5.2. Slow relaxation component above Tc
THz-TDS measurements of liquid water have demonstrated that there is a relaxation process with a time constant of 8.5 ps at room temperature, and the time constant shows a power-law temperature dependence as (T − Ts)−1 (Ts = 228 K) above the melting point of water [32]. The time constant (~12 ps) of the slow component in the present study is comparable with that observed in liquid water at room temperature. Furthermore, almost all the water molecules in the reverse micelle are bulk-like. Hence, the slow component is considered to correspond to the relaxation process observed for liquid water due to the collective motions of water molecules. The power-law temperature dependence of the relaxation time indicates that the cooperative nature of the water's motions increases with decreasing temperature, that is, a larger hydrogen-bond network is formed for the relaxation process at lower temperatures; hence, the timescale of the process increases. Such a mechanism is valid for water confined in large reverse micelles. However, the confinement of the reverse micelles should affect the mechanism, and hence, it is reasonable that the slowing down of the relaxation process with decreasing temperature is enhanced in the reverse micelle compared with that in liquid water.
THz spectroscopy of water in AOT reverse micelles was performed at room temperature [13]. The study reported that there are two relaxation processes (~0.1 ps and ~1 ps) of water and that the two time constants hardly depend on w0 in the w0 range from 1 to 25. Moreover, the slower process was discussed in terms of the confinement effect on collective motions in liquid water. On the other hand, various experimental studies with varying w0 have demonstrated that most of the water molecules in AOT reverse micelles are bulk-like at w0 = 25 and that the properties of the water are close to those of liquid water [6,7,10]. Accordingly, it seems to be more reasonable that the time constant due to the slower relaxation at w0 = 25 is closer to that of liquid water (i.e., 8.5 ps), and the former is larger than the latter owing to slowing down by the confinement effect [7,19]. For comparison, the time constant in the present study is ~12 ps at room temperature, which is a reasonable value. The small time constant of ~1 ps derived in literature [13] could be ascribed to the lower limit of the spectral range examined, i.e., 0.2 THz; meanwhile, the lower limit in the present study and in the studies of liquid water [32] is 0.1 THz. A study with a lower limit of 0.1 THz for the reverse micelle at lower w0 values is needed, and is currently in progress in our laboratory.
5.3. Slow relaxation component at and below Tc
A dramatic drop in the time constant of the slow component at Tc in Fig. 6(a) is expected to be due to water shedding from the reverse micelle. Water excluded from the reverse micelle is free from the confinement of the reverse micelle.