Dynamics of Water in Partially Crystallized Solutions of Glass …
191
PVP, where two water relaxations were observed, as discussed in Sect. 7. In the next
section, we will discuss the impact of all these results on bulk water.
8 The Implication of the Present Results for Bulk Water
In this section, we discuss likely implications of the findings presented above for
water in both solutions and hard confinements. In the case of water in “ordinary
solutions” (i.e., solutions with only a single water relaxation), we found that the
dynamic crossover temperature of the water relaxation is strongly affected by the
glass transition temperature. The dynamic crossover occurs at T g of the solution
irrespective of the degree of water crystallization. This observation suggests that the
dynamic crossover is not an intrinsic property of the water in the solution, but rather
caused by an immobilization of the solute matrix at T g , which imposes a confinement
effect of the water intercalated in the matrix. Thus, water in these kinds of solutions
behaves as confined water only below T g , and the water is furthermore affected by
interactions with the solute matrix, causing the water relaxation to be slower at low
water contents. This also implies that the water relaxation becomes slower if the
effective water concentration decreases by freeze-concentration, as occurs during
crystallization at a high temperature. This strong influence of the local environment
on the water relaxation in these kinds of solutions indicates that the water relaxation
cannot be considered as an intrinsic property of water, and therefore not be of strong
relevance for bulk water.
However, for solutions exhibiting two water relaxations, such as solutions of
PVP, the crossover temperature of the fast water relaxation is independent on the ice
level and therefore independent on the T g value. This is also the case for water in
hard confinements, as can be seen in Ref. [33] where the degree of crystallization
can be regulated by the pore size. Furthermore, in these two types of systems also
the time scale and activation energy of the water relaxation are unaffected by the
amount of ice in the systems. These findings indicate that the dynamical properties
of this amorphous water can be considered as intrinsic properties of supercooled
or glassy water. Hence, bulk water is then expected to exhibit similar dynamical
properties in the same temperature range. Indeed, Amann-Winkel et al. [57] observed
a dielectric relaxation process of low-density amorphous ice (LDA) that is almost
identical to this “universal” relaxation of confined water. This strongly suggests that
the low-temperature water relaxation we observe in both solutions exhibiting two
water relaxations and in hard confinements is the same as for bulk water. Moreover,
DSC studies of both crystalline and amorphous ices show that the calorimetric feature
observed for LDA at 136 K [58] is identical to the calorimetric feature observed
for hydrogen-disordered crystalline ice phases at a similar temperature [59]. Since
this endothermic peak is not affected by isotope substitution of
16 O to
18 O, but
significantly shifted for D 2 O, it is also clear that this is due to an unfreezing of
molecular reorientation dynamics, rather than related to translational motions of the
oxygen atoms, which should be the case if it was a true glass-to-liquid transition.
191
PVP, where two water relaxations were observed, as discussed in Sect. 7. In the next
section, we will discuss the impact of all these results on bulk water.
8 The Implication of the Present Results for Bulk Water
In this section, we discuss likely implications of the findings presented above for
water in both solutions and hard confinements. In the case of water in “ordinary
solutions” (i.e., solutions with only a single water relaxation), we found that the
dynamic crossover temperature of the water relaxation is strongly affected by the
glass transition temperature. The dynamic crossover occurs at T g of the solution
irrespective of the degree of water crystallization. This observation suggests that the
dynamic crossover is not an intrinsic property of the water in the solution, but rather
caused by an immobilization of the solute matrix at T g , which imposes a confinement
effect of the water intercalated in the matrix. Thus, water in these kinds of solutions
behaves as confined water only below T g , and the water is furthermore affected by
interactions with the solute matrix, causing the water relaxation to be slower at low
water contents. This also implies that the water relaxation becomes slower if the
effective water concentration decreases by freeze-concentration, as occurs during
crystallization at a high temperature. This strong influence of the local environment
on the water relaxation in these kinds of solutions indicates that the water relaxation
cannot be considered as an intrinsic property of water, and therefore not be of strong
relevance for bulk water.
However, for solutions exhibiting two water relaxations, such as solutions of
PVP, the crossover temperature of the fast water relaxation is independent on the ice
level and therefore independent on the T g value. This is also the case for water in
hard confinements, as can be seen in Ref. [33] where the degree of crystallization
can be regulated by the pore size. Furthermore, in these two types of systems also
the time scale and activation energy of the water relaxation are unaffected by the
amount of ice in the systems. These findings indicate that the dynamical properties
of this amorphous water can be considered as intrinsic properties of supercooled
or glassy water. Hence, bulk water is then expected to exhibit similar dynamical
properties in the same temperature range. Indeed, Amann-Winkel et al. [57] observed
a dielectric relaxation process of low-density amorphous ice (LDA) that is almost
identical to this “universal” relaxation of confined water. This strongly suggests that
the low-temperature water relaxation we observe in both solutions exhibiting two
water relaxations and in hard confinements is the same as for bulk water. Moreover,
DSC studies of both crystalline and amorphous ices show that the calorimetric feature
observed for LDA at 136 K [58] is identical to the calorimetric feature observed
for hydrogen-disordered crystalline ice phases at a similar temperature [59]. Since
this endothermic peak is not affected by isotope substitution of
16 O to
18 O, but
significantly shifted for D 2 O, it is also clear that this is due to an unfreezing of
molecular reorientation dynamics, rather than related to translational motions of the
oxygen atoms, which should be the case if it was a true glass-to-liquid transition.