(Present Research at Toyota Physical & Chemical Research Institute)Studies on Static and Dynamic Fluctuations of Disordered Materials
and Functions Caused by Fluctuations
Keiko Nishikawa
Purpose of the Research“Fluctuation” is a concept to express the deviation from the average. Spatial molecular distributions(static fluctuations) and temporal fluctuations (dynamic fluctuations) determine the structure andphysical properties of the system and cause its subsequent development over time. I take up the mixed
state of solutions (static fluctuations) and the dynamics in phase change (dynamic fluctuations) assystems in which fluctuations are manifested in materials science. From various experiments, I observe
fluctuations from various angles and systematically, express them quantitatively, elucidate thecharacteristics of each fluctuation and the mechanisms causing them, and relate them to the function of
the fluctuation field.
Methods[Static Fluctuations]
When two types of samples are mixed, various mixing states occur, reflecting the properties andinteractions of the components. Kirkwood & Buff and Bhatia & Thornton theoretically showed thatquantitative knowledge of the mixing state can be obtained. Based on the former theory (KB theory), it
is quantitatively expressed as Kirkwood-Buff Integral (KBI), and experimentally, the threethermodynamic quantities must be obtained and combined. In the latter theory (BT theory), it isexpressed as density fluctuation and concentration fluctuation, and one of the above thermodynamicquantities (second-order derivative of Gibbs energy by concentration) can be replaced by the scattering
intensity obtained from small-angle scattering experiments, which has a strong structure-science color.
Although many researchers in the world have used KB theory to study solution structures, my group isat the forefront of using BT theory to experimentally determine the mixing state of solutions. Several
experiments have shown that BT theory's methods are superior in terms of data accuracy andapplicability. It is also shown that KBI can be linked to various fluctuations mathematically.
In addition to simple aqueous solutions of molecular liquids (e.g., aqueous alcoholic solutions), I takeup systems related to life science; for example, I elucidate how solvent fluctuations affect the dissolved
state of proteins. In addition, my group is considering the development of supercritical solution systems.
In this system, it is not possible to obtain the experimental results using KB theory, so we conduct
experiments using the method of BT theory. In addition, we have recently experienced the inconvenienceof applying BT theory to such a system. We will seek experimental methods to overcome theseshortcomings.
[Dynamic Fluctuations]Not only in physical and chemical changes in materials, but also in life phenomena, heat is always
absorbed of released when they are changing. If we can track the phenomenon with enough sensitivityto detect the heat in and out and with a scanning speed matching the change occurring, we can also
obtain information on the dynamics.
In the past, it was difficult to capture the dynamic fluctuations that occur during the phase transition
as the heat input and output. However, we have succeeded in capturing them using the instrument
developed in my laboratory and a suitable sample selection.
The instrument is an ultra-sensitive differential thermal analysis system. In the commerciallyavailable devices, the sensitivity and stability of 3 to 5 µW and the scanning speed for raising andlowering the temperature are usually 1 to 10 K/min (60 to 6 seconds for 1 K temperature change). The
instrument we developed so far has a sensitivity of 3 nW (about 1000 times better than the commercialdevice) and the slowest scanning speed is 0.01 mK/s (28 hours for 1 K temperature change). Also, theappropriate samples are ionic liquids. My group has found that many of the ionic liquids exhibit very
slow and specific phase change behaviors and are best suited to track the dynamics during phase change.
In other words, they are systems that can directly observe the dynamic behaviors of fluctuations andturbulence in the phase transition as if in slow motion mode. Using several representative ionic liquids
as samples, the phase behaviors in terms of dynamic fluctuations are elucidated by the ultra-sensitive
differential thermal analysis apparatus.
The time scales on which these dynamic fluctuations are occurring are the time domains in which
NMR excels, and we follow the dynamics from an NMR perspective by making full use of longitudinaland lateral relaxation time measurements. When applied to ionic liquids, the kinetics of each part of theconstituent ions are different, which leads to complex phase behavior due to fluctuations in the kineticsof the ions. NMR measurements are applied to representative ionic liquids in order to relate the dynamics
of phase change to the dynamics of constituent ions and atoms.
Expected Results
Many of the fluctuation phenomena in material science occur in mesoscale space. A spatial domainof this size is described as a "lost domain", and mesoscale science is the most lagging field. Academically,“fluctuation” will represent the disordered structure in terms of mesoscale and will propose a new
methodology for mesoscale structural chemistry and property science. In application, the quantificationof the heterogeneity of solution mixing by concentration fluctuations leads to the design of reaction andseparation fields from a new perspective that is sensitive to fluctuations. In other words, the mixing state
can be freely designed according to the combined substances, concentration, and temperature.
In addition, the dynamic fluctuations elucidated in ionic liquids can be considered as a slow-motion
version of the phase change behavior, and it is possible to discuss the phase change based on thefluctuations that can be applied to general materials. In terms of applications, the phase behaviors ofcomplex ionic liquids can be used as devices for thermal energy treatment such as heat storage and heatexchange systems.