Hydrogen-Bond Network Breakage as a First Step to Isopropanol Crystallization

taneously ND measurements we have designed a special sample holder made from insulating anodized aluminum. be accomplished considering both the primary and the secondary relaxation as being Cole-Davidson processes Upon cooling liquids may either crystallize or vitrify. Crystallization is a common phase transition by which molecules self-organize into crystals [1]. Water is one of the most important liquids due to its direct implication in life. Among the peculiarities of liquid water is the presence of a dynamic three-dimensional hydrogen-bond network [2–4]. The physical and chemical properties of hydrogen-bond network forming liquids are controlled by the extension of intermolecular structures resulting from hydrogen bonds [5,6]. Related to crystallization, molecular dynamics simulation in water indicated that crystal nucleation occurs after a significant reorganization of the hydrogen-bond network [4]. However, this network modification has not been observed experimentally. This is in part due to the fast freezing kinetics of water [4]. On the contrary, crystallization kinetics of isopropanol, which is also a hydrogen-bond network forming liquid, can be thermally controlled from hours to seconds [7]. The major problem to correlate the evolution of a dynamic magnitude with that of a structural one comes from simultaneity. Crystallization phenomena can be very well studied by diffraction or calorimetry, among others. The dynamics of the hydrogen-bond network can be also well characterized by means of relaxation techniques such as dielectric spectroscopy [8,9]. However, in order to find a direct relationship between structure and dynamics, simultaneity may be crucial. Our experiments involve a novel experimental setup which allows one to perform simultaneous measurements of time resolved neutron diffraction (ND) and dielectric spectroscopy (DS). DS measures the complex dielectric permittivity " "0 i"00 as a function of frequency, where "0 is the dielectric constant and "00 is the dielectric loss. By measuring as a function of temperature, dielectric spectroscopy becomes a very powerful method to study molecular dynamics in liquids [8–10] above the glass transition temperature Tg. In this case, molecular motions give rise to a characteristic relaxation process, which can be observed as a maximum [8–10] in "00. To perform simul-