as an inhibitor for freezing of water in industry. In the field of biology, the vitrifiability of EG is applied to cryopreser- vation of biological cells and tissues without significant damage. On the other hand, physicochemical properties of EG and aqueous solutions of EG have been investigated by many researchers with various techniques. In particular, there have been numerous reports on thermodynamic properties of EG-water solutions, such as excess molar volumes 1-3 and enthalpies. 4-6 The physicochemical properties of EG-water solutions should arise from the microscopic mixing state of EG and water molecules. A possible conformation of an EG molecule has been investigated by using computer calculations, such as ab initio. 7-10 Most of the investigations have shown an intramolecular hydrogen bond between the hydroxyl groups of an EG molecule, i.e., a gauche OC-CO conformation is stabilized by hydrogen bonding. Bako ´ et al. have made X-ray and neutron scattering measurements on pure EG and treated their data by using a Monte Carlo method to clarify the liquid structure. 11 They have also examined conformation of the EG molecule in gas phase by using density-functional theory (DFT). Both X-ray and neutron scattering on liquid EG and the theoretical method on EG in gas phase have reached the same conclusion that an intramolecular hydrogen bond is formed in an EG molecule with a gauche OC-CO conformation. Moreover, a plausible structure of dimer EG by hydrogen bonding has been proposed from the DFT results. Many molecular dynamics (MD) investigations have also been made on pure EG. 12-15 The X-ray and neutron scattering measurements by Bako ´ et al. 11 and the MD simula- tions 14,15 on pure EG showed that an EG molecule is hydrogen bonded with three to four neighbor molecules. For EG-water binary solutions, NMR measurements have been made on EG-water solutions to determine the self-diffusion coefficients of both EG and water molecules 16 and the rotational correla- tion time of H 2 17 O. 17 It has been indicated that the dynamic hydration number (5.7) of EG, which is estimated from the correlation time determined, is comparable with that (5.8) for methanol. There have been several reports of MD simulation on EG-water solutions to elucidate their structure. 18-20 Despite these efforts, the structure of EG-water solutions has not yet been clarified at the molecular level by using X-ray and neutron scattering techniques. In the present investigation, to clarify the relation between thermal properties of EG-water binary solutions and their * To whom all correspondence should be addressed. E-mail: takamut@ cc.saga-u.ac.jp. ² The Graduate University for Advanced Studies. ‡ Saga University. § High Energy Accelerator Research Organization (KEK). | Fukuoka University. 12372 J. Phys. Chem. B 2006, 110, 12372-12379 10.1021/jp061456r CCC: $33.50 © 2006 American Chemical Society Published on Web 06/03/2006